Research On Key Technology Of High Power Converter With Energy Storage In Microgrid

The microgrid is widely attractive because of energy shortage and environmental pollution.The various Distributed-Generators(DGs)are controlled by microgrid as power generation equipment.As a kind of special DGs in microgrid,the Energy-StorageConverter(ESC)need to have the following functions: using energy storage equipment to stabilize output power fluctuation of intermittent energy and improve power quality;establishing stable AC bus voltage for DGs and load in microgrid under off-grid mode;possessing the essential protection function when the grid fails.At present,some problems and challenges arise in the application of microgrid,they are: output current harmonics of high power ESC lead to poor power quality in microgrid;unequal output power distribution and circulating current of the parallel converters,which influence the efficiency and stability of the parallel system;When islanding protection,Low-VoltageRide-Through(LVRT)and High-Voltage-Ride-Through(HVRT)occur,the ESC's capability of supporting and controlling for microgrid becomes worse;the fluctuation of Neutral-Point-Potential(NPP)of multilevel converters in microgrid increases the output voltage harmonics,and seriously influences the stability of the system.These problems of high power ESC limit the application and promotion of microgrid.This article takes the ESC in microgrid as the research object,and the key control technology of ESC are studied.The main work of this article is as follows:1.The control structure of the microgrid is discussed,and the basic control methods and key problems of the ESC are summarized.Using the ESC to stabilize intermittent power fluctuation is studied;by analyzing the operation characteristics of a microgrid,a microgrid energy management system and a power control strategy for ESC are designed.The above research contents are applied to a microgrid demonstration project with master-slave structure,which improves the stability of microgrid.2.Suppression of ESC output current harmonic.ESC has an important influence on the power quality of microgrid,and suppression of the ESC output current harmonics is studied in this paper.For a single converter,a harmonic extraction algorithm named as Quadrature-Sine-wave-Extractor(QSE)is proposed.By constructing the orthogonal sine wave model of the periodic signal,the QSE algorithm transforms the harmonic extraction into the design of the state observer.Based on the discrete state model,the QSE controller can be designed directly in the discrete domain.QSE has advantages of no phase displacement and can offset mutual influence among each component,which can reduce current harmonics and ameliorate system dynamic performance.For multiple ESCs in parallel operation,the Carrier-Phase-Shifted(CPS)is used to control the output current ripple phase of the ESC,which can eliminate the harmonics of the total output current at switching frequency,and effectively reduce the THD of total output current.3.Study on the parallel operation and circulating current suppression of ESC.When the droop control algorithm used for parallel ESCs in low voltage microgrid,line impedance characteristics and the difference of output voltage leads to unequal output power distribution and large circulating current between the parallel ESCs,which influences the stability of parallel systems.An improved droop control strategy based on virtual impedance is proposed.By adjusting the given value of the droop control in real time,the output voltage of the ESC remains stable in off-grid mode and accepts the power given from the microgrid master controller under grid-connected mode.The virtual inductance and inductance voltage drop compensation are added to change the output impedance of the ESC to inductive,which overcome the problem of coupling between the output active power and reactive power of ESC in the low voltage microgrid,and effectively realize the equalization of output power and guarantee the stable operation of parallel ESCs.A microgrid experimental platform is built to realize the operation of parallel ESCs under the two modes of grid-connected and off-grid.For circulating current between the parallel ESCs of independent control and operation in microgrid,the frequency characteristics of the Zero-Sequence-Circulating-Current(ZSCC)are analyzed,and the QPR and PI controllers are used separately to reduce the dc and 3,9 and 15 harmonic components of ZSCC.Experimental results prove the effectiveness of the proposed method.4.The technology of islanding protection,LVRT and HVRT for ESC in case of grid failure are studied.Firstly,aiming at the shortcoming of traditional island protection algorithm that affects output power quality,a control strategy based on disturbance reactive power decoupled is proposed.The disturbance component of reactive power is extracted by two vector angles to avoid the influence of disturbance component on the output active power.Then for LVRT of parallel ESCs,a control strategy that separating voltage positive and negative sequence and compensating reactive power during grid faults is designed.The effect of negative sequence components caused by the asymmetric LVRT is eliminated,the parallel ESCs can output reactive power to support the grid voltage and remian stable operation during LVRT.Finally,in view of the impact of grid voltage rise on the output capability of ESC,a control strategy that increases the controllable area of the ESC by changing the DC bus voltage is designed during HVRT.The tests of islanding protection,LVRT and HVRT are carried out according to the relevant grid standards,and the results show that the proposed strategy has strong practicability.5.The Space-Vector-Pulse-Width-Modulation(SVPWM)technology and NPP balance control strategy for T-type Neutral-Point-Clamp(T-NPC)Three-Level-Converter(TLC)are studied.The realization process of T-NPC SVPWM is discussed,and the output power quality of TLC is analyzed.Aiming at the NPP fluctuation of TLC,an OptimizedVirtual-Space-Vector-PWM(OVSPWM)is proposed.By controlling the operational time of positive and negative vectors for synthesizing the virtual vector,the OVSPWM can solve the disadvantages of the traditional Virtual-Space-Vector-PWM(VSPWM)that cannot inhibit the deviation of NPP caused by external factors,and effectively reduce the uncontrollable area with the high modulation ratio.A 125 kW T-type TLC prototype is developed,the simulation and experimental results show that the proposed control strategy reduces the NPP fluctuation effectively,and TLC possesses the advantages of higher voltage level,lower harmonic currents against two level converter...