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Parallel Control Strategy Of Inverter Based On Virtual Impedance

Posted on:2016-11-04Degree:MasterType:Thesis
Country:ChinaCandidate:Y YuFull Text:PDF
GTID:2322330479452886Subject:New Energy Science and Engineering
Abstract/Summary:PDF Full Text Request
This paper focuses on the study of normal running respectively in the stand-alone and parallel operation of UPS in the way of SPWM. Based on introduction about the tendency and reliable indicators of UPS development, several aspects are studied detailedly in this paper, such as mathematical model of circuit, nonlinear load, unbalanced load, circulating current characteristics of parallel system, active and reactive power distributing equally, etc.A mathematical model and transfer function of three-phase inverter with open-loop control are derived in abc and dq reference frame, also the disadvantage of it is shown to us. So the dual-loop control system is introduced. Repetitive control also plays an important role in decreasing THD when there is nonlinear load. Unblanced load needs to be considered in three-phase half-bridge inverter,a method of seperating positive and negative sequence component in dq reference frame is introduced.Based on the normal running in the stand-alone operation, the output power characteristics of inverter in parallal operation is analyzed deeply, the mathematical model of the parallel system is setted up. The circulating current is relevant to two factors:(1)difference of open circuit voltage;(2)difference of equivalent output impedance. We modulate the amplitude and phase angle(frequency) via the error of active and reactive power.But the equivalent output impedance of inverter is often not purely inductance,it has resistive and inductance component. So the coupling between error of active and reactive power and amplitude and phase of reference voltage is existed, there is a serious objection and a poor accuracy in the conventional droop control method. Based on analyse for this, a concept of virtual impedance is presented, by which control quantity and controlled quantity of load sharing control scheme can be decoupled, and circulating current caused by difference of output impedance can be sharply weaken.
Keywords/Search Tags:SPWM inverter, Negative sequence component, Virtual impedance, Circulating current, Output impedance, Active power, Reactive power, PR controller
PDF Full Text Request
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