Research On The Key Issues Of Distribution Static Synchronous Compensator

With the development of national economy and the improvement of science and technology,the non-linear,impact and unbalanced loads are increasing rapidly in low-voltage distribution network recently,which causes the power quality degradation of the distribution network and has brought a lot of power quality problems such as: low power factor,harmonic distortion,voltage flicker,three-phase imbalance and so on,especially the imbalance problem in the three-phase four-wire distribution terminal.Distribution static synchronous compensator(DSTATCOM)is a kind of dynamic compensation device with comprehensive compensation capacity,and it is usually applied to solve the complex and variable power quality problems in low-voltage distribution network.The three-level DSTATCOM has many advantages,which is quite suitable for the four-wire distribution network environment.Therefore,it has broad application prospect in the three-phase four-wire distribution network.Mathematical model is the basis for the theoretical analysis of control object and engineering design of control systems.Comparative analyses of the main topologies of three-phase four-wire three-level DSTATCOM are conducted in this paper.Three-phase four-wire DSTATCOM with the three-level three-leg capacitance center-split structure is selected as the main research object.Besides,the equivalence between the DSTATCOM with three-leg capacitor-split and the DSTATCOM with four-leg is derived and proved.Meanwhile,the unified mathematical models of two four-wire DSTATCOM compensation systems are established,which provides the basis for the transplant of control system.From the scalar model of DSTATCOM compensation system,the vector model of the system is built by introducing the concept of complex vector,so that the mapping between the stationary coordinate system and the rotating coordinate system is simplified,which provides a great convenience for the analysis and design of control system.In terms of the more severe imbalance of neutral point,a “component superposition model” is established by the symmetric component method,which can clearly describe the change of the midpoint potential.This model will provide the theoretical basis for the research and design of the neutral point potential balance control strategy in the three-level DSTATCOM with capacitor center-split structure.In order to control the zero-sequence component of output voltage of the capacitor center-split three-level DSTATCOM better,deep research of three-dimension space vector modulation(3D-SVM)technology is conducted in this paper.Through the comparative analysis of two kinds of classic 3D-SVMs in traditional orthogonal three-dimension coordinate system,this paper points out the fundamental reason for the complex calculation of 3D-SVM based on α-β-γ coordinate system.Taking the advantages and disadvantages of the two classic 3D-SVMs into account,a new 3D-SVM modulation strategy based on non-orthogonal three-dimension coordinate system is proposed.Distribution of the basic voltage space vector can form a number of spatial tetrahedrons in the non-orthogonal three-dimension coordinate system,according to the feature information of these spatial tetrahedrons the novel 3D-SVM’s three key steps,which synthetic vector selection,function time calculation and switch sequence determination,can be realized by logical judgment and simple algebraic operation.The proposed algorithm has advantages,such as simple design,short calculation time and university for level numbers.Besides,the design of this algorithm is more conducive to achieve the performance optimization control of the multi-level inverter.The neutral point potential imbalance of the three-level DSTATCOM must be considered and resolved.Based on the “component superposition model” of neutral point potential established in the paper,a neutral point potential control strategy based on vector replacement is added to the non-orthogonal 3D-SVM.In this control strategy,the influence of vector replacement scheme on the zero sequence component of the output voltage is taken into account while controlling the neutral point potential balance.By introducing the neutral point potential balance measure factor,the effective vector replacement scheme will be quantified and evaluated.In this case,the selection process of vector replacement scheme is greatly simplified.Then according to the calculation result of measure factor,corresponding control scheme is implemented to suppress the imbalance of neutral point potential.As a four-wire dynamic compensation device of distribution network,the three-level capacitor-split DSTATCOM is also responsible for the imbalance and harmonic control while making compensation for the reactive power.In the DATACOM comprehensive compensation,the traditional control program consisting of PI control and current decoupling based on the synchronous rotation coordinate system has some shortcomings,like ineffective decoupling control and the complex calculation of multi-coordinate transformation.For this reason,the instruction current tracking control strategy based on the resonant controller in the stationary coordinate system is researched in this paper.A comparative,detailed analysis of the dynamic and static characteristics as well as the system stability of two concrete implementations of the resonant controller,i.e.the proportional resonant controller(PR)and the vector resonance controller(VR),is carried out.It is demonstrated that the two kinds of resonance control can both realize steady-state control without static error,but the current loop adopted VR controller can get better dynamic response when the load is abruptly changed.This paper further analyzes the problems existing in the realization of the controller engineering,and points out the reasons for the error introduction,then it provides the corresponding compensation measures.When the DSTATCOM works as dynamic compensation equipment in the grid,there will be mutual impacts between the device and the power grid.In this paper,the influences of power system failure and self-fault of compensation device on the running status of DSTATCOM is analyzed in detail.When a power system failure occurs,the steady-state equivalent model and transient switch model are established after the entire compensation is accessed by system.Then the compensation capability of DSTATCOM device under the power system fault condition is researched,and the situations that will threaten the safe operation of device are described.In view of the most common short-circuit and open-circuit faults of the power devices in DSTATCOM,the fault-tolerant control strategies aiming at reactive power compensation or imbalance management respectively are put forward.Finally,based on the DSTATCOM control strategy proposed in this paper,a three-phase four-wire DSTATCOM experimental prototype with three-level three-leg capacitor-split is developed.Simulation and experiments of the prototype show that the non-orthogonal three-dimension space vector algorithm is simple and its computation time is short.The current tracking control system of DSTATCOM based on resonant controller can effectively achieve the compensation for asymmetric load and harmonic.The fault-tolerant control strategy will maintain the continual operation of the device in a certain failure range,so that the device design has achieved the desired purpose.