Chunju Fan

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

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 Hybrid Current-Limiting Circuit for DC Line Fault in 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 DCCBs 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 HCLCs topology has also been verified by a simplified and scaled test circuit.

An adaptive current protection for distributed network with wind generators

With the integration of wind generators in the distributed network, the power flow and fault current characteristic have been changed. The negative sequence impedance of the doubly fed induction generator (DFIG) is quite different from the positive sequence impedance after the crowbar circuit is activated. This characteristic poses a big challenge on the operation of the protection system. Traditional setting principle for the current protection is based on the assumption that the positive and negative sequence impedances of sources are the same, which is not satisfied any more. The influence of this unique characteristic on the short-circuit current under different fault types and fault positions is analyzed and a correction factor is introduced to modify the setting values of the instantaneous overcurrent protection and time delay instantaneous overcurrent protection adaptively. Finally, simulated results using DIgSILENT PowerFactory demonstrate the validity of the proposed adaptive scheme for the current protection in the distributed network.

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

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.

Distance protection optimization strategy for distribution systems with battery energy storage system

Battery Energy Storage System (BESS) is a type of clean energy, which is able to enhance energy efficiency. However, the connection of the BESS with distribution systems has an impact on the conventional protection. This paper presents the BESS operation characteristics and its impact on distance protection. Optimal strategy of Zone II is analyzed from the perspective of impedance originally. The BESS has three operation statuses which will be studied separately. When a fault occurs at different locations of feeders downstream, the fault current of the BESS increases nonlinearly. By defining two original concepts, critical impedance and maximum protection impedance, the nonlinear output current is classified into two parts. Protect access capacity and protection sensitivity are analyzed under both situations. Modifying value m is presented to solve this protection-related problem. Minimum and maximum values of m are calculated to avoid overreach or blinding operation. After optimized by m, Zone II protection can trip correctly. Analysis and simulation results, based on two practical distribution systems, verified the effectiveness of the optimization strategy.