Research On The Control And Protection Technology Of Parallel Inverter

With the development of national economy and technology, the power supply in modern industry and agriculture, transportation, aerospace and other fields are required to be more reliable and redundant and have higher power level. On the other hand, the distributed power supply technology has been widely studied to solve the problem of energy crisis.As a key technology to enhance the reliability and capacity of power supply, parallel inverter has achieved rapid development. This paper mainly studies the parallel control as well as the short-circuit protection of the inverter.Firstly, a mathematical model of the inverter is established. On the basis of dual-loop control, the model of the equivalent output impedance of the inverter has been deduced. Then the impact of control parameters, digital control delay and transformer on the output impedance of the inverter is analyzed. And ultimately, a set of parameters are chosen which make the output impedance mainly resistive. The relationship between the power and output impedance is analyzed based on the equivalent model of parallel inverter. Finally, this paper proposed a control strategy which uses active power to regulate voltage and reactive power to regulate frequency.The principle to realize power equipartition and the limitation of the parallel control strategy with and without interconnection lines are analyzed. In order to inhibit circulation, virtual impedance is introduced to solve the mismatch of the impedance in droop control. However, an inappropriate impedance could bring much larger voltage drop. Finally, an adaptive virtual impedance parallel control scheme which integrates the wired and wireless parallel control is proposed. It is turned out that the scheme not only improves the reliability of the parallel system with communication lines, but also enhances the capability of limiting circulation and modifies the performance of the output waveform quality.To avoid power outage of the inverter in case of short-circuit fault, a combined current limiting strategy which integrates software and hardware is introduced. And it ensures the current is limited within the scope of security quickly. Then a fault recovery scheme by using the change of output voltage and output impedance comprehensively is analyzed. And thus, quickly and smoothly transition to the voltage source mode after fault resection is achieved.A parallel prototype is finally established in the laboratory to verify the different parallel control strategies introduced in this paper. The experiment result verifies the effectiveness of the capability to restrain the circulating current. Also, the short circuit current limiting and failure recovery experiment is conducted in the wired and wireless system. The results show that the inverter can limit the current rapidly and effectively, as well as achieve smooth mode switch.