Transient Recovery Voltage of Generator Circuit Breaker 2

Part 2: Transient Recovery Voltage Simulation of a Medium Voltage Generator Circuit Breaker during System Source Fault

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

Transient Recovery Voltage (TRV) for the GCB is the voltage that appears across the terminals of the GCB after interruption of fault current. TRV is a critical parameter for the fault current interruption by the GCB. In adequate TRV rating may cause failure to interrupt fault current. TRV study is carried out to ensure that the GCB interrupts fault current safely and prevent damage to vital equipment/ personnel in the generating station. TRV values are usually included in the procurement specification of the GCB.

System Source Fault (SSF) TRV is generated when the GCB interrupts fault current, occurring due to a fault between the GCB and the generator.

Based on the single line diagram of the generating station a system block diagram is prepared using the Specialized Power System subsection of the Simscape Electrical section. The block diagram includes equivalent resistance, inductance and capacitance of the medium voltage system on either side of the GCB. The block diagram also includes resistance, inductance and capacitance of the high voltage system.

A simulation is carried out and TRV is mesured in the scope. TRV waveform for all the phases are displayed in the scope. The phase having maximum TRV is selected for signal processing. Other phases are no longer displayed. TRV signal is then processed by zooming time axis till the first peak of the TRV, after fault current interruption, is displayed. The scope data was logged into the workspace. The data in the work place was plotted. The data for the phase having maximum TRV value was selected for exporting to an excel file. The TRV curve was replotted in excel. The peak value of the TRV (uc) was added. A tangent to the upper part of the TRV curve, passing through the origin, was added. A vertical line, from the point of intersection of the tangent and peak value line of the TRV to the time axis is drawn, which indicates time to TRV peak value (t3). Another tangent is drawn to the lower part of the TRV curve, indicating time delay (td). The parameter uc/t3 is computed, which indicates the rate of rise of recovery voltage (RRRV).

Simulation results are indicated in the model.