prEN IEC 60076-4:2023
(Main)Power transformers - Part 4: Lightning impulse and switching impulse tests of power transformers and reactors
Power transformers - Part 4: Lightning impulse and switching impulse tests of power transformers and reactors
Leistungstransformatoren - Teil 4: Leitfaden zur Blitz- und Schaltstoßspannungsprüfung von Leistungstransformatoren und Drosselspulen
Power transformers - Part 4: Lightning impulse and switching impulse tests of power transformers and reactors
Močnostni transformatorji - 4. del: Vodilo za testiranje atmosferske udarne napetosti in preklopnega impulza - Močnostni transformatorji in dušilke
General Information
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Standards Content (Sample)
SLOVENSKI STANDARD
01-september-2023
Močnostni transformatorji - 4. del: Vodilo za testiranje impulza strele in
preklopnega impulza - Močnostni transformatorji in dušilke
Power transformers - Part 4: Guide to the lightning impulse and switching impulse testing
- Power transformers and reactors
Leistungstransformatoren - Teil 4: Leitfaden zur Blitz- und Schaltstoßspannungsprüfung
von Leistungstransformatoren und Drosselspulen
Transformateurs de puissance - Partie 4: Guide pour les essais au choc de foudre et au
choc de manoeuvre - Transformateurs de puissance et bobines d'inductance
Ta slovenski standard je istoveten z: prEN IEC 60076-4:2023
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
14/1109/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 60076-4 ED2
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2023-07-07 2023-09-29
SUPERSEDES DOCUMENTS:
14/1060/CD, 14/1061C/CC
IEC TC 14 : POWER TRANSFORMERS
SECRETARIAT: SECRETARY:
United Kingdom Ms Stephanie Lavy
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:
Other TC/SCs are requested to indicate their interest, if any,
in this CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft for
Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which
they are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some Countries”
clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is the final stage for
submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).
TITLE:
Power transformers - Part 4: Guide to the lightning impulse and switching impulse testing - Power
transformers and reactors
PROPOSED STABILITY DATE: 2026
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.
IEC CD 60076-4 IEC:2020 – 2 –
NOTE FROM TC/SC OFFICERS:
IEC CD 60076-4 IEC:2020 – 3 –
1 POWER TRANSFORMERS
3 Part 4: Lightning impulse and Switching impulse Tests
4 of Power Transformers and Reactors
6 FOREWORD
7 1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising all national
8 electrotechnical committees (IEC National Committees). The object of the IEC is to promote international co-operation on
9 all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities,
10 the IEC publishes International Standards. Their preparation is entrusted to technical committees; any IEC National
11 Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and
12 non-governmental organizations liaising with the IEC also participate in this preparation. The IEC collaborates closely with
13 the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between
14 the two organizations.
15 2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an international
16 consensus of opinion on the relevant subjects since each technical committee has representation from all interested
17 National Committees.
18 3) The documents produced have the form of recommendations for international use and are published in the form of
19 standards, technical specifications, technical reports or guides and they are accepted by the National Committees in that
20 sense.
21 4) In order to promote international unification, IEC National Committees undertake to apply IEC International Standards
22 transparently to the maximum extent possible in their national and regional standards. Any divergence between the IEC
23 Standard and the corresponding national or regional standard shall be clearly indicated in the latter.
24 5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment
25 declared to be in conformity with one of its standards.
26 6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent
27 rights. The IEC shall not be held responsible for identifying any or all such patent rights.
28 International Standard IEC 60076-4 has been prepared by IEC technical committee 14: Power
29 transformers.
This International Standard cancels and replaces the first edition published in 2002 and constitutes a
technical revision of that document.
30 This edition includes the following technical changes with respect to the previous
edition:
• Lightning impulse tests in presence of a relative overshoot value of 5% and higher (Clause 7.1.2).
• Test voltage function when performing chopped wave (Clause 7.2.1.1).
• Switching impulse tests on 3 phase transformers, test connections (Figure 5 and Figure 6)
• Glaninger circuit, clause A.3 (Annex A).
• New Annex C. Examples of oscillograms with peak voltage overshoot.
31 The text of this standard is based on the following documents:
FDIS Report on voting
32 Full information on the voting for the approval of this standard can be found in the report on voting
33 indicated in the above table.
34 This publication has been drafted in accordance with the ISO/IEC Directives, Part 3.
35 Annexes A, B and C are informative only.
IEC CD 60076-4 IEC:2020 – 4 –
37 IEC 60076 consists of the following parts, under the general title Power transformers:
38 Part 1: General
39 Part 2: Temperature rise
40 Part 3: Insulation levels, dielectric tests and external clearances in air
41 Part 4: Lightning impulse and Switching impulse Tests – Power Transformers and Reactors
42 Part 5: Ability to withstand short-circuit
43 Part 8: Application guide
44 Part 10: Determination of sound levels
45 Part xx: to be update by TC14 secretary
46 The committee has decided that the contents of this publication will remain unchanged until 20xx. At this
47 date, the publication will be
48 • reconfirmed;
49 • withdrawn;
50 • replaced by a revised edition, or
51 • amended.
IEC CD 60076-4 IEC:2020 – 5 –
54 POWER TRANSFORMERS
56 Part 4: Lightning impulse and Switching impulse Tests
57 Power Transformers and Reactors
62 1 Scope
63 This part of IEC 60076 gives guidance and explanatory comments on the existing procedures for lightning
64 and switching impulse tests of power transformers to supplement the requirements of IEC 60076-3. It is
65 also generally applicable to the testing of reactors (see IEC 60076-6), modifications to power transformer
66 procedures being indicated where required.
67 Information is given on waveforms, test circuits including test connections, earthing practices, failure
68 detection methods, test procedures, measuring techniques and interpretation of results.
69 Where applicable, the test techniques are as recommended in IEC 60060-1 and IEC 60060-2.
70 2 Normative references
71 The following documents are referred to in the text in such a way that some or all of their content
72 constitutes requirements of this document. For dated references, only the edition cited applies. For
73 undated references, the latest edition of the referenced document (including any amendments) applies.
75 IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
76 IEC 60060-2, High-voltage test techniques – Part 2: Measuring systems
77 IEC 60076-3, Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in
78 air
79 IEC 60076-6, Reactors
80 IEC 61083-1, Instruments and software used for measurement in high-voltage impulse tests – Part 1:
81 Requirements for instruments
82 IEC 61083-2, Digital recorders for measurements in high-voltage impulse tests – Part 2: Evaluation of
83 software used for the determination of the parameters of impulse waveforms
TM
84 IEEE Std C57.98 -2011, IEEE Guide for Transformer Impulse Tests
TM
85 IEEE Std 4 -2013, IEEE Standard for High-Voltage Testing Techniques
IEC CD 60076-4 IEC:2020 – 6 –
88 3 General
89 This standard is primarily based on the use of conventional impulse generators for both lightning and
90 switching impulse tests of transformers and reactors. The practice of switching impulse generation with
91 discharge of a separate capacitor into an intermediate or low-voltage winding is also applicable.
92 However, the method which employs an additional inductance in series with the capacitor to provide
93 slightly damped oscillations transferred into the high-voltage winding is not applicable.
94 Alternative means of switching impulse generation or simulation such as d.c. current interruption on an
95 intermediate or low-voltage winding or the application of a part-period of power frequency voltage are
96 not discussed since these methods are not as generally applicable.
97 Different considerations in the choice of test circuits (terminal connections) for lightning and switching
98 impulse tests apply for transformers and reactors. On transformers, all terminals and windings can be
99 lightning impulse tested to specific and independent levels. In switching impulse test, however, because
100 of the induced voltage transferred, a specified test level may only be obtained on one winding (see IEC
101 60076-3).
102 Whilst, on reactors, lightning impulse tests is similar to that on transformers, i.e., all terminals can be
103 tested separately, different considerations apply and different problems arise in switching impulse tests.
104 Hence, in this standard, lightning impulse tests are covered by a common text for both transformers
105 and reactors whilst switching impulse test is dealt with separately for the two types of equipment.
106 4 Specified waveforms
107 The voltage waveforms to be used normally during lightning and switching impulse tests of
108 transformers and reactors are given in IEC 60076-3, IEC 60076-6 and the methods for their
109 determination are given in IEC 60060-1.
110 5 Test circuit
111 The physical arrangement of test equipment, test object and measuring circuits can be divided into
112 three major circuits:
113 – the main circuit including the impulse generator, additional waveshaping components and the test
114 object;
115 – the voltage measuring circuit;
116 – the chopping circuit where applicable.
117 This basic arrangement is shown in Figure 1.
118 The following parameters influence the impulse waveform;
119 a) the effective capacitance C , and inductance of the test object, L ; C is constant for any given design
t t t
120 and any given waveform, L is also a constant for any given design. The effective L , however, may
t t
121 be influenced by the terminal treatment. It varies between the leakage inductance L for short-
s
122 circuited terminals and L for open-circuited terminals. More details in this respect are given in 7.1
o
123 and 7.3 and in annex A;
124 b) the generator capacitance C ;
g
125 c) waveshaping components, both internal and external to the generator, R , R , R , C (plus, where
si se p L
126 applicable, the impedance of a voltage divider Z );
127 d) the stray inductance and capacitance of the generator and the complete test circuit;
128 e) chopping equipment, where applicable.
IEC CD 60076-4 IEC:2020 – 7 –
129 f) Non-linear elements in the transformer, which can cause differences between impulses at different
130 voltage levels
131 The front time T is determined mainly by combination of the effective surge capacitance of the test
132 object, including C , and the generator internal and external series resistances.
L
133 The time to half-value T is, for lightning impulses, primarily determined by the generator capacitance,
134 the inductance of the test object and the generator discharge resistance or any other parallel resistance.
135 However, there are cases, for example, windings of extremely low inductance, where the series
136 resistance will have a significant effect also on the wavetail. For switching impulses, other parameters
137 apply; these are dealt with in clause 8.
138 The test equipment used in lightning and switching impulse applications is basically the same.
139 Differences are in details only, such as values of resistors and capacitors (and the terminal connections
140 of the test object).
141 To meet the different requirements of the waveform for lightning and switching impulses, due
142 consideration has to be given to the selection of the impulse generator parameters, such as capacitance
143 and series and discharge (parallel) resistances. For switching impulses, large values of series resistors
144 and/or load capacitors may be necessary, which will result in significant reduction of the efficiency.
145 While the output voltage of the impulse generator is determined by the test levels of the windings with
146 respect to their highest voltage for equipment U for the test object, the required energy storage
m
147 capability is essentially dependent on the inherent impedances of the test object.
148 A brief explanation of the principles of waveform control is given in Annex A.
149 The arrangement of the test plant, test object and the interconnecting cables, earthing strips, and other
150 equipment is limited by the space in the test room and, particularly, the proximity effect of any
151 structures. During impulse tests, zero potential cannot be assumed throughout the earthing systems
152 due to the high values and rates of change of impulse currents and voltages and the finite impedances
153 involved. Therefore, the selection of a proper reference earth is important.
154 The current return path between the test object and the impulse generator should be of low impedance.
155 It is good practice to firmly connect this current return path to the general earth system of the test room,
156 preferably close to the test object. This point of connection should be used as reference earth and to
157 attain good earthing of the test object it should be connected to the reference earth by one or several
158 conductors of low impedance (see IEC 60060-2).
159 The voltage measuring circuit, which is a separate loop of the test object carrying only the measuring
160 current and not any major portion of the impulse current flowing through the windings under test, should
161 also be effectively connected to the same reference earth.
162 In switching impulse tests, since the rates of change of the impulse voltages and currents are much
163 reduced compared with those in a lightning impulse test and no chopping circuit is involved, the
164 problems of potential gradients around the test circuit and with respect to the reference earth are less
165 critical. Nevertheless, it is suggested that, as a precaution, the same earthing practices should be
166 followed as used for lightning impulse tests.
167 Electromagnetic compatibility (EMC):
168 Power transformers are more and more fitted with control and protection devices, which are sensitive
169 in regard of overvoltage, caused by (fast) transients.
IEC CD 60076-4 IEC:2020 – 8 –
170 Potential differences, caused by special groundings at lightning and switching impulse tests the
171 different grounding of the control-and protection device (in regard of safety) can damage electronic
172 parts.
173 Examples of affected device:
174 - Mainly large transformers are fitted with (computerized) condition monitoring systems.
175 - Cooling equipment (fans, pumps) is driven in dependence of transformer load and transformer noise.
176 Such an operation is controlled by electronic device etc.
177 During impulse tests it is recommended to disconnect all electric and electronic equipment installed on
178 the transformer.
179 6 Verification of the impulse voltage measuring system before a test
180 It is not the intention of this standard to give any recommendation on measuring systems or their
181 calibration but, of course, the apparatus which is used should be approved in accordance with IEC
182 60060. Before a test, an overall check of the test circuit and the measuring system may be performed
183 at a voltage lower than the reduced voltage level. In this check, voltage may be determined by means
184 of a sphere gap or by comparative measurement with another approved device. When using a sphere
185 gap, it should be recognized that this is only a check and does not replace the periodically performed
186 calibration of the approved measuring system. After any check has been made, it is essential that
187 neither the measuring nor the test circuit is altered except for the removal of any devices for checking.
188 Information on types of voltage dividers, their applications, accuracy, calibration and checking is given
189 in IEC 60060-2.
190 7 Lightning impulse tests
191 7.1 Waveforms
192 7.1.1 General
193 The values of waveform specified may not always be obtainable. In the impulse tests of large power
194 transformers and reactors, of low winding inductance and/or high surge capacitance, wider tolerances
195 may have to be accepted.
196 7.1.2 Front time T1
197 The surge capacitance of the transformer under test being constant, the series resistance may have to
198 be reduced in an attempt to obtain the correct front time T or rate of rise, but the reduction should not
199 be to the extent that oscillations on the crest of the voltage wave become excessive. If it is considered
200 desirable to have a short front time (preferably within the specified limits) then oscillations and/or
201 overshoots may have to be accepted. In such an event, a compromise between the extent of allowable
202 oscillations and the obtainable front time is necessary. In general, the test circuit should be arranged
203 in such way, that the overshoot/oscillation is minimal.
204 If the relative overshoot (ß’) exceeds 5% here are the tests options (ref. IEC 60076-3 Edition 3.0 2013-
205 07; clause 13.2.1) :
206 • For windings receiving chopped wave(s) lightning impulse tests, T1 can be increased up to 2.5
207 µs to reduce the overshoot (ß).
208 • During the tests, if the relative overshoot (ß’) exceed 5% at full voltage level, the test voltage
209 function should be applied in accordance with IEC 60060-1 to determine the test voltage value.
210 • Irrespective of the overshoot value, it is permissible to apply the requirements of IEC 60060-1
211 Annex B to evaluate the parameters of the lightning impulse.
212 In case of a ß’ higher than 5 %, it is allowed to proceed with the test when the test voltage function is
213 enabled.
IEC CD 60076-4 IEC:2020 – 9 –
214 NOTE 1: When the manufacturer does not have the software with the test voltage function according to IEC 60060-1, the
215 second option of IEC 60076-3 with a larger front time, less overshoot and additional chopped wave test should be chosen.
216 Examples of oscillograms having overshoot are shown in Annex C.
217 NOTE 2: In case of high beta value and an overshoot with frequency above 500 kHz, the test voltage function significantly
218 reduces the test voltage value U compared to the peak value of the recorded curve U . This may lead to higher electrical
t e
219 stress and possible breakdown of the insulation system.
220 In general, manual evaluation of the test voltage is unreliable for lightning impulse tests of
221 transformers. If the manufacturer does not have the software with the test voltage function
222 according to IEC 60060-1, the buyer should be informed of this at the offer stage.
223 7.1.3 Non-linear elements
224 In some cases, the active parts of power transformers are protected with non-linear elements (surge
225 arresters). These components can cause different waveforms, depending on the voltage level of the
226 impulse. The required test sequences are defined in IEC 60076-3. Generally, it must be clarified: The
227 parameters of the waveform of an impulse have to be adjusted at the reference voltage level. Further
228 adjustments on 100 % voltage level are not allowed, if there are changes in the shape of the curve,
229 this has to be accepted.
230 NOTE: As specified in 60076-3, 3 references impulses are recommended prior the 100% full waves impulses. These
231 impulses are:
232 1. Impulse reference between 50% and 60% of the full test wave.
233 2. Impulse reference between 60% and 75% of the full test wave.
234 3. Impulse reference between 75% and 90% of the full test wave.
236 7.1.4 Time to half-value T2
237 For large power transformers and particularly the intermediate and low-voltage windings thereof, the
238 may not be achievable within the value set by the tolerance. The inductance
virtual time to half-value T
239 of such windings may be so low that the resulting waveform is oscillatory. This problem may be solved
240 to some extent by the use of large capacitance within the generator, by parallel stage operation, by
241 adjustment of the series resistor or by specific test connections of the terminals of windings not under
242 test or, in addition, of the non-tested terminals of windings under test.
243 If lightning impulse tests are carried out on phase-terminals of a delta-winding, the not-tested terminals
244 of that winding can be resistance earthed.
245 If the neutral of a star-connected winding is tested, the phase-terminals can be resistance earthed.
246 If the phase terminals of a star-connected winding are tested, the neutral-terminal has to be solidly
247 grounded or grounded through a low-ohmic shunt.
248 When resistance earthing of any non-tested line terminal is employed, it is necessary to ensure that the
249 voltage to earth appearing on any non-tested terminal does not exceed
250 – 75 % of the rated lightning withstand voltage of that terminal for star-connected windings;
251 – 50 % of the rated lightning withstand voltage of that terminal for delta-connected windings (because
252 of the undershoot voltages to earth on the delta terminals – see also 7.4).
253 When the waveform is oscillatory due to extremely low inductance and/or small impulse generator
254 capacitance, the amplitude of the undershoot should not exceed 50 % of the test voltage. With this
255 limitation, guidance for selecting impulse generator capacitance and adjusting waveforms is given in
256 annex A.
IEC CD 60076-4 IEC:2020 – 10 –
257 7.2 Impulses chopped on the tail
258 7.2.1 Time to chopping
259 Different times to chopping T (as defined in IEC 60060-2), will result in different stresses (voltage and
c
260 duration) in different parts of the winding(s) depending on the winding construction and arrangement
261 employed. Hence, it is not possible to state a time to chopping which is the most onerous either in
262 general or for any particular transformer or reactor. The time to chopping should be between 3 µs and
263 6 µs. A time, between 2 µs and 6 µs can be accepted, provided that the peak value of the lightning
264 impulse wave is achieved before the chop, as required by IEC 60076-3, clause 13.3.1.
265 Oscillograms or digital recordings of chopped waves, are only comparable up to the times to chopping.
266 7.2.1.1 Test voltage function when performing chopped wave
267 In general, for liquid immersed transformers the chopped wave is 110% of the full wave, while in dry
268 type transformers the chopped wave is 100%.The k-factor (test voltage function) evaluation of a
269 chopped wave of 110% of the full wave (as recommended in 60060-1, Annex B), should give a peak
270 value U of 110% of the full wave.
t
271 When the calculated test value Ut is inconsistent, the following steps are recommended:
272 a) Front chopped lightning impulse:
273 Not applicable to chopped waves on transformers.
274 b) Tail chopped lightning impulse with chopping time occurring after the peak value:
TM
275 Voltage reduction ratio Method (Ref. IEEE Std 4 -2013)
276 • Apply a reduced full wave (RFW).
277 • The test voltage function should provide the test voltage value U and the peak value U of
t e
278 the original recorded curve. If Ue is not available, it could be graphically determined on the
279 recorded oscillogram.
280 • Find the Voltage Reduction Ratio. Rv = U / U
t e
’
281 • Apply a full voltage chopped wave, having a recorded voltage U
e
’
282 • The calculated U t is defined as:
’ ’
283 U = Rv * U
t e
284 c) The value of front time T1 of the reduced full wave (RFW) is used to determine the T1 value of
285 the chopped wave.
286 7.2.1.2 Test voltage function and presentation of test results
287 When the test voltage function is enabled, the following test results should be displayed:
288 • U is the test voltage
t
289 • ß’ is the relative overshoot magnitude
290 The following optional value should be available for display:
291 • U the peak value of the original recorded curve
e
292 7.2.2 Rate of collapse and amplitude of reversed polarity of the chopped impulse
293 The characteristic events during chopping are largely dependent on the geometrical arrangement of the
294 chopping circuit involved and on the impedance of the chopping circuit and of the test object, all of
295 which determine both the rate of collapse and the amplitude of the undershoot.
296 In IEC 60076-3, the undershoot has been limited to 30 % of the amplitude of the chopped impulse. This,
297 in fact, represents a guideline for the arrangement of the chopping circuit and may entail the introduction
298 of additional impedance Z in this circuit to meet the limit (see Figure 1).
c
299 The chopping loop, however, should be as small as possible to obtain the highest rate of collapse, but the
300 undershoot should be limited to less than, or equal to 30 %. On multiple layer windings, the layer impedance
301 may damp the collapse normally to the extent that it does not oscillate around zero (see Figure B.20).
302 The recommendation in IEC 60076-3 to use a triggered-type chopping gap is made because of its
303 advantage in obtaining consistency of the time to chopping, thereby facilitating the comparison of
IEC CD 60076-4 IEC:2020 – 11 –
304 oscillograph or digital recordings not only before but also after chopping. The latter part will only be
305 comparable for reasonably identical times to chopping.
306 7.3 Terminal connections and applicable methods of failure detection
307 7.3.1 Terminal connections
308 It is essential that the terminal connections of the test object and the earthing practices employed relate
309 to the method of failure detection adopted.
310 Connections for impulse tests are detailed in IEC 60076-3 for transformers and in IEC 60076-6 for
311 reactors. Normally the non-tested terminals of the phase winding under test are earthed and the non-
312 tested phase windings are shorted and earthed. However, in order to improve the wavetail T ,
313 resistance earthing of the non-tested windings may be advantageous (see clause 5 and 7.1) and, in
314 addition, the non-tested line terminals of the winding under test may also be resistance earthed.
315 In addition to the methods of waveform adjustment in 7.1, the following factors have to be considered:
316 a) if a terminal has been specified to be directly earthed or connected to a low-impedance cable in
317 service, then that terminal should be directly earthed during the test or earthed through a resistor
318 with an ohmic value not in excess of the surge impedance of the cable;
319 b) earthing through a low-resistance shunt for the purpose of impulse current measurements may be
320 considered the equivalent of direct earthing.
321 When non-linear elements or surge diverters – built into the transformer or external – are installed for
322 the limitation of transferred overvoltage transients, the impulse test procedure should be as defined in
323 IEC-60076-3. External non-linear element should be disconnected for tests.
324 7.3.2 Applicable methods of failure detection
325 Failure detection is normally accomplished by examination of the oscillograms of digital records of the
326 applied test voltage and the impulse current.
327 Different transients can be recorded and used separately or in combination, as shown in Figure 2.
328 These are listed a) to e) below. It is essential, in acceptance testing, to record at least one of these
329 transients in addition to the applied test voltage:
330 a) the neutral current (for star and zigzag connected windings of which the neutral is earthed during
331 the test);
332 b) the winding current (for all other windings and star and zigzag connected windings of which the
333 neutral may not be earthed during the test);
334 c) the current transferred to an adjacent shorted and non-tested winding, sometimes referred to as
335 capacitively transferred current;
336 d) the tank current;
337 e) the voltage transferred to a non-tested winding.
338 The sum of items a), c) and d) or of items b), c) and d), is sometimes referred to as line current.
339 When testing reactors, both of the shunt and series types, items c) and e) are inapplicable; item d) may
340 be applied but only as an additional means of transient recording since it is likely to be less sensitive
341 than when used in transformer testing.
342 7.4 Test procedures
343 The relevant test sequences for full-wave tests or for full and chopped-wave tests are given in IEC
344 60076-3.
IEC CD 60076-4 IEC:2020 – 12 –
345 The preferred method of test is that of direct application although in special cases where the
346 intermediate or low-voltage winding cannot, in service, be subjected to lightning overvoltage from the
347 system connected to it, the "transferred surge" method may alternatively be employed. The impulse test
348 of the low-voltage winding is then carried out simultaneously with the test of the associated high-voltage
349 winding. In these conditions, the waveform of the transferred voltage does not conform with that
350 specified in IEC 60076-3. It is more important to try to obtain the required voltage level by means of
351 termination resistors of sufficiently high value. However, this may not always be possible even with the
352 highest values of resistors. In this test, high inter-phase voltages may occur on delta-connected
353 windings and the danger of overstressing inter-phase insulation, internal or external, may limit the
354 voltage that can be applied to the low-voltage winding. The appropriate limits may be established by
355 transient analysis with a low-voltage recurrent surge generator.
356 By their nature, non-linear protection devices connected across the windings or its parts may cause
357 differences between the reduced full-wave and the full-wave impulse oscillograms. Proof that these
358 differences are indeed caused by operation of these devices should be demonstrated by making three
359 reduced full-wave impulse test at different voltage levels to show the trend in their operation. To show
360 the reversibility of any non-linear effects, the same reduced full-wave impulses should follow up the
361 full-wave test voltage in a reversed way.
363 One example in conformity with 60076-3:
364 50%, 65%, 80%, 100%, 100%, 100%, 80%, 65%, 50%.
366 Test methods for transformer neutrals are given in IEC 60076-3. The direct method, involving an
367 impulse voltage applied to the neutral with all line terminals earthed, permits a longer duration of
368 wavefront, up to 13 µs. In this case, the inductive loading of the generator is significantly increased and
369 it may be difficult to achieve times to half-value set by the tolerances. Impedance earthing of the non-
370 tested terminals of the winding under test may then be applied.
371 7.5 Recording of tests
372 7.5.1 General
373 Digital recording systems should be used for the recording of lightning impulse voltage and current
374 waveforms.
375 NOTE: Digital recorders provide a much higher resolution than analogue measuring equipment. This might lead to more (small)
376 differences that need to be explained. Nevertheless, it is recommended not to use analogue measuring equipment anymore,
377 since this is no longer state of the art.
378 7.5.2 Digital recording systems
379 The requirements for digital recorders are given in IEC 61083-1.
380 It should be emphasized that for the purpose of presenting results for acceptance by comparison of
381 traces, the waveforms obtained by digital measurements should be produced from the raw data and not
382 subjected to any mathematical processing, filtering, smoothing, etc…,except the rescaling of the
383 oscillograms for comparison purpose.
384 It is equally important to use the raw data for non-standard waveform evaluation.
385 Figures B.18, B.19 and B.20 show significant differences in amplitude and front time T and time to half
386 value T evaluations.
387 IEC 60076-3 requires simultaneously the measurement of
388 a) the applied voltage;
389 b) at least one of the transients listed in 7.3.2;
390 Hence, at least two independent recording channels are necessary.
391 While the applied voltage is uniquely defined, the choice of the other characteristic to be recorded is
392 dependent on the selection of the method of failure detection.
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393 7.5.3 This clause is intentionally left blank
394 7.5.4 Digital recording
395 7.5.4.1 Recording of waveforms
396 The principle of digital recording is the measurement of voltage or current waveforms by taking samples
397 during the test at regular time intervals. These samples should be presented directly as raw data for
398 evaluating waveform parameters (see 7.5.3.1) and also for the assessment of test results based on the
399 comparison of recordings taken at reduced and full impulse voltage levels (see 7.5.3.2). Additionally,
400 the recorded data may also be processed by wave analysing algorithms, for example, for fault analysis
401 in recordings (see clause 10).
402 During impulse tests, high electromagnetic fields are produced in the vicinity of the test set-up.
403 Protection of the sensitive electronic devices in the digital recording system, the entire processing
404 equipment and its power supply against these fields is required.
405 7.5.4.2 Recording of the impulse voltage waveform
406 a) Determination of the impulse voltage waveform
407 The preferred period for the presentation of data for the records taken for waveform determination
408 during preliminary adjustment of test-circuit parameters is ≤10 µs for the wavefront record (longer
409 presentation times may be necessary when testing transformer neutrals). The wavetail record
410 should permit the evaluation of the time to half-value and, on occasions, the amplitude of reversed
411 polarity.
412 IEC 61083-1 specifies a 9-bit, 60 MHz digitizer as the minimum resolution of the digitizer for the
413 recording of impulse voltage and current waveforms. When zooming in on 10 µs time-periods or
414 less for the evaluation of the wavefront or for the evaluation of chopped impulses, the use of a 10-
415 bit digitizer and 100 MHz sampling frequency should be considered.
416 Historically, waveform evaluation is based on oscilloscopic records, engineering rules and eye
417 evaluation of waveform parameters. With the application of digital recorders in high-voltage testing of
418 power transformers, a warning with respect to amplitude and time parameters should be given with
419 respect to the evaluation of non-standard waveforms. In particular, when testing high-power-rated low-
420 voltage windings with resulting unipolar overshoots with frequencies less than 0,5 MHz, IEC 61083-2 is
421 not applicable for the amplitude evaluation of such non-standard waveforms. Errors in excess of 10 %
422 have been observed due to the built-in curve smoothing algorithms in the digitizers (see Figures B.18,
423 B.19 and B.20).
424 In such cases, careful evaluation of the raw data plots using engineering judgement is required. A
425 parallel measurement of the test voltage by a peak voltmeter according to IEC 61083-1 is highly
426 recommended.
427 b) Applied impulse test voltage wave recording
428 In order to determine the amplitude of the test wave and to permit detection of any fault which may
429 be present
430 – for full waves, the period for the presentation of sampled data should not be less than 100 µs;
431 – for chopped waves, a period for presentation of 10 µs to 25 µs is usually found sufficient.
432 Sampling frequencies of 10 MHz to 20 MHz per channel of the digitizer normally suffice because
433 the maximum frequencies of part winding resonance normally do not exceed 1 MHz to 2 MHz. If
434 high frequencies are observed in the voltage or current traces these are due to parasitic resonance
435 in the measuring circuit or noise in the earthing system. It is therefore recommended that higher
436 sampling frequencies (as mentioned before) be used to discriminate noise in the measuring circuit
437 from the actual behaviour of the test object.
438 For wave analysis, it is important to take samples over the complete waveform until the wave is
439 completely damped, using the maximum available memory of the digitizer. It is important to
440 programme the digitizer in such a way that a sufficient number of samples is present to determine
441 the virtual starting-point of the wave.
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442 It is, furthermore, important to use the maximum available resolution of the input amplifiers of the
443 digitizer. For that reason, a number of 50 % pre-shots may be needed to determine the optimum
444 range for the amplitude of the voltage wave and/or the offset for each channel.
445 7.5.4.3 Recording of the impulse current
446 Impulse current is normally the most sensitive parameter in failure detection. Therefore, the recorded
447 current waves are the main criteria of the test result. The stored data in the memory of the digitizer,
448 however, allows for any other presentation of the same recording by zooming in or out at different
449 time scales.
450 In order to benefit from additional mathematical investigation tools, such as transfer function analysis
451 (see clause 10), for the examination of the test results, it is important that the same recording time for
452 the recording of the impulse current and voltage is used.
453 8 Switching impulse tests
454 8.1 Special requirements
455 The response of transformers and reactors to switching impulses is very different because transformers
456 have a complete magnetic circuit and the relatively long duration of the switching impulse therefore
457 allows the establishment of a significant amount of core flux (see IEC 60076-3). This is not the case for
458 reactors for which, in addition, waveform problems and test procedures are different. Therefore, the two
459 items of equipment are dealt with separately.
460 8.2 Transformers – Switching impulses tests
461 8.2.1 Waveforms
462 As indicated in IEC 60076-3, there are no strict values specified for the front time of a switching
463 impulse wave. It should, however, be sufficiently long to ensure essentially uniform distribution of
464 voltage. This normally requires a time to peak T of at least 100 µs. It is determined by the effective
p
465 winding capacitance, any load capacitance and the series resistances.
466 The wavetail is influenced not only by the usual waveshaping components but also by a probable saturation
467 of the core. For most transformers, at full test level, the exponential decay of the wavetail is interrupted by
468 a sudden fall through zero, at a variable time after the crest, due to core saturation. Therefore, the time to
469 half-value is not used to specify the wavetail of the applied switching impulse. Instead, the waveform is
470 defined by its time above 90 % T and by the requirement of the time to first zero passage T . T ≥ 200 µs
d z d
471 and T ≥ 1 000 µs, are defined in IEC 60076-3. These quantities are illustrated in Figure 3a.
z
472 The time taken to saturate the core is dependent on the core size, its initial state of magnetization and
473 the level and waveform of the applied voltage. Unless the core magnetization state is identical before
474 each switching impulse application at a given voltage level, identical waveforms on successive
475 applications will not be obtained. In addition, identical waveforms at reduced and full test levels cannot
476 be obtained. See 8.2.3 for test procedure which reduces the effects of core saturation.
477 Core saturation does not usually occur on reduced-level voltage applications and may not even occur
478 on full-level applications. When it does occur, its effect on the voltage waveform may be large or small
479 depending on the amount of saturation involved. For this reason, when switching impulses are applied
480 from the high-voltage side of the transformer, it is possible to establish T and T from the reduced
p d
481 voltage applications. T cannot be established until the first full-level voltage application is made. When
z
482 switching impulses are performed from the low-voltage side of the transformer, only T can be
p
483 established from reduced voltage applications. In this case, T and T can only be determined from full
d z
484 test-level shots.
485 It should be noted that there may be significant differences in the shape of the wavetail on different
486 limbs of a transformer due to the different reluctances of the magnetic circuit involved.
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487 8.2.2 Terminal connections and applicable methods of failure detection
488 8.2.2.1 Terminal connections
489 In order to comply with the requirements of IEC 60076-3, there is only one admissible test connection
490 for three-phase transformers. This connection is shown in Figure 4, which indicates that the neutral
491 should always be earthed and the terminals of the non-tested phases preferably interconnected. (This
492 interconnection of non-tested terminals is not necessary for tra
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