Research Of Grid-connected Current Loop Control And Off-grid Generation Application For Power Control System

Energy storage system(ESS)can serve as both generation unit and load unit according to application demand.It plays an important role in renewable energy power generation,grid peak-load regulation,electric vehicles and micro-grid application,and it is also a great important part of the future smart grid.This paper focus on the control of modular paralleled power control system(PCS),with the space vector modulation(SVM)and dc-link capacitor voltage balancing,grid-connected current loop control,and off-grid voltage source control of the PCS as the main study topic,and the detailed contents are as follows:1)The current status of global renewable energy development is introduced first,followed by the renewable energy status of our country and the application demanding of large-scale ESS.Besides,the application status and development trend of large-scale ESS are introduced,from two the energy storage method and PCS topology are both introduced aspects.At last,this paper summarizes the difficulties and key points for the PCS control system.2)In view of the great advantage of multi-level VSC in application of the high-power multi-modular paralleled PCS,and the dc-link capacitor voltage of multi-level VSC based on SVM is analyzed.Firstly,the principle and implementation of SVM is introduced based on two-level VSC,and this method is introduced to the voltage balancing of three-level VSC,with the nearest three vector space vector modulation(NTV-S VM)based redundant space vector and virtual space vector modulation(VSVM)are analyzed.Furthermore,the voltage balancing of five-level diode clamped converter(5L-DCC)is studied,and the NTV-SVM based on objec-tive function optimization,vector decomposition space vector modulation based on hexagon one(VDSVM-Hl),the optimized vector decomposition space vector modulation based on hexagon one(OVDSVM-Hl),and the improved vector decomposition space vector modula-tion(IVDSVM)are analyzed for their voltage balancing capability.The voltage balancing is achieved with the three methods,irrespective of the power factor,and the converter can working stably even under over modulation.3)With respect of the current loop control of PCS with balanced grid voltage,the influ-ence of cross coupling of current loop in positive synchronous frame(PS-SRF)is analyzed,and two kinds of decoupling methods are introduced and the effect of filter parameters error on decoupling effect is discussed.Secondly,the effect of one-step delay of digital control on the decoupling of current loop is discussed,and the delay compensation method is analyzed for state feedback decoupling proportional-integral(PI)and complex-proportional-integral(cPl)controller respectively.In addition,the ability of gird voltage disturbance suppression for both state feedback decoupling PI and cPI controller are analyzed,and the effectiveness of grid volt-age feed-forward control and active damping to improve the system anti-disturbance ability is discussed.4)In order to effectively control the PCS current under unbalanced and distorted grid voltage,the instantaneous power calculation and reference current generation method are in-troduced.Secondly,the decoupling effectiveness of double synchronous reference frame con-trol(DSRFC)based on state feedback decoupling PI and cPI controller is analyzed,and the sensitivity to output filter parameters is analyzed.Thirdly,the transient response of RCs and PS-SRF-PI/cPI controller are discussed,and the relationship between RCs and PS-SRF-PI/cPI controller is clarified.In addition,the optimal stability criterion of RCs control system is an-alyzed,and the reason for closed-loop singularity generation is introduced.Besides,the delay compensating strategy of current loop based on RCs and its effectiveness are discussed.Finally,the controller design in discrete time domain is introduced,and the advantages of PS-SRF-DcPI and DVPI controller are analyzed,also the time delay compensation method and closed-loop dead-time compensation are introduced.5)For the output voltage source control of the PCS working in the off-grid mode,the vector model of the system in the discrete domain is deduced firstly.With the root-mean-square control of the output voltage is taken into account,the requirements of the PCS control system are a.Secondly,the limitation of single-loop instantaneous voltage control is discussed in detail,and the necessity of virtual damping to suppress the resonance of system is clarified.Thirdly,the optimal gain design criterion of the inner loop of the double closed-loop control structure is discussed.With the purpose of obtaining the maximum stability and improve the damping,an inner loop optimal gain design method is proposed.In addition,the optimal damping of the controlled object is taken into consideration,and the essence of the current inner loop is further clarified.Finally,the design of voltage controller in the discrete time domain is discussed in detail.Starting with positive-sequence voltage control of fundamental component,the delay compensation of PS-SRF voltage controller and the critical damping gain design method are introduced.Furthermore,the DVPI-LC controller is obtained,which is implemented in the stationary frame and avoiding the coordinate transformation.At last,the DVPI-LC controller is extended to the harmonic components,with the purpose of controlling the negative sequence and harmonic voltage.