## 1 Introduction  
Diverse concerns in terms of security arise in operating the modern power system. Operating points of the power system are frequently brought to the system's operating limits [1]. This trend mainly comes from various reasons. One of them is the reduced system's security margin by the competitive environment of modern electricity markets, and another reason is the usual compromise for new generation over system security investments resulting to stressed conditions of the system operation. Furthermore, system protective devices which are initially designed to meet the security needs of the old network are now subjected to increasingly unpredictable scenarios and therefore making it prone to malfunction. The trend seems to be more severe in those power systems with increased load and generation. It is of paramount importance to achieve economic operation of the power system without violating the secure operating limits [2] such as thermal limits, voltage limits, and fault level limits. To efficiently obtain the solution for the problem, several formulation and solution techniques for optimal power flow (OPF) were proposed in the literatures [3; 4; 5; 6]. The tool of OPF can be enhanced further to consider new operating limits for the secure operation of emerging power systems, and it is important to come up with the formulations for the specified OPFs and their solution techniques. In the planning and operational planning stages for power systems, excessive fault current levels at some locations should be avoided using adequate countermeasures [7; 8]. In the literature, there are several countermeasures for fault current security. When the expected fault currents are over the ratings of the corresponding circuit breakers (CB), the re-installation of circuit breakers with higher ratings is desirable. However, in the operational planning stage, that might not be viable, so other countermeasures are needed to be considered. Bus splitting and installing current limiting reactors (CLR) at critical lines are regarded as other options to maintain system operating limits [7]. This paper mainly deals with the problem of determining countermeasures against excessive fault currents in power systems using an optimization method. Practically, line faults are mostly concerned in operation, and to evaluate the total fault current on the location, a dummy bus needs to be added in advance and bus fault currents are calculated. When it comes to line faults, one of important issues is how to locate faults. In [9], a fault location method based on traveling wave principle was developed and applied to BC Hydro 500 kV networks. The fault location algorithm in [10] employed Thevenin equivalents using measured voltage and current information at both sides of the faulty line. In [11], a direct method based on three phase circuit analysis for unbalanced distribution systems was proposed. However, when making decision on countermeasures against excessive fault currents, bus fault currents in the study area are determined for fault levels in the operational planning stage, because it is to come up with countermeasures to cover severest conditions. In [12; 13], the formulation by fault level constrained optimal power flow (FLC-OPF) was proposed. It was proposed to take into account the limits of protective devices. The FLC-OPF method directly incorporates the constraints of expected fault currents in the OPF. The primary purpose of this specified OPF was to determine the optimal locations and capacities of new generators that would not violate the existing network and fault level constraints. The advantage of the direct inclusion of fault level constraints in the optimal power flow is that shadow costs are produced, connecting the overall optimum of the objective function with operational and fault current constraints. Furthermore, this feature could also be used in the design of a reinforcement planning mechanism, which would assess the return of investments on both network and protection equipment.