Research On Coordination Of Protective Relays In Distribution Systems With Distributed Generations

During the last two decades, the deregulation of the power industry, environmental issues, energy crisis and, in particular, a growing need for exploitation of renewable energy sources and combined heat and power (CHP) technologies have laid the foundations for an increased penetration of distributed generations (DGs) in distribution systems. In distribution systems with some DGs, as the DGs could impose great impacts on the system performance in many aspects both postitively and negatively, the protection issue is considered as the largest technical barrier for the further applications of distributed generations. Traditionally, distribution systems have been designed to operate radially with one supply source so both the power flow and short-circuit current are single-directional, which makes the protection issue easier. When distributed generation units are introduced in distribution systems, the power flow and short-circuit current would be changed to be bi-directional. As the share of distributed generation increases, distribution systems are more similar to transmission systems, thus more complex protection system design will be demanded. However, it is very expensive and time-consuming to replace the old protection systems. Given this background, it is very important to investigate the problems about protective relays in distribution systems with distributed generations. Although much research work has been done and some methods even developed and applied in the actual power systems, there are still many important issues to be solved. This dissertation focuses on several important, yet difficult, problems associated with protective relays in distribution systems with DGs, such as the power flow computation in distribution systems, the characteristics of distributed generations and their impacts on the protective relays in distribution systems, the optimal coordination of overcurrent relays in distribution systems with DGs. These problems are investigated systematically in this thesis and some significant results obtained.First, a new Back/Forward Sweep Method accommodating arbitrary numbing of nodes is developed, and applied to the power flow calculation in distribution systems. In the proposed method, an algorithm similar to the parallel-stream technique in digital signal processing (DSP) and the interruption-waiting technique in single chip microcomputers, named as the parallel and interruption-waiting method, is developed. The developed method is superior to some existing methods in the computational efficiency, and a further extension to deal with mesh networks and PV buses are made at last.Secondly, the Newton-Raphson power flow method is not very efficient and sometimes could even not converge when it is applied to distribution systems. The Back/Forward Sweep method is inconvenient to deal with mesh networks and PV buses in distribution systems although its convergence is good. Given this background, two modified Newton methods, i.e., the Newton-Downhill method and Broyden method, are developed. The research results show that the computational efficiency and convergence of the two proposed methods are much better than those of the Newton-Raphson method. The methods are also extended for distribution systems with DGs.Thirdly, the characteristics of DGs are described in detail, especially for the renewable energy generating units. The models of different kinds of DGs in power flow calculation are presented, and the impacts of DGs on protective relays in distribution systems investigated systematically from twelve aspects.Fourthly, the protective relay coordination problem in distribution systems is investigated with directional overcurrent relays taken as an example, and formulated as a Mixed Integer Nonlinear Programming (MINLP) problem. A mathematical model describing this problem is developed, and the well-developed differential evolution algorithm employed to find the optimal solutions. The proposed method could clear faults quickly with the coordination constraints respected.Fifthly, a novel adaptive protection scheme for the coordination optimization of overcurrent relays is proposed, and applied to settle down the impacts of the fluctuant power output of renewable generations on protective relays in distribution systems. In this scheme, the off-line calculation is employed to save the time for adjusting the relay setting. The relay setting could be obtained by the optimal coordination model, with an objective of keeping the protection system at the optimal state during the output power change of the renewable energy generation units.Sixthly, since the system configuration may change after a fault occurs, the fault current through the overcurrent protective relays could then change, thus the operation time of relays would be changed and further make the protection coordination problem more complicated. Given this background, a mathematical model, considering the system configuration change, for the optimal coordination of overcurrent relays in distribution systems with DGs is developed, and simulation results show that the developed method is correct and efficient.Finally, conclusions are made based on the research outcomes, and directions for further research indicated.