Research On Power Conditioning System For Battery Energy Storage And Its Control Schemes

Energy storage system(ESS) can serve as both generation unit and load unit according to different application. For its unique bidirectional properties, it will play an important role in the solutions to integrate renewable energy generation into grid. In this paper, battery energy storage is selected as the researched objective. According to the technical level of battery system construction nowadays, the power conditioning system(PCS) based on parallel converter modules is proposed to be as the interface of grid-connected battery energy storage system(BESS). The study is focused on the control schemes of PCS and operation strategies of BESS, the main contents are as follows:(1)The existing typical energy storage technologies are reviewed in a wide range of domestic and foreign representative literature, the basic principles,current status and to-be-solved key technologies of some popular technologies are generalized. The application modes of energy storage technologies in the traditional power system and renewable energy generation integration are summarized. In particular, the key technologies of large capacity BESS for renewable energy generation are analyzed, from the aspect of configuration mode, the battery system, PCS and system integration.(2)The linear dynamic models in two-phase coordinates of PCS,including grid-connected isolation transformer, are established.The models reveal the effect of the transformer on the performance of PCS.Three typical linear current control strategies for grid-connected converter are compared.The essential of the PI controller in dq coordinates and its relationship with PR controller in ap coordinates is reyealed.Based on transfer function model, concern on steady error, dynamical response and grid disturbance rejection, the performance of three different linear current schemes are compared theoretically.The digital implementation of the resonant regulator is investigated, which improves the adaptation of the regulator on the variable grid frequency.(3)The dynamic circulating current models in different coordinates are established respectively, which reveals the generation mechanism of circulating current. Based on double Fourier analysis, the spectrum models of PWM converter module output voltage and current are established. Through the established spectrum models, the effection of inter-module interleaved carrier on common-mode voltage, parallel current and circulating current of the PCS is studied, revealing the essence that interleaved carrier can eliminate grid-connected current harmonics, while increase the equivalent switching frequency. The distributed control laws and structures for circulating control are analyzed, and a composite circulating current suppressing scheme based on distributed structure is designed for low frequency and high frequency component respectively.(4)Using impedance modeling techniques and the analysis method of circuit network, the centralized equivalent circuit model of PCS is derived.The concept of interconnected impedance ratio is introduced, and the interaction stability issues of interconnected system between PCS and grid is studied from the aspect of equivalent open loop transfer function, using the Nyquist criterion. Furthermore, the harmonic interaction issues among the converter modules, between the inverter module and the grid is analyzed with the principle of circuit series parallel resonance. As a result, the potential reason for harmonic interaction is revealed.The design of grid-connected control scheme, from the perspective of the input admittance,is proposed. As an example, a design method of grid voltage feedforward with notch filters is given, which can improve system stability and steady-state performance.(5)Considering the application of energy storage coordinated with renewable energy generation, the smoothing scheme of PCS for battery energy storage is given. The impact of asymmetry grid voltage on major aspects of traditional grid-connected control scheme is focused, then an improved control scheme is given, using line voltage for symmetrical components extraction and phase-locked loop and setting symmetric current as grid-connected current control objectives, which enhances its adaptability to the non-ideal grid. The operation strategy of PCS under serious voltage drop is explored, a unified voltage support scheme of BESS under both symmetrical and asymmetrical grid voltage drop is proposed,both positive and negative sequence reactive power compensation can be achieved.Furthermore,a method of current limiting is designed, achieving a reasonable distribution of positive and negative sequence current,as a result,the overcurrent protection and shutdown of PCS can be avoided, and the reactive power capacity utilization of PCS is optimized.(6) The BESS experimental platform coordinated with renewable energy generation is established. The communication network structure based on Ethernet is designed, and related communications software is developed with Socket in the PC Windows platform, achieving the critical information exchange between key devices. Based on two parallel converter modules the PCS prototype is established, the software of PCS prototype is developed,including the hardware driver code and main control schemes, the effectiveness of the related control schemes for PCS is validated initially.