Breaker Switching Analysis for Wind Power Plants
By: Bikash Poudel, Senior Consultant
Vacuum Circuit Breaker (VCB) switching for protection and control operations in electric power systems may cause high frequency transients overvoltage that can affect the transformers nearby. These transients exhibit phenomenon such as current chopping, pre-strike and re-strike. Studies done by EnerNex have shown that such high frequency transients have caused transformer failures. For wind power plant transformers protection, such studies are also recommended by IEC/IEEE 60067-16.
Vacuum circuit breakers are understood to be capable of initiating a phenomenon described as current chopping. The physics of the vacuum circuit breaker allow for a smaller space to be utilized in the interruption of current in a vacuum. It is well well-known that these devices can interrupt (chop) current. This is a different behavior than typical air circuit breakers, which normally allow current arcing following contact separation until a natural zero crossing occurs. Usually, the current chopping phenomenon is not troublesome. However, there are specific circuit configurations that can cause problems. The most common concern results from the use of vacuum interrupters to de-energize unloaded transformers or other highly inductive circuits. In this case, the inductive current to the transformer is interrupted, causing a transient overvoltage.
Higher transient overvoltage can result from transient frequency current chopping. This is also known as virtual current chopping. In this case, if the vacuum interrupter contacts have only separated a very short part of their normal travel when the current reaches the 2-10 amps instantaneous value, there is a possibility that the transient recovery voltage (TRV) of the interrupter will be exceeded by the overvoltage transient of the initial current chopping. If the TRV is exceeded, then there will be restriking of the interrupting device. After restriking, the current is now flowing through the interrupter includes 60 Hz and transient frequency components. The transient frequency component is dominant and the next zero crossing of the current will initiate another interruption of current. This results in a sequence of events that repeats itself with escalating transient magnitudes. Virtual current chopping does not occur very often, however, the resulting transient voltages may be quite high.
Pre-strikes are a breakdown of the vacuum dielectric during closing of a vacuum circuit breaker. A pre-strike occurs when an arc current flows for a short period-of-time before mechanical contact closure. The pre-strike phenomenon is very complex and difficult to predict. The transient currents and voltages are dependent on many factors, such as circuit breaker characteristics, dielectric properties, surge impedance of the circuit components, and high-frequency current interrupting capability.
Transient voltages during pre-strikes are a result of the interaction (resonance) between a transformer inductance and a capacitance network. The susceptibility to the resonant frequency phenomena depends on cable length and other capacitive parameters and on the transformer inductance.
Suppose the current through a 34.5 kV vacuum circuit breaker is chopped at 6 Amps at 0.0202 s in a wind plant substation. An example transient waveform at the 34.5 kV substation are shown in Figure 1. The figure shows waveforms for four different home run cables i.e. a short conductor, a 100m cable, a 500m cable and a 3km cable. Studies can show that wind plants have adequate line capacitance to suppress transients caused by current chopping of VCB.
Figure 1 Transient voltage at 34.5kV Bus after VCB current chopping at 6 Amps
Figure 2 shows the breaker current and first padmount transformer voltage during breaker re-strike and pre-strike cases. Here too, it is evident that the line capacitance has suppressed the transient overvoltage.
Figure 2 34.5 kV Transient BKR current and XFMR voltages for BKR re-strike and pre-strike cases
For some wind plant configurations, it may be that the collector cable capacitance provides mitigation of breaker switching concerns. However, there may be some configurations where overvoltage mitigation is necessary. There are several mitigation alternatives for controlling the high-frequency transients and very steep overvoltage that can overstress the insulation system of the electrical equipment. The most popular protection method is MOV surge arresters connected at the terminals of transformers and switchgear. Surge arresters provide overvoltage protection; however, they may not adequately limit very high rate-of-rise (dv/dt) transient voltages. Surge arresters do not filter the high-frequency oscillations and they do not eliminate reflected waves.
In addition to surge arresters, there are several mitigation alternatives that can control the rate-of-rise of the transient voltages. This is beneficial because severe dv/dt transient voltages can damage the first few turns of insulation of dry-type transformers and motors. Additional mitigation options include surge capacitors, snubbers, ZORC Surge Suppressors, and series inductances. Snubbers are R-C filter networks that include fuses, capacitors and resistors. The ZORC Surge Suppressor is a combination of resistors, capacitors, and Zinc Oxide (ZnO) surge arresters.