EFFECT OF SFCL ON

DISTRIBUTION

POWER QUALITYby

Shahram Najafi, Prof. Vijay K. Sood, Ahmed HosnyUniversity of Ontario Institute of Technology, Oshawa, Ontario

Electrical Power and Energy Conference (London-Canada)

October 2012

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Outline Background

Faults and High Currents

SFCL as a Potential Solution

Power Quality

Voltage Sag

Harmonic Distortion

Case Study Model and Conditions

Simulation Results

Different Fault Types & SFCL Performance

Switching Distortion

Conclusion2

Background

Continuous growth of demand resulted in higher fault

current levels

An electrical fault occurs when current flows through an

abnormal or unintended path

Mitigation of fault current levels using newer technology

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Potential Solution

Responds to faults instantaneously, improves power

quality, occupies less space, have less power losses in

comparison to conventional fault current limiters

Extends the life of many protective devices, more reliable

and allows the usage of existing switchers and circuit

breakers

SFCL Operation principals and controller

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SFCL Operation Principal

Consisting of two or more parallel connected circuit

branches

RC snubber circuit is connected across each power

electronic switch for mitigating the effect rate-of-change

of (di/dt) during the switching ON instant

The SFCL is required to have low impedance under

normal conditions but to have high impedance under fault

conditions

The speed of the intervention must be high enough5

SFCL Controller In normal operating conditions, the

control scheme always triggers the

IGBT ON, since modulated output

wave (Vm) is higher than the

sawtooth waveform (ST)

Vm > ST

On detection of a fault, the

conducting IGBT is switched OFF

and the fault current is diverted to

the limiting impedance since

modulated output wave (Vm) is less

than the sawtooth waveform (SW)

Vm < ST

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System Parameters & Conditions

PARAMETERS VALUE

Utility Voltage 115 kV

Utility Voltage Source Impedance 1%

Transformer #1 Voltage 115:15 kV

Transformer#1 Power & Impedance 25MVA & 4.5%

Transmission Line Z1=0.3101+j0.909 Ω, Z0=0.7186+j4.317

Ω

Transformer #2 Voltage 15:4.16 kV

Transformer#2 Impedance 20MVA & 9%

Load Rating 10MW, 4.16 kV & 0.92 PF lagging

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• Initiation of fault at 0.0167 sec and fault lasts for

0.05 sec (3 cycle), and then cleared at 0.0667 sec

ONE-LINE DIAGRAM OF DISTRIBUTION

NETWORK USING SFCL

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SFCL Reference Current

The selection of the reference current is limited to the

pick-up current of over-current relays, and the maximum

current interrupting capability of the IGBT

And accordingly limiting impedance Zlim will be designed

Using Kirchhoff’s voltage law, the line current through the

SFCL is given by,

....................(1)

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lim

lim

−

=

T

utility

Z

VI

SFCL Limiting Impedance

ZT-lim is the total impedance from utility point up to the

load including the point of fault

....(2)

Where,

ZTrnsf1+utility is cumulative impedance of transformer# 1 and the utility,

ZTransf2 is transformer# 2 impedance,

Zload is the load impedance,

Zfault is the fault impedance (in this paper the fault is assumed to be bolted

with zero impedance)

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]//)[( 21limlim faultTransfloadutilityTrnsfT ZZZZZZ +++=+−

SFCL Limiting Impedance Cont.

But, Ilim is the desired limited maximum faulted current

value during fault

........(3)

where, Iratd is the rated current value and

‘desired limited value’ is represented by a number

multiplied by the pu rated current value

So, the limiting impedance that will result in desired

limited fault current can be calculated as:

....(4)

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value)limited desired(*lim ratdII =

]//)[( 21limlim faultTransfloadutilityTrnsfT ZZZZZZ ++−=+−

Line Currents: Without & With Limiter

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Phase “a” current waveform for

symmetrical fault at bus B without SFCL;

fault inception time = 16.67ms, ∆t = 10µs,

Zfault =0

Phase “a” line current waveform for

symmetrical fault at bus-bar B using SFCL;

fault inception time = 16.67 ms, ∆t = 10µs,

Zfault =0

Current Waveforms Through SFCL

A three-phase to ground (abc-g) fault with zero fault impedance is

simulated at bus-bar B, just after the SFCL

The Simulated current waveforms through the SFCL components are

presented

ID1-D3

ID2-D4

IIGBT IZnO

ILL

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Distortion in Voltage Waveform

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Voltage waveform for symmetrical fault at bus-bar B with the SFCL; fault inception time = 16.67ms, Zfault=0

Voltage magnitude for symmetrical fault) at bus-bar B with/without the SFCL; fault inception time = 33.33ms, Zfault=0

With SFCL

Without SFCL

Current and Voltage Distortion

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Although the SFCL provides the desirable current limiting function, it

exhibits harmonic characteristics that need to be carefully studied

The Total Harmonic Distortion (THD) in the current wave is calculated

as 40.11% and that in the voltage is 125%

Per-unit frequency spectrum for phase

“a” current in case of symmetrical fault

at bus-bar B, Imax = 3.55 kA

Per-unit frequency spectrum for phase

“a” voltage in case of a symmetrical

fault at bus-bar B, Vmax=10.59 kV

A Single-Phase to Ground (a-g) Fault, Zfault=0

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Current waveforms through

the SFCL for line-to-ground

(a-g) fault at bus-bar B

Voltage waveforms for

line-to-ground (a-g)

fault at bus-bar B

Voltage waveforms for

line-to-ground (a-g)

fault at bus-bar C,

fewer switching actions

& lower magnitudes

Single phase to ground (the most frequent of occurrence fault in power system) is also

shown below

Conclusions The SFCL has been used and implemented in this paper

using EMTP program to study the impact of SFCL

SFCL effectively suppressed the fault voltage and

mitigated fault current which decrease the short circuit

stress on the network

Analyzing transient behavior of the semiconductor switch

assist to improve power quality, to decrease energy

dissipation and to reduce the stress on system equipment

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Conclusions Cont.

The SFCL however exhibits harmonic generation due to

the switching of the IGBT

Some alternative control circuits to alleviate this problem

are under investigation

In future work, the coordination of the SCFL and the

existing circuit breaker elements in the studied power

system will also be investigated

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THANK YOU

QUESTIONS ?

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