Generator Protection Schemes


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The following are the main protection schemes adopted for our generator.
1. Generator Differential Protection
2. Generator & Transformer Differential Protection
3. Loss of Field or Loss of Excitation Protection
4. Negative Sequence or Current Unbalance Protection
5. Over Fluxing or Over Excitation Protection
6. Over Current Protection
7. Stator Earth Fault Protection
8. Rotor Earth Fault Protection (64R)
9. Restricted Earth Fault Protection
10. Backup Impedance Protection
11. Low Forward Power Protection
12. Reverse Power Protection
13. Pole Slip Protection
14. Pole Discrepancy Protection
15. Local Breaker Back Protection
16. Bus Bar Protection
17. Over Frequency Protection
18. Under Frequency Protection
19. Over Voltage Protection


1.GENERATOR DIFFERENTIAL PROTECTION:
Setting : 0.5 Amp Time : Instantaneous
It is one of the important protections to protect generator winding against internal faults such as phase-to-phase and three phase-to-ground faults. This type of fault is very serious because very large current can flow and produce large amounts of damage to the winding if it is allowed to persist. One set current transformers of the generator on neutral and phase side, is exclusively used for this protection. The differential protection can not detect turn-to-turn fault and phase to ground within one winding for high impedance neutral grounding generator such as ours. Upon the detection of a phase-to-phase fault in the winding, the unit is tripped with out time delay.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.

Once the differential protection operated, the unit can not be taken into service unless the generator winding is thoroughly examined by the maintenance staff of any internal faults

2.GENERATOR-TRANSFORMER DIFFERENTIAL PROTECTION :
Setting : 0.75 Amp Time : Instantaneous
It protects 11KV bus duct, 11/0.440KV unit auxiliary transformer, 11/20KV step-up transformer against internal faults such as phase-to-phase and three phase-to-ground faults. This type of fault is very serious because very large current can flow and produce large amounts of damage to the winding if it is allowed to persist. One set current transformers of the generator on neutral side and another set current transformer on 220KV side after transformer, is exclusively used for the protection. Upon the detection of difference in current between these current transformers, the unit is tripped with out time delay.One the generator-transformer differential protection operated, the unit can not be taken into service unless the 11KV bus duct, unit auxiliary transformer, power transformer are thoroughly examined by the maintenance staff for any internal faults.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.


3.LOSS OF FIELD OR EXCITATION PROTECTION :
Setting : K1-2, K2-1, K3-2 Trip after 2 Sec.
When the synchronous machine with excitation, is connected to the grid, it generates reactive power along with active power to the grid and the rotor speed is same as that of grid frequency. Loss of field or loss of excitation results in loss of synchronism between rotor flux & stator flux. The synchronous machine operates as an induction machine at higher speed and draws reactive power from the grid. This will result in the flow of slip frequency currents in the rotor body as well as severe torque oscillations in the rotor shaft. As the rotor is not designed to sustain such currents or to withstand the high alternating torques which results in rotor overheating, coupling slippage and even rotor failure.
A loss of excitation normally indicates a problem with the excitation system. Some times it may be due to inadvertent tripping of filed breaker, open or short circuit of field winding or loss of source to the exciter. If the generator is not disconnected immediately when it loses excitation wide spread instability may very quickly develop and major system shutdown may occur.
When loss of excitation alarm annunciates at annunciation panel, the machine may probably be running with less excitation at leading MVAR power. Increase the excitation on the machine until it reaches on lagging MVAR power. The machine trips on the same protection along with alarm resynchronize the machine and try to stabilize at required MVAR power. If not possible, trip the machine immediately and inform to the maintenance staff for thorough checking of the Automatic Voltage Regulator (AVR) and its associated parts.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.

4.NEGATIVE SEQUENCE OR CURRENT UNBALANCE PROTECTION :
Setting : Alarm – 75% of 12s Time - 5 Sec.
Trip – 75% of 12s Time - 300 Sec.
When the machine delivering the equal currents in three phases, no unbalance or negative phase sequence current is produced as the vector sum of these currents is zero, when the generator is supplying an unbalanced load to a system, a negative phase sequence current is imposed on the generator. The system unbalance may be due to opening of lines, breaker failures or system faults. The negative sequence current in the stator winding creates a magnetic flux wave in the air gap which rotates in opposite direction to that of rotor synchronous speed. This flux induces currents in the rotor body, wedges, retaining rings at twice the line frequency. Heating occurs in these areas and the resulting temperatures depend upon the level and duration of the unbalanced currents. Under these conditions it is possible to reach temperatures at which the rotor material no longer contain the centrifugal forces imposed on them resulting in serious damage to the turbine-generator set. Any machine as per design data will permit some level of negative sequence currents for continuous period.
An alarm will annunciate at annunciation panel if negative sequence currents exceeds a normal level. Reduce the MVAR power on the machine if necessary load also and keep the machine for some time till the alarm vanishes at annunciation panel. If the machine trips on the Negative sequence protection never take the machine into service until the temperatures on the rotor parts settle down to its lower value. Resynchronize the machine to the grid after considerable time under grid & feeder parameters are within limits. If the unit trips again on the same protection, stop the machine after consideration time so as to cool down the rotor parts and inform to the maintenance staff for thorough examination of the system.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
Status : a. Unit is at FSNL.

5. OVER FLUXING OR EXCITATION OR VOLTS PER HERTZ PROTECTION:
Setting : Alarm – 1.17 Time - 10 Sec.
Trip – 1.17 Time - 30 Sec.
Per unit voltage divided by per unit frequency commonly called Volts/Hertz is a measurable quantity that is proportional to flux in the generator or step-up transformer cores. Moderate over fluxing (105-110%) increases core loss resulting in increase of core temperatures due to hysterics & eddy currents loss. Long term operation at elevated temperatures can shorten the life of the stator insulation. Severe over fluxing can breakdown inter-laminar insulation followed by rapid local core melting. Over fluxing normally can be caused by over speed of the turbine or over excitation during Off-line condition, and load rejection or AVR mal-functioning during On-line condition.
If alarm annunciation panel, Increase/Reduce the speed of the turbine to rated generator speed (3000RPM) and reduce the generator voltage to rated during Off-line condition. Reduce the MVAR power on the generator during On-line condition. If the machine trips on over fluxing protection during On-line, Keep the machine at FSNL till the grid parameters stabilize and resins. Again the machine trips on the same stop the machine for examination of the AVR & Governor systems by maintenance staff.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.

6.OVER CURRENT WITH VOLTAGE RESTRAINT PROTECTION :
Setting : Alarm – 85% Time - 10 Sec.
Trip – 100% Time - 0.5 Sec.
Normally generators are designed to operate continuously at rated MVA, frequency and power factor over a range of 95 to 105% rated voltage. Operating the generator at rated MVA with 95% voltage, 105% stator current is permissible. Operating of the generator beyond rated KVA may result in harmful stator over current. A consequence of over current in winding is stator core over heating and leads to failure of insulation.
If alarm annunciates at annunciation panel, Reduce the stator current to the below the rated by reducing the MVAR power on the machine. When the trips on the same protection, Resins the machine after keeping the machine at FSNL for some time, and keep the stator current below the rated.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
Status : a. Unit is at FSNL.

7 STATOR EARTH FAULT PROTECTION :
Setting : 70% Time - 5 Sec.
Normally the generator stator neutral operates at a potential close to ground. If a faulty phase winding connected to ground, the normal low neutral voltage could rise as high as line-to-neutral voltage depending on the fault location. Although a single ground fault will not necessarily cause immediate damage, the presence of one increases the probability of a second. A second fault even if detected by differential relay, may cause serious damage. The usual method of detection fault is by measuring the voltage across the secondary of neutral grounding transformer (NGT). Here are two over lapping zones to detect stator ground faults in a high impedance grounded generator system, the two zones are put together cover 100% stator winding for earth faults. A fundamental frequency neutral over voltage relay covers about 0-95% of the stator zonal winding for all faults except those near the neutral. Another third harmonic neutral under voltage relay covers remaining 96-100% of the stator zone 2 winding on neutral side.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
8.ROTOR EARTH FAULT PROTECTION (64R) :
Settings : Less than 80K ohm
Any rotor field winding of the generator is electrically isolated from the ground. Therefore the existence of one ground fault in the field winding will usually not damage the rotor. However the presence of two or more ground faults in the winding will cause magnetic and thermal imbalance plus localized heating and damage to the rotor metallic parts. The rotor earth fault may be caused due to insulation failure of winding or inter-turn fault followed by localized heat.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
9.RESTRICTED EARTH FAULT PROTECTION:
Settings : 0.1 Amp. Time : Instantaneous
It is similar to generator differential protection in working. It protects the high voltage winding of 11/220KV power transformer against internal faults. One set current transformers of the power transformer on neutral and phase side, is exclusively used for this protection. The protection can not detect turn-to-turn fault within one winding. Upon the detection of a phase-to-phase or phase-to-ground fault in the winding, the unit to be tripped without time delay.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
Once the restricted earth fault protection operated, the unit can not be taken into service unless the transformer winding is thoroughly examined by the maintenance staff for any internals faults.
10.BACKUP IMPEDANCE PROTECTION:
Settings ; K1-3, K2-0.71 Time – 1.5 Sec.
As in name implies, it is used to protect the generator from supplying the over loaded or faulty system. It is backup protection of the generator over current protection. In measures ratio of the voltage and current supplied by the generator and initiates trip signal when the measured impedance is less than the preset value.
If the machine trips on the Backup protection, never take the machine into service until the temperatures of the generator settle down to its lower value. Resynchronize the machine to the grid after considerable time when grid & feeder parameters are within limits.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
11.LOW FORWARD POWER PROTECTION:
Setting : 0.5% Time : 1 Sec.
The generator will not develop output power when turbine input is less than the no load losses and motoring action develops on the turbine. The generator is able to generate power, usually 55 to 10% of generator capacity, within pre-determined time after closing of 220KV breaker.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
Status : a. Unit is at FSNL with potential.
The unit trips on the low forward protection, Resins the machine and increase input power to the turbine as quickly as possible within low forward power time setting. Even after two to three attempts, the machine is tripping on the same protection; probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same.
12.REVERSE POWER PROTECTION:
Setting : 0.5% Time - 2.0 Sec.
It is backup protection to the low forward protection. Motoring of a generator will occur when turbine output is reduced such that it develops less than no-load losses while the generator is still on-line, the generator will operate as a synchronous motor and driving the turbine. The generator will not be harmed by synchronous motoring and a steam turbine can be harmed through over heating during synchronous motoring if continued long enough. The motoring of the turbine output can be detected by reverse power protection. The avoid false tripping due to power swings a time delay is incorporated before tripping signal is generated.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
The unit trips on the reverse power protection. Resins the machine and increase the input power to the turbine as quickly as possible within low forward power time setting. Even after two to three attempts, the machine is tripping on the same protection; probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same.

13.POLE SLIP OR OUT-OF-STEP PROTECTION:
Setting : 6.9 ohm.
When a generator loses synchronism, the resulting high current peaks and off-frequency operation may cause winding stresses, pulsation torques and mechanical resonances that have the potential danger to turbine generator. Therefore, to minimize the possibility of damage, it is generally accepted that the machine should be tripped without time delay preferably during the first half-slip cycle of the loss of synchronism condition. The electrical center during loss-of-synchronous conditions can occur in the generator as a result of increased impedance of the generator while decrease system impedance. The protections normally applied in the generator zone such as back-up impedance, loss of excitation etc., will not protect a generator during loss of synchronism under normal generator conditions.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
The unit trips on the Pole slip protection, Resynch the machine after stabilization of the grid parameters

14.POLE DISCREPANCY PROTECITON:
Setting : 0.5 Sec.
If One or two poles of generator breaker fail to close during synchronization, all poles of the breaker trip on this protection. It may be due to mechanical failure of the breaker un equal distribution of closing signal to the breaker from protection system.
Relays acted : a. Flag operation at 220KV Breaker panel.
b. Indication at Annunciation Panel.
Consequences : a. tripping of 220KV breaker
Status : a. Unit is at FSNL with potential.
The generator breaker trips on the pole discrepancy protection, Resynch the generator. Even after two to three attempts, the machine is tripping on the same protection, probably the generator breaker is faulty. Inform to maintenance staff for rectification of the same.

15.LOCAL BREAKER BACKUP PROTECTION:
Setting : 25% Time : 0.8 Sec.
For most of the faults, the generator breaker involves tripping the generator from the system. Failure of the breaker to open probably results in loss of protection and other problems such as motoring action or single phasing, If one or two poles of the generator breaker fail to open due to mechanical failure in breaker mechanism, the result can be a single phasing and negative phase sequence currents inducted on the rotor. The LBB protection is energized when the breaker trip is initiated after a suitable time interval if confirmation of the confirmation of breaker tripping from three poles is not received. The energized tripping signal from LBB protection will trip all 220KV generator breakers and all 220KV feeder breakers through Bus-bar protection.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay for all units.
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker of all units.
Status : a. all Units are at FSNL.
Once the LBB protection operated, the entire station is in dark. First restore all essential services to all units such as lube oil system and turning gear etc., from battery backup and. Checkup the faulty 220KV breaker and isolate the breaker from the system by opening the both side of the isolators.
After restoring all services from station supply, Close 220KV feeder breakers first and take all units into service one after the other duly co-coordinating with the DE/LD.
Since it involves complex operation, it is necessary to get help from maintenance staff for restoring the normally in the station. Never attempt to close the faulty 220KV generator in panic as it causes permanent damage to the generator and transformer.

16.BUS BAR PROTECTION:
Setting : 0.8 Amp.
There are mainly three protection zones namely called generator zone, bus duct transformer zone, 220KV breakers zone. The protection of generator zone and bus duct & transformer zone are covered in previous schemes. All 220KV breakers at switchyard will come under Bus-Bar protection. Functioning of this scheme is similar to the generator differential protection or generator-transformer differential protection. It measures all incoming currents from the generators at 220KV side and all outgoing currents in 220KV feeders, and initiates trip signal if it detects any deviation more than the preset value as the algebraic sum of all currents at 220KV bus must be less than the preset value. It isolates all 220KV generator breakers and all 220KV feeder breakers connected to 220KV bus.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay for all units.
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker of all units.
Status : a. all Units are at FSNL.
Once the Bus-Bar protection operated, the entire station is in dark. First restore all essential services to all units such as lube oil system and turning gear etc., from battery backup and 6.6/0.44KV Stage – II reserve power supply. Checkup the entire 220KV switch yard for any wire snapping or equipment damage.
After restoring all services from station supply, Close 220KV feeder breakers first and take all units into service one after the other duly co-ordinating with the DE/LD.
Since it involves complex operation, it is necessary to get help from maintenance staff for restoring the normalcy in the station. Never attempt to restore the 220KV supply at switch yard in panic unless the entire system is thoroughly examined and satisfy yourself as it causes permanent damage to the equipment or injury/death to the person working at switch yard.

17.OVER FREQUENCY PROTECTION:
Setting : 52 Hz Time - 2 Sec.
For a generator connected to a system, abnormal frequency operation is a result of a severe system disturbance. The generator can tolerate moderate over frequency operation provided voltage is within an acceptable limits. The machine operated at higher speeds at which the rotor material no longer contain the centrifugal forces imposed on them resulting in serious damage to the turbine-generator set. The abnormal over frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller or severe grid disturbance or sudden load through off.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker

c. Stop command to Turbine thro’ Mark-IV
Status : a. Unit is at coasting down.
The unit trips on the over frequency protection, Resins the machine. Even after two to three attempts, the machine is tripping on the same protection; probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same.

18.UNDER FREQUENCY PROTECTION:
Setting : 48 Hz Time : 2.0 Sec.
For a generator connected to a system, under frequency operation is a result of a severe system disturbance. The generator can tolerate moderate under frequency operation provided voltage is within an acceptable limits. The machine operated at lower higher speeds causes severe over fluxing in the generator-transformer. The abnormal under frequency on the machine may be due to improper speed control adjustment or disoperation of the speed controller.
Relays acted : a. Flag operation at Protection panel.
b. Indication at Annunciation Panel
Consequences : a. NIL
Status : a. Unit is at lower speed with potential.
Increase governor speed until machine reaches full speed. Even after two to three attempts, the machine are running at lower speed, probably the governor of turbine is faulty. Inform to maintenance staff for rectification of the same.
19.OVER VOLTAGE PROTECTION :
Setting : a. 110% Time - 2.0 Sec.
b. 120% Time - 0.3 Sec.
Generator voltage is at present value under normal operating conditions as selected by operator in AVR. If it parts from preset value, May be due to AVR mal-functioning or a system disturbance. Severe over voltage can cause over fluxing and winding insulation failure. The over voltage protection can be considered as a backup to the Volts-per-Hertz protection.
Relays acted : a. Flag operation at Protection panel.
b. Acting of Master relay
c. Indication at Annunciation Panel.
Consequences : a. Tripping of 220KV breaker
b. Tripping of Field breaker
Status : a. Unit is at FSNL without potential.
Raise the generator voltage slowly with manual mode in AVR and keep generator voltage within the limits of normal voltage. If it is unable to control the generator voltage, trip the field breaker and inform to the maintenance staff for rectification of the AVR.






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