Abstract. City grid connecting urban residents to the electric power source via high-voltage transmission grid is a critical part of the power grid. As the urban electricity demand grows, how many electrical load city grids can supply is becoming a widely concerned problem. Various kinds of loadability problems in power system have been widely studied by several researchers during the past decades, including voltage stability, available transfer capability and power supply capability of distribution system, etc. But the existing studies and methods are not so applicative for the maximum loadability (ML) determination of 220kV city grid. In order to improve the calculation accuracy and to accelerate the calculation speed, a simplifying scheme is proposed to decouple 220kV city grids into different kinds of typical connection forms after comprehensive analysis of its connection structure. Then a non-linear mathematical model for maximum loadability determination of typical connection forms of 220kV city grid considering static security is derived from the optimal power flow (OPF) theories. According to the optimization model, a self-adaptive differential evolution algorithm with built-in Newton-Raphson (N-R) method is presented for searching the optimal solution. Finally, a case study is conducted to test the validity of the optimization model and the preciseness and efficiency of optimization algorithm. IntroductionThe maximum loadability of 220kV city grid refers to the max level of electrical load that 220kV city power grid can supply for long term operation consideration while satisfying all the equipment limitations and operating constraints. Nowadays the 220kV city grid which having a relatively complicated structure is becoming the trunk part of city grids. The precise determination of the maximum loadability is of great importance to the department of power system designing and operating of power supply bureaus for the indication of load margin and the active constraint. Base on the determination of ML, power grid designers are able to make enhancement of reliability of city grids by making up the bottleneck problem. For another aspect, power grid operators can also make amelioration to the utilization of existing resources.