CURRENT INTERRUPTION USING HIGH VOLTAGE AIR-BREAK DISCONNECTORS

CURRENT INTERRUPTION USING HIGH VOLTAGE AIR-BREAK DISCONNECTORS PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op dinsdag 16 maart 24 om 16. uur door David Francis Peelo geboren te Dublin, Ierland

Dit proefschrift is goedgekeurd door de promotoren: prof.dr.ir. R.P.P. Smeets en prof.ir. L. van der Sluis CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Peelo, David F. Current interruption using high voltage air-break disconnectors / by David F. Peelo. Eindhoven : Technische Universiteit Eindhoven, 24. Proefschrift. ISBN 9-386-1533-7 NUR 959 Trefw.: hoogspanningsschakelaars / boogontladingen / elektrische doorslag. Subject headings: switchgear / switching / circuit-breaking arcs / electric breakdown.

A great flame follows a tiny spark. Dante Alighieri (1265 1321) To my wife Anna-Lena, my children Anna-Maria and Nicholas and my granddaughters Anna and Emelie i

SUMMARY High voltage air-break disconnectors are intended for use as isolators and as such are operated under energized conditions. The disconnectors will therefore be required to interrupt unloaded transformer magnetizing, capacitive or loop currents in air dependent on the circumstances and the practices of individual utilities. Each of these switching duties is unique in terms of the arc-circuit interaction, arc sustainability and arc extinction. This research investigates this arc behaviour with particular emphasis on the loop switching case. The interruption of unloaded transformer magnetizing currents of 1 A or less is mainly dependent on achieving a sufficient disconnector contact gap spacing to withstand the transient recovery voltage. For currents greater than 1 A, thermal effects come into play and will promote longer arcing times. Inrush current may occur, also having the effect of prolonging the arcing time but not the arc length. The interruption process is one of repetitive breakrestrike with associated restriking overvoltages. The impact of the overvoltages on the transformer insulation structures is a matter for consideration. For capacitive currents of 1 A or less, successful interruption is dependent on achieving the minimum disconnector contact gap spacing to withstand the recovery voltage and on the ratio of the source and load side capacitances C S /C L. For currents greater than 1 A, thermal effects add to the complexity of the interruption process. The longest arcing times and highest restriking overvoltages occur when C S /C L < 1. The explanation for this lies in the magnitude of the equalization voltage immediately after restriking relative to the source voltage and the associated restrike current magnitudes. A number of arcing modes can be identified dependent on the current magnitude and C S /C L. The loop switching case is more complex with current interruption having the obvious dependency on the current magnitude and the loop impedance. The switching duty is one of current commutation from one circuit to a parallel circuit and arc extinction follows an initial arc instability. The research shows that the condition for arc instability is similar to that for an arc in a DC circuit. Potential for arc modelling is examined with a view to enabling simulation of this duty. The research is principally based on tests and observations on vertical break and centre-break type disconnectors. The extension of the research results and conclusions to double-break and pantograph type disconnectors is discussed as is suggestions for further research into the subject. iii

SAMENVATTING Titel: Het schakelen van stromen met hoogspanningsscheiders in open-lucht opstelling Hoogspanningsscheiders hebben als functie netdelen te isoleren en worden geschakeld onder spanning. Scheiders zullen derhalve kleine stromen moeten onderbreken die oftewel capacitief van karakter zijn, oftewel inductief (als magnetiseringsstroom van onbelaste transformatoren) oftewel het gevolg zijn van een commutatie schakeling, afhankelijk van de praktijken in de diverse energiebedrijven. Elk van deze schakelhandelingen is uniek in termen van wisselwerking tussen boog en circuit, de mogelijkheden tot in stand blijven van de boog en uiteindelijk de onderbreking. Deze studie onderzoekt dit booggedrag in scheiders in open-lucht opstelling, met vooral aandacht voor het commutatief schakelen: het forceren van bedrijfsstroom uit een netdeel naar een parallel geschakelde tak. Het onderbreking van magnetiseringsstromen in onbelaste transformatoren van 1 A of minder wordt vooral bepaald door het bereiken van een afstand tussen de scheider contacten, voldoende groot om de transiënte wederkerende spanning te kunnen weerstaan. Bij stromen groter dan 1 A gaan thermische processen een rol spelen, die langere boogtijden zullen veroorzaken. Inrush stromen kunnen optreden; deze zullen de boogtijd verlengen, maar vergroten niet de lengte van de boog. Het onderbrekingsproces wordt hier gekenmerkt door een opeenvolging van onderbrekingen en herontstekingen, met de daarbij behorende overspanningen. De gevolgen van deze overspanningen op de isolatie van transformator wikkelingen moeten in acht genomen worden. In het geval van capacitieve stromen van 1 A of minder, wordt een geslaagde onderbreking ten eerste bepaald door het bereiken van een minimale afstand tussen de scheider contacten om de wederkerende spanning te kunnen weerstaan en ten tweede door de verhouding van bron- en lastzijde capaciteit Cs/Cl. Voor stromen groter dan 1 A, maken thermische processen het onderbreken complexer. De langste boogtijden en de hoogste overspanningen als gevolg van herontstekingen treden op wanneer Cs/Cl < 1. De verklaring hiervoor moet gezocht worden in de grootte van de vereffeningsspanning meteen na de herontsteking ten opzichte van de bronspanning enerzijds, en in de grootte van de bijbehorende stromen anderzijds. Een aantal verschijningsvormen van de boog kan worden vastgesteld, afhankelijk van de grootte van de stroom en de verhouding Cs/Cl. Het commutatief schakelen is ingewikkelder, waarbij de stroom onderbreking wordt bepaald door de grootte van de stroom en de impedantie van de lus waarin de commutatie plaats vindt. De schakelhandeling bestaat uit commutatie van stroom uit een circuit naar een parallel circuit waarbij het doven van de boog het gevolg is van een aanvankelijke instabiliteit. Dit onderzoek toont aan dat de voorwaarde voor het optreden van een degelijke instabiliteit analoog is aan die van een gelijkstroom boog. De mogelijkheden van boog modellering zijn onderzocht met het oog op simulatie van deze schakelhandeling. Dit onderzoek richt zich met name op beproevingen en waarnemingen van scheiders met vertikaal bewegende armen, en scheiders met een centrale scheiding. De extrapolatie van de onderzoeks resultaten en -conclusies naar scheiders met dubbele onderbreking en pantograaf scheiders wordt behandeld, en is als aanbeveling voor verder onderzoek op dit gebied neergelegd. iv

CONTENTS Summary... iii Samenvatting...iv 1. High voltage air-break disconnectors...1 1.1 Introduction...1 1.2 Standards...6 1.3 Bus and station arrangements...8 1.4 Perspective...1 1.5 Objective of the research...1 2. Literature review...13 2.1 Introduction...13 2.2 Transformer magnetizing currents...16 2.3 Capacitive currents...19 2.4 Loop currents...2 2.5 Free burning arcs in air...22 2.6 Conclusions...3 3. Interrupting transformer magnetizing current...33 3.1 Introduction...33 3.2 Analysis...34 3.3 Restriking and its consequences...36 3.4 Inrush currents...36 3.5 Auxiliary interrupting devices...4 3.6 Conclusions...45 4. Interrupting capacitive currents...47 4.1 Introduction...47 4.2 Analysis...47 4.3 Auxiliary interrupting devices...49 4.4 Field experience...49 4.5 Video record review...5 4.6 Capacitive current switching tests 23...63 4.7 Conclusions...7 5. Loop switching...71 5.1 Introduction...71 5.2 Loop switching tests 1999 2...71 5.2.1 Initial current, loop impedance and arcing time...72 5.2.2 Arc video record analysis...77 5.2.3 Application perspective...84 5.3 Electrical characteristics of the arc...93 5.4 Conclusions...11 v

6. Discussion on use of other disconnector types...113 7. General conclusions and suggestions for further research...117 8. References...119 Annex A Annex B Transformer transient recovery voltages...125 EHV transformer switching case study...129 Annex C Video still images: Figures C1 to C6...137 Annex D Auxiliary interrupting devices and capacitive currents...147 Annex E Annex F Comparative analysis of loop switching tests by Andrews, Janes and Anderson...157 Influence of weather...163 Acknowledgments...167 Curriculum Vitae...168 vi

Section 1 High voltage air-break disconnectors 1.1 Introduction The function of air-break disconnectors in high voltage power systems is to provide electrical and visible isolation of one part of the system. The isolation generally takes two forms: 1. Isolation related to normal day-to-day operation of the power system. For example, shunt reactors required only during light load periods are switched out using circuit breakers and then isolated by disconnectors during peak load periods. 2. Isolation related to repair or maintenance on transmission lines or station equipment such as transformers, circuit breakers and so on. In the latter regard, the disconnectors are a major contributer to personnel safety. In North America and no doubt similarly elsewhere, power system safety practices require a so-called guaranteed point of isolation with a visible break and a disconnector mechanically locked in the open position meets this requirement. If the disconnector is motor-operated, then the electrical circuit of the operator is also visibly isolated by means of a knife switch or a removable fuse link. To serve the purpose of isolation, disconnectors are required to have a greater voltage withstand capability across the open gap than to ground. The purpose of this is to ensure that surge voltages originating in the power system or due to lightning activity will more likely cause flashover to ground than across the open gap. At system voltages of 245 kv and below, this requirement adds at least 1% above the line to ground voltage withstand capability. 1 At system voltages of 3 kv and above, the requirement is stated as a bias voltage test, i.e. an AC voltage applied to one side of the disconnector and a switching or lightning surge applied to the other. To achieve isolation disconnectors are operated under energized conditions and will thereby interrupt current, the type of current being dependent on the circumstances. This interruption of current in air by disconnectors is the subject of this thesis. To establish the background for the subject, this section further provides an overview of the following: the different types of high-voltage air break disconnectors in use; the type and ranges of current to be interrupted by disconnectors; how the subject is treated in standards; typical bus and station arrangements. The section concludes with a perspective on the need for this work and the objectives of this research effort. 1

Section 1 High voltage air-break disconnectors come in a variety of types and mounting arrangements. The four most commonly used types are: Vertical break type Centre side break type Double side break type Pantograph type Of these types, the vertical break type is the most used and is the type primarily considered in this thesis. The vertical break disconnector is shown in Fig. 1.1. The active parts of the disconnector are the hinge end assembly, the blade and the jaw end assembly. The left-most insulator rotates to open or close the disconnector. The blade is shown open to about a 6 angle from the closed position, the hinge-end being to the left and the jaw end to the right. This disconnector type is usually horizontally mounted (base frame horizontal as shown in Fig. 1.1) but can also be vertically mounted (base frame and active parts vertical) or, at medium voltage, inverted mounted (base frame horizontal with active parts also horizontal but underneath). Standard phase spacings are used and overhead clearances must be such as to accommodate the fully open disconnector blade. Fig. 1.1 Vertical break disconnector horizontally mounted Courtesy of HAPAM B.V. The centre break disconnector is shown in Fig. 1.2. The active parts consist of two blades which make and break at the centre and both insulators rotate to open or close the disconnector. This disconnector type is used mainly where overhead clearances are restricted but also where low substation profiles are desired. Because the blades are reaching horizontally, the phase spacing must be increased above standard values and the disconnectors thus require a larger area than the vertical break type. 2

High voltage air-break disconnectors The double break disconnector is a variation on the centre break type and is shown in Fig. 1.3. The active parts consist of two jaw assemblies, one at each end, and a rotating blade. The centre insulator rotates to open or close the disconnector. The disconnector requires an area somewhat greater than that for a centre break disconnector. The pantograph switch, shown in Fig. 1.4, is used quite widely outside of North America and Fig. 1.2 Centre break disconnector only to a very limited degree within North America. The Courtesy of HAPAM B.V. active parts consist of a fixed stirrup arrangement attached to busbar at the top, a scissor type blade and a hinge assembly at the bottom. The smaller of the two insulators rotates to open or close the disconnector. This disconnector type clearly requires the least area and, in addition to providing isolation, also provides transitions from high to low busbars. Because disconnectors are expected to interrupt certain current levels as discussed later in this section, it is desirable to avoid arcing on the main contacts or on corona shields (refer to Fig. 1.1). For this reason disconnectors, usually by customer request, are equipped with arcing horns as shown in Fig. 1.5. The blade is provided with an arcing tip and on opening the current commutates from the main contacts to the arcing contacts thus achieving their purpose. On closing, making (prestriking) occurs on the arcing horns. In North American terminology, a disconnector equipped with arcing horns is referred to as a horn gap disconnector and requiring of a larger phase spacing than a disconnector without arcing horns. The arcing horns do not contribute to current interruption but rather provide only a location for the arc to Fig. 1.3 Double break disconnector root itself. Courtesy of HAPAM B.V. 3

Section 1 The interrupting capability of disconnectors can be increased by the addition of auxiliary interrupting devices. These devices include gas-blast devices (no longer in use), vacuum switches, commutating devices and, most relevant to this work, whip type devices as shown in Figs. 1.6 and 1.7. These devices achieve a fast moving contact effect when the whip releases and are widely used in North America to interrupt transformer magnetizing current and small capacitive currents. The devices are discussed further in sections 3 and 4 and in Annex D. Fig. 1.4 Pantograph disconnectors High voltage air-break disconnectors Courtesy of HAPAM B.V. do not have current interrupting ratings. However, by virtue of the fact that the disconnectors have a fixed and a moving contact, they have a certain current interrupting capability. In brief here but discussed in detail later, the currents to be interrupted are as follows: Transformer magnetizing currents: The current at high voltage is usually less than 2 A equivalent rms for modern low loss transformers and is non-sinusoidal with a high 3rd harmonic content. The recovery voltage that appears across the disconnector after current interruption is the difference between the source side power frequency voltage and the transformer side damped low frequency (less than 3 Hz) oscillation. Capacitive currents: For busbars with connected instrument transformers, the current is less than 1 A but with some exceptions in the range 1 to 2 A (EHV series capacitor bank platforms). For short lines, currents up to 2 A Fig. 1.5 Vertical break disconnector jaw assembly showing the main contacts and the arcing horn Courtesy of HAPAM B.V. 4

High voltage air-break disconnectors Fig. 1.6 Quick-break whip type device on 115 kv vertical break disconnector (closed position) Courtesy of Pacific Air Switches Corporation Fig. 1.7 Quick-break whip type device operation (position just prior to release of whip) Courtesy of Pacific Air Switches Corporation may be applicable. The recovery voltage that appears across the disconnector after current interruption is the difference between the source side power frequency voltage and the trapped DC charge related voltage on the busbar or line. Loop currents: Loop currents can be 1 A or more dependent on individual utility practice. The switching duty is a commutation or transfer of current from one circuit, such as a busbar or transmission line, to a parallel circuit. In the case of disconnectors, this is a natural commutation driven by arc voltage. As the arc voltage builds up, the current in the disconnector is gradually reduced to zero by transfer to the parallel circuit. The power frequency voltage that appears across the disconnector after total current transfer is referred to as the open circuit voltage or, for the case of current transfer between busbars, as the bus-transfer voltage. For loop switching between transmission lines, the open circuit voltage can be as high as 6 or 7 kv, but in most cases is in the order of a few kv. For loop switching between busbars, the bus-transfer voltage is less than 1 V. 5

Section 1 1.2 Standards Disconnector standards recognize the existence of current interrupting capability. The International Electrotechnical Commission (IEC) defines a disconnector as: 1 A mechanical switching device which provides, in the open position, an isolating distance in accordance with specific requirements. NOTE: A disconnector is capable of opening and closing a circuit when either negligible current is broken or made, or when no significant change in the voltage across the terminals of each of the poles of the disconnector occurs. It is also capable of carrying currents under normal circuit conditions and carrying for a specified time currents under abnormal conditions such as short-circuit. Two additional notes are applicable: NOTE 1: Negligible current implies currents such as the capacitive currents of bushings, busbars, connections, very short lengths of cable, currents of permanently connected impedances of circuit-breakers and currents of voltage transformers and dividers. For rated voltages of 42 kv and below, a current not exceeding.5 A is a negligible current for the purpose of this definition; for rated voltages above 42 kv and currents exceeding.5 A, the manufacturer should be consulted. No significant change in voltage refers to such applications as the bypassing of induction voltage regulators or circuit-breakers. NOTE 2: For a disconnector having a rated voltage of 52 kv and above, a rated ability of bus-transfer current switching may be assigned. With respect to Note 2 above, the applicable rated bus-transfer voltages are given in Annex B of reference 1 and for convenience shown below: 6

High voltage air-break disconnectors Bus-transfer is loop switching between busbars within a substation. Gas insulated disconnectors are those associated with gas insulated switchgear or GIS as it is commonly known. The loops formed by such switchgear are short compared to those found in air insulated switchgear arrangements and hence the lower recovery or bus-transfer voltages. In North America, the term disconnecting or disconnect switch is used instead of disconnector. Such a device is defined by the Institute of Electrical and Electronic Engineers (IEEE) as: 2 A mechanical switching device used for changing the connections in a circuit, or for isolating a circuit or equipment from the source of power. NOTE: It is required to carry normal load current continuously and also abnormal or short-circuit currents for intervals as specified. It is also required to open or close circuits when negligible current is broken or made, or when no significant change in the voltage across the terminals of each of the switch poles occurs. The definitions are very similar both recognizing an ability to break negligible current. Only IEC, however, states specific values: up to.5 A of capacitive charging current and, for specific disconnectors, a bus-transfer ability of 16 A against open-circuit voltages of 1 V to 3 V (Annex B of reference 1). An earlier version of an IEEE standard included the following note: 3 A disconnecting switch and a horn-gap switch have no interrupting rating. However, it is recognized that they may be required to interrupt the charging current of adjacent buses, supports and bushings. Under certain conditions, they may interrupt other relatively low currents, such as: 1. Transformer magnetizing current. 2. Charging currents of lines depending on length, voltage, insulation and other local conditions. 3. Small load currents. Horn-gap disconnectors generally have wider phase spacings. The implication is that such are used to break currents and that some accommodation should be made for the reach of the arc towards other phases or grounded structures. In fact, this note recognizes that disconnectors are commonly used in North America to break small capacitive currents, transformer magnetizing currents and loop currents. The standard was originally an American National Standards Institute (ANSI) standard and was revised in 1992 to become an IEEE standard. At that time, the above-discussed note was removed. The reason for this was that a guide on current interruption had been developed 4. The guide was based on the work of Andrews et al 5 and Peelo. 6 For reasons discussed later in sections 3 and 5 and Annex E, the guide should be viewed as questionable. 7

Section 1 1.3 Bus and station arrangements It is evident that, essentially from conception, disconnectors were assumed to have a current interrupting capability. Bus arrangements in turn exploited this capability thus avoiding the use of more expensive load-break switches or circuit breakers. Examples of such bus arrangements are shown in Figs. 1.8 to 1.11. The bus arrangement and required disconnector current interrupting capability are as follows: Fig. 1.8 shows a single bus arrangement common at generating stations and the disconnectors are expected to have unloaded transformer switching capability. Fig. 1.9 shows a double bus arrangement common outside of North America and the disconnectors shown are expected to have a bus-transfer loop switching capability. Fig. 1.1 shows an H-bus arrangement and the disconnects shown are expected to have an unloaded transformer switching capability. Fig. 1.11 shows a so-called Jones-type subtransmission network arrangement common Fig. 1.8 Single bus arrangement in North America. Circuit breakers CB2 and CB5 are normally open and to take line section LS1 out of service the sequence would be: close CB2, open disconnector B to loop switch the load current and then open disconnector A to switch out the short line length. Disconnectors at the transformers are used to switch the unloaded units. Fig. 1.9 Double bus arrangement Fig. 1.1 H-bus arrangement 8

High voltage air-break disconnectors Fig. 1.11 Jones-type sub-transmission network arrangement 9

Section 1 1.4 Perspective To put the foregoing discussion in perspective, we can state: 1. There is added-value in utilizing the (inherent) current interrupting capability of disconnectors; in fact, without that capability power systems would be difficult, if not impossible, to operate. 2. Surveys conducted by the IEEE in 1949 and 1962 indicate the range of currents involved in the past which is not to say that they are applicable according to today s practices. 7, 8 The noted magnetizing currents in particular reflect the high loss transformers that were once common and now being replaced by larger low loss units with quite different transient recovery voltage characteristics. The surveys are discussed further in subsection 2.1. 3. Despite the often-cited work of Andrews et al, 5 practice appears to be one of trial-anderror, i.e. if it worked once then it can be done again under the same circumstances. Establishing rules is not a trivial task, particularly given that the interrupting medium is atmospheric air and the arc is free-burning. 4. Looking to the future, deregulation is a major driver to operate power systems more effectively with fewer planned outages, even short-term outages. In addition, this is expected to be achieved with existing equipment and puts more onus on breaking circuits using disconnectors. 5. An overriding concern is personnel safety. Many disconnectors are manually operated with varying types of mechanical operators and the switching is subject to ongoing complaint and discussion. 1.5 Objective of the research The use of air-break disconnectors to interrupt current has a mainly trial-and-error basis. The reason for this is that little or no research effort has been devoted to current interruption in atmospheric air and to the behaviour of the associated free burning arcs. Unlimited propagation of the arc is obviously not permissible and its representation is not only as a time varying electrical element but also as a time varying physical element in space. The cases of interest are those of interrupting transformer magnetizing current, capacitive currents and loop currents and are addressed in this work. As shown in the literature review in section 2, no consideration has been given up to now to the conditions that must be satisfied in order for the current to be interrupted and such consideration is the purpose of this thesis. Consequently, the goal of this research is: to investigate and interpret free burning arc behaviour from an engineering perspective; 1

High voltage air-break disconnectors to advance the engineering basis for the use of air-break disconnectors for the abovenoted switching duties; to determine arc model parameters for the loop switching case to enable simulation of this switching duty. This research work is based on experiences and practices provided by BC Hydro, Bonneville Power Administration, Kinectrics Inc. (formerly Ontario Hydro Research), Manitoba Hydro and Puget Sound Energy. Testing associated with the work was performed during the period 1999 23 at Powertech Laboratories Inc. in British Columbia, Canada and at the high power laboratories of Eindhoven University of Technology and KEMA in The Netherlands. Disconnector information was provided by HAPAM B.V. of Bunschoten, The Netherlands, who also made two disconnectors available for testing purposes at KEMA, and by Pacific Air Switches Corporation of Forest Grove, Oregon. 11

Section 2 Literature review 2.1 Introduction Literature relating to current interruption using air-break disconnectors is quite sparse and comes almost exclusively from North American sources. The reason for this is probably the historically long practice of using disconnectors for this purpose as compared to the other parts of the world. Further literature of relevance is that relating to free-burning arcs in air and sourced from North America, Europe and Japan. The review is divided into four topics: transformer magnetizing currents, capacitive currents, loop switching and finally free-burning arcs in air. First, however, surveys of North American utility practices are considered. Surveys of actual practices were conducted by the AIEE (American Institute of Electrical Engineers later renamed to IEEE) in 1949, 7 the IEEE in 1962 8 and by IREQ in 1979. 9 In the 1949 survey, fifty-nine utilities of the time provided responses on successful interruption of magnetizing current (Fig. 2.1) and successful or unsuccessful interruption of line charging current (Fig. 2.2). Magnetizing current (A) 16 14 12 1 8 6 4 2 5 1 15 2 25 Transformer voltage (kv) Fig. 2.1 1949 Survey: magnetizing current interruption using horn-gap disconnectors 7 1951 AIEE now IEEE Respondents were asked to define successful operation and fifty replied as follows: 13 - arc is interrupted without operation of the protective relays or system short circuit but perhaps after disconnector is fully open. 37 - arc is interrupted before disconnector is fully opened. Further comment or advice included recommending wider phase spacing and overhead clearances; using only vertical break disconnectors; and using operating mechanisms that allow the blades to open quickly, i.e. not the gear reduction type of mechanism. 13

Section 2 25 Charging current (A) 2 15 1 5 1% successful 9 to 99% successful 75 to 8% successful 5 1 15 System voltage (kv) Fig. 2.2 1949 Survey: interruption of line charging current using horn-gap disconnectors 7 1951 AIEE now IEEE The IEEE 1962 survey was more comprehensive including breaking loop currents and recognizing the use of auxiliary arc quenching devices developed in the 195s. These devices ranged from vacuum switches and quick-break (whip-type) devices to a blast device that actually blasted air, N 2 or SF 6 gas at the arc. The results of the survey based on responses from seventy-one utilities are shown in Figs. 2.3, 2.4 and 2.5. F1 16 Magnetizing current (A) 14 12 1 8 6 4 2 1 2 3 4 5 Transformer voltage class (kv) 1% successful 9 to 99% successful 6 kva rule Power (6 kva rule) Fig. 2.3 1962 Survey: magnetizing current interruption using air break disconnectors 8 1966 IEEE F1 The survey notes the existence of the 6 kva rule where the interrupting limit is given by: 3 current voltage across the open switch. The source of the rule is unknown and its use will be reviewed later. 14