Impacts on and benefits of innovations for protection of series capacitor banks in HV/EHV-systems

Deregulation trends and right of way issues are forcing the pace of series compensation of transmission lines. Many installations in the USA are being refurbished, since the existing installations can no longer cope with the increased short circuit level. The installed gaps are mostly of the self triggering types and the available MOV-energy is not sufficient to handle the short circuit current. Even the capacitors have to be replaced, since they contain environmentally harsh PCB. In some other cases, there is a need to improve the system dynamic performance and avoid subsynchronous oscillations due to their damaging effects on turbogenerators. TCSC (thyristor controlled series capacitors) schemes are being used more often in the form of FACTS-technology to improve system dynamics. The platform equipment is also undergoing a transition, since the utilities prefer maintenance free equipment and less electronics at the high voltage level. There is also a strong desire to keep the banks switched on (not bypassed; improved life time of bypass breakers through avoidance of unnecessary close-open duty cycle) even for line currents up to zero. All these requirements have led to a rethinking regarding the control and protection practices of series capacitor installations. This paper describes innovative solutions implemented in some of the installations and the experience gained during system operation.