Research On Integrated Protection Of Distribution Systems With Multi-access Distributed Generators

With the rapid development of the global economy, energy and environmentalissues get more seriously, distributed generation is mainly based on renewable energyand is the vicinity of clean and environmentally friendly economical efficient andreliable power generation. It can effectively alleviate the energy and environmentalproblems which are caused by rapid economic. Micro-grid contains several DGs andcan take full use of primary energy. Its high flexibility makes it possible to track loadvariation well, which is helpful for economical and effective operation of power grid.However, after a large number of distributed generators connected, the traditionalradial structure of the distribution network changed into the two-way flow powerdistribution network, in which the traditional current protections are not suitable asusual. They may not meet the demands or even maloperation due to low short-circuitcurrent level. In this paper, the protection schemes for the distribution system withdistributed generation are studied in this dissertation.Since the majority distributed generators are connected to power grid throughpower electronic devices, its output characteristics are closely associated with thecontrol strategy such as the obvious nonlinear characteristics. As the fault behavior isabsolutely different from prefault state, fault current provided by IBDG rises inseveral circles, and because of current limitation the fault current is relatively little,which may lead to malfunctioning of overcurrent protection. Thevenin equivalentmodel which replaces the generator with constant voltage and fixed impedance cannotbe used. The current transient process is caused by current loop of the controller; theduration is related to response speed of the controller.The wavelet transform, continuous wavelet transform, discrete wavelet transformand multi-resolution wavelet analysis theory are introduced in this paper. the db serieswavelet are focused on and the polarity characteristics of the high-frequency transientcurrent at each CT of the distribution network with distributed generators are analyzedusing wavelet transform. In this paper, the protection based on transient currentpolarity comparison is proposed. Simulations of different fault types and faultsituation are preceded in allusion to the protection principle. Results of thesimulations proved the correctness of the principle. Integrated network protection based on bus-line integrated protection is presentedin this paper. The integrated protection units are installed at each bus in thedistribution network with multi distributed generators. The fault location isrecognized by comparing the polarity of each line connected to the bus. Bycommunicating the comprehensive information on each bus can identify the specificfault location when fault occurs in the distribution system with multi distributedgenerators. Theoretical analysis and simulation results show that the use of transientcurrent polarity comparison based on wavelet transform to determine the polarity ofthe relative position of the fault is feasible.