The increasing penetration rate of intermittent power, such as wind or photovoltaic power and electric vehicle load, leads to voltage being more vulnerable to intermittent power fluctuation and impact charging load. The background of quantitative voltage security assessment is analyzed. A new voltage deviation security margin index is presented, which takes into account both the duration of voltage deviation and the degree of voltage deviation. The new security index has clear physical concept, better linearity and smoothness. Characteristics of the index have been analyzed based on the voltage data obtained from simulation of IEEE 39-bus system and actual power grid. Based on WAMS, the quantitative voltage security assessment method has been deployed online.1