IEC 60885-3:2015

IEC 60885-3 ®
Edition 2.0 2015-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electrical test methods for electric cables –
Part 3: Test methods for partial discharge measurements on lengths of extruded
power cables
Méthodes d'essais électriques pour les câbles électriques –
Partie 3: Méthodes d'essais pour la mesure des décharges partielles sur des
longueurs de câbles de puissance extrudés

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IEC 60885-3 ®
Edition 2.0 2015-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Electrical test methods for electric cables –

Part 3: Test methods for partial discharge measurements on lengths of extruded

power cables
Méthodes d'essais électriques pour les câbles électriques –

Partie 3: Méthodes d'essais pour la mesure des décharges partielles sur des

longueurs de câbles de puissance extrudés

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.060.20 ISBN 978-2-8322-2582-0

– 2 – IEC 60885-3:2015 © IEC 2015
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references. 6
3 Terms, definitions and symbols . 6
3.1 Terms and definitions . 6
3.2 Symbols used in Figures 1 to 14 . 6
4 Overview . 7
4.1 General . 7
4.2 Object . 7
4.3 Problem of superposition of travelling waves for long lengths . 7
5 Partial discharge tests . 10
5.1 Test apparatus . 10
5.1.1 Equipment . 10
5.1.2 Test circuit and instruments. 10
5.1.3 Double pulse generator . 10
5.1.4 Terminal impedance . 10
5.1.5 Reflection suppressor . 10
5.2 Setting up the test circuit . 10
5.2.1 Determination of characteristic properties of the test circuit . 10
5.2.2 Terminal impedance . 10
5.2.3 Determination of superposition of travelling waves . 11
5.2.4 Reflection suppressor . 11
5.2.5 Calibration of the measuring system in the complete test circuit . 11
5.2.6 Sensitivity . 11
5.3 Measurement procedures . 11
5.3.1 General . 11
5.3.2 Short cable lengths including type test lengths . 12
5.3.3 Long cable lengths tested without a terminal impedance . 12
5.3.4 Long cable lengths tested with a terminal impedance . 13
5.3.5 Long cable lengths tested with a reflection suppressor . 14
5.4 Voltage levels/partial discharge limits . 15
5.5 Double pulse behaviour and plotting the double pulse diagram . 15
5.6 Requirements for the terminal impedance . 16
5.6.1 General . 16
5.6.2 RC element . 16
5.6.3 RLC element series resonance circuit . 17
Bibliography . 21

Figure 1 – Discharge site exactly at the cable end remote from the detector (x = l) . 7
Figure 2 – Discharge site at a distance x = x – Travelling waves . 8
Figure 3 – Attenuation of PD pulses along the cable . 8
Figure 4 – Superposition and attenuation of PD pulses . 9
Figure 5 – Input unit Z connected in series with the coupling capacitor, C . 17
A K
Figure 6 – Input unit Z connected in series with the cable, C . 18
A x
Figure 7 – Bridge circuit . 18

Figure 8 – Connection of the terminal impedance Z . 18
w
Figure 9 – Connection of the reflection suppressor, RS . 19
Figure 10 – Connection of the double pulse generator into the measuring circuit in
Figure 5 . 19
Figure 11 – Double pulse diagram type 1 without negative superposition . 19
Figure 12 – Double pulse diagram type 2 with negative superposition between t and t . 20
1 2
Figure 13 – Double pulse diagram type 3 with negative and positive superpositions
between t and t . 20
1 2
Figure 14 – Connection of the double pulse generator for the test circuit in Figure 9
with the reflection suppressor . 20

– 4 – IEC 60885-3:2015 © IEC 2015
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL TEST METHODS FOR ELECTRIC CABLES –

Part 3: Test methods for partial discharge measurements
on lengths of extruded power cables

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
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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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
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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between 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
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 60885-3 has been prepared by IEC technical committee 20:
Electric cables.
This second edition of IEC 60885-3 cancels and replaces the first edition, published in 1988
and constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
• The definition of sensitivity as twice the background noise level has been removed and
replaced by a practical assessment of sensitivity based on the minimum level of
detectable discharge.
• References to measurements of pulse heights in mm on an oscilloscope have been
replaced by measurements of partial discharge magnitude in pC.

• The order of the clauses has been revised in line with the general numbering scheme of
IEC standards and to provide clarity in order to facilitate its practical use. Section 3 of the
first edition (Application guide) has been removed as it is considered that background
information is better obtained from the original references as listed in the bibliography.
The text of this standard is based on the following documents:
FDIS Report on voting
20/1560/FDIS 20/1587/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 60885 series, published under the general title Electrical test
methods for electric cables, 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 website 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.
– 6 – IEC 60885-3:2015 © IEC 2015
ELECTRICAL TEST METHODS FOR ELECTRIC CABLES –

Part 3: Test methods for partial discharge measurements
on lengths of extruded power cables

1 Scope
This part of IEC 60885 specifies the test methods for partial discharge (PD) measurements on
lengths of extruded power cable, but does not include measurements made on installed cable
systems.
Reference is made to IEC 60270 which gives the techniques and considerations applicable to
partial discharge measurements in general.
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
amendments) applies.
IEC 60270:2000, High-voltage test techniques – Partial discharge measurements
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60270 apply.
3.2 Symbols used in Figures 1 to 14
a discharge magnitude measured with the calibrator at the end near to the detector
a discharge magnitude measured with the calibrator at the end remote from the detector
Ccal calibrator
CK coupling capacitor
Cx power cable
D detector
I double pulse generator
l length of the power cable
M coaxial signal cable
Q discharge magnitude
R1R2 matching resistors
RS reflection suppressor
v propagation velocity of partial discharge
V voltage indicator
W power supply
Z impedance/filter
Z input unit
A
Z terminal impedance
W
4 Overview
4.1 General
Partial discharge measurements shall be carried out using the test techniques specified in
IEC 60270.
4.2 Object
The object of the test is to determine the discharge magnitude, or to check that the discharge
magnitude does not exceed a specified value, at a specified voltage and a declared minimum
sensitivity.
4.3 Problem of superposition of travelling waves for long lengths
Short lengths of cable behave in the same way as a single capacitor in that the discharge
magnitude can be measured directly by considering the cable as a single capacitor. However
longer cables behave like a transmission line and PD pulses travel away from their source in
both directions along the cable, in the form of a wave. On reaching the remote end from the
measuring equipment, the pulse will be reflected with the same polarity if the end is open
circuit. The reflected pulse will then travel back along the length of cable and arrive at the
detector at a time after the directly received pulse. If the time between the arrival of the two
pulses is short (the time difference depending on the length of the cable) then the detection
instrument may give a false response, indicating either a larger or smaller magnitude of
discharge than was actually the case. The methods detailed in this standard allow correct
measurement of partial discharges under these conditions.
Figures 1 to 4 illustrate the behaviour of travelling waves and possible superposition effects.
Q
V
C
K
Void
x
x = l
D Z
A
IEC
Figure 1 – Discharge site exactly at the cable end remote from the detector (x = l)

– 8 – IEC 60885-3:2015 © IEC 2015
a) at the time of the discharge V V
Q/2 Q/2
C
K
Void
l – x
x
x = x
x = l
Z
D
A
t = 2 (l – x )/V
V V
b) after reflection
Q/2 Q/2
C
K
Void
x = x
D
Z
A
IEC
Figure 2 – Discharge site at a distance x = x – Travelling waves
2,0
1,5
a (x = 0)
a (x = l)
1,0
0,8
0,6
0,5a
0,4
= end open
= end with characteristic impedance
0,2
0,1
x  (m)
2 000
0 500 1 000 1 500
x = 0 x = l
IEC
Figure 3 – Attenuation of PD pulses along the cable
Relative response
a)
2,0
Positive superposition
1,5
(i) < 30%
a
1,0a
1,0
0,8
0,7a
0,6
0,5a
0,4
= end open
= end with characteristic impedance
0,2
0,1
x  (m)
0 500 1 000 1 500 2 000
l = 440 m
k
b)
l = 640 m
2,0
1,5
Positive superposition
1,0
(i) < 30%
1,0a
0,8
0,6
0,7a
0,5a
0,4 2
Negative superposition
= end open
0,2
= end with characteristic impedance
0,1
x  (m)
2 000
0 500 1 000 1 500
l = 190 m
k
l = 220 m
l = 600 m
IEC
Figure 4 – Superposition and attenuation of PD pulses
Relative response
Relative response
– 10 – IEC 60885-3:2015 © IEC 2015
5 Partial discharge tests
5.1 Test apparatus
5.1.1 Equipment
The equipment consists of a high-voltage alternating voltage supply having a rating adequate
to energise the length of cable under test, a voltmeter for high voltages, a measuring circuit, a
discharge calibrator, a double pulse generator and, where applicable, a terminal impedance or
reflection suppressor. All components of the test equipment shall have a sufficiently low noise
level to achieve the required sensitivity. The frequency of the test supply shall be in the range
45 Hz to 65 Hz with a waveshape approximating to a sinusoid with the ratio of peak to r.m.s.
values being equal to √2 with a maximum tolerance of 5 %.
5.1.2 Test circuit and instruments
The test circuit includes the high voltage power supply, test object, the coupling capacitor and
the HV and PD measuring equipment. The measuring circuit consists of the measuring
impedance (input impedance of the measuring instrument and the input unit which is selected
to match the cable impedance), the connecting lead and the measuring instrument. The
measuring instrument or detector includes a suitable amplifying device, an oscilloscope, or
other instrument to indicate the existence of partial discharges and to measure the apparent
charge. The measuring system shall comply with IEC 60270.
5.1.3 Double pulse generator
A double pulse generator is an instrument producing two equal pulses (with the same
apparent charge) following each other within a time interval which can be varied between
0,2 µs to 100 µs. The rise time of the pulses shall not exceed 20 ns (10 % to 90 % of peak
value); the time between 10 % values of the front and the tail shall not exceed 150 ns. The
pulses may be synchronized with the power frequency.
5.1.4 Terminal impedance
A terminal impedance is an impedance, equal in value to the characteristic impedance of the
test object, which is connected to the open end of the cable remote from the detector. It may
be a combination of resistance and capacitance (R & C) or resistance, capacitance and
inductance (R, C & L). The components shall be suitable for operation at the test voltage to
be applied to the cable under test. Additional requirements are specified in section 5.6.
5.1.5 Reflection suppressor
This is an electronic switch which is designed to block the input of the measuring instrument
from pulses reflected from the open end of the cable. This is achieved by blocking the input
for a fixed time after the first pulse is received.
5.2 Setting up the test circuit
5.2.1 Determination of characteristic properties of the test circuit
The characteristic properties of the test circuit should be determined under the conditions to
be used. The test circuits normally used for connections to a single cable end are those
shown in Figures 5, 6, 7, 8 and 9. Similar test circuits are also applicable when both ends of
the cable conductor are connected together; in this case the two ends of the metal cable
screen shall also be connected together.
5.2.2 Terminal impedance
If a terminal impedance is connected to the remote end of the cable under test, with an
impedance value equal to the characteristic impedance of the cable then the cable will behave

as if it is of infinite length and there will be no reflected wave. The circuit for connection of a
terminal impedance is shown in Figure 8. The values (RC and L where applicable) of the
components of the terminal impedance and its suitability for the type of cable under test
should be demonstrated using the procedure described in 5.6. This check should be carried
out when the test circuit is set up and also when any changes are made to the circuit.
5.2.3 Determination of superposition of travelling waves
If a terminal impedance is not used, it is necessary to determine the properties of the test
circuit with respect to superposition of travelling waves. A double pulse generator is
connected according to Figure 10 and a double pulse diagram is plotted (see 5.5 and
Figures 11, 12 and 13). This check should be carried out when the test circuit is set up and
also when any changes are made to the circuit.
5.2.4 Reflection suppressor
The purpose of using a reflection suppressor is to obtain a double pulse diagram of Type 1
corresponding to Figure 11. Using the arrangement shown in Figure 14, the efficiency of the
reflection suppressor should be checked by plotting a double pulse diagram (see 5.5 and
Figures 11, 12 and 13), when the test circuit is set up and also when any changes are made
to the circuit.
5.2.5 Calibration of the measuring system in the complete test circuit
Calibration of the measuring system in the complete test circuit shall be carried out in
accordance with Clause 5 of IEC 60270:2000. The calibrator used shall comply with
IEC 60270. For long lengths of cable (> 100 m) there is an additional requirement that the
calibrating capacitance shall be not greater than 150 pF.
5.2.6 Sensitivity
The sensitivity of the measuring system is defined as the minimum detectable discharge
pulse, q (in picocoulombs – pC) that can be observed in the presence of background noise.
min
Value of q shall be determined by evaluation of the background noise level and shall be no
min
more than twice the apparent noise level, h (h is the noise reading on the measuring
n n
instrument).
Therefore:
q = x × k × h
min n
where k is the scale factor and x is the ratio of the minimum detectable discharge to the
background noise. The maximum allowed value of x is 2. Typically values of x of between 1,25
and 1,5 should be achievable.
The maximum values of sensitivity shall be determined according to 5.4.
5.3 Measurement procedures
5.3.1 General
The selection of the test circuit depends on whether the cable sample may be considered as a
short length (see 5.3.2) or a long length (see 5.3.3, 5.3.4 and 5.3.5). The test circuit shall be
discharge free in order to achieve the required sensitivity (see 5.2.6). Calibration does not
necessarily have to be done with the HV supply on (see 5.2.5). During the partial discharge
measurement, individual pulses clearly identifiable as interference may be disregarded.

– 12 – IEC 60885-3:2015 © IEC 2015
5.3.2 Short cable lengths including type test lengths
5.3.2.1 Requirements
For short lengths the cable may be considered similar to a lumped capacitance. The limitation
on length where this is not acceptable depends upon the test circuit used, however it may be
assumed that cable lengths of up to 50 m (or 100 m, if both ends of the cable are connected
together) behave as a lumped capacitance and therefore superposition of reflected waves
need not be taken into account. For longer lengths whether they can be treated as a lumped
capacitance shall be determined using the double pulse diagram as described in 5.5. The
maximum length which can be considered as a lumped capacitance is defined as l This may
k;
be as low as 100 m or even greater than 1 000 m, depending on the particular measuring
system in use.
The test circuits normally used are those in Figures 5, 6 and 7.
5.3.2.2 Verification of sensitivity
The determination of the scale factor k for the measurement of the apparent charge shall be
carried out in accordance with Clause 5 of IEC 60270:2000. Therefore the partial discharge
calibrator shall be connected in parallel with the cable at the end remote from the detector.
5.3.2.3 Test procedure
The measurement shall be made only at one end of the cable.
The test parameters shall be selected according to 5.4.
5.3.3 Long cable lengths tested without a terminal impedance
5.3.3.1 General
For long cable lengths (>50 m or >100 m with ends connected), tested without a terminal
impedance, it is necessary to plot a double pulse diagram.
5.3.3.2 Requirements
For cable lengths in excess of l it may still be possible to test without a terminal impedance
k
provided superposition and attenuation phenomena are taken into account.
A double pulse generator is connected according to Figure 10 and a double pulse diagram is
plotted (see 5.5 and Figures 11, 12 and 13). This shall be carried out when the test circuit is
set up and also when any changes are made to the circuit.
A test without terminal impedance is permitted where the double pulse diagram is either
– type 1 (Figure 11), or
– type 2 and type 3 (Figures 12 and 13) but where the cable length, l, lies outside the limits
2l ≤ l ≤ 2l .
1 2
(See 5.5 for the determination of l and l .)
1 2
For lengths inside these limits an alternative test circuit should be used or the procedures
described in 5.3.4 or 5.3.5 should be adopted.
The test circuits normally used are those shown in Figures 5, 6, 7 and 9.

5.3.3.3 Verification of sensitivity
The determination of the scale factor k for the measurement of the apparent charge shall be
carried out in accordance with Clause 5 of IEC 60270:2000. Therefore the partial discharge
calibrator shall be connected in parallel with the cable at the end near to the detector.
For the determination of the attenuation correction factor, the partial discharge calibrator shall
be connected to each end in turn in parallel with the cable with the same setting of the
amplifier and calibration charge. The following values shall be recorded:
– a discharge magnitude measured with the calibrator at the end near to the detector;
– a discharge magnitude measured with the calibrator at the end remote from the detector.
and a are used to determine a correction factor F to allow for attenuation. It is given by:
a
1 2
F = 1 if a ≥ a
2 1
a
F=
a
2 if a < a
2 1
5.3.3.4 Test procedure
The measurement shall be made twice by connecting the high voltage end of the coupling
capacitor to each end of the cable in turn. The measured discharge magnitudes A and A
1 2
shall be determined and the higher value A (pC) selected. With the correction factor F, the
max
discharge magnitude q(pC) is:
q = A × F
max
The voltage levels used when measuring the highest discharge magnitude A shall be
max
selected according to 5.4.
NOTE Only if the double pulse diagram is of type 1 (see Figure 11) and a ≥ a , a measurement of A(pC) is
2 1
sufficient when both cable ends are connected together (see 5.3.2). The discharge magnitude is then: q = A.
5.3.4 Long cable lengths tested with a terminal impedance
5.3.4.1 General
For long cable lengths (>50 m or >100 m with ends connected), tested with a terminal
impedance, it is not necessary to plot a double pulse diagram.
5.3.4.2 Requirements
To eliminate superposition errors, cables of length greater than l may be tested with a
k
terminal impedance as shown in Figure 8. This method may be used with all detectors and all
cable lengths provided that the impedance Z meets the requirements specified in 5.6. The
w
suitability of the impedance for the cable under test shall be demonstrated using the
procedure described in 5.6.
5.3.4.3 Verification of sensitivity
The partial discharge calibrator shall be connected to each end in turn in parallel with the
cable with the same setting of the amplifier and calibration charge. The following values shall
be recorded:
– a (pC) the discharge magnitude measured with the calibrator at the end near to the
detector. This need not be measured if the procedure in 5.3.4.4 b) is sufficient;

– 14 – IEC 60885-3:2015 © IEC 2015
– a (pC) the discharge magnitude measured with the calibrator at the end remote from the
detector.
For the determination of the scale factor k for the measurement of the apparent charge in
accordance with Clause 5 of IEC 60270:2000, the value a (pC) with the partial discharge
calibrator connected in parallel with the cable at the end remote from the detector shall be
used.
5.3.4.4 Test procedure
The test procedure is as follows.
a) When it is required to determine the value of the partial discharge magnitude as closely as
possible, the high voltage end of the coupling capacitor shall be connected to each end of
the cable in turn and both measured discharge magnitudes A (pC) and A (pC)
1 2
determined. The discharge magnitude q (pC) is given by:
A × A
1 2
q= q ×
cal
a × a
1 2
where q is the calibration discharge magnitude (pC).
cal
b) When it is sufficient to check that the discharge magnitude does not exceed a specified
value, the measurement may be made with the high voltage end of the coupling capacitor
connected to one end of the cable only. In this case the calibration pulse is injected only
at the end of the cable connected to the terminal impedance remote from the detector (a ).
With the measured discharge magnitude A1 (pC) and the scale factor k the discharge
magnitude q (pC) is given by:
q = k × A
2 1
The voltage levels used when measuring the discharge magnitudes A and if necessary A
1 2
shall be selected according to 5.4.
5.3.5 Long cable lengths tested with a reflection suppressor
5.3.5.1 General
For long cable lengths (>50 m or >100 m with ends connected), tested with a reflection
suppressor, it is necessary to plot a double pulse diagram.
The connection of the reflection suppressor is shown in Figure 9.
A double pulse generator is connected according to Figure 10 and a double pulse diagram is
plotted (see 5.5 and Figures 11, 12 and 13). This shall be carried out when the test circuit is
set up and also when any changes are made to the circuit.
5.3.5.2 Requirements
When using a reflection suppressor the double pulse diagram shall be type 1 (see Figure 11).
5.3.5.3 Verification of sensitivity
See 5.3.2.2.
5.3.5.4 Test procedure
See 5.3.2.3.
5.4 Voltage levels/partial discharge limits
The test voltages, partial discharge sensitivity and partial discharge limits shall be determined
in accordance with the requirements in the standard for the type of cable.
5.5 Double pulse behaviour and plotting the double pulse diagram
The double pulse plot is affected by variations in each circuit component. It is important that
the double pulse plot be obtained for the precise conditions to be used in the high voltage
test.
NOTE The test cable is not connected whilst the double pulse plot is being plotted, the double pulse plot depends
solely on the measuring system and test circuit, excluding the cable.
The power cable is replaced by a resistive load having the maximum characteristic impedance
for extruded cables (generally R = 40 Ω). The double pulses are injected in the same
max
position as the calibration pulses for the various test circuits shown in Figures 5, 6 and 7.
Figure 10 shows, as an example, the double pulse generator connected to the test circuit of
Figure 5.
The following conditions should apply:
a) The double pulse generator should satisfy the requirements of 5.1.3. In some cases the
dials of the double pulse generator may have numeric (e.g. 0 to 9) markings for pulse
separation, in which case it will be necessary to use a suitable oscilloscope to calibrate
these scales in terms of µs; the required accuracy is ±3 % or 50 ns whichever is the
greater. The overall output impedance should approximately match the characteristic
impedance of the cable, which is typically in the range of 20 Ω to 40 Ω. To achieve this it
may be necessary to add external resistors in parallel to or in series with the output.
Experience has shown that the double pulse plot may be reliably obtained in the following
ways:
– The simplest method is to connect the double pulse generator across the high voltage
capacitor C and the measuring impedance Z with wires not longer than 3 m.
K A
– For longer connections a coaxial cable should be used (see Figure 10). In this case
two adapter resistors R and R are necessary to ensure that the system approximately
1 2
matches the characteristic impedance of the cable, which is typically in the range of
20 Ω to 40 Ω.
b) The capacitor C and the other high voltage components of the test circuit should be the
K
same and have the same connections as those used in the high voltage test.
c) The matching unit or detector impedance Z to be used in the high voltage test should be
A
used to obtain the double pulse plot.
d) The detector amplifier D should be used with the gain setting and amplifier frequency
response selected for the high voltage test. For accurate measurement of the changes in
pulse magnitude caused by superposition distortions, the output of the detector amplifier D
should be displayed on an external oscilloscope (for example the oscilloscope used in 5.5
a)).
The time interval of the double pulse generator should be set to 100 µs and the discharge
magnitude of the partial discharge detector to the two pulses A should be measured.
The time interval should then be reduced from 100 µs to 0,2 µs; for different values of an
interval t measured between maximum peaks of the two pulses, the maximum discharge
magnitude A should be measured. Particular attention should be given to areas of positive
t
and negative superposition. Values of A /A should then be plotted as a function of t to
t 100
obtain the double pulse diagram. Examples of diagrams are in Figures 11 to 13.
The value t where A /A = 1,4 on the initial positive superposition should be determined
k t 100
from the plot. Times t and t where A /A ≤ 1,0 at all areas of negative superposition
1 2 t 100
should be determined. Taking into account the errors of measurement, areas of negative
superposition with a maximum magnitude up to –10 % may be ignored.

– 16 – IEC 60885-3:2015 © IEC 2015
The cable lengths l , l and l corresponding to t , t and t should be calculated using the
k 1 2 k 1 2
formula l = 0,5 × t × v. The mean propagation velocity is v and typical values for most
extruded cable lie between 150 m/µs and 170 m/µs. On request the propagation rate shall
be measured by injecting a calibration pulse into a cable not having a terminal impedance
and measuring the time delay between incident and reflected pulse.
The cable lengths l < l can be considered as short lengths. These may be as low as
k
100 m and even higher than 1 000 m.
Lengths between 2l and 2l have to be tested with a terminal impedance (see 5.3.4.2) or
1 2
under modified conditions of the test circuit (for example D, Z , C ) to alter l and l to
A K 1 2
more suitable values. Alternatively, it is possible to effectively double the value of l by
k
connecting both ends of the cable together.
5.6 Requirements for the terminal impedance
5.6.1 General
The terminal impedance Z , shown in Figure 8 comprises either RC or RLC elements which
w
are selected on the basis of experimental evaluation.
5.6.2 RC element
The following measurement shall be used to prove the suitability of the terminal capacitor C .
w
The RC element shall be connected in parallel with the cable across the end remote from the
detector. The capacitive component shall be short-circuited and the ohmic component shall be
adjusted to correspond to the characteristic impedance of the cable. Subsequently the
calibrator shall also be connected to the end remote from the detector and the measured
discharge magnitude a shall be determined.
With the same amplifier setting, the short circuit of the capacitive component of the terminal
impedance shall be removed.
The removal of the short circuit of the capacitor (C ) shall not change the discharge
w
magnitude a by more than ±15 %.
For PD detectors having a cut-off frequency lower than 2 MHz, a reasonable estimate for the
value of the capacitance C (high voltage coupling capacitor of Z ) may be obtained using the
w w
following formula:
C ≥ 0,5
w
R × f
w m
where
R is the ohmic component of the terminal impedance (corresponding approximately to the
w
characteristic impedance of the cable);
f is the mean measuring frequency of the detector (arithmetic mean of the upper and
m
lower limiting frequencies of the detector).
For PD measuring instruments having a wide-band amplifier with an upper cut-off frequency
more than 2 MHz in connection with an electronic integrator unit, C can be estimated on the
w
basis of the relation:
3 T
J
C ≥
w
R
w
T is the time duration of the original PD pulse (in general smaller than 0,2 µs).
J
5.6.3 RLC element series resonance circuit
The following measurement shall be used for proving the suitability of the resonant circuit at
the respective measuring frequency.
With the terminal impedance removed an ohmic resistor corresponding to the characteristic
impedance of the cable shall be connected to the end remote from the detector in parallel with
the cable. Furthermore the calibrator shall be connecte
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