Research On Control Methods For Inverters Of Distributed Generation

It has become one of the toughest problems that how to combine distributed generation with the traditional power system technology.This problem is being solved with the development and maturity of the mircogrid.The controls of multiple distributed generations strengthen the stability of the system by using strategies and devices.The main goals to maintain the stable and efficient operation of the system are frequency,voltage regulation and power flow.Microgrid can operate both in island mode and grid-connected mode.Droop control is often adopted when in island mode.Inverters can control the output of generations in accordance to the relationship of active power and frequency,reactive power and voltage.As the droop control strategy is realized under the inductive environment of the high voltage line,it can no longer be effective when in resistive atmosphere.To start with,the operating characteristics and volt-ampere characteristics of the photovoltaic cells are analyzed.MPPT is studied,the P&O method of constant step length and variable step length are adopted to simulate and verify the tracking effect.The PQ control strategy of inverters is studied when distributed generations are connected to the grid.The working principle of sine pulse width modulation and the design of filter parameters are introduced,SPLL model is built.The dual loop feedback regulation structure of current and voltage is established in U/f control when in island mode.The droop control strategy is detailedly studied,and the power distribution between each DG supply and the control of the frequency of the system and voltage regulation are introduced.The outcome of simulation of PQ control and U/f control meet the needs of microgrid.Aiming at the problem of frequency instability and power distribution disorder caused by traditional droop control in low voltage resistive circuit,an improved droop control method is proposed with frequency automatic restorable regulation and new power flow distribution device based on the analysis of the principle of power distribution of each micro power supply and frequency deviation.By using the small-signal model to analyze the stability of the system,the control parameters of the improved control mothed are confirmed.The simulation outcomes exhibit a good performance of power flow distribution and system frequency stability of the proposed control method when in resistive atmosphere of island mode.Last but not least,the switching process of two different working conditions of micro power grid-connected/off-grid were simulated,the results show that the system with the improved method works quite well during the switching process.