Key Issues Research On Paralleled Multi-Inverter System

With the development and utilization of renewable energy, the scaling of microgrid composed of distributed generation systems and energy storing devices, e.g., photovoltaic (PV), wind power, micro gas turbine, fuel cell, are becoming much larger. Research on control of Multi-inverter parallel is the main focus as the key technique, which can improve the reliability of microgrids.This paper proposes an improved control strategy to eliminate DC circulating current in the paralleled Uninterrupted Power Supply (UPS) systems. The active and reactive power control has good performance on restricting AC circulating current. However, it is unable to suppress DC circulating current effectively. Based on conventional active and reactive power control for load sharing in paralleled UPS systems, a DC circulating current suppression loop is introduced to each paralleled inverter module, which is used to eliminate the DC component of output voltage in each paralleled inverter module. The DC circulating current control is decoupled since each inverter module only regulates its own DC component. The proper design of multi-loop controller parameters for DC suppression loop ensures excellent steady-state and dynamic performances. The proposed DC circulating current suppression method is verified by experimental results of a paralleled three-phase UPS inverters system.PV inverters without the isolation transformer become more attractive due to higher efficiency and lower weight. However, it may exist DC offset current problem and is critical to power system. In this paper a novel control strategy to suppress DC current injection of PV inverter to the grid is investigated. It is based on idea of accurately sensing the DC offset voltage of PV inverter. Since DC component of the inverter output can be eliminated, DC injection to the grid can be effectively suppressed. Finally the control scheme is verified by the experiment.The novel control strategy has two significant features. The first is that differential amplifier is used to sample the DC offset voltage between the two bridge-leg middle points of Full-Bridge inverter, which avoids the zero-drift resulted from Hall effect sensors. The second one is that DC suppression loop can contain inverter output disturbances. Therefore the DC current injected to the grid can be well suppressed.This paper establishes the mathematical model of parallelled Multiple inverters under grid-connected mode, makes research on the resonant mechanism of Multi-inverter parallel, and suppresses the resonant by active damping method. The resonant problem has an influence on the development of distributed generation system based on renewable energy in a large scale. In order to restrict the resonant of Multi-inverter parallel under grid-connected mode, we establishes the equivalent mathematical model of the closed-loop control circuit for single inverter by the Thevenin Equivalent Principle firstly, then analyzes the resonant mechanism based on the established control mathematical model. To effectively suppress the resonant of Multi-inverter parallel, we present the active damping control method by the feedback of capacitor current of LCL filter, and establishes the mathematical model of Multiple inverters with the active damping control. Finally, the presented control method is verified by simulation with the Matlab/Simulink tools.Uninterruptible Power Supply (UPS) is widely used in industry and electronics field in order to improve power quality. According to the design rule, the maximum load current which inverter in the UPS can endure is three times of the rated load current. When UPS is applied to impulsive load, such as electric motors or rectifiers, the instantaneous current may reach five or seven times of the rated load current. Therefore, the huge impulsive current will result in inverter modules breakdown. However, the distortion and interruption of output voltage will occur when UPS is applied to instantaneous impulsive load. This paper proposes a novel dynamic control strategy to resist impulsive load. When the impulsive load is imposed on UPS instantaneously, the impulsive current can be quickly sensed by the digital controller and the supplying power for the load is switched from inverter to bypass immediately. Therefore, the output voltage of UPS can be maintained uninterruptible. The proposed control strategy is verified by a30KVA UPS prototype.