Unified iterative method to calculate power flow of the interconnected system with Fractional Frequency Transmission System

Fractional Frequency Transmission System (FFTS) is a new approach to integrate large-scale, remote wind power into grid. It adopts lower frequency, such as 50/3Hz, to transmit remote wind power to the load center directly. Therefore, it could reduce the voltage influence caused by the intermittence of wind power. In order to analysis the Stable characteristics of FFTS, power flow analysis of this system should be conducted. However, traditional Newton-Raphson method cannot be utilized directly due to two reasons. First, the node admittance matrix should be modified according to the exact operation frequency in lower frequency side (LFS). Second, the cycloconverter is installed as the converter to transmit the power from LFS to grids. So the model of cycloconverter must be incorporated. Therefore, a new unified iterative method is proposed in this paper to calculate power flow of FFTS. With this method, the admittances of the transformers and transmission lines in LFS are modified according to their operation frequency before the admittance matrix is built. Moreover, base on the model of cycloconverter, the Jacobi matrix also needs to be extended. Case studies are implemented to prove the valid of this method.