Rotating electrical machines - Part 27-3: Dielectric dissipation factor measurement on stator winding insulation of rotating electrical machines

IEC 60034-27-3:2015 provides guidelines for the test procedures and the interpretation of test results for dielectric dissipation factor measurements on the stator winding insulation of rotating electrical machines. These guidelines are valid for rotating electrical machines with conductive slot coatings operating at a rated voltage of 6 kV and higher. This standard applies to individual form-wound stator bars and coils outside a core (uninstalled), individual stator bars and coils installed in a core and complete form-wound stator winding of machines in new or aged condition. This edition includes the following significant technical changes with respect to the previous publication IEC TR 60894:1987: - inclusion of digital measurement of dissipation factor and capacitance; - inclusion of limits for dissipation factor values; - detailed description of measuring techniques; extension of scope to complete windings.

Drehende elektrische Maschinen - Teil 27-3: Messung des dielektrischen Verlustfaktors an der Ständerwicklungsisolierung drehender elektrischer Maschinen

Machines électriques tournantes - Partie 27-3: Mesure du facteur de dissipation diélectrique sur le système d'isolation des enroulements statoriques des machines électriques tournantes

L'IEC 60034-27-3:2015 fournit des lignes directrices pour les procédures d'essai et l'interprétation des résultats d'essai concernant les mesures du facteur de dissipation diélectrique sur l'isolation des enroulements statoriques des machines électriques tournantes. Ces lignes directrices sont valables pour les machines électriques tournantes à revêtement d'encoche conducteur dont la tension assignée est supérieure ou égale à 6 kV. Cette norme s'applique aux barres et bobines de stator individuelles préformées situées à l'extérieur d'un noyau (non installées), aux barres et bobines de stator individuelles installées dans un noyau et aux enroulements statoriques préformés complets des machines à l'état neuf ou vieilli. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'IEC TR 60894:1987: - intégration de la mesure numérique du facteur de dissipation et de la capacité; - indication de valeurs limites pour le facteur de dissipation; - description détaillée des techniques de mesure; - extension du domaine d'application aux enroulements complets.

Električni rotacijski stroji - 27-3. del: Merjenje faktorja dielektričnih izgub v izolaciji statorskega navitja električnih rotacijskih strojev (IEC 60034-27-3:2015)

Ta del standarda IEC 60034 podaja smernice za preskusne postopke in interpretacijo rezultatov merjenja faktorja dielektričnih izgub v izolaciji statorskega navitja električnih rotacijskih strojev. Te smernice se uporabljajo za električne rotacijske stroje s prevodno prevleko, ki obratujejo pri nazivni napetosti 6 kV ali več.
Ta standard se uporablja za posamezna predhodno oblikovana navitja statornih palic in navoje zunaj jedra (nemontirane), posamezne statorne palice in navoje, montirane v jedru, ter celotna predhodno oblikovana statorna navitja novih ali rabljenih strojev.
Ta mednarodni standard se uporablja za vse vrste vakuumsko impregniranih ali smolnatih (polno obremenjenih) zalepljenih palic, navojev in celotnih navitij. Ne uporablja se za neimpregnirane posamezne palice in navoje ali neimpregnirana celotna navitja.
Podane so zahteve glede lastnosti faktorja dielektričnih izgub posameznih predhodno navitih statornih palic in navojev strojev pod napetostjo 6 kV ali več pri preskušanju z izmenično napetostjo 50 Hz ali 60 Hz.

General Information

Status
Published
Publication Date
23-Jun-2016
Technical Committee
Drafting Committee
Current Stage
6060 - Document made available
Due Date
24-Jun-2016
Completion Date
24-Jun-2016

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SLOVENSKI STANDARD
SIST EN 60034-27-3:2016
01-november-2016

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Rotating electrical machines - Part 27-3: Dielectric dissipation factor measurement on

stator winding insulation of rotating electrical machines (IEC 60034-27-3:2015)

Drehende elektrische Maschinen - Teil 27-3: Messung des dielektrischen Verlustfaktors

an der Ständerwicklungsisolierung drehender elektrischer Maschinen (IEC 60034-27-

3:2015)
Machines électriques tournantes - Partie 27-3: Mesure du facteur de dissipation

diélectrique sur le système d'isolation des enroulements statoriques des machines

électriques tournantes (IEC 60034-27-3:2015)
Ta slovenski standard je istoveten z: EN 60034-27-3:2016
ICS:
29.160.01 Rotacijski stroji na splošno Rotating machinery in
general
SIST EN 60034-27-3:2016 en

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

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SIST EN 60034-27-3:2016
---------------------- Page: 2 ----------------------
SIST EN 60034-27-3:2016
EUROPEAN STANDARD EN 60034-27-3
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2016
ICS 29.160
English Version
Rotating electrical machines - Part 27-3: Dielectric dissipation
factor measurement on stator winding insulation of rotating
electrical machines
(IEC 60034-27-3:2015)

Machines électriques tournantes - Partie 27-3: Mesure du Drehende elektrische Maschinen - Teil 27-3: Messung des

facteur de dissipation diélectrique sur le système d'isolation dielektrischen Verlustfaktors an der

des enroulements statoriques des machines électriques Ständerwicklungsisolierung drehender elektrischer

tournantes Maschinen
(IEC 60034-27-3:2015) (IEC 60034-27-3:2015)

This European Standard was approved by CENELEC on 2016-01-20. CENELEC members are bound to comply with the CEN/CENELEC

Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC

Management Centre or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation

under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the

same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,

Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,

Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2016 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.

Ref. No. EN 60034-27-3:2016 E
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SIST EN 60034-27-3:2016
EN 60034-27-3:2016
European foreword

The text of document 2/1803/FDIS, future edition 1 of IEC 60034-27-3, prepared by IEC/TC 2

"Rotating machinery" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC

as EN 60034-27-3:2016.
The following dates are fixed:
(dop) 2016-12-24
• latest date by which the document has to be implemented at
national level by publication of an identical national
standard or by endorsement
(dow) 2019-06-24
• latest date by which the national standards conflicting with
the document have to be withdrawn

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such

patent rights.
Endorsement notice

The text of the International Standard IEC 60034-27-3:2015 was approved by CENELEC as a

European Standard without any modification.

In the official version, for Bibliography, the following note has to be added for the standard indicated :

IEC TS 60034-27 NOTE Harmonized as CLC/TS 60034-27.
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SIST EN 60034-27-3:2016
EN 60034-27-3:2016
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

NOTE 1 When an International Publication has been modified by common modifications, indicated by (mod),

the relevant EN/HD applies.

NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is

available here: www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60060-1 - High-voltage test techniques - EN 60060-1 -
Part 1: General definitions and test
requirements
IEC 60060-2 - High-voltage test techniques - EN 60060-2 -
Part 2: Measuring systems
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SIST EN 60034-27-3:2016
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SIST EN 60034-27-3:2016
IEC 60034-27-3
Edition 1.0 2015-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Rotating electrical machines –
Part 27-3: Dielectric dissipation factor measurement on stator winding
insulation of rotating electrical machines
Machines électriques tournantes –
Partie 27-3: Mesure du facteur de dissipation diélectrique sur le système
d’isolation des enroulements statoriques des machines électriques tournantes
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.160 ISBN 978-2-8322-3061-9

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 7 ----------------------
SIST EN 60034-27-3:2016
– 2 – IEC 60034-27-3:2015 © IEC 2015
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms and definitions ...................................................................................................... 7

4 Theory and measuring techniques ................................................................................... 8

4.1 Dielectric dissipation factor measurement ............................................................... 8

4.2 Analogue Schering bridge ..................................................................................... 10

4.3 Transformer ratio arm bridge................................................................................. 11

4.4 Digital phase shift measurement ........................................................................... 12

5 Test procedures ............................................................................................................ 13

5.1 General ................................................................................................................. 13

5.2 Winding bars and coils .......................................................................................... 15

5.2.1 Test object preparation .................................................................................. 15

5.2.2 Guarding techniques ...................................................................................... 15

5.2.3 Measuring procedure ..................................................................................... 17

5.3 Complete windings ................................................................................................ 17

6 Test results ................................................................................................................... 18

6.1 General ................................................................................................................. 18

6.2 Winding bars and coils .......................................................................................... 19

6.3 Complete windings ................................................................................................ 20

7 Test report ..................................................................................................................... 21

7.1 General ................................................................................................................. 21

7.2 New coils, bars and winding .................................................................................. 21

7.3 Operational aged winding ..................................................................................... 22

Annex A (informative) Relationship between power factor and dissipation factor .................. 24

Bibliography .......................................................................................................................... 26

Figure 1 – Parallel circuit and vector diagram ......................................................................... 8

Figure 2 – Series circuit and vector diagram ........................................................................... 9

Figure 3 – Dielectric losses with increasing voltage (schematic) ........................................... 10

Figure 4 – High voltage Schering bridge – Basic circuit ......................................................... 11

Figure 5 – Transformer ratio arm bridge ................................................................................ 12

Figure 6 – Schematic test set-up of a digital dissipation factor measuring system with

principle current oscillogram ................................................................................................. 13

Figure 7 – Example of a curve of tan δ versus voltage ratio U/ U measured in voltage

steps of 0,2 U ...................................................................................................................... 14

Figure 8 – Arrangement with guard rings electrodes on test objects with insulation gap

(example of preferred insulation gap and guard ring electrode dimensions) .......................... 16

Figure 9 – Application of guard ring electrodes on top of stress control coating .................... 17

Figure A.1 – Phasor diagram ................................................................................................ 24

Table 1 – Maximum values of dielectric dissipation factor of single bars and coils in

new condition with guard ring electrodes up to a rated voltage of U = 21 kV ........................ 19

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SIST EN 60034-27-3:2016
IEC 60034-27-3:2015 © IEC 2015 – 3 –

Table A.1 – Comparison between correlating values of dielectric power factor cos φ

and dielectric dissipation factor tan δ and their difference ..................................................... 25

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SIST EN 60034-27-3:2016
– 4 – IEC 60034-27-3:2015 © IEC 2015
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ROTATING ELECTRICAL MACHINES –
Part 27-3: Dielectric dissipation factor measurement on stator
winding insulation of rotating electrical machines
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 interna-

tional co-operation on all questions concerning standardization in the electrical and electronic fields. To this

end and in addition to other activities, IEC publishes International Standards, Technical Specifications, Tech-

nical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publica-

tion(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested in the

subject dealt with may participate in this preparatory work. International, governmental and non-governmental

organizations liaising with the IEC also participate in this preparation. IEC collaborates closely with the Interna-

tional 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 inter-

ested 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 misinter-

pretation 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 be-

tween any IEC Publication and the corresponding national or regional publication shall be clearly indicated in

the latter.

5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity

assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any

services carried out by independent certification bodies.

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 ex-

penses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC Publica-

tions.

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 60034-27-3 has been prepared by IEC technical committee 2:

Rotating machinery.

This first edition cancels and replaces the first edition of IEC TR 60894 published in 1987.

This edition constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous

edition:
a) digital measurement of dissipation factor and capacitance included;
b) limits for dissipation factor values given;
c) detailed description of measuring techniques;
d) extension of scope to complete windings.
---------------------- Page: 10 ----------------------
SIST EN 60034-27-3:2016
IEC 60034-27-3:2015 © IEC 2015 – 5 –
The text of this standard is based on the following documents:
FDIS Report on voting
2/1803/FDIS 2/1804/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.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 60034 series, published under the general title Rotating electrical

machines, can be found on the IEC website.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data re-

lated to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
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SIST EN 60034-27-3:2016
– 6 – IEC 60034-27-3:2015 © IEC 2015
INTRODUCTION

This International Standard provides guidelines for dielectric dissipation factor measurements

on form-wound stator bars or coils as well as for complete windings.

The dielectric dissipation factor is a measure of the dielectric losses in the stator winding in-

sulation. Measurement of dielectric dissipation factor is an appropriate means of assessing

the quality of new and also aged stator winding insulation of rotating electrical machines. Es-

pecially, the method is useful for assessing the uniform quality of manufacturing and the die-

lectric behaviour of the insulation as a whole. For aged stator windings, the dielectric dissipa-

tion factor provides information about insulation condition.

The dielectric dissipation factor measurements give no indication of the distribution of loss

within the insulation and – in contrast to off-line partial discharge measurements – do not

permit localization of weak points of the insulation system.

The main principle is to measure the dielectric dissipation factor over a range of voltages and

to derive different characteristic dielectric loss parameters as basis for the evaluation.

Empirical limits verified in practice can be used as a basis for evaluating the quality of stator

winding insulation systems in manufacturing. Furthermore, trend evaluation, e.g. diagnostic

tests as part of the functional evaluation of insulation systems or in connection with servicing

and overhaul of rotating machines, can also provide information on ageing processes, neces-

sary further measures and intervals between overhauls. However, such trend evaluations

cannot be used to predict the time to failure of a stator winding insulation.
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SIST EN 60034-27-3:2016
IEC 60034-27-3:2015 © IEC 2015 – 7 –
ROTATING ELECTRICAL MACHINES –
Part 27-3: Dielectric dissipation factor measurement on stator
winding insulation of rotating electrical machines
1 Scope

This part of IEC 60034 provides guidelines for the test procedures and the interpretation of

test results for dielectric dissipation factor measurements on the stator winding insulation of

rotating electrical machines. These guidelines are valid for rotating electrical machines with

conductive slot coatings operating at a rated voltage of 6 kV and higher.

This standard applies to individual form-wound stator bars and coils outside a core (unin-

stalled), individual stator bars and coils installed in a core and complete form-wound stator

winding of machines in new or aged condition.

This International Standard applies to all kind of vacuum impregnated or resin-rich (fully-

loaded) taped bars, coils and complete windings. It is not applicable to non-impregnated indi-

vidual bars and coils or non-impregnated complete windings.

Requirements for the dielectric dissipation factor characteristics of individual form-wound sta-

tor bars and coils of machines with rating voltages from 6 kV and higher when tested with

50 Hz or 60 Hz alternating voltages are given.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and

are indispensable for its application. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amend-

ments) applies.

IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements

IEC 60060-2, High-voltage test techniques – Part 2: Measuring systems
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
rated voltage

voltage or voltage range between lines at the terminals (also called line-to-line voltage) as-

signed, generally by a manufacturer, for a specified operating condition of a machine

3.2
dielectric dissipation factor
tan δ

tangent of the dielectric loss angle δ (complement of the insulation power factor angle) at pre-

determined values of temperature, frequency, and voltage or dielectric stress

Note 1 to entry: Other terms sometimes used for this property are tan delta, loss tangent, dielectric loss factor or

dielectric power factor. Between the dielectric dissipation factor and the power factor (the cosine of power factor

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SIST EN 60034-27-3:2016
– 8 – IEC 60034-27-3:2015 © IEC 2015

angle or the sine of the dielectric loss angle) a physical difference exists, but the two measurements are very near-

ly the same, when the dielectric dissipation factor is lower than 100 × 10 (see 4.1).

Note 2 to entry: Although the dielectric dissipation factor tan δ is expressed in absolute value in this standard, it is

also expressed in percentage in other documents.
3.3
delta tan delta
∆ tan δ

the difference in dielectric dissipation factor measured at two successive test voltages in

steps of 0,2 U intervals
3.4
tan delta tip-up
and 0,2 U

the difference in dielectric dissipation factor measured at the two voltages 0,6 U

N N

Note 1 to entry: Dielectric dissipation factor differences with other voltage steps than mentioned in 3.3 and 3.4

may be used but the limits suggested in Table 1 will not be valid in that case.
4 Theory and measuring techniques
4.1 Dielectric dissipation factor measurement

As defined in 3.2, the dielectric dissipation factor tanδ is the tangent of the dielectric loss an-

gle δ (complement of the insulation power factor angle φ) at a predetermined voltage U, fre-

quency and temperature. The dielectric loss of the insulation system can be represented by

either a parallel (C , R ) or a series (R , C ) equivalent circuit diagram of elements respec-

p p s s
tively (see Figure 1 and Figure 2).
tanδ =
ωC R
p p
I I I = U/R
C R R p
I = ωC U
C p
R U
IEC
Key
C parallel capacitance
R parallel resistor
ω 2πf angular frequency
I current in capacitive path
I current in resistive path
U voltage at insulation system
I total current through insulation system
Figure 1 – Parallel circuit and vector diagram
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SIST EN 60034-27-3:2016
IEC 60034-27-3:2015 © IEC 2015 – 9 –
U I
tanδ = ωC R
s s
U = IR
R s
U = I/ω C
C s
U R
R s
C U
U C
C s
IEC
Key
C series capacitance
R series resistor
U voltage at insulation system
I total current through insulation system
U voltage at capacitance
U voltage at resistor
Figure 2 – Series circuit and vector diagram

Comparison of the dielectric dissipation factor tan δ and the sometimes otherwise used insula-

tion power factor cos φ show that these values are very nearly the same, if the dielectric dis-

sipation factor tan δ is less than 100 × 10 , which may be presumed for all modern stator

winding insulation systems.

NOTE The preferred and exclusive used loss characteristic in this standard is the dielectric dissipation factor

tan δ. But in order to make possible a comparison between insulation power factor cos φ and dielectric dissipation

factor tanδ values, a table is given in Annex A.

As shown in Figure 1, the vector of insulation current I can be divided in two perpendicular

components, which represent a capacitive current I (90° leading to voltage U) and a resistive

current I (in phase with voltage U). The phase shift angle δ is caused by a resistive compo-

nent in addition to the capacitive component of the insulation. The dielectric dissipation factor

tanδ can be expressed in the following equation:
tanδ=ωC R =
s s
ωC R
p p

The capacitive component C or C represents the lossless capacitance of the tested insula-

S P

tion while the resistive component R or R summarizes the different kind of losses. The loss

S P

characteristics under consideration are those mainly relating to the main ground-wall insula-

tion between the conductor structure (including inner conductor shield, if such exists), the

conductive slot coating and the earthed enclosure. In the case of measurements on single

stator bars or coils, only that part of the insulation which is dielectrically in series with the

ground-wall insulation enters into the measurement result because guard ring electrodes can

be used. In the case of dissipation factor measurements on complete windings, the action of

the stress control coating and ambient surface condition have to be considered. These influ-

encing factors may be important when comparing test results from different measurements.

Dielectric dissipation factor measurement at voltages below the inception of partial discharges

represents the magnitude of dielectric losses in the solid insulation (dielectric absorption and

conductive losses) and the conditions of electrical contact to the earthed measuring electrode.

The dielectric dissipation factor component arising from the dielectric losses generally chang-

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SIST EN 60034-27-3:2016
– 10 – IEC 60034-27-3:2015 © IEC 2015

es very little with voltage, but a significantly higher than normal loss measured indicates some

difference in the structure of the insulation, such as may arise from incorrect resin composi-

tion or inadequate cure.

When the test voltage is raised two different types of dielectric losses increase (see Figure 3):

• dielectric losses of the solid insulation material (polarization, conductivity);

• partial discharges within gaseous inclusions (voids) in the insulation structure cause an

increase in dielectric dissipation factor and increasingly larger number of voids begin to

undergo discharge with rising applied voltage. The value of dielectric loss and therefore

the value of tan δ will continue to increase.
Total dielectric loss
Ionization loss
Solid losses
(dielectric polarization
and conductivity)
Test voltage
IEC
Figure 3 – Dielectric losses with increasing voltage (schematic)
4.2 Analogue Schering bridge

Measurement is carried out by means of the analogue Schering bridge or an equivalent type

of bridge like a transformer ratio arm bridge (see 4.3) or by means of modern digital meas-

urement facilities (see 4.4). A variable amplitude alternating voltage supply is used, having

sufficient rating to provide the measured voltage across the capacitance of the test object and

complying with the requirements of IEC 60060-2. Figure 4 shows the basic circuit diagram for

a high voltage Schering bridge, when measuring a stator winding bar or coil with an assumed

lossless capacitance C and resistive losses of R , using a test circuit with guard ring elec-

x x

trodes. The high voltage branch of the bridge includes the high voltage standard capacitor

(C ) with very low dielectric losses. The Schering bridge instrument itself consists of the low

voltage branches with variable sets of resistive (R and R ) and capacitive (C ) decades of

1 2 1

high precision. The balanced condition of the bridge, which is a necessary requirement for

correct measurement, is monitored by a sensitive “Null indicator” (see Figure 4).

Dielectric losses
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SIST EN 60034-27-3:2016
IEC 60034-27-3:2015 © IEC 2015 – 11 –
Test object with
guard ring electrodes
High voltage x
standard x
capacitor
Null indicator
High voltage
AC supply
C R
1 1
A B
IEC
Key
Positions of earthing switch: A for testing coils or bars not earthed
B for testing windings in earthed condition
C Capacitance of standard capacitor C Capacitance of test object
0 x

C Variable capacitance of balancing branch 1 R Variable resistance of balancing branch 2

1 2
R Variable resistance of balancing branch 1 R Resistance of test object
1 x
Figure 4 – High voltage Schering bridge – Basic circuit

The analogue Schering bridge is very sensitive to disturbances produced by stray capacitance

to earth potential. Therefore it is recommended to use double screened coaxial measuring

cables with an active screen potential compensator, i.e. Wagner earth circuit.

Today, most analogue bridges like high voltage Schering bridge use an automated balancing

procedure for that part of the bridge equipment which includes the low voltage branches with

the variable bridge elements
...

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