Study On The Steady State Characteristic Of UHVDC Hierarchical Connection Mode To AC System

With the constrction of the high-voltage direct current(HVDC) system, multi-infeed HVDC embedded in one AC system has emerged in east China power grid and south China power grid. However, multi-infeed HVDC system need adequate voltage support capability provided by received power grid. Multi-infeed HVDC system will make received power grid more denser and value of short-circuit current of the received power grid seriously. Also, it can not lead power flow to be in a reasonable distribution according to demand of the load center.Ultra high-voltage direct current(UHVDC) Hierarchical connection mode is that inverters in a DC line have access to different voltage level grid, such as 1000 k V and 500 kV. Compared with the multi-infeed mode, UHVDC Hierarchical connection mode can improve the voltage support capability of the received power grid and make a reasonable allocation of the total power between 1000 kV layer and 500 kV layer, improving distribution of the power flow. It is a kind of innovative connection mode and the research on it is less. In this paper, we mainly study on the steady state characteristic of UHVDC Hierarchical connection mode. The works and innovative achievements in the dissertation are as follows.(1)Compared UHVDC Hierarchical connection mode with the mode of traditional HVDC system, the difference is that inverters of UHVDC Hierarchical connection mode have access to different voltage level grid. So we study the equivalent circuit, control method and stable mathematical model of HVDC system.(2)UHVDC Hierarchical connection mode to AC system is successfully bulit in PSCAD. Then, the mathematical method of the MIIF(multi-infeed interaction factor) under UHVDC hierarchical connection mode is analyzed, and the correctness of the method is verified by PSCAD simulation. The effect of coupling impedance, equivalent impedance and different DC control mode adopted by two groups inverters on MIIF are analyzed.(3)According to some papers, the method to calculate the multi-infeed short circuit ratio(MISCR) of multi-infeed HVDC system is also applied to Hierarchical-infeed Short Circuit Ratio(HISCR)of UHVDC Hierarchical connection mode. Then, getting HISCR of UHVDC Hierarchical connection mode to AC system according to the MIIF that we get before.(4)In UHVDC Hierarchical connection mode, you can make a reasonable allocation of the total power between 1000 kV layer and 500 k V layer and improve distribution of the power flow by the way to change parameter of received power grid. However, it will change MIIF of UHVDC Hierarchical connection mode to AC system. Finally, analyzing the impact on the commutation failure when we improve distribution of the power flow. And it may increase the risk of commutation failure in one AC system when the converter of other AC system is in the state of commutation failure.