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Chapter 6: Protection Schemes of Generators, Transformers, Motors and Feeders -10 marks

Protection scheme of generator:

A protection scheme of an alternator is a system of protective devices and measures designed to ensure the safe
and reliable operation of the alternator, as well as to protect it from electrical and mechanical faults.

Different types of faults on alternator:

1. Stator winding faults
2. Rotor faults
3. Mechanical faults:
• Vibration
• Mechanical shock
• Bearing failure
4. External faults

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1. Stator winding faults:

• Stator winding faults in generators refer to any defects or failures in the stator winding that can cause disruptions in
the normal operation of the generator.
• These faults occur mainly due to the insulation failure of the stator coils. The main types of stator winding faults are
Phase to earth faults, Phase-to-phase faults, and inter-turn faults .

Main causes of stator winding faults:

There are several reasons why stator winding faults can occur in generators. Some of the most common causes include:

1. Electrical insulation failure: The electrical insulation used in stator windings can deteriorate over time due to several
factors, including heat, moisture, and chemical exposure. Insulation failure can lead to short circuits or grounds in
the winding.
2. Overvoltage: High voltages can cause insulation breakdown in stator windings, resulting in short circuits or grounds.
3. Overloading: Operating a generator at or beyond its rated capacity can cause excessive heat in the stator winding,
leading to insulation breakdown and winding faults.
4. Mechanical stress: Mechanical stress on the stator winding, such as vibration, thermal expansion, or misalignment,
can damage the winding and cause faults.
5. Manufacturing defects: Poor manufacturing processes or faulty materials can result in stator winding faults in newly
installed generators.

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Effect of stator faults of alternator:

Stator faults occur due to failure of the winding insulation. The heat generated by these faults can cause serious damage
to the laminated core of the Stator. This may require expensive re-insulation and rebuilding

Stator winding faults can have several negative effects on an alternator, including:
1. Reduced output voltage: This is because the winding faults can cause an imbalance in the magnetic fields that
produce the electrical power.
2. Increased temperature: This is because the faults can cause more current flowing through the wire and more heat
being generated.
3. Reduced efficiency: This is because the winding faults can cause additional losses in the magnetic fields and increase
the amount of energy needed to produce the same amount of electrical power.

4. Increased vibration: This is because the faults can cause an imbalance in the magnetic fields, which can lead to
mechanical vibrations in the stator.

5. Increased noise: This is because the faults can cause an imbalance in the magnetic fields, which can lead to
mechanical vibrations that produce audible noise.

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Protection of stator winding faults

1. Differential Protection scheme:
Differential protection is a protection method that detects any phase-to-phase fault in the stator winding by comparing
the current entering and leaving the winding. If there is any difference, it trips the generator.
• The Mertz-Price protection scheme is a differential protection
scheme used for protecting large generators and transformers
against internal faults, such as winding-to-winding faults or
winding-to-ground faults.
• The CTs are placed at opposite ends of the stator winding, so
they measure the current entering and leaving the winding. CTs
compare the currents entering and leaving the winding.
• Under normal operating conditions, the currents entering and
leaving the stator winding should be equal, and differential relay
will not trip.
• However, in the event of a fault, such as a winding-to-winding
Fig: differential protection (Mertz price protection) of alternator
fault or a winding-to-ground fault, the currents will become
unbalanced, and the differential relay will trip to isolate the
fault.
• The Mertz-Price protection scheme is designed to be sensitive
enough to detect even small internal faults, while also being
selective enough to avoid tripping for external faults, such as
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faults in the transmission system.

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2. Overcurrent Protection: Overcurrent protection is a basic protection method that detects any overcurrent in the stator
winding and trips the generator before any further damage occurs.

3. Overvoltage and Under voltage Protection: Overvoltage and under voltage protection are protection methods that
protect the alternator from overvoltage and under voltage conditions in the stator winding.

4. Thermal Protection: Thermal protection is a protection method that detects the temperature of the stator winding and
trips the generator if the temperature exceeds the allowable limit

5. Earth Fault Protection: Earth fault protection is a protection method that detects any fault between the stator winding
and earth. It trips the generator if any fault is detected.

Figure: earth fault protection scheme

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Rotor faults of alternator:

• A rotor fault in an alternator refers to any type of problem or malfunction that occurs in the rotor assembly, which
is a key component of the alternator.
• The rotor is a rotating part of the alternator that consists of a core, winding, and slip rings.
• Its main function is to generate a magnetic field that interacts with the stator winding to produce electrical power.

Faults in the rotor circuit may be either earth faults (conductor to earth faults) or inter- turn faults The main causes
of the rotor faults include:

1. Earth faults or inter-turn faults, which are caused by severe mechanical and thermal stresses on the winding
insulation.

2. Overheating of the rotor, which can result from unbalanced currents, low power factor, high stator voltage, or
low cooling fan speed.

3. Insulation breakdown, winding deformation, or rotor cracking, which can occur due to thermal cycling,
vibration, or corrosion.

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Rotor earth fault protection system

Effect of rotor earth fault:
-non-uniform production of flux in
A rotor earth fault in an alternator refers to an electrical fault that occurs when a part
rotor
of the rotor winding makes contact with the metal frame of the alternator, causing a
-non uniform production of emf
current to flow to the earth. This fault can occur due to insulation failure, damage to
on stator
the winding, or other reasons.
-vibration produced in rotor.

Rotor earth fault protection method:

i. Rotor earth fault protection by using high resistance method:

• When one earth fault occurs in the rotor then it is not

necessary that the system completely trip, only the relay
indicates the fault has occurred.
• As a result, generator is taken out of service.
• Figure shows the high resistance method for protection of
alternator,
in this method, high resistance is connected across the field
winding of rotor
• Mid point of the resistor is grounded through sensitive relay
• When the fault occurs the relay detects the fault and send
tripping command to the CB.
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ii. Rotor earth fault protection by using AC and DC injection method:

• The Rotor Earth Fault Protection Device consists of a current

injection device that applies a DC or AC voltage to the rotor
winding by means of a slip ring fitted on the rotor.

• The current is applied to the rotor through a current

limiting resistor (In case of a DC injection) or capacitor (in
case of an ac injection).

• In normal conditions, No current through resistor or

capacitor is zero, relay is inoperative.

• when earth fault occurs, earth fault will senses by the relay
and relay operates.

The relay can be made more sensitive in the case of

DC injection as there is no issue of leakage current
through capacitance as in the case of AC injection
through the capacitor
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2. Loss of excitation (Or field failure) Protection:

Loss of excitation can be following cause:

• Loss of field to main exciter
• Accidently tripping to the field breaker
• Short circuit in the field winding
• Loss of ac supply to excitation system

Due to loss of excitation, speed of rotor increases, as the magnitude of flux decreases and machine operate as induction
generator and also reverse reactive power flow in generator.

Protection from loss of excitation:

When the voltage regulator detects a loss of excitation, it sends a signal to the protection relay. The protection
relay then initiates the necessary protection action, which may include tripping the generator or reducing the load
on the generator. The excitation system may also be shut down to prevent further damage to the generator.

3. Protection against rotor overheating (Temperature sensor and alarm)

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Protection of alternator against mechanical failure

Mechanical conditions can have a significant impact on the operation of an alternator, and as a result, protection systems
are required to detect and prevent any damage caused by these conditions. Here are some common mechanical conditions
that can affect alternators and the corresponding protection systems:

1.Vibration:
• Vibration can cause damage to the alternator's stator winding, rotor, bearings, and other components.
• To protect against this, vibration sensors can be installed to detect excessive vibration and alert the operator of the issue.

2.Mechanical shock:
• Mechanical shocks can occur due to sudden changes in the load or due to external impacts.
• To protect against this, shock sensors can be installed to detect sudden impacts and alert the operator of the issue.

3.Bearing failure:
• Bearing failure can cause the rotor to move out of alignment, leading to vibrations and potential damage to the
alternator.
• Regular maintenance:
• Avoid overloading the alternator:
Bearing failure • Replace the belt and pulleys when necessary:
can be protect • Ensure proper cooling:
by • Check for misalignment:
• Use high-quality bearings
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External faults that can occur on an alternator include:

• Overloading: This occurs when the alternator required to supply more load then limit load. This can cause the
alternator to overheat and fail.

• Overvoltage: due to lightning or switching of system

• Short circuit: due to short circuit occurs outside of alternator of the system. This can cause the alternator to
overheat and fail.
• Loose connections: This can cause a drop in voltage
• Broken or worn belts: The alternator is powered by a belt that connects it to the engine. If the belt breaks or wears
out, the alternator will not be able to produce electricity.
• Damaged wiring: If the wiring in the electrical system is damaged , it can cause a drop in voltage or a short circuit,
which can affect the performance of the alternator.
• Failure of the regulator: The regulator is responsible for regulating the voltage produced by the alternator. If the
regulator fails, it can cause the alternator to produce too much or too little voltage, which can damage the electrical
system it is powering.

These external faults can be caused by various factors such as age, wear and tear, poor maintenance, and
environmental conditions. Regular maintenance and inspection can help prevent these issues and ensure the
alternator operates correctly.
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Protection of power transformer: Types of faults and protective schemes: Over current, Earth fault, Differential protection,
Buchholz devices, Winding Temperature Protection

Power transformer:
A power transformer is a device that is used to transfer electrical energy from one circuit to another, typically by means of
electromagnetic induction. It is a static electrical device that is designed to transform electrical power from one voltage
level to another, without changing the frequency.

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