Fast and reliable DC fault detection is one of the main challenges for modular multilevel converter (MMC) based DC grid with DC circuit breakers (DCCBs). This paper extracts the high-frequency components in transient voltages by wavelet transform and proposes a fault identification method based on the difference of square of transient voltages to identify the faulted lines for DC grids using overhead lines. Meanwhile, a faulted pole discrimination method based on the difference between the change of positive and negative pole voltages is presented. A line protection scheme including detection activation, fault identification, faulted pole discrimination, and post-fault re-closing is designed. Using only the local measurements, the scheme can realize the protection of the whole line without communication and has the capability of fault resistance endurance and anti-disturbance. The proposed method is tested with a four-terminal MMC-based DC grid in PSCAD/EMTDC. The selection methods of threshold values are presented and the impact of DCCB operation on the reliability of DC fault protection is analyzed. Simulation results verify the fast detection and reliability of the designed DC line protection scheme.

/pdf/a-transient-voltage-based-dc-fault-line-protection-scheme-1d2d2xwe9h.pdf

A Transient Voltage-Based DC Fault Line Protection Scheme for MMC-Based DC Grid Embedding DC Breakers

This paper proposes a fast dc short-circuit fault detection method for HVdc grids, which relies on the voltage across the dc fault-current-limiting reactor. In the proposed approach, primary detection uses only the single-terminal dc reactor voltage information and is shown to be able to identify the faulted line as well as the faulted pole within 1 ms. The backup detection is primarily required for high resistance faults and uses communication between the two sides of the line. Its speed is dependent on the travel time of signals between the two ends, but will operate, as shown in the paper, typically in 2 ms or less. The approach is justified using analytical calculations and exhaustively validated via electromagnetic transient simulation for pole-to-pole and pole-to-ground faults, different fault resistance, fault locations, and dc reactor values. The validation shows that the method is able to identify all genuine dc line faults, but is not prone to misoperation with ac line faults, different loading levels, and so on.

A Fast DC Fault Detection Method Using DC Reactor Voltages in HVdc Grids

This paper provides a detailed analysis of the temporal development of fault currents in a multiterminal high-voltage direct-current (MT-HVDC) grid composed of a bipolar converter configuration. The sequence of events following the occurrence of a pole-to-ground fault is identified, divided into three distinct periods; namely, submodule capacitor discharge, arm current decay, and ac in-feed periods. The critical parameters that have a significant impact on the fault current in each period are discussed. The impacts of various parameters of the HVDC grid such as the size of the current limiting reactor, ac grid strength, as well as the location of the fault within the grid are studied through PSCAD/EMTDC simulation. Then, a fault current interruption process using models of various HVDC circuit breaker technologies and the resulting stresses are studied. Both serve as important inputs to define test procedures. It is found that the HVDC circuit breakers are subjected to not only dc current and voltage stresses but also energy stress. These stresses are translated into test requirements.

Analysis of Faults in Multiterminal HVDC Grid for Definition of Test Requirements of HVDC Circuit Breakers

The fast dc line protection is one of the key techniques for the multi-terminal flexible dc grid whose main technical difficulty lies in quick and reliable fault identification. The traditional protection principles for the ac system are not suitable for the flexible dc grid, and those for the conventional dc transmission system can also not be applied directly. Therefore, the frequency-domain traveling-wave boundary characteristics after faults are analyzed first in this paper to find the critical condition for distinguishing the internal and external faults as well as the forward and backward faults. And then, a novel single-ended transient-voltage-based protection strategy for flexible dc grid was proposed, which mainly consists of the fault identification criterion and the direction criterion. The protection scheme used wavelet transform to extract transient voltage energy. In order to eliminate the influences of the wavelet transform boundary effect and converter blocking, the principle to select the proper data window was proposed. Compared with the existing traveling-wave-based dc protection, the proposed method has a higher reliability and stronger ability to endure high transition resistance, which was verified by lots of simulation cases based on the PSCAD/EMTDC.

A Novel Single-Ended Transient-Voltage-Based Protection Strategy for Flexible DC Grid

Multi-terminal HVDC is an important new technology for bulk power transfer from large sources of remote renewables to large load centers. Recently MTDC grids have become more technically feasible due to improvements in VSC based converters, which can reverse currents and generally have greater control than LCC. Furthermore, emerging DC breakers technology unlocks the possibility of de-energizing only the faulted section of an MTDC without losing the whole grid. The protection relaying in such grids is not trivial: it must be very reliable, robust to the converter control system׳s effect on the post fault transients and act quickly, before the fault current develops beyond the presently modest interrupt capability of DC breakers. Within this challenging environment, this paper discusses in detail the important considerations for designing MTDC protection, including accurate modeling of the converter topology, associated control scheme and the DC link itself, supported by the results of novel transient simulation studies. This context is then used to appraise the suitability of the existing protection of HVDC and MTDC grids and suggests future work in the area.

Design of protection schemes for multi-terminal HVDC systems

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.

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

The high fault current of dc line is a major threat to multiterminal voltage-source-converter-based HVDC (VSC-HVDC) system. However, dc circuit breaker (DCCB) with large capacity and fast breaking speed is still under development. Therefore, fault current limitation is vital for the multiterminal VSC-HV DC system. This paper proposes a simple and easily applied hybrid current-limiting circuit (HCLC) at dc side, which consists of a current-limiting inductor (CLI) and an energy dissipation circuit (EDC) in parallel with the CLI. The CLI is designed to reduce the requirement for the DCCB's capacity and breaking speed. The EDC, which consists of thyristor-controlled resistors, is proposed to reduce the stress on energy absorption element (metal oxide arrester) in DCCB and to accelerate the fault current interruption. The design and discussion about the HCLC parameters are performed in detail. By employing the proposed HCLC, dc line fault in the multiterminal system can be isolated effectively with existing DCCB technology, and fast system restoration without power interruption of healthy part can be achieved. Numerous simulations with real-time digital simulator and comparisons with traditional schemes have demonstrated the promising performance of the proposed HCLC. The effectiveness of the HCLC's topology has also been verified by a simplified and scaled test circuit.

A Hybrid Current-Limiting Circuit for DC Line Fault in Multiterminal VSC-HVDC System

With the rapid development of renewable energy, multi-terminal flexible DC grids composed of modular multilevel converters have been widely used in power systems. To ensure the safe and stable operation of a DC grid, this paper proposes a DC line protection scheme for MMC-based DC grids using AC/DC transient information. In the proposed scheme, the difference between the transient voltage root mean square ratios (DTVR) on both sides of the current-limiting inductor of DC lines is used to identify the DC line fault, and the instantaneous zero sequence voltage (IZSV) of the AC side of the converter station is used to discriminate the faulted poles. According to the theoretical analysis result, a fault criterion setting method suitable for faulted pole discrimination is proposed. The proposed scheme is based on local information and tested in a PSCAD/EMTDC model of a four-terminal MMC-based DC grid. The simulation results show that the scheme can fast and accurately identify faults.

A DC Line Protection Scheme for MMC-Based DC Grids Based on AC/DC Transient Information

HVDC lines play an important role in transmission power over long distance, while faults frequently occur on DC lines due to complex terrain and terrible weather conditions, which is a major cause of HVDC outages. Traveling-wave-based protection and voltage derivate theory are used as the primary protection for HVDC transmission lines to detect a ground fault, and differential protection is employed as backup protection. However, traveling-wave-based methods and voltage derivate theory still have shortcomings and current differential protection operates with long time delay. Thus, protection with high reliability is very important and necessary to ensure the security of HVDC transmission lines. By analyzing the direction characteristics of specific frequency current at DC line ends, a novel pilot protection principle for HVDC transmission lines is proposed. The specific frequency current is adopted through spectrum analysis of DC-filter and their directions are analyzed during internal and external fault, and it is found that the directions of specific current detected at DC line ends are the same during internal fault, while their directions are opposite during fault outside of DC lines. Therefore, the direction difference of specific frequency current can be used to identify external fault from internal fault. An improved backup protection scheme for HVDC transmission line is also proposed by resetting two voltage derivative criterions in traveling-wave-based protection. Simulation results show that the proposed protection scheme is valid and feasible, and is of capability to distinguish fault with high transition resistance.

A novel pilot directional protection scheme for HVDC transmission line based on specific frequency current

High-voltage direct current system based on voltage source converters (VSC-HVDC) is widely recognized as a feasible solution to cope with increasing electricity demand and integrate renewable energy sources. One of the most important issues still to be addressed is the protection of the system under DC line fault, especially for the situations using overhead lines due to higher fault frequency. This paper proposes a new fault detection method for DC line in VSC-HVDC system using the correlation of currents at DC-link capacitor branch and DC line end. In this approach, Pearson correlation coefficient is used to measure the correlation. Fault detection can be achieved easily by calculating the Pearson correlation coefficients at both ends of DC line. Simulation results using PSCAD/EMTDC demonstrate that the proposed method is able to detect the DC line fault with high selectivity and high robustness to fault resistance and noise, and it is suitable for multi-terminal VSC-HVDC system.

Jian Liu

Papers

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

A Hybrid Current-Limiting Circuit for DC Line Fault in Multiterminal VSC-HVDC System

A DC Line Protection Scheme for MMC-Based DC Grids Based on AC/DC Transient Information

A novel pilot directional protection scheme for HVDC transmission line based on specific frequency current

A fault detection method for DC lines in VSC-HVDC system based on current correlation