A novel non-unit protection scheme for HVDC transmission lines based on multi-resolution morphology gradient

This paper presents a novel non-unit protection scheme for the protection of LCC-HVDC transmission lines. The fault-generated travelling waves at the faulty line ends are filtered by a filter–reactor unit. The filtered current waveforms are then processed by a multi-resolution morphological gradient and are used by a fast non-unit protection scheme for LCC-HVDC transmission lines. The proposed method does not require communication links between transmission line ends and uses only current signals with a low sampling rate of 10 kHz. In addition, the proposed scheme is computationally efficient, making it suitable for practical applications. The performance of the proposed protection scheme is validated by comprehensive simulation studies as well as field data on a bipolar LCC-HVDC system. Both simulation and field data test results verify the accurate performance of the proposed method for different internal fault conditions, including high fault resistances up to 1000 Ω. Furthermore, the proposed scheme is robust against external DC and AC faults, change of sampling frequency and data window length, non-ideal faults, change of power flow, change of DC filter parameters and smoothing reactor, change of operation mode, and measurement noise.