The document provides information about a course on power system protection that covers topics like protection relays, schemes for transformer, generator, transmission line and capacitor bank protection. It also discusses switchgear testing and protection of circuit breakers. The objective of power system protection is to isolate a faulty section from the healthy system using circuit breakers tripped by protection relays. The document then provides a brief overview of the history and subsidiaries of the M.M. Ispahani group in Bangladesh. It concludes with descriptions of a substation visited, including its single line diagram, equipment functions and a brief overview of tap changing methods.

1. Introduction:
The course Power System Protection covers almost everything related to protection system in
power system including standard lead and device numbers, mode of connections at terminal
strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles
of various power system protection relays and schemes including special power system
protection schemes like differential relays, restricted earth fault protection, directional relays
and distance relays etc. The details of transformer protection, generator protection,
transmission line protection & protection of capacitor banks are also given. It covers almost
everything about protection of power system.
The switchgear testing, instrument transformers like current transformer testing voltage or
potential transformer testing and associated protection relay are explained in detail. The close
and trip, indication and alarm circuits different of circuit breakers are also included and
explained under power system protection.
Objective of Power System Protection:
The objective of power system protection is to isolate a faulty section of electrical power
system from rest of the live system so that the rest portion can function satisfactorily without
any severer damage due to fault current.
Actually circuit breaker isolates the faulty system from rest of the healthy system and this
circuit breakers automatically open during fault condition due to its trip signal comes from
protection relay. The main philosophy about protection is that no protection of power system
can prevent the flow of fault current through the system, it only can prevent the continuation
of flowing of fault current by quickly disconnect the short circuit path from the system.
History of M. M. Ispahani limited of Bangladesh:
M. M. Ispahani Limited also known as the Ispahani Group, is a Bangladeshi conglomerate.
Headquartered in Chittagong, it is owned by the Ispahani family. The group owns Bangladesh’s
largest tea company, as well as other major food brands in the country. It also has interests in
shipping, real estate, textiles, jute and hotels.
The group was awarded the prestigious Bangladesh Business Awards in 2003. According to
the The Daily Star, M. M. Ispahani is one of the most respected business concerns in the
subcontinent. A brief history of this company is given below;
Type: Private
Industry: Conglomerate
Founded: 1820
Founder: Mirza Mohammed Ispahani
Headquarters: Chittagong, Bangladesh

2. Key people: Mirza Ali Behroz Ispahani, Chairman
Products: Tea, consumer foods, textiles, yarn, jute, shipping, real estate
Subsidiaries:
M.M. Ispahani Limited
Ispahani Foods Limited
Pahartali Textile and Hosiery Mills
Broad Band Telecom Services Limited
South East Trading Limited
Islamia Eye Hospital
Orient Airways
As a part of our academic curriculum we have visited Pahartali Textile and Hosiery Mills
which is a subsidiaries of M. M. Ispahani group. Established in 1954 This mill located at
Ispahani Complex, Zakir Hossain Rd, Chittagong 4202, Bangladesh.
Brief on Substation:
A substation is a part of an electrical generation, transmission, and distribution system.
Substations transform voltage from high to low, or the reverse, or perform any of several other
important functions. Between the generating station and consumer, electric power may flow
through several substations at different voltage levels.
Substations may be owned and operated by an electrical utility, or may be owned by a large
industrial or commercial customer. Generally sub-stations are unattended, relying on SCADA
for remote supervision and control.
A substation may include transformers to change voltage levels between high transmission
voltages and lower distribution voltages, or at the interconnection of two different transmission
voltages. The word substation comes from the days before the distribution system became a
grid. As central generation stations became larger, smaller generating plants were converted to
distribution stations, receiving their energy supply from a larger plant instead of using their
own generators. The first substations were connected to only one power station, where the
generators were housed, and were subsidiaries of that power station.
Types of Substation:
According to service requirements substation may be classified into-
1. Transformer Substation
2. Switching Substation
3. Power factor correction Substation
4. Frequency changer Substation
5. Converting Substation
6. Industrial Substation

3. According to constructional features the substations are classified as-
1. Indoor Substation
2. Outdoor Substation
3. Underground Substation
4. Pole mounted Substation
Pahartali Textile and Hosiery mills Sub-station:
This sub-station is a 33/11 KV, 10/14 MVA sub-station. The 33 KV line is taken from the
Khulshi sub-station which we have visited previously. By using the 10 MVA transformer
voltage is stepped down to 11 KV so from the classification of transformer it can be said that
it is a transformer sub-station. This sub-station constitutes equipment’s like lightening arrester,
line earth switch, current transformer, potential transformer, circuit breaker (33KV & 11KV),
power transformer, over current relay, earth fault relay, surge relay, buchholz relay, alarm
relay, oil temperature indicator and multifunction static programmable meter. A schematic
diagram of the sub-station is given below;
Figure: Schematic diagram of pahartali textile and hosiery mills sub-station.

4. Pahartali Textile and Hosiery Mills Equipments & Its
Functions:
Figure: Single line diagram for 33/11 KV, 10/14 MVA sub-station

5. Function of different equipment’s from the single line
diagram is given below:
Lightening Arrester:
Lightening arrestors are the instrument that are used in the incoming feeders so that to
prevent the high voltage entering the main station. This high voltage is very dangerous to the
instruments used in the substation. Even the instruments are very costly, so to prevent any
damage lightening arrestors are used. The lightening arrestors do not let the lightening to fall
on the station. If some lightening occurs the arrestors pull the lightening and ground it to the
earth. In any substation the main important is of protection which is firstly done by these
lightening arrestors. The lightening arrestors are grounded to the earth so that it can pull the
lightening to the ground. The lightening arrestor works with an angle of 30° to 45° making a
cone.
Line Earth Switch:
The use of line earth switches is to send any trapped charge in the line into ground after the line
is switched off. Mainly what happens, when the line is switched off, some charges remain trapped in
line due to their own capacity and also due to influence of side by transmission line. This trapped charge
is dangerous if someone goes for maintenance of that very line. So it is very much required to neutralize
that much charge by using line earth switch.
Instrument Transformer:
Instrument transformers are used to step-down the current or voltage to measurable
values. They provide standardized, useable levels of current or voltage in a variety of power
monitoring and measurement applications. Both current and voltage instrument transformers
are designed to have predictable characteristics on overloads. Proper operation of over-current
protection relays requires that current transformers provide a predictable transformation ratio
even during a short circuit.
These are further classified into two types which are discussed below.
a. Current Transformers
b. Potential Transformers

6. Current Transformer:
Current transformers are basically used to take the readings of the currents entering the
substation. This transformer steps down the current from 800 amps to 1 amp. This is done
because we have no instrument for measuring of such a large current. The main use of this
transformer is
a. Distance Protection
b. Backup Protection
c. Measurement
A current transformer is defined as an instrument transformer in which the secondary
current is substantially proportional to the primary current (under normal conditions of
operation) and differs in phase from it by an angle which is approximately zero for an
appropriate direction of the connections. This highlights the accuracy requirement of the
current transformer but also important is the isolating function, which means no matter what
the system voltage the secondary circuit need to be insulated only for a low voltage.
The current transformer works on the principle of variable flux. In the ideal current
transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and
opposite to the primary current. But, as in the voltage transformer, some of the primary current
or the primary ampere-turns are utilized for magnetizing the core, thus leaving less than the
actual primary ampere turns to be transformed into the secondary ampere-turns. This naturally
introduces an error in the transformation. The error is classified into current ratio error and the
phase error.
Potential Transformer:
There are two potential transformers used in the bus connected both side of the bus.
The potential transformer uses a bus isolator to protect itself. The main use of this transformer
is to measure the voltage through the bus. This is done so as to get the detail information of the
voltage passing through the bus to the instrument. There are two main parts in it;
a. Measurement
b. Protection
The standards define a voltage transformer as one in which the secondary voltage is
substantially proportional to the primary voltage and differs in phase from it by an angle which

7. is approximately equal to zero for an appropriate direction of the connections. This in essence
means that the voltage transformer has to be as close as possible to the ideal transformer.
In an ideal transformer, the secondary voltage vector is exactly opposite and equal to
the primary voltage vector when multiplied by the turn’s ratio.
In a practical transformer, errors are introduced because some current is drawn for the
magnetization of the core and because of drops in the primary and secondary windings due to
leakage reactance and winding resistance. One can thus talk of a voltage error which is the
amount by which the voltage is less than the applied primary voltage and the phase error which
is the phase angle by which the reversed secondary voltage vector is displaced from the primary
voltage vector.
Bus Bar:
The bus is a line in which the incoming feeders come into and get into the instruments
for further step up or step down. The first bus is used for putting the incoming feeders in la
single line. There may be double line in the bus so that if any fault occurs in the one the other
can still have the current and the supply will not stop but in the pahartali sub-station only one
bus bar is present. Bus bar can also be defined as, in electrical power distribution refers to thick
strips of copper or aluminum that conduct electricity within a switchboard, distribution board,
substation, or other electrical apparatus. The size of the bus bar is important in determining the
maximum amount of current that can be safely carried. Bus bars are typically either flat strips
or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface
area to cross sectional area ratio. The skin effect makes 50-60 Hz AC bus bars more than about
8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current
applications. A hollow section has higher stiffness than a solid rod of equivalent current
carrying capacity, which allows a greater span between bus bar supports in outdoor
switchyards. A bus bar may either be supported on insulators or else insulation may completely
surround it. Bus bars are protected from accidental contact either by a metal enclosure or by
elevation out of normal reach.
Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly onto
any metal chassis of their enclosure. Bus bars may be enclosed in a metal housing, in the form
of bus duct or bus way, segregated-phase bus, or isolated-phase bus.

8. Circuit Breaker:
The circuit breakers are used to break the circuit if any fault occurs in any of the
instrument. These circuit breaker breaks for a fault which can damage other instrument in the
station. For any unwanted fault over the station we need to break the line current. This is only
done automatically by the circuit breaker. There are mainly two types of circuit breakers used
for any substations. They are;
a. SF6 circuit breakers
c. Vacuum circuit breakers.
At our visited site two vacuum circuit breaker is being used one is 33 KV VCB and
another is 11 KV VCB, In vacuum circuit breaker vacuum (degree of vacuum being in the
range from to torr) is used as the arc quenching medium. Since vacuum offers the highest
insulating strength, it has far superior arc quenching properties than any other medium. VCB
are being employed for outdoor application ranging from 22kv to 66kv. This range suits the
sub-station we visited.
Transformer:
A transformer is a static machine used for transforming power from one circuit to another
without changing frequency. This is a very basic definition of transformer. Generation of electrical
power in low voltage level is very much cost effective. Hence electrical power is generated in low
voltage level. Theoretically, this low voltage level power can be transmitted to the receiving end. But
if the voltage level of a power is increased,the current of the power is reduced which causes reduction
in ohmic or I2
R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in
capital cost of the system and it also improves the voltage regulation of the system. Because of these,
low level power must be stepped up for efficient electrical power transmission. This is done by step up
transformer at the sending side of the power system network. As this high voltage power may not be
distributed to the consumers directly, this must be stepped down to the desired level at the receiving
end with the help of step down transformer. These are the uses of electrical power transformer in the
electrical power system. Now, the transformer used in the pahartali sub-station is a step down
transformer as it step down the AC voltage level from 33 KV to 11 KV. This step down transformer is
rated at 10 MVA. A diagram for power transformer is given below;

9. Figure: A power transformer.
Buchholz Relay:
In the field of electric power distribution and transmission, a Buchholz relay is a safety device
mounted on some oil-filled power transformers and reactors, equipped with an external
overhead oil reservoir called a "conservator". The Buchholz relay working principle of is very
simple. Buchholz relay function is based on very simple mechanical phenomenon. It is
mechanically actuated. Whenever there will be a minor internal fault in the transformer such
as an insulation faults between turns, break down of core of transformer, core heating, the
transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO.
The gases produced due to decomposition of transformer insulating oil will accumulate in the
upper part the Buchholz container which causes fall of oil level in it. Fall of oil level means
lowering the position of float and thereby tilting the mercury switch. The contacts of this
mercury switch are closed and an alarm circuit energized. Sometime due to oil leakage on the
main tank air bubbles may be accumulated in the upper part the Buchholz container which may
also cause fall of oil level in it and alarm circuit will be energized. By collecting the
accumulated gases from the gas release pockets on the top of the relay and by analyzing them
one can predict the type of fault in the transformer. More severe types of faults, such as short
circuit between phases or to earth and faults in the tap changing equipment, are accompanied
by a surge of oil which strikes the baffle plate and causes the mercury switch of the lower
element to close. This switch energized the trip circuit of the circuit breakers associated with
the transformer and immediately isolate the faulty transformer from the rest of the electrical
power system by inter tripping the circuit breakers associated with both LV and HV sides of
the transformer. This is how Buchholz relay functions.

10. Oil temperature indicator:
These are generally precision instruments. A temperature indicator of power transformer is
specially designed for protection of transformer in addition to its temperature indication and
cooling control features. This device performs three functions which are given below;
1) These instruments indicate instantaneous temperature of oil and windings of transformer.
2) These also record maximum temperature rise of oil and windings.
3) These instruments operate high temperature alarm at a predetermined value of allowable
temperature limit.
Figure: An oil temperature indicator
Tap changing:
Off-circuit designs (NLTC or DETC):
In low power, low voltage transformers, the tap point can take the form of a connection
terminal, requiring a power lead to be disconnected by hand and connected to the new terminal.
Alternatively, the process may be assisted by means of a rotary or slider switch. Since the
different tap points are at different voltages, the two connections cannot be made
simultaneously, as this would short-circuit a number of turns in the winding and produce
excessive circulating current. Consequently, the power to the device must be interrupted during
the switchover event. Off-circuit or de-energized tap changing (DETC) is sometimes employed
in high voltage transformer designs, although for regular use, it is only applicable to
installations in which the loss of supply can be tolerated. In power distribution networks,
transformers commonly include an off-circuit tap changer on the primary winding to
accommodate system variations within a narrow band around the nominal rating. The tap
changer will often be set just once, at the time of installation, although it may be changed later
during a scheduled outage to accommodate a long-term change in the system voltage profile.

11. On-load designs (OLTC):
Also called on circuit tap changer or On Load Tap Changer (OLTC). For many power
transformer applications, a supply interruption during a tap change is unacceptable, and the
transformer is often fitted with a more expensive and complex on-load tap-changing (OLTC,
sometimes LTC) mechanism. On-load tap changers may be generally classified as either
mechanical, electronically assisted, or fully electronic.
Isolator:
The use of this isolator is to protect the transformer and the other instrument in the line. The
isolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line.
Thus an isolator is used after the bus also for protection. Isolator is an off load device that is
why it is often called off load isolator.
Figure: An isolator
Trip circuit supervision scheme:
Trip circuit supervision in Circuit breakers is a vital part of any protection scheme. If the trip
relay fails to operate, it may result in upstream tripping or even in damage to equipment. Trip
circuit supervision makes sure that the tripping coil of a circuit breaker is always in the healthy
condition. The Trip circuit supervision is particularly important in breakers which have only
one trip coil. The Trip circuit supervision relay continually measures the resistance of the trip
coil of circuit breakers. It also measures the control voltage of the trip coil and gives and alarm
when the control voltage falls to low levels. The Trip circuit supervision relay injects a constant
current through the trip coil of the breaker and measures the voltage drop across the coil. Thus,
the relay is able to measure the resistance of the coil. The Trip circuit supervision relays can
also monitor more than one breaker coil. If the Trip circuit supervision Relay detects a fault,
it activates the breaker failure logic which can activate a backup breaker if installed or cause
the tripping of upstream breakers.

12. Over current relay:
In an over current relay or o/c relay the actuating quantity is only current. There is only one
current operated element in the relay, no voltage coil etc. are required to construct this
protective relay. In an over current relay, there would be essentially a current coil. When normal
current flows through this coil, the magnetic effect generated by the coil is not sufficient to
move the moving element of the relay, as in this condition the restraining force is greater than
deflecting force. But when the current through the coil increased, the magnetic effect increases,
and after certain level of current, the deflecting force generated by the magnetic effect of the
coil, crosses the restraining force, as a result, the moving element starts moving to change the
contact position in the relay. Although there are different types of over current relays but basic
working principle of over current relay is more or less same for all.
Figure: An induction disk type over current relay
Earth Fault Relay:
Earth fault relay is a relay which is sense the faults between the phase and Earth. Earth fault relay's
function is, it calculates the vector sum of all the three currents flowing in R Y B lines. If the vector
sum is not zero then there is a fault
current flowing in particular line. Since these lines are connected to load say generator then, more than
the rated value of current flowing through windings, hence windings will damage so to prevent these
type of damages EFR is used.

13. Differential relay:
The differential relay is one that operates when there is a difference between two or more similar
electrical quantities exceeds a predetermined value. In differential relay scheme circuit, there are two
currents come from two parts of an electrical power circuit. These two currents meet at a junction point
where a relay coil is connected. According to Kirchhoff Current Law, the resultant current flowing
through the relay coil is nothing but summation of two currents,coming from two different parts of the
electrical power circuit. If the polarity and amplitude of both currents are so adjusted that the phasor
sum of these two currents, is zero at normal operating condition. Thereby there will be no current
flowing through the relay coil at normal operating conditions. But due to any abnormality in the power
circuit, if this balance is broken, that means the phasor sum of these two currents no longer remains
zero and there will be non-zero current flowing through the relay coil thereby relay being operated. In
current differential scheme, there are two sets of current transformer each connected to either side of
the equipment protected by differential relay. The ratio of the current transformers are so chosen, the
secondary currents of both current transformers matches each other in magnitude. The polarity of
current transformers are such that the secondary currents of these CTs opposes each other. From the
circuit is clear that only if any nonzero difference is created between this to secondary currents, then
only this differential current will flow through the operating coil of the relay. If this difference is more
than the peak up value of the relay, it
will operate to open the circuit breakers
to isolate the protectedequipment from
the system. The relaying element used
in differential relay is attracted
armature type instantaneously relay
since differential scheme is only
adapted for clearing the fault inside the
protected equipment in other words
differential relay should clear only
internal fault of the equipment hence the protected equipment should be isolated as soon as any fault
occurred inside the equipment itself. They need not be any time delay for coordination with other relays
in the system. Relay (2 phases and 1 Earth fault for 3 phase system). Under voltage and over
voltage relays. Neutral Current Unbalance Relays are for both Alarm and Trip facilities breaker
control switch with local/remote selector switch, master trip relay and trip alarms acknowledge
and reset facilities.

14. Oil Surge Relay:
The oil surge relay is connected in between OLTC chamber and conservator (separate for
OLTC) with Breather. A separation should be arrange such that the oil in OLTC chamber and
transformer chamber should not be mixed together. In case any on load tap changer a problems
developed during operation a gas is developed and that gas will actuate the oil surge relay and
the relay activates other circuits and main circuit will be opened with alarm indication and flag
indication.