Research On Steady State Analysis Method Of AC/DC United Grid

In our country, coal resources and renewable energy sources such as wind power and solar energy distribution are extremely uneven and far away from load center, the energy they generated need massive and long distance transmit. HVDC has been widely used in power large-scale, long-distance transmission and the regional power grid interconnection, because of the carrying capacity without being limited by the transport distance and realizing asynchronous interconnection, etc. But the traditional HVDC can only realize the inverter DC side "point to point" connection, the DC line have no redundancy. With the development of the high voltage direct current circuit breaker, putting on the DC side of inverters,it will forming the "one to more" and "more to one" network structure, that is the DC grid.By establishing a wide-area and wide range of AC/DC united network framework, it can realizing power flow freely between different regional power grid and flexible large-scale renewable energy access. It is also of great significance in intensive developing the renewable traditional energy and improving the stability of power system operation.At the beginning of this paper, we studys the DC grid possible topologies, basing on thhe comparison between the CSC and VSC static characteristic, we conclude that VSC is more suitable for DC grid. By comparing the static influence of DC gird by using master-slave control and power droop control, we conclude that power droop control is better for flexible adjustment and stable operation of the DC grid.Secondly, because of the traditional AC/DC system has so many problems, such as more than the poor communication subroutine inheritance, extension exists, slow operation, we put forward the AC/DC united grid tide decoupling fast algorithm,which is basing on the VSC converter. The algorithm decoups the AC gird and DC gird, iteratively and calculation respectively, and has the advantages of rapid convergence, small computational complexity. For DC power flow equation is nonlinear and needs iterate, large amount of calculation, when using the droop control.We put forward the improvement way of voltage droop control flow algorithm. The flow algorithm realize the DC grid current equations linearization, and avoids the iterative and improving the computational speed greatly.Thirdly. this paper decoupling AC and DC gird and according to the static characteristics of VSC and CSC. Then solving AC grids and DC grid power flow separately, alternately by adding Jacobi matrix. So as to put forward the suitable for CSC-VSC hybrid AC/DC united grid trend of alternating iterative algorithm, the algorithm convergence, fast computing speed quickly.At last, using the DC model of AC grid and DC grid network model, and basing on AC power and DC power grid decoupling, we puting forward a AC/DC power grid static security analysis of fast algorithm. The algorithm can solve the DC grid first, then modified AC power grid node power injection and solve the AC power grid DC trend equation. The algorithm, need not iteration, programming and the computation workload small, calculation speed, and can communicate static security analysis such as the generator exit, circuit disconnection, inverter exit and DC line break.