• Energy Research

The modular multilevel converter (MMC) topology is very suitable for high voltage direct current (HVDC) system to achieve feasible power control, favorable black start capability as well as long-distance power transmission. This paper proposes a novel current-limiting method based on the coordination of virtual impedance control (VIC) and resistive fault current limiter (R-FCL) for MMC-HVDC, and it aims to effectively decrease the DC fault current level and assist the DC circuit breaker (DCCB) to remove the fault reliably and smoothly. Firstly, the MMC-HVDC’s topology and control strategy are expounded, and the modeling method of the VIC and the R-FCL is presented. According to the DC fault evolution, the time sequence coordination scheme of the VIC and the R-FCL for the MMC-HVDC is put forward. Then, considering the introduction of the proposed current-limiting method, the fault characteristics of the MMC-HVDC are theoretically deduced, and the capacity design of the VIC and the R-FCL is conducted. Using MATLAB, a typical ±320 kV MMC-HVDC system with VIC, R-FCL, and DCCB is built, and comparative simulations are performed. The results indicate the proposed approach can strongly limit the DC fault current within the acceptable range, and reduce the electrical stress and thermal stress in the MMC and DCCB. The proposed approach possesses better techno-economics than the R-FCL alone to solve the DC fault problem, and a more efficient protection of the MMC-HVDC is realized.

Concerning the development of a micro-grid integrated with multiple intermittent renewable energy resources, one of the main issues is related to the improvement of its robustness against short-circuit faults. In a sense, the superconducting fault current limiter (SFCL) can be regarded as a feasible approach to enhance the transient performance of a micro-grid under fault conditions. In this paper, the fault transient analysis of a micro-grid, including distributed generation, energy storage and power loads, is conducted, and regarding the application of one or more flux-coupling-type SFCLs in the micro-grid, an integrated technical evaluation method considering current-limiting performance, bus voltage stability and device cost is proposed. In order to assess the performance of the SFCLs and verify the effectiveness of the evaluation method, different fault cases of a 10-kV micro-grid with photovoltaic (PV), wind generator and energy storage are simulated in the MATLAB software. The results show that, the efficient use of the SFCLs for the micro-grid can contribute to reducing the fault current, improving the voltage sags and suppressing the frequency fluctuations. Moreover, there will be a compromise design to fully take advantage of the SFCL parameters, and thus, the transient performance of the micro-grid can be guaranteed.

In smart substation, with regard to the realization of substation area backup protection (SABP) which can execute the backup protection function for whole substation, the information communication network transmitting and sharing digital signals will undoubtedly play an important role. In this paper, the realtime performance of a smart substation's process level network, which receives voltage/current sampled values (SV) message and sends generic object oriented substation event (GOOSE) message for SABP, was studied. Firstly, the structure and function of SABP were introduced, and its demands for the communication network's working performance were proposed. Besides, taking a practical 220kV smart substation for example, the process level network's structure scheme was designed for SABP, and then the time-delay characteristics of the messages including both SV and GOOSE were estimated. According to the research results, under the condition that the actuating signals of SABP come from the process level network, the time-delay characteristics of this information communication network can satisfy the requirement for the rapidity of action of backup protection. In the end, some measures which may help to reduce transmission delay further were presented in brief.

Abstract Due to the continually expansion in size of electricity distribution networks, the demand for the reliability of power supply is on the increase, and in the case of transferring load, a closed-loop operation is usually needed. However, in the course of the closed-loop operation, a larger surge current may be induced, and this current can cause the equipped over-current protection take action incorrectly. In this paper, a flux-coupling type superconducting fault current limiter (SFCL) is suggested to suppress the surge current caused by the closed-loop operation. The structure and principle of the suggested SFCL are introduced, and considering the SFCL′s current-limiting impedance, the mathematical process for calculation of the surge current is presented. Furthermore, according to the simulation analysis of a practical 10 kV power distribution network integrated with the SFCL, the surge current′s dynamic characteristics and the SFCL′s performance behaviors are both assessed. From the demonstrated results, the SFCL′s positive effects on reducing the surge current and improving the system′s robustness against the closed-loop operation can be verified. At the last of the paper, a few technical discussions regarding the SFCL′s engineering application are conducted, and some useful conclusions related to the practicability of experimentally building the SFCL are obtained. PACS codes: 85.25.Am; 84.30.Jc.

Electric vehicle sharing provides an effective way to improve the traffic situation and relieve environmental pressure. The government subsidy policy and the car-sharing operator’s pricing strategy are the key factors that affect the large-scale application of electric vehicle sharing. To address this issue, a subsidy and pricing model for electric vehicle sharing based on the two-stage Stackelberg game is proposed in this paper according to the current situation in China. First, an electric vehicle sharing operation mode under government participation is constructed. Then, a two-stage Stackelberg game model involving the government, the car-sharing operator and the consumers is proposed to determine the subsidy rates and pricing strategies. The improved particle swarm optimization algorithm is used to obtain the Nash equilibrium of the model. Also, the influence of private car cost and shared travel comfort on subsidy rates and pricing strategies is analyzed. Finally, the simulation of electric vehicle sharing in a town of China is carried out to investigate the performance of the proposed subsidy and price model. The simulation results show that the model rationally formulates subsidy policies and pricing strategies of the electric vehicle sharing to balance the interests of the three participants, mobilizing users’ enthusiasm while guaranteeing the benefits of the government and operator, making the overall benefit optimal.

Virtual synchronous generators (VSGs) present attractive technical advantages and contribute to enhanced system operation and reduced oscillation damping in dynamic systems. Traditional VSGs often lack an interworking during power oscillation. In this paper, a coordinated control strategy for multiple VSGs is proposed for mitigating power oscillation. Based on a theoretical analysis of the parameter impact of VSGs, a coordinated approach considering uncertainty is presented by utilizing polytopic linear differential inclusion (PLDI) and a D-stable model to enhance the small-signal stability of system. Subsequently, the inertia and damping of multiple VSGs are jointly exploited to reduce oscillation periods and overshoots during transient response. Simulation, utilizing a two-area four-machine system and a typical microgrid test system, demonstrates the benefits of the proposed strategy in enhancing operation stability and the anti-disturbing ability of multiple VSGs. The results conclusively confirm the validity and applicability of the method.

In regards to the cogeneration system in Northern China, mainly supported by combined heat and power (CHP) plants, it usually offers limited operation flexibility due to the joint production of electric and thermal power. For that large-scale wind farms included in the cogeneration system, a large amount of wind energy may have to be wasted. To solve this issue, the utilization of the electric energy storages and the thermal energy auxiliaries are recommended, including pumped hydro storage (PHS), compressed air energy storage (CAES), hydrogen-based energy storage (HES), heat storage (HS), electric boilers (EB), and heat pumps (HP). This paper proposes a general evaluation method to compare the performance of these six different approaches for promoting wind power integration. In consideration of saving coal consumption, reducing CO2 emissions, and increasing investment cost, the comprehensive benefit is defined as the evaluation index. Specifically, a wind-thermal conflicting expression (WTCE) is put forward to simplify the formulation of the comprehensive benefit. Further, according to the cogeneration system of the West Inner Mongolia (WIM) power grid, a test system is modelled to perform the comparison of the six different approaches. The results show that introducing the electric energy storages and the thermal energy auxiliaries can both contribute to facilitating wind power integration, and the HP can provide the best comprehensive benefit.

Directing at the hybrid high-voltage direct-current (HVDC) system with line commutated converter (LCC) and voltage source converter (VSC), it is of significance to investigate a feasible approach to isolate the DC-line fault rapidly and validly. In this study, the performance comparison of employing AC and DC circuit breakers for removing the DC-line fault is conducted. Firstly, the theoretical model and control strategy of the hybrid HVDC system are depicted. Then, the transient characteristics of the fault currents contributed by the LCC and VSC are analyzed. Based on the MATLAB platform, the isolation behaviors of the AC and DC breakers are compared, and herein the peak value of the DC fault currents and the breaking time are selected as the performance indexes. In addition, different action delays of the circuit breakers are taken into consideration. From the simulation results, it is found that using the DC circuit breakers is better than employing the AC breaker to address the DC-line fault, and the isolation time can be controlled within several milliseconds. Finally, some beneficial technical discussions for the fault isolation methods are performed, and it is descried to provide possible improvement directions.