Yang Zilong

Study on control strategy of energy storage system in photovoltaic microgrid

The randomness and fluctuation of photovoltaic power generation could hurt the stability of power grid. Application of energy storage technology can effectively alleviate the impact of distributed generation on the grid and improve its anti-interference capability. A control strategy of battery energy storage system is proposed to help to carry out peak load leveling of micro grid and restrain the power fluctuation. The power value of each period is determined by PV power forecasting and the state of charge of batteries. Batteries are controlled by the energy management system to optimize the load curve of micro grid and carry out the peak load leveling in this strategy, as a result, economic benefits of photovoltaic power generation are improved. In addition, a spare branch of batteries are controlled to charge and discharge to compensate short term power fluctuation according to the real time power sampling results and the quality of electric energy is improved. A photovoltaic micro grid system is used for modeling, and is simulated by MATLAB/Simulink. Simulation results show that the control strategy is effective.

Measurement and investigation of PCC Point harmonics in a grid-tied PV station

This paper addresses the PCC-point harmonics in a parallel-inverter-based photovoltaic station. Firstly, the harmonic measurement of a grid-tied PV station has been implemented. The voltage and current waveforms present some significant characteristics, such as lower frequency, power correlation and inter-harmonic. The most of these harmonic currents flows through the inverter IGBT bridges and filter inductance branches, rather than the filter capacitor branches. These harmonics are 3-phase symmetric and contain 3rd and 6th harmonic components. These harmonics are not produced by the resonances related to the output LCL filters of PV inverters. This study proposes that the resonances between inverter and grid impedance are the dominant causes of the harmonics.

Investigation of active voltage control methods for photovoltaic inverters connected to network

Photovoltaic power connected the distribution network has changed the power flow, resulting in voltage rise at PCC. Various methods have been proposed to limit the voltage within standard. Different methods were compared in this paper to identify their advantages and disadvantages. First, pure active power was injected in order to study the influence on voltage of each node; then every active voltage-control method was used in a simulation distribution network model to study the static and dynamic properties of the method, and a novel optimal active voltage-control method was brought up, which would meet both static and dynamic properties better.; then a series of experiments on grid-connected inverters was operated on test-bed to verify the feasibility of the proposed method. At last the summary came into conclusion, and their applicable occasions and the shortcoming of reactive power voltage control were pointed out.

Analysis of harmonic resonances among parallel grid-connected inverters

The resonances in parallel grid-connected PV inverters were studied. Considering the effect from the step-up transformers and grid impedance, the transfer functions of different inputs to the output were derived; furthermore, the deficiencies of conventional inverters were pointed out, and a novel method was proposed, which can effectively suppress the harmonic current output, thus avoiding the resonance phenomenon, without any changes of the topology. Then compared Block Diagrams before and after applying the proposed method, theoretically proved the correctness of the virtual conductance method; Finally, simulation models were built to restore resonance phenomenon and to verify the feasibility of the method.