**1. Introduction**

At present, the exhaustion of resources and environmental problems are continuously intensifying due to the indiscriminate use of fossil fuels. As a result, the Paris Convention, which will replace the Kyoto Protocol, which is scheduled to expire in 2020, was adopted at the Paris Climate Change Conference in France in 2015 with a focus on converting renewable energy sources into alternative energy sources for fossil fuels worldwide. Accordingly, the deployment of Distributed Generation (DG) based on Power Conditioning Systems (PCS) such as photovoltaic generation and wind power generation are rapidly increasing worldwide [1,2].

In this way, PCS-based DG without inertia may have some negative effects on reliability and power quality in power systems if it is introduced on a large scale. In particular, when a temporary fault occurs in distribution systems with many PCS-based DGs, they should be disconnected from the Point of Common Coupling (PCC) before Circuit Breaker (CB) reclosing. Then the voltage becomes unstable. Moreover, all DGs interconnected to distribution systems should wait more than 5 minutes for reconnection after restoration and cannot supply the pre-bid power in power market during that period [3,4]. In such a case, power trading cannot be performed, thereby leading to not only economic damages, but also unstable system operation such as power quality, etc. [5,6].

In this regard, the only existing studies for rapid restoration from faults are as follows: a multi-stage/multiple micro-grid technique for dividing the islanding operation section [7,8], an islanding operation method engaging the droop control of the DG when the fault section is isolated [9–13], a voltage control strategy that involves switching between the grid-connected mode and islanding operation mode for the VSC (Voltage-Sourced Converters) type of DGs [14], a Multi Agent System (MAS) technique for dividing switches, DGs, and load into agents for quick fault restoration [15–20], and a fault restoration method for minimizing the number of switch operations for load transfer [21]. However, these papers have focused on permanent faults and temporary faults have not been considered. That is, there are no measures against the voltage problems caused by the opening of CBs or recloser upon fault occurrence, disconnection of all DGs, and temporary inrush current problems caused by their reclosing.

On the other hand, in the conventional FRT(Fault Ride Through) regulations for grid code, when a fault occurs in transmission systems, the time required for fault detection and isolation is around 150 ms to 160 ms, and after that it takes about 2 s to return to the steady-state voltage. Taking this into consideration, DG should continue to operate within this range [22–26]. Control methods for complying with FRT operating conditions in which DG keeps operating during an accident are currently proposed and applied [27,28]. However, when a temporary fault occurs in distribution systems, CB operates within 5–6 cycles (about 0.1 s) immediately after a temporary fault occurs and then returns to the steady-state operation by performing CB reclosing within 0.5 s. As this abnormal condition is severe compared with the fault situation in transmission systems, FRT for temporary faults in distribution systems could not be considered until now and there were no suggestions to solve it. In addition, there is a problem with determining whether the fault is temporary or permanent before CB reclosing.

Therefore, in this paper, a temporary fault ride-through method in distribution systems is proposed. Section 2 describes the method of restoration from a fault in conventional distribution systems, and Section 3 describes some problems during restoration processes. Section 4 proposes a control method where all DGs connected to distribution systems can continue to operate even if a temporary fault occurs. In Section 5, the proposed method is verified through simulation by PSCAD/EMTDC and analysis.

### **2. Conventional Restoration Method for Temporary Faults**

In distribution systems, the reclosing operation of a CB or RC(Recloser) is applied in order to determine whether there has been a fault, temporary or permanent [29–31]. The general protection method in distribution systems is described below.

The protection devices to break fault current in distribution systems are composed of a CB at the substation outlet and reclosers on distribution lines, which can detect a fault and reclose. In the following the characteristic operation of a CB and recloser is described.
