Study On Micro-grid Interconnection Control Technology And Optimization

In recent years, for the realistic condition that it is occurring the utility grid faultlarge-scale blackout, the serious environmental pollution, and exhaustion of energysources, the micro-grid consisting of renewable clean micro-sources such as wind powergeneration、 solar power generation, and energy storing device not only can rungrid-connected with the utility grid, but also can be separated with the utility grid into theisolated network operation mode. Based on the stabilization, flexibility and easy to control,the micro-grid are gotten more and more attention and widely used. Grid-connectedinverter is the core part in the micro-grid; its performance plays an important role in theefficient and stable operation of system. In the process of micro-grid’s parallel operation,in order to distribute the electrical and thermal stress of the inverter power supply modulesuniformly and reasonably, the voltage、frequency and phase angle of the inverter powersupplies must be kept synchronization. At the same time, the grid-connected inverter itselfis nonlinear, which could cause the system’s voltage, current wave, and waveformdistortion, lead to harmonic pollution of whole micro-grid. For the above situation ofmicro-grid connected system, this paper studies the micro-grid connected controltechnology and its optimization.For the micro-grid system’s single-phase LCL grid-connected inverter, a kind ofrobust iterative learning controller is designed. Based on the output power droopcharacteristics of inverter, the current sharing among the inverters is gotten. Iterativelearning strategy is suitable for repeated tracking control and periodic disturbance inhibit,and the controller is designed combining with robustness index, so that it has the ability toovercome the uncertainty of system parameters. Compared with the repetitive control,robust iterative learning control can get high precision output waveform, and enhance thetracking ability for waveform, and solve effectively the distortion problem of the outputsignal.For the problems that the parameters of multi-grid inverters are unknown or slowtime-varying in the micro-grid, first of all, a kind of power droop characteristics is adopted to achieve power distribution among inverters, meanwhile the virtual reactance isintroduced to solve the problems that when the line is showed resistive, there are being thestrong coupling phenomenon between active power and reactive power, and theperformance of power droop control is improved. The voltage reference which is got byvirtual power droop control has been transformed into current reference. Considering theimprecise and changes of system parameters, an adjustable PID controller based onadaptive internal model is adopt, and high precise control of current is achieved. Thesimulation results show that the control scheme has good steady and dynamicperformance.Aiming at the micro-grid which is provided power by wind power, because of theserious frequency fluctuations and intermittent wind power, frequency control should becarried out on the perform client. Regarding the micro-grid as the uncertainties system,this paper proposes optimization control scheme based on mixed H2/H∞objective, andadopts the simplex to optimize PID controller parameters. The Simplex method based onmixed H2/H∞objective function, not only resolves the problem that the PID controller hasdifficulty in parameter tuning, but also improves the robustness of the system and thereal-time optimal control, and ensures the stable operation and making full use of power inthe micro-grid.