ANURAG ENGINEERING COLLEGE

(AN AUTONOMOUS INSTITUTION)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

POWER SYSTEM PROTECTION

III B. Tech, II Semester
BY
JIBILIKAPALLY SUMAN
ASSISTANT PROFESSOR

1
30-November-2024
 UNIT-III
PILOT RELAYING SCHEMES, AC MACHINES
AND, BUS ZONE PROTECTION
• Pilot Relaying Schemes
1. Wire Pilot protection
2. Carrier current protection.

• AC Machines and Bus Zone Protection

1. Protection of Generators
2. Protection of transformers
3. Bus-zone protection
4. Frame leakage protection.
2
 Introduction
•Pilot wire is an interconnecting channel used to send information
between both ends of a transmission line for protection.

•Use in power systems: It helps in quick fault detection by

comparing electrical signals at both ends of the line.

•Types of pilot relaying schemes:

1. Wire Pilot – Uses buried cables or telephone lines; suitable for
distances up to 30 km.
2. Carrier-Current Pilot – Uses high-frequency signals (50 kHz–700
kHz) on the power line; cost-effective for long distances.
3. Microwave Pilot – Uses high-frequency radio waves (450 MHz–
10,000 MHz); works for up to 150 km but needs a clear line of
sight.
2
 Wire Pilot Protection Scheme
•Wire pilot relaying uses two wires to carry signals between both ends of a
transmission line.
•It works on the differential protection principle, comparing CT secondary
currents at both ends.
•A single-phase current is derived from three-phase currents to reduce the number
of pilot wires needed.

•Advantages: Less expensive for short lines (under 15-30 km), simpler, and more
reliable.

•Limitations: Signal attenuation (weakening)due to capacitance and resistance

limits the distance.

•Usage: Recommended for short, important lines; for long lines, carrier-current
schemes are preferred.

•Two main operating principles:

1.Circulating current principle – Uses amplitude comparison, suitable for multi-
2
ended lines.
2.Balanced voltage principle – Less commonly used in wire pilot schemes.
 Wire Pilot Protection Scheme(cont..)

Circulating Current Protection Schemes

2
 Wire Pilot Protection Scheme(cont..)
Circulating Current Protection Schemes(cont..)
This scheme is a differential protection method used for transmission lines. It works by
comparing the current at both ends of the protected section using pilot wires.

Working Principle:
•Current transformers (CTs) are installed at both ends of the line.
•The secondary sides of these CTs are connected using two pilot wires.
•Under normal conditions or an external fault, the currents at both ends are equal, and no
current flows through the relay.
•If an internal fault occurs, the currents become unequal, causing a current to flow through the
relay, which trips the circuit breaker to isolate the faulty section.

Advantages:
•Fast and accurate fault detection.
•Simple and reliable due to minimal components.
•Suitable for short transmission lines (up to 30 km).

Limitations:
•Pilot wire costs increase with distance.
•Signal attenuation due to resistance and capacitance in long lines.
2
•Not suitable for long-distance transmission lines (carrier-current schemes are preferred).
 Wire Pilot Protection Scheme(cont..)
Balanced Voltage (Opposed Voltage) Pilot Wire Protection Scheme

2
 Wire Pilot Protection Scheme(cont..)
Balanced Voltage (Opposed Voltage) Pilot Wire Protection
Scheme(cont..)
•This scheme operates based on voltage balance between the two ends of a
transmission line.
•No current flows through the pilot wires under normal conditions or during
external faults.
•The relay coil is connected in series with the pilot wire.
•Current transformers (CTs) are placed at both ends of the transmission line.
•Under normal conditions or during an external fault, the voltages at both ends
remain equal and opposite, preventing any current flow through the pilot wires.
•If an internal fault occurs, the CT polarity at the remote end reverses, creating
an unbalanced voltage that causes current to flow through the pilot wires and the
relay coil.
•This current activates the relay, which trips the circuit breaker to isolate the
fault.
•A capacitor is sometimes added to tune the circuit to the fundamental
frequency.
•This scheme is suitable for short transmission lines with pilot loop resistance
up to 400 Ω. 2
 Wire Pilot Protection Scheme(cont..)

Translay Scheme

2
 Wire Pilot Protection Scheme(cont..)

Translay Scheme(cont..)

•The Translay scheme is a balanced voltage protection scheme with an

added directional feature for improved fault detection.
•It uses an induction disc-type relay at both ends of the protected
transmission line.
•The secondary windings of the relays are interconnected in opposition
using pilot wires, creating a balanced voltage system.
•The upper magnet of the relay has a summation winding that receives
input from current transformers (CTs).
•Under normal conditions or external faults, no current flows through the
pilot wires or the lower relay magnet, so no operating torque is produced.
•During an internal fault, current flows through the pilot wires and the lower
relay magnet, generating torque that activates the relay and trips the circuit
breaker.
•The flux interaction between the two electromagnets (upper and lower) is
responsible for relay operation.
•This scheme is suitable for fairly long pilot wires with a loop resistance of
2

up to 1000 Ω.
•It is classified as a phase comparison voltage-balanced scheme.
 Wire Pilot Protection Scheme(cont..)
Half-Wave Comparison Scheme

Working of the Scheme:

•Similar circuit connection to

the circulating current scheme,
but with a different operating
principle.
•The relay has only an
operating coil, with no
restraining coil.
•Rectifiers are used to allow
current flow only during an
internal fault.
•Resistances (RA and RB) are
slightly higher than the pilot
loop resistance (Rp).
•CTs at both ends (A and B) are 2

connected to ensure the correct

voltage polarity.
 Wire Pilot Protection Scheme(cont..)
Half-Wave Comparison Scheme(cont..)
Operation Under Normal & External Fault Conditions
•When voltage at A is positive, resistance RB is short-circuited by a
rectifier.
•The voltage at B becomes negative, so no relay operates.
•When voltage at B is positive, resistance RA is short-circuited, and
the voltage at A is negative.
•Again, no relay operates in this condition.

Operation During Internal Fault

•Voltage at both relays is positive during the positive half cycle, so
both relays operate.
•During the negative half cycle, voltage at both ends is negative,
preventing operation.
•Half-wave rectifiers are added across each relay coil to maintain
current flow during the negative half cycle.
•Non-linear resistors protect CTs from overvoltages when CTs would
otherwise be open-circuited. 2
 CARRIER CURRENT PROTECTION
Introduction
• Widely Used Protection Scheme – Carrier current protection is the
most commonly used method for Extra High Voltage (EHV) and Ultra
High Voltage (UHV) power lines.

• Carrier Signal and Frequency Range – A high-frequency carrier signal

(50 kHz to 700 kHz) is directly coupled to the power line for protection.

• Power Level of Carrier Signal – The carrier signal operates at a power

level of about 10-20 W.

• Power Line as Carrier – The power line itself is used for transmitting
the carrier signal, making it a power line carrier scheme.

• High-Speed Protection – With the expansion of power systems, high-

speed protective schemes are essential, and carrier current schemes
provide faster and more efficient protection.
2

• Advantages Over Distance Protection – Carrier current schemes are

superior to distance protection schemes, as they ensure simultaneous
tripping of circuit breakers at both ends, avoiding system instability.
 CARRIER CURRENT PROTECTION(cont..)
Introduction(cont..)
• Types of Carrier Schemes – Two main types:
• Carrier-blocking scheme – Prevents relay operation using the
carrier signal.
• Carrier intertripping scheme – Uses the carrier signal to initiate
tripping.

• Cost and Reliability – Carrier current schemes are more cost-effective

and reliable for long lines compared to wire pilot schemes, despite the
higher cost and complexity of terminal equipment.

• Additional Uses of Carrier Signal – The carrier signal can also be

used for telephone communication, supervisory control, and telemetry,
reducing overall costs.

• Two Key Operating Techniques –

i. Phase Comparison – Compares the phase angles of currents at both
ends; a 180° phase difference indicates a fault. 2

ii. Directional Comparison – Compares the direction of power flow at

both ends to detect internal faults.
 CARRIER CURRENT PROTECTION(cont..)
Phase Comparison Carrier Current Protection
Working Principle

•This scheme compares the phase angle of the current entering one end of
the protected line section with the current leaving the other end.
•If the currents are in phase (0° difference), it indicates an external fault or
normal operation (no tripping).
•If the currents are 180° out of phase, it indicates an internal fault, triggering
the relay to trip the circuit breaker.

Parts Used and Their Functions

• Line Trap
1. A parallel resonant circuit that blocks the carrier signal from flowing into
adjacent line sections while allowing power frequency current to pass.
• This ensures that the carrier signal remains within the protected zone.
• Carrier Transmitter & Receiver
1. Each end of the line has a transmitter and receiver to send and receive high-
frequency signals.
2. These signals help determine whether there is an internal or external fault.
2
• Coupling Capacitor
1. Connects the transmitter/receiver to the power line while withstanding high
voltage.
2. It also prevents direct contact between power equipment and sensitive relays.
 CARRIER CURRENT PROTECTION(cont..)
Phase Comparison Carrier Current Protection(cont..)
• Inductance (Tuning Reactor)
1. Grounds the transmitter/receiver at power frequency but keeps them insulated
at carrier frequency, ensuring the signals remain effective.
• Fault Detectors
1. Detect faults and ensure the carrier signal is sent only when a fault occurs to
prevent unnecessary relay operation.
• Comparator
1. Compares the phase difference between signals at both ends.
2. If no signal is received (due to an internal fault), it triggers the tripping relay.
• Relay & Circuit Breaker
1. When the comparator confirms an internal fault, it activates the auxiliary
tripping relay, which then operates the circuit breaker to isolate the fault.

Operating Mechanism
•During Normal Operation or External Fault
• The output voltages at both ends are 180° out of phase due to opposite CT
connections.
• A continuous carrier signal is transmitted and received, blocking the relay
from tripping.
2
•During an Internal Fault
• The polarity of the output voltage at one end reverses, causing a phase shift.
• The carrier signal disappears for one half-cycle, allowing the relay to trip the
circuit breaker.
 CARRIER CURRENT PROTECTION(cont..)
Phase Comparison Carrier Current Protection(cont..)
Advantages of Phase Comparison Protection
✔ Fast and Reliable – Ensures high-speed tripping for faults inside the protected section.
✔ No Impact from Power Swings – Does not trip due to system swings, load fluctuations, or
zero-sequence currents from parallel lines.
✔ Better Selectivity – Trips only for internal faults, reducing unwanted shutdowns.
✔ Cost-Effective for Long Lines – Uses the power line itself for signal transmission, avoiding
expensive pilot wires.
✔ Supports Multi-Function Use – Carrier signals can also be used for communication,
control, and telemetry.

Disadvantages of Phase Comparison Protection

✖ Limited Distance Protection – The accuracy decreases for very long
transmission lines due to signal propagation delay and phase shifts caused by
capacitance.
✖ Requires Backup Protection – It only provides primary protection, so a
distance relay is needed as a backup.
✖ Complex Equipment – Needs precise tuning of line traps, filters, and coupling
devices, making installation and maintenance complicated. 2

✖ Errors Due to CT Performance – Current transformer (CT) errors and through

current effects may affect phase comparison accuracy.
 CARRIER CURRENT PROTECTION(cont..)
Phase Comparison Carrier Current Protection(cont..)

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection
•Unit protection schemes do not provide backup protection for the
adjacent line section.
•Distance protection schemes can provide backup protection, but
they do not ensure high-speed protection for the entire line.
•In distance protection, circuit breakers do not trip simultaneously
at both ends for end-zone faults, which can cause instability.
•The best solution is to combine unit protection and distance
protection, using carrier signals to interconnect distance relays at
both ends.
•This combined scheme provides instantaneous tripping for the
entire line and also offers backup protection.
•There are three types of such schemes:
i. Carrier transfer or intertripping scheme – Uses a carrier signal
to directly trip the relay at the remote end.
ii. Carrier acceleration scheme – Speeds up distance relay 2

operation to provide faster tripping.

iii. Carrier blocking scheme – Uses a carrier signal to block
unnecessary tripping for external faults.
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme
The following are important types of transfer tripping schemes.
(a) Direct transfer tripping (Under-reaching scheme)
(b) Premissive under-reach transfer tripping scheme
(c) Premissive over-reach transfer tripping scheme

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
a. Direct Transfer tripping(Under-reaching scheme)

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
a. Direct Transfer tripping(Under-reaching scheme)(cont..)
1.Three-step distance relays are placed at both ends of the protected line to detect faults at
different zones.
2.When a fault occurs at F3 (near end B), the Zone 1 relay at B trips the circuit breaker
instantly.
3.However, the circuit breaker at A does not trip immediately because the fault is outside
its primary protection zone.

4.To ensure instantaneous tripping at A, a carrier signal is sent from B to A, triggering

the receive relay (RR) at A.
5.The RR relay at A activates the trip circuit, causing the circuit breaker at A to trip
immediately, avoiding delays.
6.If a fault occurs at F1 (near end A), the Zone 1 relay at A trips its circuit breaker and
also sends a carrier signal to B, ensuring instant tripping at B.
7.If a fault occurs at F2 (mid-section of the line), the circuit breakers at both A and B trip
simultaneously without delay.
8.The scheme provides backup protection for adjacent lines, as Zone 2 and Zone 3 relays
operate with a time delay to cover faults beyond the primary zone.
2
9.A major disadvantage is the risk of undesired tripping due to malfunctions or accidental
triggering of the carrier signal channel.
10.The carrier signal is transmitted over the faulty line, leading to signal attenuation,
which may reduce reliability in some cases.
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
b. Permissible Under-reaching scheme

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
b. Permissible Under-reaching scheme(cont..)
Need for Supervision
•In direct transfer tripping schemes, there is a risk of false tripping due to accidental or
malfunctioning carrier signals.
•To prevent this, the receive relay (RR) is supervised by the Zone 2 relay (Z2).
Placement of Zone 2 Relay
•The Zone 2 relay contact (Z2) is placed in series with the receive relay (RR) in the
trip circuit.
•This means that both relays must close for the circuit breaker to trip.
Operation During an Internal End-Zone Fault
•If a fault occurs in the end-zone, the Zone 2 relay (Z2) detects it and closes its
contact.
•The carrier signal is received from the other end, causing the receive relay (RR) to
close.
•Since both Z2 and RR are now closed, the trip circuit is completed, and the circuit
breaker trips instantly.
Prevention of False Tripping 2

•If there is no actual fault, the Zone 2 relay (Z2) remains open.
•Even if an accidental carrier signal is received, the receive relay (RR) alone cannot
trip the circuit because Z2 is open.
•This prevents false tripping due to signal malfunctions.
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
b. Permissible Under-reaching scheme(cont..)
Signal Sending Arrangement
1. The Zone 1 relay is responsible for sending the carrier signal to the remote end when it detects a
fault.
2. The carrier signal confirms the fault and initiates a trip at the other end.
Trip Circuit Logic
1. A solid-state logic circuit is used to process signals and control the tripping mechanism
efficiently.
2. It ensures that only valid fault conditions trigger the trip operation.
Flow of Operation
1. Step 1: Fault occurs in the end-zone.
2. Step 2: Zone 1 relay at the faulted end detects it and sends a carrier signal.
3. Step 3: At the other end, the Zone 2 relay (Z2) checks if the fault is within its range.
4. Step 4: If Z2 detects the fault, it closes its contact.
5. Step 5: If the carrier signal is received, the receive relay (RR) closes.
6. Step 6: With both Z2 and RR closed, the circuit breaker trips, clearing the fault.
Transmission of Carrier Signal
1. The carrier signal is transmitted over the faulty line section.
2. This may cause signal attenuation, reducing its strength.
Advantages of this Scheme
1. Prevents false tripping by ensuring that both Z2 and RR must operate.
2. Provides fast protection for end-zone faults. 2
3. Allows proper coordination between relays at both ends of the line.
Disadvantage
1. Signal attenuation occurs because the carrier signal is sent over the faulty line, which may affect
reliability.
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
c. Permissible over-reach transfer tripping scheme

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
i. Carrier transfer or intertripping scheme(cont..)
c. Permissible over-reach transfer tripping scheme(cont..)
1.In this scheme, the Zone 2 relay is used to send a carrier signal to the other end of the
protected line.
2.A directional relay supervises the receive relay (RR) contact to ensure that tripping
happens only if the fault is within the protected section.
3.The trip circuit includes both the Zone 2 relay and the receive relay (RR), as shown in
Fig. (a).
4.The Zone 2 relay must be directional (such as a MHO relay) to prevent tripping for faults
outside the protected section.
5.This scheme is also called a Directional Comparison Scheme because it uses
directional relays at both ends to compare fault conditions.
6.Direct transfer tripping is not used in this scheme because the Zone 2 relay may send
a carrier signal even for external faults within its reach.
7.The signal sending arrangement is shown in Fig. (b). The Zone 2 relay sends the signal
when a fault is detected.
8.A solid-state logic circuit is used to control the trip operation, as shown in Fig.(c).
9.The Over-Reach Transfer Scheme works by extending the Zone 2 protection beyond
the protected section to improve fault detection.
10.In this scheme, the carrier signal is transmitted over the faulty line, which can cause2

signal attenuation and reduce communication effectiveness.

 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
ii. Carrier acceleration scheme(cont..)

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
ii. Carrier acceleration scheme(cont..)
•In this scheme, the carrier signal extends the reach of the Zone 1 relay to Zone
2, allowing it to detect end-zone faults.
•When an end-zone fault occurs, the relay at one end trips and sends a carrier
signal to the other end.
•This scheme uses a single measuring unit for both Zone 1 and Zone 2 (MHO
unit).
•The Zone 1 relay is responsible for sending the carrier signal to the other end
when a fault is detected.
•The receive relay (RR) contact is connected to a range change relay, as shown in
Fig. (a).
•When the carrier signal is received, the range change relay immediately
extends the reach of the MHO unit from Zone 1 to Zone 2, which helps in clearing
the fault at the remote end faster.
•If the carrier signal fails, the fault is still cleared, but it takes longer because the
Zone 2 relay operates with its normal time delay.
•This scheme is not as fast as the permissive transfer tripping schemes, because
extra time is needed for the MHO unit to adjust its range.
•However, this scheme is more reliable, as the Zone 2 relay only operates when it
actually detects a fault, avoiding false tripping due to accidental or faulty carrier 2

signals.
•The carrier signal is transmitted over the faulty line, so the effectiveness of this
scheme depends on the carrier signal working properly. If the carrier fails, end-
zone faults take more time to be cleared.
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
iii. Carrier blocking scheme

2
 CARRIER CURRENT PROTECTION(cont..)
Carrier Aided Distance Protection(cont..)
iii. Carrier blocking scheme(cont..)
•The carrier signal is used to block the relay from operating during external faults to avoid
unnecessary tripping.
•If a fault occurs inside the protected line section, no carrier signal is transmitted, allowing
the relay to operate normally.
•This scheme is especially useful for protecting multi-ended transmission lines, where
multiple substations are connected.
•The Zone 3 relay looks in the reverse direction and sends a blocking signal to stop the
Zone 2 relay at the other end from operating during an external fault.
•When a fault occurs at F1, Zone 1 relays at both ends (A and B) detect the fault and clear it
instantly. No carrier signal is sent because the Zone 3 relay does not see this fault.
•When a fault occurs at F2 (end-zone fault), Zone 1 at B and Zone 2 at A detect the fault. The
Zone 1 relay at B clears the fault immediately, and the Zone 2 relay at A clears it instantly
through the receive relay (RR) and Z2, as shown in Fig. (a). No carrier signal is transmitted
since it is an internal fault.
•The Zone 2 relay at A has two operating times:
•Instantaneous operation (through Z2 and RR).
•Delayed operation (through T2).
•When a fault occurs at F3 (an external fault), it is detected by the forward-looking Zone 2
relay at A and the reverse-looking Zone 3 relay at B.
•Since F3 is outside the protected zone, the Zone 1 relay for line BC should clear the 2fault.
To prevent the Zone 2 relay at A from tripping, the Zone 3 relay at B sends a carrier signal
to block the operation of the Zone 2 relay at A.
•If the fault at F3 is not cleared instantly by line BC relays, the Zone 2 relay at A will operate
after a time delay to provide backup protection.
 GENERATOR PROTECTION

(i) Stator protection (ii) Rotor protection (iii) Miscellaneous

(b)
a) Field (b) Loss (c) (a) (c)
(a) Over
(b) (c) Stator- ground- of Protection Overvoltage Protec
Percentage Protection spee
overheating fault excitation against protection d tion
differential against protecti protection rotor
protection prote agains
protection stator on overheating ction t
inter-turn motori
faults ng
d) Protection
against
vibration 2
 GENERATOR PROTECTION(cont..)
i. Stator Protection
(a) Percentage Differential Protection
 GENERATOR PROTECTION(cont..)
(a) Percentage Differential Protection(cont..)

Principle:
Percentage differential protection is a protective scheme used for generators above 1 MW. It primarily
protects the generator windings from internal faults, such as:
•Phase-to-phase faults
•Phase-to-ground faults
This scheme is also known as biased differential protection or longitudinal differential protection. It
works by comparing the currents entering and leaving a protected zone (e.g., a generator winding) using
Current Transformers (CTs).
If the difference in current (known as differential current) is within a safe limit, the relay does not operate.
However, if the difference exceeds a predefined setting, the relay trips to isolate the faulty section.

Working Against Internal and External Faults:

1. External Fault Condition (Stable Condition – No Tripping):
In case of an external fault (outside the generator winding), the CTs at both ends of the generator produce
equal secondary currents with the same polarity.
These currents cancel each other in the operating coil of the relay.
Since no unbalanced current flows, the relay does not operate, ensuring stability.
✅ Relay remains OFF → No unnecessary tripping

2. Internal Fault Condition (Fault inside the Generator – Tripping Occurs):

When a fault occurs within the generator winding, the current distribution changes.
The polarity of one CT reverses, causing the currents from both CTs to add up in the relay’s operating coil.
This produces a large differential current, leading to relay operation.
The relay trips the circuit breaker, isolating the faulty generator from the system.
🚨 Relay operates → Faulty generator is disconnected
 GENERATOR PROTECTION(cont..)

(b) Protection against Stator Interturn Faults

Principle:
• Longitudinal percentage differential
protection does not detect stator interturn
faults (faults between turns of the same
winding). To protect against such faults,
transverse percentage differential
protection is used.
• This scheme is mainly applied in generators
with parallel windings that are separately
brought out to the terminals. It works by
comparing the currents in different parallel
winding sections of the same phase.
• This protection method is also known as
split-phase protection.
 GENERATOR PROTECTION(cont..)

(b) Protection against Stator Interturn Faults(cont..)

Working Mechanism:
✅ When the generator is healthy (No fault condition):
•The current in both parallel windings of a phase is equal.
•The CTs in each parallel winding produce equal secondary currents.
•These currents cancel out in the operating coil, and the relay does not operate.
🚫 No tripping → Normal operation continues

✅ When an inter-turn fault occurs (Fault condition):

•A fault between turns in the same winding creates an imbalance.
•The current in the affected winding reduces, while the other winding carries more current.
•This imbalance creates a differential current, which flows through the relay operating
coil.
•The relay trips the circuit breaker, isolating the generator from the system.
🚨 Relay operates → Faulty generator is disconnected

Application and Limitations:

✔ Used in hydro-generators that have parallel windings in each phase.
✔ Acts as a backup protection for phase faults.
✔ Detects stator interturn faults that are not detected by longitudinal differential protection.
 GENERATOR PROTECTION(cont..)

(C) Stator Over Heating Protection

Principle:
Stator overheating can occur due to:
•Cooling system failure (hydrogen/water cooling
malfunction)
•Overloading
•Core faults such as short-circuited laminations or core
bolt insulation failure
To prevent damage, modern generators (above 2 MW)
use two methods for overheating detection:
1.Comparing the inlet and outlet temperatures of the
cooling medium (hydrogen/water).
2.Using temperature sensors embedded in the stator
slots to detect overheating.

Working Mechanism:
✅ Temperature Sensing Method (Large Generators - Above 2 MW)
•Temperature sensors such as thermistors, thermocouples, or resistance temperature
detectors (RTDs) are embedded at various points inside the stator slots.
•These sensors are connected to a multi-way selector switch, which checks each sensor one by
one.
•A Wheatstone bridge circuit processes the signals from the sensors.
•When the temperature exceeds a preset maximum limit, the relay activates an alarm.
🚨 Alarm sounds → Operator is alerted to overheating
 GENERATOR PROTECTION(cont..)
(C) Stator Over Heating Protection(cont..)

✅ Cooling Medium Temperature Comparison (Large Generators - Above 2 MW)

•The inlet and outlet temperatures of the cooling medium (hydrogen or water) are continuously
monitored.
•If the difference between them is too high, it indicates inefficient cooling or excessive heating.
•This triggers an alarm or protective action.
🔍 Continuous monitoring → Early fault detection

✅ Bimetallic Strip Method (Small Generators)

•In small generators, a bimetallic strip is placed in the stator circuit.
•It is heated by the secondary current of the CT (Current Transformer).
•If the temperature rises excessively, the strip bends and operates the relay.
•Limitation: This method does not detect cooling system failures.
⚠️ Simple but less reliable for cooling system faults

Rotor Temperature Measurement:

•Unlike stator temperature sensors, thermocouples are not embedded in the rotor because slip ring
connections would become complex.
•Instead, rotor temperature is determined by measuring its winding resistance using an ohm-meter
type instrument, calibrated in temperature.
•This instrument is energized by the rotor voltage and current.
🛠 Accurate rotor temperature measurement without slip ring complications
 GENERATOR PROTECTION(cont..)
(ii)Rotor Protection

a) Field Ground-fault Protection

The field circuit of a generator operates ungrounded, meaning a
single ground fault does not immediately affect its operation.
However, it does increase the risk of a second ground fault,
which can cause serious damage.
A second ground fault leads to:
•A portion of the field winding getting bypassed.
•An increase in current in the remaining field winding.
•Unbalanced air-gap flux, leading to:
• Magnetic force imbalance on opposite sides of the rotor.
• Eccentricity in the rotor shaft.
• Vibrations and mechanical stress on the rotor.
•Even if the second fault does not create severe magnetic
imbalance, the arcing at the fault point causes local heating,
slowly distorting the rotor and leading to eccentricity.

Working Mechanism:
✅ Single Ground Fault Condition (No Immediate Tripping)
•A DC voltage is applied between the field circuit and earth using a polarized moving iron relay (as
shown in Fig.).
•If a single ground fault occurs, the relay detects the fault but does not trip the machine.
•Instead, an alarm is sounded, alerting operators to take corrective action.
•Immediate steps are taken to reduce the load on the faulty generator and shut it down safely before a
second fault occurs.
🚨 Alarm → Load reduction → Safe shutdown
 GENERATOR PROTECTION(cont..)
(ii)Rotor Protection

a) Field Ground-fault Protection(cont..)

✅ Second Ground Fault Condition (Severe Fault - Tripping Required)
•If a second ground fault occurs, part of the field winding is short-circuited, leading to increased current
and magnetic imbalance.
•This results in vibrations and rotor eccentricity, which can cause mechanical damage.
•To prevent further damage, the protection relay trips the generator, disconnecting it from the system.
⚠️ Relay trips → Generator shutdown → Prevents damage

Field Fault Protection in Brushless Machines:

•In brushless generators, the main field winding is inaccessible for direct monitoring.
•If a partial field failure occurs due to a short-circuited turn, it is detected by an increase in field current.
•A severe short-circuit (such as a diode failure) is detected by a relay monitoring the exciter control
circuit.
🔍 Field current monitoring → Detects faults in brushless generators

Conclusion:
Field ground-fault protection is essential for preventing rotor damage due to unbalanced magnetic forces.
✔ A single field ground fault is not critical, but it raises the risk of a second fault.
✔ Single fault → Alarm → Load transfer & shutdown (prevents severe damage).
✔ Second fault → Magnetic imbalance → Tripping required.
✔ Brushless generators use field current monitoring to detect faults.
 GENERATOR PROTECTION(cont..)
b) Loss of Excitation

Principle:
When a generator loses excitation, it stops
producing its own magnetic field and starts
operating as an induction generator. This
leads to:
•A slight increase in speed of the generator.
•Heavy induced currents in the rotor, which
can cause overheating.
•Stator overheating, due to the generator
drawing magnetizing current (reactive
power) from the system.
•A negative impact on system stability, as
the generator starts consuming reactive
power instead of supplying it.

⚠️ Round-rotor generators (without damper

windings) are more vulnerable to rotor
overheating.
✔ Salient pole generators (with damper
windings) handle the loss of excitation better,
as the damper windings carry the induced
currents.
 GENERATOR PROTECTION(cont..)
b) Loss of Excitation (cont..)
Working Mechanism:
✅ Field Failure Causes:
•Excitation system failure
•Faulty field breaker operation

✅ Effects of Field Failure:

•The generator's impedance locus shifts from the first to the fourth quadrant (as shown in
Fig.).
•This specific impedance movement is unique to loss of excitation.
•The machine draws reactive power, which can affect system stability.
✅ Protection Scheme:
•Offset mho relay or directional impedance relay is used to detect field failure.
•When the impedance trajectory moves into the characteristic zone of the relay, it detects the
fault.
•The relay trips the field breaker and disconnects the generator from the system to prevent
further damage.
🚨 Relay operates → Generator disconnected → Prevents overheating & system
instability
Application and Limitations:
✔ Large systems with automatic voltage regulators may allow the generator to run as an
induction generator for a few minutes without harm.
✔ Protection is essential for large modern generators, as a failure in excitation can lead to
severe overheating and instability.
 GENERATOR PROTECTION(cont..)
(c) Protection against Rotor Overheating because of Unbalanced Three-phase
Stator Currents

When the generator experiences unbalanced stator currents, a

negative sequence component is produced. This induces double-
frequency currents in the rotor iron, which can cause severe
overheating.
🚨 Causes of Unbalanced Stator Current:
1️⃣ Stator winding fault
2️⃣ Unbalanced external fault (not cleared quickly)
3️⃣ Open circuit in one phase
4️⃣ Failure of one circuit breaker contact
The ability of the rotor to withstand unbalanced conditions is
determined by the equation:
 GENERATOR PROTECTION(cont..)
(c) Protection against Rotor Overheating because of Unbalanced Three-phase
Stator Currents (cont..)

Working Mechanism:
✅ Negative Sequence Filter & Relay (Fig.)
•A negative sequence filter extracts the I₂ component from the unbalanced current.
•This signal is fed to an overcurrent relay that operates based on the I₂²t characteristic.
•The relay has long operating time settings (0.2 s to 2000 s) to match the generator's
tolerance.
✅ Alarm and Delayed Action:
•When the negative sequence current exceeds a set threshold, an alarm is activated.
•A time-delay relay (adjustable from 8% to 40% of I₂) prevents false alarms due to short-
duration unbalanced loads.
•If the unbalanced condition persists, the relay trips the generator to prevent rotor
overheating.
🚨 Relay operates → Alarm → Timer → If prolonged, generator trips
 TRANSFORMER PROTECTION
Types of Faults Encountered in Transformers
1. External Faults (or Through Faults)
•These faults occur outside the transformer but may still affect its operation.
•The transformer must be disconnected if primary protective devices fail to clear
these faults in time.
•Protection Methods:
• Time-Graded Overcurrent Relays: Used as backup protection for external
faults.
• Thermal Relays: Detect sustained overload conditions and provide an alarm
to prevent overheating.
2. Internal Faults
Internal faults occur within the transformer itself and require immediate attention.
They are classified into two types:
(i) Short Circuits in Transformer Windings and Connections
•These faults are severe and can cause immediate damage.
•They can be detected at the winding terminals by monitoring voltage or current
imbalances.
•Types of Short-Circuit Faults:
• Line-to-Ground Faults
• Line-to-Line Faults
• Interturn Faults (within HV and LV windings)
(ii) Incipient Faults
•These faults start as minor issues but gradually develop into major problems.
•They are not easily detected by standard short-circuit protection systems since they
do not create an immediate voltage or current imbalance.
 TRANSFORMER PROTECTION
Percentage Differential Protection
 TRANSFORMER PROTECTION
Percentage Differential Protection(cont…)
1. Purpose and Application
•Used for protecting large power transformers (5 MVA and above).
•Detects internal short circuits within the transformer.
•Limitation: Cannot detect incipient faults (gradual faults like insulation degradation or
overheating).

2. Working Principle
•Uses Current Transformers (CTs) on both primary and secondary sides of the transformer.
•Operating & Restraining Coils:
• O (Operating Coil): Activates the relay during internal faults.
• R (Restraining Coil): Prevents unnecessary tripping during normal and external faults.
•Under normal conditions or external faults:
• The CT currents from primary and secondary sides oppose each other, preventing
relay operation.
•Under internal fault conditions:
• The polarity of CT voltage on the secondary side reverses, making both currents
flow in the same direction, which activates the relay.

3. CT Connections and Phase Shift Correction

•CT connections are designed to correct the 30° phase shift in Y-Δ transformers.
•CTs on the star side of the transformer are connected in delta.
•CTs on the delta side of the transformer are connected in star.
•Why?
• Ensures correct phase alignment.
• Eliminates zero-sequence current, preventing false tripping
 TRANSFORMER PROTECTION
Percentage Differential Protection(cont..)
4. Special Cases for Star/Star Transformers
•If the star point is NOT earthed:
• CTs can be connected in star on both sides.
•If the star point is earthed:
• CTs should be connected in delta to avoid unwanted relay operation during external faults.
•General Rule:
• CTs on star windings → Delta connection.
• CTs on delta windings → Star connection.

5. Relay Settings for Transformer Protection

•Higher settings than alternators due to:
• On-load Tap Changer (OLTC):
• Transformer tap changes cause an unbalanced current in the relay.
• CT ratio remains fixed, causing variations in transformation ratio.
• No-load Current:
• A transformer has a no-load current, which could cause false tripping if settings are too low.
• Typical settings:
• Alternator Protection: 10% (operating) & 5% (bias).
• Transformer Protection: 40% (operating) & 10% (bias).
6. Conclusion
•Percentage differential protection is the most reliable method for detecting internal faults in large
transformers.
•Proper CT connections ensure phase shift correction and prevent false tripping.
•Relay settings must account for tap changers and no-load conditions to ensure stability and accurate fault
detection.
 TRANSFORMER PROTECTION
Overheating Protection
1. Transformer Rating and Temperature Considerations
•Transformer rating depends on temperature rise above an assumed maximum ambient temperature.
•Sustained overload is not allowed if the ambient temperature reaches the assumed maximum.
•Overloading is permissible at lower ambient temperatures, but it must not cause winding overheating.
•The maximum allowed winding temperature is 95°C.
2. Protection Against Overload
•Overload protection is based on winding temperature measurement.
•The thermal image technique is used to monitor and control winding temperature.
3. Thermal Image Technique
•A temperature sensing device is placed in transformer oil near the top of the tank.
•A Current Transformer (CT) on the LV side supplies current to a small heater placed in a pocket inside
the transformer.
•The heater mimics the temperature rise of the main winding, ensuring accurate temperature tracking.
4. Use of Silistor for Temperature Sensing
•A silistor (silicon resistor) is used as a heat-sensitive element.
•It is integrated with the heating element in a thermal-molded material, forming a thermal replica of the
transformer winding.
•Placement:
• Installed 25 cm below the transformer tank top, where the oil temperature is highest.
5. Temperature Measurement and Control
•The silistor acts as an arm of a resistance bridge supplied by a stabilized DC source.
•Functions of the system:
• An indicating instrument measures temperature based on the bridge’s out-of-balance voltage.
• Cooling system control: Activates cooling pumps and fans to manage overheating.
• Overheat warnings: Generates alarms for overheating conditions.
• Transformer trip mechanism: If temperature exceeds the safe limit, it trips the transformer circuit
breakers to prevent damage.
 TRANSFORMER PROTECTION
Protection Against Magnetizing Inrush Current
1. What is Magnetizing Inrush Current?
•When an unloaded transformer is switched on, it draws a large initial magnetizing current.
•This initial current can be several times the rated current of the transformer.
•Since the inrush current flows only in the primary winding, the differential protection may
misinterpret it as an internal fault.
2. Characteristics of Inrush Current
•Harmonic content in inrush current differs from a fault current:
• DC component: 40–60%
• Second harmonic: 30–70%
• Third harmonic: 10–30%
• Higher harmonics progressively decrease.
•Third harmonic and its multiples do not appear in CT leads due to cancellation in delta winding.
•Second harmonic content is significantly higher in inrush current than in fault current, making it
useful for distinguishing between them.
3. Harmonic Restraint Differential Protection
•A high-speed biased differential relay with a harmonic restraint feature is used.
•Working principle:
• Filters out harmonics from the differential current.
• Rectifies them and adds them to the percentage restraint.
• A tuned circuit (XCXL) allows only fundamental frequency current to flow through the operating
coil.
• DC and second harmonics are diverted into the restraining coil.
• Relay does not operate if the second harmonic exceeds 15% of the fundamental current.
•Operating time: About 2 cycles.
 TRANSFORMER PROTECTION
Protection Against Magnetizing Inrush Current(cont..)
4. Limitations and Overcoming Challenges
•Harmonics and DC offsets also appear in
fault current, especially if CT saturation
occurs.
•A harmonic restraint relay may fail to
detect a fault if significant harmonics are
present.
•Solution: Use an instantaneous
overcurrent relay (high-set unit):
• Set above the maximum inrush
current.
• Operates within 1 cycle during
heavy internal faults.
5. Alternative Method: Harmonic Blocking
Scheme
•Uses a separate blocking relay whose
contacts are in series with the biased
differential relay.
•Blocking relay operation:
• It blocks the differential relay when
second harmonic is more than 15%
of fundamental.
• If second harmonic falls below 15%,
it allows the differential relay to trip.
 TRANSFORMER PROTECTION
Buchholz Relay
1. Purpose of Buchholz Relay
•It is a gas-actuated relay used to detect incipient faults in transformers.
•Incipient faults are minor initial faults that can develop into major faults over time.
•It is used as a supplementary protection to biased differential protection.
•It cannot detect short circuits inside the transformer winding or at the terminals.

2. Working Principle
•Slowly developing faults generate heat, which decomposes the insulation materials (solid or liquid).
•This decomposition produces inflammable gases, which accumulate inside the relay chamber.
•When a certain amount of gas is collected, the relay activates an alarm.
•The type of gas formed helps in identifying the nature of the fault.

3. Gas Analysis for Fault Diagnosis

4. Construction and Placement

•The Buchholz relay is installed between the transformer tank and the conservator.
•It consists of a chamber that accommodates the relay and floats that detect gas accumulation.
 TRANSFORMER PROTECTION
Buchholz Relay(cont..)

5. Operating Mechanism
•Gas Accumulation:
• When gas accumulates, the oil level drops, causing the float to move down.
• This triggers an alarm to alert the operator.
•Severe Faults:
• A large gas volume activates the lower float, which trips the circuit breakers to
disconnect the transformer.
 BUSZONE PROTECTION
Differential Current Protection
 BUSZONE PROTECTION
Differential Current Protection(cont..)
1. Principle of Operation
•Based on Kirchhoff’s Current Law (KCL):
• The algebraic sum of all currents entering and leaving the busbar zone must be zero.
• If the sum is not zero, it indicates a fault within the busbar zone.
•The relay is connected to trip all circuit breakers in case of a bus fault.

2. Working Mechanism
•Under Normal Conditions:
• The sum of the currents entering and leaving the bus remains zero.
• The relay does not operate.
•During a Bus Fault:
• The current sum becomes non-zero, indicating a fault inside the busbar zone.
• The relay trips the circuit breakers, isolating the faulty section.

3. Drawback of Basic Differential Scheme

•A false operation may occur during an external fault due to CT saturation.
•When a CT saturates, its secondary current is reduced, disturbing the balance.
•This leads to an incorrect non-zero sum, causing unwanted relay operation.

4. Solutions to Prevent False Operation

•High Impedance Relay Scheme:
• Uses a resistor to limit the effect of CT saturation.
• Ensures that the relay operates only for internal faults.
•Biased Differential Scheme:
• Introduces a restraining current to distinguish between internal and external faults.
• Prevents relay operation for external faults, even if CT saturation occurs.
 BUSZONE PROTECTION
High Impedance Relay Scheme
 BUSZONE PROTECTION
High Impedance Relay Scheme(cont..)
1. Principle of Operation
•Employs a high impedance relay for differential protection.
•A sensitive DC polarized relay is used in series with a tuning circuit.
•The relay is designed to respond only to the fundamental component of the
differential (spill) current from the CTs.

2. Features of the Tuning Circuit

•Filters out DC and harmonics, making the relay stable even under heavy external
faults.
•Ensures the relay operates only during internal faults by responding only to the
fundamental frequency.

3. Protection Against Excessive Voltages

•Non-linear resistance (Thyrite):
• Prevents high voltage surges during internal faults.
•High set overcurrent relay:
• Connected in series with the non-linear resistance.
• Provides fast operation during heavy faults.
• Its pick-up value is set high to prevent false operation on external faults.

4. Advantages of High Impedance Protection

•More stable during external faults compared to conventional differential protection.
•Eliminates false trips caused by CT saturation.
•Fast operation under severe internal faults due to the high set relay.
 FRAME LEAKAGE PROTECTION
1. Principle of Operation
•Used for metal-clad switchgear
installations.
•The frame is insulated from the
ground, and insulation above 10
ohms is acceptable.
•Effective for isolated-phase
construction type switchgear, where
all faults involve the ground.

2. Working Mechanism
•The relay detects leakage currents
flowing through the switchgear frame.
•If a fault occurs, the leakage current
passes through the relay, triggering
protection.
3. Components of the Scheme
•Check relay:
• Prevents false operation due to spurious currents.
• It is energized from a CT connected in the neutral of the system.
•Overcurrent relay:
• If a neutral check relay is used, an instantaneous overcurrent relay is employed.
• If there is no neutral check relay, an inverse time delay relay is used for protection.

4. Advantages of Frame Leakage Protection

•Highly effective for indoor installations.
•Prevents false trips by using a check relay to filter spurious currents.
•Better suited for metal-clad switchgear, ensuring enhanced ground fault detection.