Control Strategy Research Of Three-phase BUCK Dynamic Capacitor(D-CAP)

With the advancement of technology and the development of industry, users aredemanding an increasingly higher power quality. But at the same time, the application ofvarious non-linear loads makes it more deteriorated. As a result, the technology of reactivepower compensation and harmonic suppression is getting more and more attention as wellas development. As for low voltage situations in the industrial area, in order to avoid finefrom the State Grid and to increase productivity, users have to take the improvement ofpower quality seriously, so they are also very concerned about the technology of reactivepower compensation and harmonic suppression. With the advent of active solutions likeSTATCOM and APF, power quality has already been improved effectively. But manyindustrial users don’t choose the two devices just because that they are too expensive.Besides, they are more prone to failure (mainly happens on electrolytic capacitors andswitches of the DC side). This paper mainly takes into consideration of industrial users’requirements of the reliability and value for money, and by getting inspiration from staticcapacitor, it proposes an entirely new solution for the compensation of reactive power andharmonic suppression—BUCK Dynamic capacitors (D-CAP). This method has noelectrolytic capacitors and can response to the system reactive power dynamically, onwhich basis it suppresses a part of harmonic components. It also has a low cost and canwork reliably.This paper first introduces the basic structure of BUCK Dynamic capacitors(D-CAP), which is composed by direct AC converter (DACC) and film reactive powercompensation capacitors in parallel. Then it obtains the basic working principle of dynamiccapacitor by making an analysis with the BUCK DC-DC circuit, from which we come tothe conclusion that by adjusting the switch duty cycle of dynamic capacitor, the entiredevice can be seemed as an equivalent of adjustable capacitor for the external circuit，whichplays the role of reactive power compensation. This is also the name origin of the dynamiccapacitor.Next this paper derives the basic control equations of BUCK Dynamic capacitors(D-CAP) by modeling for it, from which it deduces that BUCK Dynamic capacitors (D-CAP) has two working conditions-only compensating for reactive power orcompensating for both reactive power and harmonics. The difference is that when dutycycle function is in the form of constant term, it only compensates for reactive power, butwhen duty cycle function is in the form of constant term plus some even harmonics, itcompensates both reactive power and harmonics.To achieve the above two conditions, reasonable control strategy is needed. In orderto make the dynamic capacitor compensate for reactive power, we first need to detect thereactive power demand of the system. So this paper gives an introduction of commondetection methods of reactive power and harmonics. Then it gets the constant term of dutycycle function from the detected reactive DC current. At last it verifies the feasibility andeffectiveness of using BUCK Dynamic capacitors (D-CAP) to realize the compensation ofreactive power by simulation model.In the process of making the complex compensation strategy which includes bothreactive power and harmonic compensation, this paper first introduces the concept ofvirtual quadrature source(VQS) and even harmonic modulation (EHM). Based on the twoconcepts, this paper takes the specified times harmonic current control method undermulti-synchronous rotating coordinate system according to the basic equations of thedynamic capacitor as well as by making an analogy with the specified times harmoniccurrent control method of APF. Specifically, it realizes the control of (N-1)theven harmoniccomponents in the duty cycle function by detecting the Nthodd harmonic components of theharmonic current. At last, we build a simulation platform based on the specified controlstrategy. Through the detailed simulation of conditions as compensating for harmoniccomponent of5thonly, both5thand7thas well as compensating for5th,7th,11thand13th,theeffectiveness of control strategy has been proved.Finally, the feasibility, effectiveness and dynamic of BUCK dynamic capacitor(D-CAP) is verified according to the simulation results.