Enhancement of critical loading for electric power distribution systems

Abstract

In this thesis it has been tried to explore the possibilities of enhancement of critical loading of radial electrical power distribution networks. It has been found that the critical loading can be enhanced by optimal conductor size selection, optimal capacitor placement, and network reconfiguration. To apply these corrective steps on the distribution networks an efficient load-flow solution method is required so that the actual condition related to node voltage and its angle, branch current, real power loss, reactive power loss, branch losses etc. of the network can be studied and analyzed. A new efficient method is proposed for load-flow solution of radial distribution networks. Simple transcendental equations are used to relate the sending-end voltage, receiving-end voltage and voltage drops in each branch of the distribution system. The effect of charging capacitance of the line has been incorporated in load-flow solution. A computer algorithm is developed in such a way that there is no need to adopt any sequential node numbering scheme for the solution of the networks. The angle of the receiving-end voltage is also computed along with the magnitude of the voltage. It is an iterative method. The flat voltage (1p.u.) start from substation to every end-node is considered. The voltage magnitude and angle are updated after each successive iteration and the voltage drops are then computed by using the new obtained values of voltage magnitude and angle. The comparison of speed and memory requirement by the proposed method with the other methods has been verified to show its efficiency. To investigate the present state of loading and loadability limit, a line loadability index is derived. The change in resistance with the change in operating temperature of the branch conductors is incorporated in the computation of voltage stability index and line loadability index. The voltage stability index computes the voltage stability limit of the radial distribution network in the fault conditions or at the time of load contingency, it identifies the most sensitive node of the distribution network and gives information about time within which the voltage collapse will occur for above said conditions. The line loadability index is based on the maximum permissible steady state operating temperature of the line conductors. The loadability limit of the distribution network is supposed to be within the maximum permissible operating temperature of the conductor. Line loadability index gives the loadability limit of the network in normal steady state conditions and identifies the most sensitive branch of the network. The values of voltage stability limit and line loadability limit are not unique for a distribution networks, the values might be different for the same network in different ambient conditions. So a very practical aspect is incorporated in the proposed work. The importance of the proposed voltage stability index is discussed in context of over current protection. The proposed voltage stability index and line loadability index can be used for voltage stability study and the loadability margin, optimal conductor size selection, capacitor placement studies, reconfiguration of distribution networks, over current protection system design etc. at planning stage as well as in operation of the electrical power distribution system. The current carrying capacity, critical loading and voltage stability of distribution networks depend upon the conductor sizes applied to each branch of distribution network. Hence the optimal conductor size selection is utmost necessary. A method for optimal conductor size selection is proposed in this work. The conductor sizes are first selected according to the current carrying capacity of the available conductor sizes, then upgraded through economical optimization. After selecting the economical conductor sizes, the constraints regarding the maximum limit of branch operating temperature and the minimum voltage level at the end nodes are applied and the up gradation of the candidate branches is carried out. While selection of the optimal conductor size for each branch of the distribution network, the effect of change of resistance with the change of temperature is incorporated. This incorporation leads to exact computation of real power losses of the network, actual thermal behavior of branch conductor in practical situations and optimal conductor size selection according to weather conditions of the region. The results are compared with the existing methods to establish the goodness of the proposed approach. The critical loading and voltage stability are related to the system’s voltage profile. The voltage profile of distribution network can be improved by reducing the reactive component of power by optimal capacitor placement. Capacitor placement problem is dealt with different way in this work. Capacitors are placed at the nodes where the reactive power demand is occurred. The capacitor sizes are selected on the basis of economical analysis. The shunt capacitor is more economical when it is placed as near as possible to the load causing low power factor. In the proposed, method it has been computed at first the summation of real power losses from substation to each node. The node having highest value of losses is taken as the candidate node and the smallest capacitor size is placed at that node and the objective function is again computed. The level of reactive power compensation is increased till it generates the economical benefits. All the nodes are tried in this manner. The results are compared with the existing method and the proposed method is found better in each aspect. The enhancement of critical loading and voltage stability can be achieved from the existing network through network reconfiguration. The reconfiguration of distribution network is dealt with different point of view in this work. The possibilities of network reconfiguration are identified from the chronological load curves of the laterals of distribution network. The load can be shifted from one lateral of distribution network to the other lateral due to the seasonal change in load demand. The enhancement in critical loading and enhancement in voltage stability after network reconfiguration are shown with the help of a test system.