G. VIJAYA KUMAR* et al. ISSN: 2250–3676
[IJESAT] INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE & ADVANCED TECHNOLOGY Volume-2, Issue-5, 1528 – 1533
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VIABLE SYSTEM PHENOMENA OF SFCL AND PROTECTIVE
COORDINATION FOR SMART GRID APPLICATIONS
G. Vijaya Kumar1, J. Prakash Kumar
2
1 Student, Dept. of. EEE, St. Martin’s Engineering College, Hyderabad, A.P, India, [email protected]
2Associate Professor, Dept. of. EEE, St. Martin’s Engineering College, A.P, India, [email protected]
Abstract This paper proposes the possible effect on the reduction of fault current and its protective coordination for upcoming smart grid.
Fault levels in electrical distribution system are rising due to the increasing presence of distributed generation, and this rising trend is
expected to continue in the future. Superconducting fault current limiter is a promising electrical equipment to reduce the excessive
fault current in electrical power system effectively. This paper describes the optimal positioning of SFCL for which a resistive type
SFCL model was implemented and protective coordination was investigated. In addition a typical smart grid model including a
distributed generation of Photovoltaic model was designed to determine the performance of SFCL.
Index Terms: Fault current, smart grid, superconducting fault current limiters, photovoltaic system, protective
coordination, etc.
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1. INTRODUCTION
Electrical power systems are designed such that the impedance
between generating sources and loads are relatively low which
assists in maintenance of a stable system. However, a
significant drawback of the low interconnection impedance is
that large fault currents can develop during power system
disturbances. In an effort to prevent damage to existing system
equipment and reduce customer down time, different fault
current interrupting devices are there. But the ever-increasing
levels of fault current will soon exceed the interruption
capabilities of existing devices [1]. SFCLs have the capability
of rapidly increasing its impedance, and thus limiting high fault
currents, where the conventional protection equipment can
operate safely.
SFCL limits fault current in a network using the behavioral
characteristics of superconductors. It can detect the fault
current in the quench process without any additional device
and limit the fault current by using the resistivity of the
superconducting material [3]. A significant advantage of the
proposed SFCL is the ability to remain virtually invisible
offering negligible impedance to the grid under normal
operation until a fault event occurs. Further advantages offered
by SFCL are:
High quality energy supply with low fault currents.
Increased security of supply by means of coupled
busbars.
Greater flexibility with respect to network operation
and configuration.
No increase in short-circuit power and therefore no
additional investment for new installations.
Smart grid is the integration of information and communication
technology in to the electrical transmission and distribution
networks. Smart grid delivers the electricity to the consumers,
to enable more efficient use of electricity, as well as the more
efficient use of the grid to identify the supply demand
imbalances instantaneously and detects faults in a self healing
process [12]. Key features of the smart grid:
Self –healing
Empowers and incorporates the consumer
Provides power quality
Accommodates a wide variety of supply and demand.
Asset management
Incorporating more renewable energy
Promoting energy independence
Distributed generation and renewable generation are the
enabling technologies that make smart grid deployments
possible by the integration of micro-grids as well as customer
premises with the utility infrastructure. However, the
challenges associated with integration of DG are excessive
increase in fault current and the islanding control [6]. In this
respect, the SFCL is a promising device for the protective
G. VIJAYA KUMAR* et al. ISSN: 2250–3676
[IJESAT] INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE & ADVANCED TECHNOLOGY Volume-2, Issue-5, 1528 – 1533
IJESAT | Sep-Oct 2012
Available online @ http://www.ijesat.org 1529
device. However, to apply the developed SFCL to power
system for practical use, the following are considered.
Optimal position of SFCL
Optimal SFCL resistance
Protective coordination of the protective devices with
SFCL.
Therefore, this paper highlights an optimal position of SFCL
and protective coordination is focused. In addition,
photovoltaic (PV) system that is connected to distributed
network which serves as distributed generation is deployed.
2. SELECTION OF OPTIMAL POSITION OF SFCL
2.1. Resistive SFCL model
Resistive SFCL uses the superconducting material as the main
conductor carries the current under normal operation. When a
fault occurs, the current increases which cause the
superconductor to quench thereby, increasing its resistance and
also it depends on the operating temperature of
superconducting material. This increase in resistance of
superconducting material yields a voltage across the
superconductor to reduce the fault current.
To simplify the analysis, a resistive type SFCL was modeled as
shown in the fig.1 by considering the fundamental parameters.
First, current through the model is measured and its RMS value
is calculated and compared with the characteristic table [5].
Whose parameter values are: response time = 2 ms, minimum
impedance = 0.01 Ohms, maximum impedance = 20 Ohms,
pick-up current = 550 A.
Fig-1. SFCL model
If current through the SFCL is greater than triggering current
level, SFCL resistance increases to maximum level as shown in
fig.2.which is drawn between SFCL resistance and Fault
current in which the triggering current is 550 A.
Fig-2. Resistance of the SFCL vs. Fault current
2.2. Power System Model
Fig.4. depicts the single line diagram of the power system
consisting of a conventional power plant, transmission
systems, distribution system, load points and distributed
generation with photovoltaic model in order to explain the
optimal position and effective coordination of SFCL.
The power system is composed of a 100 MVA conventional
power plant designed of 3-phase synchronous machine,
connected to a transmission line through a step up transformer.
At the substation, voltage is again stepped down and supplied
to high power industrial loads and low power domestic loads
(LP) by means of separate distribution branch networks. The
PV system is directly connected to the branch networks
through transformer. Simulation is carried out by considering
artificial fault as indicated in the fig.4 at distribution grid,
customer grid, and transmission line whereas the location of
SFCL is at DG, integration point, branch network and
substation.
2.3. Structure of PV System
Solar cell is a fabricated p-n junction in a thin layer of
semiconductor where the radiant energy from sun is converted
in to electricity through photovoltaic effect. Fig-3 Shows a PV
system and its building blocks. A dc-link capacitor is
connected in parallel to PV array in the dc side terminals of
voltage-source converter which is controlled by sinusoidal
PWM [8][9]. In general a PV array is a combination of ns PV
panels electrically connected in series to generate more DC
voltage and intern they are connected in np parallel strings to
ensure large power generation. The terminal equation for the
voltage and current of the array is as follows.
𝐼𝑝𝑣 = 𝑛𝑝𝐼𝑝ℎ − 𝑛𝑝𝐼𝑠 𝑒𝑥𝑝 𝑞𝑣𝑑𝑐
𝑘𝑡𝐴𝑛𝑠 − 1 (1)
Fig- 3. PV System Model
Where Iph is a light generated current, Is is the cell saturation of
dark current, q (= 1.602 x 10-19
c) is the unit electric charge, k
(=1.38 x 10-23
J/K) is Boltzmann’s constant, t is working
temperature of p-n junction, A is the ideal factor.
G. VIJAYA KUMAR* et al. ISSN: 2250–3676
[IJESAT] INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE & ADVANCED TECHNOLOGY Volume-2, Issue-5, 1528 – 1533
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Conventional
Power Plant
CB CB
CB
CB
CB
CB
DG
LP 1 LP 3LP 2Industrial loads
Location 1
SFCL at Substation
Location 2
SFCL at Branch Network Location 3
SFCL at DG
Location 4
SFCL at Integrated Point
Fault 1
Fault 2
Fault 3
--- Transformer
--- SFCL
CB --- Circuit Breaker
LP --- Load Point
DG --- Distributed Generation
Fig-4. Single line diagram of Power System
Fig-5. Subsystem implementation of PV array
G. VIJAYA KUMAR* et al. ISSN: 2250–3676
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The photon current Iph depends on cell’s working temperature
and solar irradiation level, which is described as
𝐼𝑝ℎ = 𝐼𝑠𝑐 + 𝐾𝑡 𝑡 − 𝑡𝑟𝑒𝑓 𝑠
100 (2)
Where Isc is cell’s short circuit current, Kt temperature
coefficient and S is the solar irradiation level in KW/m2. Based
on (1) power delivered by PV array is
𝑃𝑝𝑣 = 𝑛𝑝𝐼𝑝ℎ𝑉𝑑𝑐 − 𝑛𝑝𝐼𝑠𝑉𝑑𝑐 𝑒𝑥𝑝 𝑞𝑣𝑑𝑐
𝑘𝑡𝐴𝑛𝑠 − 1 (3)
Fig.5 shows the subsystem implementation of the PV array
using (1), (2) & (3).
3. RESULTS AND DISCUSSION
The case system in Fig.4 is studied with circuit breaker as
protection device. The failure rate of circuit breaker was affected
by the fault current reduction and also depends on the location of
SFCL. Generally, the fault current is injected from the LV side
of the transformer to fault point and hence the location is
postulated on LV side of transformer or feeder.
3.1. Optimal position of SFCL
When a fault occurs the high current is injected into the fault
point from the conventional power plant as well as from DG. If
the SFCL is located at location 1 or location 2, the fault current
from the conventional power plant to fault point (Distributed
grid) is limited by SFCL, and hence the fault current
contribution from DG was increased which is higher than the
NO SFCL condition. Therefore, Location 1 and Location 2 have
increased DG fault current instead of reducing. If the SFCL is
placed at the integration point of DG with grid, the fault current
from DG is successfully limited by 68%. If the fault occurs in
transmission line, the fault current from DG flows in reverse
direction and hence SFCL in location 1 and location 2 reduces
the fault current, but the majority of the faults in a power system
might occur in distributed grid.
Fig.6-8. shows the fault current from DG for four different
locations of SFCL in case of fault in (a) Distributed Grid, (b)
Customer Grid, and (C) Transmission line. SFCL designed to
(a)
(b)
(c)
(d)
(e)
Cur
rent
(A
)
Time (s)
Fig-6. DG fault currents in case of fault in distribution grid (a)
NO SFCL, (b) SFCL at Substation, (c) SFCL at Branch network,
(d) SFCL at Integration point, (e) SFCL at Substation and DG.
Protect micro-grid should not be expected to cater for faults in
transmission line. Therefore, the SFCL at integration point is the
optimal position which reduces the fault current than the other
locations for the faults in distributed grid and customer grid,
whereas for the faults in transmission line, SFCLs at location 1
and location 4 are suitable. But multiple SFCLs in a micro grid
are not economical and less efficient than the strategically
located single SFCL.
2.2. Protective coordination of SFCL
Fault currents are drastically increased due to rapid growth of
power system and might exceed the breaking limits of protection
equipment. Therefore to maintain stable and reliable operation
of power system, fault current should be reduced and controlled.
Where as in high voltage system resistive SFCL requires a more
number of series connections of superconducting modules and to
increase the current carrying capability parallel connection
should be considered.
G. VIJAYA KUMAR* et al. ISSN: 2250–3676
[IJESAT] INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE & ADVANCED TECHNOLOGY Volume-2, Issue-5, 1528 – 1533
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(a)
(b)
(c)
(d)
(e)
Cu
rren
t (A
)
Time (s)
Fig-7. DG fault currents in case of fault in customer grid
(a) NO SFCL, (b) SFCL at Substation, (c) SFCL at Branch
network, (d) SFCL at Integration point, (e) SFCL at Substation
and DG.
Therefore, due to increase in length of superconductors, line
losses increases. Hence the coordination with conventional
protection system to apply fault current limiters in power system
plays an important role due to above all conditions.
Even though the SFCLs are installed in the existing electrical
network, due to which the fault currents could be altered which
affects protective coordination level between protection
schemes. The protective coordination depends on the pick-up
level of fault current limiters, optimal position and impedance
offered by SFCL. Fig. 4 shows the application of SFCL in the
micro grid. In this case protection coordination is required
between SFCL and circuit breaker. The prerequisite for this
condition is that altered fault current is enough to operate the
circuit breaker. But, in few cases the fault current is reduced too
much by SFCL in which circuit breaker is not able to find the
tripping point. Due to reduced fault, current the operating time
was delayed which intern increases the fault clearing time, but
coordination time interval 𝛥T increases.
(a)
(b)
(c)
(d)
(e)
Cu
rren
t (A
)
Time (s)
(c)
(d)
Time (s)
(e)
Fig-8. DG fault currents in case of fault in transmission line (a)
NO SFCL, (b) SFCL at Substation, (c) SFCL at Branch network,
(d) SFCL at Integration point, (e) SFCL at Substation and DG.
Therefore to achieve the asset management, the vision of Smart
Grid, by using existing protection devices in addition with SFCL
with proper coordination is used to reduce the drastically
increasing fault current [2]. Hence in order to operate the SFCL
before the circuit breaker operation, the pick-up level of SFCL
should be smaller than recloser and this is achieved only when
recloser action is delayed. Fig. 9 shows at t1 SFCL was activated,
then after a delay the recloser begins to operate at t2. Here the
time difference between SFCL ON and recloser ON is very
important.
SFCL ONRecloser ON
T
Fault Current
Fig-9. Protective Coordination curve
G. VIJAYA KUMAR* et al. ISSN: 2250–3676
[IJESAT] INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE & ADVANCED TECHNOLOGY Volume-2, Issue-5, 1528 – 1533
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CONCLUSION
Strategic location of SFCL and protective coordination between
SFCL and circuit breaker had been investigated. Typical results
show the effectiveness of SFCL in reducing the fault current to
improve the stability and reliability of the existing power
system. The power system along with a micro grid having a
photovoltaic system was modeled and PV array has developed
with MATLAB/ Simulink
Apart from some complex obstacles which can be overcome for
their commercialization, Superconducting fault current limiter is
a new pattern to conventional electric networks
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BIOGRAPHIES
G. Vijaya Kumar received the B.Tech degree
in Electrical and Electronics Engineering from
the J.N.T.U, Hyderabad, India in 2008. He is
currently pursuing the M.Tech degree from
St. Martin’s Engineering College, Hyderabad.
His research interests include Superconducting
Fault Current Limiters, distributed generation,
Smart grid and renewable energy.
J. Prakash Kumar was born in Hyderabad,
India. He received the B.Tech and M.Tech
degrees in Electrical Engineering from JNTU,
Hyderabad, India. He was an Associate
Professor in Dept. of EEE at St. Martin’s
Engineering College. His research interests
include power system protection, monitoring
and control development in Digital Protective
relays and Smart grid.