Research On Parallel Control Strategy And Power Quality Active Control Strategy Of Inverters In Microgrid

Microgrid can reconcile the contradiction between distributed generation(DG) and the utility grid, and is also an effective way to improve the efficiency of distributed generation. Microgrid can operate in both grid-connected and islanding modes. So it can offer more reliable power to loads. Most DG and energy storage systems are connected to the grid through power electronic interfacing inverter. Microgrid inverter is the key component in microgrid power generation system, the similar topology to power quality compensator and the flexible control strategies can offer a convenient way to microgrid power quality control. Researching parallel control strategies and active power quality control method of microgrid inverter under different operating conditions, can not only actively guarantee the safe and reliable operation of microgrid inverter itself, but also take the initiative to improve the microgrid power quality. It is of extremely importance to theoretical research value and practical significance for constructing high power quality and high reliability microgrid.This paper is finished by financial support of the key project of National Natural Science Fund(51237003) "multi-inverter parallel and power quality control method of microgrid" and the basic project of National Natural Science Fund(51477045) "the research of power quality emission characteristics of typical electricity load containing micro supply". The research contents including microgrid inverter power distribution control, the parallel control strategies and control method of power quality of microgrid inverter under unbalanced or harmonic environment. The major work and innovations in this dissertation are listed below:(1) Based on the discussion that the difference of line impedance has influence on conventional power distribution droop control method, an improved robust droop multiple loop control method, which can improve the parallel process an d realize high precision power distribution is proposed. The proposed method includes the outer power loop and the inner voltage and current loop. In the voltage and current loop, the controller parameter design method is applied, which aims to suppress th e the adverse effects of load change. In the outer power loop, the improved robust droop control strategy is proposed. Compared with the conventional robust droop control, the improved robust droop can reduce the impact current, speed up the transition pro cess and does not affect the effect of precise power distribution. Simulation and experimental results demonstrate that the control strategy is correct and effective.(2)In order to improve the microgrid power quality under large feeder impedance environment, to realize high precise power distribution and to restrain circulation current, a control method, which can directly control the voltage at the installation point, is proposed for inverter parallel operation. The proposed method can transform large feeder impedance into small large feeder impedance, and weaken the adverse effects of feeder impedance on grid side on circulation suppression and microgrid voltage, without measuring the value of physical feeder impedance. The simulation and experiment indicate that the proposed method can improve the distribution precision of load current and the microgrid bus power quality under large feeder impedance condition.(3)The synchronous frame universal equivalent model of the controller in stationary frame is deduced, and the proposed equivalent model not only can reflect the control performance of the input signal at different frequency in stationary frame, but also provide theoretical support for modeling and frequency-domain analysis of the synchronous frame of controller in stationary frame. A compensation strategy of unbalanced voltage, which does not need to separate the positive and negative sequence components, is presented. Through controlling the positive and negative sequence output impedance of the inverter, the proposed method can improve the positive sequence power distribution, compensate the negative sequence voltage of common bus, and restrain negative sequence circulation current,(4) An active harmonic control strategy, which can select the operation mode flexibly according to the output capacity of inverters, is proposed. The active harmonic control strategy can restrain the negative impact on grid output current from harmonic disturbances. The active harmonic control strategy can also realize the effective compensation of local harmonic current without harmonic detection. The model of control system and frequency-domain analysis indicate the control performance of the propose strategy. The simulation and experiment results verify the effectiveness of the proposed strategy.(5) The internal connection between output power is constant and output current is sinusoidal of the grid-connected inverter is deduced under unbalanced voltage condition. On this basis, a coordinate control strategy of power/current quality in stationary frame is proposed. Through changing the current reference coefficient, the inverter output power variation and current quality can be controlled effectively, which is applying the instantaneous voltage to calculate current reference directly. The proposed method does not need to synchronous frame transformation, not need to separate the voltage/current positive and negative sequence to calculate the reference, not need phase-locked loop, and not need any low-pass filter or notch. Moreover, it can be implemented simplely, and easily applied on the condition of unbalanced voltage.