Transformer Protection.

1. Presented by
Dr.Selvarasu Ranganathan
Assistant Professor
School of Electrical Engineering and Computing
Adama Science and Technology University, Adama. .
TRANSFORMER PROTECTION

2. Transformers
• Transformers are the major power system components which are used at
various voltage levels with the capacity varying from 1kVA to >600 MVA.
• Transformers are widely used devices to transfer power from one circuit to
another circuit magnetically with out physical coupling.
• Any damage to this component results in complete loss of power to one
system.
• The transformer should be protected from the faults to avoid break down of
the transformer which may lead to major interruption in power
transformation.
• To protect it from the fault the typical faults in the fault should be known.

3. Possible Transformer Faults
Transformer
Faults
Internal Faults
(Winding faults)
Phase to Phase
fault
Phase to earth
fault
Inter-turn fault
External Faults
(Through faults)

4. Transformer Internal Faults
•Breakdown of the insulating oil.
•Insulation failure of winding.
•Over heat generated in the coils
due to failure of insulation etc.

5. Transformer External Faults
High Voltage Disturbance in Power Transformer are of two kinds
1.Transient Surge Voltage
•Arcing ground if neutral point is isolated.
•Switching operation of different electrical equipment.
•Atmospheric Lightening Impulse.
2. Power Frequency Over Voltage
As we know that, voltage V = 4.44Φ.f.N ⇒ V ∝ Φ, increased voltage causes
proportionate increase in the working flux. This therefore causes, increased in iron loss
and dis-proportionately large increase in magnetizing current. The increase flux is
diverted from the transformer core to other steel structural parts of the transformer.
Core bolts which normally carry little flux, may be subjected to a large component
of flux diverted from saturated region of the core alongside. Under such condition, the
bolt may be rapidly heated up and destroys their own insulation as well as winding
insulation.

6. Transformer External Faults Contd…
Protection against lighting surges lighting arrester can be provided.

7. Transformer Types
There are different kinds of transformers such as
•Two winding or three winding power transformer,
•Auto transformer,
•regulating transformers,
•Earthing transformer,
•Rectifier transformers etc.
Different transformers demand different schemes of transformer
protection depending upon their importance, winding connections, earthing
methods and mode of operation etc.
•It is common practice to provide Buchholz relay protection to all 500 kVA and
above transformers.
•While for all small size distribution transformers, only high voltage fuses are
used as main protective device.
• For all larger rated and important distribution transformers, over current
protection along with restricted earth fault protectionis applied.
•Differential protection should be provided in the transformers rated above 5
MVA

8. Typical Protections in Transformer
Typically transformer protections are classified as two types.
1) Main protections
2) backup protections.
Main protections:
These are immediate fault clearance protections which
acts instantaneously to clear the faults in the transformer.
Differential protection
Over current protection
Back up protection:
These protections will acts as a back up to the main
protections i.e. if the main protection fails corresponding back up
protection will act and safe guard the device from fault.
Over voltage
Volts/hertz
Negative phase sequence relay

9. Differential Protection
•Generally Differential protection is provided in the electrical
power transformer rated more than 5MVA. The Differential Protection of
Transformer has many advantages over other schemes of protection.
•The faults occur in the transformer inside the insulating oil can be detected
by Buchholz relay. But if any fault occurs in the transformer but not in oil then it
can not be detected by Buchholz relay.
•Any flash over at the bushings are not adequately covered by Buchholz relay.
•Differential relays can detect such type of faults. Moreover Buchholz relay is
provided in transformer for detecting any internal fault in the transformer but
Differential Protection scheme detects the same in more faster way.
•The differential relays normally response to those faults which occur inside the
differential protection zone of transformer.

10. Differential Protection Contd…
Basic Considerations for Transformer Differential
Protection:
• Line Current Transformer Primary Ratings
•Current Transformer Connections
•The neutrals of C.T star and power transformer stars
are grounded.
•Selection of C.T and Power transformer tapings.

11. Differential Protection Contd…
Line Current Transformer Primary Ratings
Consider a two winding, 11kV/132kV, 30MVA Power Transformer.
The rated line current on the 11kV side of the Transformer is:
Irated = MVA / Line Voltage
= 30000000 / (11000)(sqrt 3)
= 1574.59A
Thus, choose a CT with a primary rating of 1600/1.
The line current on the 132kV side of the Transformer is:
Irated = 30000000 / (132000)(sqrt 3)
= 131.22A
Thus choose a CT with a primary rating of 200/1.
Notice that in both cases, the current that the Bias Coils in the relay is
1A at both ends. Any deviation of this 1A will causes an imbalance within the
relay, which will consequently cause protection to operate and isolate the
Transformer.

12. Differential Protection Contd…
Current Transformer Connections

13. Merz-Price Protection for Star-Delta
Transformer

14. Merz-Price Protection for Star-Delta
Transformer Contd…

15. Merz-Price Protection for Star-Star
Transformer

16. BUCHHOLZ RELAY
•Buchholz relay is a type of oil and gas actuated protection relay
universally used on all oil immersed transformers having rating more
than 500 kVA.
• Buchholz relay is not provided in transformer having rating below
500 kVA from the point of view of economic considerations.

17. Why Buchholz relay is used in transformers?
Buchholz relay is used for the protection of transformers from the faults occurring inside the
transformer. Short circuit faults such as inter turn faults, incipient winding faults, and core faults may
occur due to the impulse breakdown of the insulating oil or simply the transformer oil. Buchholz relay
will sense such faults and closes the alarm circuit.
Working principle
Buchholz relay relies on the fact that an electrical fault inside the transformer tank is accompanied by
the generation of gas and if the fault is high enough it will be accompanied by a surge of oil from the
tank to the conservator
Whenever a fault occurs inside the transformer, the oil in the transformer tank gets overheated and
gases are generated. The generation of the gases depends mainly on the intensity of fault produced.
The heat generated during the fault will be high enough to decompose the transformer oil and the
gases produced can be used to detect the winding faults. This is the basic principle behind the working
of the Buchholz relay.
Construction
Buchholz relay can be used in the transformers having the conservators only. It is placed in the pipe
connecting the conservator and the transformer tank. It consists of an oil filled chamber. Two hinged
floats, one at the top of the chamber and the other at the bottom of the chamber which accompanies
a mercury switch each is present in the oil filled chamber. The mercury switch on the upper float is
connected to an external alarm circuit and the mercury switch on the lower is connected to an
external trip circuit.
Buchholz Relay Contd..

18. Buchholz Relay Contd..

19. Buchholz relay is employed for the following types faults,
• Core bolt insulation failure
• Local-overheating
• Entrance of air into the oil
• Less of oil due to leakage
• Short circuited lamination due to burns.
Buchholz Relay Contd..

20. Advantages of Buchholz relay
• The advantage of using a Buchholtz relay is that they indicate incipient faults.
For example, It indicates the faults between turns or heating of core which
enables us to take the transformer out of service before severe damage
occurs.
• Buchholtz relay gives an alarm when the oil level reduces below a certain
level due to leakage of oil from transformer.
• Buchholtz relay gives an audible warning which informs the operator that
there is some fault in the transformer.
• Type of insulation failure of the transformer can also be detected by testing
the gas.
Buchholz Relay Contd..

21. Limitation of Buchholz relay
1. This type of relay can be used only for transformers with conservators.
2. The relay does not operate when the faults are above the oil level.
3. Buchholz relay does not protect the cables associated with the transformers
below. A separate protection has to be employed for the cables.
4. Due to economic considerations, this relay is not employed for transformers
above 500 kVA.
5. Setting of the mercury switch cannot be too sensitive, otherwise there can be a
false operation due to vibrations,earthquakes. mechanical shocks to the pipe.
6. The operation of this new is unsatisfactory due to slow operating time. (Nearly
0.2 to 0.1 seconds).
Buchholz Relay Contd..