A Protection Principle of LCC-VSC Three-Terminal HVdc System Based on Instantaneous Boundary Impedance

In the field of hybrid multiterminal high-voltage direct current (HVdc) transmission, an inevitable problem is the short availability of fault information. Namely, only 1-ms complete fault information can be used to identify faults. To overcome this limitation, this study introduces the concept of instantaneous boundary impedance and proposes a dc line protection method. First, a dc transmission line is divided into three types of zones, namely, internal, forward-external, and backward-external zones. Then, the instantaneous boundary impedance is analyzed and compared under normal operation and different types of faults. The results indicate that under normal operation, the instantaneous boundary impedance is zero; under a dc line fault, the instantaneous boundary impedance is less than zero; and finally, under an external fault, the instantaneous boundary impedance is greater than zero. The proposed principle is validated using the Wudongde line-commuted converter-voltage-source converter three-terminal hybrid HVdc real-time digital simulation test system. The experimental results demonstrate that the proposed protection method is effective under different types, locations, and impedances of faults and signal-to-noise ratios within a period of 1 ms. Compared to the existing methods, the proposed protection method can achieve better performance in withstanding fault impedance (600 Ω) and has better robustness against noise interference (10 dB).