Improvement Of Power Flow Calculation Method Considering Coupling Of The AC/ DC System

With the increasing power demand of the society and the improvement of the economic and technological level of the power system, the direct current transmission has been developed rapidly. As a powerful complement to AC transmission, the DC transmission line and AC transmission line together constitute the modern power transmission system. Our country has put the "2-AC and 4-DC" UHV project into operation, and the "2-AC and 1-DC" UHV project has also started construction. In the future, China will built super large AC and DC interconnection network which is nationwide, having UHV power grid as the backbone network, including high-voltage transmission grid and all levels of the distribution network, layered and divisional, with clear structure.As the basis for the comprehensive analysis of AC / DC power system, the research on the power flow calculation of AC / DC power system has very important practical significance. In this paper, we first describe the steady-state input and output mode of the very important converter device in the HVDC transmission system, and show its basic equation. Then we introduces and analyzes the advantages and disadvantages of two kinds of commonly used AC / DC power flow calculation methods, which are unified iterative method and alternating iteration method. The equivalent method is chosen as basic method since it has advantages of the both two methods in convergence and computational speed.Consideration of the coupling of AC/DC system is the most outstanding contribution of the equivalent method. In this method, AC Jacobian matrix needs correction. However, the use of traditional Newton type equivalent method sometimes does not meet the computational speed. Current injecting power flow calculation method has comparable convergence with the Newton algorithm and higher calculation speed.This is because the former Jacobian matrix in each iteration requires only recalculating the block diagonal elements. Current injection algorithm is also based on the modified equation set, so it can be introduced in the coupling coefficients. In this paper, new coupling coefficients are obtained by using linear transformation. The current injection model is applied to the power flow calculation of AC/DC hybrid system. The convergence of the algorithm is guaranteed and the calculation speed is improved.In general methods, the converter transformer initial positions are given, and adjustment is need only when the voltage or control angle exceed limit. In practical power system, converter transformer tap adjustment and angle parameters control can combine to reduce the converter's reactive power, improving power factor. Multiplied by the continuous angle cosine value, the discrete variable turns to be continuous. Within the feasible range, the discrete variable could be determined before continuous one by only once calculation.Using MATLAB program, the numerical results verify the effectiveness of the improved method in this paper.