2014 - CIGRE - PAST, PRESENT AND FUTURE OF IEC.pdf

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Past, Present and Future of IEC

And IEEE High-Voltage and HighCurrent Testing Standards

Working GroupD1.35

August 2014


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PAST, PRESENT AND FUTURE OF IEC

AND IEEE HIGH-VOLTAGE AND HIGH

CURRENT TESTING STANDARDS WG D1.35

Contributors

Y. Li, Convener (AU), J. Hällström, Secretary (FI)

W. Larzelere (US), A. Bergman (SE), J. Rickmann (US), W. Hauschild (DE), R. Diaz (AR), F. Garnacho

(EP), T. McComb (CA), S. Okabe (JP), Y. X. Zhang (US), A P. Elg (SE)

Copyright © 2014

“Ownership of a CIGRE publication, whether in paper form or on electronic support only infersright of use for personal purposes. Are prohibited, except if explicitly agreed by CIGRE, totalor partial reproduction of the publication for use other than personal and transfer to a thirdparty; hence circulation on any intranet or other company network is forbidden”.

Disclaimer notice

“CIGRE gives no warranty or assurance about the contents of this publication, nor does itaccept any responsibility, as to the accuracy or exhaustiveness of the information. All impliedwarranties and conditions are excluded to the maximum extent permitted by law”.

ISBN : 978-2-85873-292-0


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Past, present and future of IEC and IEEE high‐voltage and high current testing standards

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Past, present and future of IEC and IEEEhigh-voltage and high current testing

standardsTable of Contents

1 INTRODUCTION ........................................................................................................................................ 5

2 HISTORY OF HIGH-VOLTAGE AND HIGH-CURRENT TEST STANDARDS ......................................... 6

2.1 History of DC high-voltage tests and measurements ............................................................................ 6

2.1.1 DC high-voltage test and measurement systems ............................................................................ 6

2.1.2

DC high-voltage supplies ................................................................................................................. 7

2.1.3 Future work on DC test and measurement systems ........................................................................ 7

2.2 History of AC test sources and measurement systems ......................................................................... 7

2.2.1 Future work on AC test and measurement systems ........................................................................ 8

2.3 History of impulse voltage tests and measurements ............................................................................. 8

2.3.1 Early history and definition of impulse waveforms ........................................................................... 8

2.3.2 Definitions of lightning impulse peak voltage ................................................................................... 9

2.3.3 Introduction of test voltage function................................................................................................ 10

2.4 Use of sphere-gaps as measurement devices .................................................................................... 10

2.5 Reference Measuring Systems ........................................................................................................... 11

2.6 History of IEEE Standard 4 - the sister Standard to IEC 60060 series ............................................... 13

3 OUTLINE OF MAJOR CHANGES MADE IN RECENT EDITIONS OF IEC AND IEEE HIGH-VOLTAGE AND HIGH-CURRENT STANDARDS ........................................................................................... 14

3.1 IEC 60060-1:2010................................................................................................................................ 14

3.2 IEC 60060-2:2010................................................................................................................................ 15

3.3 IEC 62475:2010 ................................................................................................................................... 15

3.4 IEC 61083-2:2013................................................................................................................................ 15

3.5 IEEE Std 4-2013 .................................................................................................................................. 15

4 DISCUSSION OF IMPORTANT DEFINITIONS AND REQUIREMENTS ................................................ 17

4.1 Lightning impulse test voltage and test voltage function ..................................................................... 17

4.2 Definitions of switching impulse ........................................................................................................... 19

4.3 AC test voltage .................................................................................................................................... 21

4.4 Atmospheric correction factor .............................................................................................................. 22

4.4.1 Introduction ..................................................................................................................................... 22

4.4.2 Formulae of relevant parameters for atmospheric correction factor calculation ............................ 22

4.4.3 The converse iterative procedure ................................................................................................... 23

4.4.4 Iterative procedure for determining atmospheric correction factor at high altitudes ...................... 24


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4.4.5 Atmospheric corrections for AC voltages ....................................................................................... 30

4.5 Requirements for measurement systems ............................................................................................ 32

4.5.1 Structure of IEC 60060-2:2010 ....................................................................................................... 32

4.5.2 Calibration and estimation of measuring uncertainty ..................................................................... 32

4.5.3 System calibration by calibrations of components ......................................................................... 33 4.6 Measurement Uncertainty ................................................................................................................... 34

4.6.1 The approach in IEC 60060-2:2010 ............................................................................................... 34

4.6.2 Uncertainty Requirement Differences between IEC 60060-2:2010 and IEEE Std 4-2013 ............ 34

4.6.3 Relationship between uncertainty and tolerance ........................................................................... 36

5 DIFFERENCES BETWEEN THE LATEST REVISIONS OF IEC60060 SERIES AND IEEESTD 4 ................................................................................................................................................................ 37

5.1 General structure of IEEE Std 4-2013 ................................................................................................. 37

5.2 Summary of differences ....................................................................................................................... 37

5.2.1 Estimation of uncertainty ................................................................................................................ 37

5.2.2 Definition of AC peak voltage ......................................................................................................... 37

5.2.3 Linearity test ................................................................................................................................... 37

5.2.4 Highest frequency in impulse voltage test circuit ........................................................................... 37

5.2.5 Physical characteristics of reference impulse voltage dividers ...................................................... 37

5.3 Conclusion ........................................................................................................................................... 37

6 IMPROVEMENTS AND POSSIBLE ADDITIONS IN FUTURE REVISIONS ........................................... 38

6.1 DC voltage ........................................................................................................................................... 38

6.2 AC voltage ........................................................................................................................................... 38 6.3 Lightning impulse voltage .................................................................................................................... 39

6.3.1 General considerations .................................................................................................................. 39

6.3.2 Further investigation of the test voltage function for SF6 ............................................................... 40

6.3.3 Further investigation of the test voltage function for oil .................................................................. 40

6.3.4 Further investigation of the test voltage function for air gaps ........................................................ 41

6.3.5 Test voltage function for multiple insulation materials .................................................................... 43

6.3.6 Generation and measurement of UHV lightning impulse ............................................................... 43

6.4 Improvement of atmospheric correction factor calculations ................................................................ 43

6.4.1 General remarks ............................................................................................................................. 43

6.4.2 Differences between IEC standards ............................................................................................... 44

6.4.3 Future work on atmospheric correction factors .............................................................................. 50

6.5 Review of voltage drop, AC and DC for future development .............................................................. 51

6.6 Waveforms of lightning impulse voltage and lightning impulse current ............................................... 52

6.6.1 Introduction ..................................................................................................................................... 52

6.6.2 Observation of atmospheric lightning strike voltage waveforms .................................................... 52

6.6.3 Observation of atmospheric lightning strike current waveforms .................................................... 52

6.6.4 Conclusion ...................................................................................................................................... 53

6.7 Improvement of measurement systems .............................................................................................. 53


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6.7.1 Calibration of UHV impulse measurement systems ....................................................................... 53

6.7.2 Uncertainty and risk assessment ................................................................................................... 55

6.7.3 Examples of risk assessment ......................................................................................................... 55

6.7.4 Measurement software ................................................................................................................... 58

7 CONCLUSION .......................................................................................................................................... 58

8 REFERENCES ......................................................................................................................................... 59


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1 INTRODUCTIONRecently, IEC and IEEE have published a number of revised and new standards for high-voltage and high-current testing. These standards include IEC 60060-1:2010, IEC 60060-2:2010, IEC 61083-2:2013,IEC 62475:2010 and IEEE Std.4-2013. Significant changes and additions have been introduced to theserevised and new standards. Many members of CIGRE WG D1.35 were involved in the revision and

development of these standards as members of IEC TC 42 and IEEE PSIM Subcommittee HVTT,particularly in the development of the new techniques and new procedures that are now adopted in thestandards. This Guide has been written by members of CIGRE WG D1.35 to give high-voltage test engineersa broader knowledge of how to apply the latest high-voltage and high-current testing standards.

In the preparation of this Guide, the contributors have tried to point out areas of difficulty in interpretation ofcertain clauses of these standards that should be considered for future revisions to make HV testingstandards more clear and user friendly. This Guide first presents a brief account of the history of thesestandards with the aim to allow readers to gain a better appreciation of the technical background. Thefollowing sections summarize the major changes made to the standards in their latest revisions to provide ageneral picture of the revisions. The individual sections provide detailed information on the importantrequirements and procedures that have now been incorporated into the standards. They also describe someof the specific technical background with a list of published references. Finally, some discussion is given on

the practical implications of these changes. Practical examples are provided to illustrate some of the newtechniques and new procedures. The guide also lists areas of possible improvements to the standards forfuture revisions.


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2 HISTORY OF HIGH-VOLTAGE AND HIGH-CURRENT TEST STANDARDS

2.1 His tory o f DC h igh-vo l t age tests and m easurements

2.1.1 DC high-vol tage test and measurement systems

The requirements for DC test and measurement systems and DC test procedures have not changedsignificantly over last few decades. This is because the main objectives of DC tests have not fundamentallychanged. Insulation systems most frequently tested with DC voltages are mass impregnated paper-oilinsulated cables, DC insulators, DC bushings, DC converter assemblies and DC rectifier transformers.Testing insulators and bushings are performed under dry, wet and polluted conditions whereas most testobjects are tested only under laboratory conditions. Recently, polymeric cables have been introduced for DCapplications and have necessitated an expansion of DC testing capabilities to include partial dischargemeasurements. CIGRE WG D1.55 has been formed to study HVDC partial discharge testing. Now we alsosee interest in DC testing of Gas Insulated Switchgear and the new CIGRE Joint Working GroupJWG D1/B3.57 has been recently formed (2014).

The main test procedures for DC have remained more or less the same over a long period of time. The testsinclude: the withstand voltage test, the disruptive-discharge voltage test and the assured disruptive-discharge voltage test. Withstand tests are used for testing non-self-restoring insulation, such as oil-paperinsulation in a high-voltage cable as well as self-restoring insulation, such as found with the insulatingsurfaces of insulators. The other tests are more specifically for self-restoring insulation.

The main technical challenges for DC voltage generation equipment are:1. To produce a test voltage with sufficiently low ripple voltage to meet the standards. Ripple voltage

magnitude can be more prominent at the higher test voltages due to loading by streamer activity orhigh leakage currents.

2. To assure that the measuring system can record the test voltage and the ripple voltage magnitudeaccurately when short duration current pulses, or persistent repetitive current pulses from the load,are present. These current pulses are normally random and recurring and can adversely affect thevoltage stability (i.e. cause voltage dips) of the test equipment supply.

3. To supply sufficiently high current values for charging long lengths of shielded power cable, inreasonable times, at very high voltages.4. To supply large test current pulses, containing significant electric charge values (Q), such as those

produced in pollution testing. The issue here is to avoid erroneous test results due to large voltagedrops that may influence the flashover behavior.

5. To supply sufficiently high current when sudden changes of impedance in the test object occur, suchas the sudden decrease of impedance prior to the breakdown of an insulator. The concern is toavoid excessive voltage drops that could affect the test results.

6. The introduction of polarity reversal tests in short time frames.7. The introduction of DC partial discharge testing requirements.

Because of the high instantaneous current magnitudes required for some DC voltage tests, the technicalchallenge for DC voltage measurement systems is not only its ability to measure the DC voltage with

sufficient accuracy (or sufficiently low uncertainty), but also having adequate accuracy for measuring rippleand voltage drops due to the sudden change of test object impedance [1].

Unfortunately, the present standards have little to say specifically about how to deal with these problems,mainly because the current pulse demands are not specified. It should be noted that for most DC testingsystems it is not necessary to have a dedicated ripple voltage measurement system built in if the testconditions are without corona discharges or time varying currents, such as those found during testing ofpolluted insulators. A type test of a new DC test system equipment will show the ripple characteristics for thesteady state load current and if the test current is within the rating of the system for a given ripple voltagethen the standards will be met. This also means that purely resistive dividers are adequate for these testsystems to measure the DC voltage. If the equipment is used for tests where high magnitude, time-varyingcurrent pulses are anticipated and corona from test connections is likely, then a resistive/capacitive dividerwith a bandwidth of up to 10 kHz can provide transient data to actually measure voltage drops and ripplemagnitude. The question remains: how much voltage drop for how long a time will affect the test results or

performance of the apparatus being tested? These questions require input from the relevant apparatuscommittees who set test voltages and protocols.


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A DC test with a voltage polarity reversal in a short time has been increasingly performed in recent years fortesting components in HVDC transmission systems since this is a normal operation of some DC links. Thisprovides a challenge for the DC voltage generation and measurement systems because the energy of thetest object must be discharged quickly and safely, and the sources be configured for the opposite polarity.This is often done with resistor networks and mechanically inverting diodes in the system. However, no testprocedures and requirements for associated measurement systems have been specified in the latest editions

of IEC 60060 series. Test procedures for polarity reversal tests are given in apparatus standards forconverter transformers, cables and others.

2.1.2 DC high-vol tage suppl ies

DC high-voltage supplies (generators) for high-voltage testing introduced in the early days did not have theadvantage of modern silicon rectifiers, which can be connected in series for producing very high voltages.DC was produced using mercury vapor electron tubes that acted as diodes or by Kenotron tubes. Since thetubes required filaments that had to be supplied with power at high voltages, special, cumbersome isolationtransformers had to be fitted to provide power at high voltage levels. This problem led to practical limitationson how high a voltage could be produced - especially if any significant current was required.

With the introduction of solid-state rectifiers it became possible to make multiplier circuits for higher DCvoltages. Voltage multiplier circuits invented by Heinrich Greinacher in 1914 were originally developed for

physics research. These systems were not particularly well suited for capacitor charging applications or fastresponse, voltage regulated power supplies, due to the inherent high impedance of the circuit. Since thevoltage multiplication was via transfer of energy through capacitors, these power supply designs havecurrent limitations, based on the size of the capacitors used.

In the mid 1970’s a new style of DC UHV power supplies was introduced into the market. The inventor wasStanley G. Peschel and the design was granted a United States Patent [2]. This design uses a cascade oflow impedance transformers feeding a cascade of voltage doublers in series. This design overcame thelimitations of capacitor–diode multiplier circuits and could provide stable, well-regulated DC sources at a veryhigh voltage. This design, first used in high-voltage and high-current electron beam accelerators, has largelyreplaced multiplier circuits for HV apparatus testing, where large charging currents are required and theyprovide good voltage regulation in the presence of streamers. This use of cascade doubler circuits has theadded advantage of being able to produce relatively low ripple voltages with high overload capacity for

charging large capacitive loads such as installed power cables. Due to the modular nature of the design,systems for use in the UHV range are possible and are more compact than multiplier designs.

2.1.3 Future work on DC test and measurement sys tems

Future work now is directed to the following (see Section 6.1 for detailed discussions):

Better characterization of measuring systems for voltage drop and ripple; Techniques of Measuring partial discharges under DC stress and what impact it has on the source

requirements; Recommendations of equipment ratings for UHV tests where pollution or wet testing is performed; Possible revisions of standards for normal dielectric tests in the UHV range.

2.2 His tory o f AC test sour ces and measurement sy stems

The history of AC test sources and measurement systems dates back to the turn of the 20 th century whenhigh-voltage AC was first used for transmission and distribution of electric energy. In the early days,conventional transformers of higher voltages than the service voltage were used to “proof” test AC systemcomponents. Insulation coordination studies over the years developed test levels for various apparatus in thesystem and higher safety factors which led to higher test equipment voltage ratings. Over time, increasinglylarger capacitive test loads at higher voltages were produced, especially in the case of polymeric cables.Whereas early HV cables of paper-oil insulation could be tested with DC voltages due to the reasonablylinear voltage distribution based on resistivity, voltage distribution of plastic cables is mainly dependent onthe permittivity of the insulating materials. For testing these voltages, a variety of measurement devices weredeveloped including inductive potential transformers, electrostatic voltmeters, capacitive dividers,compressed gas capacitors and standardized sphere gaps. Over time these devices became the standardsfor measurements with traceability established by comparisons or calculations or other low voltage means.

In the late 1960’s new test systems were introduced with high power capability and test voltages went up tocover 800 kV class transmission. For testing equipment, HV sources used some form of inductive reactance


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in the test system to compensate for the capacitive reactance of the test load and limit test power demands.Limitations of expanding test transformers to very high voltage and current levels led to modular resonantsystems that are seen everywhere today. Ferranti, LTD of the UK, using moving core inductor designs, firstintroduced series resonant systems. Unfortunately these designs had poor efficiency and still requiredsignificant power from the mains to energize a test object. In 1973, all of this changed with the introduction ofthe modern, high “Q” series resonant designs invented by Richard F Schutz and Stanley G Peschel [3].

These modern designs were 5 to 10 times more efficient than earlier designs. Modern variable inductanceseries resonant systems were also unique in that they could be constructed with very high voltage and powerratings. Variable inductance, high Q series resonant systems are now the industry standard for the majorityof production and research testing and are supplied by a number of companies.

Currently, test sources for AC are now being developed using variable frequency with fixed inductors ascompared to variable inductors with fixed frequency. These designs have specific advantages for testing thenew generation of very long installed lengths of high-voltage power cables.

On the measurement side of UHV AC, the ability to make gas capacitors rated up to 1200 kV allowed directcalibrations for most applications. Use of sphere gaps declined due to inconvenience, insufficient accuracyfor many applications and voltage limitations. Potential transformers are available for relatively high voltagesbut are expensive. Capacitive voltage dividers are readily available today and have been proven to be stableand sufficiently accurate (low uncertainty) for UHV measurements. Today, most AC test systems use

capacitive voltage dividers to measure the voltage.In parallel with the development of modern AC test systems has been the development of higher voltagereference measurement systems with stable scale factors and demonstrable linearity.

2.2.1 Future work on AC test and measurement sys tems

Future work is now directed to the following (see Section 6.2 for detailed discussions):

Calibration of UHV level Reference Measuring Systems; Calibration of VLF test systems; Methods to check linearity in the UHV range; AC source requirements for the UHV range.

2.3 His tory o f impulse vo l tage tests and measurements

2.3.1 Early hist ory and definit ion of impul se waveforms

It is generally known that the practice of impulse voltage testing dates back to the early 20 th century. According to HV measurements pioneer Nils Hylten Cavallius [4], the first dielectric tests with high impulsevoltages, were performed by F. W Peek Jr. around 1915 [5]. The first measurement of the impulse shape,using a continuously pumped oscillograph, was performed by Gabor in 1927 [6]. It is generally consideredcorrect that testing with simulated lightning and the measuring of voltage and currents that could affect thetransmission system in a negative way were being studied and simulated in laboratories roughly 100 yearsago. F. W Peek Jr. worked in Pittsfield, Massachusetts for General Electric Company and his book, DielectricPhenomena in High Voltage Engineering , was first published in 1915. The book was a result of the need toexplain the consequences of naturally occurring lightning on the apparatus being built for power

transmission – especially transformers. In his third edition, published in 1929, the author refers to thetremendous amount of laboratory and field data that had already been accumulated by then and a separatechapter was devoted just for the benefit of engineers studying lightning phenomena. In Impulse-VoltageTesting , by W.G Hawley, published in 1959 [7], we find the results of 30 early years of work in studying theeffects of a variety of impulse voltage dividers designs, impulse recorders and the mathematical analysis ofdozens of test circuits. Clearly, much work was done by pioneers in high-voltage technology to determinehow to best measure impulse voltages even in the early days.

The question of standardizing high-voltage testing and measuring techniques, in general, has beendiscussed within IEC since 1922. The first work in this area was in connection with insulator testing sincethere were failures of these exposed devices and a solution was needed for the problem. In 1934, asubcommittee was set up to deal with impulse voltage testing leading to first edition of IEC 60, Generalspecifications for impulse voltage tests. The second edition, High-voltage test techniques, was issued in

1962. The counting of editions was restarted, when the content of IEC 60 was distributed among four parts ofa new IEC 60 series. First edition of part 1 of the series (High-voltage test techniques – General definitions


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2.3.3 Introduction of test voltage function

The black dots in Figure 1 show the response of insulation to oscillations superimposed on a lightningimpulse according to a recent study [14]. As a result, the new IEC 60060-1:2010 [15] defines a test voltagefunction [16]:

,

which is shown by the blue line in Figure 1. This formula is used to calculate the effective peak voltage as afunction of the frequency of the oscillation where f is the frequency in MHz. The introduction of this functionremoves the problems related to the stepwise change at 0.5 MHz and makes a smooth transition that has amathematical definition.

Figure 1: Amplitude calculation factor for a range of oscillation frequencies superimposed

on a lightning impulse, together with the old and new test voltage functions.

Figure 1 shows that for oscillations below 100 kHz, the peak voltage is taken as the actual peak. Forfrequencies above 4 MHz, the oscillations are discarded and have no impact on the calculated peak voltage.

The results of this recent experimental work have also led to the development of a well-defined newprocedure for calculating the parameters of impulse test voltage. This procedure has now been adopted byIEC 60060-1:2010 and IEEE Std 4-2013. A more detailed discussion on this procedure is given inSection 4.1.

Future work in this area involves proving that the dielectric stress produced by various superimposed

oscillation frequencies on normal wave shapes can be normalized for various apparatus in terms ofinsulation performance and that the key parameters can be consistently calculated. See chapter 6.3 fordetailed discussion on future work in this area.

2.4 Use of s phere-gaps as measur ement devices

The sphere-gap is a device that has been used for calibrating high-voltage measuring systems for over half acentury [17, 18, 19, 20, 21, 22, 23]. However, due to the availability of economical voltage measuring deviceswith better measurement repeatability, reduced measurement uncertainty due to influence of atmosphericconditions and ease of use, the use of sphere-gap has been in decline. A sphere-gap does not providesufficiently low values of uncertainty to qualify as a Reference Measurement System. Sphere-gaps also onlygive one piece of information — peak voltage value. Other measurement systems now provide informationon time parameters of test voltages. There is still an IEC standard (IEC 60052) describing its use but it is

only suited to measurement checks, not the calibration of measurement devices. It is intended that sphere-gaps will be used primarily as a device for Performance Checks of high-voltage AC and impulse

22.211 f f k

0

0.2

0.4

0.6

0.8

1

1.2

0.01 0.1 1 10

f [MHz]

IEC 60:1962

Measured

IEC 60060-1:2010


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measurement systems approved in accordance with IEC 60060-2:2010 [24]. Sphere-gaps can also be usedfor calibrating measurement systems that do not require an uncertainty of voltage measurement of < 3 %,but today most users must meet this uncertainty requirement and most technical committees referenceIEC 60060-2 for the uncertainty requirement and the requirement is < 3 % for the test voltages in mostcases.

Summary of Sphere Gap Considerations for Voltage Measurement

1. Simple to construct but expensive for larger voltage ratings.2. Can only be set for one voltage at a time.3. Only measures peak voltage.4. Requires careful mechanical alignment.5. Range of operation for a given sphere size is limited.6. Can be influenced by particles in the air.7. Uncertainty of measurement > 3 % for AC & LI voltages.8. Requires little maintenance over long life time.9. Requires more test area space than voltage dividers.

Conclusion and Recommendations

Sphere-gaps are useful devices for checking high-voltage measuring systems or to provide overvoltageprotection as a protective gap during tests, but less convenient for almost any other HV testing needs.

2.5 Reference Measuri ng Syst ems

The concept of using a Reference Measuring System for calibration is as old as the concept ofmeasurement. However, the requirement to use a Reference Measuring System for calibration was notpresent in the earlier versions of IE 60 and IEEE Std. 4 before 1994. The calibration of an ApprovedMeasuring System was primarily based on low-voltage components methods. In case of impulse dividers, aslong as the total response time was less than 200 ns, the standards stated that the dividers would meet themeasurement error requirements for the peak voltage and time parameters. In 1980s several researchers atdifferent HV laboratories around the world independently found that the total response time had no definiterelationship with the impulse parameter measurement errors. As a result, a few national metrology

laboratories decided to build their own high precision reference dividers and conducted an internationalround-robin study with the dividers. The study formed the base of 1994 edition of IEC 60060-2 that requiredusing reference dividers to calibrate approved impulse measuring system [25, 26, 27, 28]. This standardspecified that the preferred method of calibrating a complete measuring system, including the divider, thetransmission system, any secondary divider/attenuator, the measurement instrument and any measurementsoftware, is by comparison against a Reference Measuring System. This concept is applicable to impulse, AC and DC voltages and the standard specified a minimum voltage rating for a reference divider of 20 % ofthe rating of the system being calibrated to determine its scale factor. Performance at levels higher than thecalibration level was to be verified by a linearity test.

The measurement uncertainty of a Reference Measuring System is normally significantly lower than that ofan Approved Measuring System used in high-voltage testing. The low value of uncertainty of a ReferenceMeasuring System is established through measurements traceable to National Standards. Comparisons

among Reference Measuring Systems are also often conducted to ensure their measurement uncertaintyand several international round robins have been conducted to confirm this [28, 29, 30, 31, 32, 33, 34].

Prior to the introduction of the requirement for the calibration of a complete measuring system against aReference Measuring System, IEC 60-2 allowed the calibration of a high-voltage measuring system to beperformed by calibration of its components. The method of component calibration was kept in the 1994edition of IEC 60060-2, as well as in the 2010 edition of IEC 60060-2. However, the comparison andcalibration against a Reference Measuring System or Approved Measuring System is specified as thepreferred method of calibration. The Reference Measuring System or Approved Measuring Systemcomparison calibration is generally much more efficient than evaluation by the component method. It shouldbe noted that low voltage “check” methods have also been introduced for Performance Checks ofmeasurement systems.

The direct comparison calibration method against a Reference Measuring System rated for the full voltage of

a system being calibrated is the best method and it is easier to demonstrate traceability to NationalStandards for scale factor and linearity.


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Table 1 is a summary of the requirements for measurement uncertainties of Reference Measuring Systems,along with the requirements for measurement uncertainties of Approved Measuring Systems, specified in thepresent edition of IEC 60060-2:2010 and IEC 62475:2010 [35].

Table 1: Requirements for measurement uncertainti es of Reference Measuri ng Syst ems

Type of test voltage Measurement quantityUncertainty limit of

Reference MeasuringSystems

Uncertainty limit of Approved Measuring

Systems

DC voltage Average value 1 % 3 %

AC Voltage Peak/√2 value 1 % 3 %

Full/Tail choppedlightning voltage

Test voltage (U t)) 1 % 3 %

Front time (T 1) 5 % 10 %

Time to-half-value (T 2) 5 % 10 %

Time to-chopping (T c) 5 % 10 %

Front chopped lightningvoltage

Test voltage (U t) 3 % 5 %

Time to-chopping (T c) 5 % 10 %

Switching impulsevoltage

Test voltage (U t) 1 % 3 %

Time to-peak (T p) 3 % 10 %

Time to-half-value (T 2) 3 % 10 %

Impulse Currents(Exponential andrectangular)

Peak current value Not specified 3 %Time parameters Not specified 10 %

Future work

Future Work in the area of Reference Measuring Systems should be focused on demonstrating theuncertainty limits achievable, especially for calibrating equipment for testing in the UHV range.


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2.6 Histo ry of IEEE Standard 4 - the s i st er Standard to IEC60060 seri es

IEEE Std 4-1995 was the previous version of the general IEEE standard dealing with High Voltage Testingand Measurement Techniques. This standard was built on the work done by pioneers in the field of HVtesting: Pete Bellaschi, Frank Creed, Nils Hylten Cavalius, Kurt Feser, Gianguido Carrara to name a few.

The current revision, published in 2013, is the eighth edition of this document as a separate standard. Thesubject had been addressed in the earliest Standardization Report of the American Institute of ElectricalEngineers (AIEE) in 1889 and had been substantially elaborated upon in the subsequent reports issued from1902 to 1933. In 1922 it was decided to issue separate sections for measurement of test voltages by AIEE.The first edition of IEEE Std 4 was published in 1928. It is interesting that as we are going to highertransmission voltages for UHV many of technical problems solved for lower voltage systems must beinvestigated again for UHV.

In the years prior to the last revision of the IEEE Std 4, which is referred to by many North Americanapparatus committees in their HV testing standards, much has changed in the global marketplace withrespect to the power equipment manufacturing industry. Testing has become more automated, more preciseand more informative about the quality of the insulation systems being evaluated. In addition, moreawareness of the corresponding IEC standards has developed, namely IEC 60060 parts 1 and 2, “High-

voltage Test Techniques”. Now it is common for North American manufacturers to use both IEC and IEEEstandards in testing their products, many of which enter the global marketplace. Increasingly data taken inone location may be instantly transmitted electronically to other facilities continents away for analysis andreview. The need for standardization has never been more important to the reliability of components of theelectrical grid.

In preparing the latest revision for IEEE Standard 4, the committee members took every effort to try to alignthe revised technical content with the revised technical content of IEC 60060 parts 1 and 2 and the newstandard IEC 62475 for High-current test techniques. This has been a difficult task as the three IECstandards (four including IEC on-site testing) have been evolving simultaneously with the IEEE revision.Fortunately, IEC revisions are now fixed and the two standards are aligned for the most critical requirements.Despite close relationships between IEEE and IEC organizations, there are still several small differencesbetween IEEE and IEC that are noted in later sections.


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3 OUTLINE OF MAJOR CHANGES MADE IN RECENT EDITIONS OF IECAND IEEE HIGH-VOLTAGE AND HIGH-CURRENT STANDARDS

This section outlines the major changes in recent editions of high-voltage and high-current test standards.Some of these changes are described in detailed in Section 4.

The relevant standards for this discussion are:IEC 60060-1 Ed. 3.0 (2010), High-voltage test techniques – Part 1: General definitions and testrequirements.

IEC 60060-2 Ed. 3.0 (2010), High-voltage test techniques – Part 2: Measuring systems.

IEC 61083-2 Ed. 2.0 (2013), Instruments and software for measurement in high-voltage and high-currenttests – Part 2: Requirements for software for impulse test.

IEC 62475 Ed. 1.0 (2010), High-current test techniques - Definitions and requirements for test currents andmeasuring systems.

IEEE Std 4-2013 High Voltage Test Techniques.

3.1 IEC 60060-1:2010 The definitions for basic lightning impulse voltage parameters, U p, T 1 and T 2, have not changed. The

test voltage function (sometimes known as the k-factor) as described in the literature published in theperiod leading up to the approval of the standard, has been introduced to enable more accurate andconsistent determination of the test voltage and time parameters of lightning impulses withsuperimposed oscillations of any frequency content. A number of new definitions related to this newprocedure have been added, e.g., test voltage function, extreme value, relative overshoot andaverage rate of rise.

The peak value of an alternating voltage is now defined as half of the peak-to-peak voltage. Theearlier definition, “maximum value”, could lead to misinterpretation for cases where even harmonicsof the test source are present. This harmonic distortion may lead to different positive and negativepeak values. A maximum value of 2 % is allowed for the difference between positive and negative

peak values. The test voltage value is the peak value divided by √2. No changes in the definition of switching impulse voltage parameters have been introduced inIEC 60060-1:2013, and the previous definition of time to peak has been retained, i.e. “time intervalfrom the true origin to the time of maximum value of a switching-impulse voltage”.

IEEE Std 4-2013 and IEC 60060-1:2010 differ slightly on evaluation of switching impulse. For thecase of a standard switching impulse, with the time to peak being 250 µs ± 50 µs, IEC identifies asimplified method using a mathematical formula adopted from IEEE Std. 4-1995. This method is alsogiven by IEEE Std 4-2013, but is stated as the definition.

For non-standard switching impulses, IEC clearly states that other methods of evaluation should beused, e.g. “For non-standard impulses, the time to peak can be determined by various methods ofdigital curve fitting dependent on the actual shape”.

Formulae have been introduced for the parameters of the atmospheric correction factor to make thecomputer calculation of the correction factor feasible.

The iterative procedure for calculating the atmospheric correction factor of a test voltage isintroduced, and is intended for voltage withstand tests. This procedure is intended to reduce theerror of the correction factor due to the error in the estimated U50 (50 % probability breakdownvoltage) that is needed for the calculation. The error becomes significant when the correction issignificant.

Only one wet test procedure has been retained. Test procedures for combined and composite voltages have been elaborated more in details. AC and DC Artificial pollution test procedures are removed as they are now specified in IEC 60507.


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3.2 IEC 60060-2:2010

Estimation of common components of uncertainties in high-voltage measurements are now specifiedin more details.

Requirements on voltage linearity tests are more clearly specified. New requirements on dynamic performance, i.e., frequency response of power frequency AC voltage

measurement systems, have been added. Step response parameter evaluation methods for characterization of voltage dividers and current

shunts have been moved into an informative annex, with a number of definitions related to the stepresponse measurement having been revised.

An informative annex on evaluation of the dynamic performance of impulse measurement system bythe convolution method has been added

The standard sphere-gap is no longer accepted as a Reference Measuring Device but can still beused for Performance Checks.

Calibration procedure for DC -systems using a rod/rod gap is removed. Measurement of impulse currents has been transferred to a new standard IEC 62475:2010, High-

current test techniques – Definitions and requirements for test currents and measuring systems.

3.3 IEC 62475:2010 The current measurement part of this standard covers more applications than the relevant sections

in the obsolete 60060-2:1994, which covered only measurement of impulse current waves as used inarrestor testing.

The new standard also covers requirements for testing with any type of high current as well as givingthe requirements for a high-current measurement system.

The types of high currents which have been added now include: steady-state direct current, steady-state alternating current, short-time direct current, short-time alternating current, and impulse current.

The standard also covers current measurement in high-voltage dielectric testing. The standard has adopted a similar structure to that of IEC60060-2:2010 Estimation of measurement uncertainties is specified similarly to that in IEC 60060-2:2010.

3.4 IEC 61083-2:2013 IEC 61083 part 2 has been updated with a new Test Data Generator (TDG) to evaluate impulse

measuring system software. The new version provides more impulse voltage waveforms, and includes waves with different

overshoot amplitudes and frequencies. Frequencies are selected to prove performance around thetransition frequency of 500 kHz. The new TDG helps users prove that their software is makingcorrect evaluations of the key parameters in a consistent and comparable way.

Reference values of lightning impulse waveforms have also been revised according to the newdefinition of the impulse test voltage in IEC 60060-1:2010

Waveforms of impulse currents are added to the TDG to represent a range of those used for arrestortesting that have been added over the last few years. More current waveforms, including lightningcurrent impulses, are now included, in order to cover the range of current waveforms used in the

new standard IEC 62475:2010. An annex on estimation of uncertainty contribution of software (waveform parameter calculation) has

been added.

3.5 IEEE Std 4-2013

IEEE Std 4-2013 continues to cover both the requirements for testing and the requirements formeasurement systems, which are covered separately by IEC 60060-1:2010 and IEC 60060-2:2010.

Efforts have been made to harmonize IEEE Std 4-2013 with the two parts of IEC 60060, in terms ofprinciples and fundamental requirements. Minor differences, however, still exist. A summary of thedifferences is given in Section 5.2.

The new edition of IEEE Std 4 still contains a significant amount of tutorial information to givepractical suggestions to the test engineer.


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The readers are also drawn to the attention of a closely related standard, IEC 60060-3:2006 [36] for on-sitehigh-voltage testing. Many of techniques and procedures are similar to those specified in the abovestandards, but with variations to suit conditions and limitations of on-site tests.


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4 DISCUSSION OF IMPORTANT DEFINITIONS AND REQUIREMENTSThis chapter discusses important definitions and requirements that have either undergone significantchanges or deserve close attentions of users for correct interpretation and better use of the standards.

4.1 L ight n ing imp ulse test vo l tage and test vo l t age func t ion

As discussed in earlier sections, the impulse analysis procedure provided in IEC 60060-1:2010 andIEEE Std 4-2013 allows determination of impulse parameters without subjective determination of theexistence or characteristics of overshoot and its frequency. The new procedure in the standards is also moreprecisely specified in order to eliminate discrepancies of impulse parameters obtained by different softwarepackages. A summary of the major steps of the procedure is given in Figure 2.

Evaluation of tail chopped impulse parameters requires the knowledge of the waveform of a full impulsetaken in the same circuit configuration. This is because double exponential curve fitting will not produce acorrect base curve for chopped impulses since the front is not really a perfect exponential and the tail doesnot exist on a chopped wave. A full impulse waveform is therefore needed to aid in the calculation. A fullimpulse from the same test circuit at a reduced voltage level is usually available in the impulse test of mostapparatus before the tail chopped impulses. Therefore, for evaluation of tail chopped impulses, steps 1 and 2in Figure 2 are performed using this full reference impulse; the base curve obtained is then scaled ornormalized to match the amplitude of the tail chopped curve for the remaining steps of the procedure.

For a front chopped lightning-impulse voltage, the test voltage curve is the recorded curve without furtherprocessing of the waveform.

An integral part of requirements for impulse voltage tests in IEC 60060-1:2010 are the requirements forsoftware specified in IEC 61083-2:2013, Instruments and software used for measurement in high-voltageand high-current tests - Part 2: Requirements for software for impulse tests . The Test Data Generator (TDG)of IEC 61083-2:2013 is a software package for generation of test data. The TDG produces digital records ofa number of different impulse waveforms for testing an impulse measurement software package.IEC 61083-2:2010 specifies the reference values of test impulses generated from the TDG. TheIEC 61083-2:2013 also provides error limits for acceptance of software being evaluated for measuringdifferent types of impulses.

After the new Test Voltage Function (k-factor) was proposed, a number of evaluation software programswere tested in round-robin tests. The work was first performed within CIGRE WG D1.33, but later the activitygradually moved to IEC TC 42 Maintenance Team (MT) 07. The main task of the latest work was to revisethe original Test Data Generator to include more wave shapes for more comprehensive testing of impulsevoltage and impulse current evaluation algorithms. Annex B and Annex C of IEC 60060-1:2010 are two ofthe products of MT07 work.

In addition to LI and SI impulse shapes in the TDG, IEC 61083-2:2013 also includes data sets for OscillatingLightning Impulse waveforms (OLI) and Oscillating Switching Impulse waveforms (OSI) in accordance withIEC 60060-3:2006 (High-voltage test techniques - Part 3: Definitions and requirements for on-site tests) [36].Impulse current waveforms defined in IEC 62475:2010 (High-current test techniques – Definitions andrequirements for test currents and measuring systems) have been added to the TDG too.

1. Data startingfrom 20% on thefront to 40% onthe tail (pink) ofthe measureddata (blue) aretaken for curvefitting.

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2. Doubleexponential basecurve (green) isfitted to the dataselected in step 1(pink).

3. Residual curve(orange) isobtained bysubtracting thebase curve(green) from themeasured curve(blue).

4. Residual curve(orange) isfiltered accordingto the test voltagefunction to getthe filteredresidual curve(brown).

5. Filteredresidual curve(brown) is addedback to the basecurve (green) toget the testvoltage curve(red).

6. Test voltagecurve (red) is

shown togetherwith themeasured curve(blue). U p, T 1 andT 2 are calculatedfrom the testvoltage curve.

Figure 2: Evaluation of the test voltage curve according to IEC 60060-1:2010.

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4.2 Def in i t ions of swi t ch ing impu lse

The definition of time to peak for switching impulse in IEC 60060-1:2010 is “time interval from the true originto the time of maximum value of a switching-impulse voltage”. It has been found that it is often difficult todetermine this time to-peak value, Tp, even from digital records of impulses with low uncertainty, due to thefact that the digitally recorded voltage values in the impulse peak region can be approximately equal for a

long period of time, and many data points of the same amplitude will be found around the peak. Toovercome the problem of calculating the Tp value, the empirical formula introduced in the previous edition ofIEEE Std 4 and retained in IEEE Std 4-2013, is now included in IEC 60060-1:2010 for evaluation of switchingimpulses that conform to the standard waveform.

The formula takes the following form:

T p = KT AB (1)

where K is a dimensionless constant given by

K = 2.42 – 2.08x10-3T AB + 1.51 x 10-4 T 2

where T AB and T 2 are in microseconds and T AB = t 90 – t 30 (see Figure 3) and the numerical constants 2.08and 1.51 have dimension s-1.

The difference between the T p values calculated from formula (1) and the true T p as defined in Clause 8.1.3of IEC 60060-1:2010 for the standard switching impulse waveform of 250/2500 µs is negligible. Taking theTDG waveform SI-A1 of IEC 60183-2:2013 as an example, the difference between the value calculatedusing formula (1) and the reference value given by IEC 60183-2:2013 is less than 0.85 % (see Table 2). Thereference values given in IEC 60183-2:2013 were determined by averaging the T p values obtained in around-robin test, by different software packages that calculate the T p values in accordance with Clause 8.1.3of IEC 60060-1:2010, i.e. the T p value that corresponds to the maximum voltage of the impulse waveform. Ifthe impulse parameters remain within the tolerance ranges of the standard switching impulse, i.e. T p remainsbetween 200 µs and 300 µs, and T 2 remains between 1000 µs and 4000 µs, the maximum deviation of T p values calculated, using formula (1), from the reference T p value is less than 3 %, which is negligible for thepurposes of high-voltage testing.

However, since formula (1) was defined for standard switching impulses only, if the waveform deviates

significantly from the standard switching impulse, the deviation of T p values can be significantly larger thatthe permitted measurement uncertainty of T p, which is ±10 % and for those cases other methods ofevaluation must be used. However, for most testing situations this is not an issue.

Figure 3: Illustration of defined switching impulse voltage parameters.

U

1,0

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0,3 A

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Table 2: Tp values given b y TDG of IEC61083-2: 2010 and th ose calcu lated usi ng f ormul a (1)

TDG Case No.TDG T p

(µs)TDG T 2

(µs)Formula (1) T p

(µs)T p Error

(% of TDG T p)

IS-A1 250.7 2512 248.8 -0.76

SI-A2 19.89 1321 15.30 -23.1

SI-A3 43.1 3987 35.99 -16.5SI-A5 218 2407 221.8 +1.8

Note that waveforms S1-A2 and S1-A3 are non- standard switching impulses

The errors of T p values obtained by using formula (1) for some practical waveforms are shown in Figure 4 and Table 3. The reference values are obtained according to the definition in Clause 8.1.3 ofIEC 60060-1:2010, using double exponential fitting to find the peak.

Results in Table 2 and Table 3 show that when the standard switching impulse voltages are used,formula (1) is sufficiently accurate for meeting the uncertainty requirement of high-voltage testing.

Table 3: Tp values obtained using f ormul a (1) as compared to Tp values by double-exponential f i t t ing

Waveform No. T p by fitting(µs)

T 2 by fitting(µs)

Formula (1) T p (µs)

T p Error(% of fitted T p)

1 174.71 2481.95 169.66 2.9

2 212.51 1491.09 210.84 0.8

3 237.13 3998.58 238.21 0.5

4 287.93 4032.71 291.34 1.2

5 288.55 1504.95 285.00 1.2

6 312.51 2546.15 308.06 1.4

Figure 4: Errors of T p values of practical waveforms obtained using formula (1) with the T p values of fitted waveforms as the references.


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It has been found, however, that errors of Tp values obtained by other methods, for example, by curve fitting,would not be significantly lower, in some cases could be higher than those of formula (1). A number of curvefitting methods have been tested by various laboratories. One method was to fit the complete waveform fromthe origin of the impulse to a double exponential function. Another method [37] is to fit from the instant of85 % of the peak value on the front to the instant of 95 % of the peak value on the tail (denoted as top fit). Itwas found that the difference in the Tp values obtained by these two fitting method for the last waveform in

Table 3 is 1.7 %. Furthermore, when the time parameters are within the tolerance range of the standardswitching impulse, the maximum errors of Tp values obtained by the fitting methods for the TDG waveformsare of similar magnitude to that of the formula (1), with reference to the reference values as given inIEC 61083-2:2013. Table 4 shows the errors of the two fitting methods.

Table 4: Tp errors of two f i t t ing methods for st andard switchi ng impulses

TDG Case No.TDG T p

(µs)

Error of T p (complete fit)(% of TDG T p)

Error of T p (top fit)

(% of TDG T p)

Error of T p (Formula (1))(% of TDG T p)

IS-A1 250.7 +0.52 +0.53 -0.76SI-A5 218.0 +3.2 -4.7 +1.8

In conclusion, the difference of T p values of standard swit ching impu lses obtained by the fitting methods inaccordance with Clause 8.1.3 of IEC 60060-1:2010 and the formula given in Clause 8.2.3.1 ofIEC 60060-1:2010 and Clause 8.1 of IEEE Std 4-2013 are not significant for the purposes of high voltagetesting. Therefore, for standard switching impulse measurement, formula (1) is recommended due to thesimplicity of its implementation as no complicated nonlinear curve fitting and the related softwaredevelopment are needed.

Non-Standard Switching Impulse Wave shapes

The calculation errors using formula (1) can be significant for calibration of the measurement systemsystems using the non-standard switching impulses. For the non-standard switching impulses given inIEC 601083-2, the formula yields T p values outside the acceptance limits of the reference values. Fitting

methods may be utilized for these special waveforms by agreement between parties. In any case, thesewaveforms would generally not be acceptable for apparatus testing by relevant apparatus committees thatreference IEC 60060 for standard wave shapes and their evaluation is left to those relevant apparatuscommittees to define.

4.3 A C test vo l tage

A change in the new standards is for a new definition of the AC peak voltage. As mentioned earlier the ACpeak voltage is now defined in IEC 60060-1:2010 as the “average of the magnitudes of the positive andnegative peak values, as opposed to, “the maximum value” in the 1989 edition.

The definition of peak voltage in IEEE Std 4-2013, remains the same as in its previous version,IEEE Std 4-1995, which is essentially identical to the corresponding definition in IEC 60060-1:1989.

The practical impact of the difference between IEC 60060-1:2010 and IEEE Std 4--2013 is insignificant in thevast majority of AC tests, where the AC voltage waveforms are symmetrical to the zero voltage level.

In rare cases, when the voltage waveform becomes unsymmetrical, e.g., with the presence of evenharmonics produced by power electronics voltage sources, if a peak voltmeter is used to measure voltage,the difference between the peak voltage reading from a voltmeter that detects the maximum of the ACvoltage and that from a voltmeter that reads the average of the positive peak and the negative peak can bedifferent. IEC 60060-1:2010 requires that the difference between the positive peak and negative peak valuesshall be less than 2 %.

For voltage sources that meet this 2 % requirement, the maximum difference in the measured peak voltagebetween IEC 60060-1:2010 and IEEE Std 4-2013 would be 1 % if there was an non-symmetrical AC shapedue to electronic converter loads. Nearly all High-voltage AC testing produces symmetrical waveforms sothis is not an issue for most testing.


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The requirement for the crest factor, which is defined as the ratio of the peak voltage to the rms value,remains the same for the new editions of both IEC 60060-1 and IEEE Std 4. The ratio between peak and rmsshould be within 5 % of √2.

The crest factor has been used for some time as a determinant of waveform distortion especially in the peakarea of the test voltage. With modern instruments being capable of more accurately measuring harmonics,the value of total harmonic distortion, THD, can now be easily measured. It is tempting to use this parameterto characterize the wave-shape, but it must however be considered, that THD does not directly relate tochanges in the crest value and that THD is not a suitable measure of crest distortion.

A voltage drop of up to 20 % is still considered acceptable in the revised standard during the AC tests suchas the disruptive voltage test, the pollution test and the wet test.

4.4 Atmos pher ic correct ion factor

4.4.1 Introduction

The atmospheric correction factor is used for correcting the disruptive discharge voltage of insulation under aparticular test atmospheric condition to a voltage under the standard atmospheric conditions. This sectiondescribes in detail some of the additions and changes in relation to atmospheric correction factor as defined

in IEC 60060-1:2010.This chapter also lists the newly introduced formulae in IEC 60060-1:2010 that replaced the graphs inIEC 60060-1:1989. These formulae facilitate calculation of atmospheric correction factors by computersoftware.

The atmospheric corrections defined in IEC 60060-1:2010 are valid for air-gaps and clean insulators. Ifatmospheric corrections are to be calculated for surface discharge tests, the results obtained with themethod in IEC 60060-1:2010 have to be treated with caution.

In IEEE Std 4-2013, two methods of calculating atmospheric correction factor are used. Method 1 is thesame as IEC 60060-1:2010, and is recommended for new equipment. Method 2 has been used in the pastand may be valuable for repeated tests on existing equipment designs.

4.4.2 Formulae of relevant parameters for atmos pheric correction factor calcul ation A number of formulae have been introduced for convenient implementation of computer calculation ofatmospheric correction factor values. Changes have also been made to a few formulae as a result of thelatest revision of IEC 60060-1. These changes are highlighted here to help users to make necessarychanges to their calculation procedures.

4.4.2.1 Formulae of Exponents for air density correction and humidity correction

In IEC 60060-1:1989, the value of the exponent for air density correction, m, and the value of the exponentfor humidity correction, w , needed to be obtained from graphs. In IEC 60060-1:2010, formulae of these twoexponents, as functions of parameter g , have been included to facilitate atmospheric correction factorcalculations by computer programs. The formulae are especially useful when the iterative procedure (see4.4.3 and 4.4.4) has to be used. The relationship between g and m, and that between g and w , are both

described by piecewise functions listed in Table 1 of the IEC 60060-1 Ed.3.0, which is reproduced below asTable 5.

Table 5: Values of exponents, m for air density correction and w for humidit y correction,as a func tio n of the parameter g

Table 1 of IEC 60060-1:2010g m w

2,0 1,0 0


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The definitions for g , m and w in IEC 60060-1:2010 remain the same as in IEC 60060-1:1989. The graphs form and w are however still given for those who still want to use them.

4.4.2.2 The voltage type parameter k

The parameter for voltage type, k , in the case of “direct voltage”, was expressed a linear function of “ h/δ” inIEC 60060-1:1989 (See Figure 3 of IEC 60060-1:1989), where h is the absolute humidity of ambient air andδ is the air density. However, the function k in the “direct voltage” is slightly non-linear. The non-linearfunction has now been adopted in IEC 60060-1:2010.

The other change is that the applicable range of humidity for k has been extended for the DC case and theimpulse case. The extension of the humidity range was largely based on consensual decisions, withconsiderations given to the historical experimental data, practical issues and impact on the validity of thecorrected test voltage.

The changes related to parameter k are summarized below:

IEC 60060-1:1989 Ed. 2.0:

“Direct voltage: k = 1 + 0,014(h/δ − 11), for 1 g/m3 < h/δ < 13 g/m3”

“Impulse voltage: k = 1 + 0,010(h/δ − 11), for 1 g/m3 < h/δ < 15 g/m3”

IEC 60060-1:2010 Ed. 3.0:

“DC: k = 1 + 0,014(h/δ – 11) – 0,00022(h/δ – 11)2 for 1 g/m3 < h/δ < 15 g/m3”

“Impulse: k = 1 + 0,010(h/δ − 11) for 1 g/m3 < h/δ < 20 g/m3”

In the AC voltage case, k remains unchanged.

“AC: k = 1 + 0,012(h/δ − 11) for 1 g/m3 < h/δ < 15 g/m3”

4.4.2.3 Air humidity

The calculation of the atmospheric correction factor requires determination of the absolute humidity of air, h.

The value of h can be determined by the reading of a dry bulb thermometer and the reading of a wet bulbglass thermometer. However, IEC 60060-1:2010 also includes a conversion formula that allows calculation ofthe absolute humidity of air, h, from the relative humidity and the air temperature. This allows convenientmeasurement of h using electronic sensors measuring relative humidity.

,

where h is the absolute humidity in g/m3,

R is the relative humidity in percent and

t is the ambient temperature in °C.

The uncertainty of measurement of R using electronic sensors would normally be 1 % to 2 % relativehumidity, and hence at least the same uncertainty in the value of h. Determination of absolute humidity usingwet and dry thermometers is not expected to alter the uncertainty significantly (neglecting the uncertainty ofthe temperature measurement). The sensitivity of the uncertainty of atmospheric correction factor (and hencethe corrected test voltage) to the uncertainty of h is low (see D.7.1.3 of IEEE Std 4-2013). As shown in TableD.2 (IEEE Std 4-2013), an uncertainty of 1 g/m3 in h would only lead to a relative uncertainty of the correctedtest voltage of 1.1/559, which is approximately 0.2 %.

4.4.3 The conv erse iterative procedure

The atmospheric correction factor, K t, is used for both breakdown tests of given breakdown probability andwithstand tests. In most cases of product testing, atmospheric correction is performed for voltage withstandtests, with AC, DC, lightning impulse or switching impulse voltage, at an altitude less than the service altitude

of the equipment under test.

)273(4615,0

11,6 2436,17

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e Rh

t

t


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The procedure in IEC 60060-1:2010 is derived from experimental data of 50 % probability flashover voltage,however the correction often needed is for the corrected test voltage to be applied in a withstand test. Thewithstand test voltage is defined as the voltage with 10 % probability of disruptive discharge, U 10.IEC 60060-1:2010 assumes the same correction factor applies to both U 50 and U 10.

The calculation of the atmospheric correction factor requires a known value of U 50 of the insulation undertest. In a flashover test, U 50 can be determined and therefore the value of U 50 does not cause significant errorin the correction factor if the experiment is performed correctly. However, U 50 is usually unknown in awithstand test and has to be estimated for calculation of the correction factor. The iterative procedureintroduced in IEC 60060-1:2010 (Annex E) is to reduce the error in the correction factor, hence the correctedtest voltage, due to the error of the estimated U 50. The error in the calculated correction factor is oftensignificant in comparison with the required tolerance of the test voltage in cases where the correction islarge, i.e., where the correction factor deviates from unity by more than the tolerance of the specified testvoltage. The iterative procedure in IEC 60060-1:2010 recalculates U 50 and U 10 until both converge to constantvalues, and hence reduce the error caused by the initially estimated U 50.

Atmospheric corrections lately attracted more attention due to the introduction of UHV AC and DC systems,of which some are located at high altitudes. The corrections are necessarily large for test voltages to be usedat high altitudes.

The examples in section 4.4.4 give the calculated correction factors using the standard procedure, where ameasured 50 % probability disruptive-discharge voltage in given conditions may be converted to the valuewhich would have been obtained under standard reference atmospheric conditions. Calculations by theconverse iterative procedure are also given to show the differences of the calculated correction factors tothose obtained in accordance with standards used for insulation coordination or apparatus standards.

It should be noted that the iterative procedure always leads to the lowest value in altitude correction factor(=1/K t), which are closer to the altitude correction factors calculated using the methods of the otherstandards, IEC 60071-2:1996 (Insulation coordination: Application Guide) and IEC 61869-1:2007 (InstrumentTransformers, General requirements). It should also be noted that for lightning impulse tests on someequipment, such as instrument transformers, no atmospheric corrections are to be applied according to therelevant standards.

4.4.4 Iterative procedure for determini ng atmosph eric correct ion factor at high altit udes

When the converse procedure is used for determining the atmospheric correction factor, where a withstandtest voltage is specified for standard reference atmosphere (Clause 4.3.1, IEC 60060-1:2010) and must beconverted into the equivalent value under the test conditions, an iterative procedure described inIEC 60060-1:2010 may need to be used. The application of the iterative procedure is necessary if thecorrection factor K t is lower than e.g. 0.95, for reducing the error of calculated correction factor for a highaltitude test site. In the iterative calculation procedure, the correction factor K t is calculated by iteration until itconverges to be within a predetermined limit, i.e.:

(i) = 1.1 × (i-1) = 1.1 × (i-1) × with being the specified test voltage.The iteration is continued until

1 , where i is the number of the iteration.Figure 5 to Figure 8 show atmospheric correction factors K t, calculated with the non-iterative procedure andthe iterative procedure, keeping the humidity correction factor to unity. These results are then essentially theinverse of the values of the altitude correction factor used in the insulation coordination standard(IEC 60071-2:1996) and some equipment standards such as IEC 61689-1.

The calculations were performed for AC, lightning impulse (LI) and switching impulse (SI) voltages with gapdistances of 3800 mm and 2000 mm and for DC voltage for 4000 mm and 2000 mm. The first number in thelegend denotes the gap distance with the 2nd number being the specified test voltage level in kilovolt. For the2000 mm gap, 806 kV is the AC flashover voltage level (the voltage level of assured disruptive discharge)and 651 kV is the AC withstand test voltage level. For the 3800 mm gap, 1245 kV is the AC flashover voltagelevel and 1047 kV is the AC withstand test voltage level.

For AC (Figure 5) and switching impulse (Figure 8) voltages, the iterative procedure yields higher values ofthe atmospheric correction factor than the standard procedure. The atmospheric correction factors for tests


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at the withstand level are larger than those for tests at the flashover level. The atmospheric correction factorswith the larger gap distance (3800 mm) are larger than those with the smaller gap (2000 mm).

For DC voltages (Figure 6) the iterative procedure yields larger altitude correction factor values for tests onlyat the withstand voltage level. The other results are very similar.

For lightning impulse voltages (Figure 7) the correction factors are the same irrespective of the altitude and

the calculation procedures for the smaller gap (2000 mm). For the larger gap (3800 mm), the correctionfactors for tests at the flashover voltage level are the same for both calculation procedures and are the sameas those of the smaller gap, only the correction factor at the withstand level obtained from the iterativeprocedure is larger.


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Figure 5: Atmospheric correction factors K t for AC voltage, obtained with the non-iterative

procedure and the iterative procedure.


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Figure 6: Atmospheric correction factors K t for DC voltage, obtained with the non-iterativeprocedure and the iterative procedure.


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Figure 7: Atmospheric correction factors K t for lightning impulse voltage, obtained with the

non-iterative procedure and the iterative procedure.


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Figure 8: Atmospheric correction factors K t for switching impulse voltage, obtained with the

non-iterative procedure and the iterative procedure.


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In most cases of applying atmospheric corrections, these are used for withstand testing of either AC, DC,lightning impulse or switching impulse of equipment at an altitude less than the altitude the equipment will beused in service. Here the iterative procedure always leads to the lowest altitude correction factor (1/K t).

4.4.5 Atmo spheric correct ions for AC volt ages

Note 3 of clause 4.3.3 “Application of correction factors” in IEC 60060-1:2010 points out that the peak valuehas to be used in correcting power frequency voltages, because the discharge behavior is based on thepeak value. Apparatus standards however specify RMS values based on the system voltage for which theequipment is to be used. If the specified RMS test voltage is not converted into the peak value, thecalculation of the correction factors becomes erroneous and not consistent.

Figure 9Figure 9 shows the effect of using the RMS value instead of the peak value when calculatingatmospheric corrections using the standard procedure. The RMS voltage value of 570 kV and 460 kVcorrespond to a peak voltage value of 806 kV and 651 kV respectively for the 2000mm gap, 880 kV and 740kV correspond to 1245 kV and 1047 kV respectively for the 3800 mm gap.

For smaller gaps the atmospheric correction curves for the peak values of withstand and breakdown voltageconverge at an altitude of 2500 m, but the difference between the correction factor using the peak value andthe correction factor using the RMS value can be as large as 19 % at 4000 m. For larger gaps the correction

factors become much larger when using the RMS value. The difference can be as large as 30 % as is thecase for the 3800 mm gap at withstand level for an altitude of 4000 m; even at an altitude of 2000 m thedifference is 8 %. For the iterative procedure at withstand level the trend is similar with smaller deviations,from 6 % at 2000 m to 12 % at 4000 m.

This is an important note in IEC 60060-1:2010, which can easily be missed.


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Figure 9: Atmospheric correction factorsK

t for AC voltage, comparison of Peak voltage toRMS voltage.


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4.5 Requi r ements for measurement sys tems

4.5.1 Stru ct ure of IEC 60060-2:2010

Requirements for measurement systems are specified in IEC 60060-2:2010. The main structure ofIEC 60060-2:2010 is similar to that of the previous edition IEC 60060-2:1994, although there have been

significant additions of contents to some clauses, for example, the additional requirements in Clause 5 onestimation of measurement uncertainties. Table 6 compares the main clauses of Edition 2.0 and Edition 3.0of IEC 60060-2. Term “AMS” in Table 6 denotes “Approved Measuring System” and “RMS” denotes“Reference Measuring System”. Both terms are defined in IEC 60060-2.

Table 6: Comparison of the main clauses of editions 2.0 (1994) and 3.0 (2010)

IEC 60060-2:1994, Ed. 2.0 IEC 60060-2:2010, Ed. 3.0

3. Definitions and symbols

4. Qualification of AMS

3. Terms and definitions

4. Qualification of AMS

5. Acceptance tests on components

6. Performance tests on AMS

5. Tests and test requirements for AMS,

including the uncertainty estimation

7. Measurement of DC voltage

8. Measurement of AC voltage

9. Measurement of LI voltage

10. Measurement of SI voltage

6. Measurement of DC voltage

7. Measurement of AC voltage

8. Measurement of LI voltage

9. Measurement of SI voltage

11. Measurement of impulse currents Transferred to IEC 62475

12. Reference Measuring Systems (RMS) 10. Reference Measuring Systems (RMS)

Annex A: Accreditation systems

Annex B: Record of performance

Transferred to 4.1 and 4.4

Amendment 1/Annex H: Uncertainty estimation Annex A: Uncertainty estimation (GUM)

Annex B: Example for uncertainty calculation

Annex C: Step response measurement Annex C: Step response measurement

Annex D: Convolution

Annex D: Temperature rise of resistors

Annex E: RMS bibliography

Annex F: Summary of tests

Tables transferred to chapters 6 to 9

The two old clauses on acceptance and performance tests have been combined into the new Clause 5. Thisnew clause gives detailed requirements on the determination of the measuring uncertainty. The summarytables for approving the HV measuring systems of different types of test voltages have been transferred fromthe old Annex F to the relevant clauses on individual voltage types. The old chapter on impulse current

measurement has been transferred to the new standard IEC 62475:2010.The content of six of the seven annexes of the old standard have been, where appropriate, transferred to themain text of the new standard. Only the annex on step response measurement (Annex F, now C) remainsand is complemented by an informative annex (Annex D) on convolution.

4.5.2 Calibr ation and estimatio n of measuring uncertaint y

It is strongly expressed in IEC 60060-2:2010 that the preferred calibration procedure is the comp

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