| In recent years,with the continuous development of new generation power systems and the widespread application of power automation equipment,voltage-type power quality issues,represented by voltage sag,have caused many adverse impacts on the development of various industries.Dynamic Voltage Restorer(DVR),as a partial power compensated series governance device,has become one of the effective ways to solve the above problems.Compared with the back-to-back structure,the structure of DVR powered by energy storage system(ESS)has a low degree of coupling with the power grid side during voltage sag compensation,and is more practical in engineering.Although energy storage DVRs have been studied or commercially applied,there are still many difficulties to be solved in terms of operational optimization and control performance.This paper focuses on the key techniques related to the operation and control optimization of three-phase DVR powered by ESS.Firstly,the steady-state compensation strategy and optimization technique of three-phase energy storage DVR are studied.The mathematical models of power level and control level of DVR are established,based on which the definition and influencing factors of compensation capability of DVR powered by ESS are analyzed.Aiming at the problem that traditional compensation strategies for DVR have a large demand for compensation amplitude in asymmetric sag conditions,an optimal zero-sequence voltage injection strategy(OZVIS)for DVR is proposed for three-phase three-wire power supply systems,which can achieve the equalization effect of DVR three-phase output voltage to improve the steady-state compensation capability of the device.At the same time,in order to enhance the practicality of the proposed strategy,a digital implementation process based on analytical geometry modeling ideas is proposed.The proposed strategy can be integrated into any compensation strategy in a "plug-in" manner,achieving plug and play.The feasibility of the optimal zero-sequence voltage injection strategy is verified through hardware-in-the-loop simulation and prototype experiments.Secondly,a flexible compensation strategy(FCS)based on dual phase locked loops(PLL)is proposed to solve the problem of the smoothness of compensation caused by transient disturbances in the power grid during DVR compensation,effectively resolving the contradiction between high-quality load operation requirement and optimal energy management for DVR.Firstly,the causes of sudden changes in the phase angle of the load supply voltage are analyzed,and it is pointed out that the load-side phase jump phenomenon can be caused by primary-side system impedance differences,short circuit fault distance,transformer winding structure,and secondary-side DVR compensation methods.To solve this problem,the idea of transforming the flexible compensation problem facing the entire stage into the one facing the transient disturbance of the power grid is proposed.Based on this idea,the optimal compensation target is decoupled from the real-time compensation command at different time scales,and a flexible compensation strategy based on a dual-speed PLL is proposed,which can flexibly adjust the degree of flexibility according to the characteristics of sensitive loads.Further,in view of the problem of DVR output voltage exceeding the limit in unbalanced sag conditions,an adaptive adjustment method is proposed to alleviate the contradiction between energy storage capacity and load voltage quality.The effectiveness of the proposed method is verified by experiments.Thirdly,the voltage sag detection algorithm and optimization technique necessary for DVR compensation are studied.Aiming at the problem that traditional voltage sag detection algorithms cannot adapt well to non-ideal power grid environments such as unbalanced sag,frequency deviation,phase angle jump,and low-order harmonic distortion,a novel voltage sag detection method based on selective harmonic extraction algorithm(SHEA)is proposed.Discrete spacestate models for SHEA are established using orthogonal basis vectors,and parameter design domains are analyzed and determined from different performance perspectives,realizing the decoupling and separation between target extraction frequency points and interference frequency points,and effectively improving the robust performance of the detection algorithm.In addition,in order to improve the dynamic performance of the proposed detection algorithm,an optimization method based on mid and low frequency gain compensation is proposed,and the overshoot parameter of system convergence is used to constrain the risk of erroneous judgment of the algorithm.Simulation and prototype experiments are used to verify the proposed theory.Finally,the problem of fast mode switching for DVR is studied.The limitations of traditional switching methods are analyzed and elaborated,and it is pointed out that the reasonable utilization of current information is the key to the successful commutation of the bypass SCR in DVR.And a fast switching strategy(FSS)based on current feedback to force SCR commutation is proposed,which is easy to implement in engineering,and can achieve seamless switching of DVR from bypass state to compensation state.The feasibility of applying current loop in series type devices is analyzed,the operating mechanism of this strategy is described,and the design method of control parameters related to system performance is emphatically analyzed.At the same time,through quantitative analysis,it is known that the fundamental voltage drop introduced by the proposed strategy has a small negative impact on the quality of load power supply.Finally,the effectiveness of the proposed strategy is verified through hardware-in-the-loop simulation and prototype experiments,and the proposed strategy has good load adaptability. |
PDF Full Text Request