## 1 Introduction
In [14], an allocation method of fault current limiters using OPF with fault current level constraints was proposed. However, because of the crisp fault current constraints, some optimal solutions might not be achieved in some scenarios. In [15; 16], network reconfigurations algorithm were proposed to reduce the expected fault currents in the power system. The network reconfiguration algorithm allowed countermeasures such as bus splitting and transmission line opening. The manipulation of the network topology might cause the reduction of fault currents allowing the satisfactions of CB limits and causing the method to eventually converge at an optimal solution. However, for the cases where the system's initial condition have critically excessive fault current levels and the resource set of countermeasures is not enough to cover the initial fault current violation, the network reconfiguration algorithm might not be able to provide a set of feasible solutions. Fuzzy-based approaches in solving allocation and control problems in power systems are proven to be robust [17; 18]. This paper adopts the fuzzy set theory to deal with the fault level constraints (FLCs) for the systems with critically excessive fault current levels. By applying the fuzzy enforcement [19; 20; 21] to the FLC-OPF, slight violations in the fault level constraints can be allowed to achieve adequate solutions even though there are not enough resources for reducing the fault current levels. The purpose of this paper is to formulate and test a method that would aid in the decision-making of which countermeasure such as bus splitting and/or CLR will be applied to reduce the fault current level of the network. Consequently, given the set of pre-defined candidate locations for the two countermeasures, the optimal location and allocation of reactance values are determined. Furthermore, to model the two kinds of countermeasures at the same time, variable reactances are inserted on the candidate locations. In this paper, a decomposition method is employed for the purpose of simultaneously considering both countermeasures, while the variable reactances are the decision making variables. In the subproblem of this decomposition method, the optimization tool of fuzzy fault level constrained optimal power flow (FFLC-OPF) is performed to acquire the optimal operating point with the given network topology considering both traditional constraints and fault level constraints. On the other hand, the main problem is done to identify the optimal settings of the variable reactances for the countermeasures considering sensitivities of fault currents and generation costs with respect to the incremental of each reactance. These two problems are iteratively solved until the convergence tolerance in terms of fault current violations from the given FLCs is reached. This paper provides illustrative examples in applying the decomposition method to the modified 28-bus system. The main features of the proposed method are (i) the use of a decomposed approach for determining adequate countermeasures and an OPF operating point that is within fault current levels, (ii) that of two types of fault current countermeasures such as bus splitting and current limiting reactors, (iii) that of FFLC-OPF that incorporates both traditional constraints and fault level constraints in solving for the power flow as well as the application of fuzzy concept to the FLCs that enhances the search for feasible solutions, and (iv) that of sensitivity analysis in determining which countermeasure contributes to the decrease of fault current levels.