The current day-ahead reactive power optimization does not take voltage stability into account. And it cannot deal with discrete variables and time-coupled constraints accurately. In order to remedy this situation

a day-ahead reactive power optimization model is presented with dynamic reactive power reserves as one of the multiple objective functions and constraints

and a branch and bound-interior point method is used to solve the problem. The discrete variables achieve discrete values through a branch and bound tree and the time-coupled constraints are met by rational branching and pruning principles. As a result

the day-ahead reactive power optimization problem is decomposed into a series of static reactive power optimization problems with only continuous variables. Numerical tests with IEEE 30-bus system and IEEE 118-bus system have verified the effectiveness of the proposed model and method. This work is supported by National Natural Science Foundation of China (No. 51077042)

National High Technology Research and Development Program of China (863 Program) (No. 2011AA05112) and Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) of China (No. 20120094110008).