Impacts of Shunt Reactors on Transmission Line Protection

Shunt reactors are installed to offset the capacitive effect of transmission lines and therefore improve the voltage profiles of transmission lines. In addition, they also help regulate the volt/VAR of power systems. Specific implementations of shunt reactors may greatly differ between utilities. Reactors can be placed on a section of the transmission line or on the adjacent bus. Current transformers (CTs) may be installed on the reactors, or the line protection devices may rely on bus CTs. Each philosophy presents challenges for reactor, line, and/or bus protection schemes. This paper examines typical implementations for shunt reactors, considers their advantages and disadvantages, and presents protection requirements and solutions required by each implementation. For example, a line-installed reactor can be kept within the line protection zone or taken out of the line protection zone by measuring its current and effectively subtracting it from the total line current. In the former case, the reactor represents a shunt branch within the zone with the possibility of inrush current, effect of zero-sequence feed into the zone during external ground faults, and so on. In the latter case, we need to consider the performance of reactor CTs during inrush conditions and ground faults forcing the zero-sequence fault current in the reactor and the potential impact of CT errors or CT saturation on the security of line protection, especially if sensitive protection elements are used. We begin the paper by examining why shunt reactors are used on power lines and then discuss how to calculate the size of the reactors. Next, we see what impact these shunt reactors have on the power system. Lastly, we investigate the impact shunt reactors have on line protection schemes.