Analysis And Compensation On Power Quality Of Distribution Network With Distributed Generation

The power electronics based Distributed Generation(DG)has the characteristics of complex transfer capability,large power fluctuation and high nonlinearity.While ignoring its influence,the large-scale DG integration has great impacts upon the distribution network,such as harmonic,resonance,voltage fluctuation,unbalance and so on.These problems will disturb the reliability of power system and deteriorate the power quality for customers.Therefore,some studies on the analysis and compensation of the power quality problems considering the grid-connected inverters have been highlighted in this dissertation.In the power quality problem analysis part,there are four items are researched.They are the bifurcation characteristic of DG's full bridge inverters,the harmonic interaction between nonlinear loads and DG,the harmonic resonance of the distribution network with DG,the voltage variation of the distribution network when DG integrated asymmetrically.1.DG's full-bridge inverters may lose stability and inject high or low frequency harmonic components into the distribution network via fast-scale or slow-scale bifurcation due to its high nonlinearity.In this dissertation,the state-space averaging model and discrete-time model of the three-phase full-bridge inverter under Sinusoidal Pulse Width Modulation(SPWM)and Space Vector Pulse Width Modulation(SVPWM)are established respectively in synchronous rotating frame to analyze its slow-scale and fast-scale instabilities theoretically.2.The study on output characteristics of inverters with nonlinear loads can not only guide the optimized-design of inverters but also obtain quantitative analyses of the power system.The models of inverters are constructed based on the two-dimension Fourier transform.Meanwhile the models of nonlinear loads are constructed based on the modulation theory and the hybrid modeling method in time-frequency domain.The harmonic characteristic of inverters with nonlinear loads is estimated by solving the models' functions in frequency domain with the preconditioned damped Jacobian-free Newton-Krylov method.3.The harmonic resonance can be caused by the capacity difference,inverters'nonlinearity and high order filters.So the modal analysis method is adopted to determine the contribution degree of each network component to a harmonic resonance phenomenon.And the propagation or impact areas of each harmonic resonance are revealed.Besides,the guidelines for relieving the harmonic resonance in distribution network are provided.4.After the DG with random output power connecting to the grid asymmetrically,the voltage fluctuation and unbalance problem will be caused easily.By using the probabilistic power flow algorithm,such as the Monte Carlo method and the unscented transformation method,the probabilistic load flow problem can be transferred into the deterministic power flow problem and solved after several iterations.Furthermore,by using sequence-component method,the asymmetric power flow problem can be solved.By using step iterative method,the droop controlled nodes are converted to conventional nodes.Then the droop power flow problem can be solved by the Newton-Raphson method.In the power quality problem compensation part,according to the influence and propagation mechanism of power quality problems,not only the control strategies of DG are improved to make active compensation.But also a new DFACTS device based on Nine-Switch Inverter(NSI)is proposed to make passive compensation.So the compensation methods for the power quality problems are suggested as below.1.For the controlled voltage source type DG,a novel phase compensation control strategy,which formed by the phase compensation quasi proportional complex integral controllers and phase compensation quasi proportional resonance controllers,is proposed to provide independent control for every harmonic component.So the response speed and stability margin of voltage control loop can be optimized.For the controlled current source type DG,a composite space vector hysteresis current controller based on modulation time allocation is proposed.So the current control loop can make coordination between accuracy and dynamic response.2.A novel double injection type hybrid active power filter based on NSI,whose configuration with a dual-port power converter bridging both sides of the distribution transformer,is proposed for power quality enhancement in distribution network.By using this new DFACTS device,the DC side voltage of NSI can be controlled while power quality compensating.And the transformer can be protected from the harmonic interference.The further works,including an adaptive theory based improved FBD algorithm and a discontinuous space vector modulation method based on token instruction,are given to improve the performance of the new DFACTS device.In conclusion,this dissertation discussed the generation mechanisms and the compensation methods of the power quality problem in distribution network with DG.Due to the complexity and particularity of power quality problems with high penetrated DG,this dissertation can only provide a reference for this research.And it's hard to cover every field of the power quality issue.Further surveys on the power quality compound control system,the multi-objective coordinated control strategy and the optimal configuration method should be carried.