Research Of Load Security Domain Based On The Voltage Level Constraint

The Voltage stability in power system is very complex. Before then there are many literatures writing about the voltage stability indices, but due to the complexity of the power system and the coupling between the state variables, the application effect of the voltage stability indices are undesirability in the practical operation, for example the voltage stability index based on local information of the line LP,Lq,FV SI,LP N,LQ N,LP P and LQ P, the actual operation shows that the result of those indexes are not match with the P ? Vcharacteristic curve. The derivation of those indexes are depended on the various forms of change of the power equation and the quadratic equation has solution. The analysis of the mathematical derivation of those indexes discover that the state variables of the quadratic power equation is intercoupling with the coefficient, it not meet the prerequisite of the Ouida theorem, that is to say the variable is unrelated with the coefficient of the equation. So the indexes which are derived from the derivation of mathematics theory are not rigorous enough, the result of the index is discriminating with the practical state of voltage stability.At present, the study about the load margin of the static voltage stability is based on the idea of decoupling of the state variables. But the relation of the state variables in practical system is intercoupling. So it is difficult to exactly solve the load margin of the stability critical point.Generally, the power system could bear the incidents. But when the system security level is weak and entering alert status, it will possibility cause voltage instability under the incidents. So, it has more practical significance to estimate the load margin of the voltage alert condition. This paper which is based on the Thevenin equivalence theorem proposes the security domain boundary of the load apparent power algorithm and solves load power stability domain of the current operating condition under the same voltage level. This paper adopted 3 machine 9 bus systems'simulation example to illustrate that if the load power is within the range of stable interval, the load node voltage will above the constraint level, the more the distance between the operating point and the boundary of Security domain, the higher the voltage level. If the load power is out of the range of Security domain, the load node voltage will below the constraint level, the more the distance between the operating point and the boundary of Security domain, the lower the voltage level. This method is simple and the physical meaning is clear, and provides a reference for the operating personnel.