Research On Relay Protection Strategy Of Parallel Connected Inverter Supply System

Nowadays the use of parallel-inverter supply system is becoming increasingly wider and deeper,and the most common method used to enable the inverters to survive the short-circuit fault is shutting down the machine as soon as the fault occurs.However,this strategy would cut off the power supply of the healthy loads.It does not meet the expectation of high reliability of the power supply system.Aiming at clearing the fault selectively and improve the reliability of the system,this paper researches the relay protection strategy of parallel-inverter supply ststem.The paper firstly analysis the three-phase short circuit fault and phase-to-phase short circuit fault happening in the three phase combined inverter supply system.The analysis shows that if the inverter is still controlled as voltage source,the current will be very large which could destroy the switch transistor.As a result,the inverter must be controlled as current source and the current must be controlled to a safe value.This article analysis the half-period average method and the method based on the dq transformation of positive and negative sequence compents.In addition,the paper simulates this two methods in the MATLAB,which proves the dq-transformation method is better.This paper employs the strategy which combines the instant constraining based on hardware circuit and the constraining means based on software.The current constraining strategy proposed in the paper can limit the current to a safe value as soon as the fault happens and guarantees the output current as sinusoidal as possible.When the fault is cleared by the protection devices,the inverter supply system must recover to normal fast and smoothly.This paper analysis the smooth-recovery methods in the single-inverter supply system, wire-parallel inverters supply system and wireless-parallel inverters supply system.The recovery strategies proposed in this paper are effective,which ensure the inverter recover to normal fast and smoothly as soon as the fault is cleared.After solving the three key issues,this paper proposes two over-current protection strategies,namely the relay protection method based on constant current and the method based on the droop relationship between the current and impedance.When applying the protection methods to the supply system,the faults can be cleared selectively,which meets the demand of the relay protection system.At last,relevant experiments are carried out on the three phase inverter plate in the laboratory,and the results prove the effectiveness of the proposed strategies in this paper.