Research On The Control Of High Voltage Cascaded Energy Storage Power Conversion System

Energy production of wind and photovoltaic is fluctuating. Interconnecting these intermittent sources to the utility at a large scale would pose a challenging problem. An energy storage system could compensate the active-power fluctuation, and the battery energy storage system, interconnected to the utility grid by power electronic converter, could provide reactive power to the grid when necessary.The unique structure of the cascaded H-bridge(CHB) converters allow it to synthesize a high ac output voltage from several independent low dc voltages, which make it proper for connecting either renewable energy sources or energy storage devices to high voltage grid as a battery energy storage power conversion system(PCS).As the interface between the battery unit and the grid, the CHB energy storage PCS must be capable of inputting/outputting active and reactive power according to the grid dispatch both on the balance and unbalance grid voltage condition. Because the dc-buses of the H-bridge cells among the CHB converters are split and isolated, therefore the CHB energy storage PCS should be capable of balancing the SOC of battery units among the energy storage system. The CHB converter consists of many H-bridge cells, in order to improve the reliability of the whole system, the CHB energy storage PCS should be able to continue to work when one of the H-bridge cells is in fault.In this paper, a dual current control scheme for cascade H-bridge energy storage PCS under balance and unbalance grid voltage is presented and the algorithm for the detection of the positive and negative sequences component of the grid voltage and PCS current is reviewed. An analytical expression for each phase power and whole power of the PCS is drived, which provides the link beteen the power of the PCS to the positive and negative sequence of the grid voltage and PCS current, and a zero-sequence-voltage is added to the three-phase ac voltage of the PCS to balance each-phase power of the PCS on unbalance condition. Methods to balance the three-phase battery clusters SOC based on the negative sequence voltage injection and zero sequence voltage injection are both analyzed, and the zero-sequence-voltage injection method is employed. An extra ac component is added to the ac voltage of the H-bridge to balance the battery unit SOC among each phase. When one of the H-bridge cells is in fault, an neutral shift fault-tolerant control is introduced to ensure that the CHB energy storage PCS continuous operation and maintain SOC balancing of the remaining battery units. In addition, a simulation model based on MATLAB/SIMULIK is implement and the simulation results which verify the validity of the context control scheme is presented.