Back-to-back DC transmission technology can be used in the interconnection of asynchronous grids. The characteristics of back-to-back DC transmission system and its applications in the world are introduced in the paper. Asynchronous interconnection back-to-back HVDC project between Yunnan Grid and the main grid of China Southern Power Grid (CSG) is the first asynchronous interconnection HVDC project in CSG. The system schemes, including unit connection modes, main operation modes, main devices parameters and land area of converter station are presented. Finally, the key technologies and innovation points of the asynchronous interconnection project are summerized.

The bipolar short-circuit fault at DC line of MMC-HVDC transmission system is a serious threat to the safe and stable operation of the system. The transient process of the fault is studied and the fault characteristics are analyzed in detail. Firstly, an equivalent circuit model of MMC sub-module dynamic casting process is put forward for the pre-blocking stage of the converter. The simulation results show that the equivalent circuit model can accurately and effectively reflect the high-frequency switching process of the MMC sub-module. Alternating discharge of the sub-module capacitance makes the DC-side fault current rise more quickly. Secondly, a mathematical description equation is established with the equivalent circuit for the post-blocking stage of the converter, and a three-stage analysis method is proposed for the post-blocking stage of the converter. The analysis shows that after the inverter is blocked, in the currents of each bridge arm, there is a non-periodic component of the bridge arm inductance at the moment of blocking in addition to the non-periodic component of the capacitor discharge before the blocking; when the DC-side outlet fails, the peak value of the fault current may appear after the inverter is blocked. PSCAD/EMTDC model simulation verified that the theoretical analysis result is correct.

In view of the advantages and challenges of using DC circuit breakers for fault isolation, this paper introduces the research status of DC current limiting solutions. When there is a bipolar short-circuit fault current at the DC side of a modular multi-level converter (MMC), the fault current will rise to a dozen or even dozens of times of the rated current in two or three milliseconds, because of a certain action delay of the DC circuit breaker, it is necessary to set up current limiter in DC circuit. The parameter of current limiter is analyzed the above fault occurs, taking advantage the characteristics that a hybrid flow through branch has low on-state losses, fast operating speeds, and the ability of the resistive-capacitive element to smooth the voltage rise rate of the current limiter, a resistance-capacity hybrid current limiter for DC power grid is prensented, the working principle of the current limiter and internal switch voltage stress are analyzed. Finally, a four-terminal DC grid model and the proposed current limiter model are built in the PSCAD/EMTDC simulation platform, the simulation results show that the proposed current limiter can reduce the fault current and shorten the protection time, and the correctness of the theoretical analysis and the feasibility of the current limiting scheme are verified.

The subsynchronous oscillation (SSO) problem has been captured in power system with PMSG based wind farm connected to HVDC transmission system. In this paper, the installation position of the subsynchronous damping controller (SSDC) is determined by analyzing the participation factors of the dominant mode. Besides, a design method of SSDC is provided. The configuration and the performance indicator is discussed, and then the optimization algorithm is developed based on genetic algorithm and electromagnetic transient simulation. Finally, the proposed design method is validated by simulation in PSCAD/EMTDC.

In view of the fact that the noise of power capacitor devices in conventional high voltage direct current (HVDC) transmission projects is prominent and needs to be controlled from the source, test circuits for noise test in accordance with the actual operating conditions of power capacitor units are proposed, including a bridge circuit which can load power frequency and multiple harmonic currents simultaneously, and a test circuit which can load DC voltage and multiple harmonic currents simultaneously. Meanwhile, the technical scheme of the harmonic power supply, which is the key equipment in the circuit, is illustrated. The feasibility of the circuits are verified by the simulation waveform. Based on the proposed circuit, a power capacitor noise test platform is set up, and the noise test of multiple power capacitor units in converter station are carried out. The comparison between the test results and the theoretical values proves that the test circuit can meet the noise test requirements of power capacitor units in HVDC projects.

Potential difference in large grounding grid has not been concerned so far. By simulating analysis of a serious fault of the 500 kV breaker mis-operation during a 500 kV current transformer grounding fault in Pannan Power Plant in Guizhou province, the paper points out that abnormal high ground potential difference would likely interfere the signals in secondary control cable, cause secondary system relay protection mis-operation, and lead to a latent accident extension. A corresponding solution to reduce grounding grid potential is presented to reduce the risk of mis-operation or failure to operate. The scheme is suitable for both of the optimizing design and safety evaluation of large grounding grid of power plant, converter station or substation. To adapt to the developing requirements for grounding grid safety and streamline, single index that focuses only on grounding impedance should be discarded, and multiple safety indices should be attended to, such as ground potential rise, potential difference within grounding grid, step voltage and touch voltage.

This paper presents a comprehensive evaluation concept for multi-area interconnected grid. A comprehensive evaluation system of multi-area interconnected power grid with renewable energy considering uncertainties is constructed from the aspects of power supply access side, network itself, network connection and user side. This paper analyzes the uncertainty of each evaluation index from the aspects of change amount, change rate and sensitivity degree. It uses the fuzzy comprehensive evaluation method and the analytic hierarchy process to comprehensively judge and solve the model. The results of cases study show that the method can be used to judge the uncertainty in the actual operation of the interconnected power grid, which is instructive for the operation of power grid and the evaluation of online status.

After the Fukushima nuclear accident, the Japanese government passed a new round of power reform programs and proposed the full liberalization of entry to electricity retail business. At present, China is in the key stage of power system reform. Researching and analyzing the practical experience of Japan electricity retail reform will be of significance for deepening of electric power reform in China. According to the electricity market reform plan, electricity retail market of Japan was fully liberalized in April 2016. In order to ensure the smooth implementation of the reform, the Japanese government adopted measures to simplify the procedures and requirements for the establishment of electricity retail companies, establish an independent electricity market supervision committee, strengthen supervision of the electricity retail market, and establish last resort mechanism. After the reform in Japan, more than 400 power retail companies have been established which promote the cross-regional electricity trading. The enlightenment to electric power reform in China is as follows: carrying out the electricity retail choice with legislation, maintainig the unified dispatch management system of power grid in China, and accelerating the construction of electricity wholesale markets.

Subsynchronous oscillation threatens the security and stability of power grid connected with wind farms. Subsynchronous oscillation suppression of grid-connected wind farms is one of the most important and practical issues in the study of the stability of grid-connected clean energy. FACTS /VSC-HVDC and its additional damping controller, subsynchronous damping controller and subsynchronous filiter of the control loop on the converter of wind power generator, and other measures are four types of main measures to suppress subsynchronous oscillation of grid-connected wind farms. This paper reviews and compares the research findings of these four types of suppression measures in the last ten years. The mechanism and designing steps of subsynchronous damping controller to suppress subsynchronous oscillation are also described in detail. Finally, the existing problems in the study of the subsynchronous oscillation suppression measures of the grid-connected wind farms are summarized. The paper also points out the direction for future research.