A new sequential power flow algorithm for AC/DC systems including independent multiterminal DC subsystems

This paper presents a new algorithm based on the sequential method for power flow calculation in multiterminal AC/DC systems. The proposed approach differs from other similar studies in the literature by handling two main properties. Firstly, a real equivalent circuit model is considered for the under-load tap changer transformers (ULTCs) of DC converters for the first time in the literature. Hence, new DC equations are obtained. Thus, exact and accurate results can be obtained for practical applications using the proposed algorithm. Tap values adjustment effects of the ULTCs connected to DC converters are included into the Jacobian matrix instead of the bus admittance matrix in the sequential AC power flow algorithm, as well as other ULTCs in the AC system. For this aim, new equations for calculating power and Jacobian matrix elements are obtained. Secondly, more than one independent multiterminal DC subsystem in terms of DC connection is included in the study. DC power flow calculation is performed for each DC subsystem independently in the algorithm. Thus, modularity and flexibility are provided for including or removing any DC subsystems without needing to change the proposed algorithm. The proposed approach has been tested on a modified IEEE 20-bus AC/DC test system. The proposed AC/DC power flow algorithm was written and performed in MATLAB. The results show that the proposed approach is accurate and reliable in convergence.