Research On Power Quality Control Of Microgrid With Distributed Generation

Renewable energy resources are effective to relieve global energy resource crisis and enhance the ability of energy source sustainable development. Solar and wind energy development have caught common attention recently. Distributed generation (DG) makes use of all kind of renewable energy resources, which have been becoming the key way of renewable energy development. Microgrid is an important linkage between distributed generators and the utility grid and has been widely favored because of its high reliability and flexibility. Microgrid can reduce the influence on the security, stability and reliability of the utility grid operation because of its randomness and instability, which is also the significant research content of the smart grid. Although the microgrid has many advantages, however, due to the special structure and operation mode the microgrid faces the power quality issues, which have significant influence on the security and stability of the microgrid operation. As a conventional approach, ancillary devices can be installed in the microgrid to overcome the power quality problems. These ancillary devices, however, are undesirable because of their high installation costs. Therefore, the power quality control of the microgrid with the distributed generators is an innovative and challenging research topic.Some detailed and deep studies on the harmonics, voltage deviation and frequency deviation control to provide ancillary service of microgrid have highlighted in this dissertation by using distributed generation interfacing converter. This dissertation is finished by the financial support of The National High Technology Research and Development Program of China (863 Program) and the key research projects of the State Grid Corporation.1. In order to mitigate the harmonics introduced by the nonlinear load, a novel communication-less harmonic compensation approach through distributed generation interfacing converter in a multi-bus microgrid is proposed. The control approach of each individual DG unit was designed to use only feedback variables of the converter itself that can be measured locally. In the proposed approach, the adjacent bus voltage is indirectly derived from the measured DG converter output voltage, DG line current and line impedance. A voltage closed-loop controller and a current closed-loop controller are designed to achieve both functions of DG real power generation and harmonics compensation. Therefore, the traditional harmonic measurement devices installed at the bus as well as the long distance communication between the bus and the DG converter are not required. The proposed approach can compensate the current harmonics, mitigate the buses voltage distortion and enable the customer devices to be operated in normal conditions within the multi-bus microgrid, and meanwhile relieve the burden of power quality regulator installed at the point of common coupling (PCC). Matlab simulations and experimental results are presented to show the operational effectiveness of the proposed approach.2. By considering bus voltage deviation produced by the droop controller itself and the voltage drop on the line impedance, a bus voltage compensation method in the islanding microgrid using the improved power sharing control scheme among the distributed generators without communication compensation algorithm is proposed in this dissertation. The control scheme of each individual DG unit is designed to only use the locally measured feedback variables and the obtained line impedance to calculate the bus voltage. Therefore, the traditional voltage measurement devices installed at the bus as well as the communication between the bus and the DGs are not required. The proposed control scheme can maintain the bus voltage amplitude within the allowed range even to some extent assuming the inaccurate line impedance parameters, meanwhile, can achieve the proper power sharing in the islanding microgrid. The experimental results obtained under accurate and inaccurate line impedance are presented respectively to show the performance of the proposed control scheme in an islanding microgrid.3. Droop control is an effective power sharing control scheme of distributed generation inverters in islanding AC microgrid, which can stabilize the grid as long as the distributed generation systems have sufficient residual power. On the contrary, many loads have equipped with interfacing power conversion circuits to improve the operational efficiency, which in fact can also provide the regulation function for stabilizing grid during transient process. This dissertation therefore proposes a frequency-dependent trimming scheme for insensitive active power electronics loads in microgrid to control grid frequency together with DGs. The frequency-dependent power electronics loads trimming scheme can effectively reduce the frequency variation. Meanwhile the frequency-dependent power electronics loads trimming scheme can release the burden of fast energy storage systems in microgrid and then reduce the construction cost of microgrid. Control wise, the load trimming scheme is also a communication-less control theory, which only assumes the local information. Matlab simulations are presented to show the effectiveness of the proposed theory in an islanding microgrid.4. In order to attenuate the double-line-frequency dc-link voltage ripple and improve the power quality of the converter output current, dc-link voltage control method for the two-stage power converter during the asymmetrical voltage drop is proposed in this dissertation. The proposed control method could to some extent attenuate the double-line-frequency dc-link voltage ripple to keep the dc-link voltage in the safe operational range by shifting the double-line-frequency power ripple to the front-end dc input source, which can be achieved by intentionally employing a bi-directional dc-dc converter according to the voltage drop detection. The theoretical findings were verified by Matlab simulations and the constructed experimental prototype.