POWER SYSTEM PROTECTION
AND SWITCHGEARS
EE256
Unit I:
• Introduction
Principles and need for protective schemes –
Nature and cause of faults – types of fault –
per unit representation - Analysis of
Symmetrical fault – Current limiting reactors.
CTs and PTs and their applications in their
protection schemes.
Why do we need protection?
• Electrical apparatus operates at various
voltage levels and may be enclosed or placed
in open.
• Under abnormal operating conditions
protection is necessary for
Safety of electrical equipments.
Safety of human personnel.
Why A System Needs Protection?
• There is no ‘fault free’ system.
• It is neither practical nor economical to build a
‘fault free’ system.
• Electrical system shall tolerate certain degree
of faults.
• Usually faults are caused by breakdown of
insulation due to various reasons: system
aging, lighting, etc.
Purpose of Protection System
• Minimize damage
• Leave unaffected equipment in-service
• Maintain equipment operating limits
• Maintain electrical system stability
Abnormal conditions:
• Short circuits in the transmission or distribution
line
• Over voltages due to switching or lightning
• Over speeding of generators or motors
• Loss of excitation of machines
• Over heating of stator and rotor of the machine.
• Insulation breakdown between the inter-turn
coils of the winding
• Low level oils in the transformer and circuit
breakers
Nature and causes of faults
•
•
•
•
•
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Breaking of conductors
Failure of insulation
Mechanical failure
Accidents
Excessive internal and external stress
High degree of pollution on an insulator string
(<insulation strength)
• OH lines-perching of birds
Accidental short circuiting by snakes
kite strings
Tree branches
ice and snow loading
Non-system faults,
In correct setting
Incorrect connection
Human error(maintenance)
Faults and types
• A fault in a circuit is any failure which
interrupts with the normal flow of current.
• The faults are associated with abnormal
change in current, voltage and frequency of
the power system. The faults may cause
damage to the equipments, if it is allowed to
persist for a long time.
Faults occur in a power system due to insulation failure of
equipments, flashover of lines initiated by a lightening stroke,
permanent damage to conductors and towers or accidental faulty
operations.
Types
• Symmetrical fault:
Three phase fault
• Unsymmetrical faults:
short circuit
Single line-to-ground fault
Line-to-line fault
Double line-to-ground fault
Open circuit
Single phase open circuit
Two phase open circuit
Winding faults
Sources of Asymmetrical fault are:
Effects of faults
The fault must be cleared as fast as possible. Many equipments may be
destroyed if the fault is not cleared rapidly. The dangerous of the faults
depends on the type of the fault, as example the three phase short
circuit is the most dangerous fault because the short circuit current is
maximum. Some of the effects of short circuit current are listed here
under.
 Due to overheating and the mechanical forces developed by faults,
electrical equipments such as bus bars, generators, transformers
will be damaged.
 Negative sequence current arises from unsymmetrical faults will
lead to overheating.
 Voltage profiles may be reduced to unacceptable limits as a result
of faults.
 A frequency drop may lead to instability
Fault statistics
Element
% of total fault
OH lines
50
UG cables
9
Transformers
10
Generators
7
Switchgears
12
CTs, PTs, Relays ,Control equipment, etc
12
Per unit representation
• In a power system different power equipment
with different voltage and power levels are
connected together through various step up or
step down transformers. However the presence
of various voltage and power levels causes
problem in finding out the currents (or voltages)
at different points in the network.
• To alleviate this problem, all the system
quantities are converted into a uniform
normalized platform. This is called the per unit
system .
In a per unit system each system variable or quantity is
normalized with respect to its own base value. The units
of these normalized values are per unit (abbreviated as
pu) and not Volt, Ampere or Ohm. The base quantities
chosen are:
 VA base ( Pbase ): This is the three-phase apparent power
(Volt-Ampere) base that is common to the entire circuit.
 Voltage Base ( Vbase ): This is the line-to-line base voltage.
This quantity is not uniform for the entire circuit but gets
changed by the turns ratio of the transformer.
• The per unit value of any electrical quantity is
defined as the ratio of the actual value of the
quantity to its base value expressed as a
decimal.
Advantages of per unit representation
 The per unit impedance referred to either side of a single
phase transformer is the same.
 The per unit impedance referred to either side of a three
phase transformer is the same regardless of the three
phase connections whether they are Y-Y, Δ-Δ or Δ-Y
 The chance of confusion between the line and phase
quantities in a three phase balanced system is greatly
reduced.
 The manufacturers usually provide the impedance values
in per unit.
 The computational effort in power system is very much
reduced with the use of per unit quantities.
Reactors
• Reactors are equipment of transformer family. A
reactor has a predominantly inductive coil.
Reactors are used in the power system network
for current limiting and for compensation of
reactive power.
There are two types of reactors:

Series reactors-connected in series for current
limiting

Shunt reactors- connected in shunt, for
compensation of reactive power.
Current limiting reactor
• Series Reactors (CLR) are necessary for
limiting short circuit currents, for limiting
inrush currents while switching-in, for limiting
current surges with fluctuating loads, for
smoothing the current waveform, for giving
stored energy for satisfactory operation of
converters, neutral grounding reactors, etc
Advantages of CLR
• To limit the flow of short circuit current.
• Protect the equipment from over heating as
well as failure due to destructive mechanical
forces.
• Increases the chances of continuity of supply.
• They permit the installation of C.B of lower
ratings.
Current limiting reactor
These are inserted in series with the line , to limit the
current flow in the event of a short circuit.
Continue….
• Fault current < breaking current capacity- C.B
have enough breaking
• Fault current > breaking current capacityreplace high breaking capacity C.B or put CLR
• The essential requirements of CLR is that
reactance should not reduce due to saturation
under short circuit condition.
Types:
 Dry type air core reactor
 Oil immersed air core reactor
Dry type :
 For moderate voltages (up to 33KV)and power ratings
 the cheapest type of current limiting reactors is usually the simple
 naked dry-type reactor iron core and any enclosure, cooled by natural air
circulation.
 The magnitude of the inductance of these reactors is normally in the order
of millinery. The inductance remains constant when short circuit flows
through the reactor.
 There is no decline in the inductance due to saturation in an iron core.
 It occupies large space.
 The absence of an iron core makes the winding capacitance to earth quite
small, which gives the advantage that the voltage distribution within the
winding deviates just moderately from linearity during transient voltage
conditions.
Oil- immersed type:
 Dry-type reactors for higher voltages may not be suitable in heavily
polluted areas because of the risk of dielectric failure. In such cases, oilimmersed reactors might be more reliable.
 To avoid excessive heating in the tank a frame of laminated core steel must
enclose the oil-immersed reactor winding.
 The dimensioning of the reactor must be such that the inductance is
sufficiently large when short circuit current flows through the reactor and
when saturation may occur in the core.
 The cost of an oil-immersed reactor will be considerably higher than of a
dry type, while the oil-immersed reactor might be less space consuming.
Advantages oil immersed type:
Higher factor of safety against flash over
Smaller size
High thermal capacity
Location of reactors:
In series with each generator
In series with each feeder
In bus-bars
What is Instrument Transformer ?
• A transformer that is used in conjunction with
a measuring instrument.
• It utilizes the current-transformation and
voltage transformation properties to measure
high ac current and voltage.
Importance of Instrument
transformers
• In dc circuits for current and voltage
measurement we use ammeters and voltmeters.
• For measurement of high current ,it is usual to
use low range ammeter with suitable shunt.
• For measurement of high voltage, low range
voltmeter are used with high resistance
connected in series.
• But for measurement of high A.C. current and
voltage we cannot use these methods.
• We use specially constructed instrument
transformers.
Types of instrument transformers
These instrument transformers are of two
types:• Current transformers
• Potential transformers
Current Transformers
• Current transformer normally known as c.t. is a
step up transformer.
• These are used with low range ammeter to
measure current in high voltage alternating
circuits where it is not practical to connect
instrument and meters directly to lines.
• This is step up transformer because when we step
up the, voltage increases and current decreases.
• The current is step down in a known ratio called
current ratio.
Construction of C.T.
• C.T. has a primary coil of one or more
turns of thick wire connected in series
with the line whose current
is to be measured.
• The secondary consist of large number of turns of fine wire, is
connected across the ammeter terminals.
Working
• If a current transformer has primary to secondary current ratio of
100:5 then it step up the voltage 20 times and step down the
current 1/20 times of its actual value.
• If we know the current ratio I1/I2 and the reading of a.c. ammeter,
the current can be calculated.
Current = ratio × ammeter reading
Importance of short ckt.
• Ammeter resistance is very low ,the current
transformer normally works short circuited.
• If for any reason the ammeter is taken out of
secondary winding then the secondary winding must
be short ckted with the help of short ckt switch s.
• If this is not done, then due to high m.m.f. will set up
high flux in the core and it will produces excessive core
loss which produce heat and high voltage across the
secondary terminals Hence the secondary of current
transformer is never left open.
Potential transformer
• A PT is a step down transformer having many
primary turns but few secondary turns.
• In step down the voltage decreases and
current increases, thus voltage can be easily
measured by using low range voltmeter.
• The voltage is stepped down in known ratio
called voltage ratio.
Construction and working of P.T
Construction
 A potential transformer has many primary
windings but few number of secondary windings
that makes it step down transformer.
Voltmeter is connected to secondary winding
usually voltmeter of 150 v is suitable.
Working
 Primary terminals are connected across the line to which the voltage is to
be measured.
 The voltmeter gives the transformed value of voltage at secondary.
 The deflection of voltmeter when divided by transformed ratio gives the
actual voltage at primary.
 Line voltage = deflection / trasf. Ratio
 Where transformation ratio = V2/V1
Precaution for P.T.
• Since the secondary of p.t. is connected to
relays, their ratings are usually 40 to 100
Watts.
• For safety purpose the secondary should be
completely insulated from the high voltage
primary and should be in addition grounded.
Types of P.T.
• Some types of p.t. are
Shell type
Dry type
Oil type
Errors in instrument transformer
There are 2 types of errors
 Ratio error
 Phase angle error
Ratio error:
N1/N2~=I1/I2 ,N1/N2`=V1/V2
because of magnetizing and core loss components of the exciting current.
Transformation ratio is not constant but depends upon the load current,powerfactor of
load and exciting current of the transformers.
• For a certain transformer design, the burden capability
depends on the value of the short-circuit impedance. A low
value for the short-circuit impedance (a high quantity of
copper) means a high burden capability and vice versa. The
burden capability must always be referred to a certain
accuracy class.
• If 200 VA, class 1 is performed with a certain quantity of
copper, the class 0.5 capability is 100 VA with the same
quantity of copper, on condition that the turns correction
is given values adequate to the two classes.
• The ratio between accuracy class and burden capability is
approximately constant. This constant may be called the
“accuracy quality factor” K of the winding
Applications
Circulating current differential protection
Over current phase fault protection
Distance protection
Assignment
• Explain various types of instrument
transformer
• Analysis of symmetrical fault
Date of submission-22-7-11