1. An automatic voltage control method based on a cooperative game theory, comprising acts of: S 1 , establishing a multi-objective reactive voltage optimizing model (1) of a power system: 
 min EI ( u 0 ,x 0 ) 
 min SI ( x 1 ,. .. ,x N C ) 
 s.t. g 0 ( u 0 ,x 0 )=0 
 g k ( u 0 ,x k )=0 
 u ≦u 0 ≦ū 
 x ≦x 0 ≦ x 
 SI ( x 1 ,. .. ,x N C )≦ SI Base 
 k= 1,. .. , N C (1) where k is a series number of a power system state, N C is the number of contingencies, u 0 is a control variable vector, x 0 is a state variable vector in a pre-contingency state, x k is a state variable vector in a k th post-contingency state, a constraint equation g 0 (x 0 ,u 0 )=0 is a power flow equation of the power system in the pre-contingency state, g k (x k ,u 0 )=0 is a power flow equation of the power system in the k th post-contingency state, u is a lower limit of the control variable vector, ū is an upper limit of the control variable vector, x is a lower limit of the state variable vector in the pre-contingency state, x is an upper limit of the state variable vector in the pre-contingency state, EI is an economy index of the power system, SI is a security index of the power system, and SI Base is a security index of the power system before an automatic voltage control instruction is performed, min means minimize and s.t. means subject to; S 2 , resolving the multi-objective reactive voltage optimizing model (1) into an economy model (2) and a security model (3), wherein the economy model (2) is: 
 min EI ( u 0 ,x 0 ) 
 s.t. g 0 ( u 0 ,x 0 )=0 
 u + ε u ≦u 0 ≦ū+ ε u 
 x + ε x ≦x 0 ≦ x + ε x (2) where ε u is a relaxation vector of the lower limit u of the control variable vector, ε u is a relaxation vector of the upper limit ū of the control variable vector, ε x is a relaxation vector of the lower limit x of the state variable vector in the pre-contingency state, and ε x is a relaxation vector of the upper limit x of the state variable vector in the pre-contingency state; and the security model (3) is: 
 min SI ( x 1 ,. .. ,x N C ) 
 s.t. g k ( u 0 ,x k )=0 
 SI ( x 1 ,. .. ,x N C )≦ SI Base 
 k= 1,. .. , N C (3); S 3 , solving the economy model (2) and the security model (3) based on the cooperative game theory to obtain the automatic voltage control instruction; and S 4 , performing an automatic voltage control for the power system according to the automatic voltage control instruction.