DC low-voltage distribution system

Electrical Power and Energy Systems 105 (2019) 521–528

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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes

Development of protective schemes for hybrid AC/DC low-voltage distribution system

T

⁎

Chul-Ho Noha, Chul-Hwan Kima, , Gi-Hyeon Gwonb, Muhammad Omer Khana, Saeed Zaman Jamalia a b

College of Information of Communication Engineering, Sungkyunkwan University, Suwon, Republic of Korea Yonam Institute of Technology, Jinju, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Hybrid AC/DC low-voltage (LV) distribution system Low-voltage DC (LVDC) distribution system Power system protection Protective coordination Transient analysis

As the interest in DC-based power system increases, the low-voltage DC (LVDC) distribution system is gaining attention. Until now, however, the related studies and developments are still not mature to commercialize the system. This corresponds to the protection system as well. In fact, previous studies have dealt with protection schemes targeting only the LVDC distribution system although the system is closely influenced by the lowvoltage AC (LVAC) distribution system–hybrid AC/DC low-voltage (LV) distribution system. However, most literatures reporting the hybrid AC/DC LV distribution system do not focus on the protection scheme. Thus, in the work, the protective schemes for the hybrid AC/DC LV distribution system are developed and coordinated. The transient analysis on the events in both systems is first conducted; subsequently, the protection schemes for each system are developed. Finally, two proposed protection schemes are coordinated using the state diagram and their performances are verified through simulations using the electromagnetic transients program (EMTP).

1. Introduction CURRENTLY, DC-based power systems are gaining attention in distribution and transmission systems. This trend resulted from the development of power electronic converters, introduction of distributed generation (DG), and increase of digital loads. Therefore, researchers have conducted various studies to develop conventional power systems that can accommodate these changes. In particular, low-voltage DC (LVDC) distribution systems are still in the development stage because some challenges need to be resolved [1–3]. One of them is to develop novel protection schemes for the LVDC distribution system, considering the fault characteristic due to power electronic converters. In this regard, many studies have dealt with various protection issues. Refs. [1–4] studied the aspect that should become a priority in the protection scheme for DC faults. Based on the fault analysis, they have concluded that the protection scheme for DC faults can detect and isolate DC faults rapidly, primarily with the current derivative. Refs. [5–6] proposed the protective coordination for the LVDC distribution system, and [7–8] considered the interconnections with DGs. Additional topics such as the detection of high-impedance fault, current-limiter, and fault location are discussed in [9–12]. As reviewed, however, most protection schemes proposed in the existing literatures targeted only the LVDC

⁎

distribution system itself. In the case DC network is being coupled to an AC network, LVDC distribution systems are generally supplied through two conversion stages: a transformer and an AC/DC converter. In this regard, the lowvoltage AC (LVAC) distribution system for AC loads is on the upstream of the LVDC distribution system with respect to efficiency and cost. Such a hybrid AC/DC low-voltage (LV) system, called the hybrid AC/DC LV microgrid or hybrid AC/DC LV distribution system according to their topologies, have been studied in many literatures so far. However, the existing literatures rarely discuss the protection scheme for the hybrid AC/DC LV distribution system. Refs. [13–21] presents the methods to control power flow in a hybrid AC/DC LV system. The control schemes to improve the system power quality are discussed in [22–24]. Moreover, [25–28] proposed cooperative control schemes among converters in a hybrid AC/DC LV system. Moreover, the existing protection schemes for DC systems are not suitable for protecting the hybrid AC/DC LV distribution system. That is because they hardly consider the impacts by disturbance in the upstream AC systems. The work therefore proposes the protective schemes for the hybrid AC/DC LV distribution system based on transient phenomena by disturbance in both LVAC and LVDC distribution systems. And then, the performance is verified by simulations using the electromagnetic

Corresponding author. E-mail address: [email protected] (C.-H. Kim).

https://doi.org/10.1016/j.ijepes.2018.08.030 Received 20 May 2018; Received in revised form 31 July 2018; Accepted 22 August 2018 0142-0615/ © 2018 Elsevier Ltd. All rights reserved.

Electrical Power and Energy Systems 105 (2019) 521–528

C.-H. Noh et al.

transients program (EMTP). The proposed protective scheme coordinates two protection systems for both LVAC distribution system and LVDC one. In this regard, each protection system adopts typical factors: current magnitude for LVAC distribution system and current derivative for LVDC one. Thus, the main contribution of this work is how to coordinate two protection systems by using new circuit breaker (CBIT) in front of an AC/DC converter, which is proposed on the basis of the transient analysis in this paper. Especially, the state diagram is applied to discriminate the operation of individual protection systems and coordinate them. This work achieves not only the protection of each distribution system similar to the conventional ones, but also assures the normal operation even under fault conditions of other distribution system. Especially, the protective weak point of the existing protection schemes for DC distribution system, which cannot protect some transient phenomena due to the events in LVAC distribution system, can be solved in this study as well. The remainder of this paper consists of four sections. In Section 2, the transient analysis performed according to the events in the LVAC and LVDC distribution system is discussed (LADS and LDDS stand for LVAC and LVDC distribution systems hereinafter). Section 3 proposes the protective schemes for the hybrid AC/DC LV distribution system and coordinates them. In this section, each protection scheme for both distribution systems is described initially. Subsequently, in Section 4, the performance of the proposed scheme is verified by simulations using the EMTP. Finally, conclusions are presented in Section 5.

Open state LADS

ACCB

AC load Transformer ACCB

AC load

~ =

RAC,line+LAC,line

=

AC/DC converter

=

iDC

CDC

Rfault

VS

-

Fig. 2. Current flow due to DC fault.

2.1. Transient analysis by events in LDDS This section presents the results of the transient analysis under the DC fault state and the DCCB opening state shown in Fig. 2, which shows the equivalent circuit of one phase in the LADS. When a fault occurs in the LDDS, the capacitor (CDC) in the AC/DC converter starts to discharge, resulting in a capacitive discharge current (iDC) along a series RLC path in Fig. 2 [31]. It has a very short time constant and contribute to high fault currents [32]. To prevent the semiconductor switches from being damaged, they are deliberately opened at that time. The manufacturers define its maximum overcurrent capability to be 2–3 times the nominal current during various milliseconds [29,33]. The current increases and decreases dramatically while the capacitor voltage (Vc) discharges, and is in the form of an underdamped sinusoidal current expressed by

iDC (t ) = e−(Req /2Leq) t A sin(ωt + α ),

(1)

where Req and Leq denote the equivalent values of resistance and inductance in a fault current loop, respectively. A represents the current magnitude and ω represents the resonant frequency. For reference, (1) is the modified form from the original one by using a formula of a trigonometrical function. Finally, α denotes the damping factor. The equation of voltage transients for VC can be expressed as integral form of (1) because it is directly related with the capacitive discharge current. The fault is contributed by both the energy stored in the line inductance (LDC,line) and the AC source immediately after Vc is fully discharged, causing the current flow through the AC/DC converter as shown in Fig. 2 [34]. The two currents can be respectively expressed by

iL (t ) = −IL0 e (−Req, L /2Leq, L)(t − tC )

(2)

is (t ) = Vs / R eq, S + (Is0−Vs / R eq, S ) e (−Req, S / Leq, S )(t − tC )

(3)

where IL0 and IS0 denote the initial value of the current resulting from the discharge of LDC,line and AC source, respectively. Req,L, Leq,L, Req,S, and Leq,S represent the equivalent values of resistance and inductance on the corresponding paths, respectively. In addition, VS means the AC input voltage level and tC represents the time when Vc is fully discharged. Since the semiconductor switches are opened, the anti-parallel diodes become the path for the currents, resulting in the diodes being damaged. Thus, the DC fault must be interrupted before Vc is fully discharged. Moreover, the DC fault adversely affects the LADS by a higher current and voltage sag. This means that the circuit breaker (ACCB) in the LADS could be opened if the DC fault is not interrupted quickly enough. Next, the DC fault is interrupted by opening the circuit breaker in the LDDS. Subsequently, two distribution systems are isolated from each other and the LADS can be normally operated with a smaller load current than that under the prefault state. This is because only the AC loads are supplied.

DC load

DC/DC converter

= DCCB

=

LDDS

LS Vc

LDDS

DCCB

AC/DC converter

+

The concept of a hybrid AC/DC LV distribution system is illustrated first in this section. A hybrid AC/DC LV distribution system includes an upstream LADS supplying the LDDS, as shown in Fig. 1. The hybrid AC/ DC LV distribution system includes a transformer and an AC/DC converter at the input terminals of the LADS and LDDS, respectively. The transformer plays role to supply LVAC power to LADS by stepping down input voltage level, but it is regarded as the ideal voltage source in this paper. On the other hand, the AC/DC converter is controlled using a pulse width modulation (PWM) technique used to rectify the AC voltage into a DC voltage. In particular, the AC/DC converter is to be protected in the development of the protective schemes for a hybrid AC/DC LV distribution system. Moreover, DC/DC converters are installed in front of DC loads, which is to transform voltage level to the required level by using a duty ratio control scheme. The two types of circuit breakers in Fig. 1 represent a mechanical circuit breaker for the LADS (ACCB) and a solid state circuit breaker for the LDDS (DCCB). The solid state circuit breaker is applied due to the requirements for rapid interruption of DC fault. The solid state circuit breakers have become valid options for protecting DC fault because of its fast operation speed while the mechanical circuit breaker has the limitation [29]. Last, it is adopted TN-S grounding system connected with the middle point of AC/DC converter. That is because the fault detection is easier in the TN-S grounding system than in IT one [30]. In addition, the target system has low possibility to be touched by human contrary to telecom power systems or the system of customer-end.

Utility

After Vc discharges RDC,line+LDC,line

2. Transient analysis in hybrid AC/DC LV distribution system

LADS

While Vc discharges

DC load

Fig. 1. Concept of a hybrid LV distribution system. 522

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Current by VS

LADS

Open state

Current by discharge of CDC

AC/DC converter

Current flow under AC temporary fault

LDDS

AC/DC converter

LADS

LDDS RDC,line+LDC,line

RDC,line+LDC,line

+

+ ACCB

RAC,line+LAC,line

RAC,line+LAC,line

LS CDC

VS

LS

Vc

CDC

VS

Rfault § 0

Vc

Rfault

-

-

Fig. 4. Current flow during the reclosing process after clearing AC fault.

Fig. 3. Current flow by three-phase short circuit fault in LADS.

2.2. Transient analysis by events in LADS

3. Protective schemes for hybrid AC/DC LV distribution system

In this section, the transient phenomena are analyzed according to events such as the fault occurrence in the AC side, and the opening and reclosing of the ACCB. When a fault occurs in the LADS, the bulk of the fault current tends to flow towards the fault location in general. Thus, the impact on the downstream LDDS is slight with respect to the current. Although an unbalanced voltage could be induced at the input terminal of the AC/ DC converter according to the fault type, a variety of solutions are available to produce a normal output, as proposed in [35–37]. Besides, it gives little influence on the system because the AC power cannot be supplied to LDDS through AC/DC converter by the following operation of ACCB. In a three-phase short-circuit fault, however, the power supply to the LDDC is instantaneously prevented because the source current rarely flows downstream of the fault location. Furthermore, the semiconductor switches are deliberately opened due to the discharge current of Vc flowing toward the fault location via the semiconductor switches as shown in Fig. 3. The current flows along the series RLC circuit such that it can be expressed in an equation similar to (1). Only the values of R, L, and C are decided by the corresponding path. The ACCB is tripped according to the time-current curve after an AC fault occurs. Once the ACCB is opened, the LDDS cannot be supplied, except by the DG interconnected with the LDDS. Therefore, similar to the three-phase short-circuit fault, the LDDS could not be supplied. Moreover, the CDC discharge could result in the deliberate opening of the semiconductor switches in the same manner as in Fig. 3. Nevertheless, the current by CDC discharge flows toward the load side if the semiconductor switches have already been opened before the ACCB opens. From the result of the transient analysis above, it is evident that the process from the AC fault occurrence to the opening of the ACCB could cause the deliberate opening of the semiconductor switches in the AC/DC converter. Finally, in the LADS, the ACCB reclosing scheme is performed to identify whether the fault is temporary. During the reclosing process, an inrush current flows in the LADS whenever the ACCB recloses. In an AC temporary fault, as shown in Fig. 4, this inrush current flows completely into the AC/DC converter and could damage the anti-parallel diodes. Thus, a hybrid AC/DC LV distribution system cannot operate normally although the fault is identified as temporary. However, in a permanent fault, the anti-parallel diodes are not damaged by the inrush current because the inrush current flows along the fault path. However, the ACCB is locked out such that the whole hybrid AC/DC LV distribution system experiences power outage. Moreover, the LDDS cannot be operated normally even though the DG is interconnected with the LDDS. This is because an abnormal loop path due to the AC fault still exists and the LDDS is not isolated from the LADS. In conclusion, the LADS should be kept isolated from the AC/DC converter during the reclosing process and after the fault is identified as permanent.

This section first proposes the protection schemes for each distribution system by introducing an additional solid state circuit breaker (CBIT) before the AC/DC converter. This CBIT is introduced with two main purposes: (1) a backup protection for a DC fault in the case the DCCB is malfunctioned, (2) a protection of VSC and LDDS from AC fault and the operation of ACCB, other than resumption of LADS. Subsequently, the coordination for two proposed protection schemes is developed to prevent the proposed protection schemes from causing malfunction to each other. 3.1. Protection scheme for LDDS The protection scheme for the LDDS is proposed to improve the reliability by interrupting the DC fault rapidly. This scheme consists of the main protection with DCCB and a backup protection with CBIT; their schematic diagrams are shown in Fig. 5. The main protection protects the AC/DC converter as well as the LADS against a DC fault. Thus, the DCCB must interrupt the DC fault before tC to prevent the inflow of IL and IS. As the detection factor for the main protection, in this study, the current derivative (diDC/dt) is used because iDC dramatically increases within a few microseconds after a DC fault occurs. This factor can response under initial condition of the capacitive discharge current [38]. Moreover, this method can prevent the ACCB from malfunctioning because the required interruption time (tACCB) in the LADS is generally much longer than the corresponding time (tDCCB) in the LDDS. The following equations, (4) and (5), show the criterion to detect a DC fault and the relation among each time point, respectively. In (4), diset/dt is the setting value, determined to satisfy (5) in the given system conditions. (4)

diDC / dt > diset / dt Satisfied

IS

LADS iIT

CBIT Block

Unsatisfied

Vc

~ =

DCCB

= diDC/dt

=

DC load

=

DC load

Trip

(a) Main protection Trip

IS

CBIT

LADS iIT

Satisfied

Vc

~ =

DCCB IL

Failure

diDC/dt

=

Trip

(b) Backup protection Fig. 5. Schematic diagram of protection scheme for LDDS. 523

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Utility

(5)

tDCCB < tC ≪ tACCB

ACCB

For the backup protection, CBIT operates when the DCCB fails. As the detection factor, Vc is used because the rate of change of Vc is relatively less than that of iDC [39]. The criterion for detecting a DC fault is expressed by

Intertrip iAC

(6)

VC < Vset

This section proposes the protection scheme for the LADS. In this scheme, the main purpose is to isolate the LADS from the downstream of the hybrid AC/DC LV distribution system. For the AC/DC converter, the inflow of inrush current due to the ACCB reclosing scheme can be blocked through this scheme. In addition, from the LDDS perspective, the protection scheme could enable the DC loads to be soundly supplied by the DGs even though an AC fault occurs or the ACCB is opened. This type of protection scheme is realized by an intertripping scheme between the ACCB and CBIT as shown in Fig. 7. The intertripping scheme is the controlled tripping of a circuit breaker to complete the circuit isolation of a piece of apparatus in sympathy with the tripping of another circuit breaker [40]. In the basic intertripping scheme, the signal is transferred to the corresponding circuit breaker when a circuit breaker is tripped. In this study, a permissive intertripping method is used, in which whether the specific condition is satisfied is checked before the corresponding circuit breaker is tripped. This scheme generally takes approximately 1.5 ms in the distribution system [40]. The time is defined as tinter. When an AC fault occurs, the ACCB is tripped by the overcurrent relay according to the conventional time-current curve. Once the ACCB opens, the signal from the sending end (ACCB) is transferred to the receiving end (CBIT), which triggers the permissive intertripping scheme. To trip CBIT, the given specific criterion is checked whether it satisfies

In this protection scheme, CBIT must not operate when the DCCB interrupts the DC fault within tDCCB. However, Vc decreases gradually despite the timely operation of the DCCB, which results in the malfunction of CBIT. In this regard, additional criterion is applied to the backup protection besides (6), which is based on the current characteristic in the LADS under a DC fault. The CBIT therefore works with the assumption that the main protection fails, if both (6) and (8) are met simultaneously. (8)

Here, iIT is the RMS value of the current at the input terminal of the AC/DC converter. As the setting value for backup protection, iblock is set to be higher than the rated current in the LDDS. Finally, Fig. 6 illustrates the transient phenomena for the detection factors during the process from fault occurrence to the operation of the protection scheme. In Fig. 6, iset means the setting value for the ACCB. In addition, tdelay(#) is marked in the figure, which represents the time physically required to open a circuit breaker. Specifically, the sign, “*” means the delayed time by the inverse-time relay in the LADS. When a DC fault occurs, both iDC and iIT increase and Vc decreases. Thus, the DCCB can be opened according to (4) as the main protection (black solid line) and other criteria don’t affect to the protection system of LDDS. On the other hand, in case the DCCB cannot work (blue mixed – solid and double dotted – line), CBIT works as the backup protection according to the criteria (6) and (8) in the second and third graph.

Main protection

Fault Occurrence

iAC < iset − o

DCCB open diset/dt

iset − o < iAC < iset − c

tdelay(1) tDCCB

tCBIT

Vset tdelay(2) t0

tDCCB

tCBIT

tACCB iset

iIT

tdelay(3)*

iblock

Backup protection Fault Occurrence

DCCB failure

(10)

where iset_c is the setting value for reclosing CBIT, which is determined to be higher than the rating value in the LADS. Since iAC is measured at the input terminal of the LADS and CBIT is still opened, the setting value (iset_c) is decided based on the rating current for the AC loads. The delayed time is the same as that by opening the CBIT. If CBIT is reclosed following the ACCB, the starting current due to the AC/DC converter appears and could malfunction the protective devices in the hybrid AC/ DC LV distribution systems. Thus, in the proposed protection scheme, this starting current is ignored by introducing the dead time (tdead) after CBIT recloses. A series of the processes for the proposed scheme result in the change in iAC, as illustrated in Fig. 8 except the impact by the starting current. Additionally, the intertripping signal is included in Fig. 8. The expression of states (open and reclose) and time (t0–t4) in Fig. 8 are associated with the ACCB. When a AC fault occurs, iAC increases and the ACCB can be opened according to the specific time-current curve. Once ACCB works, the CBIT is intertripped according to (9). While conducting a reclosing scheme of ACCB, the CBIT is lock-out or closed according to

tACCB

Vc

(9)

where iset_o is the setting value for intertripping CBIT, which can be set to be much lower because the current does not flow after the ACCB is opened. In addition, some delay occurs in the process, which includes tinter and the time (trms) to calculate the RMS value of the AC current (iAC). Next, the sending end (ACCB) sends the signal continuously to inform that the ACCB state is changed during the reclosing of the ACCB. Once CBIT is opened by (9), CBIT is not closed until an AC fault is identified as temporary. That is, CBIT keeps open. The criterion where CBIT is reclosed is expressed by

diDC/dt

t0

Communication link

LDDS

Operation Criteria

3.2. Protection scheme for LADS

(7)

iIT > iblock

AC load

~ =

Fig. 7. Schematic diagram of protection scheme for LADS.

where Vset is the setting value. Vset is decided based on the required interruption time (tCBIT) of CBIT. To decide tCBIT, two important conditions should be considered. First, the primary purpose of CBIT is to prevent the ACCB from malfunctioning. The other is that CBIT must operate after the DCCB as a backup protection. That is, in the given system circumstances, Vset should be determined to satisfy (7).

tDCCB < tCBIT < tACCB

CBIT

CBIT open

Fig. 6. Detecting factor waveforms according to the protection scheme in LDDS. 524

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ACCB operation

Open p

Reclose

Fault Occurrence

Temporary Fault

Open p

Reclose

Open p

Table 2 States according to sequence of LDDS protection scheme.

Reclosing Fault Clearance

t0

State

t3

t2

t1

iset_c

iAC

S1 S7

iset_o Permanent Fault

t0

t1

t2

t3 Reclosing

t4

t0

t1

t2

t3

t4

Fault Occurrence

ACCB

Signal

Sequence of LDDS protection scheme

t4

SACCB

SIT(AC)

SIT(DC)

1 1

1 1

1 0

Normal state Backup protection: Must be ‘0’

S1 (111)

Open of ACCB / 1

Charge of CDC / 1

Transfer the signal & Check the condition for intertriping CBIT

CBIT

t0+trms+tinter

S2 (011)

t3+trms+tinter Standby(Open) Standby(Open)+Lockout

Fig. 8. Waveform of iAC and intertripping signal according to the protection scheme in LADS.

S3 (001)

(10). In the case of temporary fault (black solid line) in Fig. 8, rated iAC flows through the operation of ACCB and CBIT in order. On the other hand, in the case of permanent fault (blue mixed – solid and double dotted – line), the CBIT is lock-out.

S7 (110)

Discharge of CDC / 1

Intertrip CBIT / 0

CBIT operation

Discharge of CDC / 0

S4 (010) Intertrip CBIT / 0

Discharge of CDC / 0

Reopen of ACCB / 0

S5 (000)

Reclose of CBIT / 1

S6 (100)

Reclose of ACCB / 0

Completion of reclosing scheme / 0

Lock out

3.3. Protective coordination for hybrid AC/DC LV distribution system

Fig. 9. State diagram for controlling CBIT in protective coordination for hybrid LV distribution system.

The proposed protection schemes in Section 3.1 and 3.2 typically require the operation of CBIT. Two different signals could be transferred to CBIT such that the method to select the correct signal is necessary. For this purpose, three possible cases are preferentially discussed. The first case is when CBIT opens as a backup protection against a DC fault. When the DCCB fails to interrupt a DC fault current, the trip signal (SIT(DC) = 0) is transferred to CBIT. However, from the LADS perspective, the trip signal (SIT(AC) = 0) for CBIT cannot be transferred because the ACCB is still kept closed. That is, the signals for CBIT do not coincide with each other. Another case is the inconsistency of trip signals for CBIT at the fault interruption juncture. Once an AC fault occurs, CBIT is opened following the ACCB. Although both SIT(DC) and SIT(AC) with zero values are transferred to CBIT, the time delay could appear between them. Finally, when CBIT is reclosed after an AC fault is identified as temporary, the signal (SIT(AC) = 1) to reclose CBIT is transferred to CBIT through the intertripping scheme. However, the signal (SIT(DC) = 1) to reclose CBIT is not the output because CDC in the AC/DC converter cannot be charged until CBIT recloses. In this case, the inconsistency between two signals occurs as well. As the countermeasure for the inconsistency between the signals from the LADS and LDDS, in this study, the protective coordination to be controlled according to the combination of the signals (SACCB) for the ACCB, SIT(AC), and SIT(DC) is proposed. First, it presents the actual signal values according to the sequence of the proposed protection schemes through Tables 1 and 2. The values 1 and 0 in the two tables represent

the block signal and trip one, respectively. As shown in two tables, the combination of three signals (SACCB, SIT(AC), SIT(DC)) informs which value should be output and transferred to CBIT when SIT(AC) and SIT(DC) are inconsistent with each other. Although different values are required in the two protection schemes, in state S7, a clearly distinguished state transition is found: S6 to S7 in the LVAC protection scheme and S1 to S7 in the LVDC. Thus, the two proposed protection schemes can be coordinated through the logic expressed by the mealy state diagram in Fig. 9. For reference, it is marked “j/k” during each state transition, which denotes the event/output (sigCBIT). In Fig. 9, the state transition from S1 to S7 via other states describes the LVAC protection scheme. It includes two different state transitions from S2 to S5 because it cannot estimate the time that satisfies the corresponding criteria according to the fault conditions. In addition, the state transition from S5 to S6 and reverse are repeated until the reclosing scheme is completed or an AC fault is identified as temporary. If the reclosing scheme is completed, two protective devices (ACCB and CBIT) are locked out. Otherwise, the state is transited to S6 via S7. Nevertheless, a direct state transition from S1 to S7 illustrates the LVDC backup protection scheme. 4. Simulation and verification 4.1. Test system and simulation conditions

Table 1 States according to the sequence of LADS protection scheme. State

S1 S2 S3 S4 S5 S6 S7

Signal

In this section, a test hybrid AC/DC LV distribution system is modeled as the radial type, as shown in Fig. 10. In addition, a photovoltaic (PV) system is included in the LDDS to verify that the proposed protective schemes and their coordination work well even when interconnected to the DG. The detailed data of the test system and the setting values for the protective schemes are summarized in Table 3. The setting values are determined based on the criteria described in Section 3. Additionally, the reclosing scheme is assumed as a 2 fast 2 delay operation (2F2D) in this study, and its delayed time is arbitrarily determined to lessen the simulation time. In this study, only four representative cases are presented in the

Sequence of LADS protection scheme

SACCB

SIT(AC)

SIT(DC)

1 0 0 0 0 1 1

1 1 0 1 0 0 1

1 1 1 0 0 0 0

Normal state Open of ACCB by AC fault Intertrips CBIT: Must be ‘0’ Keeps CBIT: Must be ‘1’ Open of CBIT by intertripping scheme Reclose of ACCB Identification of temporary fault: Must be ‘1’

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AC grid 1.0

signal

Transformer (22.9/0.38kVAC, 60Hz)

LVAC bus

0.0

2km

1km

S1

ACCB

ACCB

S2 ~ S6

Point A

Fault 1200 occurrence

iAC [A]

AC load

1500

AC load

~

(200kW, 380VAC/1500VDC)

=

200kW

0 1000

Point B

600

800

200

400 -200 0.5

DC load

VC [V]

DCCB

= converter

PV

iAC(a) iAC(b) iAC(c)

2000 iIT(a) iDC 1600 VC 1200

LVDC bus

1km

DC/DC

Re-startup of AC/DC converter

300

DCCB

=

S1

600

iDC & iIT[A]

1km

Fault clearance

S7

900

CBIT

AC/DC converter

sigACCB sigDCCB sigCBIT

ACCB opens & recloses

(MV supply)

1.0

1.5

2.0

0 3.0

2.5

Time [s]

Fig. 10. Test hybrid AC/DC LV distribution system.

Fig. 11. Simulation results in case I. Table 3 Simulation conditions.

0.164 Ω/km 0.26 Ω/km

Load capacity

AC loads DC loads

50 kW for each point 100 kW

AC/DC converter

Ls CDC

1 mH 5000 μF

Photovoltaic system

Output voltage Output power

1500 VDC 50 kW

Setting values for protective schemes

diset/dt Vset iblock iset_o iset_c

30 kA/s 500 V 180 A 50 A 500 A

2F 2D

0.1 s 0.3 s

Delayed time in reclosing scheme

signal

Resistance Reactance

0.0 S2 ~ S6

Lock out

S5

1500

Fault 1200 occurrence

iAC(a) iAC(b) iAC(c)

900 600 300 0 1000 600

800

200

400 -200 0.5

Fault conditions

2000 iIT(a) iDC 1600 VC 1200

VC [V]

Table 4 Simulation cases. #

1.0

S1

iDC & iIT[A]

Distribution line constant

sigACCB sigDCCB sigCBIT

ACCB opens & recloses

Input values

iAC [A]

Parameter

1.0

1.5

Time [s]

2.0

2.5

0 3.0

Fault location

Fault type/duration

Fault resistance

Case I Case II

Point A

Ground fault/Temporary (0.5 s) 3Ф short circuit fault/Permanent

0.1 Ω 0.1 Ω

Fig. 12. Simulation results in case II.

Case III Case IV

Point B

Pole-to-Pole fault/Permanent Pole-to-Pole fault (DCCB failure)/ Permanent

10 Ω 0.1 Ω

resistances ranging from the bolted fault to 10 Ω. The only representative simulation results among them are included herein. 4.2. Simulation results and discussion

various fault conditions, which are arranged in Table 4. The first two cases (cases I and II) are conducted for the AC fault, which are subject to the verification that the proposed protection scheme works well regardless of the fault duration. The other two cases (cases III and IV) are performed for the DC fault, which focus on the main and backup protection against the DC fault. To verify the applicability and generality besides the performance of the proposed protective schemes, additional simulations are performed in terms of various fault types as well as fault

The simulation results are presented in Figs. 11–14. Each figure includes the signal for the circuit breakers (sigACCB, sigDCCB, and sigCBIT), the waveforms of the detection factors (iIT(Ф), diDC/dt, and Vc) and the currents (iDC, iAC(Ф)) in both the LVAC and LDDS as occasional demands. For cases I and II, it is verified that sigACCB and sigCBIT are normally output in the given fault conditions, as shown in Figs. 11 and 12, respectively. Both figures are discussed step by step as follows. 526

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step. (3-1) Finally, CBIT recloses (S1 via S7) or is locked out (Lock out via S5) according to the fault duration by intertripping with the ACCB. For case I, the AC fault is cleared at 1.5 s so that it is identified as a temporary fault through the following reclose of ACCB. And then, the CBIT is also reclosed. In Fig. 11, at this time (at about 2 s), a high starting current is observed after CBIT is reclosed (S1 via S7). In the proposed protective coordination, both the ACCB and CBIT are not tripped during this period because of the specific duration (tdead) that allows the overcurrent during the re-startup of the AC/DC converter. After its completion (at about 2.5 s), it can estimate that the hybrid AC/DC LV distribution system operates normally through the waveforms in the bottom-most figure in Fig. 11. (3-2) On the other hand, a high starting current is not appeared while the ACCB works it reclosing scheme until about 2.5 s. That is because CBIT is locked out as shown in Fig. 12 (Lock out via S5). In conclusion, the power by AC source can’t be supplied to both AC and DC loads. It is validated that the proposed protective schemes and their coordination works well regardless of the fault type through case II in Fig. 12.

sigACCB sigDCCB sigCBIT

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Next, for cases III and IV, it is also verified that both sigDCCB and sigCBIT are normally output in the given fault conditions through Figs. 13 and 14. In particular, the bottom-most figures in both figures include the waveform for the magnified range because the capacitive discharge current lasts for only a few milliseconds. Both cases correspond to the LDDS protection scheme under the DC fault, which are discussed in detail as follows. According to the well-controlled signals, in case III of Fig. 13, the DCCB opens immediately after a DC fault occurs (S1). Thus, the capacitive discharge current (iDC) in the third plot is interrupted within 1 ms after a DC fault occurs. Additionally, due to the rapid operation of DCCB, the voltage across a dc-link capacitor (VC) in the second plot decreases slowly. However, in Fig. 14, the CBIT opens as a backup protection when DCCB fails (S1 → S7). The state transition from S7 to S1 does not appear since the permanent fault is only represented herein. Since the CBIT operates at about 7 ms after a DC fault occurs, for case IV of Fig. 14, the VC in the second plot is fully discharged. The time difference between the main protection and backup protection can be observed. For both cases (case III and IV), it is confirmed the LADS can be normally operated through two figures (Figs. 13 and 14) even if the DC loads cannot be supplied by AC source. In addition, case III validates that the proposed protective schemes and their coordination are effective for a wide range of fault resistance. In conclusion, the proposed protection schemes can not only protect the distribution systems against their internal faults, but also prevents a distribution system from an instantaneous discontinuity of supply by fault occurrence or the operation of protective devices in the other distribution system when applying the proposed protective coordination to the hybrid AC/DC LV distribution system. That is, the reliability of the hybrid AC/DC LV distribution system can be improved by decreasing the power outage area.

0 1.020

1.015

Fig. 13. Simulation results in case III.

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500 0 0.990

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2500 iDC [A]

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VC [V]

iAC(a) iIT(a) VC

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400

50 1.015

0 1.020

Fig. 14. Simulation results in case IV.

(1) Both cases correspond to the LADS protection scheme under the AC fault. When the ACCB opens because of the AC fault (S1 → S2), CBIT opens as well after a delayed time (S2–S5). During this period (from 1 s to 1.1 s), the AC rms values (iAC(Ф), iIT(Ф)) in second/third plot increase. (2) Subsequently, the CBIT is kept open while the ACCB is conducting the reclosing scheme (S5 ↔ S6). While the ACCB is repeating ON/OFF (from 1.1 s to 1.5 s), all the measured detection factors (iIT(Ф), diDC/dt, and Vc) in third plot keep the value of zero. On the other hand, the AC rms values (iAC(Ф)) in second plot shows the change of magnitude according to the operation of the ACCB. In the both cases, the simulation results equal to each other until this

5. Conclusions This paper proposes the protective schemes in a hybrid AC/DC LV distribution system and verifies its performance using EMTP. To this end, the transient analysis for events in both the LVAC and LVDC distribution system is first conducted such as in the fault occurrence and protective devices. Based on the analysis results, it is conclude that the events in one distribution system can affect the other system as well as the corresponding distribution system. The mutual effects will disturb the normal power supply to customers and further damage the hybrid AC/DC distribution system. In particular, the AC/DC converter, which plays an important role as the interface between both distribution 527

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systems, could be easily damaged by the overcurrent. Thus, in this study, the protection schemes for each distribution system are preferentially developed, and subsequently coordinated based on the state diagram with a set of signals transferred to the protective device (CBIT). In this regard, the difference in state transition must be distinguished according to the mechanism of each protection scheme. This work achieves not only the protection of each distribution system, but also assures the normal operation even under fault conditions of other distribution system. Especially, the protective weak point of the existing protection schemes for DC distribution system, which cannot protect some transient phenomena due to the events in LVAC distribution system, can be solved in this study as well. Finally, it is concluded that the proposed protective schemes are effective in hybrid AC/DC LV distribution systems as indicated by the simulation results using the EMTP. The simulation results provide the theoretical validity for the proposed research results, but it could be further requested to check the applicability to the practical system. Thus, it will be planned to conduct the experimental measurements in the near future.

[16]

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Acknowledgment [24]

This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20184030202190) and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2015R1A2A1A10052459).

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