Review of DC Microgrid System with Various Power Quality ...
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Majlesi Journal of Mechatronic Systems Vol. 8, No. 3, September 2019
35
Review of DC Microgrid System with Various Power Quality
Issues in Real Time Operation of DC Microgrid Connected
System
Ila Rai1, S. Ravishankar2, R. Anand3
1,2,3-Department of Electrical and Electronics Engineering, Amrita School of Engineering, Bengaluru, Amrita Vishwa
Vidyapeetham, India.
Email: [email protected], [email protected] , [email protected]
Received: May 2019 Revised: June 2019 Accepted: July 2019
ABSTRACT:
This paper presents an overview of the various microgrid architectures of DC microgrid and power quality issues
including harmonic currents, inrush currents, bus fault,circulating currents and grounding of the system. A microgrid
utilizing a DC bus can avoid various power conversion steps, and hence minimize the loss incurred as compared to AC
bus. The high frequency semiconductor devices with greater voltage blocking capacities are essential components of
DC microgrid. With the advent of new power electronic devices and circuits, the practical implementation of DC
microgrid still suffers from various power quality issues and device limitations. This paper investigates and compares
the power loss and harmonics for different semiconductor devices with the help of MATLAB and the experimental
findings indicate that SiC device implementation significantly increases energy efficiency and enhances microgrid
power quality.
KEYWORDS: DC Microgrid, Harmonic Analysis, Inrush Current, Bus Fault, Power Quality.
1. INTRODUCTION
The energy landscape is rapidly changing because
electrical systems are balancing a range of challenges
around the globe.Environmental concerns and gradual
depletion of fossil fuel gave rise to a growing increase in
the penetration of distributed generators that incorporate
renewable energy sources, energy storage systems, and
new types of loads like electric vehicles and heat pumps
in the present-day electrical systems [1]. The concept of
microgrid is proposed as a possible answer to control the
impact of distributed generation and to make
conventional grid more suitable for large scale
deployment of distributed generation since it can work
independently, or in grid-connected mode [2]. Even if
notable progress has been made in enhancing the
performance of AC microgrids over the previous
century, DC microgrids have been acknowledged as
more appealing for various uses owing to greater
effectiveness, more natural interface to many kinds of
RES and ESS, better compliance with consumer
electronics, etc. The control system for DC microgrid is
less complicated as there are no issues related to reactive
power and frequency regulation. Sources can be directly
connected to DC microgrid through a controllable
electronic interface converter and regulation of common
DC bus voltage is the main challenge for proper control
operation [3]. The power quality issues of DC
microgrids differ in many respects from those in grid-
connected AC microgrids. The idea of power quality
issues in DC microgrid comes from the power electronic
converters which is the pillar of DC microgrid structure
which are often overlooked in DC microgrid. DC
microgrid, an emerging technology, needs to tackle
these issues to make its practical implementation
possible. The essential DC microgrid power quality
issues acknowledged in the literature are harmonic
currents, inrush current, fault current, and grounding,
voltage transient, Electro-Magnetic Interference (EMI),
communication challenge, voltage unbalance issue and
challenge to minimise the circulating currents [4].
To enrich the DC microgrid research field, these
issues are critically reviewed on real time operation and
reported in this paper.
The content of this paper is presented as follows: In
section II complete overview of different architectures
are discussed to motivate the analysis of DC microgrid
power quality issues. In section III DC microgrid
interfaced with AC grid is analysed. Section IV
discusses various power quality issues that have been
recognized as significant for design of DC microgrids.
Section V provides conclusions and summarizes the
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power quality problems that must be considered in the
design of a DC microgrid.
2. OVERVIEW OF DC MICROGRID SYSTEM
The DC microgrids are classified into different
categories according to voltage polarity, grid
architecture and bus topology.
A. Voltage polarity of DC microgrid:
DC microgrids are using two-wire systems
(unipolar) or three-wire systems ((bipolar) currently for
power transmission [5-8]. The available voltage levels
are different in numbers in these two configurations. The
DC microgrid can be unipolar or bipolar according to
voltage level. The unipolar System provides only one
voltage level while bipolar system provides three
voltage level for consumer.
DC microgrid with unipolar system
DC microgrid with bipolar system
DC microgrid with unipolar system:
Unipolar system transmits energy at single voltage
level over the bus. Positive and negative poles of the bus
are used to connect various sources and loads as shown
in fig.1.Selection of voltage level is very important as
there is only one voltage level for different loads. Power
transmission capability increases if the selected voltage
level is high, but it requires more no. of DC-DC
converters. Another issue with high voltage level is
increased safety risk. At low voltage level these issues
can be minimised and can be a conceivable answer for
off-grid houses in remote zones. The unipolar system
does not face the problem of asymmetry between the
poles and utilises less no. of DC-DC converters and
hence simple to implement.
DC microgrid with bipolar system:
The DC migrogrid with bipolar system is a three-
wire system and consists of positive line, negative line
and neutral line as shown in Fig. 2. This system can
provide option to choose three voltage level: +Vdc, -Vdc
and 2Vdc as any of the two wires can be used for power
supply. The bipolar system provides better dependability
and accessibility of the power during fault since the
power can be transmitted using other two wires under
fault condition. Bipolar System offers more flexibility
for connecting different loads, but unequal distribution
of loads can create unbalance in the system. System
unbalance can be minimised with the help of voltage
balancer and suitable control for power converter [9].
DC microgrid architecture and grounding scheme will
depend on unipolar and bipolar topology in future.
Fig.1. Unipolar DC Microgrid
Fig.2. Bipolar DC Microgrid
B. DC microgrid architecture:
The integration of renewable energy sources such as
wind energy and solar energy with energy storage
systems in DC Microgrid scheme is continually growing
with enhanced demand for electricity, making it a very
significant form of electrical distribution scheme. The
power generation capability of renewable energy
sources relies on weather conditions and is inherently
unsure and variable. Therefore, interfacing of DC
microgrid with the AC grid is of great significance to
improve DC microgrid system reliability and
accessibility. A DC microgrid can be interfaced with AC
grid in many ways such as:
Radial type of configuration
Ring or loop type of configuration
Interconnected type of configuration
Each type of configuration has its own advantages
and disadvantages and in view of these different
arrangements various types of DC microgrid
architectures are possible. In recent years different types
of DC microgrid architecture is already discussed. This
section reviews different configurations in detail which
are mentioned above.
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Radial type of configuration
DC bus and AC grid are connected together at the
end providing only one route for power flow in this
configuration. Each load is linked to the grid using just
one route. The DC bus scheme used may be either
unipolar or bipolar depending on its requirements and
applications. The radial architecture is appropriate for
low voltage applications and can be used in residential
buildings. Radial configuration prevents additional
conversion and reduces distribution losses as grid
interface and loads can be installed close to each other.
Single-bus topology is most frequently used in a DC
microgrid system as it is appropriate for low voltage.
Microgrid can be linked straight to the energy storage
system and no. of battery cells in series can be decided
based on the load voltage [10], [11]. Single bus topology
is dynamically stable but suffers from voltage
fluctuation in the common DC bus because of the linked
energy storage system which limits its implementation
[12]. Another problem is unregulated battery charging as
it needs to coordinate with several parallel converters
linked to microgrid which generates an imperfection in
bus voltage. This increases the wear and tear of batteries
and the imperfection in bus voltage leads to circulating
current problems and also causes uneven loading of
converters[13][14]. On the other hand, when connected
via converter, energy storage system provides flexibility
in control, the system's performance and reliability can
also be improved. Using multiple battery stacks for
energy storage system, the scheme reliability can be
further enhanced, and this system can therefore be used
to supply loads in a variety of areas. This topology is
most widely discussed in the literature [15–17]. Despite
its attractive features, this type of topology suffers from
several technical problems. Since energy is provided via
a single bus, careful design is required for the control
parameters and circuits. A multiple bus structure is
suggested to provide the practical solution for these issue
[18], [19]. Multi-bus configurations of DC microgrid
can provide higher reliability and availability compared
to single bus topology and an alternate solution for
technical problem associated with single bus system. In
case of failure faulty buses can be isolated and each
microgrid can share the power from its neighbouring
microgrids [20-22]. Power transfer between all DC
buses can be controlled by monitoring local voltage
deviations at nominal value using digital communication
technology [23].
This concept can also be used for a multi-DC
microgrid scheme. Two or more microgrids can be
linked in series or parallel via DC bus in multi-DC
microgrid system and each microgrid can have its own
energy storage and renewable energy sources together
with different loads. The parallel radial architecture
offers more reliable operation as only defective buses
can be isolated and normal operation in healthy buses
continues [24]. This type of configurations has added
benefits such as different voltage level (in bipolar) and
simplicity. All nearby buses (multi-bus architecture) can
share energy in this type of configuration. Buses
connected after and before a faulty bus cannot share the
energy with the other part of the arrangement in a multi-
bus scheme [25] [26].The series connected radial
architecture does not provide flexibility in fault
conditions. In a series radial single bus system, a fault
may disturb all the consumer connected to it and in multi
bus system the buses connected after and before a faulty
bus cannot share the power with the other part of the
system [27].
Ring or Loop type of configuration
A ring type of configuration consists of more than
one path between the customers and grid interface and it
can overcome the limitations of radial configuration.
The faulty bus can be disconnected through switches
which are connected at the ends of each DC bus. This
offers resilience to the system during fault condition. All
the buses including their interface with other
neighbouring buses are controlled by an Intelligent
Electronic Device (IED) [28]. The IED isolates the
faulty bus from the system after detecting the fault in that
bus and gives a provision for a different path to supply
power during fault condition. This type of distribution
system is better than radial system and can be used for
industrial applications. Both radial and ring type
configuration mainly depend on the grid supply. The
drawback of the DC migrogrid system with radial and
ring type configurations is that it cannot get required
supply during fault condition in AC grid.
Reconfigurable topology is suggested to improve
system flexibility during faults or periods of
maintenance. This type of configuration can be either of
ring type or zonal type [29]. The common dc bus linked
to the load can be supplied bidirectionally in ring type
setup to provide alternative route to the DC bus during
fault. Multiterminal DC (MTDC) system is proposed in
[30–32] for HVDC system.
Multiple input terminals are used to supply DC
distribution network and connected in mesh type of
configuration. Power flow in this setup is though flexible
but more complex than in other configurations since the
system has various power transmission routes.
Interconnected type of configuration
In interconnected type of configuration, the DC bus
is connected to grid through more than one supply. It
ensures an alternative supply from AC grid in case of
fault in one or more feeders. This improves the
dependability of DC microgrid system. The system can
be connected in two ways:
Mesh type configuration
Zonal type configuration
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Mesh type configuration is also known as
multiterminal grid configuration and each AC grid
interface is connected to DC grid through different AC-
DC converter. This system offers better reliability in
comparison with the radial or the ring type configuration
because of more no. of accessible AC feeder.
This type of architectures is best suited for High
Voltage Direct Current (HVDC) system and mostly used
in either wind farms or for transmission and distribution
of underground urban sub system [8].
There are different techniques of detecting and
isolating the faulty DC bus. A new technique known as
"handshaking technique" is proposed to locate and
isolate the defective DC bus. This technique can restore
the system even if there is no internal communication in
AC-DC converters [33].
A zonal type of configuration has been proposed in
[34-35]. In this configuration distribution system setup
is partitioned into different zones and each zone consists
of generation, energy storage system, power converter
and switchgear to supply power to a group of loads. Two
redundant DC buses are connected to each zone and
these buses get power from distributed sources of energy
and AC grid connected to it. This type of configuration
has better reliability compared to mesh configuration.
This system offers various options for load energy
supply, and power can be provided during fault
condition to load through another feeder. This setup
provides greater flexibility as the number of switches
linked to this scheme is large and has modular
configuration which is suitable for distribution planning.
This model is primarily used in shipboard power
supplies [36-37]. Reconfigurable topology can be used
in zonal type of configuration. Multiple DC distribution
units are split into different zones and linked in series in
reconfigurable topology system.
3. DC MICROGRID INTERFACED WITH AC
GRID
The microgrid should function reliably under two
conditions: either in isolated from or connected to the
utility grid. Most of the DC microgrid is interconnected
to existing AC grid via converter and proposed for
bidirectional power flow to enhance system stability.
The microgrid would be designed and operated such that
it represents as a single predictable load or generator to
the grid at the point of interconnection. Hence, it is
essential to analyse how electrical power flows between
DC and AC networks in a DC microgrid system. There
are various types of AC-DC converter topologies used
for grid interface. Diode and controlled rectifier
topology, Active Front End (AFE) topology, Special
topologies are few of them. Diode and controlled
rectifier topology work on unidirectional power flow
and suffers from low order harmonics present in line
currents. A single-phase rectifier or a three-phase
rectifier is used for system requirements while passive
filters are connected to the front side of the rectifier to
improve the quality of line currents [38]. Active Front
End (AFE) topology uses bidirectional power flow
converter which delivers a high quality sinusoidal line
current. A suitable damping technique is required to
enhance the system's stability. AFE converters
connected in parallel to a common DC bus in the scheme
can produce circulating current during changing states.
Either switching frequency can be improved or PWM
patterns can be regulated to decrease the circulating
current [39]. To improve the power factor and quality of
line current a diode rectifier with boost converter is used
as special topology. This topology proves to be cost
effective with better reliability as less no. of switches are
used [40]. Some research has been performed in the
recent literature, focusing on the power architectures of
the interconnecting energy conversion devices between
DC microgrids and AC grids. The interconnecting
system can be categorized into three arrangements, i.e. a
single, parallel, and combined converter system. Small
scale system uses single bidirectional converter for
connecting DC microgrid to AC grid as power
generating capacity is less. Parallel interface converters
can be used to increase the power capability exchange
between DC microgrids and AC grids [41].Combined
converter system mostly used in hybrid AC and DC
microgrid.
4. POWER QUALITY ISSUES IN DC
MICROGRID
The idea to study about power quality issues in a DC
microgrid arise from the many power electronic
converters which form the backbone of the system and
are often overlooked. DC microgrid, an emerging
technology, needs to tackle these issues to make its
practical implementation possible. Some real case issues
such as fault current, inrush current, grounding and
harmonics due to converter are discussed in this section.
The DC microgrid can either be connected to an AC grid
interface to improve the dependability and accessibility
of the system or it can work in an islanding mode without
any grid interface. Therefore, in DC microgrid power
quality issues can either come from AC grid or from
internal connection. The various issues related to power
quality in DC microgrid are:
(A) Harmonic currents
(B) Inrush current
(C) Bus faults and voltage unbalance
(D) Circulating currents
(E) Grounding
A. Harmonic currents:
The DC bus serves as a link between multiple
electronic power converters. Harmonics can result from
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the nonlinear effects of the various electronic power
converters on a DC bus [42]. Power electronics devices
connected in a DC microgrid mostly work at high
frequency. These converters draw distorted current
(non- sinusoidal) from main supply. According to the
Fourier analysis any non-sinusoidal signal will contain
harmonics in addition to the fundamental [43]. These
Harmonics due to distorted current drawn by converter
can create a severe power quality problem in DC
microgrid. Current harmonics can produce unwanted
effects such as improper EMI, resonance currents and
voltage oscillations which can be very harmful for the
system. Furthermore, although DC systems work on
unity power factor, the harmonics produced in the
system can reduce its value [43].Converters are used to
increase or decrease the magnitude of voltage and
sometimes used to invert the polarity if required. The
switch is realised with the help of power MOSFET and
diode. Thyristor, IGBT, BJT are also utilised according
to the application. To minimise switching and
conduction losses ultrafast 1200V IGBT is available in
market and widely used in renewable technology.
Silicon Carbide (SiC) MOSFET known as SiC-based
power electronics are also very much in use for
renewable technology. It can reduce the size and
switching losses in power system by 50% focussing
especially on the high-power electronics application
such as power utilities, smart grids, high-power
industrial drive, and renewable energy panel [44]. In this
paper, the performance of Si MOSFET, Cree 1200V-
SiC MOSFET (C2M0080120D) C2MTM and Infineon
1200V- IGBT Cool MOSTMCFDA power Transistor
(IPW65R110CFDA) are analysed and compared for
conduction loss and harmonic content according to
IEEE STD - 519. Table -1 presents the value of DC side
grid voltage, Total harmonic distortion and harmonics
present in the system using different devices. We can
observe SiC MOSFET based converter presents lower
power loss and higher efficiency compared with Si
MOSFET and 1200V IGBT. DC side and AC side
harmonics are also compared. It is observed that device
using SiC MOSFET doesn’t have 5th and 7th harmonics
on DC side .AC side comparison shows that SiC
MOSFET based device has low harmonic content
compared to Si MOSFET and 1200V IGBT based
devices. Hence smaller current harmonics of the
converter based on SiC MOSFET produces better power
quality output. The Application of SiC device will
greatly reduce the power loss and harmonic content and
hence improve the efficiency of the DC microgrid. Since
packing density is very high for SiC based device, it will
open up the opportunities for better system design
optimization minimizing the size and weight of power
converter.
5. RESULT BASED ON SIMULATION DONE IN
SIMULINK
Table 1. shows the comparison of output voltage and harmonic distortion for Si MOSFET, SiC MOSFET and 1200V
IGBT.
Device Harmonics Output Voltage
SI MOSFET Total
Harmonic
Distortion
18.13% 143.9 V
H1 1.56%
H3 6.89%
H5 3.60%
IGBT Total
Harmonic
Distortion
9.62% 182 V
H1 2.09%
H3 3.30%
H5 1.61%
SiC MOSFET Total
Harmonic
Distortion
7.55% 182.7 V
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H1 1.65%
H3 2.19%
H5 0.74%
B. Inrush currents:
EMI filters are used to minimize electromagnetic
interference in DC microgrid interfaced with AC grid.
Inrush current flows through EMI filter and can create
oscillations in DC bus voltage. This will disturb the
normal operation of equipment connected to the
common DC bus [45].
Inrush Current
Fig. 3. Path for inrush current
Capacitance charging can generate an inrush current
irrespective of load and sometimes sufficient enough to
weld the contact creating arc which can damage the
system [46]. A high inrush current can cause voltage sag
in the microgrid .Despite of system design to withstand
the resulting physical stresses it can influence the
operation of other equipment connected to the system.
Small filter capacitance can reduce the amount of inrush
current but may not be sufficient for EMI generated in
the system. To minimise the effect of inrush current pre-
charge circuits and soft start circuits are proposed [47].
C. Bus faults and voltage unbalance:
A fault in DC bus can draw the current from energy
sources connected through the converters. Therefore, the
limit of fault current is decided by the power rating of
different devices such as energy sources, converters,
energy storage system and DC bus capacitor. A fault
current with low power can generate instabilities in
voltage on the different parts of the system and because
of low power fault the protection system setting can
confuse between real fault and heavy load conditions.
Series of fault can develop an arc which is difficult to
extinguish because of absence of natural zero crossing
points in DC system. Transients in DC bus are generated
because of frequent on and off connections of load and
can create voltage oscillations. Unequal distribution of
load can be responsible of creating voltage unbalance in
the system [48].
D.Circulating currents:
The load currents should be divided equally for all
converters connected in parallel with constant and same
voltage level. Any disparity in converter output voltage
will lead to initiation of circulating current. These
circulating currents will increase the amount of current
flowing through the switches. This results into higher
power ratings of power electronic switches and also
losses. The difference in current sharing because of
circulating current can cause converter overloading. The
load sharing cannot be proper as the converter with
higher output voltage must provide more load current.
Load variations, fluctuations in source energy,
parametric changes and feedback error are the major
cause of output voltage variations. A circulating current
can be initiated with very small variations in output
voltage and can cause the difference in current sharing
which in turn degrade the system performance [49].
If converters are linked to the common DC bus, then
the circulating current can be a big problem. The
circulating current can flow between the converters
because of common point of grounding at the converter
sides.
E. Grounding:
There are many literatures discussed about the
possible grounding schemes to ensure the safety of the
equipment connected to DC microgrids [50–52]. In DC
microgrid either positive or negative bus can be
grounded according to European Low Voltage Directive
[53]. The DC microgrid is usually connected with main
grid with different grounding scheme (TN, TT and IT)
to improve the system reliability. If a DC grid is
interfaced with grounded AC utility network and DC bus
conductor (positive or negative) is connected to ground,
it will create a permanent short circuit fault through
ground. An isolated DC microgrid grounding
configuration and a TN network in AC grid side can
cause a fluctuation in voltage level because of common
mode voltage generated by converters [54]. The
situation can be worst if this fluctuation enters in the DC
bus connected to the converters as it creates a current
which circulates within the converters and affects the
DC microgrid grounding system. To minimise the
unwanted circulating current is a big challenge in DC
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microgrid system design. The aim of the system
grounding is to sense fault, minimise stray currents and
to maximise the protection by minimising common
mode voltage. The grounding arrangement not only
depends on bus voltage level but also on the setup of
power electronic converters including suitable wires ,
busways and connectors[55]. Grounding schemes used
for DC microgrid are as follows:
(a) TN-S grounding arrangement (b) IT grounding arrangement
(a) TN-S grounding arrangement: In this arrangement
one of the poles is connected to protective earth. Mostly
positive pole is connected to earth as shown in fig. 4.
This type of grounding is used for low voltage system.
(b) IT grounding arrangement: When DC voltage
considered is high (about 380V-400V) then IT
grounding arrangement is preferred. The different
possible grounding arrangements are as follows:
Non-isolated type of DC bus grounding scheme
Non-isolated type of DC bus mid-point
grounding scheme
Isolated type of DC bus grounding scheme
(i) Non-isolated type of DC bus grounding scheme
The DC bus (positive bus or negative bus) is directly
attached to the earth in non-isolated type of grounding
scheme and mostly negative bus is connected to the
earth. In case the live terminal comes in straight contact
with a person, the current is decided based on the
transient current, loop impedance and body impedance
of that person. This current can be very high if the
voltage in DC link is high. The fault current can flow
through the negative bus connected to earth as it
provides a low impedance path [57].
(ii) Non-isolated type of DC bus mid-
point grounding scheme
There is another probable arrangement for
grounding, the DC midpoint grounding, mostly used for
a bipolar DC bus system. Midpoint of coupling capacitor
is connected to earth as shown in fig.5. This provides the
safety by limiting current in case of line to ground fault
.In case a human body touches the live terminal, the
current entering the body will be reduced because during
a fault condition only part of the DC voltage is exposed
to the body and hence minimising the risk.
(iii) Isolated type of DC bus grounding
scheme
The arrangement in which DC bus power return is
isolated from the equipment ground is called isolated
type of DC bus grounding scheme and used to break the
fault current loop. However, after isolation also the fault
current can enter in the both AC and DC side converter
system via stray capacitors and EMI filter capacitors.
This type of grounding arrangement cannot provide
good option to detect fault current accurately.
Fig.4. TN-S Grounding Arrangement
Fig.5. IT Mid-Point Grounding Arrangement
6. CONCLUSION
This paper discussed about different architecture of
DC microgrid including various power quality issues.
The harmonics present in the system degrade the system
performance. Hence, this paper discussed the power loss
and harmonics for different devices through real time
simulation done in MATLAB .The results are compared
between Si MOSFET, IGBT and SiC based device. The
experimental result shows that SiC MOSFET based
device has low harmonic content compared to Si
MOSFET and 1200V IGBT based devices and can
increase the efficiency of microgrids. Although DC
microgrid is not utilized fully in practical application but
with different architecture and devices it can be a
promising future for smart grid. This advancement will
lead to recommendations of new hardware designs and
different arrangements for dc bus to make the existing
power system extremely reliable and accessible.
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