Ends Of DC Lines Research Articles

Calculated DC resistance‐based pilot protection scheme for bipolar high‐voltage direct current transmission lines

The current differential protection for high-voltage direct current (HVDC) transmission lines has low reliability due to the distributed capacitive current and time delay. This paper proposes a pilot directional protection, which is based on the calculated DC resistance (CDCR) at both ends of DC lines. The characteristics of superimposed components and CDCR are analysed using the superposition theorem. The magnitude of CDCR is equal to the resistance of the converter for a forward internal fault, while it is equal to the sum of the line's DC resistance and the converter's resistance for a reverse fault. CDCR is a useful characteristic to construct the protection scheme as the resistance of DC line is significantly greater than that of the converter. Detailed simulation tests carried out on PSCAD/EMTDC under different fault conditions show that the proposed scheme is able to identify internal and external faults accurately.

An improved single-ended frequency-domain-based fault detection scheme for MMC-HVDC transmission lines

Aiming at the problems currently existing in transmission line protection for modular multilevel converter (MMC) based high voltage direct current (HVDC) grids, this paper proposes an improved single-ended fault detection scheme for MMC-HVDC transmission lines. The proposed scheme is based on the frequency-domain attenuation rate of the initial voltage traveling wave (IVTW). Firstly, the values of the IVTWs generated by the internal and external faults are calculated respectively based on the equivalent model of the four-terminal MMC-HVDC grid. Due to the boundary effect of DC inductors installed on both ends of DC lines, the attenuation rates of IVTWs with the increase of the frequency under internal and external faults have different characteristics. The detail coefficients of the IVTW in different frequency bands are extracted by stationary wavelet transform at different scales, and the amplitudes of the IVTW in different frequency bands are quantified by wavelet transform modulus maximums (WTMMs) at corresponding scales. A criterion for identifying internal and external faults is constructed based on the square of the frequency-domain equivalent attenuation rate of the IVTW. Lastly, simulation results under diverse fault conditions verify the effectiveness of the proposed fault detection scheme.

A Pilot Protection Scheme of DC Lines for Multi-Terminal HVDC Grid

Multi-terminal HVDC grid has extremely high demand for protection speed of DC lines, consequently, the sampling time window of the main protection is very short, which affects its selectivity and reliability. To make up for the insufficiency of the non-unit main protection, this paper proposes a pilot protection scheme based on the voltage polarities of current-limiting reactors at the ends of DC lines. Analysis shows that the voltage polarities of terminal reactors at the two ends of the DC lines are both positive when internal fault occurs, while they are opposite for external faults. Based on the aforementioned characteristic difference, a pilot protection criterion is built to discriminate between internal and external faults. To avoid mal-operation of the protection on healthy pole generated by the coupling effect, the polarity mutation of reactor voltage on healthy pole due to coupling effect is analyzed when single-pole-to-ground fault occurs. Based on the analysis, the fault voltage of current-limiting reactor is integrated, and its polarity can be identified based on the integration value. At last, a four-terminal modular multilevel converter (MMC)-based HVDC grid model is built in PSCAD/EMTDC, and the effectiveness of the protection scheme is verified by extensive simulations.

A novel pilot directional backup protection scheme based on transient currents for HVDC lines

Abstract For HVDC transmission lines, the backup protection cannot respond to internal faults quickly. To solve this problem, a novel pilot directional backup protection scheme is proposed in this paper. Firstly, the fault characteristic of transient currents on both sides of DC bus is analyzed. When forward traveling-wave of current reaches a DC bus, the transient current integration at the back side of the bus over a short time is larger than that at the front side. However, when backward traveling-wave reaches the bus, the current integration at the back side is smaller than that at the front side. Therefore, fault direction can be determined by comparing the transient currents on both sides of DC bus. Then, internal faults can be distinguished from external faults by comparing fault directions at two ends of DC lines, and faulty poles can be selected by comparing the transient currents of two poles. Lastly, to test the performance of the novel protection proposed in this study, a ±800 kV HVDC system is built in PSCAD/EMTDC. Comprehensive test studies show that the proposed protection scheme not only can identify internal faults and select faulty poles correctly and quickly, but also can respond to high resistance ground faults.

Transient-Voltage-Based Protection Scheme for DC Line Faults in the Multiterminal VSC-HVDC System

DC line faults are major issues for a multiterminal high-voltage direct current (HVDC) system based on voltage-source converter (VSC). The fault current increases quickly along with a large peak, and complete isolation of the faulted system is not a viable option. Therefore, protection with high selectivity and accuracy is essential. In this paper, a new protection scheme for dc line in multiterminal VSC-HVDC system is proposed, which consists of a main protection and a backup protection. Both the protection principles are based on the supplemental inductor placed at each end of the dc line. Fault identification can be achieved by calculating the ratio of the transient voltages (ROTV) at both sides of the inductor. The main protection is able to detect the fault quickly without communication, while the backup protection is a pilot method based on the ROTVs at both ends of dc line, which is employed to identify the high-resistance faults and offer a backup in case the former fails. Comparison with some previous protection methods shows that the performance of the proposed protection scheme is promising. Numerous simulation studies carried out in PSCAD/EMTDC and real-time digital simulator (RTDS) under various conditions have demonstrated that fault identification with high selectivity and strong robustness against fault resistance and disturbance can be achieved by employing the proposed protection scheme.