Loadability formulation and calculation for interconnected distribution systems considering N-1 security

•The concept of N-1 loadability for interconnected distribution systems is proposed.•N-1 loadability considers feeder and transformer N-1 contingencies and power flow.•N-1 loadability is modeled as a nonlinear optimal problem.•The model can be solved by GRG method.

Traditional distribution loadability applies to radial systems; this paper proposes a new concept of loadability considering N-1 security principle for interconnected distribution systems, in which feeders are connected with normally-open tie-switches. Firstly, this paper presents a nonlinear model to formulate the N-1 loadability, in which network topology, thermal rating of lines and substation transformers, voltage limits and losses are considered. Secondly, Generalized Reduced Gradient (GRG) method is then used to solve the nonlinear programming problem. Meanwhile, a backward/forward sweep algorithm is also employed to solve the load flow calculation in the model. Finally, both an IEEE RBTS case and a real system are given to illustrate the proposed model and calculation method; and the accuracy is verified by the traditional N-1 contingency simulation.