Chengyong Zhao

Loss calculation method of modular multilevel HVDC converters

The modular multilevel converter (MMC) is a novel voltage source converter (VSC) topology with great potential for high voltage direct current (HVDC) transmission applications. The MMC-based HVDC system has main characteristics of flexible controlled and low switching frequency so that its losses are smaller than that of the two-level VSC-based HVDC (VSC-HVDC) system. In this paper, the operating characteristics of MMC and its losses are analyzed, and a method is presented to calculate the on-state losses according to the different on-state current of upper and lower leg. Furthermore, after the impact of junction temperature coefficient in IGBT module, a method using curve fitting to calculate the switching losses of MMC is considered. Finally, the losses of MMC are calculated in MATLAB, and the corresponding loss reducing measures are obtained from the results which also show that the method proposed is reasonable and feasible.

Modeling of wind energy conversion system using doubly fed induction generator equipped batteries energy storage system

In this work the model of different parts of the wind energy conversion system using doubly-fed induction generator (DFIG) is set up and implemented in PSCAD/EMTDC. The two quadrants DC converter using battery energy storage system is employed for the DC-Link Voltage control in normal and fault operating conditions. To verify the control strategies and Low Voltage ride through (LVRT) capabilities a 600 KW DFIG Wind turbine system under 500 ms short circuit fault is tested. Finally, it is proved that the use of battery energy storage system (BESS) improves the power quality during the normal operation and enhances the turbine security level in the grid connected under fault conditions and quickly brings back the stability of the system.

Key technologies of three-terminal DC transmission system based on modular multilevel converter

A three-terminal DC transmission system based on modular multilevel converter (MMC) is developed to solve the problem of power supply for distributed islands or cities. The system, with flexible features, may generate high quality output voltage and reduce construction costs and power losses in the converter stations. However, the three-terminal DC system based on MMC faces more complex situations in operation and its control strategies. In this paper, the mathematic model of three-terminal DC system based on MMC is deduced. Then according to the structure of MMC, a method of carrier phase-shift SPWM is introduced. As the system-level control, the decoupling controller for three-terminal DC system based on MMC is designed. A multi-point DC voltage control strategy based on DC voltage error method, which doesnt need communication between converter stations, is proposed to improve the system reliability. By simulation with PSCAD/EMTDC, the feasibility and effectiveness of the proposed model and control strategies are verified.

Study on the reliability of real-time simulation

The DC line fault which occurred on the pole 2 of Guizhou-Guangdong HVDC of China South Grid company on January 15th, 2007 is simulated successfully by using the HVDC simulation model of RTDS that is essentially identical with the actual control and protection system. We use the transient wave calculation procedure in LabVIEW to compare the similarity indices of the simulation results and the fault recordings. The comparison shows that the RTDS models of CSG are precise and reliable. Thus the simulation model of the AC-DC system can exactly reflect operation characteristics of real networks.

Simulation study of the Zhoushan project as a three-terminal DC transmission system

This paper presents a simulation study of the proposed three-terminal DC transmission system in Zhoushan Archipelago located in East China Sea. In this paper, two kinds of topologies of converter are compared to select the suitable one and the DC voltage level is determined. Then, station controller based on direct current control strategy is designed. A multipoint DC voltage control strategy based on DC voltage error method is proposed to improve reliability of the system. An additional frequency control strategy is put forward to make it possible for VSC-HVDC system to take part in the frequency control. Finally, the three-terminal DC transmission system model is developed in PSCAD/EMTDC. In Results from the simulation study show that the three-terminal DC Transmission system based on modular multilevel converter (MMC) is technically feasible for Zhoushan project.

The analysis and simulation of commutation failure and protection strategies

Studying the HVDC typical fault-the commutation failure and its protection measures will give some guidance for HVDC engineering, also, it will improve the AC-DC power system safety and reliability with great significance. Based on the analysis of commutation failure of HVDC, the article studies the effect of the commutation failure when the commutation buses have different kinds of AC faults on the base of CIGRE HVDC benchmark model, with the simulation of PSCAD/EMTDC. This in-depth analysis of the causes of commutation failure, the type and characteristics of the fault will be done in this paper. Basis for the characteristics of HVDC system, combined with AC transmission relay protection tuning principle, conservation and protection action principle approach, are given to promoting HVDC transmission should take measures to return to normal. The simulation results show that the HVDC transmission recovers from commutation failure effectively after the fault is cleared due to the high controllability of the system and the reasonable protection strategies.

Integration of wind farm into power system using VSC-HVDC transmission system

In this work, wind farm with VSC-HVDC connected to the grid are developed including wind side grid converter, grid side converter, DC cable and the emulated circuit control model. During fault in the AC grid DC link voltage increases rapidly and this would cause the HVDC link to trip for over voltage. To solve this problem, the location of the AC fault can be transferred to the wind farm side by reducing the voltage at the wind farm side HVDC converter terminal. This method is called the emulated short-circuiting of the wind farm side HVDC controller. To validate the models and conclusions, PSCAD/EMTDC is used. The simulation results show that the DC voltage level on HVDC line is maintained constant by using the proposed emulated short circuit control model.