Solid-State Breaker Protection in MVDC Systems

Keith Corzine Allan Overstreet Tom Baragona, P.E.

Dept. of Electrical Engineering Dept. of Elec. and Comp. Eng. Huntington Ingalls Industries
UC Santa Cruz Clemson University Newport News Shipbuilding
Santa Cruz, CA USA Clemson, SC USA Newport News, VA USA
Keith@Corzine.net Allan@Overst.net Tom.Baragona@hii-nns.com

Abstract - The development of medium-voltage DC power

distribution for future Naval ships brings questions about the
protection methods to be used in various places within the
architecture. This paper starts with overlaying the current-time
curve of a solid-state DC breaker with a traditional DC breaker.
Using both methods can address slowly encroaching faults and
rapidly occurring faults. A medium-voltage DC system
demonstrator is described which includes solid-state breaker
protection. The system is tested in the laboratory where it is
demonstrated that a hard fault can be isolated from a controlled
rectifier by the solid-state breaker within the ultra-fast time period
(sub-millisecond). With this system, faults of various time scales
(slow, fast, and ultra-fast) can be addressed using traditional
breakers, power electronic controls, and solid-state breakers.

Keywords –Z-Source Breaker; Inhibit;

Introduction
The U.S. Navy is looking toward medium-voltage DC Figure 1. Time-current curves of a traditional and SS breakers.
(MVDC) for future shipboard power distribution [1]-[5]. This
system can reduce size and weight due to factors such as easy I. MVDC DEMONSTRATOR
integration of high-speed generators and other equipment. With
some protection methods in the experimental stage, the question Newport News Shipbuilding (NNS) constructed a MVDC
arises of which protection methods to use at various places in demonstrator in the Virginia Advanced Shipbuilding and Carrier
the electrical system. For slowly encroaching faults, power Integration Center (VASCIC), Newport News, Virginia, in order
to evaluate both normal and faulted operation of a DC power
electronic controls or traditional breakers may be appropriate.
distribution system. A one-line diagram of the MVDC
For rapidly occurring faults, control of power electronic
Demonstrator is shown in Figure 2.
components may work well. For ultra-fast faults, solid-state
breakers can act quickly enough. Figure 1 shows the current- The MVDC Demonstrator consists of port and starboard
time curve for a traditional "DC breaker". This gives a range of rectifier modules, DC-to-DC converters used to supply a vital
time that the breaker will open depending on fault current load through auctioneering diodes, and motors supplied through
relative to rated value (1 pu). Often times, the traditional breaker port and starboard variable frequency drives (VFDs). A resistive
is combined with a solid-state solution as illustrated in Figure 1. load bank is used to provide incremental load to the system. The
Thus, large over-currents are handled almost instantaneously. components of the MVDC Demonstrator are connected using
Furthermore, the solid-state curve in Figure 1 can be adjusted appropriately sized cable and non-load breaking, DC contactors.
upwards or downwards using controller settings or breaker
design. This paper addresses the solid-state breaker in a ship Each rectifier module consists of a phase controlled, six-
power system through the use of an MVDC demonstrator. The pulse rectifier, three-phase firing circuit, voltage and current
demonstrator operates at low-voltage, but includes the regulator circuits, snubber assemblies, and fast acting
interconnection of multiple sources and loads. semiconductor fuses. The rectifier modules are used to convert
440 VAC site power to 600 VDC power to supply downstream
loads. Each rectifier module is rated for 225 A at 600 VDC.
The resistive load bank is rated for continuous duty and can
be loaded to 180 kW, 300 A at 600 VDC, and allows for
adjustable loading of the DC bus. The non-load breaking, DC
contactors are single-pole, single throw, normally open switches
that are operated in pairs and have a continuous rating of 350 A
at 800 VDC. Each contactor has auxiliary contacts used to

978-1-5090-4944-8/17/$31.00 ©2017 IEEE 414

 DC AC
Z-SOURCE
M
BREAKER
AC DC

DC

DC

DC LOAD AC
VITAL
BANK LOAD
DC

DC

DC

DC AC
M
AC DC

Figure 2. One line diagram of the MVDC demonstrator.

provide position information to the control system. The DC-to- The SCRs in the rectifier module can be used to isolate
DC converters are rated at 2 kW and are used to convert the 600 downstream faults by inhibiting the rectifier SCR gating pulses
VDC input provided by the port and starboard DC busses to after a fault is detected. A low impedance fault, such as a line-
supply a vital load through auctioneering diodes and an inverter. to-line fault on the DC bus, can be detected by the rapid rise of
Motor loads are supplied through port and starboard VFDs. rectifier output current and/or the drop of rectifier output
voltage. Once a low impedance fault is detected, the inhibit
One of the challenges to be addressed for the MVDC signal will commutate off the SCRs of the rectifier within one
distribution system is protection against faults on the MVDC half cycle, or 8 ms, for a 60 Hz system. While this is faster than
bus. MVDC protection devices are not commercially available a typical electromechanical circuit breaker it will still allow high
at the voltages levels envisioned for future Navy electrical values of peak current to flow to the point of fault. A solid-state
distribution systems [6]. Commercially available breaker, with opening action in the microsecond range was
electromechanical DC switchgear is available up to about 3600 investigated to improve the DC fault response time into the ultra-
VDC. An MVDC circuit breaker will need to be developed to fast region [5].
meet the Navy’s requirement for a MVDC electrical distribution
system in the 6-30 kV range. Newport News Shipbuilding, in conjunction with Clemson
University, developed a Z-source solid-state breaker for
Electromechanical air circuit breakers are not suitable for installation in the MVDC demonstrator. The Z-source breaker is
protection of MVDC systems. In a DC system, there are no rated for 600VDC, 130A. It was installed on the output of the
natural current zero crossings to enable arc extinction. While port rectifier module of the MVDC Demonstrator. The z-source
there are several techniques to force a current to zero in low breaker is used to improve the low-impedance fault response of
voltage DC applications, it is difficult to scale these methods for the phase controlled rectifier.
operation at the kilovolt level and will require the growth of arc
chute size. This will increase switchgear size and result in A schematic diagram of the z-source breaker is shown in
weight and arrangement impacts to the ship design. In addition Figure 3. The Z-source breaker is a unique form of solid-state
to the weight and arrangement impacts, electromechanical breaker that automatically responds to system faults without the
circuit breakers are relatively slow and take several milliseconds use of external sensors or control circuitry [7]. This is
(3-5 cycles or 50-83 ms for a 60 Hz system) to operate accomplished using LC circuits first used as part of a novel type
depending on the time needed to detect a fault, contact opening of inverter input circuit [8].
time, and time for the arc to extinguish. In a faulted DC system
the fault current rises to a higher value, compared to an AC
system, since it is limited only by the resistance of the circuit
and there are no current zero crossings. The DC fault current rate
of rise is dependent upon the system time constant. Since the
inductance is low for shipboard distribution systems, the rate of
rise for DC fault current is fast, requiring fast protection. MVDC
systems should have fast opening protection devices to prevent
the growth of large fault currents and corresponding bus voltage
droop.
Figure 3. Z-source solid-state breaker topology.

415
 The Z-source breaker interrupts fault current by reverse
biasing the SCR switching element without arcing and does not
require a zero current crossing. The fault is cleared in micro-
seconds by the operation of the z-source circuit. The source and
SCR switching element will not experience the fault current.
The Z-source breaker provides rapid response to low impedance,
large transient faults. Its ultra-fast opening time ensures that
fault currents can be interrupted in microseconds (compared to
one-half cycle or 8 ms) for the phase controlled rectifier which
prevents the growth of large fault currents and corresponding
MVDC bus voltage drop.
Simulations of the MVDC Demonstrator were performed for
a line-to-line fault on the port and starboard DC buses as shown
in Figure 4. The fault downstream of the starboard rectifier
module was isolated by inhibiting the rectifier module SCR gate
pulses which isolated the fault in 8 ms. The z-source breaker was
installed on the output of the port rectifier downstream of the
port rectifier module. The fault downstream of the port rectifier
module/z-source breaker isolated the fault in less than 0.4 ms,
twenty times faster than the SCRs of the starboard rectifier
module.
Figure 5 shows the simulation results of DC bus line-to-line
fault current, both with and without the z-source breaker using
the same scale. A close-up view of the simulation results of the
DC bus line-to-line fault current using the z-source breaker to
improve low-impedance fault response is provided in Figure 6 Figure 5. DC demonstrator simulation without (top) and with
and shows that the response time is 375 µs. (bottom) the Z-source breaker.

Figure 4. Simulation of the MVDC demonstrator with and without the Z-source breaker.

416
 Figure 6. DC bus fault current with the Z-source breaker.

In order to verify the simulation results of the DC bus using

the z-source breaker, the MVDC Demonstrator was modified as
shown in Figure 7. A non-load breaking, DC contactor was
connected line-to-line across the DC bus to initiate the fault. The
DC contactor has a contact resistance of 0.4 mohms and was Figure 8. Measured DC bus fault current with the Z-source
used to simulate a low impedance fault. The DC contactor was breaker.
operated from the control system. The measured result of an
actual DC line-to-line fault is shown in Figure 8. II. PROPOSED PROTECTION SYSTEM
A DC Bus protection scheme utilizing the Z-source breaker
can provide faster response time to a low impedance, line-to-line
DC bus fault compared to relying on AC side electromechanical
DC
Z-SOURCE
BREAKER
circuit breakers or the half-cycle commutation time required to
AC turn off SCRs in a rectifier module. In the proposed DC bus
protection scheme, a Z-source breaker is connected to the output
of a phase controlled rectifier and a non-load breaking, DC
150 kVA, Transformer
480:440 VAC, 3Ø, Delta- contactor is connected to the output of the z-source breaker to
Delta, Z=3.9%
interface with the DC bus as shown in Figure 9. The phase
Figure 7. DC bus line-to-line fault test circuit. controlled rectifier, Z-source breaker, and DC contactor function
as a unit and should be mounted along with the generator on a
The measured results compare closely with the simulation common subbase. Once the control circuit detects that the SCR
results and show that the use of the z-source breaker improves of the z-source breaker has commutated on, gate pulses to the
the response time for a low impedance to less than 0.4 ms. SCR are removed to prevent the SCR from commutating back
on during a fault. Upon the inception of a low impedance, line-
to-line fault on the DC bus the z-source breaker automatically
responds to the fault and isolates the fault from the source in less
than 0.4 ms. Inhibit signals are also sent to the SCRs of the phase
controlled rectifier securing the rectifier. A signal is then sent to
the DC contactor to open providing galvanic isolation.
Combining the functionality of the power converter and
protective device eliminates the need for separate switchgear to
house circuit breakers allowing for reduced space, weight,
maintenance and cost.

NON-LOAD
Z-SOURCE
RECTIFIER BREAKING, DC PORT MVDC BUS
BKR
CONTACTOR

PRIME MOVER GENERATOR

NON-LOAD
Z-SOURCE
RECTIFIER BREAKING, DC STARBOARD MVDC BUS
BKR
CONTACTOR

MODULE CONTROL

Figure 9. Proposed DC bus protection scheme.

417
 III. CONCLUSION REFERENCES
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