Measures to Mitigate Overfrequencies at an HVDC Rectifier Station

Songo rectifier station is located 6 km away from the Cahora Bassa (CB) hydroelectric power station. It was designed to transmit 1920 MW from the 2075 MW generated to the Apollo inverter station in South Africa. Frequency fluctuation at Songo is very high because the active power balance is easily destroyed after an HVDC disturbance. Faults on the three HVAC transmission lines, faults on the two HVDC transmission lines and trips of the converter bridges make Songo rectifier station Busbar I (BBI) susceptible to overfrequencies. Load rejections can be large because the generators and converter bridges operate most of the time under full load conditions. Frequency fluctuation has a negative impact on the interconnected system. It causes the tripping of the sharply tuned AC harmonic filters and consequent tripping of the HVAC bus coupler (BC) at Songo, resulting in power reduction to Eskom or to ZESA. It may also cause the tripping of the converter valve cooling pumps due to overload and consequent tripping of the entire HVDC station. To propose mitigating measures to the overfrequencies occurring at Songo BBI, the actual behaviour of the Grid Master Power Controller (GMPC) and the actual behaviour of the HVDC scheme controls were analyzed through data recorded in the last 10 years by the Transient Fault Recorder (TFR) installed at Songo. The results reveal that frequency deviations have increased in recent years at the rectifier station mainly due to the failure of the inverter station to take over current control after HVDC disturbances. This increases the magnitude of the load rejections.