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Main equipment in the power plant is Generator. It's cost is much higher than any other equipment so we will have to protect the generator from all the possible faults and errors.
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Introduction: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 supply.
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 system.
Protective system cost is 4-5%of the total cost
SWITCHGEAR Switchgear is a general term covering a wide
range of equipments concerned with switching and protection.
Eg: Circuit breaker, Isolator, Earth switch etc.
DESIRABLE PROTECTION ATTRIBUTES
Reliability SelectivitySpeed Simplicity Economics
PROTECTION ZONES 5
GE Consumer & Industrial
Multilin
1. Generator or Generator-Transformer Units2. Transformers3. Buses4. Lines (transmission and distribution)5. Utilization equipment (motors, static loads, etc.)6. Capacitor or reactor (when separately protected)
Unit Generator-Tx zoneBus zone
Line zone
Bus zone
Transformer zone Transformer zone
Bus zone
Generator
~
XFMR Bus Line Bus XFMR Bus Motor
Motor zone
MAIN EQUIPMENT FOR SWITCHGEAR OPERATION
Current transformer Potential transformer Relay Circuit breaker
7GE Consum
er & IndustrialM
ultilinVVPP
VVSS
Relay
• 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; 14400:120, 7200:120
• A 4160:120 VT has a “VTR” of 34.66
Voltage Transformers
8GE Consum
er & IndustrialM
ultilin
• 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, 600:5, 300:5
• A 2000:5 CT has a “CTR” of 400
Current Transformers
Alarm Act at Abnormal condition. Disconnect. Fast operation. Use system supply.
Simple electromechanical relay
1. Reed relay2. Latching relay3. Solid state relay4. Solid state contact relay5. Ratchet relay6. Coaxial relay7. Overload protection relay8. Forced guided contact relay9. Buchholz relay
•It like a fuse•It is a switch•Interrupt the faulty part•Operation
1.Voltage class2.Current rating3.Type of circuit breaker
1.Air breaker circuit breaker2.Miniature circuit breaker3.Air blast circuit breaker4.SF6 circuit breaker5.Low oil circuit breaker 6.Vaccum circuit breaker
SF6 CIRCUIT BREAKER
PLANT LAYOUT
GEN
UAT
AVR
220 kv bus
220 kv HVCB
6.6 KV CB
NGT
10.5 KV
220 KV
GT
EXT TR
SER TR
415 V AC
LA
Single line Diagram of generator connection
GENERATOR THEORY GENERAL OVERVIEW AND TYPICAL SYSTEM
500 MW TG ON TEST BED
NATURE OF FAULTS IN GENERATOR
Insulation failure. Tends to deteriate with rising temp. Insulation failure may cause inter-turn fault, ph
to ph or earth fault. Bring winding in to direct contact with core
plates. Any failure to restrict earth fault may result into
core plate damage. Insulation of rotor winding is also important.
Fault Occur In Generator • Stator Fault• Rotor fault• Abnormal Running Condition
1) Unbalanced Loading2) Over loading3) Over Speed4) Over Voltage5) Failure of Primer Mover6) Loss Of Excitation7) Excessive vibration8) Difference in expansion between
rotating and stationary parts9) Loss of synchronism
PROTECTION APPLIED TO GENERATOR 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.
EQUIPMENT GROUNDING
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
SYSTEM GROUNDING
Limits overvoltages Limits difference in electric potential through local area
conducting objects Several methods
Ungrounded Reactance Coil Grounded High Z Grounded Low Z Grounded Solidly Grounded
SYSTEM GROUNDING25
GE Consumer & Industrial
Multilin
1. Ungrounded: There is no intentional ground applied to the system-however it’s grounded through natural capacitance. Found in 2.4-15kV systems.
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
SYSTEM GROUNDING26
GE Consumer & Industrial
Multilin
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 sequence reactance4. Low Resistance Grounded: To
limit current to 25-400AR0 >= 2X0
SYSTEM GROUNDING27
GE Consumer & Industrial
Multilin
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 resistance
generator
NGT NGR RELAY
GENERATOR EARTHING
Stator protection:Stator faults include the following-i. Phase-to-earth faultsii.Phase-to-phase faultsiii.Inter-turn faultsFrom these phase faults and inter turn faults are
lesscommon ,these usually develop into an earth
faults.This causes-• Arcing to core• Damage of conductor and insulation
INTER-TURN FAULT PROTECTION
Stator inter-turn fault protection:
• 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 voltage CTs.
• For protection against inter-turn faults the following protection schemes are used.
(1)Cross differential protection. (2)Residual voltage protection.
mmmmmm
mmmmmm
mmmmmm
mm
mm
mm
mm
mm
mm
Loading resistor Over voltage relay
With time delay
STATOR EARTHFAULT RELAY
exciter
P.B
Field wdg
Voltage 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 of ac).
• 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
GENERATOR PROTECTION
1 ST ROTOR E/F PROTECTION (64R1)
D.C. INJECTION METHOD.
Rotor temperature alarm
• It is provided in large generators.
• It indicates the level of temperature but not the actual hot spot temperature.
• The relay measures the temperature by measuring the resistance .(as shown in fig)
GENERATOR PROTECTION
• Abnormal Operating Conditions
The "Wild"PowerSystem
G
Exciter
Loss of FieldLoss of Field
Overexcitation
Overexcitation
Overexcitation
OpenCircuits
Loss ofSynchronism
InadvertentEnergizing,Pole Flashover
AbnormalFrequency
AbnormalFrequency
BreakerFailure
ReversePower
OverPower
Loss of excitation protection:When the excitation of generator is lost it operate as aInduction generator. It derives excitation from thesystem and supply power at leading power factor. Which may cause- A fall in voltage & so loss of synchronism & system
instability. Over heating of rotor due to induction current on it.A protection having MHO characteristicis used to detect loss of field.
Differential protection of generator:
Differential protection using balancing resistor:
Modified differential protection
Modified differential protection:
• Generally protection is made for 80 to 85% of the winding.
• 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 come this.
• 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 wire.
External fault back-up protection
External fault back up protection:
• Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection.
• Induction type IDMT relay is used for this purpose.
STEAM VALVE
C.B TRIP
Protective relay
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%.
m
m
46
mm
Zc ZA
A
BC
Ia
IbIc
VZC
VZA
POSITIVE SEQ
Ia
IbIc
VZC
VZAVZA+VZC
X Y
NEGATIVE SEQUENCE
Negative phase sequence protection
Negative phase sequenceprotection:
• 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.
Negative phase sequence protection:
Exciter
FUSE T1
T2
FUSE
TRIP
SHUNT
FILED WDG
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 generators.
Over voltage may be caused due to- Transient over voltage in the transmission line
due to lightening. 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 IDMTCharacteristic
Overcurrent protection:
• Overloading of the machine causes overheating in the stator winding.
• 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.
GENERATOR PROTECTIONName Input Protecting to
Differential protection
Differential Current Stator core and winding
Stator earth fault Voltage Stator core and windingOver current Current Stator core and windingOver voltage Voltage Stator core and winding
Interturn short circuit Current Stator core and windingRotor Earth Fault Current Rotor windingOver and under
frequencyFrequency Turbine protection
Reverse power flow Voltage and current
Turbine protection
Loss of excitation Voltage and current
Power System Protection
Back up protection for lines
Voltage and current
Generator protection