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DC Microgrid Protection: Review and Challenges
Technical Report · August 2018
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SANDIA REPORT SAND2018-8853 Unlimited Release Printed August 2018
DC Microgrid Protection: Review and Challenges
Sijo Augustine, Jimmy E. Quiroz, Matthew J. Reno, and Sukumar Brahma Prepared by Sandia National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
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Issued by Sandia National Laboratories, operated for the United States Department of Energy by
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SAND2018-8853
August 2018
Unlimited Release
DC Microgrid Protection: Review and Challenges
Sijo Augustine and Sukumar Brahma
Electrical and Computer Engineering Department
New Mexico State University
Jimmy E. Quiroz
Renewable and Distributed Systems Integration
Sandia National Laboratories
P.O. Box 5800
Albuquerque, NM 87185
Matthew J. Reno
Electric Power Systems Research
Sandia National Laboratories
P.O. Box 5800
Albuquerque, NM 87185
5
Abstract
Successful system protection is critical to the feasibility of the DC microgrid system. This work
focused on identifying the types of faults, challenges of protection, different fault detection
schemes, and devices pertinent to DC microgrid systems. One of the main challenges of DC
microgrid protection is the lack of guidelines and standards. The various parameters that improve
the design of protection schemes were identified and discussed. Due to the absence of physical
inertia, the resistive nature of the line impedance affects fault clearing time and system stability
during faults. Therefore, the effectiveness of protection coordination systems with communication
were also explored. A detailed literature review was done to identify possible grounding schemes
and protection devices needed to ensure seamless power flow of grid-connected DC microgrids.
Ultimately, it was identified that more analyses and experimentation are needed to develop
optimized fault detection schemes with reduced fault clearing time.
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TABLE OF CONTENTS
1. Introduction ........................................................................................................................11
1.1. DC Microgrid Topologies ......................................................................................12 1.1.1. Single-bus DC Microgrid ......................................................................12 1.1.2. Multi-bus DC Microgrid .......................................................................13 1.1.3. Reconfigurable DC Microgrid. .............................................................14
1.2. Benefits of DC Microgrid ......................................................................................16
1.3. DC Bus Voltage Polarity and Grounding Schemes ...............................................17 1.4. Present State-of-the-Art .........................................................................................18
1.4.1. Examples of DC Microgrid Systems.....................................................18
1.4.2. DC Microgrid Protection Overview ......................................................20 1.4.3. Types of Faults in a DC Microgrid .......................................................20
2. Challenges of DC Protection .............................................................................................23 2.1. Arcing and Fault Clearing Time ............................................................................23 2.2. Stability ..................................................................................................................23
2.3. Multi-Terminal Protection .....................................................................................23 2.4. Ground Fault Challenges .......................................................................................23 2.5. Faster Speed Requirements and Communication Challenges ...............................24
2.6. Guidelines and Standards .......................................................................................25
3. DC Protection Devices .......................................................................................................32 3.1. Sensors ...................................................................................................................32
3.2. Directional Elements ..............................................................................................32 3.3. Protective Relays ...................................................................................................32 3.4. Current Interrupting Devices .................................................................................33
3.4.1. Fuses ......................................................................................................33 3.4.2. No-Fuse DC Circuit Breaker (DCCB) ..................................................34
3.4.3. Solid State DC Breakers........................................................................35 3.4.4. Hybrid CB .............................................................................................37 3.4.5. Arc-Fault Circuit Interrupter (AFCI) Devices ......................................37
4. Protection Against Faults ...................................................................................................39
4.1. General Guidelines and Best Practices for DC Microgrid Protection ...................39 4.2. Unit and Non-Unit Protection ................................................................................39 4.3. Single-Ended and Double-Ended Protection Schemes ..........................................39
4.4. Coordination–Fault Location and Isolation ...........................................................40 4.4.1. Primary and Backup Protection Schemes .............................................40 4.4.2. Communication .....................................................................................40
4.5. Inverter Control–Grid-connected and Islanded Mode ...........................................42 4.6. Principles and Methods of Protection ....................................................................43
4.6.1. Magnitude of Voltage ...........................................................................43 4.6.2. Magnitude of Current ............................................................................43
4.6.3. Impedance Estimation Method .............................................................44 4.6.4. Power Electronic De-Energization ........................................................44 4.6.5. Power Probe Unit Method .....................................................................44 4.6.6. Virtual Impedance Method ...................................................................44 4.6.7. Differential Current-Based Fault Detection ..........................................44
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4.6.8. Transient-Based Fault Protection ..........................................................45 4.6.9. Voltage and Current Derivative Supervised Protection ........................45
4.6.10. Handshaking Method ............................................................................45 4.6.11. Fault Detection Techniques for PV .......................................................45
5. Gaps and Research Needs ..................................................................................................47
6. Conclusions ........................................................................................................................49
7. References ..........................................................................................................................51
DISTRIBUTION............................................................................................................................57
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FIGURES
Figure 1 - Architecture of a representative single bus DC microgrid. .......................................... 12
Figure 2 - Single-bus DC microgrid architecture, no power electronic interface for storage. ..... 13 Figure 3 – Multi-bus DC microgrid architecture. ......................................................................... 14 Figure 4 - Ring bus based DC microgrid architecture. ................................................................. 15 Figure 5 - Ring bus based zonal DC microgrid architecture. ....................................................... 15 Figure 6 - Mesh based DC microgrid architecture........................................................................ 16
Figure 7 - Classification of DC microgrid faults. ......................................................................... 20 Figure 8 - Operating principles of DC microgrid control strategies. ............................................ 24
Figure 9 – DC microgrid standardization needs by nominal voltage level. .................................. 30
Figure 10 - Possible voltage levels of DC microgrid [18]. ........................................................... 31 Figure 11 - Summary of DC microgrid protection devices. ......................................................... 33 Figure 12 - Solid state current interrupter [1]. .............................................................................. 36 Figure 13 - The coupled-inductor DC circuit breaker [51]. .......................................................... 36
Figure 14 - Measured source and load currents under fault [51]. ................................................. 37 Figure 15 - DC microgrid test system [51]. .................................................................................. 41
Figure 16 - Load protective current limiter [1]. ............................................................................ 43
TABLES
Table 1 - Grounding configurations of the DC microgrid in grid-connected mode. .................... 18 Table 2 – Recent standard development summary ....................................................................... 25
9
NOMENCLATURE
Abbreviation Definition
AC Alternating Current
AFI Arc-Fault Interrupter
CAFI Combination Arc-Fault Circuit Interrupter
CB Circuit Breaker
DC Direct Current
DCCB Direct Current Circuit Breaker
DER Distributed Energy Resources
EMF Electro Motive Force
ESS Energy Storage Systems
FFT Fast Fourier Transform
HVDC High-Voltage Direct Current
IEC International Electrotechnical Commission
IED Intelligent Electronic Devices
IEEE Institute of Electrical and Electronics Engineers
IGBT Insulated Gate Bipolar Transistor
LVDC Low-Voltage Direct Current
MCCB Modeled Case Circuit Breaker
MPPT Maximum Power Point Tracking
MVDC Medium-Voltage Direct Current
NEC National Electrical Code
PES IEEE Power & Energy Society
PLC Power-Line Communication
PV Photovoltaic
RMS Root Mean Square
SSCB Solid State Circuit Breaker
STD Standard
VSC Voltage Source Converter
VSI Voltage Source Inverter
ZNE Zero Net Energy
11
1. INTRODUCTION
System protection is a critical component for safety, reliability, and asset protection in any
electrical system. The following are general design criteria for any protection system [1]:
• Reliability–Predicting the protective system response to faults while preventing
unnecessary tripping, such as for transients and noise.
• Speed–Removing faults and restoring normal operating conditions rapidly.
• Selectivity–Maximum continuity of service to loads, minimizing the number of loads
impacted by a fault.
• Economics–Initial and recurring costs; generally, cost increases with better operating
conditions like faster isolation of faulted line can be achieved with a solid-state circuit
breaker than conventional DCCB.
• Simplicity–Number of devices, protection zones, multi-level control for increased
reliability.
Generally, a DC microgrid covers only a small geographical area and distribution line length is
short compared to conventional AC distribution line. Therefore, DC microgrid systems can be
treated as resistive networks [2], [3]. Unlike conventional power system generators, microgrid
systems are utilizing converters (DC-AC, DC-DC, and AC-DC) to integrate sources like solar-
PV, wind, fuel cell, microturbines etc., energy storage devices and loads as shown in Figure 1.
Due to the nature of sources and converters, the microgrid systems offer less physical inertia and
this affects the system stability during disturbances / faults. Therefore, the general performance
parameters of DC microgrid can be identified as,
• Topologies (system and converter)
• Control strategies (voltage control, power sharing, maximum power point tracking
(MPPT), if PV / wind as DERs etc.)
• Power management with energy storage devices
• Protection and grounding schemes
• Power quality
• Communication protocols
• Physical and cyber-security etc.
The challenges, devices, and schemes of DC microgrid protection can be analyzed by
considering some of these parameters and are discussed in the following subsection.
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DC
DC
DC
AC
DC DC
DC
Solar PV Array
WindTurbine
MicroTurbine
FuelCell
DC
DC
DC
DC
BatterySuper
Capacitor
Distributed Energy Resources Energy Storage Devices
AC Grid
DC
AC AC
DC
DC
DC
DC
DC
DC
DC
DC
AC
DC
DC Loads
Plug-in Hybrid Electric Vehicle
Motor LoadsData Centers /
Telecom Stations
DC Home Appliances AC Loads
AC Loads
PG PPV PWT PM T PFC PBA PSC
PL1 PL2 PL3 PL4PL
DC Grid
Figure 1 - Architecture of a representative single bus DC microgrid.
1.1. DC Microgrid Topologies
Based on the DC grid connection among the different DERs and loads, the DC microgrid
topologies can be classified into three [3] and are,
1.1.1. Single-bus DC Microgrid
Single bus topology is commonly used in DC microgrid and the architecture same as shown
in Figure 1. This topology can be considered as the base topology for all multi-bus systems. This
configuration helps to regulates the DC grid voltage and increase flexibility of the DC system.
As shown in the Figure 2, the energy storage devices can be directly connected [4], [5] to the DC
grid and the DC grid voltage depends on SOC of battery pack. Telecommunication applications
are using this type of topology. The main drawback of this topology is uncontrollable DC grid
voltage and unregulated battery charging. In addition, many converters operating in parallel may
lead to circulating current and uneven loading in the power electronic converters. Compared to
the configuration shown in Figure 2, the Figure 1 topology gives less equivalent DC grid
capacitance. Therefore, careful analysis and design of circuit components and control parameters
are required. To increase the reliability of the system more battery banks can be connected to the
DC grid through power electronic converters.
13
DC
DC
DC
AC
DC DC
DC
Solar Array
WindTurbine
MicroTurbine
FuelCell
Battery Banks
Distributed Energy Resources
Energy Storage Devicesdirectly connected to DC Grid
AC
DC
DC
DC
DC
DC
DC
DC
DC
AC
DC
DC Loads
Plug-in Hybrid Electric Vehicle
Motor LoadsData Centers /
Telecom Stations
DC Home Appliances AC Loads
AC Loads
PPV PWT PM T PF C PBA PSC
PL1 PL2 PL3 PL4PL
DC Grid
Figure 2 - Single-bus DC microgrid architecture, no power electronic interface for storage.
1.1.2. Multi-bus DC Microgrid
In multi-bus DC microgrid system, each microgrid absorbs or supplies power to or from its
neighboring microgrid [6], [7]. The multi-bus configurations can be series or parallel, Figure 3
shows a series connected multi-bus system. This type of configuration facilitates the isolation of
a DC microgrid in case of failure and the communication links between DERs are used to
exchange control parameters to improve the performance and stability of the DC microgrid.
14
DC
DC
DC
AC
Solar Array
WindTurbine
DC
DC
DC
DC
BatterySuper
Capacitor
Energy Storage Devices
DC
DC
AC
DC
DC Loads
DC Home Appliances AC Loads
AC Loads
PPV PWT PBA Rca ble
PL1PL
DC DC
DC
MicroTurbine
FuelCell
AC
PM T PF C
DC
DC
DC
DC
Plug-in Hybrid Electric Vehicle
Data Centers /Telecom Stations
PL2 PL4
DC
DC
Battery
DERs DERs
DC Loads
Lca bleRca ble Lca ble
DC Microgrid #1 DC Microgrid #2 DC Microgrid #n
Figure 3 – Multi-bus DC microgrid architecture.
1.1.3. Reconfigurable DC Microgrid.
The reconfigurable topology can be categorized into, mesh / ring bus based DC microgrid [8],
[9], [10]. Figure 4 shows a ring based DC microgrid architecture. In this configuration each
microgrid nodes are connected through intelligent electronic devices (IDEs). This type of
reconfigurable topology will increase the reliability of the system. It allows easy equipment
maintenance in the DC microgrid during fault conditions. The major advantage of this
configuration is that during fault conditions alternative paths / buses are available for the power
flow.
Another type of reconfigurable topology [1] is based on dividing ring based DC microgrid into
zones as shown in Figure 5. In this topology, different DC microgrid units are connected in series
to form zonal structure. This type of connection has better flexibility and reliability. Multi-
terminal or mesh based DC grid as shown in Figure 6 is another configuration of reconfigurable
topology [11], [12]. In multi-terminal or mesh type DC microgrid, each distribution grid is
connected to several input terminals. This type of configuration is more reliable due to multiple
power flow paths.
15
DC
DC
Solar Array
PPV DC
AC
WindTurbine
PWT
DC
DC
DC
DC
PB
SuperCapacitor
Battery
PSC
DC
DC
DC Loads
DC Home Appliances PL
Intelligent Electronic Breaker
Figure 4 - Ring bus based DC microgrid architecture.
DC
DC
Solar Array
PPV DC
AC
WindTurbine
PWT
DC
DC
DC
DC
PB
SuperCapacitor
Battery
PSC
DC
DC
DC Loads
DC Home Appliances PL
Intelligent Electronic Breaker
Zone#1
Zone#2
Zone#3Zone#n
Figure 5 - Ring bus based zonal DC microgrid architecture.
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DC
DC
Solar Array
PPV DC
AC
WindTurbine
PWT
DC
DC
DC
DC
PB
SuperCapacitor
Battery
PSC
DC
DC
DC Loads
DC Home Appliances PL
Intelligent Electronic Breaker
Zone#1
Zone#2
Zone#3Zone#n
Zone#4
Zone#5
Figure 6 - Mesh based DC microgrid architecture.
The power flow in a microgrid is controlled by power electronic interface units. The different
DC microgrid configurations can operate in islanding / standalone mode or can interconnect with
AC microgrid /AC grid. If the DC microgrid is interconnected with AC microgrid, then this
connection is termed as hybrid microgrid [13]. This helps to ensure the power availability and to
increase the overall efficiency of the system.
1.2. Benefits of DC Microgrid
Microgrids are a key consideration to both the movement to more environmentally friendly
power delivery and the growing third world power market because they enable the use of
distributed energy resources (DERs) and are more feasible for rural areas [1], [14].
With the increased emergence of DC loads and generation sources has come the consideration of
the potential benefits of conversion to DC grids. Most modern electronic circuits require a DC
power supply, such as laptops and cell phones. Emerging DER technologies generate DC power,
such as solar panels and batteries.
DC microgrids could be a feasible solution for supplying power to loads during commercial grid
blackouts. A DC microgrid could allow for increased DER penetration due to the cost
effectiveness of having generation sources near the loads, eliminating the need for expensive
transmission line utilization [15]. Considering that both loads and sources could interface on a
common DC bus, reducing the stages of AC-DC power conversion, a reduction in heat losses
and cost compared to AC implementations of DER can be expected [1].
The low-voltage direct current (LVDC) microgrid can be very suitable in systems with a large
amount of sensitive electronic equipment. One main advantage of a DC microgrid over an AC
17
microgrid is that sources, loads, and other components such as energy storage can be
interconnected with simpler and more efficient power electronic interfaces. The control of AC
microgrids deals with the power flow, load sharing, voltage regulation, protection and mitigation
of various kinds of power quality issues, whereas in DC microgrids, issues such as reactive
power, skin effect, etc. are not present. Therefore, compared to AC, DC microgrids are highly
efficient, reliable, easy to control and economical [16], [17].
1.3. DC Bus Voltage Polarity and Grounding Schemes
The possible DC microgrid grounding arrangements to be considered before designing the
protection schemes. Based on the topology, the DC bus can have two type of configurations [18],
[19]
• Unipolar–In this type of systems the sources, energy storage devices and loads are
connected to a two wire (positive and negative) DC bus through converters.
• Bipolar– This configuration uses a three wire (positive, negative and neutral) DC bus
topology. The increased reliability is the main advantage in this type of DC bus
configuration.
In most of the cases, the DC microgrid is in grid-connected mode to ensure the power
availability and performance. Therefore, the DC microgrid protection issues are considerably
related to the DC bus configuration and grounding methods of both DC microgrid and AC grid.
The possible types of DC microgrid grounding [20], [21] are:
• Ungrounded
• Low-impedance grounded
• High-impedance grounded
The above grounding configurations are selected based on the DC microgrid operating mode,
(islanded or grid-connected), DC bus voltage polarity, converter topology and AC grid side
grounding. IET BS 7671 [22] standard discusses five types of grounding system: TN-S, TN-C-S,
TT, TN-C, and IT. Where,
𝑇 = Earth
𝑁 = Neutral
S = Separate
C = Combined
I = Isolated
A number of grid-connected-mode DC microgrid grounding options are discussed in [20], [23]
and are listed in Table 1. The AC grid side can have any of the above configurations and DC
system grounding should be designed accordingly to avoid converter common mode voltage and
neutral voltage fluctuations generated by the AC-DC. The voltage fluctuations in converter side
will lead to the circulating current issues in DC microgrid [24]. A set of PV array grounding
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schemes are briefed in [25] and possible grounding schemes for residential DC microgrid
systems are discussed in [26].
Table 1 - Grounding configurations of the DC microgrid in grid-connected mode.
AC grid grounding Unipolar / Bipolar DC microgrid
TT
-No solid grounding
-Solid grounding with high frequency
transformer in the AC-DC interface
TN
-No solid grounding
-Solid grounding with high frequency
transformer in the AC-DC interface
IT
-Isolated DC bus grounding
-Non- isolated DC bus grounding
-Non- isolated DC bus mid-point
grounding
1.4. Present State-of-the-Art
There are presently several examples of DC microgrids being used, mostly in the 24 V to 1500 V
range, such as the following [1], [27]:
• Residential homes, hospitals, businesses and factories synonymous with the emergence of
DC loads
• Navy shipboard power systems using redundancy architectures, power system
automation, reduced manpower requirements, and easier integration with electric
propulsion.
• Aircraft and automotive systems trending toward DC distribution systems to replace
mechanical, hydraulic, and pneumatic loads with electric loads to realize a significant
potential for increased fuel economy and performance.
1.4.1. Examples of DC Microgrid Systems
As discussed in the Section 1, DER-based DC power generation and distribution provides
significant social and economic benefits such as reduced distribution losses. It reduces the
reliance on power from the main grid and can also provide the benefits of generating,
controlling, and storing power with the economic benefits that may come from locally generated
power.
The US Department of Energy has published the definition of “zero net energy (ZNE)”
consumption for different applications [28]:
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• ZNE building–An energy-efficient building where, on a source energy basis, the actual
annual consumed energy is less than or equal to the on-site renewable generated energy.
• ZNE campus–An energy-efficient campus where, on a source energy basis, the actual
annual consumed energy is less than or equal to the on-site renewable generated energy.
• ZNE portfolio–An energy-efficient portfolio in which, on a source energy basis, the
actual annual consumed energy is less than or equal to the on-site renewable generated
energy.
• ZNE community–An energy-efficient community where, on a source energy basis, the
actual annual consumed energy is less than or equal to the on-site renewable generated
energy.
Due to the advantage described in Section 1.2, the DC microgrid concept can be seen as a viable
solution for ZNE policies and an effective support to main AC grids.
Some of the DC microgrid initiatives across the world are discussed in this section.
A radial-type, community sized DC microgrid system known as “Dunkung microgrid” in Taiwan
is discussed in [29]. It consists of three independent zones with DERs such as photovoltaic (PV),
wind, fuel-cell, and energy storage stations. This microgrid is grid-connected and supplies power
to 15 houses. To operate in islanded mode, a static switch is used to isolate the DC microgrid
from the AC grid. A detailed review on DC microgrid protection devices and their coordination
is also discussed.
DC-based power distribution architectures for commercial buildings introduced by Bosch are
analyzed in [30]. The proposed DC voltage level is 380 V and can supply power to energy
efficient buildings. The system uses an AC-DC converter for grid interface and power-line
communication (PLC) is used to exchange data for improving the performance of the system.
A PV-based LVDC home concept is discussed in [14]. This technology was developed at the
Indian Institute of Technology, Madras, and is being commercialized by Cygni Energy Private
Limited, India. The system uses PV along with battery storage for powering DC homes via a 48
V common DC bus. The loads connected to the DC bus are rated 48 V and to improve the
performance, it can be connected to the AC grid through a DC-AC converter.
Direct Current BV Ltd. [18] has designed and installed a 150kW, 350 V solar-PV based DC
office in ABN AMRO building ‘circl’ in Amsterdam. Protection against various faults and
electrical shock is designed for this project. The system is operating both in grid-connected and
islanded mode and the overall power balance is achieved using energy storage devices. Public
lighting, business park and residential area on DC smart grid is also designed and installed by
Direct Current BV [31] in Netherlands.
[32] discusses a ±750 V LVDC microgrid installation in Finland. The system is designed with
100kVA rectifying substation, 1.7 km undergrounded ±750 V cables and three customer end
DC-AC converters for residential power supply. This grid-connected LVDC system uses IT
grounding scheme on DC side and TN-C-S grounding at consumer side.
Duke energy installed a 10 kW solar DC microgrid atop Mount Sterling in a remote region in
North Carolina’s Great Smoky Mountains National Park in [33]. The project was put in to
20
energize a communications tower and enabled the return of about 13 acres of feeder right-of-way
back to wilderness area. The DC microgrid also incorporated a 95 kWh zinc battery for energy
storage.
A solar-PV based, 150kW, 380 V DC microgrid installed at School of Energy, Xiamen
University, Xiamen, China is discussed in [34]. The system consists of different DC loads like,
30kW DC air-conditioning, 40kW DC EV charging station and 20kW DC LED lightings and
energy storage devices. The system control strategies are done remotely and locally to improve
the performance. The advantages, challenges and economic analysis are also discussed.
Also some of the microgrid testbeds around the world is discussed in [35]. The analysis includes
a detailed classification of AC, DC and hybrid microgrid systems with technical and economic
advantages.
1.4.2. DC Microgrid Protection Overview
As discussed in introduction, due to the low inertia and converters behavior makes the microgrid
system potentially very sensitive to disturbances and faults. DC microgrid disturbances are
mainly due to fluctuations in load, input power variations, changes in load sharing proportions,
different maximum power point tracking (MPPT) controls among DERs, temporary faults,
communication failures/delays, disturbances in the AC grid etc. These factors may degrade
performance and are considered as frequently occurring technical/operational challenges of DC
microgrid. The DC bus voltage regulation is achieved by controlling each converter in the DC
microgrid system network by considering the control strategies and communication protocols.
Therefore, the fault clearing time is a function of line parameters and may affect the system
stability.
Another issue with the DC microgrid protection design is the discrimination of faults and other
disturbances. To achieve a better performance, the protection schemes should categorize the
disturbances (like sudden changes in the source power, load, parametric variations, errors in the
voltage and current feedback etc.) and faults as temporary or permanent. Therefore, these issues
make DC microgrid protection is a challenging task.
1.4.3. Types of Faults in a DC Microgrid
Considering system components and configurations, faults in the DC microgrid can be classified
into two major categories [29] and is shown in Figure 7.
DC Microgrid Fault
Short Circuit Fault Arc Fault
Line-Line Fault Line-Ground Fault Series Arc Fault Parallel Arc Fault
Figure 7 - Classification of DC microgrid faults.
21
As shown in Figure 3, the sources of fault current are DERs, energy storage devices, and the AC
grid (in grid-connected mode). Hence, the magnitude of DC microgrid fault current is a function
of source type the power control schemes, DC bus voltage, fault location, type of fault, fault
impedance, and type of grounding. Due to the low fault impedance, the severity and magnitude
of fault current is high if a Line-Line (L-L) fault occurs in DC microgrid systems. Depending on
the grounding configurations and type of grounding, the fault impedance may be either high or
low for Line-Ground (L-G) faults. To design an effective protection scheme, it is necessary to
identify the different fault locations within a DC microgrid system. The following sections
describe possible fault locations in a DC microgrid system.
1.4.3.1. DC Bus Faults
The faults possible within a DC bus are L-L and L-G faults. In a DC microgrid, the DERs,
energy storage devices, loads, and AC grid are connected in parallel to the common DC bus
through power electronic converters. A short circuit fault in the DC bus may damage the
components and affect the system stability, if proper protection devices are not implemented.
During L-L faults, the capacitors connected to the power converters will discharge high fault
current within a short time. This will cause a decrease in the DC bus voltage and lead to an
unstable operation of the converters, since the converters may be designed to operate in some
particular voltage range. If the system is in grid-connected mode, during a DC fault, IGBTs in
the converters will be blocked after detecting the undervoltage/overcurrent. Now the converter
will act as diode bridge rectifier and the current starts flowing from the AC grid through the anti-
parallel diodes of the voltage source converter(s) (VSC). The severity of the fault may increases
if adequate protection schemes are not designed to overcome this scenario [36].
In some DC microgrid configurations, the energy storage devices are directly connected to the
DC bus to maintain the bus voltage and system stability. Hence, the energy storage devices will
also contribute substantial amounts of fault current. This will intensify the fault conditions and
will increase the level of damage to the components in the DC microgrid.
For a L-G fault in a DC bus, the fault current depends on the grounding configurations and type
of grounding (see Section 1.3). The grounding configuration in both AC grid and DC microgrid
side determines the ground fault current and protection devices should be designed to detect a
ground fault current. For example, in a multi-terminal DC microgrid system, during L-G faults,
the DC bus capacitors will discharge quickly. Based on the grounding configurations, the fault
currents may reach the other terminals of the system and cause overvoltage at other buses and
feeders. If the DC microgrid system is in islanded mode, then the type and configuration of AC
grounding is also important to mitigate the neutral voltage fluctuations in the DC microgrid due
the common mode voltage generated at the VSC terminals.
1.4.3.2. DC Feeder Faults
DC feeder faults can also be categorized as L-L and L-G faults. As discussed in the DC bus fault
scenario, DC feeder L-L fault current magnitude is high compared to the L–G fault. During a
fault at a DC load feeder, all the DERs, energy storage devices, and the AC grid will contribute
to the fault current based on the fault impedance. If a fault occurs at any one of the source
feeders, the source at the faulted feeder will contribute more fault current. The L-G faults in DC
feeders will also have the same effect as described in the DC bus fault. Therefore, effective
22
primary and backup protection schemes should be installed at distinct locations of the DC
microgrid.
1.4.3.3. Source Faults
Arc faults mainly occur in PV-based DC microgrid systems. In PV-based systems, the main
source of power is series- and parallel-connected PV panels [37]. Variations in the stored energy,
changes in the temperature, broken cables, degradation of solder joints, failure of cable insulation
in the junction boxes, corrosion of conductors, etc., may lead to arc fault phenomenon in PV
systems. Failure of arc fault protection can lead to fire hazards in PV panels. There are many
techniques developed for series and parallel arc detection and protection for PV systems [38],
and are explained further in Section 3.4.5.
PV array L-L and L-G faults with detection techniques and protective devices are also explained
in [25]. Ground faults are categorized into i) single ground faults, and ii) double ground faults.
Failure detection of PV ground faults cause fluctuations in V-I characteristics and this may affect
the system stability because of the change in the maximum power point of the array.
23
2. CHALLENGES OF DC PROTECTION
One fundamental challenge with DC protection is that there is no zero crossing of current in DC
as in AC, therefore faults are more difficult to interrupt with fuses and circuit breakers. The
following sections describe some other main challenges of DC protection.
2.1. Arcing and Fault Clearing Time
The size, system components, and configuration are the key parameters for the selection of the
protective devices for a DC microgrid. High fault clearing time and arcing phenomena are the
main drawbacks of the conventional DC circuit breaker (CB). Therefore, to improve the
protection, solid-state circuit breakers (SSCBs) or hybrid CB technologies with less/no arcing
and minimum fault clearing time should be used. The economic feasibility of these breakers
should be considered while designing the protection system for DC microgrids.
2.2. Stability
Variations in DERs input power, disturbances in the AC grid, changes in the load power, etc.,
may cause temporary faults and disturbances in DC microgrid systems. Therefore, stability is a
major issue during the fault and restoration process. The instability may arise due to the
controllability of power converters, resistive nature of line impedances, lack of physical inertia,
etc. As a result, better system control strategies (such as virtual inertia [39], virtual impedance
[16], etc.) with good protection schemes are necessary for the stable operation of a DC
microgrid.
2.3. Multi-Terminal Protection
When considering the design of a LVDC microgrid, experience from existing DC power
systems, such as traction power systems, can be useful. However, because existing systems
largely use current-limiting rectifiers during DC faults, which only allow current to flow in one
direction, a different protection design will be needed to accommodate for the fact that DC
microgrids are AC grid-connected through converters with bidirectional power flow [40]. This
would require a more flexible protection scheme to accommodate for multiple terminals with
multi-directional power flow. Protection challenges may arise from supply-and-demand control,
such as maintaining energy storage state-of-charge, DER control, etc. [15].
2.4. Ground Fault Challenges
Power conversion devices, such as DC-DC and AC-DC converters, contain capacitive output
filters. These capacitive filters present a protection challenge in that they can rapidly discharge
into a fault, resulting in large current surges. Depending on the filter design, fault location, and
installed capacity of the converter, the current surges can have amplitudes of 10,000 to 50,000 A
[1].
For circuit breakers, the greatest challenge posed by the high capacitive discharges is
coordination, because they can cause both upstream and downstream breakers to trip, or only the
upstream breaker, increasing the loads impacted. There is also a potential of damage to the
circuit breakers due to the high currents. Additionally, because loads on a DC microgrid are
likely to have significant input capacitance, capacitor discharge from loads into adjacent faults
exacerbates the problem and can cause unwanted circuit breaker trips.
24
There are two significant operation challenges to consider with DC circuit breakers, failure to
open and the risk of welding-closed. The risk of failure to open is related to the capacitive
discharge issue, where there may be sufficient current to initiate opening, but if not sustained
long enough, may not deliver enough force for opening the contacts completely. In the case of
highly inductive systems, there is a potential for the contacts to then weld-closed during a fault.
Time/trip coordination becomes virtually impossible with circuit breakers in DC microgrids
unless larger, more expensive low voltage power circuit breakers are used, because they can ride
through initial capacitor discharges [1].
2.5. Faster Speed Requirements and Communication Challenges
Power flow management is realized by the power electronic interface units to ensure effective
extraction and storage of power from DERs and energy storage systems (ESS). This could be
achieved by selection of suitable control principles and coordination. In other words, each DER
local controller can share the control parameters/information with the other converters local
controllers. Therefore, from a communication perspective [2], operating principles of DC
microgrid control strategies are divided into three categories and is shown in Figure 8:
• Decentralized DC microgrid system
• Centralized DC microgrid system with communication network
• Distributed DC microgrid system with communication network
P1 P2 Pj
DG#1local
cont roller
DG#2local
cont roller
DG#jlocal
cont roller
P1 P2 Pj
DG#1local
cont roller
DG#2local
cont roller
DG#jlocal
cont roller
P1 P2 Pj
DG#1local
cont roller
DG#2local
cont roller
DG#jlocal
cont roller
Centralized communication
network
Distributed communication
network
Figure 8 - Operating principles of DC microgrid control strategies.
The communication networks can also be used for DC microgrid protection. For a double-ended
protection scheme (see Section 4.3) the system voltage and current information need to be shared
for fault isolation; however, time requirements for protection are much faster than controls.
25
Therefore, faster communication protocols need to be developed to improve the efficiency of the
DC microgrid.
2.6. Guidelines and Standards
One of the fundamental challenges of realizing DC microgrids is a lack of standards and
guidelines to adhere to and depend on for safety and functionality. There is also a relative lack of
practical experience from which to draw lessons learned and best practices. There are many
technical committees and sub committees working on DC microgrid standardization under IEEE,
IEC, ETSI etc. Some of the working groups are published the standards like IEEE 1547,
REbusTM, EMerge Alliance, etc.
The standardization of DC microgrid systems should be, in terms of
• System voltage (12 V, 24 V, 48 V, 110 V, 350 V, 380 V etc.)
• Communication protocols
• Grounding
• Protection and safety
• Islanding and grid-connected mode of operation
• Power quality issues etc.
• Cyber security
In [20], [27], recent updates in standard developments are discussed and are summarized in
Table 2. The first thing that needs to be addressed is the voltage levels of the DC microgrid [11].
IEC SG4 is an active project group working to develop the standards for DC microgrid systems
up to 1500 V. The protection and need of safety regulations of DC microgrid voltage levels are
shown in Figure 9 [27]. Direct current BV Ltd. [18] discusses the possible DC microgrid voltage
levels for different applications with number of cables and power handling capacity and is shown
in Figure 10. The standardization of DC microgrids should be based on the applications, which
will help to create protection standards of different DC microgrid components based on system
configuration.
EMerge Alliance [41], an open industry association for DC power distribution, recently released
a set of standards for occupied space and data/telecom. The occupied space standard is based on
24 V DC grid voltage and data/telecom standard recommends a 380 V DC grid. The
communication protocols should be based on the operating principles of DC microgrids as
discussed in Section 2.5. The other parameters needing standardization are grounding schemes
and protection, both for operating the DC microgrid in grid-connected and islanded mode. The
general procedures/standards for fault detection and isolation are important because the fault
clearing time affects the performance parameters of the DC microgrid.
Table 2 – Recent standard development summary
Org. Project No.
/ STD / TS
Working
Group Title Scope Status
P2030.10
Distribution
Resources
Integration
Standard for
DC Microgrids
for Rural and
-Design, operations, and
maintenance of a dc Active
Project
26
IEEE
WG/Remote
DC Microgrid
Remote
Electricity
Access
Applications
microgrid for rural or
remote applications.
-Requirements for
providing LVDC and AC
power to off-grid loads.
1547-2018
Standard for
Interconnection
and
Interoperability
of Distributed
Energy
Resources with
Associated
Electric Power
Systems
Interfaces
-Requirements relevant to
the interconnection and
interoperability
performance, operation,
testing, safety,
maintenance and security
considerations.
(This standard is written
considering that the DER
is a 60 Hz source.
Suitable for the design of
hybrid (AC-DC)
microgrid systems.)
Available
1547
Impact of IEEE
1547 Standard
on Smart
Inverters (white
paper)
-Smart inverter functions,
modeling, protection,
power quality, ride‐
through, distribution
planning, interoperability,
and testing and
certification.
Available
DC@Home
Intelligent
Grid
Coordinating
Committee
(IGCC)
DC@Home DC use in residential
dwellings and a LVDC
Micro-grid systems Active
Project
946-2004
DC System
Design
Working
Group
Recommended
Practice for the
Design of DC
Auxiliary
Power Systems
for Generating
Systems
-Recommended practice
include lead-acid storage
batteries, static battery
chargers, and distribution
equipment. Guidance for
selecting the quantity and
types of equipment, the
equipment ratings,
interconnections,
instrumentation, control
and protection is also
provided.
Available
P946
WG_946 - DC
System Design
Working
Group
Recommended
Practice for the
Design of DC
Power Systems
for Stationary
Applications
Active
Project
SEG 4
Standardizatio
n Evaluation
Group (SEG)-
4
Standardization of LVDC
systems up to 1500V. Active
Project
27
IEC
SEG 6
Standardizatio
n Evaluation
Group (SEG)-
6
Non-
conventional
Distribution
Networks /
Microgrids
- Rural and developing
markets that serve
potential huge market
needs (notably in Asia
and Africa) including
networks that may be
connected in the future to
a traditional /
interconnected grid.
- Facility or campus grids
capable of operating in an
isolated mode with
respect to a large
interconnected grid.
Active
Project
IEC
SEG 9
Standardizatio
n Evaluation
Group (SEG)-
9
Smart
Home/Office
Building
Systems
Standardization activities
and gaps related to
electrical installations and
communication
technologies for smart
building premises systems
in order to build up a
high-level landscape. Active
Project
SyC LVDC
Systems
Committee
Low Voltage
Direct Current
and Low
Voltage Direct
Current for
Electricity
Access
To provide systems level
standardization,
coordination and guidance
in the areas of LVDC and
LVDC for Electricity
Access.
Active
Project
IEC
62040-5-
3:2016
Uninterruptible
power systems
(UPS) –
Part 5-3: DC
output UPS -
Performance
and test
requirements
DC uninterruptible power
systems (DC UPS) that
deliver a DC output
voltage not exceeding
1500 V. Available
TS
62257:2015
Recommendatio
ns for
renewable
energy and
hybrid systems
for rural
electrification
Designed to be used as
guidelines and are
recommendations for
small renewable energy
and hybrid systems for
rural electrification.
Available
28
60364
Low-voltage
electrical
installations
Part 4-41: Protection for
safety – Protection against
electric shock
Part 4-43: Protection for
safety - Protection against
overcurrent (buildings)
Part 4-44: Protection for
safety - Protection against
voltage disturbances and
electromagnetic
disturbances (buildings)
Available
Pika
Energy REbusTM
REbus™ is a DC energy
network standard that
operates alongside
the existing AC
infrastructure, enabling
customers to build cost-
effective, scalable
renewable energy
systems.
Available
EMerge
Alliance
EMerge
Alliance
-Development of
standards within five
building space categories:
occupied space, data and
telecom space, building
services, outdoor and
whole campus/building
microgrids.
Available
ETSI
EN 300 132-
3-1
European
Telecom
Standard
Institute
Environmental
engineering
(EE); power
supply interface
at the input to
telecommunicat
ions and
datacom (ICT)
equipment
Part 3: operated by
rectified current source,
alternating current source
or direct current source up
to 400 V;
sub-part 1: direct current
source up to 400 V
Available
ITU
ITU L.1200
(2012-05)
International
Telecommunic
ation Union
Direct current power
feeding interface up to
400 V at the input to
telecommunication and
ICT equipment
Available
ITU-T
L.1201
(2014-03)
Architecture of power
feeding systems of up to
400 VDC Available
ITU-T
L.1202
(2015-04),
Methodologies for
evaluating the
performance of an up to
400 VDC power feeding
system and its
environmental impact
Available
YD/T 2378-
2011
China
Communicatio
240 V direct
current power
supply system
This standard specifies
the communication with
240 V DC power supply
Available
29
CCSA
ns Standards
Association
for
telecommunicat
ions
system components,
series, technical
requirements, test
methods, inspection rules,
signs, packaging,
transport and storage.
This standard applies to
communication bureau
station and data
communications
equipment supply to the
engine room, with a
nominal voltage of 240 V
DC power supply system.
YD/T 2556-
2013
Maintenance
requirements of
240 V direct
current power
supply system
for
telecommunicat
ions
240 V DC power supply
system, conditions of use,
items month period, index
maintenance requirements
and test methods. Available
YD/T 3091-
2016
Communication
with 240/336 V
DC power
supply system
evaluation
requirements
and methods of
running
The assessment
requirements and methods
of online running 240
/336 V DC system for
telecommunications
Available
30
Figure 9 – DC microgrid standardization needs by nominal voltage level.
31
Figure 10 - Possible voltage levels of DC microgrid [18].
32
3. DC PROTECTION DEVICES
The following sections describe some DC protection devices.
3.1. Sensors
Currently, all power converters use MOSFET or insulated gate bipolar transistor (IGBT)-based
solid state devices. These devices have high switching frequency, high current carrying, and
voltage withstanding capability. By use of additional circuits, these devices can improve the
short-circuit current withstanding capability. This reduces voltage transients during turn-off and
improve system stability.
Measurement errors may lead to the failure of fault protection systems and it may cause physical
damage of DC microgrid components. Generally the measurement devices are hall-effect
sensors, current transducers, direct current transformers etc. For example, a double ended
protection scheme (see Section 4.3) uses the measured values at various locations to analyze the
fault conditions and error in the measurement values may lead to the incorrect operation of the
protective devices. Therefore, validation of voltage and current sensors using any standard
validation algorithm is the most crucial part of any protective algorithm. This helps to
reconstruct the bad data and it can be replaced with calculated appropriate data. The error or
variations in the measurements are generally due to disturbances in the DC microgrid and there
are many schemes [42] discussed in the literature to verify the parametric uncertainties in the
measurement. This method can be used for the calibration and testing of the sensors and in actual
practice this will increase the fault clearing time.
3.2. Directional Elements
Directional comparison can be achieved by using a double-ended detection scheme [43]. The
direction of the fault current and communication network can be used to differentiate the fault
and the coordination can be done in two ways:
1. The tripping scheme such as Directional Comparison Unblocking (DCUB)
2. The blocking scheme such as Directional Comparison Blocking (DCB)
A better protection coordination can be achieved using double-ended fault direction comparison
methods. For example in DCUB, during a feeder fault, if the adjacent protection devices both
detect a fault in forward direction, they communicate and trip together. On the other hand, DCB
schemes communicate faults to the upstream device in the opposite direction of the fault to block
them from operating.
3.3. Protective Relays
Power relays mitigate many disadvantages of fuses and CBs. DC power relays can protect the
microgrid from overvoltage, overcurrent, undervoltage, time derivatives, step changes in
current/voltage, and ground faults. In most of the relays, external voltage and current sensors
monitor the real time system conditions, and if there are any deviations in the measured value
compared to the threshold value, a delay time is activated. If the measured values are still higher
even after the delay time expires, microprocessor in the relay will give a trip signal to the
contactors and will isolate the fault.
33
If the relay is set to non-latching mode, a release signal to close the contactors will give after the
set release time. The relays can be implemented with or without communication network based
on the application and DC microgrid configurations. Digital relays with microprocessor are more
popular because they can monitor and protect more than one fault condition in the DC microgrid.
In this type of relay for improving the performance and system requirements, each alarm can be
individually activated or deactivated based on the fault type. The IEEE general standard for
selecting an AC relay is C37.90-2005-IEEE standard for relays and relay systems associated with
electric power apparatus.
3.4. Current Interrupting Devices
The DC microgrid fault current interrupting devices are summarized in Figure 11 and discussed
in this section.
Figure 11 - Summary of DC microgrid protection devices.
3.4.1. Fuses
Ideally, fuses can be applied to DC systems having a high di/dt (low inductance) where the time
for the fuse to reach its melting point is minimized. Regarding reliability and simplicity, fuses
are not a satisfactory solution to DC microgrid protection due to the constraints that would need
to be considered on distribution cable length and the difficulty to predict transient effects of
opening on the microgrid voltage [1].
Fuses are the simplest and most commonly available protection devices for AC and DC systems
[29]. In DC microgrid systems, choosing fuses as a protective device is dependent on the DC
microgrid components, level of protection, and fault characteristics. Fuses are mainly used for
overcurrent (short circuit and overload) protection and the selection is based on current, voltage
ratings and current-time characteristics.
DC Microgrid Protection Devices
Fuse
• Arc Fault Circuit Interrupter (AFI)
• Combination Arc-Fault Circuit Interrupter (CAFI)
DC Circuit Breaker Protective Relay Solid State CB Hybrid CB
• Molded Case
• Vacuum CBs
• Hybrid-solid/vacuum
• DC power relays
• Digital relays
• Gate turn off thyristor (GTO)
• Emitter turn-off (ETO) thyristor
• Insulated gate bipolar transistor (IGBT)
• Insulated gate commutated thyristor (IGCT)
• Coupled inductor SSCB
• Combination of DCCB+SSCB
Arc Fault Interrupter
• Conventional
• Semiconductor
34
The tripping time characteristics mainly depends on the fault impedance [44]. Therefore, to
improve the performance of the fuse in DC microgrid systems, it necessary to accurately
calculate the deviations in the system voltage and overcurrent through the fuse from the starting
point of the fault to the completion of the fuse blowing. One of the main disadvantages of fuses
is the inability to categorize the permanent and temporary faults and therefore, the probability of
malfunction is high when there is a small overcurrent disturbance in the system.
Recently, high speed fuses or ultra-fast fuses, generally known as semiconductor fuses, are
getting much attention for power electronics applications. These fuses are rated in terms of
current, voltage, melting point and clearing 𝐼2𝑡 [45].
By considering the general facts for a DC microgrid system, the design and selection of a fuse as
a protective device should be based on the following parameters:
• System voltage
• Current rating
• Current-time characteristics (by considering temporary and permanent fault conditions)
• Breaking capacity
• Type of protection (primary or backup protection)
• Components to be protected and voltage ratings (power electronic converters, energy
storage devices, type of DC microgrid sources, load etc. )
• Any inrush characteristics of the DC microgrid (supercapacitor performance during
transients, motor start-up, etc.)
• Any other standard requirements of the DC microgrid
3.4.2. No-Fuse DC Circuit Breaker (DCCB)
The inherent arc interruption voltage developed by AC circuit breakers can be leveraged for DC
current interruption by connecting contactors in series until the sufficient voltage blocking
capability is achieved [1].
DC Circuit breakers are another protective device used in DC microgrid systems. CBs can
replace fuses and improve the operating conditions of a system. The arc extinguishing time is
longer for a DCCB due to the nature of the direct current system, where voltage is continuous.
In DCCBs, the arc voltage is a function of system voltage, self-induced voltage (back EMF) and
voltage across the arc attenuator [46]. This implies that if the DC microgrid is more inductive,
then the magnitude of arc voltage will be high and contacts of CBs should be completely opened
for clearing the fault. Most of the advanced DCCBs also offer ground fault protection features.
To provide accuracy and reliable operation, advanced DCCBs use microprocessor-based digital
sensing schemes. Hence, the selectivity of the CB can be customized based on adjustable fault
pickup settings and time delays.
In some DCCBs, advanced communication options help protection monitoring, remote on/off
trip mechanisms, etc., and this allows selectivity of CBs as a primary or backup protection
device. Molded case CBs (MCCBs) [47], [48], vacuum CBs, hybrid solid/vacuum interrupters
are the common CBs used in DC microgrid systems. The general selection criteria for DCCBs
are discussed in the following IEEE standards; however in revising for DC microgrid
35
protections, guidelines are required to consider the complexity of power electronics systems,
fault impedance and fault clearing time.
• IEEE STD C37.14-2015 (Revision of IEEE STD C37.14-2002) explains IEEE standard
for DC (3200 V and below) power circuit breakers used in enclosures.
• LVSD-WG_C37.16-LVSD–standard for preferred ratings, related requirements, and
application recommendations for low-voltage AC (635 V and below) and DC (3200 V
and below) power circuit breakers
3.4.3. Solid State DC Breakers
Solid state CBs are the most promising protective devices for DC microgrid applications because
of the ultra-high speed operation where the fault current is high and fault clearing time is very
low. There are different type of SSCBs available for DC microgrid systems [29], [49], [50], [51].
They are:
• Gate turn off thyristor based CB
• Emitter turn-off thyristor based CB
• Insulated gate bipolar transistor (IGBT) based CB
• Insulated gate commutated thyristor (IGCT) based CB
• Coupled inductor SSCB
The following advantages make SSCB as a better protective device for DC microgrid:
• High switching speed
• Fully controllable
• High voltage blocking capability
• High current carrying capability
• Low on-state conduction loss
• No arcing [52]
The application of solid state technology to circuit breakers has resulted in advantages over their
electromechanical and magnetic predecessors. The risks of arcing and mechanical wear have
essentially been eliminated, resulting in higher reliability and longer lifetimes. Faster switching
is another significant advantage, improving from milliseconds or even seconds to microseconds
[53].
Solid state breakers can autonomously sense over-current and either hold it to a limit or
immediately open and drive it to zero. Solid state breakers must have sufficient current limiting
inductance and voltage clamping circuitry to be able to withstand displaced stored energy during
interruption. Figure 12 shows an example of a simple solid state current interruption electrical
diagram.
36
Figure 12 - Solid state current interrupter [1].
The size, weight, and cost of solid state circuit breakers can be significant, although present
advancements show promise in design improvements.
In [51], a conceptual DC breaker design is proposed. The design is a variation of a solid state
breaker with enhancements that allow for automatic fault detection, bidirectional operation, and
manual opening. Figure 13 shows an electrical diagram of the basis of the coupled-inductor DC
circuit breaker.
The essential logic of the coupled-inductor DC breaker is that the capacitor (C) charges under
normal operation. Under a load-side fault condition, the capacitor then discharges through the
coupled-inductor, causing the source current to go to zero and switch S1 to turn off. An example
of measured source and output currents from the breaker under a fault are shown in Figure 14.
Figure 13 - The coupled-inductor DC circuit breaker [51].
37
Figure 14 - Measured source and load currents under fault [51].
The coupled-inductor DC breaker allows for automatic fault detection and isolation. The amount
of transient current that will operate the breaker can be set using the turns ratio of the coupled
inductor. It can also be operated as a DC switch by gating S2 and causing a discharge current,
where the fault operation sequence is induced and the switch opens. By incorporating a center-
tapped transformer and bidirectional rectifiers, the switch can also be configured for bidirectional
operation, which would be ideal for DC microgrids.
3.4.4. Hybrid CB
A thyristor-based DC hybrid CB (HCB) is discussed in [54]. This HCB is a combination of
conventional MCB with SSCB. These HCBs have different limiting behavior to detect the
permanent and temporary faults and limit the fault current within a short duration of time. In
normal operation, load current passes through a miniature circuit breaker. Upon fault occurrence,
fault current is then transferred to a limiting path through a soft transition mechanism.
3.4.5. Arc-Fault Circuit Interrupter (AFCI) Devices
As discussed in the Section 1.4.3,Source Faults in PV-based DC microgrid systems, due to the
variations in the stored energy of PV panels, changes in the temperature, broken cables,
degradation of solder joints, failure of cable insulation in the junction boxes, corrosion of
conductors, etc., arc faults can occur. In general, the arc faults can be categorized into two:
• Series arc fault
• Parallel arc fault
The PV arc fault phenomenon is discussed in [25], [55]. Because of the low fault current
magnitude, the series arc faults are difficult to identify. On the other hand, parallel arc faults
produce high current and are quite easy to detect. Prevention of fire hazards from arc faults is
necessary in a PV-based DC microgrid system. There are many schemes to differentiate these
38
two fault types and the basic idea is to use the current/voltage information to analyze the high
frequency components during the arc fault.
Arc fault protection devices consist of a fault detection mechanism and an interruption device.
Series arc faults can be measured across line or neutral cables and parallel arc faults can be
measured between line-line, line-neutral, line-ground or the neutral-ground.
The following are the popular devices [56] to detect the arc faults in DC microgrid:
• Arc Fault Circuit Interrupter (AFI)–AFIs are designed to protect the PV system only from
parallel arc faults. The AFI will also act as a CB and can protect the DC microgrid system
from the overload, short circuit faults.
• Combination Arc-Fault Circuit Interrupter (CAFI)–CAFIs are designed with added
capability. They can protect the PV system from both series and parallel arc faults along
with the overload, short-circuit faults.
39
4. PROTECTION AGAINST FAULTS
4.1. General Guidelines and Best Practices for DC Microgrid Protection
Currently, DC microgrid technology is gaining more popularity because the distribution systems
from generation to consumption level are experiencing a shift towards DC. As discussed in
Section 1.4.3, various faults in the system need to be isolated effectively by detecting the voltage
and current magnitudes and associated transients. The following are the few parameters that must
be considered in the protection design for a DC microgrid system:
• DC microgrid configuration (radial, ring, mesh, zone etc., and grid-connected or
islanded)
• Size, ratings, and components (see Section 4.4.1) in the DC microgrid.
• Configuration and type of grounding
• Control strategies and communication networks (centralized or decentralized)
• Possible type of faults and locations
• Fault detection
• Reliability, selectivity, speed, and cost of protective devices
• Reclose and restoration control schemes.
• Stability of power converters
• Scalability of the DC microgrid
The following guidelines should also be considered for the design of the DC microgrid
protection framework.
4.2. Unit and Non-Unit Protection
Generally, DC microgrid fault protection schemes can be divided into unit protection and non-
unit protection schemes. The unit protection schemes are implemented to protect specific zones
of a DC microgrid, for example, common DC bus, power electronic converters, energy storage
devices like batteries or super capacitor banks, or loads, etc. In general, current deviations (based
on Kirchhoff’s current law) are calculated in the specified zones of DC microgrids and the
corresponding zone is only protected from fault. Examples for this type of fault are differential
protection schemes and restricted earth-fault protection schemes.
Conversely, non-unit schemes basically follow a “threshold” value to detect the various faults.
These schemes also protect the DC microgrid components without defining any specific zones.
Therefore, these schemes can be used as backup protection scheme for DC microgrids. Popular
detection schemes under this category include: overcurrent, under/over-voltage, derivative of
current (di/dt), derivative of voltage (dv/dt) etc. [29], [43]. A unit-based protection can be
implemented for DC microgrid feeders where coordination of other protection devices is difficult
within the minimum fault clearing time [57].
4.3. Single-Ended and Double-Ended Protection Schemes
In a DC microgrid, voltage and current are the only two parameters available for fault detection.
To improve the DC microgrid fault detection and protection schemes, protection devices must
40
share and coordinate their sensed parameters. The protection in DC microgrids can be classified
as single-ended and double-ended schemes. The single-ended scheme uses local measurement of
voltage and current to detects faults. Popular single-ended methods of local measurement include
voltage derivative, current derivative, power transient-based schemes, traveling wave-based
detection schemes, undervoltage, and overcurrent methods. Double-ended methods use sensing
devices at both ends of DC microgrid transmission lines and communication channels to share
the voltage and current information. The main double-ended detection schemes are longitudinal
DC line current differential schemes [43] or inter-tripping.
4.4. Coordination–Fault Location and Isolation
Based on the type and characteristics of fault, the detection and coordination strategies should
identify a fault and generate a trip signal to the corresponding protection devices installed at
appropriate locations along the DC microgrid. To analyze the different coordination strategies, it
is essential to explore the concepts in the following sections.
4.4.1. Primary and Backup Protection Schemes
In a DC microgrid, the primary and back-up protection schemes can be selected based on the
configuration of the system. As mentioned earlier, the protection devices are implemented by
considering all components [58] of a DC microgrid as given below:
• Sources (protection of PV system, wind, fuel cells, etc.)
• Power converters (VSI, rectifiers and DC-DC converters)
• Energy storage devices (battery, supercapacitor, etc.)
• DC link capacitors
• DC and AC buses and feeders
• Loads
As the name indicates, primary protection is the main protection device intended for the
protection of a particular component in a DC microgrid. As an alternative solution, backup
protection should be provided for every component in the system if main protection fails. The
backup protection can be categorized as local backup protection and remote backup protection
[59].
By considering the type of fault, fault impedance and the DC microgrid component to be
protected, there may be multiple solutions for primary and back up schemes. The main thing to
consider while designing protection is scalability of the system. Therefore, the selection of
primary and backup schemes should be based on fault location, fault impedance, fault current,
fault clearing time, system voltage and load type (critical and non-critical load). Another point to
be considered when selecting primary and backup protection is the identification of temporary
and permanent faults and fault ride-through capability.
4.4.2. Communication
Adding a communication network in a DC microgrid will improve the system performance in
terms of power sharing, MPPT, protection, online system monitoring, and stability and
reliability. There are DC microgrid communication systems with centralized and decentralized
41
control strategy to optimize the system performance. Communication systems always provide the
ability to reconfigure the system during abnormal operating conditions. these communication
channels can be used to play a vital role in protection.
There are different protection solutions based on communication challenges. One method uses
IEDs installed at different zones of a DC microgrid and the data is used to monitor the fault
conditions. The communication network can be a wide area network, neighborhood area
network, building area network, home area network, and industrial area networks, etc., based on
the size and application of the DC microgrid [20].
If double-ended protection schemes are used in a DC microgrid, communication delay is the
main parameter that controls the fault clearing time. Generally, a DC microgrid covers only a
small geographical area and transmission line length is short compared to the conventional AC
transmission line. Hence, line impedance is mostly resistive and the rate of rise of fault current is
high. This limits the fault clearing time or trip time of the DC microgrid to few milliseconds
[58].
In a double-ended protection scheme, the total fault clearing time is equal to the time between
the fault instance and trip time, which includes the communication delay. The communication
delay includes propagation delay time and processing time delay of the telecommunication
equipment. Therefore, the effectiveness of the communication-based fault detection scheme
depends on the length of DC transmission line, type and severity of the fault, total time delay,
time synchronization of measured data, and data acquisition tools with high performance signal
processing capability.
In [51], the enhancements of communication and control were assessed. Using the coupled-
inductor DC circuit breaker described in Section 3.2.4, the notional DC microgrid system shown
in Figure 15 was simulated. The generators (G) are DC power supplies with droop control and
bidirectional breakers are used on line 7 (E and F) to allow power to conduct in both directions.
Nodes 6, 8, and 10 are considered load nodes with an assortment of an inverter-fed load and two
DC-DC converter loads, respectively.
Figure 15 - DC microgrid test system [51].
42
Three control strategies were simulated and analyzed: 1) central control, 2) local breaker control,
and 3) paired breaker control. With central control, all breakers can receive gate signals from a
central processor and all breakers send their input and output current to the central control. This
allows for optimal fault location and isolation. The disadvantage of central control is the more
complex communication infrastructure required and the higher risk of communication-related
failures.
Under the local control case, each breaker can be monitored using the equivalent of differential
protection, where equality between input and output current is monitored. Because this type of
control would result in tripping due to current reversal not associated with faults, logic to
reactivate the breaker would need to be built in. One problem is that faults occurring upstream of
the breakers would be interpreted as needing to be switched back on. Local control would also
not be able to isolate all fault locations. The advantage is that it requires only a simple
communication infrastructure.
Paired breaker control takes advantage of the fact that some pairs of breakers can operate with
the same control. For example, both breakers on line 4 of the test system in Figure 15 will either
both be conducting or open, and therefore can be paired. There will need to be communication
between the paired breakers. In the case of a fault between two paired breakers, both breakers
will remain open, eliminating the risk of re-closure of the downstream breaker as in local control.
Also, more fault locations can be isolated and conditions where breakers need to be switched
back on can be identified. The communication requirements for this scheme are between central
and local control.
Faults at all possible locations on the test system were simulated for with the three control
schemes. Central control provides desired results for all fault locations. Local control runs the
risk of re-energizing a fault at location 7 because breaker E is bidirectional. While the same risk
at other fault locations upstream of breakers would conceptually be avoidable because the
rectifiers don’t allow negative current, if the fault has resonance it is possible that current can be
injected back into the system, causing other breakers to trip. The only shortcoming of paired
breaker control are faults at junctions 5 and 9, because at any given time one of these junctions
will be upstream of the bidirectional breakers. Assuming the breakers at these junctions are very
close to each other, it may be a tolerable risk because a fault occurring between them becomes
less likely. That limitation aside, paired breaker control can perform desirably with a much
simpler communication architecture than central control.
4.5. Inverter Control–Grid-connected and Islanded Mode
The main advantage of a DC microgrid is that it can operate either in a grid-connected mode or
an islanded mode. This improves the system reliability and other performance parameters.
However, protection of the DC microgrid in both situations is a challenging task.
In grid-connected operation of a DC microgrid, voltage source converters (VSCs) are used to
control the power flow between AC and DC grid [60]. A fault can occur on either side of the
converters and the severity depends on AC-DC converter topology and the ability of protection
devices to clear the fault in minimum time. During a DC microgrid side fault, the anti-parallel
diodes in the VSC will start conducting and it will act as a rectifier circuit. Therefore, proper
bidirectional protection schemes should be implemented at VSC.
43
4.6. Principles and Methods of Protection
Several articles discuss fault detection and protection schemes [29], [61], [62], [63], [64] based
on different measurement and calculations. There is a range of research on DC system
protection, including DC microgrids and HVDC line protection systems, summarized in this
subsection.
4.6.1. Magnitude of Voltage
Fault detection by analyzing variations in the system voltage is one of the simplest methods for
DC microgrid protection. This can be achieved by setting a “threshold voltage value” based on
the system voltage. For example, during fault, there is a drop in the system voltage and change
in voltage is used to generate a trip signal by comparing it with the set value. It is considered a
fast detection method because it depends entirely on voltage magnitude. This method can be
categorized as a single-ended detection scheme. One of the major problems with this method is
the inability to discriminate temporary and permanent faults. Therefore, implementation of this
method is limited by the size, rating of the DC microgrid and type of connected loads.
4.6.2. Magnitude of Current
This method is similar to the fault detection by voltage magnitude method. In this scheme, the
deviations in the current value are compared with the “threshold current value.” This method
also has the advantages and disadvantages of the voltage detection method. Because of the fast
discharge of converter capacitors in the DC microgrid system, fault detection using this method
is a challenging process.
Figure 16 shows a load protective current limiter utilizing solid state switches, illustrating a
method proposed for load fault protection. Multiples of these could be used to prevent a single
load fault from causing a common bus feeding other loads to collapse. For clusters of higher
current loads, a solid state switchboard could use multiples of the current limiter similar to circuit
breakers in a power panel, with a single shared voltage clamp sized with the assumption that not
all devices will interrupt a fault at once [1].
Figure 16 - Load protective current limiter [1].
44
4.6.3. Impedance Estimation Method
Another popular scheme is the impedance estimation method. There are different ways to
estimate the impedance of the system. The simplest method is to directly use voltage and current
magnitude. The change in the line impedance is analyzed and a trip signal is generated if the
system violates the threshold limits.
Another scheme in this category is active impedance estimation explained in [65], [66]. This
method injects a short-duration spread frequency current by using a power electronic converter
to estimate the line impedance. The estimation is based on voltage and current responses and
Fast Fourier Transform (FFT) is used to analyze the formations. This method only injects current
spikes if there is any abnormality in the system. The responses contain both fault location and
severity of the fault. The main drawback of this method is that it uses extra power electronics
equipment to detect the fault and this increases the fault clearing time.
4.6.4. Power Electronic De-Energization
One of the simpler protection schemes described in [1] is relying on power conversion devices to
de-energize a fault and then sectionalizing with no-load switches. This approach would be
inherently slow because it would require time to de-energize and then re-energize buses.
4.6.5. Power Probe Unit Method
A non-iterative fault detection scheme is proposed for LVDC microgrid systems in [67]. This
method uses an external circuit (power probe unit) for analyzing the fault and also discusses the
backup protection scheme without de-energizing the DC microgrid system. The power probe unit
consists of a power source, capacitor, inductor and CB. In this method, the DC microgrid is
divided into zones and if a fault occurs, IEDs are used to detect and isolate the fault location
based on the predefined current threshold values. After the isolation, the power probe unit is used
to examine the fault status and location by sending a probe current. The return probe current is a
function of natural frequency and damping factor. Hence, distance to the fault location and fault
resistance can be found out. This method uses FFT and therefore, the controller should have high
sampling rate and signal processing capability.
4.6.6. Virtual Impedance Method
The virtual impedance method is another popular and cost-effective scheme commonly used for
power sharing in microgrid systems [24], [68]. The scheme can be modified for the protection of
the DC microgrid system. In fault conditions, the rate of rise of fault current is very high due to
the low impedance nature of the microgrid. This scheme calculates the adaptive virtual
impedance values based on the voltage and current measurement. This effectively controls the
converter pulse width and helps to restrict the flow of fault current. Additionally, a trip signal is
send to the CB to isolate the faulted zone. This method can be used with the coordination of a
fault current limiter [69].
4.6.7. Differential Current-Based Fault Detection
This method is an example of a double-ended protection scheme, where communication network
and time synchronization of data is required for efficient operation of the system. This scheme
relates the line current at both ends of the transmission line and exchanges the information
45
through the communication network. The result is compared with the threshold value to detect
the fault. This method takes a comparatively long pick up time and this deteriorates the
performance of the scheme while detecting severe faults. A differential current-based fault
detection and distance calculation is proposed in [70]. This method can detect the DC arc fault
and a cumulative sum average method can be used to detect the fault conditions.
4.6.8. Transient-Based Fault Protection
A single-ended protection scheme based on a travelling wave-based fault detection scheme is
explained in [71]. Travelling wave scheme in general uses a communication channel to detect the
fault and thus time delay will increase the fault clearing time. In this method, DC inductance at
both ends of the transmission line is used to measure the difference in fault-induced travelling
wave voltage and current. Thus, the system can identify remote line fault and bus fault by a
single-end measurement scheme. The calculation of change in power from the measurement
value will give the fault direction.
4.6.9. Voltage and Current Derivative Supervised Protection
A combination of voltage and current derivative-based fault detection scheme is described in
[71]. This method uses the current direction, rate of change of currents and rate of change of
voltages information to detect the fault. This helps classify the faults as internal or external. For
example, during a cable fault, a positive current derivative shows the fault is within the
protection zone and a negative current derivative indicates an external fault.
4.6.10. Handshaking Method
The handshaking method explained in [72] is based on local voltage and current data to detect
the multi-terminal DC fault. This scheme uses VSC with local pre-fault and post-fault
measurements to calculate the DC fault current. After determining the direction of currents in
each line, each VSC selects the DC switches to isolate the fault line. The selection rule is based
on the direction and magnitude of DC fault current through the DC switches. There are three
independent selection methods explained in [72] to ensure the reliability of handshaking method.
The advantage of this method is that without a communication network, fault line can be
identified.
4.6.11. Fault Detection Techniques for PV
A more detailed fault detection techniques for solar PV systems is discussed in [25]. In this
literature, the PV faults are classified into physical, environmental and electrical. Advantage and
limitation of conventional fault detection devices are also discussed. Model-based difference
measurement schemes are discussed in [25] where the predicted data is compared with real-time
data. The reliability of the schemes is a main drawback because the real-time values will change
with irradiation, temperature, and time. The real-time difference schemes, output signal analysis
schemes, and machine learning techniques are considered as the future fault detection methods in
PV systems.
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5. GAPS AND RESEARCH NEEDS
The DC microgrid literature review reveals that effective protection strategies, standards, and
guidelines should be developed to improve the performance of the DC microgrid. The
parameters related to the system protection are size, configuration, voltage rating, components,
and load and control strategies of the system. Therefore, while designing DC microgrid
protection, all these parameters determine the probability of occurrence of a fault and need more
attention toward the selection of proper detection schemes and devices. The gaps and research
needs for a DC microgrid protection scenario can be discussed based on:
• Fault detection
• Fault analysis
• Fault isolation
• System restoration
• Protection coordination
• Communication protocols
• Stability
As discussed in this report, the voltage and current are the two parameters available for the DC
microgrid fault detection. There are many detection techniques reported in the literature and the
popular schemes with pros and cons are discussed in Section 4. However, fast fault detection
schemes need to be developed to minimize the fault clearing time.
Fault analysis is another wide area where proper standards and guidelines are required. There
should be a clear understanding between temporary and permanent faults and the controller
needs to generate the trip command to isolate the faulted portion. Therefore, the analysis
techniques and reclosing strategies should be more focused on time and fault characteristics, and
the component that need to be protected. Isolation of a fault mainly influenced by the
performance of the protection devices are discussed in Section 3.
Due to the nature of power electronic devices and its control techniques, the DC microgrid
components are very sensitive to disturbances and faults. This may lead a voltage collapse of the
DC microgrid. Therefore, the fault clearing and restoration time should be kept to a minimum in
order to improve the performance of the system. The application of solid state technologies for
faster protection devices with low on-state resistance need to be investigated.
Without appropriate standards and guidelines it is difficult to address the DC microgrid system
restoration strategies. There should be more research on this topic to develop proper guidelines
for the closing sequence of primary and backup protection devices based on the fault
characteristics and system components. There are many communication standards available for
DC power distribution but they need to be modified for the DC microgrid system.
Protection schemes relying on a communication network generally increase the fault clearing
time. Therefore, communication protocols should be focused on the size of the DC microgrid and
system components. Due to the resistive impedance nature of DC microgrid systems and lack of
physical inertia, system stability is a major issue during fault conditions. The system stability
during fault and restoration is another topic that needs more focused investigations and
guidelines.
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6. CONCLUSIONS
In this literature review, challenges of fault detection and protection techniques for DC microgrid
systems were highlighted. The types of faults were identified with possible fault locations. A
collection of DC microgrid protection schemes reported in the literature were also reviewed.
The protection schemes are widely classified into unit/non-unit and single ended/double ended
schemes. Fault detection in DC microgrid systems are a function of system configuration,
components, size, speed, time etc. The selection of the protection devices should consider the
available fault detection scheme and above mentioned parameters.
Conventional DCCBs have many disadvantages, like more fault clearing time, arcing, etc.
However, introduction of SSCBs and hybrid CBs mitigates the disadvantages and improves the
overall performance of DC microgrid systems. The effectiveness of the protection schemes are
also impacted by the communication challenges and grounding configurations. In islanded mode,
the fault severity is more and the design of system grounding is a crucial factor for avoiding
voltage fluctuations due to the common mode voltage.
Recently, DC microgrid technology is gaining more popularity because the distribution systems
from generation to consumption level are experiencing a shift toward DC. Lack of guidelines and
non-standardization of system parameters, such as voltage ratings, grounding schemes, control
and communication protocols, protection and restoration schemes, etc. are some of the main
issues preventing the DC microgrid from being a future power solution.
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DISTRIBUTION
1 MS1033 Jimmy Quiroz 8812
1 MS1033 Abraham Ellis 8812
1 MS1140 Matthew J. Reno 8813
1 MS0899 Technical Library 9536 (electronic copy)
