Larege-Scale Power Conversion System In Battery Energy Storage Without Transformer

Because of fast response,high power density,no specific installation location and fast technology development,battery energy storage has been one of the most promising energy storage technology at present.With the increase of battery energy storage capacity,the existing power conversion system has more and more difficulties in the aspects of efficiency and cost optimization design.Therefore,better solution is extremely urgent.Transformerless large-scale battery energy system combines the low voltage battery module and multilevel converter with modular constructure,can not only access to the high voltage power grid,but also reduce the inner circulation of battery module.As a result,the cycle efficiency and reliability of whole system will be improved.However,the special structure brings many new challenges for batteries and power conversion system.In order to solve these problems,this paper studies the transformerless large-scale power conversion system in the aspects of topology,control scheme and methods of optimization design.Based on the power conversion system with cascaded H-bridge converter as object,this paper proposes unified balance control scheme,which reduces the complexity of the whole control scheme and unites the balance needs of grid voltage unbalance,module fault redundancy control and state of charge balance between phases;This paper uses the bi-directional DC/DC converter as the the interface of the battery modules and proposes the new control schem with notch filter based on the impedance model,thus suppress the second-order ripple current on the battery side;This paper studies the mechanism of common-mode current in the power conversion system with cascaded H-bridge converter and shows the estimation formula of peak value based on the common-mode current path analysis,then provides a suppression method;This paper put forward a kind of energy storage function of reactive power compensator,study the characteristics of the output power,and puts forward a new control strategy based on bangbang principle,which not only improves the control performance,but also expands the range of output power.With battery modules in each submodule,the modular multilevel converter will have a new application scenario.This paper analyzes the complex operating conditions of modular multilevel converter integrated with batteries,studies the state of charge balance methods with ac power and dc power,proposes a control scheme that can be used multiple operating conditions.This paper studies the special operating condition that modular multilevel converter integrated with batteries support dc grid continuously when ac grid fault happens,proposes a new state of charge balance method between arms.This paper replaces part of half bridge submodules with full bridge so that the new modular multilevel converter integrated with batteries can ride through the dc grid fault,studies the control before and after dc grid fault and provides a new voltage allocation method to gurantee state of charge balance between submodules.For the optimal design of the transformerless large-scale power conversion system,this paper discusses the boundary conditions of the system design,provides the estimation formula of efficiency and reliability,and puts forward the optimization design method.This paper designs the power conversion system of a 2MW/2 MWh energy storage power station,which have been put into commercial operation and achieve the efficiency of 98%.This paper uses the modular multilevel converter integrated with batteries into wind power generation,provides the control scheme and studies the performance of wind power smooth in this application scenario.