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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007
Overcurrent Protection on Voltage-Source-ConverterBased Multiterminal DC Distribution Systems
Mesut E. Baran, Senior Member, IEEE, and Nikhil R. Mahajan, Student Member, IEEE
Abstract—This paper proposes a protection scheme which utilizes modern voltage-source converters as fast-acting current-limiting circuit breakers. This paper investigates the main challenges
of detecting and localizing a fault, and interrupting it as quickly as
possible in a multiterminal dc system. A system protection scheme
consisting of smart relays associated with converters has been developed. The protection relays monitor local quantities to detect
and isolate disturbances/faults. It is shown that overcurrent-based
schemes can be adopted for these relays to meet the fast response
requirements. The effectiveness of the proposed protection scheme
is illustrated through simulations.
Index Terms—DC distribution, power-electronic converters,
protection.
NOMENCLATURE
CDCCB
ETO
PEBB
SES
VSC
Capacitor dc circuit breaker.
Emitter turnoff device.
Power-electronic building block.
Shipboard electrical system.
Voltage-source converter.
(CBs) and converter action [6]. Protection becomes more challenging for multiterminal dc lines, as dc CBs are needed to isolate the faulted section of the system. Recently, there have been
advances toward the development of dc CBs [7]–[10]. In a recent HVdc application [1], an insulated-gate bipolar transistor
(IGBT)-based dc CB was utilized.
Furthermore, the new semiconductor devices used in the
new converters have the capability to limit and interrupt fault
currents [8]. Therefore, it is possible to integrate fault-current
handling capability within the new converters and have them
behave similar to fast-acting current limiting CBs (i.e., limit and
interrupt the fault current [11]). With this new functionality for
converters, a new challenge emerges, that the protection scheme
should also be able to detect and locate faults more quickly. To
achieve this goal, this paper proposes a new protection system
using relays embedded into the converters. The application
considered is power distribution on a ship. It is shown that by
adopting overcurrent-based schemes for these relays, this type
of system can be protected against faults on the dc lines. It is
also shown that these converter-based relays can detect and
isolate faults quite fast, on the order of a few milliseconds.
II. DC SYSTEM PROTECTION
I. INTRODUCTION
T
HE emergence of voltage-source converters (VSCs) that
use self turnoff power-electronic devices makes dc distribution an attractive alternative for medium- and low-voltage
distribution in special applications. Present day applications include dc ties between two systems at medium-voltage levels [1],
[2], and dc distribution for industrial parks, space, and shipboard
electrical systems (SESs) [3], [4].
One of the main limitations of present day VSCs is that their
fault current withstand is much lower than that of thyristorbased converters, typically twice the nominal current rating of
the converter [5]. Hence, faults on a dc line fed by the VSCs
must be limited and interrupted much faster than those on a conventional HVdc system.
Protection of the dc lines requires special consideration. For
a simple two-terminal dc line with two converters, protection is
usually achieved by a combination of ac-side circuit breakers
Manuscript received May 25, 2005; revised January 24, 2006. This work was
supported by the Office of Naval Research (ONR) under Award N000014-00-10475. Paper no. TPWRD-00315-2005.
The authors are with the Department of Electrical and Computer Engineering,
North Carolina State University, Raleigh, NC 27695 USA (e-mail: baran@ncsu.
edu; nikhilmahajan@ieee.org).
Digital Object Identifier 10.1109/TPWRD.2006.877086
The basic task of the protection scheme proposed here is the
same as any other protection scheme—detect and locate any disturbance that can occur on the system and isolate the affected
area quickly. The prototype system considered for this study is
shown in Fig. 1. It is a revised version of the system considered
by the U.S. Navy for SES [12], [13], and it represents a generic
multiterminal dc distribution system. The system has two ac
sources (generators) feeding the dc line through rectifier converters. The boost-type voltage-controlled bridge rectifiers are
rated at 4 MW each, and they convert the 4160-V, three-phase
ac voltage to 7 kV dc. In this system, the dc line is a short cable
(100 m). Sectionalizers have been placed on the middle of the
line so that only one side of the line would be out of service
during a fault on the dc line. The loads are supplied through the
dc buck converters or inverters.
For protection of such a system, relays and protection devices
should be placed on the system to detect faults, interrupt fault
current, and isolate the faulted part of the system. These design
issues are addressed in the following subsections.
A. Protection Devices
To provide protection for system components (generators,
converters, and loads), the conventional approach would be to
put CBs at the input and output of the converters in order to facilitate unit protection for each device.
0885-8977/$20.00 © 2006 IEEE
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BARAN AND MAHAJAN: VSC-BASED MULTITERMINAL DC DISTRIBUTION SYSTEMS
Fig. 1. Prototype dc distribution system with CBs.
The alternative approach, which has been considered here,
is to use the converters themselves for fault current interruption. In [11], for example, it has been shown that VSCs used
for dc distribution can limit fault current and interrupt it and,
hence, they can be used as current-limiting CBs. Furthermore,
converters have overcurrent protection on the power semiconductor switches to protect the switches when the current through
a switch gets close to its limit. Therefore, this switch level protection employed on the converter can serve as a backup overcurrent protection relay.
Hence, in this study, the CBs are assumed to be embedded
within the converters. As a result, the protection zones defined
by them will not be device based, and Fig. 2 illustrates the main
zones defined by embedded CBs on the prototype dc system.
Note that there will be four main zones.
1) DC zone: The dc line supplies power to all of the loads
and, therefore, it is the most critical component for protection. It is also the one that is exposed the most to the
faults/damage. Note that since the switches in the converters will be conducting the fault interruption, the protection zone is defined by the switches of the converters
that are connected to the line side of the rectifiers, inverters, and the dc converters. The zone, therefore, includes not only the dc line but also the dc rails of the converters connected to the dc line, as the figure illustrates.
Note that to protect the dc line itself by using conventional schemes; we would need a CB at every connection point, as illustrated in Fig. 1. The proposed scheme
eliminates these extra CBs.
2) Rectifier ac side zone: A protection device is needed
at the source side terminals of the rectifier to provide
protection against faults on the input filter elements of
the rectifier, as these elements are upstream of rectifier
switches. Fig. 2 illustrates this small zone. In terrestrial
applications, the protection device at the rectifier input
would usually be an ac CB. However, for the prototype
system considered, fuses have been chosen instead of
CBs, as Fig. 2 illustrates, mainly since the fault in this
protection zone will be permanent rather than temporary. Thus, there is no need for a fast reclosing capability
that the CB can provide. Also, the compactness of fuses
is an important advantage for the shipboard systems.
3) Inverter load-side zone: The inverters are tapped off the
dc line to supply power to the ac loads. Since the inverter can provide overcurrent protection for the loads
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it serves [14]–[17], no protection device is needed at the
load side. There is no need for a CB on the source side either, since the inverter does not feed a fault on the source
side assuming a nonregenerative load (except the input
filter capacitor).
4) DC converter load side zone: Similar to the inverter, the
dc–dc buck converter, when properly designed, can provide overcurrent protection on the load side. Furthermore, if the dc–dc converter is of the two stage type, as
shown in Fig. 10, the input side switches can also provide fault isolation on the source side. Hence, with this
functionality, there is no need for a protection device at
either terminal of the dc-dc converter.
Capacitor Protection: When a fault occurs on the dc line, the
capacitors connected to the dc line (such as the filter capacitors
at rectifier output, and inverter and the dc–dc converter inputs)
start to discharge with a very short time constant and contribute
to high fault currents. Therefore, the capacitors connected to the
dc buses of converters demand special attention in the form of
their own protection. Protection of the capacitors is typically
done at a hardware level by way of snubbers [17] which limit
the rate of discharge current. The snubbers, however, do not interrupt fault current. To limit and interrupt the discharge current during a fault, a protection device is needed. For the prototype application, a CDCCB has been employed [8] as shown in
Fig. 3.
Without the snubber, the time constant of the capacitor discharge current will be around 10 s. Hence, the CDCCB should
also be very fast in order to effectively protect the capacitor from
extreme stresses and destruction [16], [17]. In [5] and [8], it is
shown that indeed an ETO-based CDCCB can be used to turn
off and interrupt fault current in less than 10 s, thereby meeting
the requirement set forth.
The operating principle of the CDCCB is based on the inherent current sensing of the ETO [5]. The measured current is
compared to a 2.1-kA threshold (maximum limit of the DCCB
kA). When the capacitor current crosses this threshold, a
hard turnoff is initiated which limits the current from further increase and interrupts the current in 3–7 s, thus protecting the
capacitor satisfactorily. Fig. 4 illustrates the capacitor discharge
current limiting using the CDCCB. This case corresponds to
the discharge of the rectifier output capacitor on the prototype
system in Fig. 2, and the simulation is done using EMTDC softs
ware [19], [20]. As the figure illustrates, the fault at
causes the capacitor discharge current to rise very rapidly, and it
crosses the threshold level of 2 kA in about 4 s, at which time,
the CDCCB interrupts the fault current. As the figure illustrates,
this is a very effective method of discharge current limiting. Its
use is warranted here for two reasons: 1) to limit the amount of
energy to be dumped by the capacitor to the dc cable and 2) to
limit the discharge currents that will flow through the converter
switches following a fault since if they are not limited, the high
discharge currents can damage the switches.
B. Fault Monitoring and Protection
For detecting faults and operating appropriate protective devices, a protection system consisting of relays integrated with
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007
Fig. 2. Zones for the prototype dc system without separate CBs.
Fig. 3. CDCCB protection.
each VSC has been considered. The goal here is to provide autonomy to the relays so that they can take fast action. The challenge to be addressed here is to provide enough intelligence for
the relays to ensure that they will make correct decisions. The
details about these relays embedded in the rectifier, buck converter, and the inverter are given below.
1) Rectifier Relay: The faults on the dc line are the most severe faults on the system. In the prototype application, the primary concern is the bus fault (short-circuiting of the two terminals of the dc line) since it will cause high fault currents from
the rectifiers.
The smoothing capacitors at the output terminals of the rectifier will react to a dc bus fault first and start discharging very
fast, as the bus fault will cause the dc bus voltage to collapse
very fast. As pointed out in the previous section, that is why the
capacitors need a very fast acting device, such as CDCCB, to
limit and interrupt this high discharge current.
A typical bridge-type boost rectifier (with antiparallel diodes)
will also feed high current to the fault, as the rectifier will act like
a diode bridge once the dc terminal voltage collapses. Hence, the
rectifier diodes need to be replaced with turnoff devices in order
to be able to interrupt the current before it gets too high [11].
To detect the faults on the dc side of the rectifier, the relay
monitors the dc output current I_R shown in Fig. 5 and uses an
Fig. 4. Capacitor discharge current during a fault and its interruption by
CDCCB. (a) Capacitor discharge. (b) Zoom-in of (a).
overcurrent protection scheme: if I_R passes a threshold value
and stays above it for a certain amount of time, it assumes that
there is a fault on the dc side of the rectifier. To improve the security of this scheme, the relay also monitors the dc bus voltage,
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BARAN AND MAHAJAN: VSC-BASED MULTITERMINAL DC DISTRIBUTION SYSTEMS
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Fig. 7. Fuse operation.
Fig. 5. Zones around the rectifier.
Fig. 6. Fault current limiting by the rectifier. (a) Generator currents. (b) Rectifier current.
and checks whether the voltage drops below 80% of its rated
value. When both the overcurrent and low-voltage conditions
are satisfied, the relay sends a trip signal to the rectifier to interrupt the current and isolate the rectifier from the dc line.
Simulation results for a dc bus fault are shown in Fig. 6. Fol, the bus capacitor discharges into
lowing the fault at
the fault with a very short time constant as seen in Fig. 4, and
the CDCCB limits and interrupts the discharge current in less
than 10 s. Meanwhile, the generator also starts contributing to
the fault current as seen by the I_A, I_B, I_C, and I_R in Fig. 6.
s,
When I_R exceeds the threshold of 1.75 kA at
the relay detects and identifies a primary dc-bus fault as seen in
Fig. 6(b), as by this time, the bus voltage has already collapsed.
The relay then sends a trip signal to turn off the rectifier switches
in order to interrupt the fault current.
Note that the fault current here is limited and brought down
to zero by the rectifier switches in a controlled manner (within
20 s) in order to limit the transient recovery voltages across the
switches to an acceptable level (see [11] for details).
The relay also monitors the input ac line currents I_A, I_B, or
I_C, to decide whether the fault is on the dc or ac side. If the fault
is on the ac side, the output current I_R will not rise. Hence, by
monitoring ac-side currents, the relay detects ac-side faults and
turns off the rectifier switches in order to isolate the fault. Note
that this action does not interrupt the ac-side fault current. The
ac-side faults are interrupted by the fuse, as illustrated below.
Rectifier Fuse: As pointed out above, fuses at the source side
of rectifiers are needed to protect the rectifier ac zone shown
in Fig. 5. The fuse provides the primary protection for the L–L
faults and the three-phase faults on the ac source side of the rectifier. The fuse will not operate for ground faults, as the system
is high resistance grounded. The fuse also provides backup protection for the rectifier relay.
The fuse should be fast enough so that the upstream protection does not trip before the fuse blows, and it should be slow
enough so that the downstream protection has enough time to
operate. Since the downstream protection in this case is much
faster than the operation times of a fuse, coordination of the
fuse with the downstream relay of the rectifier PEBB is not a
problem. For the prototype system, the current profiles are: 1)
the nominal line current is about 450 A (rms), 2) the startup
current is about 1.5 kA (rms) for about 5–10 ms (due to initial
capacitor charging via rectifier PEBB), and 3) the fault current
level is about 10–30 kA (depending on the fault being after/before the source inductor). Hence, a 500E-rated fuse is appropriate for this application, and the EJO-1 type 9F62 fuse from
GE has been considered for the prototype system.
To demonstrate the fuse operation, a fuse model was developed for EMTDC/PSCAD to represent the fuse melting and
clearing. A phase A–B fault at the rectifier input terminals has
been simulated, and the results are shown in Fig. 7. As the figure
s, the line currents
shows, following the fault at
I_A and I_B start increasing and they are limited only by the
, the fuse clears as it has
source impedance. At
dissipated enough energy for clearing, and at
s, at the
first zero crossing of I_A and I_B, the fault currents have been
interrupted. Hence, the fuse interrupts the current in about half
a cycle (about 10.4 ms), as desired.
2) Inverter Relay: Commonly used three-phase bridge-type
inverters have been considered here for the prototype system.
These inverters can be used to provide overcurrent protection
for the load-side faults [11], [14]–[17] with minimal revisions.
This is because, unlike the rectifier, the antiparallel diodes of the
inverter do not freewheel the current during a load-side fault.
Fig. 8 shows the zones of protection associated with the inverter. The inverter relay monitors the input and output currents
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007
Fig. 8. Protection zones around the inverter.
, and , , , to detect and localize the faults. Note that
the relay is responsible mainly for the faults in the load zone,
as the upstream faults in the source zone are interrupted by the
rectifiers.
To illustrate the fault current interruption by the inverter, an
overcurrent scheme is considered (although other more secure
schemes are possible [14]–[17]). The inverter relay monitors
the input current , and when the current passes the threshold
limit, the relay assumes the fault. At the same time, if any of the
output currents pass the corresponding threshold value, the fault
is localized as the load-side fault.
Note that since the system is high resistance grounded, the
first ground faults are not disruptive and, hence, only the line
and three-phase faults are considered for the prototype system.
Simulation results for a three-phase fault at the load terminals
of the inverter are given in Fig. 9. The three-phase fault causes
the output voltage to collapse and, correspondingly, the current
starts to increase. When the input current crosses the threshold
of 2.5 kA and stays above the threshold, the relay detects the
fault and turns off the switches of the inverter to interrupt and
isolate the fault. Note that the current decreases slowly to zero,
rather than being chopped when the switches are turned off.
This is due to the fact that the antiparallel diodes provide a freewheeling path for the load current after the turnoff. As a result,
there is very little transient recovery voltage on the switches.
Note also that monitoring the output currents helps the relay to
localize the fault (i.e., identify whether the fault is downstream
or internal to the inverter).
Note that the load considered here is passive. If there is a
motor load that can regenerate, a diode on the input dc bus is
needed to prevent reverse current feed to a source-side fault.
3) Buck Converter Relay: dc–dc buck converters can also
perform fault-current interruption when properly designed [11].
A buck converter with an isolation transformer provides enough
flexibility for this purpose, and it is considered for the prototype
application. The converter in Fig. 10 uses an isolation transformer with a 5:1 turns ratio to help buck the primary 7-kV dc
voltage to 800 V dc. Note that the switches do not use antiparallel diodes, and this facilitates fault-current interruption and
isolation by turning off all of the switches as will be illustrated
here.
The protection relay for the buck converter is mainly responsible for detecting and interrupting the faults in the load-side
Fig. 9. Fault current limiting by the inverter. (a) Inverter current profile during
a fault. (b) Output voltages.
Fig. 10. Zones around buck converter.
zone as Fig. 10 illustrates. The protection scheme considered for
this relay is similar to that of the inverter. The relay monitors
the input current , output current , as well as transformer
currents
and
. The relay detects the existence of a
crosses a threshold (0.5 kA). The relay localizes
fault when
the fault to the load side when the output current crosses the
.
threshold (2.5 kA) along with a simultaneous increase in
The simulation results are shown in Fig. 11 for a bus fault at
. The fault causes the curthe load-side terminal at
to increase as shown in Fig. 11(a). At
,
rent
crosses the threshold of 0.5
the relay detects the fault as
kA. Also, at
, the currents and
cross the
threshold of 2.5 kA as shown in Fig. 11(b). The relay thus identifies the fault as the load-side bus fault. The protective action
taken by the relay is to turn off the converter switches.
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BARAN AND MAHAJAN: VSC-BASED MULTITERMINAL DC DISTRIBUTION SYSTEMS
Fig. 11. Fault current limiting by a dc–dc buck converter. (a) Secondary dc bus
fault detection. (b) Secondary dc bus fault locating. (c) Diode freewheeling.
The action of hard turnoff of the switches interrupts the current from the primary side immediately, but the current on the
load side does not cease immediately. The energy stored in the
output inductor is freewheeled through the diodes of the rectifier stage. When this energy is completely dissipated, the fault is
completely interrupted. The freewheeling of the diodes is seen
in Fig. 11(c) from onwards. This freewheeling of the diodes
is important in preventing any excessive transient recovery voltages on the downstream devices [11].
C. Protection Coordination and Backup
For the proposed protection scheme to provide complete
system protection, the relays should provide: 1) proper coordination and 2) backup when a protection relay fails.
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Coordination: Coordination between relays is needed to provide selectivity (i.e., only the protective device closest to a fault
should operate to isolate the faulted part of the system [6]). On
the prototype system, for example, coordination is needed between the rectifier relay and fuses, between the two rectifier relays, and the rectifier relays and the load converter (inverter and
buck converter) relays. This coordination can be achieved by
adopting the same techniques used for coordinating overcurrent
protection devices as described in the following.
• Coordination Between Rectifier Relay and Fuse
As shown earlier, coordination between the rectifier relay
and the fuse is achieved by the proper selection of the fuse.
The fast action fuse selected for the rectifier takes 10–100
ms (depending on fault current) to interrupt and isolate the
ac zone faults, whereas the relay detects and interrupts the
dc zone faults in about 0.5 ms. Therefore, the relay coordinates with the fuse by detecting and interrupting faults in
its protection zone much faster than the source-side fuse.
• Coordination Between Rectifier Relays
Since multiple rectifiers feed the dc line, when there is a
fault on the ac side of a rectifier, the other rectifiers will
feed the fault also, as the rectifier switches can carry the
bidirectional currents. Hence, coordination between rectifier relays is needed so that when a fault is in the ac zone
of one of the rectifiers, only the relay of the corresponding
rectifier interrupts the fault.
For this coordination, we propose to employ an interface
diode on the dc side of the rectifier shown in Fig. 5. This
diode will prevent the reverse dc current flow through the
rectifier and, thus, it will eliminate the need for coordination among the rectifier relays.
• Coordination Between Rectifier and Buck-Converter Relays
The rectifier relay needs to coordinate with the relays of
the load converters (buck converters and inverters). The
coordination is facilitated by the fact that the rated current
of load converters is much lower than that of the rectifiers.
For the prototype SES, for example, the normal current of
the buck converter is 200 A and, therefore, the threshold for
its relay is set to 500 A (2.5x). Since the threshold of the
rectifier relay is 1.75 kA, the big difference in thresholds
provides sufficient margin for coordination between these
two relays.
Backup: On the prototype system, local backup protection
and the remote backup protection provide protection when the
primary protection fails. Remote backup is the main backup
scheme, and it is based on the fact that power flows from the
source (generators) toward the loads. Hence, the buck converter
relay provides backup for the inverter relay, and the rectifier
relay provides backup for the buck converter relay. Finally, the
fuse provides backup for the rectifier, as previously pointed out.
The overcurrent protection used for the switches themselves
on the converters provides also a “local backup” feature which is
unique to the dc system considered. This local protection can be
considered as a backup relay when the primary relay associated
with the converter fails. For example, if a fault occurs on the dc
line of Fig. 2, the rectifier relay provides primary protection. But
if the relay fails, the switch level protection on the rectifier then
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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007
provides local backup for the relay by turning off the rectifier
switches when the current through the switches approaches their
limits.
[9] J. Zyborski, T. Lipski, J. Czucha, and S. Hasan, “Hybrid arcless lowvoltage ac/dc current limiting interrupting device,” IEEE Trans. Power
Del., vol. 15, no. 4, pp. 1182–1187, Oct. 2000.
[10] P. M. McEwan and S. B. Tennakoon, “A two-stage dc thyristor circuit
breaker,” IEEE Trans. Power Electron., vol. 12, no. 4, pp. 597–607, Jul.
1997.
[11] N. R. Mahajan, “System protection for PEBB based dc distribution systems,” Ph.D. dissertation, Dept. Elect. Comput. Eng., North Carolina
State Univ., Raleigh, NC, Nov. 2004.
[12] K. L. Butler, N. D. R. Sarma, C. Whitcomb, H. Do Carmo, and H.
Zhang, “Shipboard systems deploy automated protection,” IEEE
Comput. Appl. Power, vol. 11, no. 2, pp. 31–36, Apr, 1998.
[13] J. G. Ciezki and R. W. Ashton, “Selection and stability issues associated with a navy shipboard dc zonal electric distribution system,” IEEE
Trans. Power Del., vol. 15, no. 2, pp. 665–669, Apr. 2000.
[14] D. Kastha and B. K. Bose, “Investigation of fault modes of voltage-fed
inverter system for induction motor drive,” IEEE Trans. Ind. Appl., vol.
30, no. 4, pp. 1028–1038, Jul./Aug. 1994.
[15] A. Ghosh and G. Ledwich, Power Quality Enhancement Using Custom
Power Devices. New York: Springer, 2002.
[16] R. Peuget, S. Courtine, and J.-P. Rognon, “Fault detection and isolation on a PWM inverter by knowledge-based model,” IEEE Trans. Ind.
Appl., vol. 34, no. 6, pp. 1318–1326, Nov./Dec. 1998.
[17] F. Blaabjerg and J. K. Pedersen, “A new low-cost, fully fault-protected
PWM-VSI inverter with true phase-current information,” IEEE Trans.
Power Electron., vol. 12, no. 1, pp. 187–197, Jan. 1997.
[18] E.-C. Nho, I.-D. Kim, and T. A. Lipo, “A new boost type rectifier for a
DC power supply with frequent output short circuit,” presented at the
Industry Applications Conf., Phoenix, AZ, 1999.
[19] Manitoba HVDC Research Centre, “EMTDC, The electromagnetic
transients & controls simulation engine”. Winnipeg, MB, Canada,
2002.
[20] Manitoba HVDC Research Centre, “PSCAD, power systems computer
aided design” . Winnipeg, MB, Canada, 2003.
[21] M. Baran and N. Mahajan, “System reconfiguration on shipboard DC
zonal electrical system,” in Proc. IEEE Electric Ship Technologies
Symp., Philadelphia, PA, Jul. 2004.
III. CONCLUSION
This paper illustrates that protection of a multiterminal dc distribution system can be simplified by utilizing the power-electronic converters’ ability to act as fault current-limiting CBs. It is
shown that by associating relays with the converters—rectifier,
inverter, and dc–dc buck converter—and by adopting overcurrent-based protection schemes for these relays, the faults on the
system can be detected and localized very fast. The simulation
results on the prototype system illustrate that these relays can
quickly detect and localize the faults within a few milliseconds.
The relays in the proposed scheme are versatile in that they can
perform both unit protection and overcurrent protection functions.
One challenge of the protection of dc systems is locating a
fault on the dc line. This is especially the case for the systems
where the dc link is rather short and ungrounded. The overcurrent-based protection schemes are thus able to localize the
faults only to the zones upstream/downstream of the converters.
This paper has focused on fault interruption and isolation. The
use of sectionalizers to minimize the interruption is the second
stage—the reconfiguration stage—of fault management, and it
has been investigated in [21].
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Mesut E. Baran (S’87–M’88–SM’05) received
the Ph.D. degree from the University of California,
Berkeley, in 1988.
Currently, he is an Associate Professor at North
Carolina State University, Raleigh. His research interests include the analysis and control of distribution
and transmission systems.
Nikhil R. Mahajan (S’01) is currently pursuing the
Ph.D. degree in electrical engineering at North Carolina State University, Raleigh.
His research focuses on the protection of powerelectronic building blocks. His research interests are
in the areas of power system protection and transmission and computer-aided system analysis.
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