This document discusses the use of superconducting fault current limiters (SFCLs) to mitigate fault currents in smart grids with different types of distributed generation sources like wind farms, solar panels, and diesel generators. The author models an SFCL and a smart grid system in MATLAB/Simulink. Simulation results show that locating an SFCL at the point where distributed generation connects to the grid (Location 3) provides the best performance, reducing fault currents from all distributed generation sources by 68-84% while also limiting fault current from the main grid. In contrast, placing SFCLs upstream in the distribution system can unexpectedly increase fault currents from some distributed generators.

1. Placement of Super Conducting Fault Current Limiters to mitigate
fault Currents in Smart Grids with different types of
Distributed Generation Sources
1
Sai Sowmya Varanasi
Ramachandra college of Engineering,4th
B.Tech saisowmya.varanasi@gmail.com
Abstract— The important application of superconducting fault
current limiters (SFCL) for upcoming smart grid is its possible effect on
the reduction of abnormal fault current and the suitable location in the
micro grids. Due to the grid connection of the micro grids with the
current power grids, excessive fault current is a serious problem to be
solved. In this paper, a resistive type SFCL model was implemented
using Simulink and SimPowerSystem blocks in Matlab. The designed
SFCL model could be easily utilized for determining an impedance level
of SFCL according to the fault-current-limitation requirements of
various kinds of the smart grid systems. In addition, typical smart grid
model including generation, transmission and distribution network with
different types of DG’s having different ratings such as wind, solar,
diesel was modeled to determine the location and the performance of
the SFCL. A 10 MVA wind farm, 675 KW solar and 3MVA diesel
generator was considered for the simulation. Three phase fault has been
simulated at distribution grid and the effect of the SFCL and its
location on the wind farm, solar, diesel generator fault currents was
evaluated. The SFCL location in Smart Grid with different types
Distributed Generation Sources have been proposed and their
performances were analyzed.
Index Terms—Fault current, micro grid, smart grid, SFCL, wind farm,
solar, diesel.
1. INTRODUCTION
Conventional protection devices installed for protection of
excessive fault current in electric power systems allows initial two
or three fault current cycles to pass through before getting activated.
But, superconducting fault current limiter (SFCL) is innovative
electric equipment [1] which has the capability to reduce fault
current level within the first cycle of fault current. The first-cycle
suppression of fault current by a SFCL results in an increased
transient stability of the power system carrying higher power with
greater stability.
Smart grid is the novel term used for future power grid which
integrates the modern communication technology and renewable
energy resources to supply electric power which is cleaner, reliable,
resilient and responsive than conventional power system. One of the
important aspects of the smart grid is decentralization of the power
grid network into smaller grids, which are known as micro grids,
having distributed generation sources (DG) connected with them.
These micro grids need to integrate various kinds of DGs and loads
with safety should be satisfied. The direct connection of DGs with
the power grid are the excessive increase in fault current and the
islanding issue which is caused, when despite a fault in the power
grid, DG keeps on providing power to fault-state network . Solving
the problem of increasing fault current in micro grids by using
SFCL technology is the main topic of this work. In this paper, the
effect of SFCL and its position was investigated considering a wind
farm, solar [5], diesel generator [6] integrated with a distribution
grid model as one of typical configurations of the smart grid. The
impacts of SFCL on the wind farm, solar, diesel generator and the
strategic location of SFCL in a micro grid which limits fault current
from all power sources and has no negative effect on the integrated
Sources was suggested.
2. SIMULLINK MODELS SET-UP
Matlab/Simulink/SimPowerSystem was selected to design and
implement the SFCL model. A complete smart grid power network
including generation, transmission, and distribution with an
integrated wind farm, solar, diesel generator model [6] was also
implemented in it. Simulink /SimPowerSystem have number of
advantages over its contemporary simulation software (like EMTP,
PSPICE) due to its open architecture, a powerful graphical user
interface and versatile analysis.
2.1. Power System Model
The modeled power system consists of an electric transmission
and distribution power system. Newly developed micro grid model
was designed by integrating a 10 MVA wind farm, 675 KW solar,
3MVA diesel generator with the distribution network. Fig.1 shows
the power system model designed in Simulink/SimPowerSystem.
The power system is composed of a 100 MVA conventional power
plant, composed of 3-phase synchronous machine, connected with
200 km long 154 kV distributed-parameters transmission line
through a step-up transformer TR1. At the substation (TR2),
voltage is stepped down to 22.9 kV from 154 kV. High power
industrial load (6 MW) and low power domestic loads (1 MW each)
are being supplied by separate distribution branch networks. The 10
MVA wind farm, 675 KW solar, 3MVA diesel generator are
connected directly through Step up transformer to the grid in Fig.1
artificial fault three-phase-to-ground fault and locations of SFCL
are indicated in distribution grid. Three prospective locations for
SFCL installation are marked as Location 1, Location 2 and
Location 3. Generally, conventional fault current protection devices
are located in Location 1 and Location 2. The output current of
wind farm, solar, diesel measured at the output of DG for various
SFCL locations have been measured and analyzed.
2.2. Resistive SFCL Model
The three phase resistive type SFCL was modeled and
considering four fundamental parameters of a resistive SFCL.
These parameters and their selected values are 1) Transition or
response time 2) minimum impedance and maximum impedance
3) Triggering current and 4) recovery time .Its working voltage is
22.9 kV. Fig. 2 shows the SFCL model developed in
Simulink/Simpower System. The SFCL model works as follows.
First, SFCL model calculates the RMS value of the passing current
and then compares it with the Lookup table. Second, if a passing
Current is larger than the triggering current level, SFCL’s resistance
increases to maximum impedance level in a pre-defined response
time. Finally, when the current level falls below the
1
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2. Fig.1 Power system model designed in Simulink/SimPowerSystem,Fault and SFCL locations are indicated in the diagram
triggering current level the system waits until the recovery time and
then goes into normal state. SFCL is located at different locations.
Fig.2 Single phase SFCL model developed in Simulink/SimPowerSystem.
2.3. Wind Turbine Generator Model
A 10 MVA wind farm[1] is composed of 5 fixed-speed induction-
type wind turbines,each having a rating of 2MVA at the time of
fault the domestic load is being provided with 3MVA out of which
2.7 MVA is provided by the wind farm. The wind farm is directly
connected with the branch network B6 through the transformer
TR3.Simulink model of wind turbine generator is shown.
Fig.3 Three phase wind turbine generator subsystem model.
2.4. Solar model
The terminal equations of the Voltage, Current and Power [5]
used for the devolpement of Solar Simulink model are given by
equations 1,2 and 3.
Where Iph is the light generated current, Is is the cell saturation of
dark current, q is the unit electric charge, k is the Boltzman’s
constant t is the working temperature of p-n junction, A is the ideal
factor. Iph is the current based on cells working temperature and
solar irradiation level described in equation.2 . Isc is the cells short
circuit current Kt is the temperature coefficient and S is the solar
irradiation level in KW/m2
. Power delivered by PV array is given
by equation(3) and PV array is modeled as single diode circuit . The
subsystem can be implemented by using equations 1, 2 and 3.
Fig.4 Solar model developed in Simulink/SimPowerSystem.
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3. The output power of pv array is 675KW which is given to the
inverter and then the ouput of model is connected directly to the
branchnetwork B6 through transformer T3. The PV simulink model
is shown in Fig.4.
2.5. Diesel Generator Model
A lower order model [6] is used for studying dynamics of internal
combustion engines. A self –tuning PID controller is developed for
a small size diesel engine and generator set.The output power of
diesel generator is 3.125MVA.The ouput of model is connected
directly to the branchnetwork B6 through transformer T3.The
Simulink model of Governor and diesel engine is shown in Fig.5.
Fig.5 Governor and diesel engine Simulink model developed
The Simulink model of Voltage control and Speed control of diesel
generator shown in Fig.6.
Fig.6 Voltage and Speed control Simulink model developed
3. RESULTS AND CONCLUSIONS
Three scenarios of SFCL’s possible locations were analyzed for
distribution grid fault occurring in the power system. The
simulation is carried by placing SFCL in Location 1,2 and 3 by
considering one DG at a time at the position shown in fig.1.
3.1. Wind Turbine Generator as DG
Fault currents of one phase where the severity is more and
different locations of SFCL are shown in Fig.7. When a three-
phase-to-ground fault was initiated in the distribution grid (Fig.1)
SFCL located at Location 1 or Location 2, fault current contribution
from the wind farm was increased . These critical observations
imply that the installation of SFCL in Location 1 and Location 2
instead of reducing, has increased the DG fault current. This sudden
increase of fault current from the wind farm is caused by the abrupt
change of power system’s impedance. The SFCL at these locations
(Location 1 or Location 2) entered into current limiting mode and
reduced fault current coming from the conventional power plant
due to rapid increase in its resistance. Therefore, wind farm which
is the other power source and also closer to the Fault is now forced
to supply larger fault current to fault point. In the case when SFCL
is installed at the integration point of wind farm with the grid,
marked as Location 3 in Fig.1 wind farm fault current has been
successfully reduced. SFCL gives 68% reduction of fault current
from wind farm and also reduce the fault current coming from
conventional power plant because SFCL is located in the direct
path of any fault current flowing towards Fault 1.
0.25 0.3 0.35 0.4 0.45 0.5
-600
-400
-200
0
200
400
600
800
Time(Sec)
Current(Amp)
A. Fault current without SFCL
0.25 0.3 0.35 0.4 0.45 0.5
-600
-400
-200
0
200
400
600
800
1000
1200
Time(Sec)
Current(Amp)
B.Fault current with SFCL at Location 1
0.25 0.3 0.35 0.4 0.45 0.5
-600
-400
-200
0
200
400
600
800
1000
1200
Time(Sec)
Current(Amp)
C. Fault current with SFCL at Location 2
0.25 0.3 0.35 0.4 0.45 0.5
-600
-400
-200
0
200
400
600
Time(Sec)
Current(Amp)
D.Fault current with SFCL at Location 3
Fig.7 Fault Currents of Wind Turbine Generator.
Comparison of fault currents when SFCL is placed at different
Locations in Fig.1 are shown in Fig.8
Fig.8.Comparison of fault currents at different locations
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4. 3.2. Solar model as DG
Fault currents of one phase where the severity is more and
different locations of SFCL are shown in Fig.9.When a three-phase-
to-ground fault was initiated in the distribution grid (in Fig. 1).The
fault without SFCL is greater when compared with SFCL placed at
different Locations the following results were observed. Once again
the best results are obtained when a single SFCL is located at
Location 3, when compared with the remaining two locations. Fault
current has been successfully reduced.SFCL gives 70% reduction of
fault current from Solar module and also reduce the fault current
coming from conventional power plant because SFCL is located in
the direct path of any fault current flowing towards Fault 1.
0.25 0.3 0.35 0.4 0.45 0.5
-1000
-500
0
500
1000
1500
2000
2500
3000
Time(sec)
Current(amp)
A. Fault current without SFCL
0.25 0.3 0.35 0.4 0.45 0.5
-1500
-1000
-500
0
500
1000
1500
Time(Sec)
Current(amp)
B. Fault current with SFCL at Location 1
0.25 0.3 0.35 0.4 0.45 0.5
-1000
-500
0
500
1000
1500
Time(Sec)
Current(amp)
C.Fault current with SFCL at Location 2
0.25 0.3 0.35 0.4 0.45 0.5
-800
-600
-400
-200
0
200
400
600
800
1000
Time(Sec)
Current(amp)
D. Fault current with SFCL at Location 3
Fig.9 Fault Currents of Solar model.
Comparison of fault currents when SFCL is placed at different
Locations in Fig.1 are shown in Fig.10.
Fig.10.Comparison of fault currents at different locations
3.3. Diesel Generator as DG
Fault currents of one phase where the severity is more and
different locations of SFCL are shown in Fig.11. In this case when a
three-phase-to-ground fault was initiated in the distribution grid (in
Fig. 1).The fault without SFCL is greater when compared with
SFCL placed at different Locations the following results were
observed. Once again the best results are obtained when a single
SFCL is located at Location 3, when compared with the remaining
two locations. Diesel fault current has been successfully reduced.
SFCL gives 84% reduction of fault current from diesel engine and
also reduce the fault current coming from conventional power plant
because SFCL is located in the direct path of any fault current
flowing towards Fault 1.
0.25 0.3 0.35 0.4 0.45 0.5
-200
0
200
400
600
800
1000
Time(Sec)
Current(Amp)
A. Fault current without SFCL
0.25 0.3 0.35 0.4 0.45 0.5
-800
-600
-400
-200
0
200
400
600
800
1000
Time(Sec)
Current(Amp)
B. Fault current with SFCL at Location 1
0.25 0.3 0.35 0.4 0.45 0.5
-800
-600
-400
-200
0
200
400
600
800
1000
Time(Sec)
Current(Amp)
C.Fault current with SFCL at location 2
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5. 0.25 0.3 0.35 0.4 0.45 0.5
-400
-300
-200
-100
0
100
200
300
400
500
Time(Sec)
Current(Amp)
D.Fault current with SFCL at Location 3
Fig.11 Fault Currents of Diesel Engine.
Comparison of fault currents when SFCL is placed at different
Locations in Fig.1
Fig.12.Comparison of fault currents at different locations
When the SFCL was strategically located at the point of integration
Location 3, the highest fault current reduction was achieved
whatever may be the DG source. The multiple SFCLs in a micro
grid are not only costly but also less efficient than strategically
located single SFCL. Moreover, at Location 3, fault current coming
from the conventional power plant was also successfully limited.
The results of fault currents at three locations with different DG
units are summarized in Table I for comparison.
Table I: Percentage change in fault currents of different DG units
due to SFCL locations
DG
type
3-Ph Distribution grid fault
No
SFCL
Location 1 Location 2 Location 3
Wind 787A
1020A
30% increased
1020A
30% increased
265A
67% decreased
Solar 2675A
1100A
59% decreased
1100A
59% decreased
800A
70% decreased
Diesel 916A
798A
13% decreased
798A
13% decreased
148A
84% decreased
4. CONCLUSION
This paper presented a feasibility analysis of positioning of the
SFCL in rapidly changing modern power grid. A complete power
system along with a micro grid (having a wind farm, solar, diesel
connected with the grid) was modeled and transient analysis for
three-phase-to-ground faults at different locations of the grid were
performed with SFCL installed at different locations of the grid.
This placement of SFCL results in abnormal fault current
contribution from the wind farm and other two sources such as solar
and diesel. Multiple SFCLs will not used in micro grid because of
their inefficiency both in performance and cost. The strategic
location of SFCL in a power grid which limits all fault currents and
has no negative effect on the DG source is the point of integration
of the DG sources with the power grid.
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