1. An automatic voltage control method based on a cooperative game theory, comprising steps of: (1) establishing a multi-objective reactive voltage optimizing model (2) 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 (2) where k is a series number of a power system state, N c is a 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, x c is a lower limit of state variable vectors in post-contingency states, x c is an upper limit of the state variable vectors in the post-contingency states, 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; (2) resolving the multi-objective reactive voltage optimizing model (2) into an economy model (3) and a security model (4), wherein the economy model (3) 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 (3) 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 (4) 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 (4); (3) solving the economy model (3) and the security model (4) based on the cooperative game theory to obtain the automatic voltage control instruction; and (4) performing an automatic voltage control for the power system according to the automatic voltage control instruction.