Open AccessArticle

An Implementation of Intelligent Fault Isolation Device for LVDC Distribution System Considering Slope Characteristics of Fault Current

Yun-Ho Kim

Kyung-Hwa Kim

Hyun-Sang You

Se-Jin Kim

Sung-Moon Choi

and

Dae-Seok Rho

Department of Electrical Engineering, Korea University of Technology & Education, Cheonan 31253, Republic of Korea

Author to whom correspondence should be addressed.

Electronics 2025, 14(1), 171; https://doi.org/10.3390/electronics14010171

Submission received: 15 November 2024 / Revised: 26 December 2024 / Accepted: 31 December 2024 / Published: 3 January 2025

(This article belongs to the Special Issue Machine Learning Applications in Predictive Monitoring of Power Grid Stability and Resiliency Enhancement)

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Browse Figures

Figure 1
Fault current characteristics with fault location. "> Figure 2
Fault current characteristics with magnitude of load. "> Figure 3
Configuration of intelligent fault isolation device. "> Figure 4
Operation mechanism of IFID. "> Figure 5
Operation method of IFID with fault current slope characteristics. "> Figure 6
Operation characteristics of IFID by existing and proposed methods. "> Figure 7
Configuration of IFID. "> Figure 8
Characteristics of voltage and current of CLR. "> Figure 9
Configuration of control board in IFID. "> Figure 10
Configuration of test device for IFID. "> Figure 11
Operation characteristics of IFID with existing method. (a) Case I; (b) case II. "> Figure 11 Cont.
Operation characteristics of IFID with existing method. (a) Case I; (b) case II. "> Figure 12
Operation characteristics of IFID with the proposed method. (a) Case I; (b) case II. "> Figure 12 Cont.
Operation characteristics of IFID with the proposed method. (a) Case I; (b) case II. ">

Versions Notes

Abstract

This paper deals with the operation method of an intelligent fault isolation device (IFID), which can detect and estimate faults in rapid and accurate ways considering the slope characteristics of fault currents with distribution line constants. Namely, the proposed operation method in IFID calculates the slope of the fault current with distribution line constants, and reduces its operation time by comparing the calculated slope value to the setting value to detect and evaluate the fault condition. Moreover, this paper implements the DC 400 V, 10 kW scaled IFID consisting of hardware (H/W) and software (S/W) sections based on the proposed operation method. The H/W section is composed of main and current limit switches, a current limit resistor, voltage and current sensors, and switching mode power supply (SMPS). Also, the S/W section consists of a control board and code composer studio (CCS) to calculate the slope characteristics of the fault current and control the semiconductor device. From the test results based on the proposed operation method, it was found that the IFID considering the slope characteristics of fault currents can detect and evaluate the fault condition and limit the fault current faster than the existing method to consider the fault current only.

Keywords:

intelligent fault isolation device; slope characteristics; protection coordination; LVDC distribution system; operation method

1. Introduction

Recently, research on the DC distribution system is being actively carried out for the purpose of reducing energy losses and improving the hosting capacity of renewable energy sources by replacing the existing AC distribution system with a DC distribution system [1,2,3,4,5]. And conventional research has focused on the DC circuit breaker to interrupt the fault current when the fault occurs in the DC distribution system. However, since the power converter devices supplying the DC sources have extremely high sensitivity, the protection coordination method is required to protect and operate the DC distribution system in a stable manner [6,7,8]. For example, when a fault occurs in the DC distribution system, the outage area may be expanded due to the rapid shut down of the main converter [9,10,11]. Specifically, the general radial-type LVDC distribution system has the problem that it is difficult to apply the same protection coordination method using the cooperation time difference and T-C curve characteristics adopted in the existing AC method. Because the magnitude of fault current has a limitation to protect coordination in a DC distribution system, as shown in reference [12,13,14], a protection coordination method is required to rapidly isolate a fault location by considering the slope characteristics of the fault current.

Therefore, this paper proposes an operation method of the IFID, which can detect and estimate the fault in rapid and accurate ways by considering the slope characteristics of the fault current depending on distribution line constants and customer load. Namely, the proposed operation method of the IFID calculates the slope of the fault current with distribution line constants and customer load, and shortens its operation time by comparing the calculated slope value with the setting value to detect and evaluate the fault condition. Moreover, this paper implements the DC 400 V, 10 kW scaled IFID consisting of H/W and S/W sections in order to validate the proposed operation method. The H/W section is composed of main and current limit switches, a current limit resistor, voltage and current sensors, and SMPS, and also a S/W section containing a CCS and control board to perform monitoring and communication. The CCS plays a key role in the control of the main and current limit switches to rapidly transfer the operation modes using the slope characteristics of the fault current based on the proposed operation method of the IFID. From the test results based on the proposed operation method, it was found that the IFID to consider the slope characteristics of the fault current can detect and evaluate the fault condition under any load variation and shorten the operation time in a fast and accurate manner compared to the existing method to consider the fault current only.

2. Slope Characteristics of Fault Current in LVDC Distribution System

When a short-circuit occurs in the LVDC distribution system, the magnitude and slope of the fault current may have different characteristics depending on the line constant of the fault location and capacity of customer load. Figure 1 shows fault current characteristics according to fault location, where graph (a) and (b) illustrate the fault currents that occurred at the first and last sections in the LVDC distribution line, respectively. As shown in graph (a) in Figure 1, the fault at the first section can lead to a large magnitude and variation (slope) of the fault current (S_fault1) at t₀ of fault time due to the low impedance of the distribution line, while the fault at the last section can result in a small magnitude and variation of the fault current (S_fault2) relative to graph (a) due to the high impedance of the distribution line, as shown in graph (b) in Figure 1 [15].

Moreover, Figure 2 shows fault current characteristics depending on the capacity of the customer load in the LVDC distribution system, which is the concept of reference [16], and graph (a) and (b) illustrate fault current characteristics in off-peak and peak loads, respectively. Graph (a) in Figure 2 illustrates the large magnitude of variation (slope) in the off-peak load, which is the no-load condition, while graph (b) shows the small magnitude of variation in the peak load. Based on the concept of the slope characteristics in the fault current, this paper proposes an operation method of the IFID to consider the distribution line constant and capacity of customer load.

3. Operation Method of IFID Considering Fault Current Characteristics

3.1. Configuration of Intelligent Fault Isolation Device

The proposed IFID for isolating fault location in a radial-type LVDC distribution system, which is adopted from the idea and concept of the reference [17], is composed of a main path and current limit path and so on, as shown in Figure 3. The main path is designed by main switches (S_M) with n switch modules in series, and the current limit path is composed of current limit switches (S_CL) and current limit resistance (CLR), where the current limit switches are the same type as the main switches. Also, the main and current limit paths adopt semi-conductor switches (MOSFETs), which are the components with high insulation voltage and rapid operation characteristics in micro-seconds, and switch modules to control the bi-directional current, which are connected through the emitters they have in common [17].

Furthermore, as shown in Figure 4, the operation modes of IFID are classified with an initial operation mode, auxiliary operation mode, main operation mode, and restoration mode. The concept of the main operation mode in the device is to operate at time interval t₁ when the fault occurs at time interval t₀ and then to clamp and consume the fault current until time interval t₂ in order to isolate the fault location. When the fault location is isolated, the switches of the main path are turned on and the switches of the current limit path are turned off, and the IFID is transferred to a normal operation mode.

3.2. Operation Method of IFID Considering Slope Characteristics of Fault Current

When a fault occurs in a DC distribution system, the outage area may be extended due to the rapid shut down of the main converter. Namely, the existing method considering only the magnitude of fault current has a limitation to protect coordination in the DC distribution system; an operation method to rapidly isolate the fault location by considering the slope characteristics of the fault current is required. Therefore, this paper proposes an operation method of the IFID based on the slope characteristics of a fault current in the LVDC distribution system. First, the slope characteristics of a fault current (S_fault(n)) considering line impedance to detect the fault in each section can be expressed as shown in Equation (1), which represents the variation of fault current from the initial time when the fault occurs to the next time step. Based on Equation (1), the operation strategy of the IFID by each section (Z(n)) is calculated, as shown in Equation (2). The IFID installed at each section detects the fault and performs a limit of fault current according to the operation strategy when the S_fault(n) is bigger than the reference value (S_ref(n)).

S_{fault} (n) = \frac{{∆ I}_{f a u l t} (n)}{∆ t} = \frac{I_{1} (n) - I_{0} (n)}{∆ t}

(1)

where S_fault(n): slope characteristics of the fault current in section n, ΔI_fault(n): variation of the fault current in section n, n: section number, Δt: time step, I₀(n): initial fault current in section n, and I₁(n): fault current in section n after Δt.

Z (n) = \{\begin{matrix} 1, i f S_{f a u l t} (n) > S_{r e f} (n) \\ 0, o t h e r w i s e \end{matrix}

(2)

where Z(n): operation signal of the IFID in section n.

Therefore, an operation method of the IFID considering the slope characteristics of a fault current in the DC distribution system can be illustrated, as shown in Figure 5. The IFID(2) installed at section 2 is rapidly operated by using the slope characteristics of the fault current when the fault occurs at the secondary feeder of section 2 in Figure 5, because the IFID can secure the time interval for protection coordination between main and section converters to isolate the outage area.

On the other hand, the operation characteristics of the IFID with the existing and proposed operation methods are obtained, as shown in Figure 6. The IFID in the existing method as reference [15], which considers the magnitude of fault current only, detects the fault and turns into the auxiliary operation mode at the β point, where the magnitude of fault current reaches the reference current (I_ref). However, the operation time of the IFID in the proposed method can be reduced by detecting the fault at the α point, where the slope characteristics of the fault current (S_fault(n)) exceed the reference value (S_ref(n)).

4. Implementation of IFID in LVDC System

4.1. H/W Section

The H/W section of the IFID in the LVDC system is composed of a main switch, current limit switch, current limit resistor, voltage and current sensors, and SMPS, as shown in Figure 7. The main switch and current limit switch for the main and current limit paths are applied with MOSFET modules, which are operated by the gate drive and SMPS in an independent way. Also, voltage sensors measure the voltages of the primary and secondary sides of the IFID, and current sensors measure the currents of the main and current limit path. The voltage and current sensors adopt an analog circuit of a first-order low pass filter (LPF) to reduce noise, as shown in Equation (3), and the cut-off frequency of the LPF is properly selected as 20 [kHz], considering the sudden inrush current and the switching frequency of the main and section converter, as shown in Equation (4).

H (s) = \frac{1}{τ s + 1}

(3)

f_{c} = \frac{1}{2 π R_{v} C_{v}}

(4)

where

H (s)

: transfer function of analog first-order LPF,

τ

: time constant,

f_{c}

: cut-off frequency,

R_{v}

: filter resistor, and

C_{v}

: filter capacitor.

Furthermore, the number of switch modules (m) connected in series is calculated by dividing the rated voltage of the IFID as the insulation voltage (V_CE) of switch modules and the load factor of insulation voltage (k). The load factor of the insulation voltage in the semi-conductor switch is assumed as 0.6 to solve the issues of system overvoltage and surges, as shown in Equation (5) [18,19].

m = \frac{V_{I F I D}}{2 k \times V_{C E}}

(5)

where m: the number of switch modules, V_IFID: the rated voltage of IFID [kV], k: the load factor of insulation voltage in the semi-conductor switch (0.6), and V_CE: insulation voltage of switch modules [kV].

And the limiting current (

I_{c c}

) for calculating the CLR is obtained by considering the limiter ratio (

α

) and margin capacity of the main and section converter, as shown in Equation (6). Therefore, the CLR is calculated by dividing the limiting current with the primary side voltage of the IFID, as shown in Equation (7).

I_{c c} \leq α I_{d c . o c} = α \frac{P_{c o n}}{V_{p r i}}

(6)

R_{c c} = \frac{I_{c c}}{V_{p r i}}

(7)

where

I_{c c}

: limiting current,

α

: limiting current ratio,

I_{d c . c c}

: margin capacity of main and section converter,

P_{c o n}

: capacity of main and section converter, and

V_{p r i}

: primary side voltage of the IFID.

Furthermore, in order to calculate the thermal capacity of the CLR, the voltage and current characteristics of the current limit path are expressed as shown in Figure 8. t₁ is the starting time of the main operation mode and t₂ is the ending time of the main operation mode. Also, the terminal voltage (

v_{C L R} (t)

) of the CLR should be designed by considering the line impedance, and also the operation time (

t_{C L R}

) of CLR should be calculated by assuming the larger than isolation time interval (

t_{C B}

) of circuit breakers, such as the medium voltage circuit breaker (MVCB) and low voltage circuit breaker (LVCB), as shown in Equation (8). Therefore, the energy of the CLR is calculated by integrating the multiplication of the voltage and current on the basis of Equation (8). Therefore, the energy of the CLR is obtained by integrating the multiplication of the voltage and current for the operation time (

t_{C L R})

, as shown in Equation (9).

t_{C L R} = t_{2} - t_{1} \geq t_{C B}

(8)

E_{C L R . m i n} = \int_{0}^{t_{C L R}} v_{C L R} (t) \times i_{d c} (t) d t

(9)

where

t_{C L R}

: the operation time of CLR,

t_{1}

: starting time of the main operation mode [ms],

t_{2}

: ending time of the main operation mode,

t_{C B}

: isolation time interval of circuit breakers [ms],

E_{C L R . m i n}

: energy of the current limit resistor,

v_{C L R} (t)

: voltage of CLR, and

i_{d c} (t)

: current of the normal operation mode.

4.2. S/W Section

The S/W section of the IFID is composed of the CCS and control board to perform monitoring and communication. The CCS tool performs debugging and monitoring functions and has real-time control of the semi-conductor component, such as IGBT and MOSFET. Also, it controls the transfer of operation modes and the current limit switch to the limit fault current based on the proposed operation method of the IFID. Namely, it calculates the slope characteristics of the fault current every time step of 10 μs, and evaluates the fault condition and then transfers the auxiliary and main operation modes when the slope of the fault current exceeds the reference value. Furthermore, the control board of the IFID consists of a digital signal processor (DSP), pulse width modulation (PWM) port, A/D port, and communication port, as shown in Figure 9. The DSP is adopted as TMS320F28335 to control each component independently, and is used by using 12 PWM ports and 16 ADC channels to control multiple voltage and current sensors simultaneously [20,21,22].

5. Case Studies

5.1. Test Conditions

The test conditions to evaluate the operation characteristics of the proposed IFID considering the slope characteristics of a fault current are assumed, as shown in Figure 10 and Table 1. The input and output voltages of the main and section converters are designed as 380 V_AC/400 V_DC and 400 V_DC/200 V_DC, respectively. Also, the rated powers of the main and section converters are 10 kW and 5 kW, respectively, and the primary DC line includes two sections. Also, test scenarios based on the line impedance and capacity of customer load are assumed as two cases, as shown in Table 2. Case I shows a small line impedance and off-peak load and case II shows a large line impedance and peak load. Moreover, the experimental test is performed by comparison between the proposed method considering the slope characteristics of a fault current and the existing method with a magnitude of fault current.

5.2. Operation Characteristics of IFID Considering Magnitude of Fault Current

Based on the test conditions mentioned earlier, the operation characteristics of the IFID in the existing method can be illustrated, as shown in Figure 11. In Figure 11, (a) and (b) are obtained by the test scenarios of case I and case II in Table 2, respectively. Graph (a) of Figure 11 shows that the fault current rises rapidly when the fault occurs at the t₁ point, and the IFID detects the setting value of a 110% rated current (27.5 [A]) at the t₂ point, and then transfers to the main operation mode at 34.57 [μs] after the t₂ point. Also, graph (b) of Figure 11 shows the same characteristics of the IFID as graph (a), except for transferring to the main operation mode at 44.68 [μs]. Therefore, it is known that the IFID with a fault current magnitude only can limit the fault current after 59.57 [μs] and 46.81 [μs], depending on the line impedance and pattern of customer load.

5.3. Operation Characteristics of IFID Considering Slope of Fault Current

Based on the test conditions mentioned earlier, the operation characteristics of the IFID considering the slope characteristics of a fault current can be obtained as shown in Figure 12. In Figure 12, (a) and (b) are obtained by the test scenarios of case I and case II in Table 2, respectively. Graph (a) of Figure 12 indicates that the fault current starts to rapidly increase at the t₁ point, and the IFID detects the fault condition before the setting value of a 110% rated current (27.5 [A]), and then transfers to the main operation mode at 33.23 [μs] after the t₁ point. Also, graph (b) of Figure 12 shows the same characteristics of the IFID as graph (a) of Figure 12, except for transferring to the main operation mode at 27.39 [μs]. Therefore, it is known that the IFID with the slope characteristics of a fault current can limit the fault current after 33.23 [μs] and 27.39 [μs], depending on the line impedance and pattern of customer load.

5.4. Comprehensive Analysis

Based on the test conditions and scenarios in Table 1 and Table 2, the comparison results between the existing and proposed operation methods of the IFID can be illustrated, as shown in Table 3. The table shows that the proposed method to consider the slope characteristics of the fault current can shorten the operation time of the IFID about 44~54[%] faster than the existing method to consider only the fault current magnitude depending on the line impedance and capacity of customer load. Therefore, it was found that the IFID to consider the slope characteristics of the fault current can detect and evaluate the fault condition and shorten the operation time in a fast and accurate manner compared to the existing method to consider the fault current only.

6. Conclusions

This paper presents an operation method of the IFID, which can detect and estimate the fault in a rapid and accurate way by considering the slope characteristics of the fault current depending on the line impedance and pattern of customer load. Moreover, this paper implements the DC 400 V, 10 kW scaled IFID consisting of H/W and S/W sections in order to validate the proposed operation method. The main results of this paper are summarized as follows:

(1): It was found that the IFID with a fault current magnitude only can limit the fault current after 59.57 [μs] and 46.81 [μs], depending on the line impedance and pattern of customer load.
(2): It was confirmed that the IFID with the slope characteristics of a fault current can limit the fault current after 33.23 [μs] and 27.39 [μs], depending on the line impedance and pattern of customer load.
(3): It was found that the IFID to consider the slope characteristics of a fault current can detect and evaluate the fault condition and shorten the operation time in a fast and accurate manner compared to the existing method to consider the fault current only.
(4): In the future study, it is necessary to perform modeling of the proposed IFID to verify its operation characteristics in contingency scenarios.

Author Contributions

All authors participated actively in the development and publication of this paper. Y.-H.K. was responsible for implementation and manuscript preparation. K.-H.K., H.-S.Y. and S.-J.K. performed the test analysis and collected data and reviewed earlier studies while D.-S.R. conducted the literature review. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government (MOTIE) (20224000000160, DC Grid Energy Innovation Research Center), and this research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MOTIE) (RS-2024-00421994, Development of performance verification techniques and safety evaluation system for LiB-UPS System unit).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Fault current characteristics with fault location.

Figure 2. Fault current characteristics with magnitude of load.

Figure 3. Configuration of intelligent fault isolation device.

Figure 4. Operation mechanism of IFID.

Figure 5. Operation method of IFID with fault current slope characteristics.

Figure 6. Operation characteristics of IFID by existing and proposed methods.

Figure 7. Configuration of IFID.

Figure 8. Characteristics of voltage and current of CLR.

Figure 9. Configuration of control board in IFID.

Figure 10. Configuration of test device for IFID.

Figure 11. Operation characteristics of IFID with existing method. (a) Case I; (b) case II.

Figure 12. Operation characteristics of IFID with the proposed method. (a) Case I; (b) case II.

Table 1. Test conditions.

Items		Contents
IFID	type	MOSFET
	drain-source breakdown voltage [V]	650
	load factor of insulation voltage [%]	60
	turn-on delay time [ns]	20
	turn-off delay time [ns]	82
	CLR [Ω]	15
mainconverter	rated capacity [kW]	10
	input voltage [V_AC]	380
	output voltage [V_DC]	400
section converter	rated capacity [kW]	5
	input voltage [V_DC]	400
	output voltage [V_DC]	200

Table 2. Test scenarios.

Cases	Line Impedance		Capacity of Customer Load
Cases	Section 1	Section 2	Capacity of Customer Load
I	1 [Ω] + 5.37 [mH]	1.5 [Ω] + 7.96 [mH]	off-peak load
II	3 [Ω] + 15.92 [mH]	1.5 [Ω] + 7.96 [mH]	peak load

Table 3. Operation characteristics of the IFID.

Cases	Operation Methods	Operation Time of IFID [μs]
I	existing method	59.57 [μs]
I	proposed method	46.81 [μs]
II	existing method	33.23 [μs]
II	proposed method	27.39 [μs]

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MDPI and ACS Style

Kim, Y.-H.; Kim, K.-H.; You, H.-S.; Kim, S.-J.; Choi, S.-M.; Rho, D.-S. An Implementation of Intelligent Fault Isolation Device for LVDC Distribution System Considering Slope Characteristics of Fault Current. Electronics 2025, 14, 171. https://doi.org/10.3390/electronics14010171

AMA Style

Kim Y-H, Kim K-H, You H-S, Kim S-J, Choi S-M, Rho D-S. An Implementation of Intelligent Fault Isolation Device for LVDC Distribution System Considering Slope Characteristics of Fault Current. Electronics. 2025; 14(1):171. https://doi.org/10.3390/electronics14010171

Chicago/Turabian Style

Kim, Yun-Ho, Kyung-Hwa Kim, Hyun-Sang You, Se-Jin Kim, Sung-Moon Choi, and Dae-Seok Rho. 2025. "An Implementation of Intelligent Fault Isolation Device for LVDC Distribution System Considering Slope Characteristics of Fault Current" Electronics 14, no. 1: 171. https://doi.org/10.3390/electronics14010171

APA Style

Kim, Y.-H., Kim, K.-H., You, H.-S., Kim, S.-J., Choi, S.-M., & Rho, D.-S. (2025). An Implementation of Intelligent Fault Isolation Device for LVDC Distribution System Considering Slope Characteristics of Fault Current. Electronics, 14(1), 171. https://doi.org/10.3390/electronics14010171

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An Implementation of Intelligent Fault Isolation Device for LVDC Distribution System Considering Slope Characteristics of Fault Current

Abstract

1. Introduction

2. Slope Characteristics of Fault Current in LVDC Distribution System

3. Operation Method of IFID Considering Fault Current Characteristics

3.1. Configuration of Intelligent Fault Isolation Device

3.2. Operation Method of IFID Considering Slope Characteristics of Fault Current

4. Implementation of IFID in LVDC System

4.1. H/W Section

4.2. S/W Section

5. Case Studies

5.1. Test Conditions

5.2. Operation Characteristics of IFID Considering Magnitude of Fault Current

5.3. Operation Characteristics of IFID Considering Slope of Fault Current

5.4. Comprehensive Analysis

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Conflicts of Interest

References

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