Decision making of bus splitting for reduction fault current level using a novel network reconfiguration algorithm with rerun optimizer

This paper presents a method of decision making on where bus splitting is needed to reduce fault current level of power systems using a network reconfiguration algorithm (NRA). After selecting the looped topology for Korean power system to increase supply reliability, maintaining transfer capability of the system is considered as one of the main factors for secure system operation, with the increased load demand each year. As the systems are heavily looped by transmission expansion for enhancing transfer capability depending on the increase in load demand, however, fault current levels might be severely increased and hence at some critical locations they can exceed short-circuit capacity of the circuit breakers. This paper focuses on countermeasures applicable to reduce fault current level. If the short-circuit capacity of the circuit breaker at the corresponding location is less than the fault current level, the fault might not be isolated in an adequate period of time and hence then it can be spread through the system. Moreover, it can result in second faults and the damage of the circuit breakers. In the literature, several countermeasures can be adopted to properly deal with the increase in fault current level. Of them, the best solution is to simply equip new circuit breakers with higher short circuit capacity at those critical locations, but it needs an amount of investment cost. When some actions need to be devised in operational planning stages, available schemes need to be deliberated such as bus splitting and line opening. Making decisions on whether to split the candidate buses to reduce fault current levels is the main target of this paper. This paper adopts a so-called fault level constrained optimal power flow (FCC-OPF), previously proposed by the authors. An FCC is a nonlinear function of the pre-fault voltage magnitude and the Thévenin equivalent impedance at the faulted location, but the equivalent impedance is an implicit function of the binary variables representing bus splitting actions. However, the formulation of FCC-OPF employs a continuous reactance (X) for the bus splitting actions, and takes the inversed U-shaped objective function to enforce the continuous X variable to the two discrete points, meaning whether to split or not. NRA of this paper depends on the results of FCC-OPF. This paper mainly discusses the application of re-run optimizer to minimize the bus splitting locations, determined by NRA. From a number of NRA simulations, it was noticed that there were several cases with the intermediate values of Xs even though the inversed U-shaped objective functions are applied in the optimization formulation, and that the adequate selection of bus splitting locations critically affects the total number of the actions, satisfying the whole fault level constraints, applied. This paper presents a novel re-run optimizer using the concept of topological information on critical fault level constraints and bus splitting actions, to effectively minimize the number of actions taken. This paper includes the results applying the NRA with re-run optimizer the Korean power system with 1,338 buses, and also discusses static security issues that need to be checked after applying the determined bus splitting.