*2.2. Recloser*

Recloser is generally installed on distribution lines and detects a fault, breaks the fault's current, and automatically performs the reclosing operation within a specified time. Reclosing is also applied because more than 80% of faults are temporary faults that are eliminated by themselves in a temporary period [32].

If the fault persists, the last reclosing function should be performed, followed by lock-out and open state. When a fault occurs on distribution lines, the upstream recloser nearest the fault point should perform the reclosing operation in accordance with specified operation obligation and satisfy the main or back-up protection relationship with CB at upstream substation. Figure 1 shows an operation example of the recloser under the occurrence of a fault between RC1 and RC2—RC1 is opened to break the fault current supplied from substation.

**Figure 1.** Configuration of the Circuit Breaker (CB) and Recloser (RC) in distribution systems.

The operating characteristics of a recloser generally operate with Two-Fast Two-Delay (2F2D). This can be accomplished by two fast trips and two delayed trips in order to eliminate fault or determine permanent lock-out. Figure 2 illustrates the 2F2D reclosing processes.

**Figure 2.** Typical Two-Fast Two-Delay (2F2D) operating scheme example of recloser.

### **3. Critical Problems in Conventional Fault Restoration Method**

### *3.1. Discontinuity of DG Supply by Anti-Islanding Function*

One of critical problems in the conventional fault restoration method is that the CB or recloser is opened when a fault occurs, and all DGs and loads on the stream line of the CB or recloser are islanding. At this time, for the safety of the human body and the protection of over-voltage due to the ratio of the power generation to the load, all DGs should detect the islanding state and be disconnected. In addition, after the fault is eliminated, distribution systems will be restored, and they will be reconnected after 5 minutes for normal operation based on grid code [32]. Then, DG cannot supply the pre-bid power to power market.

### *3.2. Power Quality Problems during Islanding Operation*

When a fault occurs in distribution systems, the CB and recloser on distribution lines operate as shown in Figure 3. In order to prevent the islanding operation of the DG due to the opening of this recloser, there are several national grid codes as seen in Table 1. Most grid codes require that the islanding state should be detected and DG disconnected within around 0.5–2 s, considering the reclosing operation of a CB or recloser. This means that islanding operation may occur for 0.5–2 s after detecting a fault, which may cause power quality problems such as frequency and voltage fluctuation beyond permissible ranges and result in equipment damage and human body danger. As shown in Figure 3, when the output of aggregated DGs is larger than aggregated loads in the islanding region, the over-voltage phenomenon occurs in a just few milliseconds after islanding operation, which may adversely affect loads, equipment, and the human body. In addition, in the case of an under-voltage, even if the fault is eliminated, a situation in which DG is disconnected occurs.

**Figure 3.** Configuration of AC grid with distributed generators. DG: Distributed Generation.



IEEE: Institute of Electrical and Electronics Engineers, IEC: International Electrotechnical Commission, VDE: Verband Der Elektrotechnik, BDEW: Bundesverband der Energie-und Wasserwirtschaft, JEAC: Japan Electric Association Code, KEPCO: Korea Electric Power Corporation.

The ratio of DG output to load in the islanding region has the largest influence on voltage problems. This is an important factor, because the voltage *Vis* during islanding is determined by the ratio as follows [37]:

$$V\_{\rm is} = \frac{\text{Total amount of power generation}}{\text{Total amount of load demand}} \tag{1}$$

The load characteristic can be classified into three types: constant power, constant current, and constant impedance. Generally, power systems have all three types of load. The voltage *Vis* of the islanding condition can be expressed as shown in Equation (2) when DG with constant power is connected to a load with constant impedance [38,39].

$$V\_{is} = V\_L \frac{\sqrt{\overline{P\_{DG}^2 + Q\_{DG}^2}}}{\sqrt{\overline{P\_L^2 + Q\_L^2}}} \tag{2}$$

*Vis* Voltage amplitude during islanding.

*VL* Voltage amplitude under steady-state.

*PDG* Output active power of DG.

*QDG* Output reactive power of DG.


Equation (2) means that the voltage during islanding is proportional to the square root of the apparent power ratio of DG to load. The research to solve this problem is currently underway on the output control of DG when islanding operation occurs. Several papers have applied the output control through the droop control of DG [11–15]. However, these controls have a slow response to prevent over-voltage, which rises rapidly due to the output control by the droop characteristic curve. Therefore, this paper considers that the voltage magnitude is controlled to be 0.5 p.u., which is the permissible voltage operation range of VRT (Voltage Ride Through), for the voltage problem during islanding operation generated by Equation (2).
