Studies On Dynamic Reactive-Power Compensation In Distribution Grid

With the rapid development of power electronic devices and power converting technique, power electronic equipments have been widely used in modern society since 1980s, and play an ever important role in enhancing the stability and security of power system, increasing transmission capability and load efficiency, and saving energy. Unfortunately these types of equipment impose nonlinear loads to the power grid, which draw harmonic and/or reactive currents in addition to active current from the power grid and do harm to the power system and other equipments connected to the power grid. As applications of power electronic equipments increasing, the problems related to harmonic pollution, electromagnetic interference (EMI), and low power factor (PF) of the power system are becoming more serious.Facing to the current status of weak grid configuration, undeveloped transmission and distribution technology, low automation level in our country, it is more important and urgent to study new technique and equipments to suppress harmonics and compensate reactive-power. This Ph.D paper focuses on the reactive-power compensation of distribution grid. As one of the most important equipments in custom power, Distribution Static Synchronous Compensator (DSTATCOM) is an effective measure to dynamically compensate reactive-power, maintain voltage stability and improve power quality of distribution grid.Based on analyzing and comparing many control strategies, multivariable inverse-system method is applied to a current indirectly controlled DSTATCOM. First, a more rational and precise nonlinear dynamic mathematical model of DSTATCOM is established by the switching function approach, which takes device losses into more consideration, i.e. the losses which are in proportion to the square of current can be equivalent to an AC side resistor, while the losses which are in proportion to the square of voltage can be equivalent to a DC side resistor. In order to achieve decoupled control easily, the state variables of the model are reconstructed, and then the dc-link voltage and reactive-current are decoupled successfully based on multivariable inverse-system method, therefore the design of PI controller is simplified greatly. A control strategy combining with state feedback and feedforward is also presented, which can fast voltage and reactive-current response and enhance anti-disturbing ability. By analyzing the stability of the decoupled pseudo linear system, it can be proved that the total system is stable. Based on the designed multivariable inverse-system controller, DSTATCOM's control under asymmetric voltages is also studied deeply. The theoretical analysis and design are verified by the simulation results.For a current directly controlled DSTATCOM, the equivalence of two current control schemes—AC main current direct control and traditional current direct control—is being proved first, i.e. the two control schemes are found to have equal control results. But the AC main current direct control scheme does not require the load's harmonic and reactive currents to be sensed and computed, thus it can result in more simplified circuit. Furthermore, the key of current direct control is the application of tracking PWM control technique, so a new current tracking PWM control method is proposed in the paper, which is called bidirectional parabola method and have a nearly constant switching frequency, then the basic principle of this new method is presented. Finally a new current directly controlled DSTATCOM system is designed and implemented, which employs techniques of AC main current direct control and bidirectional parabola current tracking PWM control. The effectiveness and rationality of this new type DSTATCOM are verified by the simulation and experiment results.Installing dynamic compensating equipment to compensate the harmonics and reactive-power in distribution grid is a kind of passive compensation. On the other hand, the capacity of the distribution compensating equipments will decrease greatly, even nothing at all, if we can improve power electronic equipments themselves which are regarded as main nonlinear loads to make them produce less harmonic and keep the input current in phase with the input voltage. Therefore, it's a more positive and active means to improve the inner performance of customs' power electronic equipments, and it is helpful to solve the problems of power quality radically. More in-depth study on the subject has been developed in this paper.Currently widely used rectifiers, which have very low power factor, are the main harmonic source in distribution grid. If we can improve these rectifiers, enable them do not produce harmonics and keep nearly unit power factor, then high power factor rectifiers can be achieved. On the basis of study of the current active power factor correction technology, a new high-performance power factor correction(HPPFC) method based on one-cycle control(OCC) is proposed in this paper, which can be used to correct power factor and suppress harmonic for single-phase rectifier load. The circuit topology and basic principle of this method are first analyzed, and the corresponding OCC control equation is derived out, then the estimating formulas and design to determine the main circuit parameters, such as the inductor, the dc-link capacitor, are given. The large signal model of the HPPFC circuit with OCC is established based on energy balance, then the PI controller of the control system is designed. The control system is corrected to be a typical II system in which the steady error is eliminated and the anti-disturbing ability is improved. All of the above analysis and design lay a theory foundation to comprehensive design of the whole system. The theoretical analysis and design are verified by prototype experiments.