Receiving-End Power Grid Voltage Stability Studies Of The First ±660kV Transmission Project

Power networks dispersed geographically are interconnected together and large mount of power is transmitted from west to east in China because of the inverse distribution of energy resources and load demands. Optimal allocation of resources and optimal supply of energy are achieved. Now, nationwide AC/DC hybrid power grid is built and complex dynamic phenomena appear.HVDC (high voltage direct current) is widely used for its significant advantages in the long-distance high-capacity transmission and power grid interconnection. Converters inherently absorb reactive power that is 50% of active power transferred under steady-state conditions. During and after faults, reactive power consumption of converters may be more for the regulation of controller. It does harm to voltage stability of ac system, especially for the weak one. For the long-distance DC project, the voltage stability of receiving-end power grid needs more attention.Yindong DC transmission project is the first±660kV project in the world. It's the comprehensive selection at the requirement of power transmission, rated current, economic and environment. Based on the discussion above and typical operation scheme of Shandong power grid in 2011, research on the voltage stability in the receiving-end power grid of DC system is carried out in detail and the main contents are as follows.The effects to voltage stability in receiving-end power grid caused by basic transmission modes, pole control modes, mode shift control and voltage dependent current order limiter (VDCOL) are analyzed in detail in the paper. Considering that VDCOL with fixed parameters is difficult to adapt the complexity of operating conditions and AC system faults in the receiving-end power grid, a dynamic VDCOL control strategy is proposed based on the voltage of AC bus of inverter substation. The parameter VH of VDCOL varies dynamically according to the voltage change of AC bus of inverter substation and it can control direct current rationally under different conditions. Simulations show that the new VDCOL is beneficial to voltage stability of Shandong power grid in China.The support of AC system to DC system during system recovery is analyzed based on short circuit capacity and short circuit ratio (SCR). Besides, the effects of different AC fault to short circuit capacity and SCR are compared. The applicability of different loads in stability analysis is discussed. Meanwhile, the influences of induction motor on voltage stability during and after faults are analyzed in detail. The voltage stability of Shandong power grid is discussed using different load composition and the critical one that causes voltage instability is sought out under typical fault.The transient voltage stability of Shandong power grid is analyzed under monopolar/bipolar block of Yindong DC project. In order to avoid the instability caused by bipolar block of DC system, emergency load shedding control strategy is proposed based on bisection method. It can find out critical quantity of load shedding accurately and quickly. Considering serious fault, after bipolar block of DC system and load shedding, the transient voltage stability of Shandong power grid is analyzed under trip of import tie-lines and disconnection of plants.The conclusions and suggestions are as follows.With respect to VDCOL with fixed parameters, the dynamic VDCOL control strategy based on voltage of AC bus of inverter substation is more beneficial to voltage stability and system recovery in Shandong power grid during transient period. When loads are composed of induction motor and impedance, voltage stability in Shandong power grid becomes worse and even voltage collapse occurs with the increase of proportion of induction motor. It is supposed that load modeling should be carried out more deeply to determine the composition of load better. Load shedding in Shandong should be taken to make system stable under bipolar block of Yindong DC system. The stability limit of Changzhi-Nanyang-Jingmen UHVAC demonstration project is the critical factor that leads to instability with loss of large active source in Shandong power grid.