Temporary Overvoltage. The temporary overvoltages (“TOV”) potentially caused by the Merchant Transmission Facility system shall be defined to include, but not be limited to, all voltage components resulting from the following causes:
Temporary Overvoltage. High voltages at the converter buses can occur due to load rejections or other cause. The HVDC system shall remain connected and provide dynamic voltage control to mitigate the voltage rise within the capability of the equipment. The requirement for ac system voltage control during an overvoltage has highest priority and therefore it is permissible to run back the active power transfer to limit the overvoltage. The converter valves shall be capable of continuing to operate under the temporary overvoltage conditions specified below, which could occur with the valves deblocked and also that the valves are capable of deblocking under the highest temporary over voltage conditions within five (5) cycles of the initiation of a fault or disturbance. The equipment shall be designed for the applicable short circuit ratio and overvoltage arising thereby. Temporary Overvoltage caused by Bipole link HVDC transmission shall be controlled to 1.4 pu or below. Temporary overvoltage caused by other equipment in the AC network shall be controlled within the limits of the capability of the deblocked converter. In case of DC line permanent fault, the converters should restart in STATCOM mode for AC grid support. In case the converter is tripped, and not possible to restart within seconds, filter tripping shall be allowed to limit overvoltage as applicable. The actual temporary overvoltage shall be determined by the TSP, but the equipment shall be designed for temporary over voltage not less than the values given above. In addition, so as to prevent operation and overstressing of the arresters, the TSP shall limit the temporary over voltages including harmonic, resonant, and ferro-resonant effects on the 400 kV AC bus bars so that: - 705 kV crest phase to ground is not exceeded by more than 3 peaks; - 565 kV crest phase to ground is not exceeded by more than 10 cycles; - 510 kV crest phase to ground is not exceeded by more than 20 cycles. In the calculation of temporary over voltages on the AC side the TSP shall allow for blocking of the complete Bipole from up to the highest steady state transmission capability of the installation. On the DC side the TSP shall allow for the maximum load rejection which could occur and which leaves converter deblocked. The converter Equipment shall be designed to withstand temporary over voltages corresponding to AC Side which are not less than 1.5 times 400 kV upto 100ms at converter stations with the converter blocked The TSP shall provide a...
