Unit-4

MERZ PRICE PROTECTION

• Merz price differential protection is used to protect the transformer from internal short circuit,
Internal ground faults and inter turn shorts.
• Transformer is a static device. Merz price differential protection is nothing but a percentage
differential protection Which works under the principle of circulating current scheme.

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MERZ PRICE DIFFERENTIAL PROTECTION FOR GENERATORS
This is most commonly used protection scheme for the alternator stator windings. The scheme is also
called biased differential protection and percentage differential protection. The figure below shows a
schematic arrangement of Merz-Price protection scheme for a star connected alternator.

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MERZ PRICE DIFFERENTIAL PROTECTION FOR GENERATORS
• The differential relay gives protection against short circuit fault in the stator winding of a generator.
When the neutral point of the windings is available then, the C.T.s may be connected in star on both
the phase outgoing side and the neutral earth side.
• But, if the neutral point is not available, then the phase side CTs are connected in a residual
connection, so that it can be made suitable for comparing the current with the generator ground point
CT secondary current.
• The restraining coils are energized from the secondary connection of C.T.s in each phase, through pilot
wires. The operating coils are energized by the tappings from restraining coils and the C.T. neutral
earthing connection.

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THE ADVANTAGES OF THIS SCHEME ARE,
• Very high speed operation with operating time of about 15 msec.
• It allows low fault setting which ensures maximum protection of machine windings.
• It ensures complete stability under most severe through and external faults.
• It does not require current transformers with air gaps or special balancing features.

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PROTECTION AGAINST ABNORMAL CONDITIONS
• There are a large number of possible faults, as well as combinations of faults, that threaten the
operation of the generator. Instances where there is no direct electrical fault in the generator but
one or more of its associated equipment develop a fault or an abnormality, may lead to an
abnormal operating condition, which may or may not be serious.

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UNBALANCED LOADING
• If there is an unbalanced loading of the generator then the stator currents have a negative sequence
component.
• The stator field due to these negative sequence currents, rotates at synchronous speed but in a direction
opposite to the direction of the field structure on the rotor.
• Thus, the negative sequence stator armature mmf rotates at a speed -Ns while the rotor field speed is
+Ns.
• Therefore, there is a relative velocity of between the two. This causes double frequency currents, of large
amplitude, to be induced in the rotor conductors and iron.
• Thus, if the stator carries unbalanced currents, then it is the rotor, which is overheated. How long the
generator can be allowed to run under unbalanced loading, depends upon the thermal withstand capacity
of the machine, which in turn depends upon the type of cooling system adopted.

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UNBALANCED LOADING

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OVER SPEEDING
• Consider that a turbo-alternator is supplying its rated real electrical power Pe, to the grid.
• Its mechanical input Pm is nearly equal to Pe, (except for the losses) and the machine runs at
constant synchronous speed Ns.
• Now, consider that due to some fault the generator is tripped and disconnected from the grid.
Thus, P, becomes zero. However, the mechanical power input Pm cannot be suddenly reduced to
zero. Therefore, we land up in a situation where the generator has full input mechanical power but
no output electrical power.
• This would cause the machine to accelerate to dangerously high speeds, if the mechanical input is
not quickly reduced by the speed-governing mechanism.

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OVER SPEEDING
• The protection against such an eventuality can be provided by
sensing the over speeding and taking steps such as operating
the steam valve so as to stop steam input to the turbine. The
speed-governing mechanism or the speed governor gf the
turbine is basically responsible for detecting this condition. The
over-speeding can also be detected either by an over-frequency
relay or by monitoring the output of the tacho generator
mounted on the generator shaft.

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LOSS OF EXCITATION
There are several possible causes due to which field excitation may be lost, namely:
• Loss of field to main exciter
• Accidental tripping of the field breaker
• Short circuit in the field winding
• Poor brush contact in the exciter
• Field circuit breaker latch failure
• Loss of ac supply to excitation system

Consider that the field excitation is lost while the mechanical input remains intact. Since the generator is
already synchronized with the grid, it would attempt to remain synchronized by running as an rnduction
generator. As an induction generator, the machine speeds up slightly above the synchronous speed and
draws its excitation from the grid.

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LOSS OF EXCITATION

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LOSS OF EXCITATION
Now, there are two possibilities. Either the grid is able to meet this reactive power demand fully or meet it
partially. If the grid is able to fully satisfy this demand for reactive power, the machine continues to deliver
active power of Pe, MW but draws reactive power of QLOE MVA and there is no risk of instability. However,
the generator is not designed as an induction machine, so abnormal heating of the rotor and overloading of
the stator winding will take place.
• The simplest method by which loss of excitation can be detected is to monitor the field current of the
generator. If the field current falls below a threshold, a loss of field signal can be raised. A complicating
factor in this protection is the slip frequency current induced in the event of loss of excitation and running
as an induction generator.
• The quantity which changes most when a generator loses field excitation is the impedance measured at
the stator terminals. On loss of excitation, the terminal voltage begins to decrease and the current begins
to increase, resulting in a decrease of impedance and also a change of power factor

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INDUCTION MOTOR PROTECTION
Induction motors come in a wide range of ratings, from fractional power motors used in tools and
domestic appliances to motors of megawatt rating .used for boiler feed pump in thermal power
stations

It is not possible to make any general statements about the protection of induction motor, since the
protection scheme depends on the size (horsepower/kW rating) of the motor and its importance in
the system. This is the reason why induction motor protection has not been standardized.

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ABNORMAL OPERATING CONDITION IN MOTOR

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PHASE FAULTS INSIDE THE MOTOR
• Protection against phase faults as well as ground faults can be provided using either fuses , or over-
current relays depending upon the voltage rating and size of the motor. Most motors will be protected
by HRC fuses.
• The fusing current should be greater than the starting current of the motor. The fuse operating time
should be less than permissible locked rotor time of the motor.
• The locked rotor time is the time for which the rotor can be safely stalled with full supply voltage
applied to the stator.

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PHASE FAULTS INSIDE THE MOTOR
• Big motors, which are high voltage. motors, will need to be provided with an over-current protection
for increased accuracy of protection.
• The thermal capability characteristic of the motor should be kept in mind while applying over-current
protection. The OC relay characteristic should be below the thermal capability characteristic.
• In case of high impedance ground faults inside the motor, the fault current may happen to be less
than the full-load current. Such faults are difficult to detect using over-current approach.
• In case of big motors whose kVA rating is more than half of the supply transformer kVA rating, the
current for a three-phase fault may be less than five times the current for locked rotor condition. In
such cases, it is recommended to use percentage differential protection.

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 PHASE FAULTS INSIDE THE MOTOR

Assume a motor is connected to a supply transformer

with 8% impedance. The maximum fault current at the
transformer secondary with an infinite source is:

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PHASE FAULTS INSIDE THE MOTOR

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GROUND FAULTS INSIDE THE MOTOR
• The arrangement for detecting high impedance ground faults is shown below. The three phase line
conductors carrying current to the motor form the primary of a transformer.
• The secondary consists of a pick-up coil wound on the core.

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GROUND FAULTS INSIDE THE MOTOR
• When the motor is running normally, the instantaneous sum of all the three line currents is zero. Thus,
there is no net flux in the core. Hence, the pick-up coil does not have any voltage induced in it.
• Now, consider a ground fault as shown. The three line currents do not sum up to zero. Thus, there is
a net primary mmf proportional to the fault current If, returning to the supply neutral through the fault
path. There is, thus, a flux in the CT core. The pick-up coil has a voltage induced which can be
sensed by an electronic circuitry or the pick-up coil can be made to drive the operating coil of a
sensitive relay.
• If an electronic circuit is used to sense the voltage developed by the pick-up coil, the current balance
relay described above can be made extremely sensitive and can detect earth fault currents down to a
few tens of milliamperes. Very high sensitivity, however, is likely to cause some nuisance tripping.

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 SINGLE PHASING
• Single phasing can occur because of a non-closure of
one pole of a three-phase contactor or circuit breaker, a
fuse failure or similar causes.
• Single phasing causes negative sequence current to flow.
The motor has a limited ability to carry negative sequence
currents, because of thermal limitations.
• Single phasing causes the motor to develop insufficient
torque, leading to stalling, making the motor to draw
excessive current and finally leads to burn out unless the
motor is tripped.
• Thus, there is a thermal limit on the amount of the
negative sequence current that can be safely carried by
the motor. The quantity I2t represents the energy
liberated as heat due to negative sequence current I2

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PHASE REVERSAL
• When there is a reversal of phase sequence, possibly due to reversal of phases, the motor rotates in
a direction opposite to its normal direction of rotation. In several applications, such as hoists and
elevators, this is a serious hazard.
• In such situations, a phase sequence detector, which is generally a part of under-voltage , over-
voltage, or a negative phase sequence protection scheme, can be used to instantaneously trip the
motor.

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OVERLOAD
• Thermal overload relays offer good protection
against short, medium, and long duration
overloads but may not provide protection against
heavy overloads.
• The long time induction over-current relays provide
good protection against heavy overloads but over-
protection against light and medium overloads.
• Therefore, a combination of both the relays
provides adequate protection. Thermal overload relays offer good protection
against short, medium, and long duration
overloads.

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OVERLOAD

Long time induction OC relays offer good Combination of thermal overload relays and OC relays
protection against heavy overloads provides complete thermal protection.

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TRANSFORMER PROTECTION -DIFFERENTIAL RELAY WITH
HARMONIC RESTRAINT
• The protective scheme for the transformer that takes care of magnetizing inrush current without
affecting the sensitivity is a percentage differential relay incorporating a harmonic restrain relay also
known as harmonic restraint differential relay.
• The magnetizing inrush currents contain high components of even and odd harmonics. The
harmonic component associated with short circuit current is negligible. This principle is used for
restraining the relay from operation during the initial current inrush. The harmonic restrain differential
relay remains inoperative to magnetizing currents and trips during fault currents.
• In this relay, the operating coil takes only the fundamental component of current. The rectified sum of
fundamental and harmonic components is given to the restraining coil. Due to this, the currents with
high harmonic content will give high restraining torque and the relay remains inoperative.

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 TRANSFORMER PROTECTION -DIFFERENTIAL RELAY WITH
HARMONIC RESTRAINT

• conceptual scheme of a harmonic restraint differential relay.

• The fundamental component of spill current is segregated with the
help of a filter and is used to develop the tripping torque. The non-
fundamental component of the spill current aids the unfiltered
circulating current in developing the restraining torque. This makes
the relay stable on inrush while at the same time not affecting its
operation in case of genuine internal faults

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TRANSFORMER PROTECTION -DIFFERENTIAL RELAY WITH
HARMONIC RESTRAINT

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THANK YOU..

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