Impact of power flow on the apparent impedance calculated by the distance protection in transmission lines

Abstract

Abstract The reach of primary protection zones of transmission lines distance relays is affected when large power flows circulate through the transmission line prior to a short circuit occurrence. This can cause improper operation of the protection system in certain scenarios. In practice, protection zone adjustments are made by considering the apparent impedances obtained from short‐circuit simulations in the line of interest using standard methods such as IEC 60909‐0 or ANSI/IEEE C37.010. However, these short‐circuit calculation methods omit the existing operating conditions prior to the fault. Therefore, this work proposes a set of equations developed for calculating the apparent impedance seen by the distance relay in view of different types of short circuits, including the effects of power flow prior to the fault. These equations are then used to establish the distance primary protection zone settings. The analysis uses the mathematical expressions proposed of voltages and currents at the relay location, and the impedance diagrams obtained from simulated short circuits. Subsequently, the settings obtained with these equations are compared to those achieved using the IEC 60909‐0 standard, allowing for the differences in settings and their effectiveness. Finally, this procedure is applied to a part of the Ecuadorian Transmission System.

Keywords

• electric impedance measurement
• failure analysis
• power transmission lines
• relay protection
• short‐circuit currents