Distance protection is widely applied in High/Ultra-High Voltage AC transmission system. Impedance relay, the core element of distance protection, may operate incorrectly under power swing, so a power swing blocking unit is needed in distance protection. In this paper, the phase difference between the measurement voltage and compensation voltage under power swing and line fault is analyzed. Based on the analysis, a power swing blocking strategy using the change rate of phase difference between measurement voltage and compensation voltage is proposed. Simulation results in PSCAD/EMTDC indicate that the proposed strategy can block the impedance relay reliably under power swing and unblock the impedance relay after detecting the fault under power swing.

Distance protection; High voltage AC transmission system; power swing; power swing blocking strategy