IEC 60076-7:2005
(Main)Power transformers - Part 7: Loading guide for oil-immersed power transformers
Power transformers - Part 7: Loading guide for oil-immersed power transformers
Is applicable to oil-immersed transformers and describes the effect of operation under various ambient temperatures and load conditions on transformer life.
Transformateurs de puissance - Partie 7: Guide de charge pour transformateurs immergés dans l'huile
Est applicable aux transformateurs immergés dans l'huile et décrit l'effet du fonctionnement pour diverses températures ambiantes et conditions de charge durant la vie du transformateur.
General Information
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Standards Content (Sample)
INTERNATIONAL IEC
STANDARD 60076-7
First edition
2005-12
Power transformers –
Part 7:
Loading guide for oil-immersed
power transformers
This English-language version is derived from the original
bilingual publication by leaving out all French-language
pages. Missing page numbers correspond to the French-
language pages.
Reference number
Publication numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series. For example, IEC 34-1 is now referred to as IEC 60034-1.
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The IEC is now publishing consolidated versions of its publications. For example,
edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the
base publication incorporating amendment 1 and the base publication incorporating
amendments 1 and 2.
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INTERNATIONAL IEC
STANDARD 60076-7
First edition
2005-12
Power transformers –
Part 7:
Loading guide for oil-immersed
power transformers
© IEC 2005 Copyright - all rights reserved
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60076-7 IEC:2005 – 3 –
CONTENTS
FOREWORD.7
INTRODUCTION.11
1 Scope.13
2 Normative references .13
3 Definitions .13
4 Symbols and abbreviations.17
5 Effect of loading beyond nameplate rating .21
5.1 Introduction .21
5.2 General consequences.23
5.3 Effects and hazards of short-time emergency loading.23
5.4 Effects of long-time emergency loading .25
5.5 Transformer size .27
5.6 Non-thermally and thermally upgraded insulation paper .27
6 Relative ageing rate and transformer insulation life .31
6.1 General .31
6.2 Relative ageing rate .31
6.3 Loss-of-life calculation.33
6.4 Insulation life.33
7 Limitations.33
7.1 Current and temperature limitations.33
7.2 Specific limitations for distribution transformers.35
7.3 Specific limitations for medium-power transformers .37
7.4 Specific limitations for large power transformers.39
8 Determination of temperatures .41
8.1 Hot-spot temperature rise in steady state .41
8.2 Top-oil and hot-spot temperatures at varying ambient temperature and load
conditions.53
8.3 Ambient temperature .63
9 Influence of tap changers .65
9.1 General .65
9.2 Short-circuit losses.67
9.3 Ratio of losses .67
9.4 Load factor .67
Annex A (informative) Calculation of winding and oil time constant .69
Annex B (informative) Practical example of the exponential equations method.73
Annex C (informative) Illustration of the differential equations solution method .83
Annex D (informative) Flowchart, based on the example in annex B .101
Annex E (informative) Example of calculating and presenting overload data .105
Bibliography.113
60076-7 IEC:2005 – 5 –
Figure 1 – Sealed tube accelerated ageing in mineral oil at 150 °C.29
Figure 2 – Thermal diagram .43
Figure 3 – Local temperature rises above air temperature in a 120 kV winding at a load
factor of 1,6 .45
Figure 4 – Local temperature rises above air temperature in a 410 kV winding at a load
factor of 1,6 .47
Figure 5 – Two fibre optic sensors installed in a spacer before the spacer was installed
in the 120 kV winding.47
Figure 6 – Zigzag-cooled winding where the distance between all sections is the same
and the flow-directing washer is installed in the space between sections .51
Figure 7 – Top view section of a rectangular winding with "collapsed cooling duct
arrangement" under the yokes .51
Figure 8 – Temperature responses to step changes in the load current.55
Figure 9 – The function f (t) generated by the values given in Table 5 .59
Figure 10 – Block diagram representation of the differential equations.61
Figure 11 – Principle of losses as a function of the tap position .67
Figure B.1 – Hot-spot temperature response to step changes in the load current .79
Figure B.2 – Top-oil temperature response to step changes in the load current .79
Figure C.1 – Plotted input data for the example .93
Figure C.2 – Plotted output data for the example .99
Figure E.1 – OF large power transformers: permissible duties for normal loss of life.111
Table 1 – Life of paper under various conditions .29
Table 2 – Relative ageing rates due to hot-spot temperature .31
Table 3 – Normal insulation life of a well-dried, oxygen-free thermally upgraded insulation
system at the reference temperature of 110 °C .33
Table 4 – Current and temperature limits applicable to loading beyond nameplate rating .35
Table 5 – Recommended thermal characteristics for exponential equations .59
Table 6 – Correction for increase in ambient temperature due to enclosure .65
Table B.1 – Load steps of the 250 MVA transformer .73
Table B.2 – Temperatures at the end of each load step .81
Table C.1 – Input data for example .91
Table C.2 – Output data for the example.97
Table E.1 – Example characteristics related to the loadability of transformers .105
Table E.2 – An example table with the permissible duties and corresponding daily loss of
life (in "normal" days), and maximum hot-spot temperature rise during the load cycle.109
60076-7 IEC:2005 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 7: Loading guide for oil-immersed power transformers
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60076-7 has been prepared by IEC technical committee 14: Power
transformers.
This standard cancels and replaces IEC 60354 published in 1991. This first edition constitutes
a technical revision of the material given in IEC 60354. Details of the changes are given in the
introduction.
The text of this standard is based on the following documents:
FDIS Report on voting
14/512/FDIS 14/520/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
60076-7 IEC:2005 – 9 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
IEC 60076 consists of the following parts, under the general title Power transformers:
Part 1: General
Part 2: Temperature rise
Part 3: Insulation levels, dielectric tests and external clearances in air
Part 4: Guide to the lightning impulse and switching impulse testing – Power transformers
and reactors
Part 5: Ability to withstand short circuit
Part 7: Loading guide for oil-immersed power transformers
Part 8: Application guide
Part 10: Determination of sound levels
Part 11: Dry-type transformers
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
60076-7 IEC:2005 – 11 –
INTRODUCTION
This part of IEC 60076 provides guidance for the specification and loading of power
transformers from the point of view of operating temperatures and thermal ageing. It provides
recommendations for loading above the nameplate rating and guidance for the planner to
choose appropriate rated quantities and loading conditions for new installations.
IEC 60076-2 is the basis for contractual agreements and it contains the requirements and
tests relating to temperature-rise figures for oil-immersed transformers during continuous
rated loading. It should be noted that IEC 60076-2 refers to the average winding temperature
rise while this part of IEC 60076 refers mainly to the hot-spot temperature and the stated
values are provided only for guidance.
This part of IEC 60076 gives mathematical models for judging the consequence of different
loadings, with different temperatures of the cooling medium, and with transient or cyclical
variation with time. The models provide for the calculation of operating temperatures in the
transformer, particularly the temperature of the hottest part of the winding. This hot-spot
temperature is, in turn, used for evaluation of a relative value for the rate of thermal ageing
and the percentage of life consumed in a particular time period. The modelling refers to small
transformers, here called distribution transformers and to power transformers.
A major change from IEC 60354:1991 is the increased use of fibre optic temperature sensors
in transformers. This has radically increased the possibilities of obtaining a proper thermal
modelling of power transformers, especially at step changes in the load current. These
possibilities have also yielded some differences between the "oil exponent x" and the "winding
exponent y" used in this part of IEC 60076 and in IEC 60076-2:1993, for power transformers:
• x = 0,9 in IEC 60076-2, and x = 0,8 in this part of IEC 60076 at ON cooling.
• y = 1,6 in IEC 60076-2, and y = 1,3 in this part of IEC 60076 at ON and OF-cooling.
For distribution transformers, the same x and y values are used in this part of IEC 60076 as in
IEC 60076-2.
This part of IEC 60076 further presents recommendations for limitations of permissible
loading according to the results of temperature calculations or measurements. These
recommendations refer to different types of loading duty – continuous loading, normal cyclic
undisturbed loading or temporary emergency loading. The recommendations refer to
distribution transformers, to medium power transformers and to large power transformers.
Clauses 1 to 7 contain definitions, common background information and specific limitations for
the operation of different categories of transformers.
Clause 8 contains the determination of temperatures, presents the mathematical models used
to estimate the hot-spot temperature in steady state and transient conditions.
Clause 9 contains a short description of the influence of the tap position.
Application examples are given in Annexes B, C and E.
60076-7 IEC:2005 – 13 –
POWER TRANSFORMERS –
Part 7: Loading guide for oil-immersed
power transformers
1 Scope
This part of IEC 60076 is applicable to oil-immersed transformers. It describes the effect of
operation under various ambient temperatures and load conditions on transformer life.
NOTE For furnace transformers, the manufacturer should be consulted in view of the peculiar loading profile.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60076-2:1993, Power transformers – Part 2: Temperature rise
IEC 60076-4:2002, Power transformers – Part 4: Guide to the lightning impulse and switching
impulse testing – Power transformers and reactors
IEC 60076-5:2000, Power transformers – Part 5: Ability to withstand short circuit
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
distribution transformer
power transformer with a maximum rating of 2 500 kVA three-phase or 833 kVA single-phase
3.2
medium power transformer
power transformer with a maximum rating of 100 MVA three-phase or 33,3 MVA single-phase
3.3
large power transformer
power transformer exceeding the limits specified in 3.2
3.4
cyclic loading
loading with cyclic variations (the duration of the cycle usually being 24 h) which is regarded
in terms of the accumulated amount of ageing that occurs during the cycle. The cyclic loading
may either be a normal loading or a long-time emergency loading
60076-7 IEC:2005 – 15 –
3.5
normal cyclic loading
higher ambient temperature or a higher-than-rated load current is applied during part of the
cycle, but, from the point of view of relative thermal ageing rate (according to the
mathematical model), this loading is equivalent to the rated load at normal ambient
temperature. This is achieved by taking advantage of low ambient temperatures or low load
currents during the rest of the load cycle. For planning purposes, this principle can be
extended to provide for long periods of time whereby cycles with relative thermal ageing rates
greater than unity are compensated for by cycles with thermal ageing rates less than unity
3.6
long-time emergency loading
loading resulting from the prolonged outage of some system elements that will not be
reconnected before the transformer reaches a new and higher steady-state temperature
3.7
short-time emergency loading
unusually heavy loading of a transient nature (less than 30 min) due to the occurrence of one
or more unlikely events which seriously disturb normal system loading
3.8
hot-spot
if not specially defined, hottest spot of the windings
3.9
relative thermal ageing rate
for a given hot-spot temperature, rate at which transformer insulation ageing is reduced or
accelerated compared with the ageing rate at a reference hot-spot temperature
3.10
transformer insulation life
total time between the initial state for which the insulation is considered new and the final
state when due to thermal ageing, dielectric stress, short-circuit stress, or mechanical
movement, which could occur in normal service and result in a high risk of electrical failure
3.11
per cent loss of life
equivalent ageing in hours over a time period (usually 24 h) times 100 divided by the
expected transformer insulation life. The equivalent ageing in hours is obtained by multiplying
the relative ageing rate with the number of hours
3.12
thermally upgraded paper
cellulose-based paper which has been chemically modified to reduce the rate at which the
paper decomposes. Ageing effects are reduced either by partial elimination of water forming
agents (as in cyanoethylation) or by inhibiting the formation of water through the use of
stabilizing agents (as in amine addition, dicyandiamide). A paper is considered as thermally
upgraded if it meets the life criteria defined in ANSI/IEEE C57.100; 50 % retention in tensile
strength after 65 000 hours in a sealed tube at 110 °C or any other time/temperature
combination given by the equation:
60076-7 IEC:2005 – 17 –
15 000 15 000 15 000
− 28,082 −
(1)
( θ + 273 ) ( θ + 273 )() 110 + 273
h h
Time (h) = e ≈ 65 000 × e
Because the thermal upgrading chemicals used today contain nitrogen, which is not present in
Kraft pulp, the degree of chemical modification is determined by testing for the amount of
nitrogen present in the treated paper. Typical values for nitrogen content of thermally
upgraded papers are between 1 % and 4 % when measured in accordance with
ASTM D-982.
NOTE This definition was approved by the IEEE Transformers Committee Task Force for the Definition of
Thermally Upgraded Paper on 7 October 2003.
3.13
non-directed oil flow
OF
indicates that the pumped oil from heat exchangers or radiators flows freely inside the tank,
and is not forced to flow through the windings (the oil flow inside the windings can be either
axial in vertical cooling ducts or radial in horizontal cooling ducts with or without zigzag flow)
3.14
non-directed oil flow
ON
indicates that the oil from the heat exchangers or radiators flows freely inside the tank and is
not forced to flow through the windings (the oil flow inside the windings can be either axial in
vertical cooling ducts or radial in horizontal cooling ducts with or without zigzag flow)
3.15
directed oil flow
OD
indicates that the principal part of the pumped oil from heat exchangers or radiators is forced
to flow through the windings (the oil flow inside the windings can be either axial in vertical
cooling ducts or zigzag in horizontal cooling ducts)
3.16
design ambient temperature
temperature at which the permissible average winding and top-oil and hot-spot temperature
over ambient temperature are defined
4 Symbols and abbreviations
Symbol Meaning Units
C Thermal capacity Ws/K
c Specific heat Ws/(kg·K)
DP Degree of polymerization
D Difference operator, in difference equations
g Average-winding-to-average-oil (in tank) temperature gradient at rated K
r
current
m
Mass of core and coil assembly kg
A
m
Mass of the tank and fittings kg
T
m
Mass of oil kg
O
60076-7 IEC:2005 – 19 –
Symbol Meaning Units
m Mass of winding kg
W
H Hot-spot factor
k Thermal model constant
k Thermal model constant
k Thermal model constant
K Load factor (load current/rated current)
L Total ageing over the time period considered h
n Number of each time interval
N Total number of intervals during the time period considered
OD Either ODAN, ODAF or ODWF cooling
OF Either OFAN, OFAF or OFWF cooling
ON Either ONAN or ONAF cooling
P Supplied losses W
P Relative winding eddy loss p.u.
e
P Winding losses W
W
R Ratio of load losses at rated current to no-load losses
R Ratio of load losses to no-load loss at principal tapping
r
R Ratio of load losses to no-load loss at tapping r + 1
r+1
R
Ratio of load losses to no-load loss at minimum tapping
min
R
Ratio of load losses to no-load loss at maximum tapping
max
s
Laplace operator
t
Time variable min
tap Number of principal tapping
r
Number of tapping r + 1
tap
r+1
tap Number of minimum tapping
min
tap Number of maximum tapping
max
V
Relative ageing rate
V Relative ageing rate during interval n
n
x
Exponential power of total losses versus top-oil (in tank) temperature rise
(oil exponent)
y Exponential power of current versus winding temperature rise (winding
exponent)
θ Ambient temperature °C
a
Yearly weighted ambient temperature
θ °C
E
60076-7 IEC:2005 – 21 –
Symbol Meaning Units
Hot-spot temperature
θ °C
h
Monthly average temperature
θ °C
ma
Monthly average temperature of the hottest month, according to
θ °C
ma-max
IEC 60076-2:1993
Top-oil temperature (in the tank) at the load considered
θ °C
o
Yearly average temperature, according to IEC 60076-2:1993
θ °C
ya
Average oil time constant min
τ
o
τ Winding time constant min
W
Bottom oil (in tank) temperature rise at rated load (no-load losses + load K
Δθ
br
losses)
Hot-spot-to-top-oil (in tank) gradient at the load considered K
Δθ
h
Hot-spot-to-top-oil (in tank) gradient at start K
Δθ
hi
Hot-spot-to-top-oil (in tank) gradient at rated current K
Δθ
hr
Top-oil (in tank) temperature rise at the load considered K
Δθ
o
Top-oil (in tank) temperature rise at start K
Δθ
oi
Average oil (in tank) temperature rise at the load considered K
Δθ
om
Average oil (in tank) temperature rise at rated load (no-load losses + load K
Δθ
omr
losses)
Top-oil (in tank) temperature rise in steady state at rated losses (no-load K
Δθ
or
losses + load losses)
Corrected top-oil temperature rise (in tank) due to enclosure K
Δθ'
or
Extra top-oil temperature rise (in tank) due to enclosure K
Δ(Δθ )
or
5 Effect of loading beyond nameplate rating
5.1 Introduction
The normal life expectancy is a conventional reference basis for continuous duty under design
ambient temperature and rated operating conditions. The application of a load in excess of
nameplate rating and/or an ambient temperature higher than design ambient temperature
involves a degree of risk and accelerated ageing. It is the purpose of this part of IEC 60076 to
identify such risks and to indicate how, within limitations, transformers may be loaded in
excess of the nameplate rating. These risks can be reduced by the purchaser clearly
specifying the maximum loading conditions and the supplier taking these into account in the
transformer design.
60076-7 IEC:2005 – 23 –
5.2 General consequences
The consequences of loading a transformer beyond its nameplate rating are as follows.
a) The temperatures of windings, cleats, leads, insulation and oil will increase and can reach
unacceptable levels.
b) The leakage flux density outside the core increases, causing additional eddy-current
heating in metallic parts linked by the leakage flux.
c) As the temperature changes, the moisture and gas content in the insulation and in the oil
will change.
d) Bushings, tap-changers, cable-end connections and current transformers will also be
exposed to higher stresses which encroach upon their design and application margins.
The combination of the main flux and increased leakage flux imposes restrictions on possible
core overexcitation [1], [2], [3] .
NOTE For loaded core-type transformers having an energy flow from the outer winding (usually HV) to the inner
winding (usually LV), the maximum magnetic flux density in the core, which is the result of the combination of the
main flux and the leakage flux, appears in the yokes.
As tests have indicated, this flux is less than or equal to the flux generated by the same applied voltage on the
terminals of the outer winding at no-load of the transformer. The magnetic flux in the core legs of the loaded
transformer is determined by the voltage on the terminals of the inner winding and almost equals the flux generated
by the same voltage at no-load.
For core-type transformers with an energy flow from the inner winding, the maximum flux density is present in the
core-legs. Its value is only slightly higher than that at the same applied voltage under no-load. The flux density in
the yokes is then determined by the voltage on the outer winding.
Voltages on both sides of the loaded transformer should, therefore, be observed during loading beyond the
nameplate rating. As long as voltages at the energized side of a loaded transformer remain below the limits stated
in IEC 60076-1, Clause 4, no excitation restrictions are needed during the loading beyond nameplate rating. When
higher excitations occur to keep the loaded voltage in emergency conditions in an area where the network can still
be kept upright, then the magnetic flux densities in core parts should never exceed values where straying of the
core flux outside the core can occur (for cold-rolled grain-oriented steel these saturation effects start rapidly above
1,9 T). In no time at all, stray fluxes may then cause unpredictably high temperatures at the core surface and in
nearby metallic parts such as winding clamps or even in the windings, due to the presence of high-frequency
components in the stray flux. They may jeopardize the transformer. In general, in all cases, the short overload
times dictated by windings are sufficiently short not to overheat the core at overexcitation. This is prevented by the
long thermal time constant of the core.
As a consequence, there will be a risk of premature failure associated with the increased
currents and temperatures. This risk may be of an immediate short-term character or come
from the cumulative effect of thermal ageing of the insulation in the transformer over many
years.
5.3 Effects and hazards of short-time emergency loading
Short-time increased loading will result in a service condition having an increased risk of
failure. Short-time emergency overloading causes the conductor hot-spot to reach a level
likely to result in a temporary reduction in the dielectric strength. However, acceptance of this
condition for a short time may be preferable to loss of supply. This type of loading is expected
to occur rarely, and it should be rapidly reduced or the transformer disconnected within a
short time in order to avoid its failure. The permissible duration of this load is shorter than the
thermal time constant of the whole transformer and depends on the operating temperature
before the increase in loading; typically, it would be less than half-an-hour.
___________
Numbers in square brackets refer to the bibliography.
60076-7 IEC:2005 – 25 –
a) The main risk for short-time failures is the reduction in dielectric strength due to the
possible presence of gas bubbles in a region of high electrical stress, that is the windings
and leads. These bubbles are likely to occur when the hot-spot temperature exceeds
140 °C for a transformer with a winding insulation moisture content of about 2 %. This
critical temperature will decrease as the moisture concentration increases.
b) Gas bubbles can also develop (either in oil or in solid insulation) at the surfaces of heavy
metallic parts heated by the leakage flux or be produced by super-saturation of the oil.
However, such bubbles usually develop in regions of low electric stress and have to
circulate in regions where the stress is higher before any significant reduction in the
dielectric strength occurs.
Bare metallic parts, except windings, which are not in direct thermal contact with cellulosic
insulation but are in contact with non-cellulosic insulation (for example, aramid paper,
glass fibre) and the oil in the transformer, may rapidly rise to high temperatures. A
temperature of 180 °C should not be exceeded.
c) Temporary deterioration of the mechanical properties at higher temperatures could reduce
the short-circuit strength.
d) Pressure build-up in the bushings may result in a failure due to oil leakage. Gassing in
condenser type bushings may also occur if the temperature of the insulation exceeds
about 140 °C.
e) The expansion of the oil could cause overflow of the oil in the conservator.
f) Breaking of excessively high currents in the tap-changer could be hazardous.
The limitations on the maximum hot-spot temperatures in windings, core and structural parts
are based on considerations of short-term risks (see Clause 7).
The short-term risks normally disappear after the load is reduced to normal level, but they
need to be clearly identified and accepted by all parties involved e.g. planners, asset owners
and operators.
5.4 Effects of long-time emergency loading
This is not a normal operating condition and its occurrence is expected to be rare but it may
persist for weeks or even months and can lead to considerable ageing.
a) Deterioration of the mechanical properties of the conductor insulation will accelerate at
higher temperatures. If this deterioration proceeds far enough, it may reduce the effective
life of the transformer, particularly if the latter is subjected to system short circuits or
transportation events.
b) Other insulation parts, especially parts sustaining the axial pressure of the winding block,
could also suffer increased ageing rates at higher temperature.
c) The contact resistance of the tap-changers could increase at elevated currents and
temperatures and, in severe cases, thermal runaway could take place.
d) The gasket materials in the transformer may become more brittle as a result of elevated
temperatures.
The calculation rules for the relative ageing rate and per cent loss of life are based on
considerations of long-term risks.
60076-7 IEC:2005 – 27 –
5.5 Transformer size
The sensitivity of transformers to loading beyond nameplate rating usually depends on their
size. As the size increases, the tendency is that:
• the leakage flux density increases;
• the short-circuit forces increase;
• the mass of insulation, which is subjected to a high electric stress, is increased;
• the hot-spot temperatures are more difficult to determine.
Thus, a large transformer could be more vulnerable to loading beyond nameplate rating than
a smaller one. In addition, the consequences of a transformer failure are more severe for
larger sizes than for smaller units.
Therefore, in order to apply a reasonable degree of risk for the expected duties, this part of
IEC 60076 considers three categories.
a) Distribution transformers, for which only the hot-spot temperatures in the windings and
thermal deterioration shall be considered.
b) Medium power transformers where the variations in the cooling modes shall be
considered.
c) Large power transformers, where also the effects of stray leakage flux are significant and
the consequences of failure are severe.
5.6 Non-thermally and thermally upgraded insulation paper
The purpose of thermally upgrading insulation paper is to neutralize the production of acids
caused by the hydrolysis (thermal degradation) of the material over the lifetime of the
transformer. This hydrolysis is even more active at elevated temperatures, and published
research results indicate that thermally upgraded insulation papers retain a much higher
percentage of their tensile and bursting strength than untreated papers when exposed to
elevated temperatures [4], [5]. The same references also show the change of DP over time of
non-thermally and thermally upgraded paper exposed to a temperature of 150 °C (see Figure 1).
60076-7 IEC:2005 – 29 –
1 200
1 000
DP
0 1 000 2 000 3 000 4 000
t
IEC 2306/05
Key
DP Degree of polymerization
t Time (h)
∆ Values for thermally upgraded paper
● Values for non-thermally upgraded paper
Figure 1 – Sealed tube accelerated ageing in mineral oil at 150 °C
Another reference [6] illustrates the influence of temperature and moisture content, as shown
in Table 1.
Table 1 – Life of paper under various conditions
Paper type/ageing temperature Life
years
Dry and free With air and
from air 2 % moisture
Wood pulp at 80 °C 118 5,7
90 °C 38 1,9
15 0,8
98 °C
Upgraded wood pulp at 80 °C 72 76
90 °C 34 27
18 12
98 °C
The illustrated difference in thermal ageing behaviour has been taken into account in
industrial standards as follows.
• The relative ageing rate V = 1,0 corresponds to a temperature of 98 °C for non-thermally
upgraded paper and to 110 °C for thermally upgraded paper.
NOTE The results in Figure 1 and Table 1 are not intended to be used as such for ageing calculations and life
estimations. They have been included in this document only to demonstrate that there is a difference in ageing
behaviour between non-thermally and thermally upgraded insulation paper.
60076-7 IEC:2005 – 31 –
6 Relative ageing rate and transformer insulation life
6.1 General
There is no simple and unique end-of-life criterion that can be used to quantify the remaining
life of a transformer. However, such a criterion is useful for transformer users, hence it seems
appropriate to focus on the ageing process and condition of transformer insulation.
6.2 Relative ageing rate
Although ageing or deterioration of insulation is a time function of temperature, moisture
content, oxygen content and acid content, the model presented in this part of IEC 60076 is
based only on the insulation temperature as the controlling parameter.
Since the temperature distribution is not uniform, the part that is operating at the highest
temperature will normally undergo the greatest deterioration. Therefore, the rate of ageing is
referred to the winding hot-spot temperature. In this case the relative ageing rate V is defined
according to equation (2) for non-thermally upgraded paper and to equation (3) for thermally
upgraded paper [7].
()θ −98 / 6
h
(2)
V = 2
15 000 15 000
−
(3)
110 + 273 θ +273
h
V = e
where θ is the hot-spot temperature in °C.
h
Equations (2) and (3) imply that V is very sensitive to the hot-spot temperature as can be
seen in Table 2.
Table 2 – Relative ageing rates due to hot-spot temperature
Non-upgraded paper insulation Upgraded paper insulation
θ
h
V V
°C
80 0,125 0,036
86 0,25 0,073
92 0,5 0,145
98 1,0
0,282
104 2,0 0,536
110 4,0 1,0
116 8,0 1,83
122 16,0 3,29
128 32,0 5,8
134 64,0 10,1
140 128,0 17,2
60076-7 IEC:2005 – 33 –
6.3 Loss-of-life calculation
The loss of life L over a certain period of time is equal to
t
N
L = V dt or L≈ V × t
(4)
n n
∑
∫
n =1
t
where
V is the relative ageing rate during interval n, according to equation (2) or (3);
n
t is the nth time interval;
n
n is the number of each time interval;
N is the total number of intervals during the period considered.
6.4 Insulation life
Reference [7] suggests four different end-of-life criteria, i.e. four different lifetimes for
thermally upgraded paper as shown in Table 3.
Table 3 – Normal insulation life of a well-dried, oxygen-free thermally upgraded
insulation system at the reference temperature of 110 °C
Basis Normal insulation life
Hours Years
65 000
50 % retained tensile strength of insulation 7,42
25 % retained tensile strength of insulation 135 000 15,41
200 retained degree of polymerization in insulation 150 000 17,12
180 000
Interpretation of distribution transformer functional life test data 20,55
The lifetimes in Table 3 are for reference purposes only, since most power transformers will
operate at well below full load most of their actual lifetime. A hot-spot temperature of as little
as 6 °C below rated values results in half the rated loss of life, the actual lifetime of
transformer insulation being several times, for example, 180 000 h.
NOTE For GSU transformers connected to base load generators and other transformers supplying constant load
or operating at relatively constant ambient temperatures, the actual lifetime needs special consideration.
7 Limitations
7.1 Current and temperature limitations
With loading values beyond the nameplate rating, all the individual limits stated in Table 4
should not be exceeded and account should be taken of the specific limitations given in 7.2
to 7.4.
60076-7 IEC:2005 – 35 –
Table 4 – Current and temperature limits applicable to loading beyond nameplate rating
...
NORME CEI
INTERNATIONALE 60076-7
Première édition
2005-12
Transformateurs de puissance –
Partie 7:
Guide de charge pour transformateurs
immergés dans l’huile
Cette version française découle de la publication d’origine
bilingue dont les pages anglaises ont été supprimées.
Les numéros de page manquants sont ceux des pages
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Numéro de référence
CEI 60076-7:2005(F)
Numérotation des publications
Depuis le 1er janvier 1997, les publications de la CEI sont numérotées à partir de
60000. Ainsi, la CEI 34-1 devient la CEI 60034-1.
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Transformateurs de puissance –
Partie 7:
Guide de charge pour transformateurs
immergés dans l’huile
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МеждународнаяЭлектротехническаяКомиссия
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– 2 – 60076-7 CEI:2005
SOMMAIRE
AVANT-PROPOS.6
INTRODUCTION.10
1 Domaine d’application .12
2 Références normatives.12
3 Définitions .12
4 Symboles et abréviations.16
5 Effet d'un régime de charge au-delà des caractéristiques de la plaque signalétique .20
5.1 Introduction .20
5.2 Conséquences générales .22
5.3 Effets et risques d'un régime de charge de secours de courte durée .22
5.4 Effets du chargement d'urgence de longue durée .24
5.5 Taille du transformateur .26
5.6 Papier d'isolation à performances thermiques améliorées ou non .26
6 Vitesse de vieillissement relatif et durée de vie de l’isolation du transformateur .30
6.1 Généralités.30
6.2 Vitesse de vieillissement relatif .30
6.3 Calcul de perte de la vie.32
6.4 Durée de vie de l'isolation .32
7 Limites .32
7.1 Limitations de courant et de température.32
7.2 Limitations spécifiques pour les transformateurs de distribution .34
7.3 Limitations spécifiques pour les transformateurs de moyenne puissance.36
7.4 Limitations spécifiques pour les transformateurs de grande puissance .38
8 Détermination des températures.40
8.1 Echauffement du point-chaud en régime permanent .40
8.2 Températures de l'huile à la partie supérieure et température du point-chaud
pour des conditions variables de température ambiante et de régime de charge.52
8.3 Température ambiante .62
9 Influence des changeurs de prises .64
9.1 Généralités.64
9.2 Pertes en charge.66
9.3 Rapport des pertes.66
9.4 Facteur de charge .66
Annexe A (informative) Calcul de la constante de temps de l’huile et de l’enroulement .68
Annexe B (informative) Exemple pratique de la méthode d’équations exponentielle .72
Annexe C (informative) Illustration de la méthode de solution des équations
différentielles .82
Annexe D (informative) Organigramme, fondé sur l’exemple de l’annexe B.100
Annexe E (informative) Exemple de calcul et de présentation des données de surcharge .104
Bibliographie.112
– 4 – 60076-7 CEI:2005
Figure 1 – Vieillissement accéléré en tube scellé dans de l’huile minérale à 150°C.28
Figure 2 – Diagramme thermique .42
Figure 3 – Echauffement local au-dessus de la température de l'air dans un
enroulement de 120 kV avec un facteur de charge de 1,6 .44
Figure 4 – Echauffement local au-dessus de la température de l'air dans un
enroulement de 410 kV avec un facteur de charge de 1,6 .46
Figure 5 – Deux sondes à fibres optiques installées dans une cale avant que la cale
ait été installée dans l'enroulement de 120 kV .46
Figure 6 – Enroulement refroidi par une circulation en zigzag où la distance entre
toutes les sections est la même et l’écran de circulation du fluide est installé dans
l’espace entre sections .50
Figure 7 – Vue de dessus de la section d'un enroulement rectangulaire avec
"disposition de canaux de refroidissement réduits" sous les culasses. .50
Figure 8 – Réponses en température aux variations en échelons du courant de charge.54
Figure 9 –La fonction f (t) associée aux valeurs données au tableau 5 .58
Figure 10 – Représentation du schéma bloc fonctionnel des équations différentielles .60
Figure 11 – Principe des pertes en fonction de la position de prise .66
Figure B.1 – Courbes comparatives des réponses en température du point chaud aux
variations en échelons du courant de charge .78
Figure B.2 – Courbes comparatives des réponses en température de l’huile supérieure
aux variations en échelons du courant de charge.78
Figure C.1 – Exemple de données d’entrée tracées .92
Figure C.2 – Données de sortie tracées pour l’exemple .98
Figure E.1 – Gros transformateurs de puissance OF: charges admissibles pour une
perte de la vie normale .110
Tableau 1 – Durée de vie du papier sous diverses conditions .28
Tableau 2 – Vitesse de vieillissement relatif due à la température du point-chaud .30
Tableau 3 – Durée de vie normale d'un système d'isolation à performance thermique
améliorée exempte d'oxygène et bien sec à la température de référence de 110 °C .32
Tableau 4 – Limites de courant et de température applicables aux charges au-delà
des caractéristiques de la plaque signalétique .34
Tableau 5 – Caractéristiques thermiques recommandées pour les équations
exponentielles.58
Tableau 6 – Correction concernant l’augmentation de la température ambiante due à
l’enceinte .64
Tableau B.1 – Périodes de charge du transformateur de 250 MVA.72
Tableau B.2 – Températures à la fin de chaque étape de charge.80
Tableau C.1 – Exemple de données d’entrée .90
Tableau C.2 – Exemple de données de sortie .96
Tableau E.1 – Caractéristiques d’exemple liées à la possibilité de charge des
transformateurs .104
Tableau E.2 – Exemple de tableau présentant les charges admissibles et la perte de
la vie quotidienne correspondante (en jours "normaux), et l’échauffement maximal du
point-chaud au cours du cycle de charge .108
– 6 – 60076-7 CEI:2005
COMMISSION ÉLECTROTECHNIQUE INTERNATIONALE
____________
TRANSFORMATEURS DE PUISSANCE –
Partie 7: Guide de charge pour transformateurs
immergés dans l’huile
AVANT-PROPOS
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La Norme internationale CEI 60076-7 a été établie par le comité d’études 14 de la CEI:
Transformateurs de puissance.
Cette norme annule et remplace la CEI 60354 publiée en 1991. Cette première édition
constitue une révision technique du contenu de la CEI 60354. Les détails des changements
techniques sont donnés dans l’introduction.
Le texte de cette norme est issu des documents suivants:
FDIS Rapport de vote
14/512/FDIS 14/520/RVD
Le rapport de vote indiqué dans le tableau ci-dessus donne toute information sur le vote ayant
abouti à l’approbation de cette norme.
– 8 – 60076-7 CEI:2005
Cette publication a été rédigée selon les Directives ISO/CEI, Partie 2.
La CEI 60076 comprend les parties suivantes, sous le titre général Transformateurs de
puissance:
Partie 1: Généralités
Partie 2: Echauffement
Partie 3: Niveaux d’isolement, essais diélectriques et distances d’isolement dans l’air
Partie 4: Guide pour les essais au choc de foudre et au choc de manœuvre –
Transformateurs de puissance et bobines d'inductance
Partie 5: Tenue au court-circuit
Partie 7: Guide de charge pour transformateurs immergés dans l’huile
Partie 8: Guide d’application
Partie 10: Détermination des niveaux de bruit
Partie 11: Transformateurs de type sec
Le comité a décidé que le contenu de cette publication ne sera pas modifié avant la date de
maintenance indiquée sur le site web de la CEI sous «http://webstore.iec.ch» dans les
données relatives à la publication recherchée. A cette date, la publication sera
• reconduite;
• supprimée;
• remplacée par une édition révisée, ou
• amendée.
– 10 – 60076-7 CEI:2005
INTRODUCTION
La présente partie de la CEI 60076 fournit des conseils pour les spécifications et les régimes
de charge des transformateurs de puissance du point de vue des températures de
fonctionnement et du vieillissement thermique. Elle fournit des recommandations pour le
fonctionnement à des régimes de charge supérieurs à la valeur assignée et un guide pour le
planificateur afin de choisir les grandeurs assignées et les conditions de charge appropriées
pour de nouvelles installations.
La CEI 60076-2 constitue la base pour des ententes contractuelles et elle contient les
exigences et les essais concernant les valeurs d'échauffement des transformateurs immergés
dans l'huile dans le cas de régime permanent aux grandeurs assignées. Il convient de noter
que la CEI 60076-2 fait référence à l'échauffement moyen des enroulements tandis que la
présente partie de la CEI 60076 se réfère principalement à la température de point chaud et
les valeurs indiquées sont données seulement à titre indicatif.
La présente partie de la CEI 60076 donne des modèles mathématiques pour juger la
conséquence de différents régimes de charge, transitoires ou cycliques, pour différentes
températures du fluide de refroidissement. Les modèles donnent le calcul des températures
de fonctionnement dans le transformateur, en particulier la température de la partie la plus
chaude de l'enroulement. Cette température du point chaud est utilisée à son tour pour
évaluer une valeur relative de la vitesse de vieillissement thermique et le pourcentage de la
vie consommée dans une période de temps particulière. La modélisation fait référence aux
petits transformateurs, ici appelés transformateurs de distribution, et aux transformateurs de
puissance.
Un changement important par rapport à la CEI 60354:1991 est l'utilisation croissante des
capteurs de température à fibres optiques dans les transformateurs. Cet usage a
radicalement accrû les possibilités d’obtenir une modélisation thermique correcte des
transformateurs de puissance, en particulier à chaque changement de palier du courant de
charge. Ces possibilités ont également mis en évidence des différences entre "l'exposant
d'huile x" et "l'exposant d'enroulement y" utilisés dans la présente partie de la CEI 60076 et
dans la CEI 60076-2:1993, pour les transformateurs de puissance:
• x = 0,9 dans la CEI 60076-2, et x = 0,8 dans la présente partie de la CEI 60076 avec un
mode de refroidissement ON.
• y= 1,6 dans la CEI 60076-2, et y = 1,3 dans la présente partie de la CEI 60076 avec les
modes de refroidissement ON et OF.
Pour les transformateurs de distribution les mêmes valeurs de x et de y sont utilisées dans la
présente partie de la CEI 60076 et dans la CEI 60076-2.
La présente partie de la CEI 60076 présente des recommandations supplémentaires
concernant les limites de charge admissible selon les résultats des calculs ou des mesures de
la température. Ces recommandations se réfèrent à différents types de régimes de charge –
régime de charge continu, régime cyclique non perturbé ou régime temporaire de secours.
Les recommandations se rapportent aux transformateurs de distribution, aux transformateurs
de moyenne puissance et aux transformateurs de grande puissance.
Les articles de 1 à 7 contiennent les définitions, les informations communes générales et les
limitations spécifiques pour le fonctionnement des différentes catégories de transformateurs.
L’article 8 contient la détermination des températures, et présente les modèles
mathématiques utilisés pour estimer la température du point chaud en régimes permanents et
transitoires.
L’article 9 contient une courte description de l'influence de la position de prise.
Des exemples d'application sont donnés en Annexes B, C et E.
– 12 – 60076-7 CEI:2005
TRANSFORMATEURS DE PUISSANCE –
Partie 7: Guide de charge pour transformateurs
immergés dans l’huile
1 Domaine d’application
La présente partie de la CEI 60076 est applicable aux transformateurs immergés dans l'huile.
Elle décrit l'effet du fonctionnement pour diverses températures ambiantes et conditions de
charge durant la vie du transformateur.
NOTE Pour les transformateurs de four, il convient de consulter le fabricant compte tenu du profil particulier de
charge.
2 Références normatives
Les documents référencés ci-après sont indispensables pour l'application du présent
document. Pour les références datées, seule l’édition citée s’applique. Pour les références
non datées, la dernière édition du document de référence s'applique (y compris les éventuels
amendements).
CEI 60076-2:1993, Transformateurs de puissance – Partie 2: Echauffement
CEI 60076-4:2002, Transformateurs de puissance – Partie 4: Guide pour les essais au choc
de foudre et au choc de manœuvre – Transformateurs de puissance et bobines d'inductance
CEI 60076-5:2000, Transformateurs de puissance – Partie 5: Tenue au court-circuit
3 Termes et définitions
Pour les besoins du présent document, les termes et définitions suivants s'appliquent.
3.1
transformateur de distribution
transformateur de puissance avec une puissance assignée maximale de 2 500 kVA en
triphasée ou de 833 kVA en monophasée
3.2
transformateur de moyenne puissance
transformateur de puissance avec une puissance assignée maximale de 100 MVA en
triphasée ou 33,3 MVA en monophasée
3.3
transformateur de grande puissance
transformateur de puissance dépassant les limites spécifiées en 3.2
3.4
régime de charge cyclique
régime de charge avec des variations cycliques (la durée du cycle étant habituellement de
24 h) qui est considéré en termes de quantité cumulée de vieillissement qui se produit
pendant le cycle. Le régime de charge cyclique peut être soit un régime normal, soit un
régime de charge de secours de longue durée.
– 14 – 60076-7 CEI:2005
3.5
régime de charge cyclique normal
une température ambiante plus élevée ou un courant de charge supérieure à la valeur
assignée est appliqué pendant une partie du cycle, mais, du point de vue du taux de
vieillissement thermique relatif (selon le modèle mathématique), cette charge est équivalente
à la charge assignée à température ambiante normale. Ceci est réalisé en tirant profit des
températures ambiantes basses ou des faibles courants de charge pendant le reste du cycle
de charge. Pour les besoins de la planification, ce principe peut être étendu aux longues
périodes de temps pendant lesquelles les cycles avec des taux de vieillissement thermique
relatifs supérieurs à l'unité sont compensés par des cycles avec des taux de vieillissement
thermique inférieurs à l'unité.
3.6
régime de charge de secours de longue durée
régime de charge résultant de panne prolongée de certains éléments du système qui ne
seront pas reconnectés avant que le transformateur atteigne, en régime permanent, une
nouvelle température stabilisée plus élevée
3.7
régime de charge de secours de courte durée
régime de charge exceptionnellement élevé à caractère transitoire (moins de 30 min) dû à
l'apparition d'un ou plusieurs événements de faible probabilité qui perturbent sérieusement le
régime de charge normal du système
3.8
point chaud
en l'absence de définition particulière, par "point-chaud " on sous-entend le point le plus
chaud des enroulements
3.9
vitesse relative de vieillissement thermique
pour une température de point-chaud donnée, la vitesse à laquelle le vieillissement de
l’isolation du transformateur est réduite ou accélérée comparée à la vitesse de vieillissement
correspondant à une température de point-chaud de référence
3.10
durée de vie de l’isolation du transformateur
durée totale entre l'état initial pour lequel l'isolation est considérée neuve et l'état final
correspondant au moment où, en raison du vieillissement thermique, des contraintes
diélectriques, des contraintes de court-circuit ou des mouvements mécaniques, qui pourraient
se produire en service normal, conduisent à un risque élevé de défaillance électrique
3.11
consommation de durée de vie en pourcentage
vieillissement équivalent en heures sur une période de temps (habituellement de 24 h)
multiplié par 100 et divisé par la durée de vie prévue pour l’isolation du transformateur.
Le vieillissement équivalent en heures est obtenu en multipliant la vitesse relative de
vieillissement par le nombre d'heures.
3.12
papier à performance thermique améliorée
papier brut de cellulose qui a été chimiquement modifié pour réduire la vitesse de
décomposition du papier. Les effets de vieillissement sont réduits soit par l'élimination
partielle des agents formant l'eau (comme dans le cyanoéthylation) soit en inhibant la
formation de l'eau par l'utilisation d’agents stabilisant (comme dans l'addition d'amine, le
dicyandiamide). Un papier est considéré comme thermiquement amélioré s'il répond aux
critères de vie définis dans l’ANSI/IEEE C57.100; conservation de 50 % de la résistance à la
traction après 65 000 heures dans un tube scellé à 110 °C ou à toute autre combinaison de
temps/température donnée par l'équation:
– 16 – 60076-7 CEI:2005
15 000 15 000 15 000
− 28,082 −
( θ + 273 ) ( θ + 273 )() 110 + 273
h h (1)
Time (h) = e ≈ 65 000 × e
Puisque les produits chimiques d’amélioration des performances thermiques utilisés
aujourd'hui contiennent de l'azote, qui n'est pas présent dans les pâtes de Kraft, le degré de
modification chimique est déterminé par mesure de la quantité d'azote présente dans le
papier traité. Les valeurs typiques de teneur en azote des papiers thermiquement améliorés
sont comprises entre 1 % et 4 % lorsqu'elles sont mesurées selon la ASTM D-982.
NOTE Cette définition des Papiers Thermiquement Améliorés a été approuvée le 7 octobre 2003 par le groupe de
travail du Comité transformateurs de l’IEEE.
3.13
circulation d'huile non dirigée
OF
indique que l'huile injectée et pompée par les échangeurs de chaleur ou les radiateurs circule
librement à l'intérieur de la cuve, et n'est pas forcée à circuler à travers les enroulements (le
débit d'huile à l'intérieur des enroulements peut être soit axial dans des canaux de
refroidissement verticaux soit radial dans des canaux de refroidissement horizontaux avec ou
sans circulation en zigzag)
3.14
circulation d'huile non dirigée
ON
indique que l'huile des échangeurs de chaleur ou des radiateurs circule librement à l'intérieur
de la cuve et n'est pas forcée à circuler à travers les enroulements (le débit d'huile à
l'intérieur des enroulements peut être soit axial dans des canaux de refroidissement verticaux
soit radial dans des canaux de refroidissement horizontaux avec ou sans circulation en
zigzag)
3.15
circulation d'huile dirigée
OD
Indique que la partie principale de l'huile pompée par les échangeurs de chaleur ou les
radiateurs est forcée et dirigée à travers les enroulements (le débit d'huile à l'intérieur des
enroulements peut être soit axial dans des canaux de refroidissement verticaux soit en zigzag
dans des canaux de refroidissement horizontaux)
3.16
température ambiante de conception
température pour laquelle sont définies les valeurs autorisées pour l’échauffement moyen des
enroulements, l’échauffement de l’huile au sommet et l’échauffement du point chaud par
rapport à la température ambiante.
4 Symboles et abréviations
Symbole Signification Unités
C Capacité thermique Ws/K
c Chaleur spécifique Ws/(kg·K)
DP Degré de polymérisation
D Opérateur de différence, dans les équations de différence
g Gradient de température entre l’enroulement moyen et l’huile moyenne K
r
(dans la cuve) au courant assigné
m Masse de l'ensemble du noyau magnétique et des bobinages kg
A
m Masse de la cuve et des accessoires kg
T
m Masse d’huile kg
O
– 18 – 60076-7 CEI:2005
Symbole Signification Unités
m Masse des enroulements kg
W
H Facteur du point-chaud
k Constante du modèle thermique
k
Constante du modèle thermique
k Constante du modèle thermique
K Facteur de charge (courant de charge/ courant assigné)
L Vieillissement total sur la période de temps considérée h
n Nombre de chaque intervalle de temps
N Nombre total d’intervalles durant la période de temps considérée
OD Soit refroidissement de type ODAN, ODAF ou ODWF
OF Soit refroidissement OFAN, OFAF soit refroidissement OFWF
ON Soit refroidissement de type ONAN ou ONAF
P Pertes fournies W
P Pertes dues aux courants de Foucault dans les enroulements exprimées p.u.
e
en valeur relative
P Pertes dans l’enroulement W
W
R Rapport entre les pertes dues à la charge à courant assigné et les pertes
à vide
R Rapport entre les pertes dues à la charge et les pertes à vide sur la prise
r
principale
R Rapport entre les pertes dues à la charge et les pertes à vide sur la prise
r+1
r + 1
R Rapport entre les pertes dues à la charge et les pertes à vide sur la prise
min
minimale
R Rapport entre les pertes dues à la charge et les pertes à vide sur la prise
max
maximale
s
Opérateur de Laplace
t
Variable de temps min
tap Numéro de la prise principale
r
Numéro de la prise r + 1
tap
r+1
tap Numéro de la prise minimale
min
tap Numéro de la prise maximale
max
V
Vitesse de vieillissement relatif
V
Vitesse relative de vieillissement pendant l’intervalle n
n
x
Puissance exponentielle des pertes totales pour le calcul de
l’échauffement de l'huile supérieure (dans la cuve) (exposant huile)
y Puissance exponentielle du courant par rapport à l’échauffement des
enroulements (exposant d’enroulement)
Température ambiante
θ °C
a
Température ambiante pondérée annuelle
θ °C
E
– 20 – 60076-7 CEI:2005
Symbole Signification Unités
Température du point-chaud
θ °C
h
Température moyenne mensuelle
θ °C
ma
Température moyenne mensuelle du mois le plus chaud, selon la
θ °C
ma-max
CEI 60076-2: 1993
Température de l'huile supérieure (dans la cuve) à la charge considérée
θ °C
o
Température moyenne annuelle, selon la CEI 60076-2:1993
θ °C
ya
Constante de temps d'huile moyenne min
τ
o
τ Constante de temps d’enroulement min
W
Échauffement de l’huile en bas (de cuve) à charge assignée (pertes à K
Δθ
br
vide + pertes dues à la charge)
Gradient du point-chaud par rapport à l’huile supérieure (dans la cuve) K
Δθ
h
pour la charge considérée
Gradient du point-chaud par rapport l’huile supérieure (dans la cuve) au K
Δθ
hi
début
Gradient du point-chaud par rapport l’huile supérieure (dans la cuve) pour K
Δθ
hr
le courant assigné
Échauffement de l'huile au sommet (dans la cuve) pour la charge K
Δθ
o
considérée
Échauffement de l'huile au sommet (dans la cuve) au début K
Δθ
oi
Échauffement de l'huile moyenne (dans la cuve) pour la charge K
Δθ
om
considérée
Échauffement de l’huile moyenne (dans la cuve) pour charge assignée K
Δθ
omr
(pertes à vide + pertes dues à la charge)
Échauffement de l'huile au sommet (dans la cuve) en régime permanent K
Δθ
or
pour les pertes assignées (pertes à vide + pertes en charge)
Échauffement corrigé de l'huile au sommet (dans la cuve) dû à une K
Δθ'
or
enceinte
Δ(Δθ ) Échauffement supplémentaire de l'huile au sommet (dans la cuve) dû à K
or
une enceinte
5 Effet d'un régime de charge au-delà des caractéristiques de la plaque
signalétique
5.1 Introduction
L'espérance de vie normale est une base de référence conventionnelle pour un service
continu à la température ambiante de conception et aux conditions de fonctionnement
assignées. L'application d'une charge supérieure à celle de la plaque signalétique et/ou d'une
température ambiante plus élevée que la température ambiante de conception implique un
degré de risque et un vieillissement accéléré. C'est l’objet de la présente partie de la
CEI 60076 d'identifier de tels risques et d'indiquer comment, dans certaines limites, les
transformateurs peuvent être chargés au-delà des caractéristiques de la plaque signalétique.
Ces risques peuvent être réduits par l’acheteur en spécifiant clairement les conditions de
charge maximales et par le fournisseur en prenant celles-ci en compte dans la conception du
fournisseur.
– 22 – 60076-7 CEI:2005
5.2 Conséquences générales
Un régime de charge d’un transformateur au-delà des valeurs de sa plaque signalétique a les
conséquences suivantes.
a) Les températures des enroulements, des calages, des connexions, des isolants et de
l'huile vont augmenter, et peuvent atteindre des niveaux inacceptables.
b) l’induction magnétique du flux de fuite en dehors du circuit magnétique augmente et
provoque un accroissement de l’échauffement par courants de Foucault dans les parties
métalliques embrassées par le flux de fuite.
c) Comme la température varie, les taux d'humidité et teneurs en gaz dans l'isolation et dans
l'huile sont modifiés.
d) Les traversées, les changeurs de prises, les connexions d'extrémité de câble et les
transformateurs de courant sont également soumis à des contraintes plus élevées qui
réduisent leurs marges de conception et d'application.
La combinaison du flux principal et du flux de fuite accru restreint les possibilités de
fonctionnement du circuit magnétique en surexcitation [1], [2], [3] .
NOTE Pour les transformateurs de type colonne siège d’un transit d'énergie provenant de l'enroulement extérieur
(habituellement HT) vers l'enroulement intérieur (habituellement BT), l’induction magnétique maximale, résultant de
la combinaison du flux principal et du flux de fuite dans le circuit magnétique, apparaît dans les culasses.
Comme le montrent les essais, ce flux est inférieur ou égal au flux produit par la même tension appliquée à vide
aux bornes de l'enroulement extérieur du transformateur. Le flux magnétique dans les colonnes bobinées du circuit
magnétique du transformateur chargé est fixé par la tension aux bornes de l'enroulement intérieur et est presque
égal au flux produit par la même tension à vide.
Pour les transformateurs de type colonnes avec un transit d'énergie provenant de l'enroulement intérieur,
l’induction magnétique maximale apparaît dans les colonnes bobinées du circuit magnétique. Sa valeur est
légèrement supérieure à celle apparaissant pour la même tension appliquée à vide. L’induction dans les culasses
est alors déterminée par la tension sur l'enroulement extérieur.
Il convient donc d’observer les tensions des deux côtés du transformateur en charge pendant les régimes de
charge au-delà des caractéristiques de la plaque signalétique. Tant que les tensions qui alimentent un
transformateur siège d’un transit de courant demeurent en dessous des limites indiquées dans l'article 4 de la
CEI 60076-1, aucune restriction d'excitation n'est nécessaire durant la charge au-delà des caractéristiques de la
plaque signalétique. Lorsque des excitations plus élevées se produisent pour maintenir la tension, en régime de
secours, dans une zone où le réseau peut toujours être maintenu en service, alors il convient que les valeurs
d’induction dans le circuit magnétique ne dépassent jamais les valeurs pour lesquelles le flux n’est plus canalisé
dans celui-ci (pour les tôles à grains orientés laminées à froid ces effets de saturation commencent rapidement au-
dessus de 1,9 T). Instantanément, les flux parasites peuvent alors causer de manière imprévisible des
températures élevées sur la surface du circuit magnétique et dans les parties métalliques voisines telles que le
système de serrage des enroulements ou même dans les enroulements, dus à la présence de composantes hautes
fréquences dans le flux parasite. Ils peuvent mettre en péril le transformateur. En général, dans tous les cas, les
temps de surcharge courts imposés par les enroulements sont suffisamment courts pour ne pas surchauffer le
circuit magnétique en surexcitation. La grande constante de temps thermique du noyau évite ce phénomène.
Par conséquent il y aura un risque de défaillance prématurée lié à l’augmentation des
courants et des températures. Ce risque peut être d'un caractère à court terme immédiat ou
résulter de l'effet cumulatif du vieillissement thermique de l'isolation du transformateur sur de
nombreuses d'années.
5.3 Effets et risques d'un régime de charge de secours de courte durée
Un régime de charge accru de courte durée conduira à une condition de service présentant
un plus grand risque de défaillance. La surcharge de secours de courte durée peut
occasionner un niveau de point-chaud dans les conducteurs, susceptible de conduire à une
réduction provisoire de la rigidité diélectrique. Cependant, l'acceptation de cette condition
pour une courte période peut être préférable à une perte d'alimentation. Ce type de charge se
produit rarement et il convient de le réduire rapidement ou de déconnecter le transformateur
rapidement afin d'éviter sa défaillance. La durée admissible de cette charge est plus courte
que la constante de temps thermique du transformateur dans son ensemble et dépend de la
température de fonctionnement avant l'augmentation de la charge; typiquement, elle serait
inférieure à une demi-heure.
___________
Les chiffres entre crochets renvoient à la bibliographie.
– 24 – 60076-7 CEI:2005
a) Le risque principal, pour les défaillances à courte durée, est la réduction de la rigidité
diélectrique due à la présence éventuelle de bulles de gaz dans une région de contrainte
électrique élevée, c’est-à-dire dans les enroulements et les connexions. Ces bulles sont
susceptibles de se produire quand la température du point-chaud dépasse les 140 °C
pour un transformateur avec une teneur en humidité dans l'isolation des enroulements
d’environ 2 %. Cette température critique diminuera à mesure que la concentration en
humidité augmente.
b) Des bulles de gaz peuvent également se développer (soit dans l’huile soit dans l'isolation
solide) à la surface des parties métalliques massives chauffées par le flux de fuite ou être
produites par une sursaturation de l'huile. Cependant, de telles bulles se développent
habituellement dans les régions à faibles contraintes diélectriques et doivent circuler dans
des régions où la contrainte est déjà supérieure avant que ne se produise une réduction
significative de la rigidité diélectrique
Les parties métalliques nues, à l'exception des enroulements, qui ne sont pas en contact
thermique direct avec de l'isolation cellulosique mais qui sont en contact avec de
l'isolation non cellulosique (par exemple, du papier aramide, de la fibre de verre) et l'huile
dans le transformateur, peuvent atteindre rapidement des températures élevées. Il
convient de ne pas dépasser une température de 180 °C.
c) Une dégradation temporaire des propriétés mécaniques à températures élevées peut
réduire la tenue au court-circuit.
d) La montée en pression dans les traversées peut provoquer une défaillance due à une fuite
d'huile. Du gaz peut aussi apparaître dans les traversées condensateurs si la température
des isolants dépasse environ 140 °C.
e) La dilatation de l'huile peut provoquer un débordement de l'huile dans le conservateur.
f) La coupure de courants trop élevés dans le changeur de prises peut être dangereuse.
Les limitations sur les températures maximales du point-chaud dans les enroulements, le
circuit magnétique et les parties structurelles sont fondées sur des considérations de risques
à court terme (voir l’article 7).
Les risques à court terme disparaissent normalement après que la charge a été réduite au
niveau normal, mais il est nécessaire qu’ils aient été clairement identifiés et acceptés par
toutes les parties concernées, comme les planificateurs, les détenteurs d’actif et les
opérateurs.
5.4 Effets du chargement d'urgence de longue durée
Ce n'est pas une condition de fonctionnement normal et son apparition est supposée être rare
mais elle peut persister pendant des semaines ou même des mois et peut mener à un
vieillissement considérable.
a) La détérioration des propriétés mécaniques de l’isolation des conducteurs sera accélérée
aux températures supérieures. Si cette détérioration se poursuit suffisamment, elle peut
réduire la durée de vie effective du transformateur, en particulier si ce dernier est soumis
à des courts-circuits ou des événements de transport.
b) D'autres parties d'isolation, et particulièrement les parties soutenant la pression axiale du
bloc des enroulements, peuvent également vieillir davantage à des températures plus
élevées.
c) La résistance de contact des changeurs de prises peut augmenter pour des courants et
des températures élevées, dans des cas extrêmes, un emballement thermique peut se
produire.
d) Les joints du transformateur peuvent devenir plus fragiles à des températures élevées.
Les règles de calcul pour une vitesse de vieillissement relative et la consommation de durée
de vie en pourcentage sont fondées sur des considérations de risques à long terme.
– 26 – 60076-7 CEI:2005
5.5 Taille du transformateur
La sensibilité des transformateurs à des conditions de charges supérieures aux
caractéristiques de la plaque signalétique dépend généralement de leur taille. À mesure que
la taille augmente, la tendance est que:
• l’induction de fuite augmente;
• les forces de court-circuit augmentent;
• la masse de l'isolation, qui est soumise à des contraintes électriques élevées, est
augmentée;
• il est plus difficile de déterminer les températures de point-chaud.
Ainsi, un grand tr
...
IEC 60076-7
Edition 1.0 2005-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 7: Loading guide for oil-immersed power transformers
Transformateurs de puissance –
Partie 7: Guide de charge pour transformateurs immergés dans l'huile
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IEC 60076-7
Edition 1.0 2005-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Power transformers –
Part 7: Loading guide for oil-immersed power transformers
Transformateurs de puissance –
Partie 7: Guide de charge pour transformateurs immergés dans l'huile
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XA
CODE PRIX
ICS 29.180 ISBN 2-8318-8372-5
60076-7 IEC:2005 – 3 – – 2 – 60076-7 © IEC:2005
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .7
3 Definitions .7
4 Symbols and abbreviations.9
5 Effect of loading beyond nameplate rating .11
5.1 Introduction .11
5.2 General consequences.12
5.3 Effects and hazards of short-time emergency loading.12
5.4 Effects of long-time emergency loading .13
5.5 Transformer size .14
5.6 Non-thermally and thermally upgraded insulation paper .14
6 Relative ageing rate and transformer insulation life .16
6.1 General .16
6.2 Relative ageing rate .16
6.3 Loss-of-life calculation.17
6.4 Insulation life.17
7 Limitations.17
7.1 Current and temperature limitations.17
7.2 Specific limitations for distribution transformers.18
7.3 Specific limitations for medium-power transformers .19
7.4 Specific limitations for large power transformers.20
8 Determination of temperatures .21
8.1 Hot-spot temperature rise in steady state .21
8.2 Top-oil and hot-spot temperatures at varying ambient temperature and load
conditions.27
8.3 Ambient temperature .32
9 Influence of tap changers .33
9.1 General .33
9.2 Short-circuit losses.34
9.3 Ratio of losses .34
9.4 Load factor .34
Annex A (informative) Calculation of winding and oil time constant .35
Annex B (informative) Practical example of the exponential equations method.37
Annex C (informative) Illustration of the differential equations solution method .42
Annex D (informative) Flowchart, based on the example in annex B .51
Annex E (informative) Example of calculating and presenting overload data .53
Bibliography.57
60076-7 © IEC:200560076-7 IEC:2005 – 5 – – 3 –
Figure 1 – Sealed tube accelerated ageing in mineral oil at 150 °C.15
Figure 2 – Thermal diagram .22
Figure 3 – Local temperature rises above air temperature in a 120 kV winding at a load
factor of 1,6 .23
Figure 4 – Local temperature rises above air temperature in a 410 kV winding at a load
factor of 1,6 .24
Figure 5 – Two fibre optic sensors installed in a spacer before the spacer was installed
in the 120 kV winding.24
Figure 6 – Zigzag-cooled winding where the distance between all sections is the same
and the flow-directing washer is installed in the space between sections .26
Figure 7 – Top view section of a rectangular winding with "collapsed cooling duct
arrangement" under the yokes .26
Figure 8 – Temperature responses to step changes in the load current.28
Figure 9 – The function f (t) generated by the values given in Table 5 .30
Figure 10 – Block diagram representation of the differential equations.31
Figure 11 – Principle of losses as a function of the tap position .34
Figure B.1 – Hot-spot temperature response to step changes in the load current .40
Figure B.2 – Top-oil temperature response to step changes in the load current .40
Figure C.1 – Plotted input data for the example .47
Figure C.2 – Plotted output data for the example .50
Figure E.1 – OF large power transformers: permissible duties for normal loss of life.56
Table 1 – Life of paper under various conditions .15
Table 2 – Relative ageing rates due to hot-spot temperature .16
Table 3 – Normal insulation life of a well-dried, oxygen-free thermally upgraded insulation
system at the reference temperature of 110 °C .17
Table 4 – Current and temperature limits applicable to loading beyond nameplate rating .18
Table 5 – Recommended thermal characteristics for exponential equations .30
Table 6 – Correction for increase in ambient temperature due to enclosure .33
Table B.1 – Load steps of the 250 MVA transformer .37
Table B.2 – Temperatures at the end of each load step .41
Table C.1 – Input data for example .46
Table C.2 – Output data for the example.49
Table E.1 – Example characteristics related to the loadability of transformers .53
Table E.2 – An example table with the permissible duties and corresponding daily loss of
life (in "normal" days), and maximum hot-spot temperature rise during the load cycle.55
60076-7 IEC:2005 – 7 – – 4 – 60076-7 © IEC:2005
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
POWER TRANSFORMERS –
Part 7: Loading guide for oil-immersed power transformers
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60076-7 has been prepared by IEC technical committee 14: Power
transformers.
This standard cancels and replaces IEC 60354 published in 1991. This first edition constitutes
a technical revision of the material given in IEC 60354. Details of the changes are given in the
introduction.
The text of this standard is based on the following documents:
FDIS Report on voting
14/512/FDIS 14/520/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
60076-7 © IEC:200560076-7 IEC:2005 – 9 – – 5 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
IEC 60076 consists of the following parts, under the general title Power transformers:
Part 1: General
Part 2: Temperature rise
Part 3: Insulation levels, dielectric tests and external clearances in air
Part 4: Guide to the lightning impulse and switching impulse testing – Power transformers
and reactors
Part 5: Ability to withstand short circuit
Part 7: Loading guide for oil-immersed power transformers
Part 8: Application guide
Part 10: Determination of sound levels
Part 11: Dry-type transformers
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
60076-7 IEC:2005 –– 6 – 11 – 60076-7 © IEC:2005
INTRODUCTION
This part of IEC 60076 provides guidance for the specification and loading of power
transformers from the point of view of operating temperatures and thermal ageing. It provides
recommendations for loading above the nameplate rating and guidance for the planner to
choose appropriate rated quantities and loading conditions for new installations.
IEC 60076-2 is the basis for contractual agreements and it contains the requirements and
tests relating to temperature-rise figures for oil-immersed transformers during continuous
rated loading. It should be noted that IEC 60076-2 refers to the average winding temperature
rise while this part of IEC 60076 refers mainly to the hot-spot temperature and the stated
values are provided only for guidance.
This part of IEC 60076 gives mathematical models for judging the consequence of different
loadings, with different temperatures of the cooling medium, and with transient or cyclical
variation with time. The models provide for the calculation of operating temperatures in the
transformer, particularly the temperature of the hottest part of the winding. This hot-spot
temperature is, in turn, used for evaluation of a relative value for the rate of thermal ageing
and the percentage of life consumed in a particular time period. The modelling refers to small
transformers, here called distribution transformers and to power transformers.
A major change from IEC 60354:1991 is the increased use of fibre optic temperature sensors
in transformers. This has radically increased the possibilities of obtaining a proper thermal
modelling of power transformers, especially at step changes in the load current. These
possibilities have also yielded some differences between the "oil exponent x" and the "winding
exponent y" used in this part of IEC 60076 and in IEC 60076-2:1993, for power transformers:
• x = 0,9 in IEC 60076-2, and x = 0,8 in this part of IEC 60076 at ON cooling.
• y = 1,6 in IEC 60076-2, and y = 1,3 in this part of IEC 60076 at ON and OF-cooling.
For distribution transformers, the same x and y values are used in this part of IEC 60076 as in
IEC 60076-2.
This part of IEC 60076 further presents recommendations for limitations of permissible
loading according to the results of temperature calculations or measurements. These
recommendations refer to different types of loading duty – continuous loading, normal cyclic
undisturbed loading or temporary emergency loading. The recommendations refer to
distribution transformers, to medium power transformers and to large power transformers.
Clauses 1 to 7 contain definitions, common background information and specific limitations for
the operation of different categories of transformers.
Clause 8 contains the determination of temperatures, presents the mathematical models used
to estimate the hot-spot temperature in steady state and transient conditions.
Clause 9 contains a short description of the influence of the tap position.
Application examples are given in Annexes B, C and E.
60076-7 © IEC:200560076-7 IEC:2005 –– 7 – 13 –
POWER TRANSFORMERS –
Part 7: Loading guide for oil-immersed
power transformers
1 Scope
This part of IEC 60076 is applicable to oil-immersed transformers. It describes the effect of
operation under various ambient temperatures and load conditions on transformer life.
NOTE For furnace transformers, the manufacturer should be consulted in view of the peculiar loading profile.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60076-2:1993, Power transformers – Part 2: Temperature rise
IEC 60076-4:2002, Power transformers – Part 4: Guide to the lightning impulse and switching
impulse testing – Power transformers and reactors
IEC 60076-5:2000, Power transformers – Part 5: Ability to withstand short circuit
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
distribution transformer
power transformer with a maximum rating of 2 500 kVA three-phase or 833 kVA single-phase
3.2
medium power transformer
power transformer with a maximum rating of 100 MVA three-phase or 33,3 MVA single-phase
3.3
large power transformer
power transformer exceeding the limits specified in 3.2
3.4
cyclic loading
loading with cyclic variations (the duration of the cycle usually being 24 h) which is regarded
in terms of the accumulated amount of ageing that occurs during the cycle. The cyclic loading
may either be a normal loading or a long-time emergency loading
60076-7 IEC:2005 –– 8 – 15 – 60076-7 © IEC:2005
3.5
normal cyclic loading
higher ambient temperature or a higher-than-rated load current is applied during part of the
cycle, but, from the point of view of relative thermal ageing rate (according to the
mathematical model), this loading is equivalent to the rated load at normal ambient
temperature. This is achieved by taking advantage of low ambient temperatures or low load
currents during the rest of the load cycle. For planning purposes, this principle can be
extended to provide for long periods of time whereby cycles with relative thermal ageing rates
greater than unity are compensated for by cycles with thermal ageing rates less than unity
3.6
long-time emergency loading
loading resulting from the prolonged outage of some system elements that will not be
reconnected before the transformer reaches a new and higher steady-state temperature
3.7
short-time emergency loading
unusually heavy loading of a transient nature (less than 30 min) due to the occurrence of one
or more unlikely events which seriously disturb normal system loading
3.8
hot-spot
if not specially defined, hottest spot of the windings
3.9
relative thermal ageing rate
for a given hot-spot temperature, rate at which transformer insulation ageing is reduced or
accelerated compared with the ageing rate at a reference hot-spot temperature
3.10
transformer insulation life
total time between the initial state for which the insulation is considered new and the final
state when due to thermal ageing, dielectric stress, short-circuit stress, or mechanical
movement, which could occur in normal service and result in a high risk of electrical failure
3.11
per cent loss of life
equivalent ageing in hours over a time period (usually 24 h) times 100 divided by the
expected transformer insulation life. The equivalent ageing in hours is obtained by multiplying
the relative ageing rate with the number of hours
3.12
thermally upgraded paper
cellulose-based paper which has been chemically modified to reduce the rate at which the
paper decomposes. Ageing effects are reduced either by partial elimination of water forming
agents (as in cyanoethylation) or by inhibiting the formation of water through the use of
stabilizing agents (as in amine addition, dicyandiamide). A paper is considered as thermally
upgraded if it meets the life criteria defined in ANSI/IEEE C57.100; 50 % retention in tensile
strength after 65 000 hours in a sealed tube at 110 °C or any other time/temperature
combination given by the equation:
60076-7 © IEC:200560076-7 IEC:2005 –– 9 – 17 –
15 000 15 000 15 000
− 28,082 −
(1)
( θ + 273 ) ( θ + 273 )() 110 + 273
h h
Time (h) = e ≈ 65 000 × e
Because the thermal upgrading chemicals used today contain nitrogen, which is not present in
Kraft pulp, the degree of chemical modification is determined by testing for the amount of
nitrogen present in the treated paper. Typical values for nitrogen content of thermally
upgraded papers are between 1 % and 4 % when measured in accordance with
ASTM D-982.
NOTE This definition was approved by the IEEE Transformers Committee Task Force for the Definition of
Thermally Upgraded Paper on 7 October 2003.
3.13
non-directed oil flow
OF
indicates that the pumped oil from heat exchangers or radiators flows freely inside the tank,
and is not forced to flow through the windings (the oil flow inside the windings can be either
axial in vertical cooling ducts or radial in horizontal cooling ducts with or without zigzag flow)
3.14
non-directed oil flow
ON
indicates that the oil from the heat exchangers or radiators flows freely inside the tank and is
not forced to flow through the windings (the oil flow inside the windings can be either axial in
vertical cooling ducts or radial in horizontal cooling ducts with or without zigzag flow)
3.15
directed oil flow
OD
indicates that the principal part of the pumped oil from heat exchangers or radiators is forced
to flow through the windings (the oil flow inside the windings can be either axial in vertical
cooling ducts or zigzag in horizontal cooling ducts)
3.16
design ambient temperature
temperature at which the permissible average winding and top-oil and hot-spot temperature
over ambient temperature are defined
4 Symbols and abbreviations
Symbol Meaning Units
C Thermal capacity Ws/K
c Specific heat Ws/(kg·K)
DP Degree of polymerization
D Difference operator, in difference equations
g Average-winding-to-average-oil (in tank) temperature gradient at rated K
r
current
m
Mass of core and coil assembly kg
A
m
Mass of the tank and fittings kg
T
m
Mass of oil kg
O
60076-7 IEC:2005 –– 10 – 19 – 60076-7 © IEC:2005
Symbol Meaning Units
m Mass of winding kg
W
H Hot-spot factor
k Thermal model constant
k Thermal model constant
k Thermal model constant
K Load factor (load current/rated current)
L Total ageing over the time period considered h
n Number of each time interval
N Total number of intervals during the time period considered
OD Either ODAN, ODAF or ODWF cooling
OF Either OFAN, OFAF or OFWF cooling
ON Either ONAN or ONAF cooling
P Supplied losses W
P Relative winding eddy loss p.u.
e
P Winding losses W
W
R Ratio of load losses at rated current to no-load losses
R Ratio of load losses to no-load loss at principal tapping
r
R Ratio of load losses to no-load loss at tapping r + 1
r+1
R
Ratio of load losses to no-load loss at minimum tapping
min
R
Ratio of load losses to no-load loss at maximum tapping
max
s
Laplace operator
t
Time variable min
tap Number of principal tapping
r
Number of tapping r + 1
tap
r+1
tap Number of minimum tapping
min
tap Number of maximum tapping
max
V
Relative ageing rate
V Relative ageing rate during interval n
n
x
Exponential power of total losses versus top-oil (in tank) temperature rise
(oil exponent)
y Exponential power of current versus winding temperature rise (winding
exponent)
θ Ambient temperature °C
a
Yearly weighted ambient temperature
θ °C
E
60076-7 © IEC:200560076-7 IEC:2005 –– 11 – 21 –
Symbol Meaning Units
Hot-spot temperature
θ °C
h
Monthly average temperature
θ °C
ma
Monthly average temperature of the hottest month, according to
θ °C
ma-max
IEC 60076-2:1993
Top-oil temperature (in the tank) at the load considered
θ °C
o
Yearly average temperature, according to IEC 60076-2:1993
θ °C
ya
Average oil time constant min
τ
o
τ Winding time constant min
W
Bottom oil (in tank) temperature rise at rated load (no-load losses + load K
Δθ
br
losses)
Hot-spot-to-top-oil (in tank) gradient at the load considered K
Δθ
h
Hot-spot-to-top-oil (in tank) gradient at start K
Δθ
hi
Hot-spot-to-top-oil (in tank) gradient at rated current K
Δθ
hr
Top-oil (in tank) temperature rise at the load considered K
Δθ
o
Top-oil (in tank) temperature rise at start K
Δθ
oi
Average oil (in tank) temperature rise at the load considered K
Δθ
om
Average oil (in tank) temperature rise at rated load (no-load losses + load K
Δθ
omr
losses)
Top-oil (in tank) temperature rise in steady state at rated losses (no-load K
Δθ
or
losses + load losses)
Corrected top-oil temperature rise (in tank) due to enclosure K
Δθ'
or
Extra top-oil temperature rise (in tank) due to enclosure K
Δ(Δθ )
or
5 Effect of loading beyond nameplate rating
5.1 Introduction
The normal life expectancy is a conventional reference basis for continuous duty under design
ambient temperature and rated operating conditions. The application of a load in excess of
nameplate rating and/or an ambient temperature higher than design ambient temperature
involves a degree of risk and accelerated ageing. It is the purpose of this part of IEC 60076 to
identify such risks and to indicate how, within limitations, transformers may be loaded in
excess of the nameplate rating. These risks can be reduced by the purchaser clearly
specifying the maximum loading conditions and the supplier taking these into account in the
transformer design.
60076-7 IEC:2005 –– 12 – 23 – 60076-7 © IEC:2005
5.2 General consequences
The consequences of loading a transformer beyond its nameplate rating are as follows.
a) The temperatures of windings, cleats, leads, insulation and oil will increase and can reach
unacceptable levels.
b) The leakage flux density outside the core increases, causing additional eddy-current
heating in metallic parts linked by the leakage flux.
c) As the temperature changes, the moisture and gas content in the insulation and in the oil
will change.
d) Bushings, tap-changers, cable-end connections and current transformers will also be
exposed to higher stresses which encroach upon their design and application margins.
The combination of the main flux and increased leakage flux imposes restrictions on possible
core overexcitation [1], [2], [3] .
NOTE For loaded core-type transformers having an energy flow from the outer winding (usually HV) to the inner
winding (usually LV), the maximum magnetic flux density in the core, which is the result of the combination of the
main flux and the leakage flux, appears in the yokes.
As tests have indicated, this flux is less than or equal to the flux generated by the same applied voltage on the
terminals of the outer winding at no-load of the transformer. The magnetic flux in the core legs of the loaded
transformer is determined by the voltage on the terminals of the inner winding and almost equals the flux generated
by the same voltage at no-load.
For core-type transformers with an energy flow from the inner winding, the maximum flux density is present in the
core-legs. Its value is only slightly higher than that at the same applied voltage under no-load. The flux density in
the yokes is then determined by the voltage on the outer winding.
Voltages on both sides of the loaded transformer should, therefore, be observed during loading beyond the
nameplate rating. As long as voltages at the energized side of a loaded transformer remain below the limits stated
in IEC 60076-1, Clause 4, no excitation restrictions are needed during the loading beyond nameplate rating. When
higher excitations occur to keep the loaded voltage in emergency conditions in an area where the network can still
be kept upright, then the magnetic flux densities in core parts should never exceed values where straying of the
core flux outside the core can occur (for cold-rolled grain-oriented steel these saturation effects start rapidly above
1,9 T). In no time at all, stray fluxes may then cause unpredictably high temperatures at the core surface and in
nearby metallic parts such as winding clamps or even in the windings, due to the presence of high-frequency
components in the stray flux. They may jeopardize the transformer. In general, in all cases, the short overload
times dictated by windings are sufficiently short not to overheat the core at overexcitation. This is prevented by the
long thermal time constant of the core.
As a consequence, there will be a risk of premature failure associated with the increased
currents and temperatures. This risk may be of an immediate short-term character or come
from the cumulative effect of thermal ageing of the insulation in the transformer over many
years.
5.3 Effects and hazards of short-time emergency loading
Short-time increased loading will result in a service condition having an increased risk of
failure. Short-time emergency overloading causes the conductor hot-spot to reach a level
likely to result in a temporary reduction in the dielectric strength. However, acceptance of this
condition for a short time may be preferable to loss of supply. This type of loading is expected
to occur rarely, and it should be rapidly reduced or the transformer disconnected within a
short time in order to avoid its failure. The permissible duration of this load is shorter than the
thermal time constant of the whole transformer and depends on the operating temperature
before the increase in loading; typically, it would be less than half-an-hour.
___________
Numbers in square brackets refer to the bibliography.
60076-7 © IEC:200560076-7 IEC:2005 –– 13 – 25 –
a) The main risk for short-time failures is the reduction in dielectric strength due to the
possible presence of gas bubbles in a region of high electrical stress, that is the windings
and leads. These bubbles are likely to occur when the hot-spot temperature exceeds
140 °C for a transformer with a winding insulation moisture content of about 2 %. This
critical temperature will decrease as the moisture concentration increases.
b) Gas bubbles can also develop (either in oil or in solid insulation) at the surfaces of heavy
metallic parts heated by the leakage flux or be produced by super-saturation of the oil.
However, such bubbles usually develop in regions of low electric stress and have to
circulate in regions where the stress is higher before any significant reduction in the
dielectric strength occurs.
Bare metallic parts, except windings, which are not in direct thermal contact with cellulosic
insulation but are in contact with non-cellulosic insulation (for example, aramid paper,
glass fibre) and the oil in the transformer, may rapidly rise to high temperatures. A
temperature of 180 °C should not be exceeded.
c) Temporary deterioration of the mechanical properties at higher temperatures could reduce
the short-circuit strength.
d) Pressure build-up in the bushings may result in a failure due to oil leakage. Gassing in
condenser type bushings may also occur if the temperature of the insulation exceeds
about 140 °C.
e) The expansion of the oil could cause overflow of the oil in the conservator.
f) Breaking of excessively high currents in the tap-changer could be hazardous.
The limitations on the maximum hot-spot temperatures in windings, core and structural parts
are based on considerations of short-term risks (see Clause 7).
The short-term risks normally disappear after the load is reduced to normal level, but they
need to be clearly identified and accepted by all parties involved e.g. planners, asset owners
and operators.
5.4 Effects of long-time emergency loading
This is not a normal operating condition and its occurrence is expected to be rare but it may
persist for weeks or even months and can lead to considerable ageing.
a) Deterioration of the mechanical properties of the conductor insulation will accelerate at
higher temperatures. If this deterioration proceeds far enough, it may reduce the effective
life of the transformer, particularly if the latter is subjected to system short circuits or
transportation events.
b) Other insulation parts, especially parts sustaining the axial pressure of the winding block,
could also suffer increased ageing rates at higher temperature.
c) The contact resistance of the tap-changers could increase at elevated currents and
temperatures and, in severe cases, thermal runaway could take place.
d) The gasket materials in the transformer may become more brittle as a result of elevated
temperatures.
The calculation rules for the relative ageing rate and per cent loss of life are based on
considerations of long-term risks.
60076-7 IEC:2005 –– 14 – 27 – 60076-7 © IEC:2005
5.5 Transformer size
The sensitivity of transformers to loading beyond nameplate rating usually depends on their
size. As the size increases, the tendency is that:
• the leakage flux density increases;
• the short-circuit forces increase;
• the mass of insulation, which is subjected to a high electric stress, is increased;
• the hot-spot temperatures are more difficult to determine.
Thus, a large transformer could be more vulnerable to loading beyond nameplate rating than
a smaller one. In addition, the consequences of a transformer failure are more severe for
larger sizes than for smaller units.
Therefore, in order to apply a reasonable degree of risk for the expected duties, this part of
IEC 60076 considers three categories.
a) Distribution transformers, for which only the hot-spot temperatures in the windings and
thermal deterioration shall be considered.
b) Medium power transformers where the variations in the cooling modes shall be
considered.
c) Large power transformers, where also the effects of stray leakage flux are significant and
the consequences of failure are severe.
5.6 Non-thermally and thermally upgraded insulation paper
The purpose of thermally upgrading insulation paper is to neutralize the production of acids
caused by the hydrolysis (thermal degradation) of the material over the lifetime of the
transformer. This hydrolysis is even more active at elevated temperatures, and published
research results indicate that thermally upgraded insulation papers retain a much higher
percentage of their tensile and bursting strength than untreated papers when exposed to
elevated temperatures [4], [5]. The same references also show the change of DP over time of
non-thermally and thermally upgraded paper exposed to a temperature of 150 °C (see Figure 1).
60076-7 © IEC:200560076-7 IEC:2005 –– 15 – 29 –
1 200
1 000
DP
0 1 000 2 000 3 000 4 000
t
IEC 2306/05
Key
DP Degree of polymerization
t Time (h)
∆ Values for thermally upgraded paper
● Values for non-thermally upgraded paper
Figure 1 – Sealed tube accelerated ageing in mineral oil at 150 °C
Another reference [6] illustrates the influence of temperature and moisture content, as shown
in Table 1.
Table 1 – Life of paper under various conditions
Paper type/ageing temperature Life
years
Dry and free With air and
from air 2 % moisture
Wood pulp at 80 °C 118 5,7
90 °C 38 1,9
15 0,8
98 °C
Upgraded wood pulp at 80 °C 72 76
90 °C 34 27
18 12
98 °C
The illustrated difference in thermal ageing behaviour has been taken into account in
industrial standards as follows.
• The relative ageing rate V = 1,0 corresponds to a temperature of 98 °C for non-thermally
upgraded paper and to 110 °C for thermally upgraded paper.
NOTE The results in Figure 1 and Table 1 are not intended to be used as such for ageing calculations and life
estimations. They have been included in this document only to demonstrate that there is a difference in ageing
behaviour between non-thermally and thermally upgraded insulation paper.
60076-7 IEC:2005 –– 16 – 31 – 60076-7 © IEC:2005
6 Relative ageing rate and transformer insulation life
6.1 General
There is no simple and unique end-of-life criterion that can be used to quantify the remaining
life of a transformer. However, such a criterion is useful for transformer users, hence it seems
appropriate to focus on the ageing process and condition of transformer insulation.
6.2 Relative ageing rate
Although ageing or deterioration of insulation is a time function of temperature, moisture
content, oxygen content and acid content, the model presented in this part of IEC 60076 is
based only on the insulation temperature as the controlling parameter.
Since the temperature distribution is not uniform, the part that is operating at the highest
temperature will normally undergo the greatest deterioration. Therefore, the rate of ageing is
referred to the winding hot-spot temperature. In this case the relative ageing rate V is defined
according to equation (2) for non-thermally upgraded paper and to equation (3) for thermally
upgraded paper [7].
()θ −98 / 6
h
(2)
V = 2
15 000 15 000
−
(3)
110 + 273 θ +273
h
V = e
where θ is the hot-spot temperature in °C.
h
Equations (2) and (3) imply that V is very sensitive to the hot-spot temperature as can be
seen in Table 2.
Table 2 – Relative ageing rates due to hot-spot temperature
Non-upgraded paper insulation Upgraded paper insulation
θ
h
V V
°C
80 0,125 0,036
86 0,25 0,073
92 0,5 0,145
98 1,0
0,282
104 2,0 0,536
...
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