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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/326957897
DC Microgrid Protection: Review and Challenges
Technical Report · August 2018
CITATIONS
0READS
266
4 authors, including:
Some of the authors of this publication are also working on these related projects:
Rapid QSTS Simulations for High-Resolution Comprehensive Assessment of Distributed Energy Resources View project
Initial Operating Experience of the 1.2 MW La Ola Photovoltaic System View project
Sijo, Augustine
New Mexico State University
9 PUBLICATIONS 124 CITATIONS
SEE PROFILE
Jimmy E. Quiroz
Sandia National Laboratories
32 PUBLICATIONS 239 CITATIONS
SEE PROFILE
Matthew J. Reno
Sandia National Laboratories
96 PUBLICATIONS 888 CITATIONS
SEE PROFILE
All content following this page was uploaded by Matthew J. Reno on 10 August 2018.
The user has requested enhancement of the downloaded file.
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.
2
Issued by Sandia National Laboratories, operated for the United States Department of Energy by
National Technology and Engineering Solutions of Sandia, LLC.
NOTICE: This report was prepared as an account of work sponsored by an agency of the United
States Government. Neither the United States Government, nor any agency thereof, nor any of their
employees, nor any of their contractors, subcontractors, or their employees, make any warranty,
expressed or implied, or assume any legal liability or responsibility for the accuracy, completeness,
or usefulness of any information, apparatus, product, or process disclosed, or represent that its use
would not infringe privately owned rights. Reference herein to any specific commercial product,
process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily
constitute or imply its endorsement, recommendation, or favoring by the United States Government,
any agency thereof, or any of their contractors or subcontractors. The views and opinions expressed
herein do not necessarily state or reflect those of the United States Government, any agency thereof,
or any of their contractors.
Printed in the United States of America. This report has been reproduced directly from the best
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
7
4.6.8. Transient-Based Fault Protection ..........................................................45 4.6.9. Voltage and Current Derivative Supervised Protection ........................45
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.
12
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.
16
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
18
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]:
19
• 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