The Key Technological Research Of Microgrid Protection

Microgrid is an automatic community of distributed generators(DGs) and loads, which can be integrated into main grids by a flexible way. It is an effective solution to solve the high penetration of DGs in distribution networks and improve the performance of DGs. Microgrid protection is an important technique to maintain stability and safety of microgrids, which is required to be effective in both grid-connected mode and islanding mode. Besides, the protection should be immune to the plug-and-play of DGs and controllable loads. This dissertation focused on the techniques of microgrid protection which is based on the microgrid architecture and fault features. Microgrid protection is divided into interconnection protection and feeder protection. Accordingly, principles and schemes for interconnection protection, DC unit protection, current differential protection based on weak communication, impedance differential protection, inverse-time low-impedance protection, and adaptive protection are proposed and analyzed in detail. On the basis of theoretical analysis, the proposed schemes are validated by the medium- and low-voltage microgrids established in PSCAD/EMTDC. The work in the dissertation is shown as follows:(1) Microgrids are divided into DC microgrid, AC microgrid, and hybrid microgrid on the basis of microgrid characteristics. The features and the application ranges of the typical microgrid structures are analyzed. The basic principle, elements and procedure for designing a microgrid are summarized. As a result, a complex microgrid structure including medium-voltage microgrid and low-voltage microgrid is proposed for the simulation of the following chapters.(2) The study proposed a new protection design interconnection protection(IP), for the interface of microgrids especially the multiple and large-scale ones. IP is intended to decouple interconnected subsystems to prevent disturbances or faults from damaging the both subsystems. IP can be classified into facility IP, circuit IP, and utility IP corresponding to the microgrid served. The deployments, features, and protection schemes of different IPs are analyzed according to the operation characteristics and requirements of the typical microgrid.(3) The study focused on the DC unit of inverter-interfaced DGs(IIDGs), introduces the structure of IIDGs, connected mode and grounding way at first. The characteristics of electrical quantities are analyzed when pole-to-pole fault and pole-to-earth fault occurred in DC unit. On one hand, protection strategies of pole-to-pole and pole-to-ground, which based on the results of fault analysis, are put forward. On the other hand, grounded supervision of non-grounding DC system is also proposed.(4) Allow for the practical conditions and technical economy, it is feasible for microgrids to adopt soft real-time communication. In the context, a current differential protection was proposed for feeders in microgrids. The method uses fault detector or starting element to detect the fault instant and solve the data synchronization of differential protection. A modified Fourier algorithm is proposed to remove the effect of DC offset during faults, which can improve the reliability and sensibility of current differential protection.(5) Distribution networks(DNs) gradually became more active and flexible with the massive integration of renewable resources. The conventional protection methods are unsuitable for the promising active DNs. A protection scheme on the basis of measured impedance, which includes an impedance differential method and an inverse-time low-impedance method, is proposed. The former approach identifies fault instant and provides time reference for the exchanged data. As primary protection, it is able to clear different types of faults as soon as possible. The latter method operates in an automatic coordination way with inverse-time characteristic. It can be used as backup protection or provide protection for single-end lines alone. Both of the methods are independent of operation modes and are able to clear faults with high sensitivity.(6) An adaptive time-dependent low-impedance(TDLI) protection scheme was presented with the aid of the advanced measurement technologies in microgrids. The scheme detects faults and calculates tripping time by using the magnitude ratio of maximum load impedance to impedance measured. TDLI protection is immune to varied fault current caused by the flexible operation modes and able to automatically adjust fault-tripping time according to the fault severity. With the existing communication infrastructures, event-triggered adaptive setting scheme updates settings of TDLI relays in real time. The setting method not only enhances the detecting capability, but also provides higher reliability and sensitivity before, during and after events in microgrids, such as tap position adjustment, DGs fluctuation and network structure change. Besides, an acceleration method on the basis of the definite-time grading technique is presented to reduce the impact of infeeds and fault impedance.