Inclusion of slack bus in Newton Raphson load flow study

Abstract

Load flow solution is an inevitable tool for power system planning and operation. Among various methods of load flow solution techniques Newton Raphson (N-R) and its derivatives are the most popular and widely used ones. In Newton Raphson method one of the generator buses is treated as slack bus which is kept out of calculation during iterations. This is because of the fact that the slack bus is to carry entire loss of the system and total loss cannot be calculated before the end of iterations. The exclusion of slack bus from the load flow iteration restricts many analyses to make. Consideration of distributed slack bus, load flow for Microgrids, study on minimization of system losses etc. cannot be performed when the slack bus is excluded from the iterations. This paper reports the development of a method of including slack bus into the Jacobians for N-R load flow study. The method involves formulating the loss equation and then devising the formula for the terms to be included with the elements of the Jacobian matrix. The modified N-R method is applied to all of the buses and the losses are calculated in each iteration and included with the slack bus. The results obtained using the modified method are compared with those using conventional method. The agreement of the results confirms the accuracy of the developments.