Non-Sustained Disruptive Discharges: Test Experiences, Standardization Status and Network Consequences

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R.P.P. Smeets, A.G.A. Lathouwers

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KEMA High-Power Laboratories, Utrechtseweg 3 10, 68 12 AR Amhem, the Netherlands

Abstract In the test practice of KEMA, more than 30% of the vacuum circuit breakers, offered for testing in 1999 have shown non-sustained disruptive discharges (NSDD), mostly within 300 ms after interruption. Although the vast majority of the observed NSDDs occurred after high current arcing, several cases of NSDD after small current were observed. It is demonstrated from short-circuit current test data, that during the occurrence of NSDD, a much larger part of the circuit is contributing to transient phenomena than just the 'local' parasitics in the immediate vicinity of the breaker. This is derived from the measured dur- ation of the conducting period during NSDD. The new standard IEC 60056 for circuit breakers makes no distinction between (a) capacitive restrike (late breakdown leading to polarity reversal of the load ca- pacitor) and (b) NSDD in the case of a vacuum switch- ing device breaking down more than half a power fre- quency cycle after capacitive current interruption. It is demonstrated that system overvoltages that accompany such 'capacitive NSDDs' can not always be neglected.

1. INTRODUCTION Occasionally, vacuum interrupters break down relativ- ely long (up to 1 s) after the interruption of current and restore insulation very soon thereafter. These 'late' breakdowns are commonly named 'Non-Sustained Dis- ruptive Discharges' (NSDD) because of the inherent characteristic of vacuum interrupters to restore the insulating state almost immediately after the start of the NSDD. Such breakdowns are thought to be initiat- ed by mechanical vibrations, leading to the release of macro-particles [ 11 or to a sudden increase of field emission level leading to breakdown [2]. In certification tests in accordance with the widely used IEC 60056, the absence of multiple (>3) NSDD's must be verified by subjecting the interrupter to the rated power frequency voltage for at least 300 ms after interruption (in contrast to 100 ms for other breakers). If there are more than three occurrences of NSDD during the entire series of test-duties, certification will not be possible. Restoration of power frequency current is never allowed. These requirements, originally formulated by STL (Short Circuit Testing Liaison) [3] as an amendment to the the present (1987 edition) of IEC 60056, will be fully integrated in the revised edition of IEC 60056, scheduled to be effective from the end of 2000.

2. EXPERIENCE WITH NSDD DURING TESTING A systematic investigation is made of the occurrence of NSDDs during one year (1999) of testing of vacuum circuit breakers (VCBs) at KEMA High-Power Laborat- ories. In that year, 133 test reports were produced on different VCBs that underwent current interruption tests. In 42 reports (32 %) mention was made of in total 134 occurrences of NSDD. In 9 reports, more than 3 NSDD occurrences were reported during the entire test series. The maximum number of NSDD after a single current interruption was 21, during the observation window lasting from % cycle of power frequency to 300 ms after the last poles have cleared. The rated voltage of the population that exhibited NSDD was between 12 - 50 kV, the rated short circuit current range was 12 - 40 kA. In fig. 1, the value of relative breaking current (= actual breaking currenthated short circuit breaking current), of the testduties in which NSDDs were observed is plotted against the VCB's rated voltage.

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From fig. 1, it must be concluded that NSDD is not necessarily a high-current phenomenon, although 86% of the NSDDs occurred with test current > 90% of the rated short-circuit breaking current. Interesting is the observation that even during capacitor bank test duties, (400 A test current), several occurrences of late restrikes occurred. Also, it is not immediately clear that NSDD is associated with VCBs in the higher rated voltage range only. The cumulative distribution of NSDD occurrence as a function of test current and test (rated system) volt- age is shown in fig. 2. During all the tests, the moment of occurrence of the

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NSDDs was recorded. In fig. 3, the histogram of this distribution is shown, illustrating that 86% of the ob- served NSDDs falls within a post-current zero interval of 300 ms. Breakdowns occurring shorter than 5 ms (50 Hz) or 4.2 ms (60 Hz) after complete interruption of all the three phases are considered to be part of the interruption process [3] and are not counted in the present survey.

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Fig. 3: Frequency of occurrence (%) of NSDD as a function of time afer complete interruption

3. HF ANALYSIS OF TEST-RESULTS In the revised IEC 60056, NSDD is defined as “a dis- ruptive discharge between the contacts of a vacuum circuit-breaker during the power frequency recovery voltage period resulting in a high-frequency current flow which is related to stray capacitance local to the interrupter”. In the following, it will be tried to quantify the terms “stray capacitance local to the interrupter”. At breakdown of the vacuum interrupter, a number of HF oscillations will be excited in the circuit. The circuit parts contributing to these oscillations are indicated in the circuit diagram of fig. 4: a generalized

HF representation of any (supply-/test-) circuit. These circuits can be reduced to a single phase diagram (with a proper choice of components) because only one vacuum gap is conducting during NSDD. At breakdown of VCB, the following currents through VCB in principle arise in a generalized circuit [4]: 0 lst parallel oscillation through parasitic inductance Lp

and capacitance Cp of the breaker itself; 0 2nd parallel oscillation, through parasitic (cable,

feedthrough) capacitances Cs, Ct near the VCB; 0 31d parallel oscillation, through source side capacitance

Cs and load impedance Z1; 4h parallel oscillation, through load side capacitance,

source inductance Lf and -capacitance Cg; 0 main circuit oscillation, through both source and load. Depending on the circuit, each oscillation path is char- acterized by its frequency (determines the frequency of the oscillating current) and surge impedance (determin- es the amplitude of the oscillating current).

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Fig. 4: Generalized circuit diagram and HF oscillation paths for current arising as a result of VCB breakdown

The duration (DN) of the NSDD is a complicated funct- ion of the circuit parameters, the voltage across the gap at the moment of breakdown and the gap’s ability to interrupt HF current. It is thus virtually impossible to predict this duration with a given circuit and breaker. The other approach, of determining from measurement what oscillations are excited - and thus what parts of the circuit are contributing to NSDD current - given a certain NSDD duration and a set of circuit parameters, is possible. This approach will be used here to quantify the spatial extent of NSDD activity in a circuit. Data are used from high-power tests of commercially developed vacuum circuit breakers, mostly offered for certification in accordance with E C 60056. One meas- ured example is given in fig. 5, where two NSDD’s (the first in phase 1 and 600 is later in phase 3) are shown. In this example, the duration of the first NSDD (ident- ified by a period of zero voltage across the VCB) DNl = 20 is and of the second: DN2 = 70 is. The data were recorded during a short circuit test with 100% of the (symmetrical) rated breaking current, with the short circuit point solidly connected to ground (Zl = 0 in fig. 4). The test-circuit’s HF characteristics are given in table 1. Analysis of the HF phenomena in the complete three-phase circuit including the parasitic components of the test circuit shows the simulated volt- ages and NSDD current as plotted in fig. 6. As can be

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Fig 5: Measured voltage across VCB at the occurrence of NSDD in phase I (upper) and phase 3

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Fig. 6: Upper part: simulated voltages across VCB, lower part: current through VCB offirst and third phase.

proving that the main circuit oscillation is essentially taking part in the events following NSDD. As an example, fig. 7 shows a very short (20 is) lasting

conducting period, with the appearance of NSDD in

table 1

In terms of the more familiar current zero dddt, from this analysis it can be concluded that di/dt has to de- crease to a value around 50 Ah, before this breaker is able to interrupt the HF current, both in phase 1 and phase 3. This tendency was found in other tests as well. From this, it can be concluded that the NSDD current (at least in this case) is not localized purely to parasitic capacitances (Cp, Cs, Ct) in the vicinity of the inter- rupter, and that NSDD is not necessarily a local phe- nomenon and may therefore interact with more distant circuit components. General rules cannot be given.

4. NSDD IN CAPACITIVE CIRCUITS An important change in the proposed new version of standard IEC 60056 with respect to the existing 1987 version is the assessment of late breakdowns in vacuum circuit breakers at the interruption of capacitive current. In the new IEC 60056, distinction is made (see table 2) based on the moment of occurrence (expressed in power frequency cycles after clearance of the last phase). Because no distinction is made between various con- sequences of the breakdown, the distinction based on the moment of occurrence may lead to peculiar (and sometimes unacceptable) situations as illustrated below.

phase 1, which in IEC certification tests should be assessed as a restrike because of its moment of occurrence, and would therefore lead to rejection of certification. Alternatively, also the opposite happens, as shown in fig. 8, resulting from a back-to-back capacitor bank test. Here, a late breakdown in the phase 1, initially leads to a dramatic load-side overvoltage (out of measured scale) and must be assessed as a NSDD in conformity with the new IEC 60056, and is therefore formally ac- ceptable. However, interpretation of this event as a cap- acitive restrike (polarity reversal of load capacitance) would be technically more appropriate.

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5. NETWORK CONSEQUENCES OF NSDD In the majority of cases, NSDD is a harmless curiosity, inherent to vacuum interrupting devices, leading to very short periods of conductivity. The length of this period is the result of a complicated interaction between inter- rupter and circuit, the circuit being not just the parasitic circuitry of the breaker itself ( lst parallel oscillation). Would the VCB be able,to interrupt the very high fre- quency current related to this oscillation, then the oc- currence of NSDD would remain hidden altogether in test laboratories where voltages are always measured at some distance (metres) from the breaker. Because of the dependence on the circuit beyond the VCB itself, in so-

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Fig. 7: Measured short breakdown with the appearance of NSDD that is assessed in IEC tests as restrike. Time:

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Fig.8: Breakdowns with appearance of a restrike causing large overvoltages, assessed in IEC tests as an

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me cases, however, unexpected consequences of NSDD must be recognized. Some cases are illustrated below.

As a result of NSDD current in one phase, flowing through more distant (non-local) circuit components, rather severe discontinuities in voltage across the VCB arise in all three phases as shown in fig. 5. It it well known that in the majority of cases, such discontinuities are associated with NSDD: they even serve as an ident- ifier of NSDD occurrences during testing [5 ] . Since most test circuits have one grounding only (either on load side or on source side), a power frequency current current by NSDD in one phase cannot occur. Several cases have been observed in which the increas- ed dielectric stress induces breakdown in a neighbour- ing phase, causing power frequency current in these two phases (see fig. 9). Such a current, though interrupted at the next current zero may interfere adversely with the

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Fig. 9: Voltage over VCB. Power frequency current in two phases initiated by NSDD. Time 2 ms/div, volt. 50 kV/div

protection system. In grounded power systems, or in the presence of phase- to-ground faults in ungrounded systems, NSDD may lead to resumption of (one loop of) short circuit current. A low surge impedance of the 2”d parallel oscillation path may be a situation that deserves attention. This is the case in extented cable networks. It is clear that such a condition is relatively far away from the standard test- circuit. Therefore, further analysis, in order to identify such conditions, is necessary.

6. CONCLUSIONS 1.

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During current interruption tests, NSDDs occur rather frequently (in > 30% of the VCB test series). The majority of NSDD (> 85%) occur within 300 ms after current interruption. The occurrence of NSDD has been conf i ied in the total tested range of rated short-circuit breaking cur- rent and rated voltage and was not confined exclus- ively to high-current tests. Many cases have been observed in which the com- plete test circuit was involved in the oscillations excited by NSDD. Defining NSDD as a purely local phenomenon is thus incorrect. In cases where a significant amount of capacitive energy is present in the circuit (such as in the switching of capacitor banks), late breakdown can lead to excessive overvoltages. Acceptance of such an event as a NSDD is potentially hazardous.

7. REFERENCES [ 11 R. Gebel, D. Falkenberg, “Mechanical Shocks as cause of late discharges in vacuum circuit breakers”, IEEE Trans. Electr. Insul., vol. 28, 1993, pp 468 - 72 [2] B. Juttner, M. Lindmayer, G. Duning, “Instabilities of prebreakdown currents in vacuum I: late breakdowns”, J. Phys. D: Appl. Phys., vol. 32, 1999, pp. 2537 - 43 [3] “STL Guide to the interpretation of IEC Publication 56: 4“ ed. 1987”, sept. 1988 [4] W.M.C. van den Heuvel, “Interruption of Small Inductive Currents in AC circuits”, Ph.D. thesis Eindhoven Univ., 1966 [5] R.P.P. Smeets, A.G.A. Lathouwers, “Current-Interruption Testing of Vacuum Sitching Devices”, IEEE Trans. Diel. and Elec. Ins., vo1.6 no.4, 1999, pp. 394 - 99

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