Fault protection of a loop type low voltage dc bus based microgrids

1. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
240
FAULT PROTECTION OF A LOOP TYPE LOW VOLTAGE
DC BUS BASED MICROGRIDS
E.G.CHINJU1
, S.RESHMILA2
1
Dept. of EEE, Sree Narayana Gurukulam College of Engineering, Kolenchery, Ernakulam, Kerala, India
2
Dept. of EEE, Sree Narayana Gurukulam College of Engineering, Kolenchery, Ernakulam, Kerala, India
ABSTRACT
The micro grid system can be divided in to AC micro grid and DC micro grid .As compared to DC system, the
traditional AC system have the years of practical experience and standards. However the problem with the AC grid still
persists. The DC micro grid system has lesser number of conversion stages and it reduces the losses, so the system
efficiency becomes increases. But the fault protection of DC system is very difficult because when a fault occurs, it
completely de-energizes the system. A loop-type dc-bus-based micro grid system against various faults is presented in
this paper. The goal of the proposed system is to detect the fault in the DC bus and to separate the faulty section only so
that the healthy portion keeps operating without any disturbances. The proposed scheme consists of one master controller
and two slave controllers which are connected at two ends of the transmission line. By using the differential relaying
principle the controller opens the power switches during fault. In addition, a converter protection is also provided which
acts as a back up protection for the feeder. The MATLAB Simulink results are presented for the proposed concepts.
Keywords: DC Distribution, Fault Protection, Micro Grids, Power System Protection, Sold State Switch
1. INTRODUCTION
The demand of electricity increases day by day and which causes an imbalance between supply and demand.
Now a days, many distributed power system have been researched and developed to meet this increasing demand by
using renewable energy resources such as photo voltaic systems, wind energy systems and other distributed energy
resources [1],[2]. The application of distributed resources in the electric power system opens new possibilities. Because
the distributed systems posses many advantages such as better operational and economical generation efficiency, capacity
to relief transmission and distribution, improved reliability , power quality and eco-friendliness.
Microgrid is an isolated electric power system that uses bidirectional communication between distributed energy
resources, loads and control systems to improve power quality, optimal use of renewable energy resources and possibility
of consumers to manage their electricity usage and minimize their expenses [3],[4]. The microgrid can be classified into
AC bus and DC bus systems depending on the characteristics of the grid supply voltage. Currently all the commercially
installed micro grids are AC because they are simple and easy to control [6]. Also they have the benefit to utilize existing
AC grid technologies. But AC system faces many drawbacks such as stability, the need for reactive power control,
reliability problems and the increasing number power electronic converters is responsible of increasing waveform
distortions and power losses [5],[7]. There is more scope for DC bus systems instead of AC systems because most
distributed generation sources such as PV, fuel cells and variable speed wind power units directly generate DC power.
Moreover the DC systems do not have the problems like skin effect, waveform distortion that AC systems do. By using
DC power as a medium, the typical ac-dc-ac power conversion stages can be reduced and which in turn reduces cost and
size of the system [7]. A diagram of DC bus micro-grid is shown in Fig 1.
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Fig.1.DC bus microgrid
However, the DC distribution system needs further development in the field of protection. Due to the lack of
guidelines and experience, protection has been challenging in DC systems [8].This paper presents a fault protection
method for low voltage DC microgrid systems. The primary goal of the proposed scheme is to detect the fault in the bus
segment between the devices and then to isolate the faulted section only, so that the other healthy segment continue to
provide power to the load and which in turn maintaining the supply continuity to the load. To achieve this, a loop type dc
bus based microgrid system is proposed. The loop type system has good efficiency if the distribution system is not so
long. The secondary function of the proposed scheme is to protect the converter from the fault occurs in the DC bus
segment. The converter protection in this scheme acts as a back up protection for the feeder protection.
1.1. Low Voltage DC Microgrid
The Low Voltage DC (LVDC) micro grid system is relatively a new concept in distribution system. As
compared to traditional AC distribution system, LVDC systems have many advantages. LVDC micro grid has been used
in office buildings with sensitive computer loads and rural power systems [9]. And also the sources, loads and energy
storage can be connected through simpler and efficient power electronic interfaces. The losses in the DC system are less
because it requires lesser number of power conversion stages and it can deliver 1.41 times more power than an AC
system for the same cable [10]. When comes to system protection, it is difficult in DC system because of no current zero
crossing.
The possible faults in a DC system are line to ground fault and line to line fault [8]. A well designed protection
method is necessary to ensure reliable operation of the LVDC microgrid. The common practice in DC system is to install
protection on AC side where AC and DC grid is connected. The protective devices commercially available for LV
systems are DC protective switch gear as well as conventional AC devices such as CBs and fuses. But during fault the
AC CBs are problematic because this scheme shut down the entire system and which causes unnecessary outages in the
healthy system [11],[112]. Fuses are slow thermal devices and they do not have the ability to distinguish whether a fault
is temporary or permanent. To overcome these limitations, now a day’s solid-state CBs such as IGBTs are used in DC
system. These are bidirectional solid state switches and they offer fast switching operation and high short circuit current
capability [13].
1.2.Converter Protection
Both AC/DC and DC/AC converters are used to interface different sources and loads to the DC bus. To achieve
high efficiency, the converter must be able to handle voltage variations in the DC grid. The internal switch fault in the
converter cause short circuit fault [7]. In most cases, this fault cannot be cleared and the only solution is to replace the
device from the system. Also due to the fault in the DC bus side, there is a chance of fault occur in the converter. The
fault current withstanding capacity of converter is only twice the converter full load rating. So the fault current must be
detected and extinguished as quickly as possible [8].
2. PROPOSED FAULT PROTECTION SCHEME
2.1. Proposed Loop Type DC Bus System
The proposed system does not require a complete isolation of the system; only the affected area is separated and
de-energized. The healthy buses keeps operating, provide a power to the load and maintaining the supply continuity. A
loop-type dc-bus-based micro grid system against various faults is presented in this paper. The loop type system offers
good efficiency if the distribution line is not so long [14]. The proposed scheme consists of one master controller and two

3. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
242
slave controllers, freewheeling branches between each line and ground. The slave controllers connected at two ends of
the transmission line. The diagram of proposed protection scheme is shown in fig.2.
Fig .2. Proposed protection scheme
The figure shows protection of one bus segment only. The implementation of proposed scheme is shown in fig
3. Snubber circuits are used to protect the solid state CBs from voltage transients.
Fig.3.Implementation of proposed scheme.
2.2. Detection and Isolation of Fault
In this scheme, the fault detection and isolation is based on the differential relaying principle. The slave
controller read the current at two ends of the line connecting two components and provides this information in to master
controller. Under normal condition, input and output currents are nearly same. If there is any fault occurs in the line,
there is a current difference. When the difference exceeds a critical value, the master controller gives an appropriate
command to open the IGBT switches, so that the faulted line is isolated from the healthy section and system can continue
to operate on the loop-type bus [7]. After isolation, the fault current in the faulted bus segment flows through the
freewheeling diodes and resistance. The freewheeling path resistance determines fault current extinction. When the
resistance is large, the fault current extinguished very quickly [7]. The path of current during fault and freewheeling time
is shown in fig 4.
(a)

4. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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(b) (c)
(d)
Fig.4.Fault current path and its extinction in proposed scheme. (a) Line to ground fault. (b) Fault current
extinction in line to ground Fault. (c) Line to line fault. (d)Fault current extinction in line to line fault.
This is the primary protection of the feeder. If this protection fails to clear the fault, the back up protection must
operate and protect the feeder. This is similar to the conventional method. There is a main switch in between converter in
the source side and DC bus. If the primary protection fails to clear the fault, the main switch just open and cut the supply
to the DC bus and hence de-energizes the complete system as like in the conventional method. In all other cases, the main
switch in the closed condition. This is also a protection for the converter. Because if the feeder protection is fails to clear
the fault, there is a large fault current in the DC bus. The converter is directly connected to the bus and so have low
impedance. When the DC link voltage becomes almost zero, the fault current will flows through the converter and leads
to failure of the converter. So the fault current must be detected and extinguished as quickly as possible
3. SIMULATION RESULTS
A MATLAB simulation has been performed for a LVDC microgrid system. It consists of two sources; three
phase AC and battery are connected to feed a common load. A Three phase 440 V supply is given to AC/DC converter
connected at the source side. A line to ground fault is created in the middle of the bus segment A only. The simulation
circuit is shown in fig.5. The input source is a three phase AC which connected to the DC bus through an AC/DC
converter. Battery source is connected between line segment B and line segment C. These two sources are connected to a
common load. The simulation parameters are given in the table 1. The main switch is connected in between the converter
and the DC bus.

5. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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(a)
(b)
Fig.5.Simulation circuit. (a)Complete diagram of Proposed scheme.(b) Line segment A
TABLE 1: Simulation parameters
A positive line to ground fault occurs in the middle of the bus segment A at 0.5ms. The load voltage with and
without protection is shown in fig.8.Without protection, the load voltage drops to zero after fault is shown in fig

6. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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6(a),when protection is used, the voltage is quickly restored after the separation of the faulted segment which is shown
fig.6(b).
(a) (b)
Fig.6.Load voltage.(a) without protection.(b)with protection
When the IGBTs in the line segment A open due to fault, the fault current freewheels and extinguished through the
resistor is shown in fig 7.
Fig.7.Fault current in the freewheeling path
Fig.8 and fig.9 shows the source side bus current and load side bus current for a line to ground fault with and
without protection. The bus current is high during faulty condition and goes to zero after separating the bus.
(a) (b)
Fig.8. Source side bus current (a) Without protection (b) With protection

7. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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(a) (b)
Fig.9.Load side bus current (a) Without protection (b) With protection
If the IGBTs in the segment A fails to clear the fault, the main switch just open and cut the supply to the DC bus
and de-energizes the complete system.Fig.10.shows the main switch operation. The gating signals to the main switch are
shown in fig.10 (a) and the output voltage is shown in fig.10 (b).
(a) (b)
Fig.10.Main switch operation under failure of protection.(a)Gating signals. (b) Load voltage
In the fig10 (a), first shows the fault, second one is status of switches in the segment A and third shows the
gating signal to the main switch. A fault is occurs at 0.5ms, from the figure it can be seen that, after the fault also the
IGBTs in the segment A are conducting. So to protect the converter and the complete system, it is necessary to switch off
the main switch immediately. So a gating signal is send to switch off the main switch, which is shown in the third figure.
Then the out put voltage goes to zero.
If the protection of line segment is proper, the main switch operation is shown in fig11.The gating signals to the
main switch and corresponding load voltages are shown in fig11.(a) and fig 11.(b) respectively.
(a) (b)
Fig.11.Main switch operation under proper protection.(a)Gating signals. (b) Load voltage

8. Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
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A fault is occurs at 0.5ms and at the same time itself, the gating signals of the switches goes to zero which is
shown in second portion of the fig11(a). Therefore the already conducting switches in the line segment turned off and
separate the segment from the healthy system. The third portion shows the gating signal. So the main switches are
conducting to provide a power to the load through other healthy bus segments. Then the output voltage restored after the
separation of the faulted segment.
4. CONCLUSION
The application of distributed resources opens new possibilities in the electric power system. There is more
scope for DC microgird systems. But the protection of DC bus is more challenging. The proposed protection scheme
avoids the complete shut down of the system and isolates the faulted section only so that the other healthy segment
continue to provide power to the load and which in turn maintaining the supply continuity to the load. A loop type DC
bus based microgrid system was utilized. This provides additional protection to the converters. If the primary protection
fails to clear the fault, the main switch which is connected in between source converter and the DC bus, just open and cut
the supply to the DC bus and hence de-energizes the complete system as like in the conventional method. This is act as a
back up protection for the feeder. The proposed protection concepts have been validated by MATLAB simulink. This
scheme can be applied to green buildings with sustainable energy resources, data center with a server array etc.
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