See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/304540272
Short Circuit Analysis Case Study & Circuit Breaker Design
Conference Paper · April 2011
CITATIONS
READS
0
3,271
1 author:
Sumit Rathor
G H Patel College of Engineering and Technology (GCET)
16 PUBLICATIONS 5 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Power Quality and Energy Management System in Smart Grid incorporating Electric Vehicles View project
All content following this page was uploaded by Sumit Rathor on 28 June 2016.
The user has requested enhancement of the downloaded file.
Short Circuit Analysis Case Study & Circuit
Breaker Design
Sumit Rathor K
Prajapati Bhavik R
Patel Anil N.
Master in Power System
BVM Engineering College
rathorsumit2006@gmail.com
Master in Power system
LD collage of engineering
er.bhavikkumar@gmail.com
Lecturer
S.P.C.E. Visnagar
anpatelee@gmail.com
Abstract: This paper presents the behavior of
system under fault condition and directs us to
designing of circuit breaker. As the short circuit
analysis is for designing power system with
considering all the conditions such as line loading,
Circuit breaker design, relay settings, transients etc.
Basically for short circuit study we have to do load
flow analysis then with considering pre fault condition
short circuit analysis studied. For case study
simulation author have use MiPOWER (PRDC) &
PsCAD/EMTDC software.
Index Terms—Fault modeling, bolted fault, arc
resistance, X/R ratio, symmetrical fault current*
I. INTRODUCTION
Short circuit studies in power system are a basic
step in planning of modern power grids. Based on
such results and studies, protective device (relay)
setting and co-ordination is being carried out,
switchgear
components
are
constructed,
manufactured and installed, etc. The study is
performed using computer software first by
modeling the system (conductors, transformers,
generators, utility sources, etc.) and then by
simulating faults.
A fault usually results in high current flowing
through the lines and if adequate protection is not
taken, may result in damages in the power
apparatus. Here the term symmetrical fault refers to
those conditions in which all three phases of a
power system are grounded at the same point. For
this reason the symmetrical faults sometimes are
also called three-line-to-ground (LLLG) faults. One
tries to limit short-circuits to the faulty section of
the electrical system by appropriate switching
devices capable of operating under short-circuit
conditions without damage and isolating only the
faulty section, so that a fault is not escalated. The
faster the operation of sensing and switching
devices, the lower is the fault damage, and the
better is the chance of systems holding together
without loss of synchronism. As the main purpose
of short-circuit calculations is to select and apply
these devices properly, it is meaningful for the
calculations to be related to current interruption
1
Here symmetrical fault current is considered for analysis.
phenomena and the rating structures of interrupting
devices. Bolted faults give the maximum shortcircuit currents and form the basis of calculations of
short-circuit duties on switching devices. Faults
involving one, or more than one, phase and ground
are called unsymmetrical faults. Under certain
conditions, the line-to-ground fault or double line-to
ground fault currents may exceed three-phase
symmetrical fault currents. Unsymmetrical faults
are more common as compared to three-phase
faults, i.e., a support insulator on one of the phases
on a transmission line may start flashing to ground,
Short-circuit calculations are, thus, the primary
study whenever a new power system is designed or
an expansion and upgrade of an existing system are
planned.
II. CALCUALTIONS OF SYSTEM QUANTITY.
A. Available line current
Under the fault condition the line current at
f
f
faulted bus is calculated as
where
V
−V
I ij
f
=
i
j
Z
ij
f
f
V j -post fault bus voltage; Iij =current flowing
between buses, Vif. =pre fault bus voltage at ith and
jth bus, Zij=impedance between two buses.
The available short-circuit current is directly related
to the size and capacity of the power sources
(utility, generators, and motors) supplying the
system and is typically independent of the load
current of the circuit. The larger the capacity of the
power sources supplying the system, the greater the
available short-circuit current (generally). The
available short-circuit current is directly related to
the size and capacity of the power sources (utility,
generators, and motors) supplying the system and is
typically independent of the load current of the
circuit. The larger the capacity of the power sources
supplying the system, the greater the available
short-circuit current (generally).
B. Total Symmetrical Fault & short circuit
current
We are interested in symmetrical fault currents. If
the envelopes of the positive and negative peaks of
the current waveform are symmetrical around the
zero axis, they are called “symmetrical current”
envelopes. The total short-circuit current available
in a distribution system is usually supplied from a
number of sources, which can be grouped into three
main categories. The first is the utility transmission
system supplying the facility, which acts like a
large, remote generator. The second includes
“local” generators either in the plant or nearby in
the utility. The third source category is synchronous
and induction motors, which are located in many
plants and facilities. The total short-circuits current
that has steady-state ac, decaying ac, and decaying
dc current components can be expressed as shown
in Equation(i).
i = idc decay+iac steady state+iac decay … …i
With
− Rωt
idc . decay = ( Iacstaedy ) Sin (α − φ ) e
X
Iacsteady
Iacdecay
=
=
2 IsSin
2 IsSin
(ω t + α − φ )
(ω t + α − φ ) e
− kt
Where
Is - is the symmetrical steady-state rms. current
magnitude
k - is a variable depending upon the mix and size
of rotational loads
t- is in seconds
The magnitude and duration of the asymmetrical
current depends upon the following two
Parameters:
a) The X/R ratio of the faulted circuit
b) The phase angle of the voltage waveform at the
time the short circuit occurs
C. Description of fault current
The greater the fault point X/R ratio, the
longer will be the asymmetrical fault current decay
time. For a specific X/R ratio, the angle of the
applied voltage at the time of short circuit initiation
determines the degree of fault current asymmetry
that will exist for that X/R ratio. In a purely
inductive circuit, the maximum dc current
component is produced when the short circuit is
initiated at the instant the applied voltage is zero (α
= 0° or 180° when using sine functions). The
current will then be fully offset in either the positive
or negative direction. Maximum asymmetry for any
circuit X/R ratio often occurs when the short circuit
is initiated near voltage zero. The initial dc fault
current component is independent of whether the ac
component remains constant or decays from its
initial value. For any circuit X/R ratio, the voltage
and current waveforms will be out of phase from
each other by an angle corresponding to the amount
of reactance in the circuit compared to the amount
of resistance in the circuit. This angle is equal to the
tan–1(2πf .L/R). For a purely inductive circuit, the
current waveform will be displaced from the
voltage waveform by 90° (lagging). As resistance is
added to the circuit this angular displacement will
decrease to zero. In a purely resistive circuit, the
voltage and current will be completely in-phase and
without an offset. In all practical circuits containing
resistance and reactance, the dc component will also
decay to zero as the energy represented by the dc
component is dissipated as I2R heating losses in the
circuit. The rate of decay of the dc component is a
function of the resistance and reactance of the
circuit. In practical circuits, the dc component
decays to zero in one to 30 cycles.
III. APPLICATION OF CURRENT SYMMERTY
DATA
In the previous discussion, a single phase
current was examined to give an understanding of
asymmetry. In a three-phase system with a bolted
three-phase fault, the sum of the current at any point
in time in the three phases must add to zero.
Therefore, if one phase has a maximum offset, then
the other two phases must have a negative offset to
balance current. The decay time constant of all
phases is the same. The maximum magnetic force
produced on a circuit element, such as a breaker,
occurs at the instant the fault current through the
circuit element is at a maximum. From an
equipment design and application viewpoint, the
phase with the largest of the fault current peaks is of
particular interest. This current value subjects the
equipment to the most severe magnetic forces. The
largest fault current peak typically occurs in the first
current cycle when the initiation of the short-circuit
current is near or coincident with the applied
voltage passing through zero. This condition is
called the condition of maximum asymmetry. In the
application of equipment that can carry fault current
such as circuit breakers, switches, transformers, and
fuses, the total available short-circuit current must
be determined. For correct equipment application,
knowledge of the minimum test X/R ratio or
maximum power factor of the applied fault current
used in the acceptance test by ANSI, NEMA, or UL
is also required. Peak fault current magnitudes are
significant for some devices, such as low-voltage
breakers, while asymmetrical rms. current
magnitudes are equally significant for high-voltage
circuit breakers. This leads to the need to develop
an X/R ratio dependent short-circuit calculation for
proper comparison to the equipment being applied.
The fault current calculation needs to take into
account the ac component and the transient dc
component of the calculated fault current to
determine the total maximum peak or rms. current
magnitude that can occur in a circuit. When the
calculated fault X/R ratio is greater than the
equipment test X/R ratio, the higher total fault
current associated with the higher X/R ratio must be
taken into account when evaluating the application
of the equipment. In this summary, it has been
shown that the effects of asymmetry are dependent
only upon the fault point X/R ratio of the circuit and
the instant of fault initiation. The references show
that the effects of the peak fault current magnitude
and the energy content of the first current cycle are
much greater than the effect of the rms. value. For
the Condition of maximum asymmetry, the rms.
value of the first cycle fault current theoretically can
be as great as 1.732 times the steady-state rms.
symmetrical fault current component. However, the
peak first cycle current for the same condition can
be up to two times the peak of the steady-state
current component, and the magnetic forces can be
four times that of the rms. symmetrical ac
component. From the equipment design viewpoint,
these peak currents and energy comparisons are the
maximum that the equipment must withstand. For
ANSI rated equipment, the maximum asymmetrical
rms. current provides this measure of maximum
capability. It is important to know the terms
defining the characteristic short-circuit current
waveforms. The test short-circuits currents used for
circuit breaker and fuse interrupting ratings have
different test procedures and power factor (X/R
ratios) requirements. For example, high-voltage
power circuit breakers use rms. current interrupting
tests at a power factor of 6.7% (X/R = 15), while
low-voltage circuit breakers use peak currents at a
power factor of 15% (X/R = 6.59). Molded case and
insulated case circuit breakers have different (from
6.7% and 15%) test power factors that must be
considered. If the calculated fault point X/R ratio is
greater than the test X/R ratio of the interrupting
device, then the calculation of equipment duty
current is affected.
IV. CASE STUDY RESULT (GETCO 220KV S/S)
As for the analysis of system under disturbance
condition case study of 220kV GETCO Mehsana
(Gujarat Energy Transmission Corporation)
substation is chosen.
A. X/R Ratio
The X/R ratio is important because it determines the
peak asymmetrical fault current. The asymmetrical
fault current can be much larger than the
symmetrical fault current. Fault is a sudden event so
it subjected to a response is called a transient, which
means that it lasts for only a short time
2
Data for the software simulation obtained from the GETCO
Mehsana (Gujarat energy transmission Corporation)
Fig 1: GETCO 220kV substation Mehsana line
diagram.(AC Load Flow)(assuming incoming lines as
generator, specify slack bus)
In power system impedance has two components.
The first is called reactance (X). Reactance depends
on two things: (1) the inductance and (2) the
frequency and second component is resistance.
p.f. = cos(tan-1(X/R))
If the power factor is unity (1), then the impedance
only has resistance. If the power factor is zero, then
the impedance only has reactance. Therefore, power
factor and X/R ratio are different ways of saying the
same thing. So as power factor decreases, the X/R
ratio increases. Right after a fault occurs, the
current waveform is no longer a sine wave. Instead,
it can be represented by the sum of a sine wave and
a decaying exponential. The decaying exponential
component added to the sine wave causes the
current to reach a much larger value than that of the
sine wave alone.
The waveform that equals the sum of the sine
wave and the decaying exponential is called the
asymmetrical current because the waveform does
not have symmetry above and below the time axis.
The actual waveform of the asymmetrical fault
current is hard to predict because it depends on
what time in the voltage cycle waveform the fault
occurs. However, the largest asymmetrical fault
current occurs when a fault happens at a point when
the voltage is zero. Then, the asymmetrical fault
current depends only on the X/R ratio, or power
factor, and the magnitude of the symmetrical fault
current. Figure 2 shows how the ratio of the peak
asymmetrical current to the RMS symmetrical
current varies with the X/R ratio. (RMS
symmetrical current equals the peak symmetrical
current divided by the square root of 2.)
What Figure 2 shows is that the peak asymmetrical
current increases with the X/R ratio.
The peak SC components can be calculated
multiplying the initial momentary SC current by a
crest factor of 2.07. These factors depend on the
X/R ratio of the driving point impedance and the
circuit breaker contact parting time. They are
different for near-to and far-from generators faults
V. CIRCUIT BREAKER DESIGN ANALYSIS
Fig2: Peak asymmetrical current as a function of
symmetrical RMS current.
10ms
10ms
I peak = 2 I sym
+ I dc
Where Isym is symmetrical AC SC component at
10ms after fault
As from the short circuit of systems 132kV bus the
various parameters with pre fault condition the
during fault and post fault component is computed
as below shows the fault data result in table and the
graph for the fault at bus number 38.
B. Results
The results shows the current magnitude as well as
the fault MVA level of the faulted bus contributed
from the generator bus.
Table I: Result of short circuit on bus no 39 of 220kV.
As previously explained the use of fault current
graph for the circuit breaker design as well as
systems
relay
and
switch
gear
settings.
Fig3: Fault current with DC decaying component.
While selecting circuit breakers it is important
to make sure that none of the capabilities are
exceeded in their application. These capabilities are
basically obtained from the short circuit current
calculations available at the equipment location.
Therefore, the starting point is the careful fault
analysis of the power system. Normally two kind of
rating considered. The first type of rating is an
interrupt rating. Devices that would have such a
rating include circuit breakers and fuses. An
interrupt rating refers to the maximum fault current
that a device can interrupt. The second type of
rating is a withstand rating.
Devices with withstand ratings are not
intended to interrupt fault current, but rather to
“ride through” a fault without damage. The rating
reflects the device’s ability to hold up during a
fault. This is usually carried out in per unit
quantities although there are other methods such as
percentage and MVA methods available. The TRV
(transient recovery voltage) is associated with the
so-called dielectric phase of the arc-interruption
phenomena. The overrating of high voltage circuit
breakers (CBs) is an ever growing problem as
power systems throughout the world tend to be
increasingly connected. The symmetrical and
asymmetrical short-circuit currents; the load
currents and the transient recovery voltage (TRV)
are amongst the most important parameters for the
analysis of CB overrating.
It is well-known that the dielectric stresses
imposed within a CB are higher when symmetric
short-circuit currents are to be interrupted. Thus,
neglecting the current asymmetry will lead to
conservative TRV values. Moreover, since the
maximum TRV value occurs before the first
current-wave peak, computation can be made using
the first current half-cycle only. Most of the faults
that occur in a real power system are non
symmetric. However, the study of symmetric three
phase faults is important because, despite the fact
that its occurrence is so rare, it is more severe from
the power system transient stability point of view
than unbalanced short circuits. In addition, this
study is useful to obtain synchronous machine
dynamic parameters and to understand the transient
behavior of electric power systems under the
occurrence of a short circuit.
VI. SINGIFICANCE OF GRAPH FOR C.B.
SELECTION
A circuit breaker has to work under different
circumstances. It’s rated in term of , The no of poles
rated, symmetrical & asymmetrical breaking
capacity, short time rating , operating duty. The
number of poles per phase of a breaker is a function
of the operating voltage. The voltage levels at
various points in a system vary depending upon the
system condition and as a result the breaker has to
operate under such variable voltage conditions. The
breaker is expected to operate at a maximum
voltage which normally is higher than the rated
nominal voltage. The rated current of a circuit
breaker is the maximum value of current in rms
amperes which it shall carry continuously without
exceeding the temperature limits of the various
parts of the breaker. The rated frequency of a
breaker is the frequency for which it is designed to
operate. application at frequencies other than the
designed,
need
special
considerations.
X/R ratio) is known sub transient current and in the
next 8 to 10 cycles it is known as transient current
and finally steady state current. The asymmetry in
the current is due to the D.C component. In case the
symmetrical breaking current is known, the making
current can be obtained by multiplying this current
by √2 to get the peak value and again by 1.8 to
include the doubling effect (i.e. D.C. component at
the first peak is almost equal to the A.C.
component). The breaking current of a breaker
depends upon the instant on the current wave when
the contacts begin to open. As shown Fig.4 the
contacts start to separate at XY. The symmetric
breaking current is given by a/√2amp and the
asymmetric breaking current is given by
(a 2 ) + b
2
2
The breaking capacity of a breaker is the product
of the breaking current and the recovery voltage.
The symmetrical breaking capacity is the product of
asymmetric breaking current and the recovery
voltage.
VII. BREAKER CALCULATION AND TRV
To perform a TRV study in digital simulation, the
first step is to build up the power system model of
the study subject (The power plant substation in this
case). The detailed representation of the substation
and nearby power system is required. The time
varying voltages at the breaker in question will be
computed, displayed and examined.
Depending on whether the user selects a 1- pole, 2pole or a 3-pole circuit breaker the calculation for
the circuit breaker varies.
3 pole =
Fig 4: shows the fault current and C.B operation.
The making current is the peak value of the
maximum current loop, including dc component, in
any phase during the first cycle of current when the
circuit breaker is closed. Then making current
corresponds to the ordinate IASYM. The capacity of a
breaker to make currents depends upon its ability to
withstand and to close successfully against the
effect of electromagnetic forces. The maximum
force in any phase is a function of the square of the
maximum instantaneous current occurring in that
phase on closing. It’s therefore, the practice to
specify making current in terms of peak value rather
than in terms of rms value. The making capacity is,
carry instantaneously at the rated service voltage. It
is known that in a particular phase the current is
maximum right at the instant short circuit takes
place, after which the current decreases. The current
in the first one or two cycles (depending upon the
SVA
3 * 80% * ( P − P )V
(P-P)V= Line to line voltage
The recovery voltage is the voltage which appears
across the terminals of a pole of a circuit breaker.
This voltage may be considered in two successive
time intervals: one during which a transient voltage
exists, followed by a second one during which a
power frequency voltage alone exists. Within a few
microseconds after current zero, current stops
flowing in the circuit. The power system response
to the current interruptions is what generates TRV.
The difference in the power system response
voltage from the source side to the load side of the
circuit breaker is the TRV. The breaking operation
is successful if the circuit breaker is able to
withstand the TRV and the power frequency
recovery voltage. TRV is then related to the power
system response to an interruption of current in a
circuit very close to a power frequency current zero.
The nature of the TRV is dependent on the circuit
being interrupted, whether primarily resistive,
capacitive or inductive.
Study of this phenomenon is extremely important to
ensure that breaker electrical insulation limits, as
defined by the appropriate standards, are not
violated due to power system characteristics. The
procedure for TRV verification and the TRV
capabilities are outlined in the IEEE standard
C37.011-1994.
Fig: 5 TRV waveform across the 132kV C.B
As in Fig.5 the TRV across the pole of the C.B of
132kV size is simulated of the EMTDC/PSCAD
software for the fault current of 9420A of
symmetrical kind of fault. The detailed result shown
in Table II.
Table II: Result of simulation for C.B wave on PSCAD.
Rating
Value
Rated Voltage
132kV
Interrupting Short Circuit Current
9.4kA
Rated Peak(E2)
219kV
Rated Time to Peak(T2)
14micro
second
VIII. CONCLUSION
Short circuit studies are performed to determine
the magnitude of the current flowing throughout the
power system at various time intervals after a fault.
The magnitude of the current through the power
system after a fault varies with time until it reaches
a steady state condition. During the fault, the power
system is called on to detect, interrupt and isolate
these faults, The duty impressed on the equipment
is dependent on the magnitude of the current, which
is a function of the time of fault initiation. Such
calculations are performed for various types of fault
such as three-phase, single line to ground fault,
double line to ground fault and at different location
of the system. The calculated short circuit results
are used to select fuses, circuit breakers and
protective relays. As from the simulation on both
MiPower and PSCAD user can model any power
system for design point of view and with accurate
modelling the result get approaches to be accurate
as verified with case study.
View publication stats
ACKNOWLEDGMENT
Authors would like to acknowledge thanks GETCO
for the allowing project work in company and kind
support during the project work.
REFERENCES
[1] IEEE Std 551™-2006 IEEE Recommended Practice
for Calculating Short-Circuit Currents in Industrial
and commercial power system.
[2] ANSI\IEEE Std. 241-1990 "Recommended Practice
for Electric Power Systems in Commercial
Buildings”.
[3] Vladimir V. Terzija “Short Circuit Studies in
Transmission Networks Using Improved Fault
Model”
[4] IEEE Std 141-1993” IEEE Recommended Practice
for Electric Power Distribution for Industrial Plants”
[5] Shui-cheong Kam “Modeling of Restriking and
Reignition Phenomena in Three-phase Capacitor
and Shunt Reactor Switching”
[6] A.M. Gole, O.B. Nayak. T.S. Sidhu, M.S. Sachdev,
"A
Graphical
Electromagnetic
Simulation
Laboratory for Power Systems Engineering
Programs", IEEE PES.
[7] ”EMTDC - The Electromagnetic Transients &
Controls Simulation Engine”, User’s Guide,
Manitoba HVDC Research Centre Inc., 2005
[8] R. Orama, ”Breakdown Phenomena of a Vacuum
Interrupter after Current Zero”, International
Conference on Power System Transients 2001, Rio
de Janeiro, Brazil, June 2001,
[9] K. Ngamsanroaj and W. Tayati,” An analysis of
switching overvoltages in the EGAT 500 kV
transmission system”, in proc. 2003 IEEE
Conference on Power Engineering, pp. 149.