IEC 60071-2

Coordination de l'isolement - Partie 2: Guide d'application

La CEI 60071-2:1996 donne des recommandations pour la détermination des tensions de tenue assignées pour les gammes I et II de la CEI 60071-1 et justifie l'association de ces valeurs assignées avec les valeurs normalisées des tensions les plus élevées pour le matériel. Traite de l'isolement phase-terre, entre phases et longitudinal des reseaux triphasés de tension nominale supérieure à 1 kV. Elle a le statut d'une norme horizontale conformément au Guide 108 de la CEI.[
]Cette version française découle de la publication d'origine bilingue dont les pages anglaises ont été supprimées. Les numéros de page manquants sont ceux des pages supprimées.

Insulation co-ordination - Part 2: Application guide

General Information

• Status: Not Published
• ICS: 29.080.30 - Insulation systems
• Technical Committee: ISO/IEC JTC 1/SC 28 - Office equipment
• Current Stage: PPUB - Publication issued
• Start Date: 19-Dec-1996
• Ref Project: SIST IEC 60071-2:1996 - Coordination de l'isolement - Partie 2: Guide d'application

Overview

IEC 60071-2, "Insulation co‑ordination - Part 2: Application guide" (3rd ed., 1996) is an application guide that supports IEC 60071-1 for the selection and justification of rated withstand voltages for three‑phase electrical systems with nominal voltages above 1 kV. It explains how to associate the required insulation levels with the standardized highest voltages for equipment (Ranges I and II), and serves as a horizontal guidance document (IEC Guide 108). The guide focuses on insulation coordination for phase‑to‑earth, phase‑to‑phase and longitudinal insulation and outlines procedures for determining representative service stresses and required testing levels.

Key topics

• Representative voltage stresses in service: origin, classification and representative overvoltages (lightning, switching, slow‑front, transferred).
• Coordination withstand voltage: insulation strength characteristics, performance criteria and co‑ordination procedures.
• Required withstand voltage: atmospheric corrections, safety factors and statistical considerations.
• Standard withstand voltages and test procedures: conversion factors and type‑test correlations.
• Special considerations: detailed guidance for overhead lines (lightning, switching, line energization) and substations (layout, transferred overvoltages).
• Annexes and examples: air clearances, temporary overvoltages from earth faults, Weibull statistics, transformer transferred voltages, lightning, gap factors and worked insulation‑coordination examples.

Applications

IEC 60071-2 is a practical guide for:

• Determining rated impulse and power‑frequency withstand voltages for equipment and installations.
• Selecting and applying surge arresters, specifying lead lengths and connection practices.
• Defining minimum air‑clearances and creepage distances to meet specified impulse withstand voltages.
• Evaluating the risk of flashover or dielectric failure under realistic service overvoltages (lightning, switching, earth faults).
• Supporting design and specification of substations, overhead transmission lines and high‑voltage equipment operating above 1 kV.

Who should use this standard

• Electrical design engineers and system planners in utilities and industry
• Substation and transmission line designers
• Manufacturers of switchgear, transformers and surge arresters
• Test laboratories and product committees establishing dielectric tests
• Standards bodies and consultants working on insulation coordination

Related standards

Important normative references include:

• IEC 60071-1 (Definitions, principles and rules)
• IEC 60‑1 (High‑voltage test techniques)
• IEC 99 series (surge arresters)
• IEC 56 (circuit‑breakers), IEC 507/815 (insulators/pollution)

Keywords: IEC 60071-2, insulation coordination, withstand voltage, overvoltages, surge arrester, phase-to-earth, phase-to-phase, substations, overhead lines.

Standards Content (Sample)

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Insulation co-ordination - Part 2: Application guideCoordination de l'isolement - Partie 2: Guide d'application29.080.01VSORãQRElectrical insulation in generalICS:Ta slovenski standard je istoveten z:IEC 60071-2SIST IEC 60071-2:1996en01-junij-1996SIST IEC 60071-2:1996SLOVENSKI
STANDARD
NORMEINTERNATIONALECEIIECINTERNATIONALSTANDARD71-2Troisième éditionThird edition1996-12Ó CEI 1996
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¾ Copyright - all rights reservedAucune partie de cette publication ne peut être reproduite niutilisée sous quelque forme que ce soit et par aucun procédé,électronique ou mécanique, y compris la photocopie et lesmicrofilms, sans l'accord écrit de l'éditeur.No part of this publication may be reproduced or utilizedin any form or by any means, electronic or mechanical,including photocopying and microfilm, without
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catalogueXFCoordination de l’isolement –Partie 2:Guide d’applicationInsulation co-ordination –Part 2:Application guideSIST IEC 60071-2:1996

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© IEC: 1996– 3 –CONTENTSPageFOREWORD.9Clause1General.111.1Scope.111.2Normative references.111.3List of symbols and definitions.132Representative voltage stresses in service.212.1Origin and classification of voltage stresses.212.2Characteristics of overvoltage protective devices.232.3Representative voltages and overvoltages.273Co-ordination withstand voltage.573.1Insulation strength characteristics.573.2Performance criterion.653.3Insulation co-ordination procedures.674Required withstand voltage.834.1General remarks.834.2Atmospheric correction.834.3Safety factors.875Standard withstand voltage and testing procedures.915.1General remarks.915.2Test conversion factors.935.3Determination of insulation withstand by type tests.956Special considerations for overhead lines.1036.1General remarks.1036.2Insulation co-ordination for operating voltages and temporary overvoltages.1036.3Insulation co-ordination for slow-front overvoltages.1056.4Insulation co-ordination for lightning overvoltages.1057Special considerations for substations.1077.1General remarks.1077.2Insulation co-ordination for overvoltages.111Tables1Recommended creepage distances.712Test conversion factors for range I, to convert required switching impulses withstand voltages to short-duration power-frequency and lightning impulse withstand voltages.933Test conversion factors for range II to convert required short-duration power-frequencywithstand voltages to switching impulse withstand voltages.954Selectivity of test procedures B and C of IEC 60-1.99A.1Correlation between standard lightning impulse withstand voltages and minimum airclearances.119A.2Correlation between standard switching impulse withstand voltages andminimum phase-to-earth air clearances.121A.3Correlation between standard switching impulse withstand voltages andminimum phase-to-phase air clearances.121C.1Breakdown voltage versus cumulative flashover probability – Single insulationand 100 parallel insulations.135SIST IEC 60071-2:1996

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© IEC: 1996– 5 –F.1Corona damping constant Kco.175F.2Factor A for various overhead lines.185G.1Typical gap factors K for switching impulse breakdown phase-to-earth.195G.2Gap factors for typical phase-to-phase geometries.197H.1Summary of minimum required withstand voltages obtained for example H.1.1.213H.2Summary of required withstand voltages obtained for example H.1.2.217H.3Values related to the insulation co-ordination procedure for example H.3.249Figures1Range of 2 % slow-front overvoltages at the receiving end due to line energizationand re-energization.392Ratio between the 2 % values of slow-front overvoltages phase-to-phase andphase-to-earth.413Diagram for surge arrester connection to the protected object.554Distributive discharge probability of self-restoring insulation describedon a linear scale.735Disruptive discharge probability of self-restoring insulation describedon a Gaussian scale.736Evaluation of deterministic co-ordination factor Kcd.757Evaluation of the risk of failure.778Risk of failure of external insulation for slow-front overvoltages as a function ofthe statistical co-ordination factor Kcs.819Dependence of exponent m on the co-ordination switching impulse withstand voltage.8710Probability P of an equipment to pass the test dependent on the difference Kbetween the actual and the rated impulse withstand voltage.9911Example of a schematic substation layout used for the overvoltage stress location(see 7.1).107B.1Earth-fault factor k on a base of X0/X1 for R1/X1 = R = 0.125B.2Relationship between R0/X1 and X0/X1 for constant values of earth-fault factor kwhere R1 = 0.125B.3Relationship between R0/X1 et X0/X1 for constant values of earth-fault factor kwhere R1 = 0,5 X1.127B.4Relationship between R0/X1 et X0/X1 for constant values of earth-fault factor kwhere R1 = X1.127B.5Relationship between R0/X1 et X0/X1 for constant values of earth-fault factor kwhere R1 = 2X1.129C.1Conversion chart for the reduction of the withstand voltage due to placing insulationconfigurations in parallel.139D.1Example for bivariate phase-to-phase overvoltage curves with constant probabilitydensity and tangents giving the relevant 2 % values.151D.2Principle of the determination of the representative phase-to-phase overvoltage Upre.153D.3Schematic phase-phase-earth insulation configuration.153D.4Description of the 50 % switching impulse flashover voltage of a phase-phase-earthinsulation.155SIST IEC 60071-2:1996

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© IEC: 1996– 7 –D.5Inclination angle of the phase-to-phase insulation characteristic in range b dependenton the ratio of the phase-phase clearance D to the height Ht above earth.157E.1Distributed capacitances of the windings of a transformer and the equivalent circuitdescribing the windings.169E.2Values of factor J describing the effect of the winding connections on the inductivesurge transference.171AnnexesAClearances in air to assure a specified impulse withstand voltage installation.115BDetermination of temporary overvoltages due to earth faults.123CWeibull probability distributions.131DDetermination of the representative slow-front overvoltage due to line energizationand re-energization.141ETransferred overvoltages in transformers.159FLightning overvoltages.173GCalculation of air gap breakdown strength from experimental data.187HExamples of insulation co-ordination procedure.199JBibliography.251SIST IEC 60071-2:1996

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© IEC: 1996– 9 –INTERNATIONAL ELECTROTECHNICAL COMMISSION––––––––––INSULATION CO-ORDINATION –Part 2: Application guideFOREWORD1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprisingall national electrotechnical committees (IEC National Committees). The object to the IEC is to promoteinternational cooperation on all questions concerning standardization in the electrical and electronic fields. Tothis end and in addition to other activities, the IEC publishes International Standards. Their preparation isentrusted to technical committees; any IEC National Committee interested in the subject dealt with mayparticipate in this preparatory work. International, governmental and non-governmental organizations liaisingwith the IEC also participate in this preparation. The IEC collaborates closely with the International Organizationfor Standardization (ISO) in accordance with conditions determined by agreement between the twoorganizations.2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible aninternational consensus of opinion on the relevant subjects since each technical committee has representationfrom all interested National Committees.3) The documents produced have the form of recommendations for international use and are published in the formof standards, technical reports or guides and they are accepted by the National Committees in that sense.4) In order to promote international unification, IEC National Committees undertake to apply IEC InternationalStandards transparently to the maximum extent possible in their national and regional standards. Anydivergence between the IEC Standard and the corresponding national or regional standard shall be clearlyindicated in the latter.5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for anyequipment declared to be in conformity with one of its standards.6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subjectof patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.International Standard IEC 71-2, has been prepared by IEC technical committee 28: Insulationco-ordination.This third edition cancels and replaces the second edition published in 1976 and constitutes atechnical revision.The text of this standard is based on the following documents:FDISReport on voting28/115/FDIS28/117/RVDFull information on the voting for the approval of this standard can be found in the report onvoting indicated in the above table.Annex A forms an integral part of this standard.Annexes B to J are for information only.SIST IEC 60071-2:1996

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© IEC: 1996– 11 –INSULATION CO-ORDINATION –Part 2: Application guide1 General1.1 ScopeThis part of IEC 71 constitutes an application guide and deals with the selection of insulationlevels of equipment or installations for three-phase electrical systems. Its aim is to giveguidance for the determination of the rated withstand voltages for ranges I and II of IEC 71-1and to justify the association of these rated values with the standardized highest voltages forequipment.This association is for insulation co-ordination purposes only. The requirements for humansafety are not covered by this application guide.It covers three-phase systems with nominal voltages above 1 kV. The values derived orproposed herein are generally applicable only to such systems. However, the conceptspresented are also valid for two-phase or single-phase systems.It covers phase-to-earth, phase-to-phase and longitudinal insulation.This application guide is not intended to deal with routine tests. These are to be specified bythe relevant product committees.The content of this guide strictly follows the flow chart of the insulation co-ordination processpresented in figure 1 of IEC 71-1. Clauses 2 to 5 correspond to the squares in this flow chartand give detailed information on the concepts governing the insulation co-ordination processwhich leads to the establishment of the required withstand levels.The guide emphasizes the necessity of considering, at the very beginning, all origins, allclasses and all types of voltage stresses in service irrespective of the range of highest voltagefor equipment. Only at the end of the process, when the selection of the standard withstandvoltages takes place, does the principle of covering a particular service voltage stress by astandard withstand voltage apply. Also, at this final step, the guide refers to the correlationmade in IEC 71-1 between the standard insulation levels and the highest voltage forequipment.The annexes contain examples and detailed information which explain or support the conceptsdescribed in the main text, and the basic analytical techniques used.1.2 Normative referencesThe following normative documents contain provisions which, through reference in this text,constitute provisions of this part of IEC 71. At the time of publication, the editions indicatedwere valid. All normative documents are subject to revision, and parties to agreements basedon this part of IEC 71 are encouraged to investigate the possibility of applying the most recenteditions of the normative documents indicated below. Members of IEC and ISO maintainregisters of currently valid International Standards.SIST IEC 60071-2:1996

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© IEC: 1996– 13 –IEC 56: 1987, High-voltage alternating-current circuit-breakersIEC 60-1: 1989, High-voltage test techniques – Part 1: General definitions and test requirementsIEC 71-1: 1993, Insulation co-ordination – Part 1: Definitions, principles and rulesIEC 99-1: 1991, Surge arresters – Part 1: Non-linear resistor type gapped surge arresters fora.c. systemsIEC 99-4: 1991, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.systemsIEC 99-5: 1996, Surge arresters – Part 5: Selection and application recommendations –Section 1: GeneralIEC 505: 1975, Guide for the evaluation and identification of insulation systems of electricalequipmentIEC 507: 1991, Artificial pollution test on high-voltage insulators to be used on a.c. systemsIEC 721-2-3: 1987, Classification of environmental conditions – Part 2: Environmentalconditions appearing in nature – Air pressureIEC 815: 1986, Guide for the selection of insulators in respect of polluted conditions1.3 List of symbols and definitionsFor the purpose of this part of IEC 71, the following symbols and definitions apply. Thesymbol is followed by the unit to be normally considered, dimensionless quantities beingindicated by (-).Some quantities are expressed in p.u. A per unit quantity is the ratio of the actual value of anelectrical parameter (voltage, current, frequency, power, impedance, etc.) to a given referencevalue of the same parameter.A(kV)parameter characterizing the influence of the lightning severity for theequipment depending on the type of overhead line connected to it.a1(m)length of the lead connecting the surge arrester to the line.a2(m)length of the lead connecting the surge arrester to earth.a3(m)length of the phase conductor between the surge arrester and the protectedequipment.a4(m)length of the active part of the surge arrester.B(-)factor used when describing the phase-to-phase discharge characteristic.Ce(nF)capacitance to earth of transformer primary windings.Cs(nF)series capacitance of transformer primary windings.C2(nF)phase-to-earth capacitance of the transformer secondary winding.C12(nF)capacitance between primary and secondary windings of transformers.C1in(nF)equivalent input capacitance of the terminals of three-phase transformers.C2in(nF)equivalent input capacitance of the terminals of three-phase transformers.C3in(nF)equivalent input capacitance of the terminals of three-phase transformers.c(m/µs)velocity of light.SIST IEC 60071-2:1996

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© IEC: 1996– 15 –cf(p.u.)coupling factor of voltages between earth wire and phase conductor ofoverhead lines.E0(kV/m)soil ionization gradient.Ffunction describing the cumulative distribution of overvoltage amplitudes,where F(U) = 1 – P(U). See annex C.3.ffunction describing the probability density of overvoltage amplitudes.g(-)ratio of capacitively transferred surges.H(m)altitude above sea-level.h(-)power-frequency voltage factor for transferred surges in transformers.Ht(m)height above ground.I(kA)lightning current amplitude.lg(kA)limit lightning current in tower footing resistance calculation.J(-)winding factor for inductively transferred surges in transformers.K(-)gap factor taking into account the influence of the gap configuration on thestrength.Ka(-)atmospheric correction factor. [3.28 of IEC 71-1]Kc(-)co-ordination factor. [3.25 of IEC 71-1]Ks(-)safety factor. [3.29 of IEC 71-1]Kcd(-)deterministic co-ordination factor.Kco(µs/(kVm))corona damping constant.Kcs(-)statistical co-ordination factor.Kf+f(-)gap factor for fast-front impulses of positive polarity.Kf-f(-)gap factor for fast-front impulses of negative polarity.k(-)earth-fault factor. [3.15 of IEC 71-1]L(m)separation distance between surge arrester and protected equipment.La(m)overhead line length yielding to an outage rate equal to the acceptable one(related to Ra).Lt(m)overhead line length for which the lightning outage rate is equal to the adoptedreturn rate (related to Rt).Lsp(m)span length.M(-)number of insulations in parallel considered to be simultaneously stressed byan overvoltage.m(-)exponent in the atmospheric correction factor formula for external insulationwithstand.N(-)number of conventional deviations between U50 and U0 of a self-restoringinsulation.n(-)number of overhead lines considered connected to a station in the evaluationof the impinging surge amplitude.P(%)probability of discharge of a self-restoring insulation.Pw(%)probability of withstand of self-restoring insulation.q(-)response factor of transformer windings for inductively transferred surges.R(-)risk of failure (failures per event).Ra(1/a)acceptable failure rate for apparatus. For transmission lines, this parameter isnormally expressed in terms of (1/a)/100 km.SIST IEC 60071-2:1996

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© IEC: 1996– 17 –Rhc(W)high current value of the tower footing resistance.Rkm(1/(m.a))overhead line outage rate per year for a design corresponding to the firstkilometre in front of the station.Rlc(W)low current value of the tower footing resistance.Rp(1/a)shielding penetration rate of overhead lines.Rsf(1/a)shielding failure flashover rate of overhead lines.Rt(1/a)adopted overvoltage return rate (reference value).Ru(kV)radius of a circle in the U+/U– plane describing the phase-phase-earth slow-front overvoltages.R0(W)zero sequence resistance.R1(W)positive sequence resistance.R2(W)negative sequence resistance.S(kV/µs)steepness of a lightning surge impinging on a substation.Se(kV)conventional deviation of phase-to-earth overvoltage distribution.Sp(kV)conventional deviation of phase-to-phase overvoltage distribution.Srp(kV/µs)representative steepness of a lightning impinging surge.se(-)normalized value of the conventional deviation Se (Se referred to Ue50).sp(-)normalized value of the conventional deviation Sp (Sp referred to Up50).T(µs)travel time of a lightning surge.U(kV)amplitude of an overvoltage (or of a voltage).U+(kV)positive switching impulse component in a phase-to-phase insulation test.U–(kV)negative switching impulse component in a phase-to-phase insulation test.U0(kV)truncation value of the discharge probability function P(U) of a self-restoringinsulation: P (U £ U0) = 0.U0+(kV)equivalent positive phase-to-earth component used to represent the mostcritical phase-to-phase overvoltage.U1e(kV)temporary overvoltage to earth at the neutral of the primary winding of atransformer.U2e(kV)temporary overvoltage to earth at the neutral of the secondary winding of atransformer.U2N(kV)rated voltage of the secondary winding of a transformer.U10(kV)value of the 10 % discharge voltage of self-restoring insulation. This value isthe statistical withstand voltage of the insulation defined in 3.23 b) of IEC 71-1.U16(kV)value of the 16 % discharge voltage of self-restoring insulation.U50(kV)value of the 50 % discharge voltage of self-restoring insulation.U50M(kV)value of the 50 % discharge voltage of M parallel self-restoring insulations.U50RP(kV)value of the 50 % discharge voltage of a rod-plane gap.Uc+(kV)positive component defining the centre of a circle which describes the phase-phase-earth slow-front overvoltages.Uc–(kV)negative component defining the centre of a circle which describes the phase-phase-earth slow-front overvoltages.SIST IEC 60071-2:1996

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© IEC: 1996– 19 –Ucw(kV)co-ordination withstand voltage of equipment. [3.24 of IEC 71-1]Ue(kV)amplitude of a phase-to-earth overvoltage.Uet(kV)truncation value of the cumulative distribution F (Ue) of the phase-to-earthovervoltages: F (Ue ³ Uet) = 0; see annex C.3.Ue2(kV)value of the phase-to-earth overvoltage having a 2 % probability of beingexceeded: F (Ue ³ Ue2) = 0,02; see annex C.3.Ue50(kV)50 % value of the cumulative distribution F (Ue) of the phase-to-earthovervoltages; see annex C.3.UI(kV)amplitude of the impinging lightning overvoltage surge.Um(kV)highest voltage for equipment. [3.10 of IEC 71-1]Up(kV)amplitude of a phase-to-phase overvoltage.Up2(kV)value of the phase-to-phase overvoltage having a 2 % probability of beingexceeded: F (Up ³ Up2) = 0,02; see annex C.3.Up50(kV)50 % value of the cumulative distribution F (Up) of the phase-to-phaseovervoltages; see annex C.3.Us(kV)highest voltage of a system. [3.9 of IEC 71-1]Uw(kV)standard withstand voltage.Upl(kV)lightning impulse protective level of a surge arrester. [3.21 of IEC 71-1]Ups(kV)switching impulse protective level of a surge arrester. [3.21 of IEC 71-1]Upt(kV)truncation value of the cumulative distribution F (Up) of the phase-to-phaseovervoltages: F (Up ³ Upt) = 0; see annex C.3.Urp(kV)amplitude of the representative overvoltage. [3.19 of IEC 71-1]Urw(kV)required withstand voltage. [3.27 of IEC 71-1]UT1(kV)overvoltage applied at the primary winding of a transformer which produces (bytransference) an overvoltage on the secondary winding.UT2(kV)overvoltage at the secondary winding of a transformer produced (bytransference) by an overvoltage applied on the primary winding.u(p.u.)per unit value of the amplitude of an overvoltage (or of a voltage) referred toUs 23.w(-)ratio of transformer secondary to primary phase-to-phase voltage.X(m)distance between struck point of lightning and substation.Xp(km)limit overhead line distance within which lightning events have to beconsidered.XT(km)overhead line length to be used in simplified lightning overvoltage calculations.X0(W)zero sequence reactance of a system.X1(W)positive sequence reactance of a system.X2(W)negative sequence reactance of a system.x(-)normalized variable in a discharge probability function P(U) of a self-restoringinsulation.xM(-)normalized variable in a discharge probability function P(U) of M parallel self-restoring insulations.Z(kV)conventional deviation of the discharge probability function P(U) of a self-restoring insulation.Z0(W)zero sequence impedance.SIST IEC 60071-2:1996

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© IEC: 1996– 21 –Z1(W)positive sequence impedance.Z2(W)negative sequence impedance.Ze(W)surge impedance of the overhead line earth wire.Zl(W)surge impedance of the overhead line.ZM(kV)conventional deviation of the discharge probability function P(U) of M parallelself-restoring insulations.Zs(W)surge impedance of the substation phase conductor.z(-)normalized value of the conventional deviation Z referred to U50.a(-)ratio of the negative switching impulse component to the sum of bothcomponents (negative + positive) of a phase-to-phase overvoltage.b(kV)scale parameter of a Weibull cumulative function.d(kV)truncation value of a Weibull cumulative function.FGaussian integral function.f(-)inclination angle of a phase-to-phase insulation characteristic.g(-)shape parameter of a Weibull-3 cumulative function.s(p.u.)per unit value of the conventional deviation (Se or Sp) of an overvoltagedistribution.r(Wm)soil resistivity.t(µs)tail time constant of a lightning overvoltage due to back-flashovers onoverhead lines.2 Representative voltage stresses in service2.1 Origin and classification of voltage stressesIn IEC 71-1 the voltage stresses are classified by suitable parameters such as the duration ofthe power-frequency voltage or the shape of an overvoltage according to their effect on theinsulation or on the protective device. The voltage stresses within these classes have severalorigins:–continuous (power-frequency) voltages: originate from the system operation undernormal operating conditions;–temporary overvoltages: they can originate from faults, switching operations such as loadrejection, resonance conditions, non-linearities (ferroresonances) or by a combination ofthese;–slow-front overvoltages: they can originate from faults, switching operations or directlightning strokes to the conductors of overhead lines;–fast-front overvoltages: they can originate from switching operations, lightning strokes orfaults;–very-fast-front overvoltages: they can originate from faults or switching operations in gas-insulated substations (GIS);–combined overvoltages: they may have any origin mentioned above. They occur betweenthe phases of a
system (phase-to-phase), or on the same phase between separated partsof a system (longitudinal).All the preceding overvoltage stresses except combined overvoltages are discussed asseparate items under 2.3. Combined overvoltages are discussed where appropriate within oneor more of these items.SIST IEC 60071-2:1996

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© IEC: 1996– 23 –In all classifications of voltage stresses, transference through transformers should be takeninto account (see annex E).In general, all classes of overvoltages may exist in both voltage ranges I and II. However,experience has shown that certain voltage classifications are of more critical importance in aparticular voltage range; this will be dealt with in this guide. In any case, it should be noted thatthe best knowledge of the stresses (peak values and shapes) is obtained with detailed studiesemploying adequate models for the system and for the characteristics of the overvoltagelimiting devices.2.2 Characteristics of overvoltage protective devices2.2.1 General remarksTwo types of standardized protective devices are considered:–non-linear resistor-type surge arresters with series gaps;–metal-oxide surge arresters without gaps.In addition, spark gaps are taken into account as an alternative overvoltage limiting device,although standards are not available within IEC. When other types of protective devices areused, their protection performance shall be given by the manufacturer or established by tests.The choice among protective devices, which do not provide the same degree of protection,depends on various factors, e.g. the importance of the equipment to be protected, theconsequence of an interruption of service, etc. Their characteristics will be considered from thepoint of view of insulation co-ordination and their effects will be discussed under the clausesdealing with the various overvoltage classes.The protective devices shall be designed and installed to limit the magnitudes of overvoltagesagainst which they protect equipment so that the voltage at the protective device and theconnecting leads during its operation do not exceed an acceptable value. A primary point isthat the voltage produced across the terminals of the arrester at any moment prior to andduring its operation must be considered in the determination of the protection characteristics.2.2.2 Non-linear resistor-type surge arresters with series gapsWhere the surge arrester comprises a silicon carbide non-linear resistor with series gap, thecharacteristics are given in IEC 99-1. However, where the arrester consists of a metal-oxidenon-linear resistor with series gap, the characteristics may differ from those given in IEC 99-1.The selection of arresters will be dealt with in IEC 99-5.2.2.2.1 Protection characteristics related to fast-front overvoltagesThe protection characteristics of a surge arrester are described by the following voltages (seetable 8 of IEC 99-1):–the sparkover voltage for a standard full lightning impulse;–the residual voltage at the selected nominal discharge current;–the front-of-wave sparkover voltage.SIST IEC 60071-2:1996

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© IEC: 1996– 25 –The lightning impulse protective level is taken as the highest of the following values:–maximum sparkover voltage with 1,2/50 µs impulse;–maximum residual voltage at the selected nominal discharge current.This evaluation of the protective level gives a value representing a generally acceptableapproximation. For more information on wave-front protection by surge arresters, referenceshould be made to IEC 99-1.NOTE – Traditionally, the front-of-wave sparkover voltage divided by 1,15 was included in the determination ofthe lightning impulse protective level. As the factor of 1,15 is technically justified only for oil-paper insulation oroil-immersed insulation like transformers, its application to other type of equipment may result in reducedinsulation margin design. Therefore, this alternative has been omitted in the determination of the lightningimpulse protective level.2.2.2.2 Protection characteristics related to slow-front overvoltagesThe protection of a surge arrester is characterized by the sparkover voltages for the switchingimpulse shapes specified in 8.3.5 of IEC 99-1.The switching impulse protective level of a surge arrester is the maximum sparkover voltagefor these impulse shapes.If the arrester contains active gaps the total surge arrester voltage exhibited by the surgearrester when discharging switching surges shall be requested from the manufacturer, becauseit may be higher than the sparkover voltage.2.2.3 Metal oxide surge arresters without gapsThe definition of such surge arresters and their characteristics are given in IEC 99-4.2.2.3.1 Protection characteristics related to fast-front overvoltagesThe protection of a metal-oxide surge arrester is characterized by the following voltages:–the residual voltage at the selected nominal discharge current;–the residual voltage at steep current impulse.The lightning impulse protective level is taken for insulation co-ordination purposes as themaximum residual voltage at the selected nominal discharge current.2.2.3.2 Protection characteristics related to slow-front overvoltagesThe protection is characterized by the residual voltage at the specified switching impulsecurrents.The switching impulse protective level is taken for insulation co-ordination purposes as themaximum residual voltage at the specified switching impulse currents.The evaluation of protective levels gives a value representing a generally acceptableapproximation. For a better definition of the protection performance of metal-oxide arresters,reference should be made to IEC 99-4.SIST IEC 60071-2:1996

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© IEC: 1996– 27 –2.2.4 Spark gapsThe spark gap is a surge protective device which consists of an open air gap between theterminals of the protected equipment. Although
spark gaps are usually not applied in systemswith Um equal to or higher than 123 kV, they have proved satisfactory in practice in somecountries with moderate lightning activity on systems operating at voltages up to 420 kV. Theadjustment of the gap settings is often a compromise between absolute protection andconsequences of spark gap operation.The protection against overvoltages is characterized by the voltage-time characteristic of thegap for the various voltage shapes, the sparkover voltage dispersion and its polaritydependence. As no standard exists, these characteristics shall be requested from themanufacturer or established by the user on the basis of his own specifications.NOTE – The fast voltage collapse and possible consequences on the insulation of windings have to be takeninto account as an overvoltage characteristic.2.3 Representative voltages and overvoltages2.3.1 Continuous (power-frequency) voltagesUnder normal operating conditions, the power-frequency voltage can be expected to varysomewhat in magnitude and to differ from one point of the system to another. For purposes ofinsulation design and co-ordination, the representative continuous power-frequency voltageshall, however, be considered as constant and equal to the highest system voltage. In practice,up to 72,5 kV, the highest system voltage Us may be substantially lower than the highestvoltage for equipment Um, while, with the increase of the voltage, both values tend to becomeequal.2.3.2 Temporary overvoltagesTemporary overvoltages are characterized by their amplitudes, their voltage shape and theirduration. All parameters depend on the origin of the overvoltages, and amplitudes and shapesmay even vary during the overvoltage duration.For insulation co-ordination purposes, the representative temporary overvoltage is consideredto have the shape of the standard short duration (1 min) power-frequency voltage. Its amplitudemay be defined by one value (the assumed maximum), a set of peak values, or a completestatistical distribution of peak values. The selected amplitude of the representative temporaryovervoltage shall take into account:–the amplitude and duration of the actual overvoltage in service;–the amplitude/duration power frequency withstand characteristic of the insulationconsidered.If the latter characteristic is not known, as a simplification the amplitude may be taken as equalto the actual maximum overvoltage having an actual duration of less than 1 min in service, andthe duration may be taken as 1 min.In particular cases, a statistical co-ordination procedure may be adopted describing therepresentative overvoltage by an amplitude/duration distribution frequency of the temporaryovervoltages expected in service (see 3.3.1).2.3.2.1 Earth faultsA phase-to-earth fault may result in phase-to-earth overvoltages affecting the two otherphases. Temporary overvoltages between phases or across longitudinal insulation normally donot arise. The overvoltage shape is a power-frequency voltage.SIST IEC 60071-2:1996

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© IEC: 1996– 29 –The overvoltage amplitudes depend on the system neutral earthing and the fault location.Guidance for their determination is given in annex B. In normal system configurations, therepresentative overvoltage amplitude should be assumed equal to its maximum value.Abnormal system configurations, e.g. system parts with unearthed neutrals in a normallyearthed neutral system, should be dealt with separately, taking into account their probability ofoccurrence simultaneously with earth faults.The duration of the overvoltage corresponds to the duration of the fault (until fault clearing). Inearthed neutral systems it is generally less than 1 s. In resonant earthed neutral systems withfault clearing it is generally less than 10 s. In systems without earth-fault clearing the durationmay be several hours. In such cases, it may be necessary to define the continuous power-frequency voltage as the value of temporary overvoltage during earth fault.NOTE – Attention is drawn to the fact that the highest voltage at power-frequency which may appear on a soundphase during the occurrence of an earth fault depends not only on the earth-fault factor but also on the value ofthe operating voltage at the time of the fault which can be generally taken as the highest system voltage Us.2.3.2.2 Load rejectionPhase-to-earth and longitudinal temporary overvoltages due to load rejection depend on therejected load, on the system layout after disconnection and on the characteristics of thesources (short-circuit power at the station, speed and voltage regulation of the generators,etc.).The three phase-to-earth voltage rises are identical and, therefore, the same relativeovervoltages occur phase-to-earth and phase-to-phase. These rises may be especiallyimportant in the case of load rejection at the remote end of a long line (Ferranti effect) and theymainly affect the apparatus at the station connected on the source side of the remote opencircuit-breaker.The longitudinal temporary overvoltages depend on the degree of phase angle difference afternetwork separation, the worst possible situation being a phase opposition.NOTE – From the point of view of overvoltages, a distinction should be made between various types of systemlayouts. As examples, the following extreme cases may be considered:–systems with relatively short lines and high values of the short-circuit power at the terminal stations, wherelow overvoltages occur;–systems with long lines and low values of the short-circuit power at the generating site, which are usual inthe extra-high voltage range at their initial stage, and on which very high overvoltages may arise if a large loadis suddenly disconnected.In analysing temporary overvoltages, it is recommended that consideration be given to thefollowing (where the 1,0 p.u. reference voltage equals: 23sU):–in moderately extended systems, a full load rejection can give rise to phase-to-earthovervoltages with amplitude usually below 1,2 p.u. The overvoltage duration depends on theoperation of voltage-control equipment and may be up to several minutes;–in extended systems, after a full load rejection, the phase-to-earth overvoltages mayreach 1,5 p.u. or even more when Ferranti or resonance effects occur. Their duration maybe in the order of some seconds;–if only static loads are on the rejected side, the longitudinal temporary overvoltage isnormally equal to the phase-to-earth overvoltage. In systems with motors or generators onthe rejected side, a network separation can give rise to a longitudinal temporary overvoltagecomposed of two phase-to-earth overvoltage components in phase opposition, whosemaximum amplitude is normally below 2,5 p.u. (greater values can be observed forexceptional cases such as very extended high-voltage systems).SIST IEC 60071-2:1996

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© IEC: 1996– 31 –2.3.2.3 Resonance and ferroresonanceTemporary overvoltages due to these causes generally arise when circuits with large capacitiveelements (lines, cables, series compensated lines) and inductive elements (transformers, shuntreactors) having non-linear magnetizing characteristics are energized, or as a result of loadrejections.Temporary overvoltages due to resonance phenomena can reach extremely high values. Theyshall be prevented or limited by measures recommended in 2.3.2.6. They shall therefore notnormally be considered as the basis for the selection of the surge arrester rated voltage or forthe insulation design unless these remedial measures are not sufficient (see 2.3.2.7).2.3.2.4 Longitudinal overvoltages during synchronizationThe representative longitudinal temporary overvoltages are derived from the expectedovervoltage in service which has an amplitude equal to twice the phase-to-earth operatingvoltage and a duration of several seconds to some minutes.Furthermore, when synchronization is frequent, the probability of occurrence of an earth faultand consequent overvoltage shall be considered. In such cases the representative overvoltageamplitudes are the sum of the assumed maximum earth-fault overvoltage on one terminal andthe continuous operating voltage in phase opposition on the other.2.3.2.5 Combinations of temporary overvoltage originsTemporary overvoltages of different origin shall be treated as combined only after carefulexamination of their probability of simultaneous occurrence. Such combinations may lead tohigher arrester ratings with the consequence of higher protection and insulation levels; this istechnically and economically justified only if this probability of simultaneous occurrence issufficiently high.2.3.2.5.1 Earth fault with load rejectionThe combination earth fault with load rejection can exist when, during a fault on the line, theload side breaker opens first and the disconnected load causes a load rejection overvoltage inthe still faulted part of the system until the supply side circuit-breaker opens.The combination
earth fault with load rejection
can also exist when a large load is switched offand the temporary overvoltage due to this causes a subsequent earth fault on the remainingsystem. The probability of such an event, however, is small, when the overvoltages due to thechange of load are themselves small and a subsequent fault is only likely to occur in extremeconditions such as in heavy pollution.The combination can further occur as a
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