With the further increase in power demand for highly concentrated load centers and the gradual formation of a power system with a high proportion of new energy sources, China will face severe risk of voltage instability. In this paper, firstly, the load bus voltage equation of simple Thevenin equivalent system is derived, and then the P-Q-V, P-V and V-Q curves are obtained respectively. Secondly, the mutual restriction relationship between active and reactive power of load is discussed when there is a real value solution to the voltage. Finally, the relationship betwween the voltage of critical point and the active and reactive power of the load is derived, and the one-to-one correspondence of the critical points between the P-V curves and the V-Q curves is proved in the corresponding interval of the active and reactive power of the load.

Critical voltage, limit power, P-V curve, static voltage stability, V-Q curve.