A new protection for hybrid cascaded DC transmission line using voltage travelling wave phase difference

A new protection for hybrid cascaded DC transmission line using phase difference of voltage travelling waves is put forward. According to fault travelling waves propagation process, the equivalent fault analysis circuits for internal fault and external fault are obtained. Then it is found that after internal fault, for either terminal of the line, at high frequency, the phase difference between the voltages at two sides of current-limiting reactor (PDVR) is approximately 0°. Whereas after external fault, the PDVR near the faulty terminal of line is approximately 90°, and at the distance faulty side, PDVR equals about 0°. Therefore, PDVR can be used to identify the faults. Simulations verify that the protection can differentiate the internal and external faults accurately, and reliably respond to high-resistance faults. It only needs to transmit the logic value of fault discrimination to two terminals, having low requirements for communicating channels. Besides, the identification criterion threshold is derived based on theory, providing the theoretical basis for the setting of on-site protection.