Transient Recovery Voltage of Generator Circuit Breaker 4

Part 4. Transient Recovery Voltage Simulation of a Medium Voltage Generator Circuit Breaker for Load Current Switching.

In a typical generating station, a medium voltage generator is connected to the medium voltage terminals of the generator transformer (T1) via 3 medium voltage single phase 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 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, where the transmission load is applied.

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

Load current switching TRV is generated when GCB interrupts the load current.

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

A simulation is carried out and TRV and load current are measured in the scopes. TRV waveforms of all the phases are displayed in the scope. The phase having maximum value of TRV is selected. The data is logged to workspace. TRV is plotted from workspace data. The plot data is then exported to an excel file. TRV is replotted in excel. The peak value of TRV (uc) is added. A tangent to the upper part of the TRV curve is added to determine time to peak (t3). Another tangent to the lower part of the TRV curve is added to determine time delay, td. The rate of rise of recovery voltage (RRRV) is computed from uc/t3. The load current waveform of the selected phase is printed for information.