Protection of Alternators and Transformers

Ques : Mention and briefly discuss some faults of an alternator.

Ans :

Faults of an alternator :

Some of the important faults which may occur on an alternator are :

(i) failure of prime-mover (ii) failure of field

(iii) overcurrent (iv) overspeed
(v) overvoltage (vi) unbalanced loading
(vii) stator winding faults

(i) Failure of prime-mover. When input to the prime-mover fails, the alternator runs as a
synchronous motor and draws some current from the supply system. This motoring
conditions is known as “inverted running”.

(ii) Failure of field. The chances of field failure of alternators are undoubtedly very rare.
Even if it does occur, no immediate damage will be caused by permitting the alternator to run
without a field for a short-period.

(iii) Overcurrent. It occurs mainly due to partial breakdown of winding insulation or due to
overload on the supply system.

(iv) Overspeed. The chief cause of overspeed is the sudden loss of all or the major part of
load on the alternator.

(v) Over-voltage. overvoltage in an alternator occurs when speed of the prime-mover

increases due to sudden loss of the alternator load.

(vi) Unbalanced loading. Unbalanced loading means that there are different phase currents
in the alternator.

(vii) Stator winding faults. These faults occur mainly due to the insulation failure of the
stator windings. The main types of stator winding faults, in order of importance are :

(a) fault between phase and ground

(b) fault between phases
(c) inter-turn fault involving turns of the same phase winding.

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Ques : Describe Merz-Price calculating current scheme for differential protection of
alternators.

Ans :

Merz-Price calculating current scheme :

Fig : Current differential protection for a 3-phase transformer

Operation. Under normal operating conditions, the current at both ends of each winding will
be equal and hence the currents in the secondaries of two CTs connected in any phase will
also be equal. Therefore, there is balanced circulating current in the pilot wires and no current
flows through the operating coils (R1, R2 and R3) of the relays. When an earth-fault or phase-
to-phase fault occurs, this condition no longer holds good and the differential current flowing
through the relay circuit operates the relay to trip the circuit breaker.

(i) Suppose an earth fault occurs on phase R due to breakdown of its insulation to earth as
shown in Fig. The currents in the secondaries of the two CTs in phase R will become unequal
and the differential current will flow through the operating relay coil (i.e. R1). Consequently,
the relay operates to trip the circuit breaker.

(ii) If a short-circuit fault occurs between the phases Y and B as shown in Fig. The currents in
the secondaries of two CTs in each affected phase will become unequal and the differential
current will flow through the operating coils of the relays (i.e. R2 and R3) connected in these
phases. The relay then closes its contacts to trip the circuit breaker.

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Ques : Briefly discuss the operation of modified differential protection for alternators.

Ans :

Modified differential protection for alternators :

Fig : Modified differential protection for alternators.

Operation. Under normal operating conditions, currents at the two ends of each stator
winding will be equal. Therefore, there is a balanced circulating current in the phase pilot
wires and no current flows through the operating coils of the relays. Consequently, the relays
remain inoperative.

(i) If an earth-fault occurs on any one phase, the out-of-balance secondary current in CTs in
that phase will flow through the earth relay ER and via pilot S1 or S2 to the neutral of the
current transformers. This will cause the operation of earth relay only.

(ii) If a fault occurs between two phases, the out of-balance current will circulate round the
two transformer secondaries via any two of the coils PA, BR, PC without passing through the
earth relay ER. Therefore, only the phase-fault relays will operate.

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Ques : What is earth fault? Describe earth fault/balanced earth fault protection for
alternator.(2nd + 3rd )

Ans :

Earth fault : When the fault current flows through earth return path, the fault is called earth
fault.

Earth fault/balanced earth fault protection :

Fig : Balanced earth-fault protection for a 3-phase alternator

Schematic arrangement. It consists of three line current transformers, one mounted in each
phase, having their secondaries connected in parallel with that of a single current transformer
. A relay is connected across the transformers secondaries. The protection against earth faults
is limited to the region between the neutral and the line current transformers.
Operation. Under normal operating conditions, no current flows the relay.

(i) If an earth-fault develops at F2 external to the protected zone, the sum of the currents at
the terminals of the alternator is exactly equal to the current in the neutral connection and
hence no current flows through the relay.

(ii) When an earth-fault occurs at F1 or within the protected zone, these currents are no
longer equal and the differential current flows through the operating coil of the relay. The
relay then closes its contacts to disconnect the alternator from the system.

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Ques : What are the common transformer faults?

Ans :

Common transformer faults: Power transformers may suffer only from :

(i) Open circuits

(ii) Overheating
(iii) Winding short-circuits e.g. earth-faults, phase-to-phase faults and inter-turn faults.

(i) Open circuits: An open circuit in one phase of a 3-phase transformer may cause
undesirable heating.
(ii) Overheating : Overheating of the transformer is usually caused by sustained overloads or
short-circuits and very occasionally by the failure of the cooling system.
(iii) Winding short-circuits: Winding short-circuits (also called internal faults) on the
transformer arise from deterioration of winding insulation due to overheating or mechanical
injury.

Ques : Write down the protection systems for transformers.

Ans :

Protection systems for transformers: The principal relays and systems used for transformer
protection are :

(i) Buchholz devices providing protection against all kinds of incipient faults.
(ii) Earth-fault relays providing protection against earth-faults only.
(iii) Overcurrent relays providing protection mainly against phase-to-phase faults and
overloading.
(iv) Differential system (or circulating-current system) providing protection against both earth
and phase faults.

Ques : Describe construction and operation of Buchholz relay for protection of

transformer.(1st + 3rd )

Ans :

Buchholz relay: Buchholz relay is a gas-actuated relay installed in oil immersed

transformers for protection against all kinds of faults.

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 Fig : Buchholz Relay

Construction. 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.

Operation. 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.

Ques : Mention the advantages and disadvantages of Buchholz relay.

Ans :

Advantages
(i) It is the simplest form of transformer protection.

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(ii) It detects the incipient faults at a stage much earlier than is possible with other forms of
protection.

Disadvantages
(i) It can only be used with oil immersed transformers equipped with conservator tanks.
(ii) The device can detect only faults below oil level in the transformer. Therefore, separate
protection is needed for connecting cables.

Ques : What are the disadvantages of Merz-Price scheme?(3rd )

Ans :

Disadvantages of Merz-Price scheme :

1. It is more expensive than the thermal type.

2. It involves a mechanism of greater complexity.

Ques : Briefly discuss earth fault or leakage protection for transformer.

Ans :

Earth fault or leakage protection :

Fig : Earth fault or leakage protection for transformer

An earth-fault usually involves a partial breakdown of winding insulation to earth. The

resulting leakage current is considerably less than the short-circuit current. The earth-fault
may continue for along time and cause considerable damage before it ultimately develops
into a short-circuit and removed from the system. Under these circumstances, it is profitable
to employ earth-fault relays in order to ensure the disconnection of earth-fault or leak in the
early stage. An earth-fault relay is essentially an overcurrent relay of low setting and operates
as soon as an earth-fault or leak develops. One method of protection against earth-faults in a
transformer is the *core-balance leakage protection.

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Ques : What factors cause difficulty in applying circulating current principle to a power
transformer?

Ans : The factors that cause difficulty in applying circulating current principle to a power
transformer is given below:

(i) 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.

(ii) There is usually a phase difference between the primary and secondary currents of a 3-
phase power transformer. Even if CTs of the proper turn-ratio are used, a differential current
may flow through the relay under normal conditions and cause relay operation.

(iii) Most transformers have means for tap changing which makes this problem even more
difficult. Tap changing will cause differential current to flow through the relay even under
normal operating conditions.

Ques : Discuss the operating principle of Merz-Price circulating current scheme for the
protection of transformers.(2nd + 3rd )

Ans :

Merz-Price circulating current scheme for the protection of transformers :

Fig : Circulating current scheme for 3-phase Δ-Δ transformer

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Merz-Price circulating-current scheme for the protection of a 3- phase delta/delta power
transformer against phase-to ground and phase-to-phase faults.

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. 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.

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