Unbalanced Power Flow Calculation Methods For Distributed Generation Systems

The distributed generation system formed by the penetration of distributed generators (DG) is the trend of future distribution systems. Distributed generators will bring on the problems such as reverse power flow, loop network and new node type that are difficult for the traditional power flow method to deal with, so the power flow methods used in traditional distribution system will not meet the requirement of distributed generation systems. In order to solve this problem, this paper carries out research wholly on the power flow algorithm for the distributed generation system.The elements in distributed generation system are first modeled. Especially the detail deduction of the admittance matrix of different winding connections of transformers is given and DGs are classified into PQ type, whose P and Q are constant; PV type, whose P and V are constant; PI type, whose P and I are constant; and P-Q(V) type, whose P is constant,V is variable and Q is limited by P and V. According the characters of each node type, this paper proposes methods of dealing with these DGs. The essence of these methods is to transform all kinds of DGs nodes into nodes which can be directly treated by traditional power flow methods at each iterative step. A Newton method considering all DGs in power flow calculation is proposed.Then a compensation-based modified Newton method suit for distributed generation system power flow calculation is introduced and implemented. Modified Newton method is based on the principle of Newton method, and its calculation process is similar to the back/forward method. Because forming of the Jacobian matrix is not involving in the calculation progress, the iteration speed is improved.At last, several sets of examples are used to test both of the proposed algorithms. The iteration characteristic and calculation speed of both algorithms are compared. It can be seen from the result that: both algorithms are applicable in large scale and multi-DGs-involved distributed generation system and the modified Newton method is more effective.