Research Of Voltage Unbalance Control Method For Microgrid With Multi Parallelly Connected Inverters

With the development of national economy, humanity’s demand for electricity isincreasing, however, the energy crisis and environmental pollution are aggravating day by day,which improves the renewable power generation technology. For dispersion, diversity andrandomness of renewable energy, distributed generation technologies become the effectiveway to make use of it. In recent years, distributed generation has attracted a wide spreadattention for its high energy utilization rate, less pollution disposal, and high reliability. Inorder to solve the negative influence of the large-scale distributed generation connected to theutility grid on its generation, the concept ‘microgrid’ is presented to regulate the distributedgenerations.Microgird mainly consists of multi distributed generations (DGs), power electronicconversion device, storage device, protection and load control devices, et al. For most DGsconnected through an interface inverter to microgrid, operation control and power qualitycontrol of microgrid with multi-inverters are the key technologies of microgrid research. Thisthesis which covers three aspects: control strategies of microsource, voltage unbalancecompensation method for microgrid with multi-inverters, and development of experimentalplatform with two three-phase inverters, is finished by financial support of the NationalNatural Science Foundation of China. The major work and novelties in this thesis are listedbelow.(1) Starting from the parallel operation of multi-inverters in microgrid, control strategyof parallel inverters based on the droop control is analyzed. In this section, it took a microgridwith two DG inverters as an example, and derived the expressions of droop characteristicwhen the inverter equivalent output impedance was inductive or resistive and the expressionof inverter output impedance with the voltage and current dual-loop control based onfeedback of inductive current. Besides, an improved doop control by introducing virtualimpedance is analyzed.(2) For the situation that unbalanced loads connected to microgrid are becomingincreasingly serious, which will lead to unbalanced voltage and have some negative impactson microgrid stability and normal operation of equipments, this paper proposes a voltageunbalance compensation method based on αβ stationary frame for parallel multi-inverters inmicrogrid. This method includes unbalanced compensation loop, active and reactive powerdroop control loop, virtual impedance loop and voltage and current control loop. Based on thepower droop control with virtual impedance, the negative-reactive-power-conductance unbalance droop control loop is introduced to synthesize and correct the command referencecurrent that is used to compensate unbalanced voltage. The voltage and current control loopadopts proportional-resonant (PR) control to realize the zero steady error control. Thesimulation results demonstrate the validity and effectiveness of the proposed method.The design and implementation of the hardware and software for experimental platformof parallel system with two three-phase inverters is introduced. The prototype experimentsshow that the proposed method doesn’t affect the power sharing effect. Meanwhile, it canbalance the output voltage of DGs and effectively realize power control and voltageunbalance compensation with good dynamic performance, which verified the correctness andvalidity of the method.