Integrating Load Flow And Short-Circuit Current Calculation For Distribution Systems Containing Distributed Generations

Along with the steady progress achieved in DG (Distributed Generation) technologies and gradual decreasing of the costs of DG, DG can offer a novel solution to many technical and commercial constraints on power network. By interconnecting with traditional power systems, DG will play a more important role in power systems in the near future. As DG is popularized, it has a great impact on node voltage, power quality, system protection, supplying reliability and short circuit of distribution network. The degree of impact is closely relative with the location and capability of DG.Traditional distribution power flow will not meet the requirement of distributed generation systems. This paper focuses on load flow and short-circuits current calculation for distribution systems containing DGs. The main works of this paper include as below:For the unbalanced three-phase load flow of the distribution system with DGs, the improved backward-forward sweep is presented.According to the interfaces between DG and power grid, as well as the operation modes and control characteristics of typical DG, including full cell, micro-turbine, photovoltaic et al, the method to deal with them in power flow calculation are presented. The essence of the method is to transform all kinds of nodes into PQ node, PI node or PV node which can be treated by traditional power flow methods. This paper puts forward the method for power flow calculation including wind power generator.Regarding the active power output of each wind power generator as the function of wind speed as well as its reactive power output as the function of both active power and nodal voltage, a PQ(V) steady state model of wind power generator based on equivalent circuit of induction generator is established, and a method to determine the initial installed capacity of shunt capacitor bank, the number of banks to be installed and the number of real-time switched banks is proposed.Based on network branch equation, according to the weakly meshed structure characteristic of a distribution network, the nodal impedance matrices are calculated directly by line impedance matrices, avoiding constitute the nodal impedance matrices of the networks. The method can be adaptable to network changes in topology.For the load flow of the distribution system with DGs, the improved backward-forward sweep is adopted. By applying superposition theorem, multi-dimensional thevenin theorem, considering characteristics of distribution system with DGs and multi-port compensation technology, this improved backward-forward sweep can handle asymmetrical three-phase and weakly meshed distribution system with DGs.This paper presents a uniform approach for power flow and fault analysis of asymmetrical three-phase, as well as weakly meshed distribution system with DGs. Based on the forward and backward sweep method, by using a uniform compensation circuit model given for all kind of faults, the algorithm can be applied in both normal and short circuit conditions,and their computation formulas and iteration steps are basically the same.An example of 13-node system is used to test the method proposed. Test result verifies the feasibility of the proposed algorithm.