Nengling Tai

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.

Improved differential protection scheme for long distance UHVDC transmission line

This study concerns the misoperation of differential protection of long distance UHVDC transmission line under external faults. The two main factors that have effect on the sensitivity of differential protection long distance UHVDC transmission line are discussed in this study: transient harmonic currents and shunt branch currents. With the improved protection scheme presented in this paper, the two factors can be removed and the reliability and sensitivity of the differential protection can be improved.

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.

Research and application of climatic sensitive short - term load forecasting

Owing to the importance of the short-term load forecasting, research in this area has resulted in the development of numerous forecasting methods. But the climatic factors will affect greatly the precision of the traditional short-term load forecasting methods, especially the temperature, humidity, wind and rain, etc.. Following the analysis of electric loads and climatic data, the paper presents their relation features and changing rules. According to that, the new forecasting model considering the climatic factors is also introduced. Test results show that the new method can obtain high precision.

An interconnection protection design for the interface of alternating current microgrids

The interface of AC microgrids plays an important part in implementing the flexible operation modes. It always manages the disconnection and reconnection between microgrids and the host distribution networks. This study proposes a new protection design interconnection protection (IP), for the interface of microgrids especially the multiple and large-scale ones. IP is intended to decouple interconnected subsystems to prevent disturbances or faults from damaging the both subsystems. IP can be classified into facility IP, circuit IP, and utility IP corresponding to the microgrid served. The deployments, features, and protection schemes of different IPs are analyzed according to the operation characteristics and requirements of the typical microgrid in IEEE Std 1547.4. Facility IP detects faults and disturbances by monitoring the power quality at the point of common coupling. Circuit IP and utility IP have alternative schemes, for example, current differential protection, distance protection, and overcurrent ...

A novel pilot protection scheme for MMC-HVDC transmission lines

Overcurrent or overvoltage problems caused by DC line faults endanger the security of modular multilevel converter based high voltage direct current (MMC-HVDC) system. To improve the reliability of fault identification for MMC-HVDC transmission lines, a novel pilot protection scheme based on DC power difference is proposed in this paper. Characteristics of the DC power difference between positive pole DC power at one terminal and negative pole DC power at the other terminal are theoretically analyzed for various faults. The analysis reveals that fault detection and fault line selection can be achieved by calculating the DC power differences. According to that, the proposed protection scheme is constructed. Simulation results in real-time digital simulator (RTDS) demonstrate that the proposed protection scheme can identify and classify MMC-HVDC transmission line faults under different fault conditions accurately.

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.

Financial compensation analysis for voltage deviation in power generation market

The enterprises managing power network are obligatory to ensure a financial quantity item for ancillary services, so as to protect the legal rights of the market participants. This paper, with its focus on the voltage deviation, develops a financial compensation mechanism, on the basis of voltage operation theory. The new approach is capable of handling various issues related to voltage management in deregulated power systems, for both the reactive and active power of generators and the reactive power of compensation equipment are incorporated into the objective function of the OPF problem. The test results of simulation and the practical operation demonstrate convincingly that the new method is applicable and hence, will enhance significantly the reliability, security, economic and fair trading of the power market.

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.