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S. Srilatha et al. / IJAIR ISSN: 2278-7844

© 2012 IJAIR. ALL RIGHTS RESERVED 24

Analysis of Positioning of Superconducting Fault

Current Limiter in Smart Grid Application

S. Srilatha1 , Dr.K. Venkata Reddy 2

1 PG-Student, Department of Electrical and Electronics Engg.JNT University, Kakinada, India

2 Assistant Professor, Department of Electrical and Electronics Engg, JNT University, Kakinada, India.

[email protected]

[email protected]

Abstract: The introduction of new generating facilities by

independent power producers and increasing load

demand can result in fault current over duty on

existing transmission system protective equipment.

Conventional solutions to fault current over duty such

as major substation upgrades, splitting existing

substations buses or multiple circuit breakers

upgrades could be very expensive and require

undesirable extended outages and result in lower

power system reliability [8]. Due to the difficulty in power network reinforcement and the interconnection

of more distributed generations, fault current level

has become a serious problem in transmission and

distribution system operations [10]. The utilization of

fault current limiters (FCLs) in power system

provides an effective way to suppress fault currents

and result in considerable saving in the investment of

high capacity circuit breakers. In this work a resistive

superconducting fault current limiter designed. To

reduce the fault current in the micro grid the designed

SFCL placed in proposed system, which consists of a

conventional power plant and 10MVA wind farm. The effect of SFCL on the micro grid, and optimal

position for SFCL to get more reduction percentage

of wind farm faults current. . Keywords: Faults currents, Smart Grids, Wind farm,

Wind turbine Induction Generator.

1. Introduction A fault condition may result in an electrical power

transmission system from event such as lighting

striking a power line, or downed trees, or utility poles

shorting the power lines to ground [6]. The fault

creates a surge of current through the electric power

system that can cause serious damage to grid

equipment.

With the increasing demand for power, electric

power systems have become greater and are

interconnected. Generation units of independent

Power producers (IPPs) and renewable energy have

been interconnected to power systems to support the rising demands [11]. As a result, faults in power

networks incur large short-circuit currents flowing in

the network and in some cases may exceed the

ratings of existing circuit breakers (CB) and damage

system equipment The problems of inadequate CB

short-circuit ratings have become more serious than

before since in many locations, the highest rating of

the CB available in the market has been used. To deal

with the problem, fault current limiters (FCLs) are

often used in the situations where insufficient fault

current interrupting capability exists [12]. Less

expensive solutions such as current limiting reactors may have unwanted side effects, Such as increasing

system losses, voltage regulation problems or

possibly could compromise system stability. Smart

grid is a modern electricity system. It uses sensors,

monitoring, communications, automation and

computers to improve the edibility, security,

reliability, efficiency, and safety of the electricity

system. Renewable energy technologies such as

photovoltaic, solar thermal electricity, and wind

turbine power are environmentally beneficial sources

of electric power generation [3]. The integration of renewable energy sources into electric power

distribution systems can provide additional economic

benefits because of a reduction in the losses

associated with transmission and distribution lines.

In this work a SFCL model is designed. SFCL is an

innovative fault current limiter. It works on the

principle of Superconducting Property. It is inactive e

under normal condition. It is in active under fault

condition; it inserts some resistance into the line to

limit the fault current. It suppresses the fault current

within first half cycle only. It operates better than Circuit breakers, Relays, because the Circuit breakers

takes minimum 2-3 cycles before they getting

activated. The effect of SFCL on micro grid fault

current observed. The optimal place to SFCL is

determined [11].

mailto:[email protected]



2. Proposed system

Fig.1. Proposed system model designed in Simulink/SimPowerSystem. Three phase to ground Fault and SFCL locations are indicated in the diagram.

The proposed system is composed of a 100 MVA,

20KV conventional power plant, composed of 3-

phase synchronous machine, connected by 200 km

long 154 KV distributed-parameters transmission line

through a step-up transformer of 20/154KV( TR1). At the substation (TR2), the voltage is stepped down

to 22.9 kV from 154KV. High power industrial loads

of 6MW and 330MVAR loads are connected to the

conventional source. The 10 MVA wind farms is

composed of three fixed-speed induction-type wind

turbines each having a rating of 3.33MVA. The wind

farm is operating at 20KV, and is this voltage is

stepped up to branch network voltages of 22.9KV

through a 20KV/22.9KV transformer (TR3). The

wind farm directly connects with the branch network

(B1) through a transformer (TR3) and is providing power to the domestic loads. The 10MVA wind farm

supplied to the customer loads at 400V, through

distribution transformers of 22.9KV/400V. Three

domestically loads are separated by each 5KM

transmission line and at each end of 5KM

transmission line a domestically load is connected

through a distribution transformer, as shown in Fig.1.

The most severe fault in the power system is three

phase to ground fault, it results in very high current in

the system. The designed fault current limiter is in

such a way that it should reduce the fault current due

to three phase to ground fault, then it can successfully reduces the fault current in micro grids for all

remaining faults. Artificial faults are marked as Fault

1, Fault 2 and Fault 3, which represent three-phase-

to-ground faults in distribution grid, customer grid

and transmission line respectively as shown in the

Fig.1.and for each and every fault the wind farm fault

current observed. To reduce this wind farm fault

current the designed superconducting fault current

limiter is placed at three scenarios for three different

faults in the power system. First, we assumed that

single SFCL was located at Location 1 (Substation). Second, single SFCL was located at Location 2

(Branch Network). Third, single SFCL was located at

Location 3 (Wind farm integration point with the

grid). Finally, in order to clarify the usefulness of

dual SFCL installed together for different locations,

SFCLs were located at Location 1 (Substation) and

Location 4 (Wind Farm) respectively.



3. Superconducting fault current limiter

Superconducting Fault Current Limiter (SFCL) is innovative electric equipment which has the

capability to reduce the fault current level within the

first cycle of fault current [11]. 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. The

concept of using the superconductors to carry electric power and to limit peak currents has been around

since the discovery of superconductors and the

realization that they possess highly non-linear

properties. More specifically, the current limiting behavior depends on their nonlinear response to

temperature, current and magnetic field variations.

Increasing any of these three parameters can cause a

transition between the superconducting and the

normal conducting regime. The current increase can

cause a section of superconductor to become so

resistive that the heat generated cannot be removed

locally. This excess heat is transferred along the

conductor, causing the temperature of adjacent

sections to increase. The combined current and

temperature can cause these regions to become

normal and also generate heat. The term “quench” is commonly used to describe the propagation of the

normal zone through a superconductor. Once

initiated, the quench process is often rapid and

uncontrolled. Though once initiated the quench

process is uncontrolled, the extent of the normal

region and the temperature rise in the materials can

be predicted.

3.1. Simulation Model of SFCL

The resistive type SFCL was modeled considering

four fundamental parameters of a resistive type

SFCL. These parameters and their selected values

are: 1) Transition or response time = 2m.sec 2)

Minimum impedance = 0.01Ω. Maximum impedance

= 20Ω 3) Triggering current and =550 A 4) Recovery

time. = 10 msec. The SFCL working voltage is

22.9kV. The maximum impedance value can be varied from 20 ohms to 27 ohms. The SFCL model

developed in Simulink/SimPower System is shown in

Fig.3.1. The SFCL model works as follows. First,

SFCL model calculates the RMS value of the passing

current and then compares it with the characteristic

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 triggering current level the system waits

until the Recovery time and then goes into normal

state. The SFCL characteristic table shown in Fig.3.1

plays a main role which consists of standard

parameter values of SFCL.

Fig.3.1 Simulink model of Single phase SFCL

The current limiting resistance value is calculated and

this value is implemented in the simulation model.

The important parameter to be given in SFCL is the

current limiting resistance value. It is stored in the

SFCL characteristic table. In order to avoid

harmonics caused by transients, filter is used. The SFCL model developed is tested in both single phase

and three phase test systems and the current

waveforms are recorded with the presence and

absence of SFCL. The simulation model of a single

phase test system with and without SFCL is shown in

Fig.3.2 and Fig.3.3 respectively and the current

waveforms are recorded. The fault current is induced

in the source directly in order to reduce the

complexity of the simulation model.

Fig.3.2.Simulation model of single phase system with

SFCL

The type of the fault induced in the model is single

phase to ground fault where it is induced through the

AC voltage source. An RMS block is needed in order

to calculate the RMS value of the incoming current

signal and to increase the impedance value according



to the limited fault current value specified in the

SFCL characteristic table.

Fig.3.3 Simulation model of single phase system

without SFCL The SFCL subsystem can be implemented in various

types of single phase test systems and the operation

can be tested. The performance of the SFCL can also

be tested in a power system generation and

distribution systems using three phase SFCL for

controlling the fault current for each phase. The

subsystem specified in Fig.3.2 is the SFCL model

designed for single phase system which is shown in

Fig.3.3. Harmonics filtration is used in order to

reduce the harmonics caused due to the abnormal

fault current. A normal first order filter is used for reducing the harmonics. The type of the filter can be

changed depending upon the application of the

system. A controlled voltage source is connected in

order to compensate the voltage sag caused due to the

induced fault current which is caused due to both

internal and external causes.

4. Results and Analysis

Four SFCL‟s possible locations were analyzed for

three different fault occurring points in the power

system depicted in Fig.1. First, we assumed that

single SFCL was located at Location 1 (Substation).

Second, single SFCL was located at Location 2

(Branch Network). Third, single SFCL was located at

Location 3 (Wind farm integration point with the

grid). Finally, in order to clarify the usefulness of

dual SFCL installed together for different locations,

SFCLs were located at Location 1 (Substation) and Location 4 (Wind Farm) respectively.

4.1. CASE I: Fault in Distribution Grid

This distribution grid fault is most frequent fault in

the power system. Three phases to ground is applied at distributed grid for 0.4s - 0.41s as shown in Fig

4.1. The total current in the system is now forced to

flow towards the fault point. The wind farm current

direction and fault current direction is same in this

fault case.

Fig 4.1. Wind farm fault current for distribution

grid fault

4.1.1. SFCL at substation (Location1)

In the case of SFCL located at Location 1

(Substation) fault current contribution from the wind

farm increased to 1025A as shown in Fig4.2. The magnitude of fault current is higher than „No FCL‟

situation, and the percentage of increment of wind

farm fault current is given by 28. These critical

observations imply that the installation of SFCL in

Location 1, instead of reducing, has increased the DG

fault current.

Fig 4.2 Wind farm fault current for SFCL at

substation in distribution grid fault

4.1.2. SFCL at Branch network (Location2)

The two locations of SFCL‟s don‟t show any

difference on the effect of rising in wind farm fault

current due to fault at distribution grid. The wind

farm fault current is about 1025A and the percentage

of increment of wind farm fault current is given by

28 as shown in the Fig 4.3. In this case also the placement of SFCL raises the wind farm fault current

instead of reducing the wind farm fault current

Fig 4.3 Wind farm fault current for SFCL at branch

network



4.1.3. SFCL at Integration point (Location3)

This location of SFCL reduces the fault current

coming from two sources. SFCL is in direct path of

fault current only. When SFCL is installed at the

integration point of wind farm with the grid, marked

as Location 3 in Fig4.4. The wind farm fault current

has been successfully reduced to265A. SFCL gives

67% reduction of fault current from wind farm and

also reduce the fault current coming from

conventional power plant because SFCL located in

the direct path of any fault current flowing towards Fault 1.


integration point

4.1.4. Dual SFCL at Substation and at Wind

farm (Location4)

With dual SFCL installed at Location 1 and Location

4, the wind farm fault current reduced to 380A and also,53% reduction in fault current from wind farm is

also observed from Fig.4.5. However, even though

two SFCLs were installed, wind farm fault current

reduction is lower than what was achieved by the

single SFCL installed at Location 3.

Fig 4.5 Wind farm fault current for dual SFCL at

Substation and at Wind farm

4.2. CASE II: Fault in Customer Grid

The wind farm fault current rises to 580A, due to

customer grid fault as shown in Fig 4.6 There is no big difference in raise in wind farm fault current due

to this fault. Because in this case just transformer

secondary side impedance is added to the primary

side impedance results in increased in impedance when compared to the fault1 case impedance ,so the

wind farm fault current is reduced somewhat less

than the first fault case.

Fig 4.6Wind farm fault current for customer grid

fault

4.2.1 SFCL at Substation (Location1) In the case of SFCL located at Location 1 (Substation) fault current contribution from the wind

farm was increased to 1050A and the magnitude of

fault current is higher than „No FCL‟ situation and

the percentage of increment of wind farm current is

given by 81 as shown in the Fig4.7. These critical

observations imply that the installation of SFCL in

Location 1, 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.

Fig 4.7 wind farm fault current for SFCL at

substation


The two locations of SFCL‟s don‟t show any

difference on the effect of raise in wind farm current

due to fault at customer grid. The wind farm fault

current is about 875A, and the percentage of

increment of wind farm current is given by 51 as



shown in the Fig 4.8. In this case also the placement

of SFCL raises the wind farm fault current instead of

reducing the raise in current due to fault.

Fig 4.8 Wind farm fault current for SFCL at branch

network


This placement reduces the fault current coming from

two sources. SFCL is in direct path of fault current

only. When SFCL is installed at the integration point

of wind farm with the grid, marked as Location 3 in

Fig.4.9. In this case the fault current is same as that

of NO SFCL case. SFCL at this location gives no

negative effect on the wind farm fault current,

because it reduces the fault current coming from conventional power plant as SFCL located in the

direct path of any fault current flowing towards Fault

2.


integration point

4.2.4 Dual SFCL at Substation and at Wind

farm (Locatio4)

With dual SFCL installed at Location 1 and Location 4 the wind farm fault current reduced to 670A. 16%

reduction in wind farm fault current is also observed

as shown in Fig.4.10. However, even though two

SFCLs were installed, wind farm fault current

reduction is lower than what was achieved by the

single SFCL installed at Location 3.

Fig 4.10 Wind farm fault current for dual SFCL at

substation and at wind farm

4.3 CASE III: Fault in Transmission Line

The wind farm current raises to 4400A due to

transmission line fault as shown in Fig.4.11. The

wind farm current in this fault case is very high when

compared to the other fault cases because the transmission line impedance is very less when

compared to the distribution grid and customer grid

transformers, results in high fault current in the wind

farm.

Fig 4.11 Wind farm fault current for transmission line

fault

4.3.1 SFCL at Substation (Location1)

SFCL positioned at Location 1(Substation) reduces

the wind farm fault current to 1540A, as shown in

Fig4.12, and the percentage reduction of wind farm

fault current is 65. This result comes from the fact

that SFCL is installed directly in the path of reverse

current being generated by the wind farm towards fault point. The currents directly coming from

conventional fault are reduced in the SFCL only due

to insertion of high resistance. So the wind farm



current reduced due to this location for transmission

line fault.


substation


SFCL positioned at Location 1(Substation) reduces

the wind farm fault current to 1320A, as shown in Fig

4.13, and the percentage of reduction of wind farm

fault current is 70. This result comes from the fact

that SFCL is installed directly in the path of reverse current being generated by the wind farm towards

fault point.

Fig 4.13Wind farm fault current for SFCL at branch

network


When the SFCL was strategically located at the point

of integration of the wind farm with the grid

(Location 3), the highest fault current 4100A,as

shown in Fig4.14,and the percentage of reduction of wind farm current is given by 7. SFCL injects current

in same direction of currents coming from both

conventional and wind farm currents.


integration point

4.3.4. Dual SFCL at Substation and at Wind

farm (Location4)

SFCL at location1 and at location4 are greatly reduce

the wind farm fault current to 1000A, as shown in

Fig4.15,and the percentage of reduction of wind farm

fault current is given by 73, due to transmission line

fault. Even though in this the wind farm fault current

is reduced better when compared to other cases, but it

is not economical and not feasible to use two SFCL‟s in the power system.

Fig 4.15 Wind farm fault current for dual SFCL at substation and at wind farm

The Percentage change in wind farm fault current for

all considered SFCL locations are tabulated below.

Table 4.1: Percentage change in wind farm fault

current due to SFCL locations

Item Fault1

Distribution Grid

Fault2

Customer Grid

Fault3

Transmission Line

Fault

current 800A 580A 4400A

SFCL

Locatio

n

Fault

Curren

t(A)

Effect

%

Fault

Curren

t(A)

Effect

%

Fault

Curren

t(A)

Effect

%

1 1025 28

increased 1050 81

increased 1540 65

decreased

2 1025 28

increased 875 51

increased 1320 70

decreased

3 265 67

decreased 580 - 4100 7

decreased

4 380 53

decreased 670 16

decreased 1000 73

decreased



5. Conclusion

The majority of faults in a power system might occur

in the distribution grid and the SFCL designed to

protect micro-grid should not be expected to cater for

the transmission line faults (Fault 3). An important

aspect to be noted here is that wind farms on

distribution side can contribute fault currents to

transmission line faults and this phenomenon must be

considered while designing the protection schemes

for the smart grid. The optimal location of SFCL is at

integration point of two generating sources, for both

distribution and customer grid faults. This 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 wind farm with the power

grid for both distribution and customer grid faults.

And SFCL should not be placed directly at the

substation or the branch network feeder. This

placement of SFCL results in abnormal fault current

contribution from the wind farm in both distribution

and customer grid faults. The optimal location of

SFCL is at the substation or at branch network feeder

for transmission line fault. And SFCL should not be placed directly at integration point of two sources.

This placement of SFCL results in abnormal fault

current contribution from the wind farm in

transmission line faults. Even though multiple SFCLs

in micro grid can reduce the wind farm current due to

faults but dual SFCL‟s are inefficient both in

performance and cost.

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