**1. Introduction**

The fault of the traction power-supply system (TPSS) directly results in the operation disturbance of railway trains because electric railways use electrical energy as an energy source. Therefore, system faults must be quickly identified and responded to, which is achieved by protective relays. In other words, protective relays are designed to monitor breakdowns and prevent breakdown impacts. Thus, they are very important for the stable operation of the system [1–4].

The autotransformer (AT) feeding method, which is applied to electric railways in many countries, supplies power to the line through an AT with twice the rated voltage of the load (i.e., train). In other words, it comprises a trolley wire (TF) that connects the neutral point of an AT with the rail to supply power to the load, and a feeder (AF) to absorb the return current of the load in the rail [5–8]. Unlike the conventional electric power system, substations that serve as a power source have a disconnected wire form instead of a network form; thus, short circuit fault at a distance from the power source can be confused with the load current. Additionally, because the load of the TPSS has a large capacity of approximately 15 [MVA] in terms of high-speed trains and rapid operation, the load current has grea<sup>t</sup> variability. Accordingly, the AC TPSS is characterized by asymmetrical single phases due to moving loads (such as trains); the TPSS characteristics di ffer significantly from conventional balanced three-phase power systems.

In addition, it is essential to establish performance verification procedures at the development stage because it is essential to ensure the reliability of the protective relay operation on railways where safety is important. However, many studies have been conducted on the fault analysis of railway systems, but little research has been conducted on the performance evaluation of relays in railways [9–12].

Generally, in power systems, the performance test of protective relay can be divided into steady and dynamic tests. A steady test is a method of injecting a specific voltage and current into a protective relay to determine whether it operates. A dynamic test tests the response characteristics of the protective relay by injecting a waveform that reflects the operating characteristics or environment of the system wherein the protective relay is equipped. A steady characteristic test of protective relay for railroads can be performed using general commercial testing devices including an amplifier that generates waveforms. However, a dynamic characteristic test can be performed through hardware in the loop simulation (HILS) in connection with the relay by simulating system operation environments through an expensive simulator such as digital real-time simulator (DRTS) [13–16]. However, DRTSs are not easy to construct, as they are expensive and require more e ffort than electromagnetic transients (EMT) simulators for transient state analysis. Additionally, the best way to test the protective relay with the correct system fault waveform is to obtain voltage and current waveforms from the protective relay installed in the actual TPSS as a common format for transient data exchange for power systems (COMTRADE) file and then inject it into the protective relay that needs to be tested.

Therefore, this paper proposes a new digital simulator for the railways called protective relay digital simulator for railway (PREDIS-R), which enables the accurate and e fficient testing of protective relay performance at a low cost at the laboratory level. The proposed digital simulator has the following configurations and features. First, it provides an environment where users can create waveforms or import them from outside. In other words, users can create the desired test waveform on a digital simulator or import a fault waveform implemented by measurement or an external EMT simulator. The waveform generated is amplified to the potential transformer (PT) and current transformer (CT) levels of the train substation via a real-time amplifier. The amplified voltage and current waveform are injected into the protective relay, and it can perform an operational performance test. Additionally, the operating test of the protective relay can monitor the pickup time and operating protective elements of the protective relay to review the correct operation of the protective relay on the users' intended fault waveform.

#### **2. Development of Protective Relay Digital Simulator for Railways**

## *2.1. Performance Scheme*

The main role of the protective relay is to analyze the waveform of the input voltage and current to determine any fault and provide output for the operation of the circuit breaker (CB). To test the performance of these protective relays, the simulator must enter a waveform that accurately reflects the characteristics of the faulty and steady states of the system and read the status information of the protective relay operating according to the input waveform.

Figure 1 shows the performance scheme of the PREDIS-R proposed herein. It was developed to be able to use fault data obtained from the field or enter waveform data obtained through presimulation based on an EMT simulation tool (i.e., power system computer aided design/electromagnetic transients with DC (PSCAD/EMTDC) [17], electro magnetic transients program (EMTP), etc.) into the protective relay simulators in the international standard COMTRADE format to simulate the fault of the system. Similarly, when external faulty waveform data input is loaded into the digital protective relay simulator used to test the performance of the protective relay, PREDIS-R generates the same waveform as the protective relay with the input waveform and outputs it.

Moreover, a function that can create a separate waveform is applied to the protective relay simulator to reflect the characteristics of the TPSS, which does not have the EMT simulation result waveform or cannot be implemented by EMT simulation. Thus, when creating a waveform internally, PREDIS-R generates the exact waveform that the user intends and outputs it to a protective relay.

The proposed PREDIS-R comprises hardware for loading external test waveforms or generating test waveforms directly showing motion information to users and software setting up test waveforms, controlling simulators, and storing and outputting operating information.

**Figure 1.** Protective relay digital simulator for railway (PREDIS-R) performance scheme.

#### *2.2. Basic Formula for Relay Setting*

The Zone 1 distance relay setting can be configured as follows in Equation (1), considering the protection range of 90%:

$$Z\_1 = 0.9 \cdot Z\_{55} \cdot L\_1 \cdot \text{CF} \tag{1}$$

where *Z1* is the relay input impedance for Zone 1, *Z55* is the unit line impedance per length based on 55 kV [Ω/km], *L1* is the line length between the substation (SS) and sectioning post (SP) [km], and CF is the conversion factor considering the CT and PT ratios.

The Zone 2 distance relay setting can be configured as follows in Equation (2), considering the protection range of 120%:

$$\mathbf{Z}\_2 = \mathbf{1}.\mathbf{2} \cdot \mathbf{Z}\_{\text{5\%}} \mathbf{L}\_2 \mathbf{\cdot} \mathbf{C} \mathbf{F} \tag{2}$$

where *Z2* is the relay input impedance for Zone 2, and *L2* is the length between two substations [km].

The input impedance of the distance relay between Zone 1 and Zone 2 is 33.83 Ω and 66 Ω, respectively.
