In a generating station the generator and
transformer are the most expensive equipments
and hence it is desirable to employ a protective
system to isolate the faulty equipment as quickly
as possible to keep the healthy section in normal
operation and to ensure uninterruptable power
The basic electrical quantities those are likely to
change during abnormal fault conditions are
current, voltage, phase angle and frequency .
Protective relays utilizes one or more of these
quantities to detect abnormal conditions in a power
Protective system cost is 4-5%of the total cost
Switchgear is a general term covering a wide range
of equipments concerned with switching and
Eg: Circuit breaker, Isolator, Earth switch etc.
1. Generator or Generator-Transformer Units
4. Lines (transmission and distribution)
5. Utilization equipment (motors, static loads, etc.)
6. Capacitor or reactor (when separately protected)
Unit Generator-Tx zone
XFMR Bus Line Bus XFMR Bus Motor
MAIN EQUIPMENT FOR
• Voltage (potential) transformers are used to isolate and step down
and accurately reproduce the scaled voltage for the protective
device or relay
• VT ratios are typically expressed as primary to secondary;
• A 4160:120 VT has a “VTR” of 34.66
• Current transformers are used to step primary system currents to
values usable by relays, meters, SCADA, transducers, etc.
• CT ratios are expressed as primary to secondary; 2000:5, 1200:5,
• A 2000:5 CT has a “CTR” of 400
Act at Abnormal condition.
Use system supply.
NATURE OF FAULTS IN
Tends to deteriate with rising temp.
Insulation failure may cause inter-turn fault, ph to ph or
Bring winding in to direct contact with core plates.
Any failure to restrict earth fault may result into core
Insulation of rotor winding is also important.
Fault Occur In Generator
• Stator Fault
• Rotor fault
• Abnormal Running Condition
1) Unbalanced Loading
2) Over loading
3) Over Speed
4) Over Voltage
5) Failure of Primer Mover
6) Loss Of Excitation
7) Excessive vibration
8) Difference in expansion between rotating
and stationary parts
9) Loss of synchronism
PROTECTION APPLIED TO
Relays to detect faults outside generator
Relays to detect faults in side generator
Over speed protections.
Temp measuring device for bearings, stator
winding, Oil temp.
Prevents shock exposure of personnel
Provides current carrying capability for the ground-fault current
Grounding includes design and construction of substation
ground mat and CT and VT safety grounding
Limits difference in electric potential through local area conducting
Reactance Coil Grounded
High Z Grounded
Low Z Grounded
1. Ungrounded: There is no intentional
ground applied to the system-however
it’s grounded through natural
capacitance. Found in 2.4-15kV
2. Reactance Grounded: Total system
capacitance is cancelled by equal
inductance. This decreases the current
at the fault and limits voltage across the
arc at the fault to decrease damage.
X0 <= 10 * X1
3. High Resistance Grounded: Limits
ground fault current to 10A-20A. Used
to limit transient overvoltages due to
arcing ground faults.
R0 <= X0C/3, X0C is capacitive zero
4. Low Resistance Grounded: To limit
current to 25-400A
R0 >= 2X0
5. Solidly Grounded: There is a
connection of transformer or generator
neutral directly to station ground.
Effectively Grounded: R0 <= X1, X0 <=
3X1, where R is the system fault
Stator faults include the following-
i. Phase-to-earth faults
ii. Phase-to-phase faults
iii. Inter-turn faults
From these phase faults and inter turn faults are less
common ,these usually develop into an earth faults.
• Arcing to core
• Damage of conductor and insulation
Stator inter-turn fault
• Inter-turn fault on the same phase of the stator
winding cannot be detected by transverse
differential protection as it does not disturb the
balance between the currents in neutral and high
• For protection against inter-turn faults the
following protection schemes are used.
(1)Cross differential protection.
(2)Residual voltage protection.
Over voltage relay
With time delay
STATOR EARTHFAULT RELAY
ROTOR E/F RELAY
Rotor earth fault protection:
• DC injection method or AC injection method.
• The dc or ac voltage is impressed between the field
circuit and ground through a sensitive overvoltage
relay and current limiting resistor or capacitor(in case
• But dc source is generally used as over-current relay in
case of dc is more sensitive than ac.
• A single earth fault in rotor circuit will complete the
path and the fault is sensed by the relay.
Rotor earth fault protection
AC Injection method
Rotor temperature alarm
• It is provided in large
• It indicates the level of
temperature but not the
actual hot spot
• The relay measures the
measuring the resistance
.(as shown in fig)
• Abnormal Operating Conditions
Loss of Field
Loss of Field
Loss of excitation protection:
When the excitation of generator is lost it operate as a
Induction generator. It derives excitation from the
system and supply power at leading power factor.
Which may cause-
A fall in voltage & so loss of synchronism & system
Over heating of rotor due to induction current on it.
A protection having MHO characteristic
is used to detect loss of field.
• Generally protection is made for 80 to 85% of the
• If any fault occurs near the neutral point then the fault
current is very small and relay does not operate.
• Modified differential protection scheme is used to over
• Two phase elements (PC and PA) and balancing
resistor(BR) is connected in star and the earth relay(ER)
is connected between the star point and neutral pilot
External fault back up
• Over-current and earth-fault protection is
provided for back-up protection of large sized
generators protected by differential
• Induction type IDMT relay is used for this
Reverse power relay
Reverse power relay scheme
REVERSE POWER PROTECTION
Failure of the prime mover of a generator set ,will
keep the set running as a synchronous
compensator, taking the necessary active power
from the net work and could be detrimental to to the
safety of the set, if maintained for any length of time.
The amount of power taken will depend on the type
of prime mover involved. It ranges from 5% to 25%.
Negative phase sequence protection
Negative phase sequence
• Unbalance may cause due to single phase
fault or unbalanced loading and it gives rise to
negative sequence current .
• This current in rotor causes rotor overheating
and damage to the rotor.
• This can be protected by negative sequence
current filter with over current relay.
Field failure protection
FIELD FAILURE PROTECTION
Loss of generator field excitation under normal running
conditions may arise due to any of the following condition.
1. Failure of brush gear.
2. unintentional opening of the field circuit breaker.
3. Failure of AVR.
When generator on load loses it’s excitation , it starts to
operate as an induction generator, running above synchronous
speed. cylindrical rotor generators are not suited to such
operation , because they do not have damper windings able to
carry the induced currents, consequently this type of rotor will
overheat rather quickly.
Over voltage protection:
Overvoltage protection is required in case of hydro-
electric or gas turbine generators but not in case of turbo
Over voltage may be caused due to-
Transient over voltage in the transmission line due to
Defective operation of the voltage regulator.
Sudden loss of load due to line tripping.
The protection is provided with an over voltage relay.
It is usually of induction pattern with an IDMT
• Overloading of the machine causes overheating in the
• This can be prevented by using over-current relay with
time delay adjustment.
• But overheating not only depends on over-current but
also the failure of the cooling system in the generator.
• So temperature detector coils such as thermistors or
thermocouples are used at various points in stator
winding for indication of the temperature.
Name Input Protecting to
Differential protection Differential Current Stator core and winding
Stator earth fault Voltage Stator core and winding
Over current Current Stator core and winding
Over voltage Voltage Stator core and winding
Interturn short circuit Current Stator core and winding
Rotor Earth Fault Current Rotor winding
Over and under
Frequency Turbine protection
Reverse power flow Voltage and current Turbine protection
Loss of excitation Voltage and current Power System Protection
Back up protection for
Voltage and current Generator protection