Dr.

Javed Ahmed Laghari

PhD, University of Malaya, Malaysia
Associate Professor
javed@quest.edu.pk
Department of Electrical Engineering
QUEST, Nawabshah, Sindh, Pakistan

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Transformers are static devices, totally enclosed and generally oil immersed.
Therefore, chances of faults occurring on them are very rare.
However, the consequences of even a rare fault may be very serious unless the
transformer is quickly disconnected from the system.
This necessitates to provide adequate automatic protection for transformers against
possible faults.
Small distribution transformers are usually connected to the supply system through
series fuses instead of circuit breakers. Consequently, no automatic protective relay
equipment is required.
However, the probability of faults on power transformers is undoubtedly more and
hence automatic protection is absolutely necessary.
Moat severe faults in transformer are the internal faults. When an internal fault
occurs, the transformer must be disconnected quickly from the system because a
prolonged arc in the transformer may cause oil fire.

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The principal relays and systems used for transformer protection are :
(i) Buchholz Relay devices provides protection against all kinds of incipient faults
i.e. slow-developing faults such as insulation failure of windings, core heating, fall of
oil level due to leaky joints etc.
(ii) Earth-fault relays provides protection against earth-faults only.
(iii) Overcurrent relays provides protection mainly against phase-to-phase faults and
overloading.
(iv) Differential system (or circulating-current system) provides protection against
both earth and phase faults.
The complete protection of transformer usually requires the combination of these
systems. Choice of a particular combination of systems may depend upon several
factors such as:
(a) Size of the transformer
(b) Type of cooling
(c) Location of transformer in the network
(d) Nature of load supplied and
(e) Importance of service for which transformer is required.

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Buchholz relay is a gas-actuated relay installed in
oil immersed transformers for protection against
internal faults.
Named after its inventor, Buchholz, it is used to
give an alarm in case of incipient (i.e. slow-
developing) faults in the transformer and to
disconnect the transformer from the supply in the
event of severe internal faults.
It is usually installed in the pipe connecting the
conservator to the main tank as shown in Figure.
It is a universal practice to use Buchholz relays on all such oil immersed transformers
having ratings in excess of 750 kVA. Its use for oil immersed transformers of rating less
than 750 kVA is generally uneconomical.

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Consider the Figure which
shows the constructional
details of a Buchholz relay.

It is placed in the

connecting pipe between the
main tank and the
conservator.
The device has two
elements. The upper
element consists of a
mercury type switch
attached to a float.
The lower element contains a mercury switch mounted on a hinged type flap located
in the direct path of the flow of oil from the transformer to the conservator.
The upper element closes an alarm circuit during incipient faults whereas the lower
element is arranged to trip the circuit breaker in case of severe internal faults. 5
The operation of Buchholz relay is as follows :
(i) In case of incipient faults within the transformer, the heat due to fault causes the
decomposition of some transformer oil in the main tank.
The products of decomposition contain more than 70% of hydrogen gas. The
hydrogen gas being light tries to go into the conservator and in the process gets
entrapped in the upper part of relay chamber.
When a predetermined amount of gas gets accumulated, it exerts sufficient pressure
on the float to cause it to tilt and close the contacts of mercury switch attached to it.
This completes the alarm circuit to sound an alarm.
(ii) If a serious fault occurs in the transformer, an enormous amount of gas is
generated in the main tank.
The oil in the main tank rushes towards the conservator via the Buchholz relay and in
doing so tilts the flap to close the contacts of mercury switch. This completes the trip
circuit to open the circuit breaker controlling the transformer.

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It is the simplest form of transformer protection.
It detects the incipient faults at a stage much earlier than is possible with other forms
of protection.

The relay is used only in oil immersed transformer with conservator tanks.
It can only detect the fault below oil level.
This relay does not protect the connecting cables. Hence separate protection is used
for the cables.

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Consider the Figure which shows the combined earth fault and over load protection
of transformer consisting of two overload relays and one earth fault relay.
The earth relay has low current setting and operates under earth or leakage faults
only. The overload relays have high current setting and are arranged to operate against
faults between the phases.
The two overload relays
are sufficient to protect
against phase-to-phase
faults.
The trip contacts of
overload relays and earth
fault relay are connected in
such a way that with the
energising of either
overload relay or earth
relay, the circuit breaker
will be tripped.
This is also known as restricted
earth fault protection of transformer.
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Merz-Price circulating -current principle is commonly used for the protection of
power transformers against earth and phase faults.
The system as applied to transformers is fundamentally the same as that for
generators. However, it needs some modification
In a power transformer, currents in the primary and secondary are to be compared. As
these two currents are usually different, therefore, the use of identical transformers (of
same turn ratio) will give differential current and operate the relay even under no load
conditions.
The difference in the magnitude of currents in the primary and secondary of power
transformer is compensated by different turn ratios of CTs.
If T is the turn-ratio of power transformer, then turn ratio of CTs on the LV side is
made T times that of the CTs on the HV side.
Fulfilled this condition, the secondaries of the two CTs will carry identical currents
under normal load conditions. Consequently, no differential current will flow through
the relay and it remains inoperative.

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Consider the Figure which shows Merz-Price circulating-current scheme for the
protection of a 3- phase power transformer against phase-to ground and phase-to-phase
faults.
The CTs on the two sides are connected by pilot wires and one relay is used for each
pair of CTs.
During normal
operating
conditions, the
secondaries of CTs
carry identical
currents.
Therefore, the
currents entering
and leaving the
pilot wires at both
ends are the same
and no current
flows through the
relays.
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If a ground or phase-to-phase fault occurs, the currents in the secondaries of CTs will
no longer be the same and the differential current flowing through the relay circuit will
clear the breaker on both sides of the transformer.
The-protected zone is limited to the region between CTs on the high-voltage side and
the CTs on the low-voltage side of the power transformer.

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.

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