Research On Protection For Microgrid And Distribution Networks With Distributed Generation

Distributed generation (DG) and microgrid have the advantages of resources and environment friendliness, flexibile and reliable power supply. With the development of bulk power system, developing distributed generation has become an inevitable trend in power systems home and abroad. Grid connected distributed generation changes the structure of distribution network, which makes the distribution system from single supply radial network to multi-source network. The power flow is no longer one way flowing from substation to the load. The changes in the distribution network have impacts on the existing protection as well. The fault current induced by DG changes the fault current magnitude and direction on distribution line, and the scope of protection, causing malfunction, misoperation or failure to operate of protective relay. Study on improving the protection system to adapt new distribution network with grid-connected distributed generations is significant to the safe and reliable operation of distribution network. This dissertation focuses on the protection issues of distribution network with distributed generations connected.Distributed generation and micro-generation in microgrid are usually renewable energy sourced power generation, which need power electronic device as interface to converter the power produced to that with the same frequency in grid. The fault characteristics of the inverter interfaced DG are different from that of traditional synchronous generators. The dissertation studies the mathematic and simulation model of inverter interfaced DG and microgrid, analyzes the fault features of the DG outlet and fault line when a variety of short circuit fault occurs in the power line, analyzes the impact on the traditional protection relays of distribution network, then studies the improvement of the protection in distribution network to accommodate DG's integration. The fault characteristics differences between the microgrid and traditional power grid make the traditional protection algorithm based on large fault current no longer suitable. On the basis of analyzing the fault features of fault line in microgrid, a new protection algorithm for microgrid is proposed. The main aspects of the dissertation are listed as follows:(1) On the basis of mathematical model of inverter interfaced distributed generation (IIDG), the simulation model is established through DIgSILENT /PowerFactory software. The fault characteristics of inverter interfaced distributed generation in two common control modes:PQ control and V/f control are analysed.(2) On the basis of mathematical model of microgrid, a simulation model of microgrid is built on the DIgSILENT/PowerFactory software platform. The fault feature in different fault conditions in microgrid lines is analysed, when microgrid operating in stand-alone or grid-connected mode.(3) The impacts of the integration of DG and microgrid on the protection performace of distribution networks are studied. An adaptive current protection algorithm for improving the original protection settings in distribution networks is proposed, which offsets the impact on fault current caused by DGs using compensating factors. The effectiveness of the algorithm is verified by simulation.(4) A new distribution network protection scheme based on multi-agent technology is proposed, including the logical framework and communication architecture of multi-agent system, the main protection algorithm, protection coordination strategy and islanding protection scheme, etc. The feasibility of the scheme is verified by simulation.(5) An improved wavelet algorithm for digital filtering is proposed based on the fault characteristics in microgrid. By using the current phase as criterion, an adaptive current phase comparison pilot protection algorithm is proposed, in which adaptive algorithm is applied. The algorithm is suitable when microgrid is operating in stand-alone and grid-connected modes. The effectiveness of the algorithm is verified by simulation.