Study On Microgrid Operation Control And Power Quality Control

Renewable energy resource is effective to relieve global energy resource crisis and enhance the ability of energy source sustainable development. Distributed generation (DG) makes use of all kind of distributed energy resource, which has been becoming the way of renewable energy resource. Distributed generation technologies have been used to relieve the construction pressure of centralized power generation facility, which is the efficient benefit for the utility grid. Although the distributed generation has many advantages, however, large-scale distributed generation connected to the utility grid would cause the influence on the security and stability of the utility grid operation because of its randomness and instability. The distributed generation would be disconnected from the utility grid when broken down, which can not play an important role in rush power supply. To solve the negative influence of the large-scale distributed generation connected to the utility grid on its operation, the concept ’microgrid’ which is presented to regulate the distributed generation and promote it friendly connected to the utility grid, can coordinate the contradiction between the distributed generation and the utility grid. The microgrid which is the important solution of the large-scale distributed generation connected to the utility, is also the significant research content of the smart grid development. The study on micogrid is corresponding with the major requirement of the national energy resource strategy.Microgrid operation control and power quality control of the microgrid and the utility grid including microgrids are the key technologies of microgrid research, which propose theoretical foundation and technical support for the microgrid practical application and the large-scale microgrids connected to the utility grid. This dissertation which cover four aspects:microgrid control, microsource control, power quality control inside microgrid (microsource grid-connected inverter composite utilization), and distribution grid power control when many microgrid connected to, is finished by financial support of The National Basic Research Program of China (973Program). The major work and novelties in this dissertation are listed below.(1) Considering the microgrid architecture and the circulating current among microsources, the combined control strategy for the micogrid is proposed and different kinds of microsources in a microgrid system act different roles and should be employed with the corresponding control methods. An improved droop control is presented to promote the response speed and the regulation precision of the output voltage of the microsource, comparing with the traditional droop control. The microgrid seamless transfer technique can suppress the urge current when the microsource is connected to, which can improve the operation stability for the microgrid and achieve the plug-and-play function for the microsource. An unified synchronization and power-output control for the microsource is proposed in order to improve the available synchronous-connection control which need to change the control strategy after the microsource synchronous connection. A simulation model of the microgrid based on several kinds of distributed source is built in MATLAB\SIMULINK. Operation characteristics of island and connection modes of the microgrid are discussed. The simulation results verify the correctness of the proposed control method.(2)Dual-loop power control strategy with an LCL filter is proposed to sever constant active and reactive power in single-phase microgrid, which consists of power outer loop and capacitor-current inner loop. To provide references of connected active and reactive power for the power outer loop, the single-phase circuit detecting method is presented based on the PQ theory. The control variables of the power outer loop are active and reactive power, which are DC. Then the power outer loop adopts PI controllers and can obtain zero steady-stat error for regulating the microsource output power, so PQ control of the single-phase microsource is realized. According to the frequency-shift characteristics of frequency-domain convolution, the transfer function of the power outer loop is derived. The open-loop and closed-loop transfer functions of the whole system are also obtained. With drawing the root locus of the closed-loop transfer function and calculating stability margin of the open-loop transfer function, the stability of the proposed power control system is discussed and the control gains of outer and inner loops are designed. The simulation and experimental results verify the correctness and effectiveness of the proposed power control strategy.(3)Two theoretical bases of composite utilization of grid-connected inverters are studied for photovoltaic microsources. On the base of analyzing two kinds of photovoltaic grid-connected control strategies (PI control and proportional resonant control), two composite control methods are proposed based on synchronous rotating coordinate transformation and stationary coordinate, respectively. And the principles of two composite controls are also analyzed respectively. From several aspects of control structure, regulating precision, digital realization and amplitude limitation, advantages and disadvantages of two composite controls are compared. Simulation research focuses on the composite control based on stationary coordinate, whose performances of photovoltaic generation, harmonic suppression, reactive power compensation, and composite utilization are discussed in detail. Besides, the influence of the composite utilization on PV generation is also studied.(4)The influence of many microgrids connected to the distribution grid on its power quality is discussed. The distribution grid would require reactive power compensation and harmonic suppression when many microgrids are connected to. According to the requirement, a power electronic hybrid system (PEHS) which consists of a thyristor controlled reactor (TCR) and a resonant impedance type hybrid active power filter (RITHAPF) is proposed for compensating reactive power and harmonic current. The PEHS working principle and control strategy are analyzed and presented, respectively. Currently, the compensated-susceptance calculation for TCR is a complete compensation manner, which easily causes over compensation. From the nonlinear relationship curve of power factor and power, what obtained is that: The power factor is closer to an unit and the relationship curve is steeper, and at that moment, the power factor increases one percent which would need more capacity of reactive power compensation. The improved compensated-susceptance calculation based on general dq transformation is proposed for TCR and PPFs. After researching the harmonic suppression principle and the mechanism of affecting the filtering performance, a selective-frequency current control with phase compensation for RITHAFP is proposed to eliminate the influence of the phase shift caused by the equivalent impedance of RITHAPF on its filtering performance. Simulation results prove that the proposed PEHS not only can dynamically compensate reactive power, but also can eliminate harmonic current. The filtering performance of RITHAPF whether phase compensation is compared.