As the interconnection of power networks becoming increasingly meshed， the assumption of weak coupled reactive power and voltage control between areas is no longer correct. If regional control systems operate uncoordinatedly， some adverse consequences may occur， such as unreasonable reactive power distribution， control oscillation or even instability. Based on sensitivity analysis， a new coordination variable named VACE is proposed. The new coordination variable can distinguish whether a disturbance occurs inside or outside the region and indicate the magnitude and direction of reactive power exchange. Using VACE as an examination index， coordinated AVC can be implemented easily. The responsibility of control is clear and the negative effect between different AVC systems can be reduced. The simulation results of the New England System show that with VACE index under consideration， control oscillation is effectively attenuated and higher reactive power reserve is achieved.