viable system phenomena of sfcl and 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 1528

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





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.





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





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.





(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





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.

viable system phenomena of sfcl and protective

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