Non-sustained Disruptive Discharges Research Articles

Particle Generation, Their Behavior and Effect on Post-Arc Breakdowns After Capacitive-Current Vacuum Arc Interruption

Post-arc breakdowns of vacuum circuit breakers during capacitive operation can be possibly caused by particles. In this work, we experimentally observe the generation and dynamics of particles in capacitive interruptions and their resultant effects on post-arc breakdowns. We further constructed a synthetic test circuit for the capacitive current switching and adopted the laser-shadow technique to observe the dynamics behaviors of particles, formed by different electrode materials and inrush currents. CuCr10 electrodes tend to generate particles with high probability and quantity, owing to their weak refractoriness and lower mechanical strength. Furthermore, we find that the higher inrush current facilitates the formation of particles. Additionally, under the influence of recovery voltages and variable charges, particles show complex and diverse dynamic behavior, such as bouncing, oscillating, levitating, lifting, and rotating. Essentially, all particles relevant to breakdowns moved at a low speed (below 0.5 m/s), besides, almost all non-sustained disruptive discharges and restrikes are concerned with the particles closed to electrodes under near-peak recovery voltage. Therefore, we think neither energy accumulation nor high-speed bombardment of particles can explain post-arc breakdowns, conversely, the joint effect of particle-induced field enhancement and resulting field electron emission is responsible for breakdowns.

Interpretation of NSDD and Restrike in Capacitive Current Switching Test on Medium Voltage Vacuum Circuit Breaker

Capacitor banks are installed in an increasing number to control power quality issues and improving the power factor tocounteract the reactive power in the transmission and distribution networks. Due to load fluctuations switching of capacitorbanks is a typical phenomenon. Capacitor bank is normally having concentrated capacitance in contrast with distributedcapacitance. It generally draws more current than unloaded cableor line in practical cases up to several hundred amperes.Hence, switching of capacitor bank causes a very high rate of rise of transient recovery voltage across circuit breakercontacts. This scenario can be simulated in testing laboratories by a voltage source connected to a circuit breaker whichagain is connected to a large capacitance in terms of IEC called capacitive current switching test. Non-sustained disruptivedischarge (NSDD) is a voltage breakdown after the vacuum circuit-breaker open the fault current which will not cause therecovery of power frequency current. Restrike is different from NSDD, the recovery of power frequency current betweenthe contacts will make the breaker failed. It’s not easy to show the distinction between NSDD and restrike during some typetests. The paper presents the study of performance evaluation of the circuit breaker during the capacitor current switchingduties. A statistical analysis of failure of circuit breaker during capacitor current switching tests are included in the paperto help the manufacturers & utilities, so that the care can be taken for trouble free services. The paper also discusses thechanges made in the IEC 62271 – 100, Amendment1:2012 for conducting the capacitive current switching tests.

Late Breakdowns Caused by Microparticles After Vacuum Arc Interruption

In vacuum circuit breakers, restrikes and nonsustained disruptive discharges are sometimes observed even up to several seconds after current interruption. These late breakdowns are triggered by a sudden increase in the field emission level and/or microparticle collision with the electrode during the application of recovery voltage. In this paper, we identify the late breakdowns triggered by the microparticles that emerged from the silver-tungsten carbide electrodes after current interruption. The combined use of a high-speed video camera and a pulse laser enabled us to conduct shadowgraph photography with a 150-ns time resolution, 10- $\mu \text{m}$ spatial resolution, and $\mu \text{s}$ framing rate for capturing the microparticle motion under the recovery voltage after current interruption. In this highly resolved photography, we observed many microparticles flying between the electrodes after current interruption. In our series of observations, we found two types of microparticle-inducing late breakdowns: those induced by small, fast microparticles whose diameter and velocity were approximately 20– $40~\mu \text{m}$ and 15 m/s, and those induced by large, slow microparticles whose diameter and velocity were approximately $100~\mu \text{m}$ and 1 m/s, respectively. Some of the large slow microparticles caused the sequential late breakdowns during the up-and-down motion between the electrodes. All the particle-induced late breakdowns occurred at the time of particle collision with the negative electrode defined by the polarity of the recovery voltage. The velocity of the large, slow microparticles was approximately 1 m/s, and these microparticles did not evaporate. Furthermore, the gradual decrease in the particle size was not observed in the continual occurrence of late breakdowns. We also observed many breakdowns caused by unknown factors. The electrode surfaces were also observed by using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructures and metal compositions on the electrode surfaces varied drastically with the observation positions.

Motion and production of microparticles in vacuum interrupter

Vacuum circuit breakers are widely used in medium-voltage class. In vacuum circuit breakers, restrikes and non-sustained disruptive discharges sometimes occur after current interruptions. It is said that these late breakdowns are triggered by the microparticles. This paper aims to investigate the relationship between the motion of microparticles and subsequent late breakdown phenomena. It does by artificially injecting microparticles into the vacuum gap and observing their motion using a highspeed video camera. We focused on the sputtering of the electrode material during the current interruption and the mechanical stress of the switching operation as being the origin of these microparticles. We found that the motion of microparticles can be grouped into two distinct patterns: bouncing and attaching. Bouncing microparticles seem to be made of metal, while most of the attaching microparticles attached themselves to the cathode, and their charge was about five times greater than that of the bouncing microparticles. The attaching microparticles seem to be positively charged by secondary electron emissions from the surface under electron bombardment due to the field emission current from the cathode. From these results, we believe that the surfaces of the attaching particles are covered with an insulating layer. From our observations of the production of microparticles by mechanical stress, we have found that the breakdown voltage of the gap drastically drops after a mechanical switching operation where microparticles are produced. The breakdown voltage is restored with repetitive mechanical switching operations and subsequent voltage application. However, the breakdown voltage drops again when the operating speed of the electrode is increased. We also found that microparticles are scattered from the cathode spots on electrodes that have not been current-conditioned, and they are hardly produced from electrodes that are current-conditioned. From these results, we believe that controlling the speed of the mechanical switching operation and the current conditioning are effective ways to prevent the microparticles from being produced.

Effect of contact surface on non-sustained disruptive discharge phenomena after current interruption in vacuum

Non-sustained disruptive discharge (NSDD) sometimes occurs when the voltage is applied for a certain time between vacuum gaps after the current is interrupted. However the mechanism of NSDD has not yet been clarified. We conducted a study to clarify this phenomenon using the contact materials CuCr, CuW, and AgWC. We applied a voltage (20[kV]-30[kV]) of AC 50(Hz) after current interruption (1 (kArms|-8 |kArms]), and checked the generating frequency of NSDD. After the current interruption test, we measured the contact surface roughness and field emission current. The result showed that surface roughness and field emission current are not related to this phenomenon and also that CuCr (melt) showed the best performance in all tests.

The Generating Mechanism of Non-Sustained Disruptive Discharges in Vacuum Interrupters

To develop vacuum circuit breaker (VCB) for higher voltage application, it may be important to understand generating mechanism and its influence of non-sustained disruptive discharges (NSDD) to the systems. So, we carried out the tests using equivalent testing circuit and observed the contacts after testing, For the test, by using commercial vacuum circuit interrupters, AC voltages of 50Hz was applied between contacts for 4 seconds after current interruption, and measured generating frequencies of NSDD vs. the voltages and vs. currents. Typical contact material used in the commercial switching equipment, such as AgWC, CuW, CuCr were tested and compared. Then CuCr's of different composition and manufacturing process are investigated. And CuCr-50 (manufactured by melting process) showed the best performance in all tests. We point out that surface condition may affect the generation of NSDD and also conditioning effect is very important.

The duration of arcing following late breakdown in vacuum circuit breakers

Vacuum interrupters occasionally show unexpected (late) breakdowns after current interruption. As a result, the interrupter conducts briefly, carrying a high-frequency current due to the discharge of various capacitances in the vicinity of the interrupter. Thanks to the capability of vacuum interrupters to interrupt such high-frequency current, the conduction period is very short (normally in the order of 10 microsecond). In the discussion around the assessment of such late break- downs (often termed non-sustained disruptive discharges in the standardisation literature) regarding the possibility of generation of overvoltage as a result of late breakdown in capacitive circuits, the duration of the conductive period is an important parameter. Experiments were carried Out, in which discharges of various single frequencies (72 kHz - 1.1 MHz) were generated following breakdown (spontaneous breakdowns as well as pre-strike) of two commercial vacuum breakers, stressed with ac voltages well above the rated value. From a statistical analysis, it is demonstrated that the dura- of the discharge depends strongly on the discharge current frequency. At higher frequencies, the duration of the discharge tends to be shorter than at lower frequencies due to the domi- nance of thermal reignitions of the arc at high values of dl/dt over dielectric reignitions, observed only at lower frequencies. For practical circuits, allowing simultaneous discharge from various circuit parts, it was observed that the highest frequency component determines the arc duration. Durations of up to 15 us were observed. This duration is too short to allow significant overvoltages to develop in standard capacitive circuits, the main candidate switching duty to be troubled with overvoltage generation.

Non-sustained disruptive discharges: test experiences, standardization status and network consequences

In the test practice of KEMA 32% of the vacuum circuit breakers offered for testing in 1999 have shown non-sustained disruptive discharges (NSDD), mostly within 300 ms after interruption. This implies, that although current has been interrupted, very brief periods of conduction occur, which seems to be an accepted inherent property of vacuum switching devices. occurred in the complete range of rated voltages (12 kV-50 kV) and short-circuit breaking current (12 kA-40 kA) of the tested devices. No clear correlation of NSDD occurrence with arcing current could be established. In 79% of the cases, less than four NSDD occurrences per test report were counted. 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 parasitic in the immediate vicinity of the breaker. This is derived from the measured duration of the conducting period during NSDD and contradicts the general notion that NSDD is just a local phenomenon, the duration of which is entirely determined by the parameters of the switching device itself. The new standard IEC 62271-100 for circuit breakers makes no distinction between (a) re-strike (late breakdown leading to polarity reversal of the load capacitor) and (b) NSDD in the case of a vacuum switching device breaking down more than half a power frequency cycle after current interruption. It is demonstrated that over-voltages that accompany such capacitive NSDDs certainly can not be neglected.

Current-interruption testing of vacuum switching devices

In the international IEC standards for medium and HV switching devices no distinction in test requirements is made between switching devices with different arc-quenching media, like SF/sub 6/, oil and vacuum. This had led to the situation that due to the different technology of arc interruption applied, various aspects of the prescribed test-procedures have a different weight in terms of severity for various types of breakers. Additional test-requirements and interpretations that are specific to vacuum switching devices have been formulated commonly by the joint major test laboratories. In this contribution, the background and practical application of several procedures, now generally adopted by certifying test-laboratories, regarding the peculiarities of vacuum switching devices are elucidated. Because the vacuum switching device has an excellent capability to interrupt current of high frequency, the main part of this contribution will be related to the consequences for test procedures of this aspect that is not encountered in SF/sub 6/ or oil breakers. In particular, the judgment of non-sustained disruptive discharges, multiple re-ignition and virtual current chopping in test-circuits is addressed. Results of a new high-resolution high-frequency current-zero measuring system are presented. This system is able to give insight into the high-frequency arc phenomena in the immediate vicinity of are interruption, and is designed to get more specific information on arc behavior during standard high-power tests.

Triggered discharges with high arc voltages in a vacuum interrupter

Studies of nonsustained disruptive discharges (NSDDs), isolated cases of which can occur in vacuum interrupters, indicate lateral discharges between the cathode and shield, which can initiate a brief discharge between the contacts. To facilitate the study of such discharges, the sample discharges were triggered by a surface discharge induced by a spark gap, built into the side of the cathode, and observed with a high-speed film camera and image-converter camera. The tests showed a cathode spot after igniting. The emitted electrons first charge the shield negatively and then are directed toward the anode. The discharge burns at a high voltage, with current ranging from 10 to 100 A. After a period of up to 400 mu s, the current demand increases abruptly; an arc discharge occurs between the contacts and discharges the capacitances near the switch. The contact gap undergoes a rapid dielectric recovery, and the restored voltage is maintained. These types of discharge were also observed with NSDDs; thus it can be assumed that the triggered discharges studied correspond to the NSDD type.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>