A New Three Phase Power Flow Solution Based On Node Injection Sequence Currents

It is common practice to assume that the power system is a balanced one when calculating its three phase power flow. However, the three phase parameters are certainly not always the same in the reality of power systems. In other words, three phases always more or less display asymmetrical characteristic. Especially in recent years, many factors lay the system in the trend of a big way and more common to have dissymmetry. Therefore, a three phase power flow program that deals with the unbalanced power system is useful and urgent to develop.Firstly, a comprehensive survey of the reports and achievements in the three phase power flow calculation methods are made in this paper. Based on analytical investigation, it is suggested to calculate three phase power flow using a symmetrical component coordinate system for convenience. The symmetrical component decoupling compensation theory is used to convert the phase-coordinate-system components to the sequence-coordinate-system components, and thus establish the models of generator, transformer and other elements in the power systems.The weakly mutual coupling between asymmetric transmission lines is a key factor influencing significantly the accuracy and convergence speed of the three phase power flow calculation. The influence is difficult to tackle. Considering this, the three phase sequence decoupling-compensation models of asymmetric transmission lines are introduced and extended from the form of power compensation to the form of current injection, which lays foundation for further using the nodal injection sequence current model.A three phase power flow calculation method is proposed based on the nodal injection sequence current model which reduces the impact of the asymmetrical power on the accuracy and converge speed of three phase power flow. Analysis of the changing characteristics of the PQ bus elements in the Jacobian matrix shows that the Jacobian matrix is approximately constant. This makes it unnecessary to calculate the Jacobian matrix repeatedly in the power flow calculation and thus speed the power flow solution.A computer program based on the newly proposed three phase power flow calculation method has been designed and developed to test its efficiency. The results of a series of test systems show that the proposed new method has low sensitivity to both power unbalance and r/x ratio compared with the existing methods.