Line-commutated converter-based-high-voltage direct-current (LCC-HVDC) transmission technology has been developing very rapidly. However, the sensitivity and reliability of HVDC line protection require further improvement. An analysis of different line fault types is conducted to show that the normalized voltage change rate without the influence of wave propagation can more effectively distinguish between internal and external faults with high fault resistances. Owing to the difficulty of obtaining an accurate solution for a traveling wave, a novel traveling-wave propagation calculation method is developed by making use of the step response of a transmission line with consideration for its frequency-dependent characteristics. A novel traveling-wave protection (TWP) scheme for two-terminal LCC-HVDC transmission lines is then proposed based on the developed traveling-wave propagation calculation method and fault analysis results. The proposed protection scheme is tested based on fault simulation data using an EMTDC/PSCAD simulation platform and the fault-recorded data from an actual undesired tripping event of conventional TWP. The results demonstrate that the proposed TWP scheme, compared with conventional TWP, can more reliably and correctly detect faults with high transition resistances under different operation modes.

This paper presents a novel time-domain fault location algorithm on HVDC transmission lines. The Bergeron line model is adopted to construct a time-domain fault location equation, considering unsynchronized two-end measurement and uncertain line parameters. The zero time reference point is set to the sample point of arrival at each end for the traveling wave-head. Time-tags of all sample points change with the zero time reference. So the time difference of two-end measurements has changed also, and equals the time difference between arrivals at two ends for the traveling wave-head. That makes it possible to mix the traveling wave theory with the Bergeron time-domain fault location method. Besides, the ratio coefficient of wave velocities between different frequencies and digital band-pass filter are presented for better performance of the proposed method. Simulations and tests demonstrate that the proposed method can locate faults accurately on the HVDC lines even in the case of loss of signals from GPS.

The deployments of inverter-interfaced distributed generators (IIDGs) in medium voltage distribution networks may cause undesirable trips or tripping failures of conventional two-terminal current differential protective relays. It is economically and technically unfeasible to reconfigure large numbers of time-synchronized sampling clocks and communication channels for each terminal inside the protected zone. In this paper, a more accurate IIDG mathematical equivalent model is established that considers the control strategies of IIDG, and the profile characteristics of the positive-sequence voltage at each point of common coupling are investigated. Then, a real-time estimation algorithm is introduced for the fault current contribution of grid-connected IIDGs. On this basis, this paper proposes a novel virtual multi-terminal current differential protection scheme. This scheme only necessitates exchanging the electrical quantities between the protective relays at both terminals via an existing two-terminal pilot channel, thus reducing the cost and requirements for communications. The feasibility of the proposed protection is demonstrated in both the PSCAD/EMTDC simulation platform and the RTDS hardware experimental laboratory. A comparison of the proposed protection alongside the conventional protection is also presented. Test results indicate that the proposed protection is widely applicable to various fault conditions, thereby improving the sensitivity and selectivity of conventional protection schemes.

The fault resistance is the key factor to affect the travelling waves and the performance of travelling-wave-based protections on HVDC transmission lines. In order to explain accurately the impacts of the fault resistance to travelling waves in theory and improve computational efficiency on the existing data of the travelling waves at given fault conditions, this paper proposed a global-sensitivity-based method to fast predict the new travelling waves with any other fault resistance. This method was built on the actual HVDC project topology and its frequency-dependent nature, and derived by the infinite-Taylor series in the phase and frequency domain, and then realized by a rational fitting technique with an order estimation scheme for the conversion to the time domain, which would overcome three limitations compared with conventional sensitivity methods. Numerical examples show that the proposed method has a huge advantage in saving CPU time to predict travelling waves in the network with any fault resistance increment over traditional simulations.

Compared with conventional HVDC systems, the hybrid multiterminal HVDC (Hybrid-MTDC) system has a more sophisticated network topology composed of line commutated converter (LCC) and modular multilevel converter (MMC) stations, which leads to DC fault discrimination being difficult and ambiguous. Furthermore, for long transmission lines, high fault resistances and different converter impedance characteristics, the reliable discrimination of external and internal faults located at the remote end and T areas is particularly demanding. Therefore, considering the frequency characteristics of DC boundaries and equivalent converter impedances and DC line models, this paper constructs a time-domain convolution power. The performance of the convolution power in various fault scenarios is mathematically elaborated. Due to its good decomposition ability for short-time nonstationary signals, wavelet decomposition is utilized to extract the specific frequency bands of traveling waves; thus, the convolution power in different frequency bands can be calculated. On this basis, a convolution power-based protection scheme is proposed for identifying external and internal faults, locating a DC bus fault and selecting faulty poles. PSCAD/EMTDC simulation results show that various DC faults can be correctly identified within 3 ms, and the proposed scheme has high reliability and sensitivity, which is beneficial for the protection of Hybrid-MTDC systems.