High-voltage direct current (HVDC) transmission line protection is becoming increasingly desirable with the expanding worldwide popularity of HVDC technologies in recent years. This paper proposes a transmission line backup protection scheme based on the integral of reactive power for HVDC systems. The directional characteristics of reactive power flow are theoretically analyzed for internal and external faults, and these characteristics are used to construct a directional protection scheme. The Hilbert transform is adopted to calculate the reactive power, which ensures a continuous output of calculation results and improves the reliability of the protection. A bipolar 12-pulse HVDC test system based on the CIGRE benchmark is modeled using PSCAD/EMTDC, and extensive simulations of various fault situations are conducted to test the effectiveness of the proposed scheme. The simulation results show that the proposed protection scheme correctly identifies internal and external faults and performs well with different fault distances and fault resistances. Furthermore, the proposed protection is insensitive to the sampling frequency, making it practical for future applications.Index Terms-directional protection, Hilbert transform, HVDC system, power system protection, reactive energy.

Differential protection is widely used for transmission lines, but its performance is affected by distributed capacitance current and current transformer (CT) saturation. Travelling wave (TW) differential protection is immune to these factors and can achieve ultra-high-speed operation. These characteristics are essential for long-distance extra/ultra-high-voltage (EHV/UHV) transmission line protection. However, the contradiction between sampling frequency and communication traffic makes it hard to ensure high sensitivity and high reliability at the same time. To solve this problem, a new TW differential protection based on equivalent travelling wave (ETW) is proposed in this paper. Wavelet transform is adopted to extract wavelet transform modulus maxima (WTMM) of TW, which are used to reconstruct ETW. Only several WTMMs need to be exchanged with the opposite terminal, so the amount of communication is largely reduced. Current energy ratio is defined and used for operation criterion to enhance the sensitivity during faults with large fault resistance and small fault inception angle. Detailed discussions on the selection of setting value ensure the reliability of the proposed protection scheme. Extensive simulations are conducted to test the performance of the protection. Simulation results verify that the protection can correctly discriminate between internal and external faults with high sensitivity and reliability.

Travelling wave based protection is becoming attractive in ultra-high voltage (UHV) transmission systems due to the requirement for high-speed protection. However, the poor transfer characteristics of coupling capacitor voltage transform- ers (CCVTs) for high-frequency voltage have strictly limited its application. A novel travelling wave based protection scheme considering the characteristics of CCVTs and current transformers (CTs) is proposed in this paper. Dyadic wavelet transform (DWT) is used to extract the polarities of voltage and current travelling waves for fault direction identification. A prototype is further developed for practical application. Extensive laboratory tests are performed to assess the performance of the developed scheme. The prototype has been applied in the 750 kV substations in China and an external fault was recorded during the operation period, validating the effectiveness and reliability of the proposed protection principle.Index Terms-directional protection, power system protection, prototype implementation, travelling wave, UHV transmission line, ultra-high-speed protection, wavelet transform. 0885-8977 (c)

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