Generator Protection Techniques

 Introduction
 Generator Faults
 Failure of Prime mover
 Failure of field
 Over-current
 Over-speed
 Over-voltage
 Unbalanced loading
 Differential Protection of Alternators
 Balanced earth-fault Protection
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Eng. Fredy F. Loita, EPE Department 3

 The modern electric power system consists of several
elements like;
Alternators (Rotating machines),
Transformers,
Station bus bars,
Transmission line,
and Other equipment.
 It is desirable and necessary to protect each element
from variety of fault conditions which may occur sooner
or later.
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Eng. Fredy F. Loita EPE Department 4

4/7/2023

 The generating stations consists of rotating machines or
generating units usually alternators. They are
responsible for generating electricity. For Tanzania,
generation voltage levels are 400V and 11kV.
 Generating units especially the larger ones are relatively
few and high in cost than other most equipment.
 They are accompanied by prime mover, excitation
system, voltage regulator, cooling system etc.
 It is therefore necessary to provide protection to cover
the wide range of faults which may occur in the modern
generating plants.
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Some of the important abnormal condition which
may occur on an alternator are;
1. Failure of prime mover
2. Failure of field
3. Over current
4. Over speed
5. Overvoltage
6. Unbalanced loading
7. Stator winding faults
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 When input to the prime mover fails, the alternator runs as a
synchronous motor and draws some current from the supply.
 This motoring condition is known as Inverted running. This
condition may be explained in the following cases;
Case 1: Failure of steam supply in turbo-alternator sets.
 Failure of steam supply in steam generator may cause
inverted running.
 If the steam is gradually restored, the alternator will pick up
the load without disturbing the system.
 If the steam failure is prolonged, the machine can safely be
isolated by the control room attendant.
 So, automatic protection is not required.
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Case 2: Hydro-generator set.
 In case of hydro-generator sets, inverted running is protected by
mechanical devices on the water-wheel.
 If water flow drops to an insufficient rate to maintain the electrical
output, the alternator is disconnected from the system.
 Therefore in this case also electrical protection is not necessary.
Case 3: Diesel Engine driven generator.
 For diesel engine driven alternators, when running inverted, draw a
considerable amount of power from the supply.
 To prevent this, the reverse power relays are employed.
 It is essential that reverse power relays have time delay in operation in
order to prevent in advert tripping during system disturbance caused by
synchronizing and phase swinging.
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 The field failure in alternator is very rare.
 Even if it occurs, no immediate damage will be caused for a short
period.
 It is sufficient to rely on the control room attendant to disconnect
the faulty alternator manually from the system bus-bars
 So there is no automatic protection against this contingency.
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 It occurs mainly due to overload on the supply system. Sometimes
due to partial breakdown of the winding insulation
 Over-current protection is considered unnecessary because of the
following reasons.
 The modern design of alternators are with high values of internal
impedances so that they will stand a complete short-circuit at
their terminal for sufficient time without serious overheating.
 Using overload protection, it might disconnect the alternator from
the plant bus-bar on account of some temporary troubles outside
the plant, therefore interfere with continuity of power
unnecessarily.
 On the occurrence of an overload, the alternators can be
disconnected manually.
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 The cause of over-speed is the sudden loss of all or the major part
of load on the alternator.
 Modern alternators are provided with mechanical centrifugal
devices on their driving shafts to trip the main valve of the prime-
mover when a dangerous over-speed occurs.
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 The field excitation system of modern alternators is so designed
that over-voltage conditions at normal running speeds cannot
occur.
 Over-voltage in alternators occurs when speed of prime mover
increases due to sudden loss of load.
 For steam turbine driven alternators, the control governors are
very sensitive to speed variations.
 They prevent occurrence of over-voltages on the generating units.
 So no need of over-voltage protection on turbo-alternator sets.
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 In case of hydro-generator, the control governors are much
less sensitive
 An appreciable time may elapse before rise in speed due to
loss of load is checked
 The over-voltage during this time may reach a value which
would over-stress the stator windings and insulation
breakdown may occur
 Over-voltage relays in this case are necessary
Disconnect the generator from the system
Disconnect the alternator field circuit
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 Unbalanced loading means different phase currents in the
alternator.
 It arises from faults to earth or faults between phases on the
circuit external to the alternator.
 The unbalanced currents may either severely burn the
mechanical fixings of the rotor core or damage the field winding.
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 Under normal operating conditions, equal currents flow through
the different phases and their algebraic sum is zero.
 Therefore, sum of the current flowing in the secondary is also zero
and no current flows the operating coil of the relay.
 However, if unbalance occurs, the currents induced in the
secondary will be different and the resultant will flow through the
relay and trip the CB to disconnect the alternator from the
system.
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 These faults occurs mainly due to the insulation failure of the
stator windings.
 The main types of stator winding faults are:
 Fault between phase and ground
 Faults between phases
 Inter-turn fault involving turns of the same winding
 Stator winding faults are the most dangerous and are likely to
cause considerable damage to the expensive machines.
 Therefore, automatic protection is absolutely necessary to clear
such faults in the quickest possible time in order to minimize the
extent damage.
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 The protection scheme for any machine is influenced by the size
of the machine and its importance in the system.
 Modern generating set are generally provided with the following
protective schemes.
Stator Protection
a) Percentage differential protection
b) Protection against stator inter-turn faults
c) Stator-overheating protection
Rotor Protection
a) Field ground fault protection
b) Loss of excitation protection
c) Protection against rotor overheating because of unbalanced
three phase stator current
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Other Protection
a) Protection against motoring
b) Protection against vibration
c) Bearing overheating protection
d) Protection against auxiliary failure
e) Protection against voltage regulator failure
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 For protection of alternators against such winding faults,
differential method of protection (also known as Merz-Price
system) is most employed due to its greater sensitivity and
reliability.
 In this scheme, currents at the two ends of the protected section
are compared.
 The currents are equal under normal conditions, but they are
different under fault conditions.
 The difference of the currents under fault conditions is arranged
to pass through the operating coil of the relay.
 The relay then closes its contacts to isolate protected section from
the system.
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1. PERCENTAGE DIFFERENTIAL PROTECTION

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Schematic Arrangement

 Identical current transformer pairs CT1 and CT2 are placed on
either side of each phase of the stator windings.
 The secondary of each set of current transformers are connected
in star ; the two neutral points and the corresponding terminals
of the two star groups being connected together by means of a
four-core pilot cable.
 Thus there is an independent path for the currents circulating in
each pair of current transformers and the corresponding pilot P.
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 Under normal operating conditions, the current at both ends of
each winding will be equal and hence the currents in the
secondary 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.
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OPERATION

 Earth fault occurs on phase R due to breakdown of its insulation
to earth as shown in the above.
 The current in the affected phase winding will flow through the
core and frame of the machine to earth, the circuit being
completed through the neutral earthing resistance.
 The currents in the secondary of the two CTs in phase R will
become unequal and the difference of the two currents will flow
through the corresponding relay coil (i.e. R1), returning via the
neutral pilot.
 Consequently, the relay operates to trip the circuit breaker. the
relay circuit is so arranged that its energizing causes opening of
the breaker connecting the alternator to the bus-bars.
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Suppose

 Short-circuit fault occurs between the phases Y and B as shown
in the short-circuit current circulates via the neutral end
connection through the two windings and through the fault as
shown by the dotted arrows.
 The currents in the secondary 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
opening the field circuit of the alternator
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Suppose

 As the relays are located close to the circuit breaker, therefore it is
not convenient to connect the relay coils to the actual physical mid-
points of the pilots.
 Under these circumstances, balancing resistance are inserted in the
shorter lengths of the pilots so that the relay tripping points divide
the whole secondary impedance of two sets of CTs into equal
portions.
 These resistances are usually adjustable in order to obtain the exact
balance.
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Schematic Diagram

 It is a general practice to use neutral earthing resistance
 This limits the destructive effects of earth-fault current
 It is therefore impossible to protect whole of the stator
windings of the star connected alternator during earth-
fault
 When an earth-fault occurs near the neutral point, there
may be insufficient voltage across the short-circuited
portion to drive the necessary current round the fault
circuit to operate the relay
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 The magnitude of unprotected zone depends upon the value of
earthing resistance and relay setting
 It is usual practice to protect only 85% of the winding because
the chances of an earth fault occurring near the neutral point
are very rare due to uniform insulation of the winding
throughout
4/7/2023
Eng. Fredy Loita EPE Department 30

 A star-connected, 3-phase, 10-MVA, 6.6kV alternator has a per
phase reactance of 10%. It is protected by Merz-Price
circulating-current principle which is set to operate for fault
current not less than 175A. Calculate the value of earthing
resistance to be provided in order to ensure that only 10% of
the alternator winding remains unprotected
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Eng, Fredy Loita EPE Department 31

4/7/2023

 An 11kV, 100MVA alternator is grounded through a resistance
of 5Ω. The CTs have ratio 1000/5. the relay. The relay is set to
operate when there is an out balance current of 1A. What a
difference of the generator winding will be protected by the
percentage differential scheme of protection.
Answer: 84.25%
4/7/2023
Eng, Fredy Loita EPE Department 34

 In small alternators, the neutral ends of the three-phase
windings are connected internally
 Hence it is not possible to use Merz-Price circulating
principle → No facility to accommodate the CTs
 Under these circumstances, it is considered sufficient to
provide protection against earth-fault only
 Balanced earth-fault scheme is used
 This scheme provides no protection against phase to
phase faults, unless they develop into earth-fault as most
of them will
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 Under normal operating conditions, the currents flowing in the
leads in the secondaries of the CTs add to zero and no current
flows through the relay
 The current in the neutral wire is also zero hence no current to
the relay through secondary of the CT connected to the neutral
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 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
 When an earth fault occurs at F1 or within the protected zone,
these currents are no longer equal and differential current
flows through the operating coil of the relay
 Then relay closes its contacts to disconnect the alternator from
the system
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 Merz-price circulating-current systems protects against a phase-to
ground and phase to phase faults. It is does not protect against turn
to turn fault on the same phase winding of the stator.
 It is because the current that this types of fault produces flows in a
local circuit between turns involved and not create a difference
between the entering and leaving the winding at its two ends where
current transformer are applied.
 In a single turn generator (e.g large steam turbine generators) there
is no necessity of protection against inter turns faults.
 However, inter turns protection is provided for multi turn generators
such as hydro-electric generators. These generators have double-
winding armatures owing to the heavy currents which may be taken
of this necessity to protect inter turn faults on the same winding.
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 The relay Rc provide protection against phase to ground and phase to
phase faults where relay R1 provide protection against inter turn
faults.
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OPERATION
 The duplicated stator windings S1 and S2 of one phase only with a
provision against inter turn faults.
 Two current transformers are connected on the circulating current
principle.
 Under normal operating conditions, the current in the stator
windings S1 and S2 are equal and so will be the current in the
secondaries of the two CTs.
 The secondary current round the loop hen is the same at all points
and no longer be equal.
 Therefore, unequal currents will be induced in the secondaries of CTs
and the difference of these two current flows through the relay R1.
The relay closes its contacts to clear the generator from the system.
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Stator- Over heating Protection
 Over heating may be caused by the failure of the cooling system,
overloading or core faults like short circuited laminations and failure
of core bolt insulation.
 Modern generator employ two methods to detect overheating both
being used in large generators ( above 2MW). In one method, the inlet
and outlet temperatures of the cooling medium which may be
hydrogen/water are compared for detecting overheating.
 In the other method, the temperature sensing elements are embedded
in the stator slots to sense the temperature. When temperature
exceeds a certain present maximum temperature limit, the relay
sounds an alarm.
 The scheme employs a temperature detector unit, relay, and
Wheatstone bridge for the purpose.
4/7/2023

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