Power Transformer Protection

Power
Transformer
Protection
Ang Sovann 1
National Polytechnic Institute of Cambodia

2
Introduction
 Transformer is a static device used to transform the
electrical energy from one alternating current circuit to
another without any change in frequency.
 Power Transformer is a vital link in a power transmission
system and distribution.

3
Introduction
 A Power Transformer is an expensive part of a power
network.
 The impact of a transformer outage due to fault is more
serious than a transmission line outage.

4
Transformer Protection
 The type of protection for the transformers varies depending
on the application and the importance of the transformer.
 Transformers are protected primarily against faults and
overloads.
 The type of protection used should minimize the time of
disconnection for faults within the transformer and to reduce
the risk of catastrophic failure.

5
Why use transformer ???
 To reduce of transmission losses.
 For increasing the low voltage level to high voltage level.
 The voltage level of a power is increased, the current of the
power is reduced which causes reduction in ohm or I2R
losses in the system.
 Low level power must be stepped up for efficient electrical
power transmission.

6
Why transformer protection ???
 To Protect the Transformer from severe damages.
 Time required to rectify the Transformer in case of
damages is more and it is very difficult.
 Operation of a power network when the power
transformer is out of service is always difficult.

7
Why transformer protection ???
 A Power Transformer fault therefore often is a more
severe disturbance for the network than an overhead
line fault which usually can be repaired rather quickly.

8
Induction Law
 The transformer is based on two principles:
1. An electric current can produce a magnetic field.
2. A changing magnetic field within a coil of wire
induces a voltage across the ends of the coil

9
Power Transformer
 Term power transformer is used to refer to those transformers used in the
generator and the distribution circuits, and these are usually rated at 500 KVA
and above.
 Power systems typically consist of a large number of generation locations,
distribution points, and interconnections within the system or with nearby
systems, such as a neighboring utility.
 The complexity of the system leads to a variety of transmission and distribution
voltages.
 Power transformers must be used at each of these points where there is a
transition between voltage levels.

10
Power Transformer
 Term power transformer is used to refer to those
transformers used in the generator and the distribution
circuits, and these are usually rated at 500 KVA and
above.
 Power systems typically consist of a large number of
generation locations, distribution points, and
interconnections within the system or with nearby
systems, such as a neighboring utility.

12
What cause transformer failure?
 Insulation Breakdown
 Over heating due to over
excitation
 Oil contamination and leakage
 Cooling system failure
 Phase to phase and Phase
to Ground Fault
 Turn to Turn Fault
 Transformer Overload

13
 Winding failures 51%
 Tap changer failures 19%
 Bushings failures 9%
 Terminal board failures 6%
 Core failures 2%
 Miscellaneous failures 13%

14
 Insulation deterioration come
from:
 Moisture
 Vibration
 Overheating
 Voltage surge
 Winding  Bushing
 Bushings deterioration come
from:
 General aging
 Contamination
 Cracking
 Internal moisture

15
 Core insulation failure
 Shorted lamination
 Core overheating
 Core problem  Miscellaneous
 CT problem
 Oil leakage
 Oil contamination
• Metal particle
• Moisture

16
Transformer Faults
Internal faults
1. Winding Failure
2. Winding inter-turn faults
3. Core insulation failure,
shorted laminations
4. Over fluxing.
External faults
1. Overloads
2. Overvoltage
3. Over heating
4. External system short
circuits.

17
Transformer Protection Scheme
Mechanical
1. Buchholz relay
2. Sudden pressure
3. Pressure relief
4. Temperature protection
Electrical
1. Bias Differential Protection (87)
2. Over Fluxing (24)
3. Over Voltage (59)
4. Under Voltage (27)
5. Neutral Unbalance (64R)
6. Restricted Earth Fault (64REF)
7. Back up O/C & E/F (50/51/67)

18
Buchhloz Relay Protection
 The Buchholz relay protects the transformer from internal
faults. It is the gas actuated relay.

20
 The Buchholz relay is placed between the main tank and
the conservator.
 Such type of relay is used in the transformer having the
rating higher than 500KVA.
 It is not used in small transformer because of economic
consideration.

21
 When the fault occurs inside the transformer, the
temperature of the oil increases. The oil evaporates in the
form of the gas.
 The generation of the gas depends on the magnitude of the
fault occurs inside the transformer.
 The internal failure occurs in the transformer either because
of the insulation breakdown between the winding or the
winding have the weak initial contact.

23
 The relay has two elements 1st stage and 2nd stage. The 1st stage
for alarm and the 2nd stage for trip.
 The 1st stage work on the accumulation of sufficient gas in the relay
chamber and the 2nd stage work on the high velocity of oil caused
due to evolution of gases as a result of high energy faults.
 The setting of 1st stage is done at collected gas volume of 300 to
500ml.
 The setting of 2nd stage is done at oil flow velocity of 100cm/s .

25
Sudden Pressure Relay Protection
 This relay is designed to detect a sudden pressure increase
caused by arcing or internal faults.

26
Sudden Pressure Relay Protection
 This protection performs better and faster than Buchholz relay 2nd
stage protection. The relay detects a sudden rise in pressure. The
alarm is set at 0.32kg/sqcm and trip at 0.6kg/sqcm per second.

27
Pressure Relief Protection
 This is used to evacuate any over pressure inside the
transformer to avoid explosion of the transformer tank.
 The pressure relief device limits the tank overpressure and
reduces the risk of tank rupture and uncontrolled oil spill,
which might also cause a fire.

29
Fire Protection
 It can occur because of deterioration of insulation in the
transformer.
 This produces arcing which in turn overheats the insulating
oil and causes the tanks to rupture; further arcing then will
start a fire.
 Fires are also initiated by lightning and occasionally by dirty
insulators on the outside of the tanks.

31
Lightning Protection
 Lightning overvoltage surges originate from atmospheric
discharges and they can reach their peak within a few
microseconds and subsequently decay very rapidly.
 The charge from the surge produces both short duration
high current impulse and long duration continuing current
impulse which affects the transformer insulation system.

33
Celica Gel Breather
 Silica gel breathers is used on the conservator of oil filled
transformers.
 The purpose of these silica gel breathers is to absorb the
moisture in the air sucked in by the transformer during the
breathing process.

35
Celica Gel Breather
 During the breathing process, the incoming air may consist
of moisture and dirt which should be removed in order to
prevent any damage.
 Hence the air is made to pass through the silica gel
breather, which will absorb the moisture in the air and
ensures that only dry air enters in to the transformer.

36
Celica Gel Breather
 Silica gel in the breather will be blue when installed and they
turn to pink colour when they absorb moisture which indicates
the crystals should be replaced.
 These breathers also have an oil cup fitted with, so that the
dust particles get settled in the cup.

38
Oil Level Gauge
 Transformers with oil conservator (expansion tank) often
have an oil level monitor.
 Usually, the monitor has two contacts for alarm.
 One contact is for maximum oil level alarm and the other
contact is for minimum oil level alarm.

39
Oil Level Gauge
 When oil level is low from fixed minimum oil level then
minimum oil level alarm is ringing.
 When oil level is high from fixed maximum oil level then
maximum oil level alarm is ringing.

42
Differential Relay Protection
 This scheme is employed for the protection of transformers
against internal short circuits. It provides the best overall
protection for internal faults.
 It compares the current entering the transformer with the
current leaving the element.
 If they are equal there is no fault inside the zone of protection
 If they are not equal it means that a fault occurs between the
two ends.

43
 It can detect the any faults occurred in the zone of protection
of transformer (CT zone).

44

45

46
 Restricted Earth Fault (REF) means an earth fault from a
restricted/localized zone of a circuit.
 The term "REF protection method " means not to sense any
earth faults outside this restricted zone. This type of
protection is prevalent in Dyn group of transformers (Delta
Primary and Star Secondary).
Restricted Earth Faults Relay Protection

47

48
 Differential protection has excellent operation in most fault
cases, but in the situations that a single phase to ground fault
that current increases slightly and causes differential
protection not to detect the fault.
 Restricted earth fault (REF) relay can be used as a
complementary of differential protection.

49
 Differential relay will operate for earth faults inside the zone
only if the earth fault current is more than the bias setting in
the relay. The normal bias setting in a differential relay is
20%.
 So, complete earth fault protection is not possible with
differential relay. That is why you need a restricted earth fault
relay with sensitive settings like 5%.

50

51
Over fluxing Protection
 A transformer is designed to operate at or below a maximum
magnetic flux density in the transformer core.
 The flux in the core increases with either increasing voltage
or decreasing frequency.
 Flux density can be formulated : B = V/F

52
Over fluxing Protection
 During startup or shutdown of generator-connected
transformers, or following a load rejection, the transformer
may experience an excessive ratio of volts to hertz.
 Over fluxing relays are so adjusted, that for every transient
over flux condition the transformer does not trip for
maintaining the power system stability.
 But it should trip as soon as the duration and severity of the
condition crosses the specified safe limit

53
Earth Fault Back up Protection
 The back up earth fault protection (51N) take current signal
from the neutral CT of the Transformer.
 This is the last back up
protection for un-cleared ground faults.

54
Overcurrent Back up Protection
 Over Current is applied against external short circuit and
excessive over loads.
 It operates whenever the current pass CTs exceed the setting
values.
 These over current are mostly Inverse Definite Minimum Time
(IDMT) or Definite Minimum Time (DMT).
 The setting is done at 300% of the rated current and no case
shall be less than 175%.

Power Transformer Protection

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