Transient Recovery Voltage of Generator Circuit Breaker 3

Part 3: Transient Recovery Voltage Simulation of a Medium Voltage Generator Circuit Breaker for 90 Degree Out of Phase Current Switching

In a typical generating station, a medium voltage generator (G) is connected to the medium voltage terminals of the generator transformer (T1) via 3 single phase medium voltage generator circuit breakers (GCB) and isolated phase busbar (IPB). One side of the GCB is connected to generator, the other side is connected to the medium voltage terminals of T1. The high voltage terminals of T1 is connected to the high voltage transmission line. The other end of the transmission line is terminated at the high voltage transmission substation.

Transient Recovery Voltage (TRV) for the GCB is the voltage that appears across the terminals of GCB after switching of the out of phase current. TRV is a critical parameter for current interruption by the GCB. Inadequate TRV rating may cause failure to interrupt current by the GCB. TRV study is carried out to ensure that GCB interrupts current safely and prevent damage to vital equipment and personnel in the generating station.

Out of phase TRV is generated when GCB switches out of phase current. Out of phase current is generated when the voltage on each side of GCB are out of phase (90 Degrees) before closing of the GCB.

Based on the single line diagram of the generating station, a system block diagram is prepared, using Specialized Power System blocks of Simscape electrical section. The block diagram includes equivalent resistance, inductance and capacitance of medium and high voltage systems.

A simulation is carried out and TRV is measured in the scope. Out of all the phases, the phase having maximum TRV is selected for signal processing. The signal is then processed by zooming time axis till the first peak of the TRV waveform is displayed. This waveform is plotted in the work space and exported to an excel file. TRV is replotted in excel. The peak value of TRV (uc) is added. Then the tangent to the upper part is added to find time to peak (t3). A second tangent to the lower part is added find the time delay (td). Rate of rise of recovery voltage (RRRV) is computed (uc/t3).

Simulation results and a phasing diagram is indicated in the file.