Yonghai Xu

A control strategy of grid-connected photovoltaic inverter under unbalanced voltage conditions

The ride-through operation of photovoltaic inverter will produce apparent harmonic currents, cause active and reactive power fluctuations as well as DC voltage fluctuation under unbalance voltage faults. This paper studied on the power control of photovoltaic inverter under unbalanced faults, and proposed an improved control strategy which can restrain the fluctuation of active power, reactive power and output current harmonics. Adding several adjusting parameters based on the existing reference current algorithm, the relationships between harmonic current, power fluctuations and parameters are analyzed. The simulation models in PSCAD/EMTDC have been established with the proportional-resonant (PR) controller used to obtain a better dynamic performance, and the effectiveness of the proposed control strategies are demonstrated by satisfactory simulation results.

Research of method for voltage sag source location in power system

Four existing voltage sag source location methods based on reactive power variation, slope of the system trajectory, real current component and distance relay are analyzed and compared in this paper. The distance relay method takes advantage of measured impedance as judging criterion and shows the best performance among them. This paper proposes an improved impedance calculation method, which uses the change rate of impedance magnitude and increment of impedance angle before and during the voltage sag event to compare with a reasonable threshold. The improved method has been verified in simulated power systems with different structures, including single-supply, double-supply and ring system. Finally, an application case occurred in an actual city power system in P.R.China is given and good characteristics of the improved method are confirmed.

Enhancement of Power Transfer Capability and Contingency Analysis through FACTS Controller

With the growing power demand, it is vital to expand power transfer capacity of transmission lines. But the new expansion in our present transmission system is confined due to financial problems, environmental concerns and health hazards due to electric and magnetic fields. Maybe its worthwhile option to use flexible AC transmission system (FACTS) for enhancement of power capacity of the transmission system. In this paper, series FACTS devices SSSC and TCSC has been implemented. The main objective of this paper is to enhance or increase power capability of the lines. In addition, the contingency analysis using TCSC has been implemented to improve the stability of system. The system in consideration is a radial network with one generator supplying the power to the load system. This paper proposes modeling and simulation of SSSC and TCSC in MATLAB/Simulink for increasing power capability of the lines.

Enhancement of Power Quality and Transient Stability Margin in Coordination with UPFC and MB-PSS Controller

Nonlinearity affects the stability, efficiency, and power quality of a power system, thereby causing low power frequency oscillation and decreasing transmission line loading capacity. The inflation of transmission systems caused by the brunt of power transfer capability on power market enterprises is crucial but the recent development of transmission systems is restrained due to coincidental deliberations and commercial complications. The use of flexible alternating current transmission system can be a significant option for improving the power capacity of transmission systems. In this study, a multi-band power system stabilizer controller is implemented in coordination with a unified power flow controller (UPFC) to enhance the power quality and dynamic voltage stability of electrical power networks. The control algorithms of the UPFC for series and shunt converters are constructed based on the MB- controller. Simulations were performed in MATLAB/Simulink software to investigate the achievements of the UPFC in enhancing dynamic voltage stability. The performance of a designed controller is explored by using a multi-machine two-area test system. Simulation results indicate that the proposed coordinated controller demonstrates an excellent capability for increasing the power quality and dynamic stability margin of the system.

Comparison of cascaded multilevel and modular multilevel converters with energy storage system

Battery energy storage system(BESS) connected to the grid by power conversion system(PCS). Currently Cascaded Multilevel Converter(CMC) and Modular Multilevel Converter (MMC) are two more advanced topologies for PCS. This paper compares CMC and MMC topologies in aspects of efficiency, harmonic characteristics, reliability, control system complexity and cost, the simulation results are presented. Suggestions for topology suitable for large power output are proposed, which can be used as a reference for the design of large-capacity PCS.

A study on the load model based on particle swarm optimization

Based on comparing between the Composite Load Model (CLM) with the Synthesis Load Model (SLM), the SLM has been adopted in this paper. In view of the load model parameter identifications characteristics of complexity and low accuracy, a parameter identification method of the SLM based on Particle Swarm Optimization algorithm was proposed and used in the specific case study. It is shown by the case that the power curves simulated are closer to the measured ones, the particle swarm optimization has a certain superiority in the aspect of load model parameter identification, and the synthesis load model is reasonable.

Self-recovery comepensation strategy for dynamic voltage regulator

A new self-recovery compensation and control strategy suitable for dynamic voltage regulator (DVR) supplied by DC-capacitor was proposed. Two compensation stages which were transient compensation stage and recovery compensation stage were included in the strategy. The maximum energy compensation strategy which could maximize the exchange active power between the equipments DC-capacitor and its outside system was proposed to the recovery stage. Moreover, the double closed-loop control strategy with load voltage and filter capacitor current was used. A stages switching control strategy which took both of the DC-capacitor voltage magnitude and source-side voltage magnitude as the criterion was applied. The simulation results of EMTDC/PSCAD show that the voltage of DC-capacitor could be recovered to the nominal value as soon as possible with the proposed strategy. And the strategy could be suitable for the compensation of several times transient voltage disturbances in short period flexibly.