Operation Algorithm for Protection Coordination Device in High-Voltage Customer with ESS for Demand Management

Abstract

Installations of an Energy Storage System (ESS) with various functions such as power stabilization of renewable energy, demand management, and frequency adjustment are increasing. In particular, ESS for demand management is being established for high-voltage customers (300 KVA–1000 KVA) who have placed an Auto Section Switch (ASS) at the connection point within the distribution system. However, a power outage may occur in the Power Receiving System (PRS) when a short-circuit fault due to insulation breakdown occurs at the ESS DC side. The reason for this breakdown is that the fault current is reduced by transformer impedance, and the ASS is opened before the DC power fuse. Therefore, using the Graphic Solution Method (GSM), this paper presents an operation algorithm for protection coordination that isolates the fault section by first operating the DC power fuse with a small fault current. Furthermore, fault analysis modeling for a PRS composed of a switchgear section, a main distribution panel, a Power Conditioning System (PCS), a power fuse, and a battery is performed through PSCAD/EMTDC. From the simulation results, it is confirmed that the fault section is quickly isolated, and power outages for high-voltage customers are prevented because the DC power fuse selected by the proposed operation algorithm of protection coordination is opened before the ASS.

1. Introduction

Government and market policies on solving environmental pollution and climate change problems are being introduced around the world. As a part of this, both in domestic and international markets, the Energy Storage System (ESS) has been introduced as an effective means to establish stable and efficient systems for power demand and supply in these areas: power stabilization of renewable energy, demand management, and frequency adjustment [1,2,3]. Among them, the ESS for demand management is added to a Power Receiving System (PRS), which may not be achieved via coordination between the protective devices. These protection coordination problems are subject to DC faults in the microgrid or DC distribution with the ESS and have been discussed in many studies. In DC microgrids, the analysis for faults that occurred on DC terminals was studied in [4]. The non-unit protection scheme for the battery energy storage system considered a vital source for microgrid operation was introduced in [5]. To address all events regarding DC microgrid protection, [6] explicitly reviewed the existing techniques along with the jurisdiction for the protection requirements. In addition, the changing of overcurrent relay settings to protect the microgrid from faults was explained in [7]. Therefore, the application of the superconducting fault current limiter for limiting the fault current on the low-voltage–direct-current system was proposed in [8]. As described above, the protective cooperation problem that may occur for high-voltage customers is not carefully studied.

The ESS for demand management is being built for high-voltage customers (300 KVA–1000 KVA) who have installed an Auto Section Switch (ASS) at the connection point within the distribution system. The ASS is commonly used to prevent faults occurring in the PRS from damaging other customers. Namely, the ASS is automatically operated for a fault current of 800 A or less; when a greater fault current is detected, the ASS is opened in a no-voltage state in coordination with a protection device (recloser) within the distribution system [9,10,11]. However, a power outage for high-voltage customers may occur when a short-circuit fault due to insulation breakdown occurs at the ESS DC side. The reason for this breakdown is that the fault current is reduced by transformer impedance, and the ASS is opened before the DC power fuse.

To solve the above problem, this paper proposes an operation algorithm for protection coordination considering the operation time of ASS and the cut-off time of the DC power fuse. At this time, a Graphic Solution Method (GSM) based on DC fault current and a Time–Current Characteristic (TCC) curve is used. In addition, fault analysis modeling for a PRS composed of a switchgear section, a main distribution panel, a Power Conditioning System (PCS), a power fuse, and a battery is performed through PSCAD/EMTDC. From the simulation results, it has been confirmed that the fault section is quickly isolated and a power outage for high-voltage customers is prevented because the DC power fuse is opened before the ASS. The structure of this paper is as follows: Section 2 analyzes a fault case that occurred in a Power Receiving System with ESS, Section 3 presents an operation algorithm for the protection coordination device, Section 4 gives fault analysis modeling of a Power Receiving System with ESS through PSCAD/EMTDC, Section 5 ascertains the validity of the proposed operation algorithm through actual case studies, and Section 6 concludes this paper.

2. Case Analysis for Power Receiving System with ESS

The PRS with ESS consists of several facilities such as a switchgear section, the main distribution panel, a PCS, a power fuse, and a battery. The switchgear section, which is composed of an ASS, a power fuse, and a main transformer (M.Tr), is used to control, protect, and isolate electrical equipment. In addition, the main distribution panel that interrupts fault currents through the Molded Case Circuit Breaker (MCCB) and the PCS section is made up of an LCL filter, IGBT modules, and a DC-link capacitor to perform battery charging and discharging. Finally, the multi-rack battery section is used to reduce peak load.

In general, a DC power fuse is installed to quickly isolate short-circuit faults occurring in the ESS DC side. Then, the TCC curve of the DC power fuse is influenced by the fault current that depends on high impedance of the PRS. Shown in detail in Figure 1, the DC fault current is decreased by high impedance of the MTr, the Induction Voltage Regulator (IVR), and the Isolation Transformer (ISOL). Consequently, the ASS can be opened before the DC power fuse because the reduced fault current exceeds the ASS setting value and the ASS operation time is shorter than the fuse operation time. Figure 2 displays voltages and currents on the ASS that opened via a short-circuit fault that occurred on the ESS DC side. The voltage of each phase is decreased to about half of the normal value and the current of each phase is increased to about three times the load current during a short-circuit fault. Hence, the ASS is opened at trip time 97 ms and the result is a power outage for high-voltage customers.

3. Operation Algorithm for Protection Coordination Device

The DC power fuse should be operated before activating the ASS to prevent power outages for high-voltage customers when a short-circuit fault occurs at the ESS DC side. At this time, the DC power fuse activates earlier than the ASS, and the charging/discharging of the ESS is immediately stopped. Therefore, after the fault is removed and the DC power fuse is replaced, the charging/discharging of the ESS is restarted. It is particularly important to estimate the proper capacity of the DC power fuse, which is calculated using the GSM.

3.1. Fuse Selection by Graphic Solution Method

As mentioned in the previous chapter 2, the protection coordination between the ASS and the DC power fuse cannot be achieved because of the reduced fault current. Thus, the capacity of the DC power fuse should be decided by considering the operation time of the ASS and the cut-off time according to fuse capacity. Currently, a GSM based on DC fault current and a TCC curve is used. Namely, the fault current curve created by Equation (1) is applied to the TCC curve displayed in Figure 3, and the appropriate coordination time between protection devices can be decided by the GSM, as per Figure 4 [12,13,14,15,16,17,18,19,20,21].

$${\mathrm{I}}_{\mathrm{f}}\left(\mathrm{t}\right)=-\frac{{\mathrm{I}}_0{\mathsf{\omega }}_0}{\mathsf{\omega }}{\mathrm{e}}^{-\mathsf{\delta }\mathrm{t}}\left(\sin \mathsf{\omega }\mathrm{t}-\mathsf{\beta }\right)+\frac{{\mathrm{V}}_0}{\mathsf{\omega }\mathrm{L}}{\mathrm{e}}^{-\mathsf{\delta }\mathrm{t}}\left(\sin \mathsf{\omega }\mathrm{t}\right)$$

(1)

where ${\mathrm{I}}_{\mathrm{f}}$: fault current, t: time, ${\mathrm{I}}_0$: initial current, ${\mathsf{\omega }}_0$: resonant frequency, δ: damping frequency, ω: angular frequency, β: phase angle, ${\mathrm{V}}_0$: initial voltage, and L: inductance.

In Figure 4, Fuse 1–Fuse 4 correspond to the TCC curve according to their fuse capacity, the green dots (ⓐ, ⓑ, and ⓒ), the mean intersection of fault currents, and their TCC curve. Section ① shows that the ASS and the DC power fuse do not operate because the fault current is less than the setting value of the DC power fuse and the duration of the fault did not reach the ASS operating time. The blue shaded section ② shows that the DC power fuse is operated before the ASS because some intersections exist between the fault current and the TCC curve; for example, power fuse 3, power fuse 2, and power fuse 1 are opened when the fault current increases to the intersection points ⓐ, ⓑ, and ⓒ, respectively. In addition, section ③ displays that the ASS is operated before the DC power fuse. The fault current intersects with the TCC curve as in section ②; however, the proper protection coordination cannot be achieved since the operation time of the DC power fuse is later than the operation time of the ASS. Accordingly, the DC power fuse that operates before the ASS should be selected from among the fuses satisfying section ②.

3.2. Operation Algorithm for Protection Coordination

Based on Section 3.1, the detailed explanation of the operation algorithm for protection coordination between protection devices is as follows:

(Step 1) The % impedance of transformers (%Z) and DC load current (${\mathrm{I}}_{\mathrm{N}}$) are assumed as initial conditions.

(Step 2) The fault current curve (${\mathrm{I}}_{\mathrm{f}}$) is created according to Equation (1) when a short-circuit fault occurs at the ESS DC side.

(Step 3) The fault current curve is applied to the TCC curve of the DC power fuse provided by the manufacturer. Then, the characteristics of protection coordination are analyzed for section ①, section ②, and section ③ separated by the GSM.

(Step 4) In section ②, the DC power fuse corresponding to the maximum value ${\mathrm{I}}_{\mathrm{f}.\max }$ along the intersections of the fault current and TCC curve is selected. For instance, if power fuse 1 is capable of breaking the maximum current, ⓒ in Figure 4 should be selected. Furthermore, the capacity of the DC power fuse should be at least 1.25 times DC load current.

The flowchart showing the above operation algorithm for the protection coordination is shown in Figure 5.

4. Fault Analysis Modeling of Power Receiving System with ESS through PSCAD/EMTDC

The following PRS is modeled to examine the effect of a short-circuit fault in the ESS DC side on the switchgear section, distribution panel section, PCS section, battery section, DC power fuse, and fault generator.

4.1. Modeling of Switchgear Section

The switchgear section involves an ASS, an AC power fuse, and a M.Tr, as shown in Figure 6. An ASS is automatically operated for a fault current of 800 A or less. When the greater fault current is detected, an ASS is opened in a no-voltage state in coordination with a protection device (recloser) on the distribution system. Moreover, an AC power fuse is used to separate the fault section and a M.Tr is used to change medium voltage into low voltage.

4.2. Modeling of Distribution Panel Section

Figure 7 illustrates the distribution panel section composed of an MCCB, an IVR, and an ISOL. Herein, an MCCB blocks fault currents such as an overload and a short-circuit fault. An IVR automatically adjusts voltage fluctuations supplied from the distribution system to supply a constant voltage to the load, and an ISOL provides galvanic isolation.

4.3. Modeling of PCS Section

The PCS performing battery charging/discharging functions consists of an IGBT module, a DC power fuse, and the like, as shown in Figure 8. The IGBT module converts DC power into three-phase AC power with a phase difference of 120°, and a DC power fuse is installed for each module to quickly isolate the fault section.

4.4. Modeling of Battery Section

Figure 9 represents the modeling of the battery section consisting of a battery and a DC power fuse. The multi-rack battery is used to reduce peak load, and the DC power fuse is used to quickly isolate a short-circuit fault from occurring at the ESS DC side.

4.5. Modeling of DC Power Fuse

A DC power fuse consisting of (+) and (−) poles is modeled in Figure 10. The comparator provided by PSCAD/EMTDC compares two currents and outputs a digital signal indicating which current is larger. A control circuit operating the comparator is also considered when the accumulated fault current is greater than the manufacturer’s fuse capacity.

4.6. Modeling of Fault Generator

The fault generator that causes a short-circuit fault at the ESS DC side is equal to Figure 11. The short-circuit fault is modeled by shorting the DC circuit’s (+) and (−) poles of the ESS.

4.7. Modeling of Power Receiving System

The fault analysis modeling for the entire PRS with an ESS for demand management is shown in Figure 12. Part A shows the switchgear section; part B represents the distribution panel section; part C displays the PCS section; part D displays the fault generator; and part E and part F represent the battery section and the power fuse in DC side, respectively.

5. Case Studies

The validity of the proposed operation algorithm for protection coordination between protection devices is ascertained through case studies.

5.1. Simulation Conditions

The operation algorithm for protection coordination between the ASS and the DC power fuse is applied for a high-voltage customer who has experienced a power outage due to a short-circuit fault at the ESS DC side.

Table 1. Simulation conditions.

Items	Components	Contents
switchgear section	ASS	25.8 kV, 200 A/15 kA
	M.TR(Δ-Y)	500 kVA, 22.9/0.38 kV %Z: 5.43
distribution panel section	MCCB 1,2	6.4 kA/42 kA
	MCCB 3	3 kA/65 kA
	IVR	750 kVA, 0.38 kV/0.38 kV %Z: 6
	ISOL(Δ-Y)	600 kVA, 0.38 kV/0.38 kV %Z: 5.09
PCS Section	AC module fuse	315 A/100 kA
	DC main fuse	500 A × 2/100 kA
	IGBT	switching frequency: 4 kHz
battery section	battery	50 S/1 P, 1000 V, 200 kWh 174 mΩ

lists the specification of the PRS, which is used under simulation conditions. As the main data, the capacity of M.Tr is 500 KVA, the load current is 12 A, the switching frequency of the PCS is 4 kHz, and the internal impedance considering parasitic capacitance and voltage source is 2.9 mΩ. In addition, the minimum operating current of the ASS is assumed to be 30 A, which is 2.5 times the load current. Moreover, the DC power fuse is composed by connecting two 500 A fuses in parallel. Finally, the battery section has a voltage of 1000 V, a capacity of 200 kWh, and a total internal resistance of 174 mΩ.

5.2. Operation Characteristics of AC Protection Devices during DC Fault

This section explores the operation characteristics of AC protection devices when a short-circuit fault occurs on the ESS DC side. Figure 13 and Figure 14 display current passing through AC power fuses and MCCBs when a short-circuit fault occurs at 0.3 s. MCCB 1, MCCB 2, and MCCB 3 do not trip at a fault current of 2.45 kA, which is smaller than the trip currents of 6.4 kA and 3 kA in Table 1. In comparison, the AC power fuse (AC module PF in Figure 14) may trip because the fault current is 0.58 kA, which is larger than the trip current of 0.315 kA in Table 1. Incidentally, the trip time of the AC power fuse is about 8 s, which is longer than the operation time of the ASS. Therefore, it was concluded that an ASS trips at 97 ms in a fault current of 43 A and a power outage occurs.

5.3. Operating Characteristics of DC Power Fuse According to High Impedance

As shown in Figure 15, three transformers installed in series were considered to simulate the high impedance. The fault current is decreased to 42 A on the ASS and 7.67 kA on the DC power fuse because impedance was increased. Accordingly, the protection coordination between the ASS and the DC power fuse is not achieved as described in Figure 16. That is, the DC power fuse is not opened depending on the inverse time-delay characteristic. On the contrary, the ASS is opened at 97 ms, since the DC power fuse is not operated, and the fault section is not separated. As a result, the high-voltage customer experiences a power outage.

5.4. Characteristics of Protection Coordination by the Proposed Operation Algorithm

The ASS may be opened first because the fault current is reduced by the high impedance of a high-voltage customer with ESS installed. To solve this problem, a DC power fuse that does not operate on a load current and operates on short-circuit current is selected through the proposed operation algorithm for protection coordination. Figure 17 shows the protection coordination characteristics for the selected DC power fuse. As shown in this figure, the DC power fuse is opened at 60.6 ms, which is faster than the ASS operating time of 97 ms. Therefore, even if a short-circuit fault occurs on the ESS DC side, the DC power fuse quickly isolates the fault section and prevents a power outage for the high-voltage customer. The above studies can be summarized as shown in

Table 2. Comparison of simulation results.

Item	Operating Time of ASS	Operating Time of Fuse	Occurrence of Power Outage
existing method	97 ms	-	O
proposed method	97 ms	60.6 ms	X

.

6. Conclusions

For a short-circuit fault at the ESS DC side, the problem was that the ASS was operated before DC power fuse, and the power outage that occurred was analyzed in this paper. To resolve this issue, the operation algorithm of protection coordination between protection devices is presented so that the DC power fuse operates even with a reduced fault current. The main research results are summarized as follows:

(1)

First, the power outage that was caused by a fault that occurred at the ESS DC side a of high-voltage customer with ESS was analyzed. The fault current was reduced due to the high impedance of the three transformers installed in series. The previously installed DC power fuse for 1000 A took too long to operate, and thus, the high-voltage customer suffered a power outage because of the ASS operation.

(2)

The DC power fuse selected by the proposed operation algorithm of protection coordination and the GSM operated at 60.6 ms, which is faster than the ASS operation time of 97 ms. Then, the fault section can be quickly separated, and power outages can be prevented by an appropriately selected DC power fuse.