A3-107 INCREASED PERFORMANCE OF CAPACITOR BANK CIRCUIT BREAKERS BY CONTROLLED OPENING

Transcription

1 , rue d'artois, F-758 Paris A3-7 Session 4 CIGRÉ INCREASED PERFORMANCE OF CAPACITOR BANK CIRCUIT BREAKERS BY CONTROLLED OPENING U. KRÜSI * P. M. JONSSON Swiss Federal Institute of Technology ABB Power Technology Produ AB ETH Zurich Ludvika (Switzerland) (Sweden) Summary: For the first time a statistical model of the dielectric withstand of the circuit breaker opening has been developed to assess the restrike probability and to allow quantitative statements of the influence of controlled opening on the restrike probability when de-energizing capacitor banks. The calculations show that even with conservative assumptions about the standard deviations, the restrike probability varies by several orders of magnitude with arcing time. In order to verify the theoretical findings, the cold gas characteristic of a SF 6 circuit breaker was measured by the use of a 6 Hz -cosine wave for different arcing times. The results indicate that a fixed arcing time of.8 ms results in an increase of test voltage of 5% and the use of the circuit breaker at frequencies of 66 Hz (compared to 5 Hz) as predicted/supported by the calculation of the restrike probability in theory. Henceforth, the restrike probability of a circuit breaker can be assessed by the use of the given test procedure and the mathematical model and the potential for increased performance of capacitor bank circuit breakers by controlled switching can be determined.. Introduction In IEC 67- (Ed. ) replacing IEC 656 the difficulty of the test duties for capacitive load switching was greatly increased and the concept of a restrike-free circuit breaker was replaced by two more appropriate categories C low and C very low expected probability of restrike during capacitive current interruption. (As a further option next edition of IEC 67- will also include a third class, C, for circuit-breakers with non-defined probability for restrikes, higher probability compared to class C). This only provides information that a circuit breaker has a restrike probability lower than a certain level but does not provide any information about possible the potential for increase performance of capacitor bank circuit breakers by controlled switching. It is a well known fact that a prolonged arcing time reduces the risk of a restrike, because of the increased contact gap when the recovery voltage appears across the opening breaker. The length of the arcing time can be adjusted by the use of a Point-on-Wave controller. Since years this technique has been successfully applied to avoid the critical reignition window when de-energising reactors. Already in 978 an attempt was made to describe the statistical nature of the dielectric gap strength of an opening oil breaker [, ]. At that time the work was not continued. For this work it provided the idea to combine a no-load characteristic test with a statistical mathematical model. This paper presents a new statistical model of the dielectric withstand to calculate the restrike probability. The model allows quantitative statements of the influence of the arcing time on the restrike probability when de-energizing capacitor banks by controlled switching. Laboratory test on a SF 6 circuit breaker were used to determine the necessary parameters for the calculation and to verify the theoretical results. * kruesi@eeh.ee.ethz.ch

2 . Capacitor bank de-energisation Capacitor banks are used to compensate reactive power for voltage control. They are usually switched on a daily basis. In the lifetime of a circuit breaker the number of operations can sum up to several thousand. Random energisation of capacitor banks is associated with high inrush currents and overvoltages. To avoid them closing resistors, combinations of resistors and reactors or controlled switching can be used. When de-energizing capacitor banks the current zero is close to a voltage maximum. The capacitor bank is therefore left almost fully charged. The recovery voltage RV follows after an initial voltage jump (depending on the strength of the supply circuit), which is accompanied by a high frequency transient of small amplitude a -cos form, which starts rising slowly (du/dt = at interruption) but will reach its maximum value of -3 p.u. half a cycle later depending on earthing conditions. The initial voltage jump is neglected in this paper because it does not affect the recovery voltage in the restrike region. A dielectric breakdown after more than a quarter of a cycle is called a restrike whereas a breakdown in the first quarter of a cycle is called a reignition. In case of capacitive loads reignitions are considered to be harmless, whereas restrikes can lead to overvoltages that should be avoided at any rate. The physics, stresses and test methods of de-energizing capacitor banks has been comprehensively studied and published by CIGRE Working Group 3.4 (Switching Test Methods) in [3, 4, 5]. 3. Model of dielectric strength In this section a model of the dielectric withstand of an opening circuit-breaker is presented, which is used to calculate the restrike probability. The model is based on the following assumptions: The influence of the arc on the dielectric strength after current zero is negligible (see section 8) The contact velocity is constant after contact separation The moment of contact separation and the rate of rise of dielectric strength are normally distributed The breakdown voltage U b at time t is approximated as the product of the rate of rise of dielectric strength and the time since contact separation at t (see ()). U b () t = ( t t ) ( t > ) () After contact separation until current interruption an arc burns and hence this time interval is called arcing time. By definition, the rate of rise of dielectric strength is the change (increase) of breakdown voltage U b of an opening contact gap (see eq. ()). dub( t) dub( t) dt () = = dt dx dx The is therefore proportional to the dielectric strength of the isolation medium and the opening velocity. An of p.u. corresponds to the steepness of the reference voltage at zero crossing: du ref ( t = ) p.u. k f Uˆ (3) = = π ref dt and is therefore proportional to the system frequency, the voltage amplitude Û ref and a factor k which is for grounded and.4 for ungrounded banks. Recombination of the gas after the interruption of comparativeliy small load currents is especially in modern SF 6 circuit breakers a matter of less than millisecond. The dielectric strength of the gap after current interruption is therefore modeled as the continuation of the prospective dielectric strength the dielectric strength of an opening gap without an arc. The dielectric strength of the gap can be defined by its breakdown voltage. To introduce the statistical behavior of the dielectric strength the breakdown voltage U b can be rewritten as (4): Ub() t = ( + ) ( t t ) ( t > ) (4) The term stands for variations in dielectric strength and contact velocity. It is assumed to be normally distributed as stated above. Figure shows the prospective and the dielectric strength before and after current zero, respectively. The statistical scatter of the breakdown voltage is indicated with a symbolic 3σ bandwidth. As soon as the withstand voltage is lower than the RV peak a reignition or restrike occurs: restrike condition: U b ( t) < RV ( t > ) (5) In Figure the area where the restrike probability is high, is marked with an ellipse.

3 4. Calculation of the restrike probability This section describes the method used to calculate the restrike probability taking into account the influence of the instant of contact separation. The procedure consists of three steps:. Neglecting scatter at first: Calculate for all instants of contact separation the minimal at which no restrike occurs.. Influence of scatter: Calculate for all instants of contact separation the probability that the is lower than the minimal. 3. Calculate the influence of the variation of the contact separation. In the first step an iterative algorithm is used to calculate the minimal min(t ) at which the restrike condition eq. (5) is just not met and no restrike occurs under the assumption that there is no scatter present. These values represent the slopes of the tangents from the instant of contact separation to the RV. For short arcing times the required minimal is higher than for longer arcing times. The result can be found in Figure. current & voltage (p.u.).5.5 3σ scatter bandwidth prospective dielectric strength current strength U b time (T) arcing time t contact separation In the second step the influences of scatter in dielectric strength and contact velocity are calculated using the results of the first step. Given the statistical properties mean and standard deviation σ which can be calculated from cold gas characteristic tests as demonstrated in section 7 it is possible to calculate the probability that the actual is smaller than the min (from the first step): min( t ( ) ) y * σ rs ( t ) = e σ π P RV restrike area idealized dielectric Fig. : Limit of dielectric strength represented by the breakdown voltage U b and the recovery voltage RV. T=/f with f=5 Hz or 6 Hz contact separation (T) Fig. : min: Minimal rate of rise of dielectric strength as a function of the instant of contact separation in order not to restrike. It is assumed that no scatter is present. The restrike probability P rs * as a function of contact separation and for a fixed 3σ of % has been calculated. The restrike probabilities P rs * were calculated using eq. (6) at discrete values separated by a t of.5 p.u. In Figure 3 the lines correspond to equal restrike probabilities. The restrike probability for an of p.u. (horizontal line at = p.u.) varies from below -5 for contact separation half a cycle prior to current zero to greater than.999 for no arcing time. In the third step the influence of scatter of the contact separation is calculated. In order to use the results from the second step it must be assumed that this scatter is independent of the others. The instant of contact separation varies due to friction, variations in the actual drive energy, idle time, etc. The standard deviation σ describes how probable it is to be a certain time ahead or later than normal. That means that the restrike probability at a certain instant of contact separation can be interpreted as the sum of the surrounding restrike probabilities from the second step multiplied by the probability that contact separation is at that instant due to the scatter σ. As calculations are of a discrete manner, the probability that contact separation takes place during a certain interval i which is centered over the min (p.u.) d y (6)

4 instant of contact separation t,i and extends half a t on both sides towards adjoining t,i- and t,i+ at which the restrike probability has been calculated can be interpreted as weights W i : t t+ i t + ( y t ) (7) σ Wi = e d y σ π t t + i t with 6 σ 6 σ i + (8) t t The resulting restrike probability is then calculated using (9) P ( t ) = W P t + i t (9) rs i i rs ( ) In Figure 4 the resulting restrike probability is given as a function of and contact separation for 3σ of % and a 3σ of.5 T (= ms at 5 Hz). In contrast to the results in Figure 3 the restrike probability for instants of contact separation close to - T. is increased because it is probable that the contact separation occurs prior to - T, which corresponds to a contact separation just before the previous current zero. A D B C A B Fig. 3: Restrike probability as a function of and contact separation for a fixed 3σ of %. A, B, C, D are explained in section 7. Fig. 4: Lines of equal restrike probability as a function of and contact separation for a fixed 3σ of % and a 3σ of.5 T. A, B are explained in section Theoretical estimation for up-rating According IEC 67-, two categories for capacitive current switching are defined: C low and C very low expected restrike probability (with new class Co to be included). Single phase capacitor bank tests require a total of 48 restrike free openings where shall be at minimum arcing time for C and another interruptions where 84 shall be at minimum arcing time for C (in addition class C also asks for pre-conditioning of the circuit-breaker by 3 short-circuit interruptions with 6 %). Opening at minimum arcing time is used to make the testing more severe because it corresponds to the moment where the restrike probability is at its maximum. A conservative estimation is to take the total number of openings to calculate the restrike probability of a successfully tested breaker: Prsc < /48 approx.. and Prsc < / approx..8. Applying these criteria to the restrike probabilities in Figure 4 it can be seen that for a C breaker the must be greater than.84 p.u. in order not to have a restrike probability greater than.8 (Figure 4, A). If the instant of contact separation would be controlled to be always at.4 T the could be reduced theoretically to about.45 p.u. without increasing the restrike probability and hence allowing a significantly increased test frequency or test voltage (Figure 4, B). These considerations apply to three phase circuit breakers with three independent drives where the arcing time can be set for each pole individually. For three phase breakers with only one drive (and without merchanical staggering), the maximum arcing time is restricted to the interval between two consecutive current zeros in different phases (equal to /6 cycle).

5 For considerations about up-rating four restrike probability values for a given are of real interest. The maximum because it represents conditions at the minimum arcing time, the mean because it corresponds to the uncontrolled opening of a circuit breaker, the minimum because this is the lowest restrike probability that can be achieved using controlled switching in single phase applications or breakers with individual drives per pole and the one at /6 cycle because it represents the three phase case with only one drive. From these values the potential for the up-rating can be calculated. When the restrike probability is kept at a constant level of - the necessary at minimum arcing time (Figure 5, A) is.83 p.u. For an uncontrolled case the can be reduced to.75 p.u., which corresponds to an increase of voltage or frequency of % (Figure 5, B). In case of a three phase breaker with one drive an of.6 p.u. is sufficient to achieve the same level of restrike probability, which corresponds to a 38 % increase (Figure 5, C). For one drive per pole the could even be reduced to.4 p.u., which corresponds to % (Figure 5, D). The other possibility is to reduce restrike probability for a certain. For an of.7 p.u. the probability can be reduced from.4 (Figure 5, E) to.4 (Figure 5, F) to 4-5 (Figure 5, G) and even less for one drive per pole. All cases under the assumption that the controlled switching system does not significantly affect the scatter assumed for the calculations or that the additional scatter was already included. Further details about controlled switching system testing and commissioning can be found in [6, 7]. 6. Laboratory test setup (p.u.) Fig. 5: Max, mean, min and for /6 cycle arcing time the restrike probability as a function of for 3σ.5 T at a fixed 3σ of %. Investigation tests have been performed on an SF 6 -gas circuit-breaker to determine the voltage withstand capability during contact opening (). The test method used was the cold characteristic test method that is a diagnostic tool that has been applied on gas circuit-breakers for years. The tests were performed under different conditions to study the influence of contact speed variation and gas pressure variation on the performance. The influence of contact burn-off was studied as well by aging the circuit-breaker by means of three short-circuit interruptions at 6 per cent of the rated short-circuit breaking current (pre-conditioning test T6 as defined by IEC 67- for Class C circuit-breakers). The test circuit used had a current limiting resistor and was arranged so that the circuit-breaker was exposed to a voltage with a -cosine wave form and with a frequency of about 6 Hz. The voltage was applied across the circuit-breaker during opening operation and at various contact distances. The circuit-breaker was not conducting any current before the voltage was applied. Both voltage polarities were tested. The maximum charging voltage for the selected circuit did not allow for voltage break downs if the "arcing time" was exceeding about 8 ms which limits the possibility for a more accurate quantification of the voltage uprating. To stress the circuit-breaker in an accurate way the recovery voltage across the circuit-breaker was shared on both terminals. Prior to opening of the circuit-breaker the charging d.c. voltage was applied to one of the terminals. At the time of discharging the pre-charged synthetic circuit a -cosine wave shaped voltage started to oscillate from the pre-charged level. 7. Test results The test results were evaluated for each test condition and a detailed presentation of the evaluation is graphically shown below for one specific test condition. restrike probability D max mean min /6 cycle C E F G B A

6 In the results presented below, the bold dotted points indicate the voltage break down versus time after contact separation while the unfilled circles show tests where the circuit-breaker withstood the voltage stress. In Figure 6 each single test in the series is plotted. The plot shows breakdown voltages (bold dots) and withstand peak voltages (unfilled circles) versus virtual arcing times (time from contact separation instant). The bold points indicate the border line of the voltage withstand performance. In order to judge the statistical probability for a possible reignition or restrike a mean value as well as the +/- 3 sigma border lines were calculated. These are shown in Figure 7. The mathematical risk of a voltage breakdown outside the 3 sigma border lines equals.3 per mil. The standard deviation was calculated as a weighted relative value based on a normalized ( norm ) at each voltage breakdown point U bd with respect to the calculated mean value of the ( mean ). U bd () tarc norm = mean where U bd is the voltage breakdown at time t arc. A normalized mean value, mean_norm was then calculated according to (): Σ( norm ) () mean _ norm = n The calculated normalized mean value of was then used to calculate a relative standard deviation: () mean_ norm norm σ = n The calculated 3 sigma was found to be per cent. In the calculation five points just after contact parting were excluded because they would increase the 3 sigma value in an inappropriate way such that the result would not represent the characteristic in the restrike area correctly. The mean value as well as the 3 sigma borderlines are drawn to intercept at origo. To evaluate the performance the recovery voltage corresponding to real capacitive current interruption can be drawn in the graph and starting after a virtual arcing time equal to zero ms (most critical arcing time). Fig. 7: 4,5 4 3,5 y =.4647x (p.u./ms) 3,5,5 Breakdown voltage p.u. +/-3 sigma,5 Linear (Breakdown voltage p.u.),, 4, 6, 8,,, 4, "Arcing time" (ms) Resulting mean value or drawn black together with the upper and lower 3 sigma border lines drawn red. In order to judge a quantified statistical secured level of restrike-free performance at interruption, typical recovery voltage wave shapes representing capacitor bank de-energising at different frequencies were entered into the graph and compared to the measured and calculated minimum withstand of the circuit-breaker. The voltage magnitude selected in the example for comparison represents the first pole to clear when de-energising an ungrounded capacitor bank (single-phase test voltage =.4 times the phaseto-ground voltage: k=.4). To evaluate the performance, the recovery voltage at real capacitive current interruption has been drawn in the Figures and starting after arcing time zero (most critical arcing time). In Figure 8 the - cosine voltage wave shape represents the recovery voltage at 5 Hz. The starting point of the recovery voltage is selected to correspond to the very minimum arcing time ( ms). The margins between the capability of the circuit-breaker and the stress at 5 Hz are judged suitable and with a very low risk for a restrike. Voltage (p.u.) Fig. 6: Withstand voltage Breakdown voltage p.u "Arcing time" (ms) Results of determination with reference gas filling pressure and increased contact speed. CB in new condition

7 Voltage (p.u.) Fig. 8: "Arcing time" (ms) Requirement for 5 Hz +/-3 Sigma Linear (Trendline) 5 Hz recovery voltage at capacitor bank switching with minimum arcing time in comparison with mean value and plus and minus 3 sigma border lines of. for restrike of about 6 per cent ( standard deviation). In order to have an idea of the performance at higher frequencies, recovery voltage corresponding to 6 Hz was added in Figure 9. The starting point for this recovery voltage do also coincide with the contact separation instant, corresponding to minimum arcing time, i.e. current zero. As can be seen in the Figure 9 there is a certain risk for a restrike at 6 Hz. The recovery voltage, starting at contact parting (bold line), touches the minus 3 sigma border line which means that the risk is about per mil for a restrike for the very minimum arcing time. Increasing the frequency further to 66 Hz would result in an estimated risk Depending on the intended system frequency and the inreased voltage the same breaker has a different value of if expressed in p.u. as defined in the section 3 eq. (3). Assuming zero arcing time and no scatter for the instant of contact separation, the corresponding restrike probabilities can be taken from Figure 3 for contact separation (vertical line on the right). The values are given in the following table: Freq Voltage k (p.u.) Prs * 5 Hz p.u..4.5 p.u. < -5 (Fig. 3, A) 6 Hz p.u p.u. 5-3 (Fig. 3, B) 66 Hz p.u..4.8 p.u..6 (Fig. 3, C) In addition to the above estimations of the risk for a restrike at different frequencies and at minimum arcing times an attempt to draw a maximized peformance was done. The uprated performance is represented by the dotted line in Figure 9 and is achieved by controlling the contact parting to result in suitable arcing times exceeding critical minimum. For conservative reason a margin of per cent betweeen recovery voltage and border line of verified withstand tests has been considered. The true border line for withstand at long contact distances (arcing times) is unknown since no voltage breakdowns were reached. A.8 ms delayed and increased - cosine voltage wave shape is drawn (dotted) in Figure 9 in comparison with the normalized 6 Hz voltage slope starting at zero arcing time. Voltage (p.u.) Fig. 9: /-3 Sigma "Arcing time" (ms) Withstand voltage Requirement for 6 Hz Possible upgrading for 66 Hz Trendline 6 Hz recovery voltage (bold) at capacitor bank switching with minimum arcing time in comparison with mean value and plus and minus 3 sigma border lines of. An additional and 5 per cent increased voltage wave shape (dotted) with 66 Hz is drawn with a delayed start of.8 ms. By avoiding arcing times less than.8 ms the recovery voltage at 66 Hz can be rised by about 5 per cent and result in a very low expected probability for a restrike. Based on the tests the risk for a restrike is estimated to correspond to a voltage breakdown outside the border of 4 standard deviations (restrike probability about ). The corresponding in p.u. is.7 p.u. (for 66 Hz and 5 per cent increased voltage) and the.8 ms arcing time is.8 T (=.8 ms 66 Hz/). In Figure 3, (D), the corresponding restrike probability can again be found and is -5 to be compared to the rather high risk (6 per cent) at the same frequency but at 5 per cent lower voltage for zero arcing time (Figure 3, C).

8 8. Discussion The results of the "cold characteristic tests", determination of the limit of voltage withstand at different contact positions during opening operation, show: - Controlling the contact parting instant at interruption of capacitive loads can: - compensate, maintain the interrupting performance, for a lower gas density. - compensate for a lower contact speed at opening. - improve the "safety" against restrikes by ensuring proper voltage withstand margins and taking care of scatter in the early stage. - make a circuit-breaker capable to operate in networks with higher frequencies if the performance is not good enough at random switching. - Ageing the circuit-breaker by 3 interruptions of T6 does not affect the performance significantly. - If restrike free performance at capacitive current switching is a limiting factor, controlled interruption is a useful tool for uprating. To verify the correctness of ignoring the arcing during the determination, the breakdown voltages at shunt reactor interruption tests have been compared to the plotted. Tests were performed with an interrupting current of A. The test results matched well the no-load cold characteristic tests. In the reignition window the breakdown voltages were well aligned to the mean value determined at cold characteristic tests. 9. Conclusions Henceforth, the restrike probability of a circuit breaker can be assessed by the use of the given cold gas characteristic test and the mathematical model. The influence of a prolonged arcing time can be calculated and thus the potential for increased performance of capacitor bank circuit breakers by controlled opening can be determined. The results encourage a new view on the application of controlled switching since modern controllers does not seem to decrease the reliability of such a controller-breaker system. A C circuit breaker equipped with a controller can pass the C tests. Alternatively, the class could stay the same while the tests are conducted at higher voltages and/or frequencies. For the tested breaker an estimated upgrading potential, increase of test voltage of about 5 per cent at an increased frequency of 66 Hz, is realistic by selecting a proper arcing time rather than random.. Acknowledgement The work presented is a result of continued investigations based on contributions received by CIGRE working group A3.7.. References [] K. Fröhlich, "Voltage Characteristic Measurements and the Switching Performance of Circuit-Breakers", CIGRE Session 978 Paris, Paper No. 3-, 3 th Aug.-7 th Sep. [] K. Fröhlich, "Oil Breaker Voltage Characteristics", IEEE PES Winter Meeting, New York, Feb. 4-9, 979. [3] CIGRE WG 3.4, Sölver C. et al., "Capacitive Current Switching State of the Art", ELECTRA, 55: pp. 3-63, August 994 [4] CIGRE WG 3.4, Bonfanti, I. et al., "Shunt Capacitor Bank Switching Stresses and Test Methods. Part I", ELECTRA, 8: pp , February 999 [5] CIGRE WG 3.4, Bonfanti, I. et al., "Shunt Capacitor Bank Switching Stresses and Test Methods. Part II", ELECTRA, 83: pp. -4, April 999 [6] CIGRE WG 3.7, "Controlled switching of HVAC circuit breakers. Guide for application. Lines, reactors, capacitors, transformers. Part I," Electra, 83:pp , Apr [7] CIGRE WG 3.7, "Controlled switching of HVAC circuit breakers. Guide for application. Lines, reactors, capacitors, transformers. Part II," Electra, 85:pp , Aug. 999.