See discussions, stats, and author profiles for this publication at: https://www.researchgate.

net/publication/376451055

Protection of Power Transformer

Presentation · December 2023

DOI: 10.13140/RG.2.2.15094.98884

CITATIONS READS

0 376

1 author:

Mohammed Nashwan
Iraqi Ministry of Electricity (MOE)
3 PUBLICATIONS 0 CITATIONS

SEE PROFILE

All content following this page was uploaded by Mohammed Nashwan on 12 December 2023.

The user has requested enhancement of the downloaded file.

University of Mosul

College of Engineering

Electrical power Engineering

Protection of Power Transformer

Prepared by :

Muhamed Nashwan Amjed

Supervision by :

Dr. Ahmed Nasser Dr. Mohammed Ali

Introduction:

The large power transformer belong to a class of very expensive and vital
components in electric power systems. Occurrence of faults can cause
damage to the equipment ,detecting fault is necessary in order to clear
fault before it increase the damage to the power transformer, so that a
major goal of transformer protection is limiting the damage to a faulted
transformer .

1 - What is the Protection ? [1]

We can define the protection as ,( it’s the art or science of continuously

monitoring the power system and detecting the fault and initiating the
correct tripping of the circuit breaker )

Protection must have the following qualities :

_ Selectivity

_ Stability

_ Speed

_ Sensitivity …….. and other properties .

The protective system can be subdivided in to three subsystem :

1 -1 Circuit breaker [1]

The type of the Circuit Breaker is usually identified according to the
medium of arc extinction. The classification of the Circuit Breakers based
on the medium of arc
extinction is as follows:
(1) Air break' Circuit Breaker. (Miniature Circuit Breaker).
(2) Oil Circuit Breaker (tank type of bulk oil)

1
(3) Gas Circuit Breaker.
(4) Vacuum Circuit Breaker

Fig (1) circuit breaker 12 kv

1-2 Transducers : [5]

Its mean the instrument transformer ( voltage and current transformer ) ,

the purpose of transducer to transform current and voltage from usually a
high value to a value easy to handle for protection's devise and for
metering , and its divided in to two types :

A ) voltage transformer ( potential transformer ):

There are basically tow type of voltage transformer used in protection

equipment .

 Electromagnetic type ( VT )

These type of transformer is similar to a power transformer , is a step

down transformer whose primary high voltage and secondary low voltage
. show fig (2)

2
 Fig ( 2) Electomagnatic voltage transfomer

 Capacitive voltage transformer ( CVT)

The primary portion consists of capacitors connected in series to split

primary voltage to convenient values. The capacitor voltage transformer
is more commonly used on extra high voltage networks. It is to have one
common primary winding and two or more secondary . show fig (3)

HV Conductor

Bushing
Capcitor unit

Tap
Auxiliarycapacitor

Ground

Fig (3) capacitive voltage transformer

B ) Current transformer : ( CT)

All current transformer used in protection are basically similar in

construction to standard transformers in that they consist of magnetically
coupled primary and secondary windings , wound in common iron core ,

3
the primary winding being connected in series with the network. Show fig
(4)

Bar type

secondary

primary

primary secondary

Wound type

Fig (4) current transformer

1-3 Relay : [1] , [2]:

As define is an electrical device designed to initiate isolation of a part of

an electric system or to operate alarm signal in the case of a fault or other
abnormal condition .

Protective relays can be divided in two type :

A ) Electromechanical Relays : in this type there are one or more coils

with a movable elements , and the operation of such relay is based upon
the effects of electric current like

4
( Electromagnetic attraction ,Electromagnetic induction ….. etc)

See fig (5) ….

Electromagnetic attraction

Electromagnetic induction

Fig (5) Electromagnetic attraction and Electromagnetic induction Relays

B ) Static Relays : in this type, the relays have no any moving part , see
fig (6) , examples about static ( digital) relays and show that can
connected it to computer .

5
 a ) Static ( digital ) relays

b ) connect the digital relay to the computer

fig (6) static ( digital) relays and show that can connected it to computer

2- Protection of Power Transformer :

we can classification the protection of power transformer in two type the

first type is the protection of power transformer from the faults occurring

6
by electrical network and the second type of protection the power
transformer is using protection's devices located in power transformer's
body .

2-2- protection of power transformer from the faults occurring by

electrical network

2-2-1- The over-current protection : [2] , [3]:

Over-current protection has a wide range of applications. It can be applied

where there is an abrupt difference between fault current within the
protected section and that outside the protected section and these
magnitudes are almost constant. Transformers are provided with over-
current protection against faults, only, when the cost of differential
relaying cannot be justified. However, over-current relays are provided in
addition to differential relays to take care of through faults. Temperature
indicators and alarms are always provided for large transformers.

Small transformers below 500 kVA installed in distribution system are

generally protected by drop-out fuses, as the cost of relays plus circuit-
breakers is not generally justified Line Protection. see fig (7) obtain the
different between electromechanical and digital over current (o/c) relay

a .) three phase o/c protection

7
b .) Electrometrical relay
 c.) Digital relay
Fig (7)a. three phase o/c protection , b. electromechanical and c. digital over
current (o/c) relay
The operating time of a relay for a particular setting and magnitude
actuating quantity can be known from the characteristics supplied by the
manufacturer. The typical characteristics are shown in (Fig. 8) . An
inverse curve is one in which the operating time; becomes less as the
magnitude of the actuating quantity is increased. However for higher
magnitudes of actuating quantity the time is constant. Definite time curve
is one in which operating time is little affected by magnitude of actuating
current. However even definite time relay has a characteristic which is
slightly inverse The characteristic with definite minimum time and of
inverse type is also called Inverse Definite Minimum Time (IDMT)
characteristics

8
Fig (8) Inverse Definite Minimum Time (IDMT) characteristics
2-2-2- Earth-Fault Protection : [1] , [2] , [3]:

When the fault current flows through earth return path, the fault is called
Earth Fault. Other faults which do not involve earth are called phase
faults. Since earth faults are relatively frequent, earth fault protection is
necessary in most cases.

When separate earth fault protection is not economical, the phase relays
sense the earth fault currents. However such protection lacks sensitivity.
Hence separate earth fault protection is generally provided. Earth fault
protection senses earth fault current. Following are the method of earth
fault protection.

1. ) Residually connected Earth-fault Relay Referring to Fig (9) In absence

of earth-fault the vector sum of three line currents

is zero. Hence the vector sum of three secondary currents is also zero.

IR+IY+IB=0

The sum (IR+IY+IB ) is called residual current The earth-fault relay is

connected such that the residual current flows through it ,in the absence
of earth-fault, therefore, the residually connected earth-fault relay does
not operate. However, in presence of earth fault the conditions is
disturbed and (IR+IY+IB) is no more zero. Hence flows through the earth-
fault relay. If the residual current is above the pick-up value, the earth-
fault relay operates. In the scheme discussed here the earth-fault at any
location near or away from the location of CT's can cause the residual
current flow. Hence the protected
zone is not definite. Such protection is called unrestricted earth-fault
protection

9
 Fig (9) Earth fault protection combined with phase fault protection

2.) Earth-fault Relay connected in Neutral to Earth Circuit

Fig. 10). Another method of connecting an earth-fault relay is illustrated

in Fig 5. The relay is connected to secondary of a CT whose primary is
connected in neutral to earth connection. Such protection can be provided
at various voltage levels by connecting earth-fault relay in the neutral-to-
earth connection of that voltage level. The fault current finds the return
10
path through the earth and then flows through the neutral to- earth
connected. The magnitude of earth fault current is dependent on type of
earthing (resistance, reactance or solid) and location of fault. In this type
of protection, The zone of protection cannot be accurately defined. The
protected area is not restricted to the transformer winding alone. The
relay senses the earth faults beyond the transformer winding hence such
protection is called ( unrestricted earth-fault protection) . The earth-fault
protection by relay in neutral to earth circuit depends upon the type of
neutral Earthing. In case of large generators, voltage transformer is
connected between neutral and earth .
Fig (10) Earth-fault protection by earth-fault-relay connected in neutral-to-earth circuit.

Fig. (11) Electromechanical and digital relay

2-2-4- Transformer Differential Protection : [3] , [6]:

Differential protection ( or Unit protection ) operation on the circulating

current system ,and it's used in power transformer protection .

A differential relay is the relay that operates when the phasor difference
of two or more similar electrical quantities exceeds a predetermined
amount .

11
 So that any type of relay connected in a certain way can be made to
operate as " Differential Relay " as fig.(14) IF
CT1 CT2
CT1 CT2

Transformer Transformer

I2
I2 I1
I1

R I1-I2 R I1+I2

Fig. (14) Transformer Differential Protection

And used Percentage differential protection because it's overcomes the

problems related with the identification of internal faults while keeping
the advantages of the basic differential In general, the operating current
in the differential relay is equal to:

 
I OP  I1  I 2 .............(3)

where,

I1, I2 are the currents on the pilot wires of the current transformers

Due to the complexities associated with transformer differential

protection, differential relays use a percentage restraint characteristic
that compares an operating current with a restraining current.
Percentage restraint increases the operate current needed to actuate the
relay based on the current flowing through the protected transformer. The
restraint setting, or slope, defines the relationship between restraint and
operate currents as shown in Fig.(15) The operating current, also called
the differential current, IOP, can be obtained from the phasor sum of the

12
 currents entering the protected element as shown in Equation 3

IOP is proportional to the fault for internal faults and approaches zero for
any operating conditions. The differential relay generates a tripping
signal if the operating current, IOP, is greater than a percentage of the
restraining current, IRT.
Iop

SLP2

Operating Region

SLP1

Restraining Region
Ipu

Irt

Fig. (15) Differential relay with dual slope characteristics

Where SLPi is the slope of the ith characteristic of the differential relay,
IP.U. the relay minimum pickup current.

Example 1: Consider a delta/star connected, 15 MVA, 33/11 kV

transformer with differential protection applied, for the current
transformer ratios shown in fig.(16). Calculate the relay currents on full
load. Find the minimum relay current setting to allow 125 percent
overload. [7]:

Solution:

The HV line current is given by

15𝑥106
Ip = = 262.43 A
√3×33×103

13
The LV line current is

15x106
Is = = 787.30 A
√3×11×103

The CT current on the HV side is thus

5
ip = 262.43× = 4.37 A
300

The CT current in the LV side is

5
is = 787.30× ×√3 = 3.41
2000

(Note that we multiply by √3 to obtain the values on the line side of the
delta connected CT’s. The relay current at normal load is therefore)

ir = ip – is = 4.37 – 3.41 = 0.9648 A

with 1.25 overload ratio, the relay setting should be

Ir = 1.25× (0.948) = 1.206 A

Plug Setting = 1.206/5 = 24.1 %

14
Fig . (16)
2-3-protection the power transformer

that using protection's devices located in power transformer's body .

2-2-1- BUCHHOLZ RELAY : [8]:

The gas and oil actuated (Buchholz) relay is designed to detect faults as
well to minimize the propagation of any damage which might occur within
oil-filled transformers supplied with oil conservator.

The relay is therefore particularly effective in case of:

•Short-circuited core laminations •Broken-down core bolt insulation

•Bad contacts •Overheating of some part of the windings

•Short-circuits between phases •Earth faults

•Puncture of bushing insulators inside tank.

SERIOUS FAULTS : when a serious fault occurs in the transformer, the

gas generation is violent and causes the oil to rush through the connecting
pipe to the conservator. In the relay this oil surge impinges on the flap
fitted on the lower float (located in front of the hole for the oil passage)
and causes the rotation of the float itself, thus operating the tripping
switch and disconnecting the transformer. The float remains in the trip
position even if the oil flow comes to a stop . see fig. (17)

Fig. (17) BUCHHOLZ RELAY

15
2-3-2-Transformer Thermal protection : [4] , [9]:

There are several aspects of heating within transformers.

by current flow in the primary and secondary windings as well as internal

connection due to ( I2 R ) , and the winding hot spot temperature, that is,
the hottest spot in a winding, is considered to be the most critical
parameter in the determination of load ability. It determines the loss of
insulation life and indicates the potential risk of releasing gas bubbles for
a severe overload condition. The hot spot is usually assumed to be near
the top of the high or low voltage winding, although there are arguments
that it could be lower (say mid-winding) within the cooling ducts,
especially if the transformer oil is not directed-flow-pumped through the
ducts ,a temperature of about ( 105oc ) is considered to be the normal
maximum working value for large power transformers, based on an
assumed maximum ambient temperature ( 40oc) ,by using Liquid
temperature indicator . Winding temperature is sensed by winding
temperature gauge , by using The winding temperature indicator relay is
a conventional oil temperature indicator supplemented with an electrical
heating element. see fig. (18)

Fig. (18) Winding temperature indicator relay

16
2-3-3- Pressure Relief Device: [10]:

Traditional explosion vents can be repaired or replaced with

modern pressure relief devise ,when the gauge pressure in the
tank reaches abnormally To 0.35-0.7 kg/cm2. The pressure relief
device starts automatically to discharge the oil. When the
pressure in the tank has dropped beyond the limit through
discharging, the device is automatically reset to prevent more
oil than required from being discharged. see fig (19).

explosion vents

pressure relief devise

Fig. (19) explosion vents pressure relief devise

17
2-3-4-The dehydrating breather:

The dehydrating breather is provided at the entrance of the

conservator of oil of power transformer The conservator governs
the breathing action of the oil system on forming to the
temperature change of the equipment, and the dehydrating
breather removes the moisture and dust in the air inhaled and
prevents the deterioration of the Transformer oil due to
moisture absorption ,See Fig. (20) The dehydrating breather
uses silica - gel as the desiccating agent and is provided with an
oil pot at the bottom to filtrate the inhaled air .

Fig. (20) Silica - gel breather

18
Finally see fig.(21) the different between transformer with good
protection and other one without protection or with bad protection

power transformer with good protection

power transformer without or with bad protection

Fig. (21)

19
 References :
1 . M.L.Soni and P.V.Gupta ," A course in Electrical power", 1983

2 . J. Lewis Blackburn and Thomas J. Domin , "Protective Relaying

Principles and Applications" , 2007

3 . "Draft IEEE Guide for protecting power transformers" , 2006

4 . Gary Hoffman and Thomas C. Tennille ," THERMAL PROTECTION

FOR POWER TRANSFORMERS "

5 . "Power system protection" , Edited by the electricity Training

Association in London , 1995

6 . Zoran Gajic' , "Differential protection for Arbitrary three-phase

power transformer" , 2008

7 . "http:// dc189.4shared.com/doc/70yui95j/preview/doc"

8 . "Google search – Bochhloz Relay filetype:doc"

9 . IEEE , Fundamental principles of transformer thermal loading and

protection , 2010

10 . www.power-eng.com

11 . P. M. Anderson , "Power system protection" , 1999.

20
View publication stats