Instrument transformers - Part 6: Requirements for protective current transformers for transient performance

Concerns requirements and tests, in addition to those given in chapter 1 of IEC 60185, that are necessary when inductive current transformers are used with electrical protective schemes in which the prime requirement is the maintenance of a defined performance up to several times the rated current when the current contains an exponentially decaying d.c. component of defined time constant.

Transformateurs de mesure - Partie 6: Prescriptions concernant les transformateurs de courant pour protection pour la réponse en régime transitoire

Décrit les prescriptions et les essais qui, en complément de ceux indiqués au chapitre 1 de la CEI 60185, s'appliquent aux transformateurs de courant inductifs destinés à être utilisés avec des systèmes de protection électrique pour lesquels il est primordial que les transformateurs de courant conservent une précision déterminée en présence d'un courant atteignant plusieurs fois le courant assigné et comportant une composante apériodique décroissante exponentiellement avec une constante de temps fixée.

Instrument transformers - Part 6: Requirements for protective current transformers for transient performance

General Information

Publication Date
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
Due Date
Completion Date

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SIST IEC 60044-6:1995

Instrument transformers - Part 6: Requirements for protective current transformers

for transient performance

Instrument transformers - Part 6: Requirements for protective current transformers for

transient performance

Transformateurs de mesure - Partie 6: Prescriptions concernant les transformateurs de

courant pour protection pour la réponse en régime transitoire
Ta slovenski standard je istoveten z: IEC 60044-6
17.220.20 0HUMHQMHHOHNWULþQLKLQ Measurement of electrical
PDJQHWQLKYHOLþLQ and magnetic quantities
SIST IEC 60044-6:1995 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST IEC 60044-6:1995
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SIST IEC 60044-6:1995
Première édition
STANDARD First edition
Transformateurs de mesure
Partie 6:
Prescriptions concernant les transformateurs
de courant pour protection pour la réponse
en régime transitoire
Instrument transformers
Part 6:
Requirements for protective current transformers
for transient performance
réservés — Copyright — all rights reserved
© CEI 1992 Droits de reproduction
rt of this publication may be reproduced or utilized in
Aucune partie de cette publication ne peut étre reproduite ni No pa

utilisée sous quelque forme que ce soit et par aucun pro- any form or by any means, electronic or mechanical,


cédé, électronique ou mécanique, y compris la photocopie et including photocopying and microfilm, without permiss

les microfilms, sans l'accord écrit de l'éditeur. in writing from the publisher.

Bureau Central de la Commission Electrotechnique Internationale 3, rue de Varembé Genève,

Commission Electrotechnique Internationale
(C` International Electrotechnical Commission
MemutyHapo tnaa 3netcrpoTexHHVecttaR
Pour prix, voir catalogue en vigueur •
For price, see current catalogue
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SIST IEC 60044-6:1995
44-6 ©I EC – 3 –
1 Scope 9
2 Normative references
3 Definitions 9
rformance requirements 4 Ratings and pe
5 Methods of specification
6 Marking of rating plate
7 Tests
35 A Basic theoretical equations for transient dimensioning
B Determination of core magnetization characteristic
C Direct tests 59
ormance criteria for current transformer for protection relaying
Guide to perf
E Determination of turns ratio error 85
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SIST IEC 60044-6:1995
44-6 0O IEC – 5 –
Part 6: Requirements for protective current transformers
for transient performance

The formal decisions or agreements of the IEC on technical matters, prepared by Technical Committees on

which all the National Committees having a special interest therein are represented, express, as nearly as

possible, an international consensus of opinion on the subjects dealt with.

2) They have the form of recommendations for international use and they are accepted by the National

Committees in that sense.

In order to promote international unification, the IEC expresses the wish that all National Committees

should adopt the text of the IEC recommendation for their national rules in so far as national conditions will

permit. Any divergence between the IEC recommendation and the corresponding national rules should, as

far as possible, be clearly indicated in the latter.

This part of International Standard IEC 44 has been prepared by IEC Technical Committee

No. 38: Instrument transformers.
The text of this part is based on the following documents:
Six Months' Rule Repo rts on Voting Two Months' Procedure Report on Voting
38(CO)78 38(00)81 & 81A 38(CO)83 38(00)86

Full information on the voting for the approval of this part can be found in the Voting

Reports indicated in the above table.

This part of IEC 44 is to be read in conjunction with IEC 185 and its Amendment No. 1.

Annexes A, B, C, D and E form an integral pa rt of this part.
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SIST IEC 60044-6:1995
44-6©IEC – 7 –

Performance criteria for class P current transformers included in Chapter III of IEC 185

relate to a steady state a.c. symmetrical primary energizing current which allows the

limiting secondary e.m.f. to be as defined in 34.5 of IEC 185. In this part of IEC 44,

requirements for protective current transformers as classified by 3.5 take account of the

additional flux linking the secondary winding due to the d.c. component of energizing

current. Strictly, the limiting condition is defined by the integral of the voltage which is

induced in the secondary winding of the current transformer in order to drive current in the

secondary loop, inclusive of winding and secondary resistance, for the specified energiz-

ing conditions. For mathematical convenience, an equivalent sinusoidal e.m.f. is used to

define the limiting condition. Refer also to annex B.
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SIST IEC 60044-6:1995
44-6 ©IEC - 9 -
Part 6: Requirements for protective current transformers
for transient performance
1 Scope

This part of IEC 44 covers the requirements and tests, in addition to those in Chapter I of

IEC 185, that are necessary for inductive current transformers for use with electrical

protective schemes in which the prime requirement for the current transformers is the

ormance up to several times the rated current when the
maintenance of a defined pe rf

current contains an exponentially decaying d.c. component of defined time constant.

2 Normative references

The following standards contain provisions which, through reference in this text, constitute

provisions of this part of IEC 44. At the time of publication, the editions indicated were

valid. All standards are subject to revision, and parties to agreements based on this part

of IEC 44 are encouraged to investigate the possibility of applying the most recent editions

of the standards indicated below. Members of IEC and ISO maintain registers of currently

valid International Standards.
High-voltage alternating-current circuit-breakers.
IEC 56: 1987,
Current transformers.
IEC 185: 1987,
3 Definitions
of IEC 44, the following definitions apply.
For the purpose of this pa rt
R.M.S. value of primary symmetrical
3.1 rated primary short-circuit current
ormance of the current transformer is
short-circuit current on which the rated accuracy pe rf
): Difference between the instantaneous values of
3.2 instantaneous error current
) and the primary
) multiplied by the rated transformation ratio (K m
the secondary current (is
current (in):
iE = Kn is - ip

When both alternating current and direct current components are present, the constituent

components are separately identified as follows:
i i i
+ cdc - (K nsac - ^pac)+ (K i nsdc - pdc)
c— cac
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SIST IEC 60044-6:1995
44-6 © I EC - 11 -
Maximum instantaneous error current, for the
3.3 peak instantaneous (total) error (é):

specified duty cycle, expressed as a percentage of the peak instantaneous value of the

rated primary short-circuit current:
i'." = 100 ï / (4-2-/
) (%)
(Eac): Maximum instanta-
3.4 peak instantaneous alternating current component error

neous error of the alternating current component expressed as a percentage of the peak

instantaneous value of the rated primary short-circuit current:
= AI 2
100 / (
Éac sac
Current transformers for protection are
protective current transformer classes:
classified according to functional performance as follows:
c ) with steady state symmetrical Accuracy limit defined by composite error (£
class P:
primary current. No limit for remanent flux.

class TPS: Low leakage flux current transformer for which performance is defined by the

secondary excitation characteristics and turns ratio error limits. No limit for
remanent flux.

class TPX: Accuracy limit defined by peak instantaneous error (É) during specified

transient duty cycle. No limit for remanent flux.
(É) during specified
class TPY: Accuracy limit defined by peak instantaneous error
transient duty cycle. Remanent flux not to exceed 10 % of the saturation flux.

class TPZ: Accuracy limit defined by peak instantaneous alternating current component

d during single energization with maximum d.c. offset at specified
error (ga
secondary loop time constant. No requirements for d.c. component error limit.
Remanent flux to be practically negligible.
(T ): That specified value of the time constant of
3.6 specified primary time constant
which the pe rformance of the current trans-
the d.c. component of the primary current on

former is based. This value may also be a rated value for class TPX, TPY and TPZ current

transformers and then will be marked on the rating plate.
): Time during which the specified accuracy
permissible time to accuracy limit (tat
is maintained during any specified energization period of a given duty cycle.

NOTE - This time will usually be defined by the critical measuring time of the associated protection

scheme. When stable operation of the protection scheme is a limiting requirement, it may also be

necessary to consider the time taken by the circuit breaker to interrupt the current.

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SIST IEC 60044-6:1995
– 13 –
44-6 © IEC
Elapsed time during a prescribed energization period
time to maximum flux

at which the transient flux in a current transformer core achieves maximum value, it being

assumed that saturation of the core does not occur.
): Time interval between interruption and
3.9 dead time (during auto -reclosing) (tfr

re-application of the primary short-circuit current during a circuit breaker auto-reclosing

duty cycle (refer also to IEC 56).
Duty cycle in which, during each
3.10 specified duty cycle (C-0 and/or C-0-C-0):

specified energization, the primary energizing current is assumed to be "fully offset" (see

) and be of rated amplitude (/psc).
note below), with the specified decay time constant (T
Duty cycles are as follows:
Single energization: C - t'- 0
Double energization: C - t'- 0 - - C - t"- 0
(both energizations in the same polarity of flux)

t' is the duration of first current flow: specified accuracy being maintained during time t'ai

t" is the duration of second current flow: specified accuracy being maintained during time t al.

NOTE - Specification of partial offset would reduce the required transient factor by an amount approxi-

mately proportional to the reduction. For this reason specification of full offset parameters is recommended.

(R b): Rated value of the secondary connected resistive
3.11 rated resistive burden
burden in ohms.
ct): Secondary winding d.c. resistance in ohms,
3.12 secondary winding resistance (R
corrected to 75 °C or such other temperature as may be specified.
Total resistance of the secondary circuit,
3.13 secondary loop resistance (Rs):

inclusive of the secondary winding resistance corrected to 75 °C, unless otherwise

specified, and inclusive of all external burden connected.
(Ts): Value of the time constant of the
3.14 rated secondary loop time constant

secondary loop of the current transformer obtained from the sum of the magnetizing and

) and the secondary loop resistance (Rs):
the leakage inductances (L s
ssc): The ratio:
short-circuit current factor (K
3.15 rated symmetrical
Kssc – /psc / /pn
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SIST IEC 60044-6:1995
44-6 ©I EC - 15 -
Ratio of the theoretical total secondary linked flux to the
3.16 transient factor (Kif):

peak instantaneous value of the a.c. component of that flux, when a current transformer is

s) is

subjected to a specified single energization and the secondary loop time constant (T

assumed to have retained a constant value throughout the energization period.
): That theoretical value representative of
3.17 rated transient dimensioning factor (Kid
the transient dimensioning necessary to satisfy the specified duty cycle.
are given in annex A.
p, s , Kif and Ktd
Mathematical relationships between T
Current transformer for which a knowledge
3.18 low leakage flux current transformer:

of the secondary excitation characteristic and secondary winding resistance is sufficient

for an assessment of its transient performance for any combination of burden and duty

cycle at rated or lower value of primary symmetrical short-circuit current up to the theo-

retical limit of the current transformer capability determined from the secondary excitation

Current transformer which does not
3.19 high leakage flux current transformer:

satisfy the requirements of 3.18 and for which an additional allowance is made by the

manufacturer to take account of influencing effects which result in additional leakage flux.

Such a current transformer is expected to satisfy a specified duty cycle.
td ) is satisfied for a C-0-C-0 duty
NOTE - In general, if the theoretical transient dimensioning factor (K

cycle, then the accuracy is maintained during a C-0 duty cycle at least up to the time at which the rated

al ), defined in 3.20, is reached.
equivalent limiting secondary e.m.f. (E
(Eat): That r.m.s. value of the
3.20 rated equivalent limiting secondary e.m.f.

equivalent secondary circuit e.m.f. of rated frequency necessary to satisfy the specified

duty cycle and derived from the following:
Rb) Isn V, r.m.s.)
ct + Eal = Kssc Ktd (R
(Uai): That r.m.s. value of
rated equivalent excitation limiting secondary voltage

sinusoidal voltage of rated frequency necessary to ensure that the rated equivalent limiting

secondary e.m.f. will be attained after due account is taken of the current transformer

construction and which, when applied to the transformer secondary winding, would result

in a magnetizing current not exceeding the maximum permissible error current appropriate

to the current transformer class.
( V, r.m.s.)
Ual = Eal F
is the factor of construction defined in 3.29.
where Fc
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SIST IEC 60044-6:1995
44-6 ©I EC – 17 –
(Ealc): That equivalent r.m.s.
3.22 equivalent secondary accuracy limiting e.m.f.

e.m.f. of rated frequency determined during a direct test when the observed error current

corresponds to the appropriate limit for the class.

NOTE - The error current is an absolute value based on the specified primary current value and is thus

not affected by any parametric changes which may have been necessary to attain the secondary error

limiting condition.
(Ualc): That r.m.s. value of
3.23 equivalent secondary accuracy limiting voltage

sinusoidal voltage of rated frequency which, if applied to the secondary winding of a

current transformer, would result in an exciting current corresponding to the maximum

permissible error current appropriate to the current transformer class.
): That peak value of the flux which would exist in a core in the
3.24 saturation flux (T s

transition from the non-saturated to the fully saturated condition and deemed to be that

point on the B-H characteristic for the core concerned at which a 10 % increase in B

causes H to be increased by 50 %.
): That value of flux which would remain in the core three
3.25 remanent flux (T r

minutes after the interruption of an exciting current of sufficient magnitude as to induce

the saturation flux (`P S) defined in 3.24 above.
remanence factor (Kr): The ratio
Kr = r / ^s
): That peak value of the secondary linked flux correspond-
3.27 accuracy limit flux (cDal
ing to E
al :
Eal / (2icf)
(Dal =
When is given in volts, r.m.s., is expressed in weber.
Eai (Dal
d: Peak value of the exciting
accuracy limiting secondary exciting current (la
(error) current appropriate to the current transformer class.
): Factor declared by the manufacturer for the design.
3.29 factor of construction (F0
The factor of construction is determined from the ratio:
Fc = Ualc / Eaic
4 Ratings and performance requirements
Standard values for rated symmetrical short-circuit current factor (Kssc)
rformance are:
Standard values of for protective current transformers for transient pe
The preferred values are underlined.
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SIST IEC 60044-6:1995
44-6 ©IEC - 19 -
4.2 Standard values for symmetrical short-circuit current
4.2.1 Rated short-time thermal current
Standard r.m.s. values, expressed in kiloamperes, are:
4.2.2 Rated primary short-circuit current
selected from the values
Preferred values are derived from the product of and K
/n sc
of IEC 44 respectively. The product need not
given in 4.1 of IEC 185 and 4.1 of this pa
be exactly equal to
Standard values for rated primary time constant (Tp)
Standard values, expressed in milliseconds, are:

NOTE - For some applications, higher values of rated primary time constant may be required. Example:

large turbo -generator circuits.
4.4 Standard values for rated transient dimensioning factor

At present there are no standard values for the rated transient dimensioning factor

because the values of this factor depend upon the application.
4.5 Standard values of rated resistive burden

Standard values of rated resistive burden in ohms for class TP current transformers,

based on a rated secondary current of 1 A are:

The preferred values are underlined. For current transformers having a rated secondary

current other than 1 A, the above values should be adjusted in inverse ratio to the square

of the current.
4.6 Error limits for TPS current transformers
. The error
The turns ratio of a TPS current transformer shall be numerically equal to 1/Kn
in this turns ratio shall not exceed ±0,25 %.

The accuracy limiting conditions are defined by the magnetization characteristic and the

shall not be less than the specified value. The
excitation limiting secondary voltage (Jai

value shall be such that an increase of 10 % in magnitude does not result in an increase in

the corresponding peak instantaneous exciting current exceeding 100 %.
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SIST IEC 60044-6:1995
44-6©IEC –21 –

When specified by the purchaser, the measured value of the peak exciting current at the

excitation limiting secondary voltage shall not exceed the specified value. If no limit is set,

the exciting current shall, in any case, not exceed that value corresponding to 10 % of

referred to the secondary side (see TPX CTs, table 1).

The excitation limiting secondary voltage defined by the purchaser is generally expressed

as follows:
KKsc (Rct+
R b)
ct is defined by
in which is a dimensioning parameter assigned by the purchaser. R

the manufacturer's design except that for some applications limits may need to be set

by the purchaser to enable co-ordination with other equipment.
Error limits for TPX, TPY and TPZ current transformers
Rs = Rct + Rb , the errors shall
With the secondary loop resistance adjusted to the value
not exceed the values given in table 1.
Table 1 - Limits of error
At rated primary current At accuracy limit condition
Phase displacement
Class Maximum peak
instantaneous error
Min Centirad
±30 ±0,9 É = 10
TPX ±0,5
±1,0 ±60 ±1,8 e = 10
180 ± 18 5,3 ± 0,6
TPZ ±1,0
Sac = 10

NOTE - For some applications, deviation from the above values may be necessary (refer also to

annex D.3). Similarly, the absolute value of the phase displacement may in some cases be of less

importance than achieving minimal deviation from the average value of a given production series.

5 Methods of specification

The methods of specification for the different CT classes are illustrated in table 2.

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SIST IEC 60044-6:1995
23 -
44-6© I EC -
Table 2 - Methods of specification
CT class
x x
x x
Rated primary current
x x
Rated secondary current x x
x x x
Rated frequency
Highest voltage for equipment
x x x
and rated insulation level x
' th
x x
Ratio to which specification applies x x
x x
K x
– x x
– – –
Ts –
see note
Duty cycle
x –
Single: t', t'a , – x
double: t', t' t
a1 , tfr' t", al
Rb X X X
x – – –
x – – –
Maximum at Uai
x – – –
x applicable – not applicable

NOTE - When the purchaser wishes to obtain compatibility between existing equipment and new units, an alignment

specification may define limiting values for certain parameters, for example s or Rif . It will be necessary, however,

to recognize possible differences between designs. Issue of the principal objectives for the intended application and

best available (name-plate) data for the existing current transformers will usually achieve acceptable results.

6 Marking of rating plate

The rating plate shall carry the appropriate information given in accordance with clause 23

of IEC 185. Table 3 identifies additional information to be included.
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SIST IEC 60044-6:1995
- 25 -
44-6 © I EC
Table 3 - Information to be included in the rating plate
TPZ to to Notes
IEC 185 IEC 44-6
3.6 1
x X X X -
p n
x 3.7 - 1
x x
x x x 3.25 -
/ x X x 3.26 -
- 3.1
x - 3.15
x x x
- 4.2
x x x -
Rb x
x x x -
Rot (at ... °C) x
Kid — x x x — 3.17
x - -
X —
— —
— -
x x x - 3.29 2
F -
x x - 3.6 5
TP - x
x - - 3.14 4,5
T - -
Duty cycle
- x x - -
single: t', t'ai
— 5
double: (, t'ai, tir, t", r
x applicable - not applicable
and K will usually be the highest values applicable
1 For multiratio protective current transformers, / ID.
to the range of ratios.
2 Data to be given if Fr,> 1,1.
may exceed 2,5 /iii depending on the values of and /PSC.
3 Values for ion

4 When s is greater than 10 s, it will usually be adequate to mark on the rating plate 5 > 10 s.

5 T , s and the duty cycles are interrelated and their indication on the rating plate could be omitted for1

low leakage flux CTs.
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SIST IEC 60044-6:1995
- 27 -
44-6 ©I EC
7 Tests

7.1 To prove compliance of the current transformer with the requirements of this standard

the following tests shall be performed as required by the test schedule - table 4.

Table 4 - Test schedule
CT class
Clause or
— — 7.2.1 1
Turns ratio error x —
Steady state ratio error
x 7.2.2 1
— x x
and phase displacement
7.2.3 1
x x x x
x 7.2.4 1
x x x
Excitation characteristic
K — — x — 7.2.5 1
— — x 7.2.6 1
x 7.3 2
— x x
Errors at limiting conditions
x 7.3.1 2
— x x
Verification of low leakage
— 7.4 3
(F < 1,1) x — —
flux design
x applicable — not applicable
1 Type and routine tests.
2 Type tests.
Special tests performed only on agreement between manufacturer and purchaser.
7.2 Type and routine tests
7.2.1 Turns ratio error

The turns ratio error shall be determined by an appropriate method. Refer to annex E.

7.2.2 Steady state ratio error and phase displacement
(Rs = Rct + Rb) ratio error and
With the secondary loop resistance adjusted to rated value
phase displacement shall be measured at rated current (Ipn).
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SIST IEC 60044-6:1995
44-6 ©I EC - 29 -
7.2.3 Determination of secondary winding resistance (Red
The secondary winding resistance shall be measured and an appropriate correction

applied if the measurement is made at a temperature which differs from 75 °C or such

other temperature as may have been specified. The value so adjusted is the rated value

for Rct.
7.2.4 Determination of secondary excitation characteristic

For type tests, an excitation curve is required up to not less than 1,1 times saturation flux.

The manufacturer may choose the test method. Some test methods are given in annex B.

For routine tests on TPX and TPY current transformers, the peak value of the exciting

current at (Jai is to be measured.

For routine tests on TPZ current transformers, the peak value of the exciting secondary

current 1 shall not exceed the value obtained from the expression given below:
/c0 7-5] +0,1 {

NOTE - For TPZ current transformers the accuracy is specified only for the a.c. component while, in the

during indirect tests, it is necessary to take into account also
determination of the permissible value of

the d.c. component of the exciting current. In the above equation, the d.c. component is represented by

(Ktd – 1) and the permissible error in the a.c. component by 0,1.
7.2.5 Determination of remanence factor (Kr)

The remanence factor (Kr) shall be determined to prove compliance with the appropriate

class limits. Refer to annex B.
Calculation of secondary loop time constant (Ts)
) shall be determined and shall not differ from the
The secondary loop time constant (Ts

declared or the rated value by more than ±30 % for class TPY and ±10 % for class TPZ

current transformers. Refer to annex B.
7.3 Type tests
7.3.1 General

Direct tests intended to be made at the specified limiting parameters and duty cycle are

type tests. Their purpose is:

- to measure the peak instantaneous error current of the CT, when subjected to the

specified duty cycle, according to 3.3 (classes TPX and TPY) and 3.4 (class TPZ);

e) (see 3.29).
- to determine the factor of construction (F
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SIST IEC 60044-6:1995
44-6 ©IEC – 31 –

The tests can be performed on a full scale model of the active part of the current trans-

former assembly inclusive of all metal housings but with reduced insulation.

Direct tests may be replaced by the secondary exciting test, if either of the following is

the current transformer is of the low leakage flux type.

To satisfy this requirement, it shall be shown by a drawing that the current transformer

has substantially continuous ring core with air gaps uniformly distributed, uniformly

distributed secondary winding, a primary conductor symmetrical with respect to rotation

and the influences of conductors of the adjacent phase outside of the CT housing and

of the neighbouring phases are negligible.
a type test report of a current transformer having substantially the same cons-
truction and rated primary short-circuit current is available.

NOTE - If, despite the information given above, direct tests are required by the purchaser, it will be stated

in the order.
7.3.2 Measurement of peak instantaneous error current

Methods for direct measurement of the peak instantaneous error current at accuracy

limiting conditions are given in annex C.

Records shall be taken of the instantaneous values of primary, secondary and error

currents in addition to the integral with time of the voltage across the secondary terminals

from which record the equivalent secondary e.m.f. can be derived at limiting conditions

(Ea, and
rt :
The following data shall be included in the type test repo
type designation;
b) year of manufacture/serial number;
rating plate markings;
d) results of secondary exciting test (see annex B);

results of direct tests including test parameters, test circuit diagram, photo(s) of test

arrangement, records and evolution of results;

declaration of the manufacturer concerning the validity to the ordered CT of the type

test made on CT having other technical data.

Correlation between direct and indirect (secondary excitation characteristic measurement)

tests may be selected from either of the following but may not be necessary if certificates

are available for a previously type-tested unit of substantially the same construction and

performance requirements.
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SIST IEC 60044-6:1995
— 33 —
44-6 ©IEC
7.3.3 Determination of factor of construction

The method for the determination of the factor of construction is given in annex D.

is valid for the current trans-
The factor of construction, obtained from the ratio

former performance at rated conditions and at the highest theoretical value of the transient

td). That is, if both C-0 and C-0-C-0 duty cycles are specified,
dimensioning factor (K
will be determined from whichever duty cycle yields the higher value.

When the factor of construction does not exceed 1,1, the design may or may not satisfy

the criteria for a low leakage flux design.

Strictly, the factor of construction relates the secondary excitation characteristics to the

current transformer performance under specified conditions only.
7.4 Special tests to verify a low leakage flux design

Direct tests to verify that a current transformer will satisfy the basic requirements of a low

shall be made at a sufficiently large number of energizing
leakage flux design (see 3.18)

current, duty cycle and burden combinations as can reasonably establish that the devia-

tion between the theoretical equivalent secondary e.m.f. and the measured value does not

exceed 10 %.

Available test experience does not permit precise specification of parametric relationships and limits

for all classes at this time.

Indirect tests to verify that the design is of the low leakage flux type may be made in addition to direct

tests at specified limits.
is determined from the secondary excitation characteristics at a value su

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