Rpp René Smeets

Experimental and Theoretical Analysis of Vacuum Circuit Breaker Prestrike Effect on a Transformer

The work presented in this paper deals with the investigation of circuit breaker prestrike effect that occurs during energizing a distribution transformer. An experimental test setup that consists of a supply transformer, a vacuum circuit breaker (VCB), a cable and a test transformer is built, and the prestrikes in the VCB are recorded. The test transformer is a prototype distribution transformer, with installed measuring points along transformer windings in each phase. Voltage oscillations are measured along the windings and transformer terminals. The transformer is modeled by lumped parameters extracted from telegraphers equations in discrete form. Voltage oscillations during switching-in operations are recorded and calculated with and without a cable installed between the VCB and the transformer. Computed voltages show good agreement with the measured voltages. Described method can be used by transformer manufacturers to estimate voltage wave forms during switching or lightning, to provide useful information for insulation coordination studies, and to investigate resonance effects in transformer windings.

Vacuum circuit breaker current-zero phenomena

Post-arc current phenomena that occur when interrupting high currents with vacuum circuit breakers have been investigated. High resolution measuring equipment has been used to measure both the post-arc current and the arc voltage in the current-zero region. Three examples of frequently observed phenomena are described. The first describes the phenomenon that in the event of a current-chopping, the current is zero for a short period of time just before the natural alternating current zero, but continues to flow afterwards, in the form of a post-arc current. The second and third example deal with the post-arc phenomena after currents that are much higher than the test breakers rated short-circuit current. These examples show a low-voltage period after current-zero. Apparently, during this post-arc period, the residual plasma between the breakers contacts conduct well. In addition to the voltage-zero period, the voltage trace in the third example also shows evidence of current-chopping. This means that the plasma conducts poorly just before current-zero, but conducts well immediately afterwards. The post-arc current model of Andrews and Varey is verified with measurements.

Disconnector switching in GIS: three-phase testing and phenomena

Certification tests in accordance with the requirements of the recent IEC 61259 standard for switching of bus-charging current by disconnecters of a (three-phase enclosed) 145 kV gas insulated switchgear (GIS) are described. These tests are primarily aimed at verifying that the discharge between the disconnector contacts does not introduce internal arc faults. Special emphasis is given to the high-frequency measuring system, used to verify a sufficiently severe waveshape of switching transients in the MHz range. Three-phase tests were performed, showing phenomena that were not demonstrated in practice before: first, it was shown that phase-to-phase voltage transients with a higher amplitude than the phase-to-ground values do actually occur. Second, the phase-to-phase capacitive coupling of the busbar system could raise the trapped charge, left after at the load side of the installation after disconnection, to values above 1 pu. In addition, measurements of induced voltages in the secondary system are reported.

Digital testing of high-voltage circuit breakers

The functionality of high-voltage circuit breakers is tested in high-power laboratories. Due to the necessary power and the physical size of the equipment, testing is rather expensive and time consuming. The steps followed so far by the authors in order to enable the digital testing of HV circuit breakers are described in this paper. At the end of the article, examples of digital testing are also presented.

The impact of capacitor bank inrush current on field emission current in vacuum

Field emission current measurements during the recovery voltage are investigated to understand the origin of restrikes in vacuum interrupters in case of the interruption of capacitive loads. Measurement and analysis of very small field emission currents (0.01 – 1 mA) from the current zero crossing until the restrike are performed both in an experimental circuit as well as in a full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of some kA and kHz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.

Vacuum Circuit Breaker Postarc Current Modelling Based on the Theory of Langmuir Probes

High-resolution measurements on the postarc current in vacuum circuit breakers (VCBs) reveal a period, immediately following current-zero, in which the voltage remains practically zero. The most widely used model for simulating the interaction between the postarc current with the electrical circuit lacks a proper explanation for this event, and hence, it needs to be complemented. We demonstrate that the breakers electrical behavior during this zero-voltage period can be explained by using the theory of a Langmuir probe. Such probes are used to investigate plasma properties such as the ion density and the electron temperature, and we extrapolate its theory to the VCB. After the voltage-zero period, when the transient recovery voltage starts to rise, the breakers electrical behavior is mainly determined by the expansion of an ionic space-charge sheath in front of the cathode. In addition to the current from the Langmuir probe model, the time change of the electric field inside the sheath gives a displacement current. Instead of solving the complicated plasma equations to find the displacement current, we use an approximation by simulating it with the aid of a voltage-dependent sheath capacitance. We programmed the model as a function block in Matlabs SimPowerSystems to facilitate its application in different electrical circuits.

Field emission currents in vacuum breakers after capacitive switching

Field electron emission current measurements during the recovery phase of vacuum interrupters are carried out to understand the origin of restrikes in case of interrupting capacitive loads. A special shunt is designed for this measurement. To compensate the capacitive current component due to the stray capacitance of the interrupter, hardware (electronic, on-line) as well as software (off-line) processing are developed for different applications. Measurement and analysis of field emission currents (0.01-5 mA) from current zero until the moment of restrike are performed both in an experimental circuit as well as in full-power test-circuits with commercially available vacuum circuit breakers (up to 36 kV rated voltage). In this context, the influence of capacitive breaking currents on the dielectric recovery of the vacuum gap and the contacts surface condition is studied. Furthermore, the influence of pre-arcing at contact closing under inrush currents in the range of several kilo-amperes and kilo-hertz on the field emission characteristics after capacitive current switching is investigated. The number of making operations as well as the amplitude of the inrush current is varied. A clear relation between inrush current during closing and field emission current after interruption was established.

Field electron emission current in vacuum interrupters after large inrush current

Field electron emission (FEE) current in the open(ing) gap of nine different designs of 36 kV vacuum interrupters under standard recovery voltage conditions was measured during full-power back-to-back capacitive switching. During closing operation, 20 kA inrush current (@ 4250 Hz) was applied. The measurement system is described, able to measure FEE current with a threshold of 30 μA but that also has to withstand a full capacitor bank discharge current of up to 40 kA after re-strike. It was concluded that in spite of the wide variety of measured FEE current magnitude (up to several mA) there is no direct relationship between magnitude of FEE current and probability of breakdown. Longer arc duration of the normal load current reduces the FEE current level after load current interruption.

The testing of SF/sub 6/ generator circuit-breakers

Generator circuit-breakers face much higher current and voltage stress than distribution switchgear. This has led to a special standard (ANSI C37.013). Strictly in accordance with this standards requirements, test circuits and parameters for a 100 kA and 120 kA (25.3 kV) SF/sub 6/ generator circuit-breaker have been defined. The circuit-breaker is equipped with capacitors at both sides of the extinction chambers. The effect of these is to reduce the TRV severity and this is quantified for the relevant switching duties. Adequate test-circuits are described. Also, the optional verification of interruption of generator-fed faults with very large DC components has been demonstrated. Herein, delayed current zero can extend arcing time. The importance of arc voltage in reducing the longer arcing time is illustrated in a calculated example.

Verification of the Short-Circuit Current Making Capability of High-Voltage Switching Devices

Switching in of short-circuit current leads to pre-arcing in the switching device. Pre-arcing affects the ability of switchgear to close and latch. In three-phase systems, making is associated with transient voltage phenomena that may have a significant impact on the duration of the pre-arcing period. An analysis of these transients is presented. It was found that pre-arcing times in three-phase systems can be considerably prolonged with respect to a single-phase situation. On the other hand, it is demonstrated that the three-phase interaction has a moderating influence on the peak value of asymmetrical current. A test circuit is described, able to perform three-phase synthetic make tests up to 245 kV at current up to 63 kA, representing all transient phenomena. Specific tests are described requiring the maximum available laboratory power: one with a circuit breaker subjected to direct test and one with a high-speed grounding switch subjected to synthetic tests.