Study On Key Technologies Of Modular Inverters Used In Household Microgrid

Household microgrid extends the exploitation and utilization of distributed renewable sources and distributed power supply. As a distributed power supply system, household microgrid could absorb distributed sources on one hand and provide reliable, qulity electric power to family cosumers on the other hand. Inverter is the core euipwment used in household microgrid. Considering the flexiable configuration of capacity, reliability, investment and maintenance, single-phase modular inverter and its combined application is the suitable soluation for household microgrid. As a result, the key technologies on single-phase modular inverter used in household microgrid, including control technology of modular inverter, islanding detection, control of parallel modular inverters and three-phase combined inverters, were researched in this paper. The main task and achievements are listed as below.(1)Voltage and current dual closed-loop control strategy using Multi-resonant PR controller is adopted for island mode of single-phase modular inverter in this paper. A power control method based on instantaneous power is proposed for grid-connected mode of single-phase inverter. By instantaneous power outer loop and inductance current inner loop control, this method achived constant active and reactive power control. Digital implementation of proposed control strategy and discretization method was researched. Tustin and modified Tustin transfer are selected as the methods to discret resenance controller at fundermental and harmonic frequency seperatly, which improves the qulity of discretized controller.(2) An islanding detection method by using frequency positive feedback and active phase shift based on FLL is proposed in this paper. This method establishes phase-frequency positive feedback according to the error between PCC instantaneous frequency from PLL and normal frequency, resulting in frequency shift step by step until out of the frequency limits. SOGI-FLL is applied to replace SOGI with constant resonance frequency considering that the frequency is time-varying during the islanding detection process in this method. Hence, the performance of the islanding detection method is enhanced. Besides, an initial low frequency variable triangular disturbance phase is used to instead constant initial phase shift in order to reduce non-detection zone and accelerate the detection process. Meanwhile, this operation solved the weaking disturbance problem caused by power regulation for constant power contolled inverter with constant initial shift phase.(3) A current sharing method for parallel inverters based on average dq current in rotating reference frame is proposed in this paper. The output current of each inverter is transferred into DC dq component by different order (Fundmental,3rd,5th,7th....) in rotating reference frame. Then current sharing can be achived indirectly by sharing d/q current component. This method is effective for both fundermental current and harmonic current. Comparing with direct instantaneous AC current sharing, this method can reduce the high requirement for communication rate of the communication system among modular inveters.(4) A Master-Slave voltage synchronous control method for parallel inverters and three-phase voltage symmetryical control method for three-phase combined inverters based on CAN bus. For parallel inverters, salve modular inverters regulate each own voltage phase according to the synchronous frame from master module and compensate freauency error caused by both oscillator error and truncation error in calculation. Hence, voltage of each inverter in the parallel system can be synchronous. With regard to three-phase combined inverters operated in island mode, the similar Master-Slave method as voltage synchronous control in parallel system based on CAN bus is applied to make three phase voltage symmetrical by regulating voltage phase and compensating frequency error. For the pre-synchronization of three-phase combined inverters before connecting to grid, a three-phase voltage control strategy using Phase-Locked-Loop in each phase is proposed. Contrast to the Matser-Slave method used for three-phase voltage symmetrical control in island mode, this method solves the problem that large instantaneous voltage asymmetry maybe exsit during pre-synchronization process and ensures continuous symmetyical three-phase output voltage.