1 Introduction

DC power distribution technologies such as smart grid systems and low-voltage direct current (LVDC) are emerging as the scale of renewable energy increases and DC loads increase. This can reduce the power conversion loss generated by existing AC distribution facilities and improve energy efficiency. [1]. However, smart grid systems that generate electricity near acceptable areas can cause many casualties such as fire and electric shock in the event of an accident caused by electrical factors. According to the recent accident caused by electrical safety factors and the "Electrical Accident Statistical Analysis" report, electrical accidents caused by arcs and leakage currents have the highest results among electrical accident cases. [2].

While it is easy to self-destruct because the zero point periodically passes in the case of an arc accident on the AC line, it is difficult to block with an overcurrent breaker because the current is operating in the normal range in the case of an arc accident on the DC line. [3]. In addition, there is no risk of fire and insulation degradation because heat is not generated in the event of a leakage current accident on the AC line, and there is no risk of electric shock as the ground flows to the ground. However, in the event of a leakage current accident on a DC track, insulation deterioration makes it easier to contact the track and the human body, and may cause a risk of electric shock when in contact [4].

Accordingly, it is necessary to secure safety against electrical accidents occurring on DC tracks, and in this paper, a failure analysis method was studied in the event of arcs and leakage currents accidents on the tracks.

2 Failure Analysis Simulator for Smart Grid System

2.1 Configuration of Simulator for Fault Analysis of Smart Grid Systems

The manufactured smart grid system fault analysis simulator consists of a power failure HMI system, a DC power failure analysis system, a fault analysis algorithm system, a line measurement section using a current sensor, and a DC leakage current detection section as shown in Fig. 1.

Fig. 1
figure 1

Simulator for fault analysis of smart grid system

Full size image

2.2 Simulator Design for Fault Analysis of Smart Grid Systems

A DC arc generator and a leakage current generator were used to input failure signals caused by generation of arcs and leakage currents into a simulator for fault analysis of a smart grid system. In addition, in the event of an arcs accident, a fault signal was detected using Rogowski CT(current transformer using Rogowski coil) and hall type CT (commonly used current transformer), and a line fault signal due to a leakage current accident was detected using CT.

As shown in Fig. 2, the signal measured through an arc measurement sensor and a leakage current measurement sensor on the line was analyzed by a simulator for fault analysis of a smart grid system. In addition, in the event of an arcs and leakage currents accident on a line, the contents of the failure signal analyzed through a monitoring system can be confirmed as a status message.

Fig. 2
figure 2

Method of detecting arc and leakage in the connection board and PCS of distributed energy

Full size image

The fault-analyzed arc is readable at the start of the arc and at the extinguishing point, and it is possible to read on which channel (track) the arc occurred. In addition, when leakage current is generated, it can be read which line between positive ( +) line and negative (−) line generated by level shift. The details of each fault analysis can be found through the monitoring system of the simulator for fault analysis in the smart grid system. (Figs. 3, 4).

Fig. 3
figure 3

Rogowski CT and hall type CT for arc detection

Full size image
Fig. 4
figure 4

Signal conversion circuit

Full size image

2.2.1 Track Fault Analysis Design in Case of Arc Accident

An arc generator satisfying the UL1699B requirements was used to analyze the track failure in the event of an arc accident on the DC line.[5].

Although instantaneous voltage or current changes occur in the event of an arc accident on DC lines, it is difficult to detect a fault signal by analyzing existing frequency information and detecting arc accidents (FFT). Accordingly, this paper proposes a method to accurately analyze the time of arc accident (start) and arc extinguishing (end) through Rogowski CT using Rogowski CT and hall type CT as shown in Fig. 5 for fault analysis in case of arc accident.

Fig. 5
figure 5

Design of leakage current generation fault signal reading

Full size image

In Fig. 3a, unlike hall type CT, Rogowski CT is composed of a non-magnetic or coreless air core, and the saturation effect disappears and reacts sensitively to external magnetic flux [6, 7]. Therefore, Rogowski CT was able to determine the exact time of accident and arc extinguishing in the event of an arc accident on the track and was used for fault signal alarms.

On the other hand, hall type CT is less sensitive than Rogowski CT due to distortion and saturation caused by residual magnetic flux because the core is composed of magnetic materials [6,7,8]. Accordingly, in this paper, a CT sensor board was manufactured to identify the fault location using hall type CT and detect arcs by location as shown in Fig. 3b.

2.2.1.1 Signal Conversion Circuit

The fault signal measured by hall type CT at the time of arc generation is unknown due to saturation, so the arc generation signal (Varc) was amplified and analyzed through a signal conversion circuit as shown in Fig. 6. The arc generation signal (Varc) detected through CT was amplified at a magnification designed by Eq. (1) using a non-inverting amplifier (OP-Amp). Amplification magnifications are 10, 30, 50, 70 and 100 times, and signals by line were analyzed for normal state (before arc generation) and post-occurrence signals on any line of the design and smart grid system.

$${V}_{out}=1+\frac{{R}_{2}}{{R}_{1}}\times {V}_{arc}$$
(1)

Varc: Arc generation signal.Vout: Arc generaion amplified signal.

Fig. 6
figure 6

Level-shift circuit diagram

Full size image
2.2.1.2 Line Failure Analysis Design in Case of Leakage Current

In the event of a leakage current accident on a DC line, a leakage current generator was used for analysis of the line failure. In addition, in order to sense the leakage current value generated in the negative and positive lines, the generation of leakage current was plotted at (−)10 mA to ( +) 10 mA.

In order to read the leakage current generation fault signal, CT for leakage current measurement is used in Fig. 5a, and after reading the leakage current generation signal value and position, it can be finally inputted to MCU through level shift part as shown in Fig. 5b. A leakage current measuring CT in Fig. 5a supplies ( +)12 v and (−)12 v to an operating voltage (IN), and a neutral point of the resistance surrounding the magnet core generates a 0 V voltage corresponding to an output signal as shown in Fig. 6a.

Accordingly, when leakage occurs on the ( +)line or (−)line, the 0 V signal outputs a positive( +) or negative(−) signal (VLK: leakage generation voltage) and is used as an input to a signal conversion circuit for level-shift in Fig. 6b.

IN: CT operating voltage.

OUT(VLK): Leakage generating voltage.

VCO: Signal conversion voltage VLV: Level-Shift voltage.

Ve: External power supply.

Figure 6b the result value corresponding to a negative signal through the circuit has a problem that the digital circuit cannot process leakage reading. Therefore, in order to read the exact location of the leakage from the (−)line, Fig. 6c was applied and an external power supply(Ve) was additionally approved so that the (−)signal is level-shift from a positive ( +) signal based on zero point (0).

2.2.1.3 Signal Processing Using Proposed Level-Shift

The input range of MCU used in this paper is 0 to 3.7 V, and the value of the (−)signal detected by CT when leakage current occurs on a (−)line cannot be read from MCU. Therefore, a level shift method is proposed to convert a negative (−) signal value into a positive ( +) signal value so that the leakage current fault signal generated in the negative (−) line can be read. A level shift is converted into 1)an original signal (leakage generation voltage, VLK) measured by CT, 2)a signal(signal conversion voltage, VCO) converted to (−)2.9 V to ( +)2.9 V, and 3)a signal level-shifted between 0 and 3.7 V which is an MCU input range.

A CT for detecting leakage current in Fig. 5a is produced together with a circuit diagram in Fig. 6a, and the occurrence position of which line the leakage current is generated can be read and how much leakage has occurred can be measured. A leakage generation voltage (VLK) can be measured at an OUT terminal of a Fig. 6acircuit when a leakage current is generated in the line. In a normal state of the line, a voltage of 0 V is measured for the leakage generation voltage (VLK), but in the case that leakage current of (−)10 mA ~ ( +)10 mA is generated, the leakage generation voltage (VLK) is output by signals of (−)5 V ~ ( +)5 V.

As shown in Fig. 7a, it can be shown as a first-order graph in the form of y = ax, which has negative and positive magnitudes while passing through the origin.

Fig. 7
figure 7

Signal processing process using level shift

Full size image

In converting a leakage generation voltage (VLK) into a level-shifted signal, since the positive ( +)signal value exceeds the reference value accepted by the MCU, conversion to (−)2.9 V ~ ( +)2.9 V is required. Therefore, in order to make the signal of Fig. 7a into the signal of Fig. 7C, it can be expressed as a signal with a small inclination like Fig. 7b. Figure 7c shows the result of converting the maximum value (−)2.9 V of a negative signal to a reference point 1.9 V while level-shifting to 0 V in Fig. 7b, and the maximum value ( +)2.9 V of a positive signal to 3.7 V, respectively.

Accordingly, based on 1.9 V, the position of (−)10 mA ~ 0 mA current corresponding to 0 ~ 1.9 V or less can be read by a leakage current generated in a DC line and 1 ~ ( +)10 mA current corresponding to 1.9 V ~ 3.7 V.

3 Experimental Result

3.1 Fault Analysis Due to DC Arc Generation

In Figs. 8 and 9, fault signals measured through hall type CT and Rogowiski CT can be identified in the event of an arc accident, C1 and C2 can be identified through hall type CT and amplified fault signals. Due to the characteristics of Rogowiski CT, it is not saturated because it is composed of coreless air cores, and the start and end of arc accidents can be identified. It was confirmed that the value accepted by the current sensor reacts sensitively compared to hall type CT waveforms. On the other hand, due to the characteristics of hall type CT, it is unclear whether saturation occurs from the start point of arc to the extinguishing point. Therefore, when comparing hall type CT and Rogowski CT, the difference between arcs end time points can be identified.

Fig. 8
figure 8

Fault signals measured by hall type CT and Rogowski CT (1000 V, 6 A)

Full size image
Fig. 9
figure 9

Fault signals measured by hall type CT and Rogowski CT (1500 V, 6 A)

Full size image

When measuring arc generation signals using hall type CT, it was difficult to accurately grasp the size of the arc signal due to CT characteristics, so five magnifications were carried out on each of four channels using amplifiers. Each magnification progressed to 10, 30, 50, 70 and 100 times. In this paper, the magnifications were designated as C1:10, C2:30, C3:50, and C4:100, respectively, and analyzed as shown in waveform Fig. 10.

Fig.10
figure 10

Arcs generation fault signal according to magnification

Full size image

In comparison with the amplified waveforms of C2 in Fig. 8 and C2 in Fig. 9, it can be confirmed that minute arc signals are sensed from arc generation to arc extinction as the amplification magnification is increased.

3.2 Identification of Leakage Current Generation Position Using the Proposed Level Shift

A leakage current generating device and a level shift circuit described in Fig. 6 are applied, and the leakage current failure diagnosis of the line is shown in Fig. 11 through a normal state of the line and a level shift proposed when leakage occurs. At the time of leakage current generation, CT original signal(leakage generation voltage, VLK), signal converted to (−)2.9 V ~ ( +)2.9 V(signal conversion voltage, VCO), and signal level shifted between 0 ~ 3.7 V which is an MCU input range can be checked by C1, C2, C3, respectively.

Fig. 11
figure 11

Leakage current fault diagnosis of a line using the proposed level shift

Full size image

Figure 11a shows each converted signal through a signal processing algorithm using level-shift proposed in this paper when the line is in normal condition, the leakage of ( +) positive and (−) negative lines occurs. In C1, when leakage current of (−)10 mA and ( +)10 mA is generated, signals of (−)5 V and ( +)5 V measured through CT can be confirmed, and waveforms converted into (−)2.9V, ( +)2.9 V for level shift can be confirmed in C2. Finally, you can check the level-shifted signal between 0 and ( +)3.7 V at C3.

In Fig. 11b, when the magnitude of the leakage current generated in a line is gradually changed, it is possible to confirm the result that the signals of leakage generation voltage (VLK), signal conversion voltage (VCO) and level shift voltage (VLV) are changed corresponding to each magnitude. As mentioned in Fig. 7 "Signal processing process using level shift", the leakage current signal measured through CT finally produces a level-shifted signal through a signal conversion process.

In addition, Fig. 11c shows leakage generation voltage (VLK) obtained through CT for measuring leakage current shown in Fig. 5a and level shift voltage (VLV) obtained through the level shift circuit diagram in Fig. 6. A negative signal (−)5 V of the leakage generation voltage (VLK) is level-shifted to 0 V, and a zero point (0) is converted to 1.9 V and a positive signal ( +)5 V to 3.7 V, respectively. Therefore, based on 1.9 V, the (−)10 mA to 0 mA current corresponding to 0 ~ 1.9 V or less shows the result of reading the position with a leakage current generated by the DC line and 1.9 V to 3.7 V.

4 Conclusions

In this paper, arcs and leakage currents generation fault signals were analyzed on the line. Using Rogowski CT, the presence or absence of arc generation (start and end of arc generation) of the track was read, and arc detection by track was carried out through Hall Type CT. In addition, when the leakage current occurred, the failure location was identified through level shift on which line, positive ( +) line or negative (−) line. Using a simulator for fault analysis of the smart grid system, the algorithm for analysis was applied to the distributed power system when an arc is generated on the line and leakage current is generated and it was organized as follows.

  1. a.

    Diagnosing track stability through DC line power analysis and real-time measurement of analog/digital signals of fault signals (arcs and leakage currents generation)

  2. b.

    The monitoring system determines the occurrence of arcs in DC lines and leakage current in positive ( +) or negative (−) lines, and analyzes the condition of the lines

  3. c.

    When an arc occurs on a DC line, the proposed signal conversion circuit can be used to detect a minute arc signal to ensure the safety of the line

  4. d.

    Improve facility safety by protecting not only smart grids but also various DC lines from accidents and fires caused by arc generation and leakage current

  5. e.

    Improvement of power analysis and fault diagnosis technology for renewable energy power systems such as smart grids and energy storage devices