Generator

Generator Protection

1. Generator Differential2. Inter turn protection.3. 95% Stator Earth fault protection4. 100% Stator Earth fault protection.5. Under frequency protection6. Over frequency protection.7. Rotor Earth fault Protection.

8. Reverse Power Protection.9. Low forward Power Protection.10. Negative Phase sequence Protection.11. Field failure Protection.12. Pole slip Protection.13. Under Voltage Protection.14. Over Voltage Protection.


15. Voltage Balance Protection.16. Dead Machine Protection.


The Requirements of Differential protection1. Sensitivity for internal faults2. Stability for External fault.

Differential protection must be sensitive to failure ofStator winding or connection insulation which can result in Severe damage to the Stator windings and Stator Core.Differential protection commonly applied for Generators above 1 MVA.Differential allows detection of winding faults with no Time Delay.The Zone of protection ,defined by location of CTs ;should be arranged to overlap protection for other items of plant such as a Bus bar or a step up Transformer.

Generator Differential Protection

Principle: Differential protection operates on the principle that the current entering and leaving a Zone of protection will be equal.Any difference between these currents is indicative of the fault being present in the Zone.Heavy through current ,arising from an external fault condition can cause one CT to saturate more than the other , resulting in a difference between the secondary current produced by each CT. The stabilising methods under these conditions are

1. Biased Differential protection2. High impedance protection.


Biased Differential : In Biasing Technique ,where the relay setting is raised as through current increases.In High Impedance technique : where the relay impedance is such element is in sufficient for the relay to operate.Biased Differential :1.In this the biasing will increase the relay setting ,such that the differential spill current is insufficient to operate the relay.2.The through current is calculated as the average of scalar sum of the current entering and leaving the zone of protection .This calculated current is used to increase the percentage Bias to increase the differential setting.


High Impedance differential Protection: In this if the relay circuit is considered to be a very high impedance, the secondary current produced by healthy CT will flow through the saturated CT. If the magnetising impedance of the saturated CT is considered to be negligible,the maximum voltage across the relay is less than its current setting. As the impedance of the relay input is relatively low, a series connected external resistor is required.To ensure the protection to operate quickly during internal fault the CT s used to operate the protection must have a knee point voltage at least 4 Vs.


Inter turn Protection

Longitudinal Differential System does not detect interturn faults Interturn fault protection not commonly provided because Faults Rare Even if they occur, they will quickly develop in to Stator Earth Faults

High Impedance Type ProtectionSettingsRelay Pick Up = Should be less than Differential Current due to Single Turn Short CircuitRstab = Same as differential Protection

Except fault Current (if) =

(MVA*103) / (1.732*KV*Xd"*2)


ZERO SEQ.VOLTAGE MEASUREMENTShort circuit of one or more turns will cause the Generated E M F to contain zero Sequence component Earth faults will also produce a zero sequence voltage. Most of the voltage will be expended on Earthing Resistor Hence,drop across the winding should be measured


95% Stator Earth fault Protection

It is current operated from a CT in the neutral earth path.This protection has two independent Tripping stages.For Directly connected Generator (With out Transformer in the neutral connection) 1.The protection must be time graded with the other earth fault protection.2. The setting employed should be less than 33% of the earth fault level.3.A setting of 5 % of the earth fault level should be applied for applications where the differential protection provides less than 95% coverage of the stator winding.


For Generator earthed Via a distribution Transformer.A sensitive 5% setting can be applied to the first tripping

stage,a short time delay can be applied to stabilise the protection against small earth currents due to VT failures or earth leakage during HV system faults.

A second tripping stage can be utilised as a High set. A 10% setting and instantaneous operation ensures fast clearance of Generator earth faults.


For indirectly connected applications , the time delayed earth fault protection in one of two ways.1. To measure earth fault current directly,via a CT in the secondary circuit of a distribution Transformer earthing arrangement.2. To measure earth fault directly via a CT in the Generator Winding.With the first mode of application , the current operated protection function(51N) may be used in conjunction with voltage operated protection(59N).


For Single line to Ground fault near the neutral end of winding , there will be proportionately less voltage available to drive the current through the ground, resulting in a lower fault current and lower neutral bus voltage.If an earth fault occurs and remains undetected because of its location (near the neutral end).A 100 % Stator earth fault protection is designed to detect earth faults occurring in the regions of Machine winding close to the neutral end.

100 % Stator Earth Fault Protection

100% Stator earth fault Protection

Principle: This works on the principle involving monitoring of the neutral side and line side components of the third harmonic voltages produced by the AC generators.AC Generators in service produce a certain magnitude of third harmonic voltages in their windings.under the healthy conditions of working the third harmonic voltage developed by the machine is shared between the phase to ground capacitive impedance at the machine terminal and the neutral to ground impedance at the machine neutral.

Under Frequency Protection

Establishes trip Points and Time Delays based on Turbine Limits* Co-Ordination with Automated Load Shedding• Failure of any Single Relay should not cause any Tripping.

Under frequency – Consequences• Generator• * Reduced output Capability• * Thermal Damages• * Over Fluxing• Turbines• * Blade Stresses• * Mechanical Resonance

Over Frequency Protection

Single over frequency stage with associated timer.It should be set above sustainable over frequency level with a time delay sufficient to overcome transient over frequency followed by load rejection.Over frequency protection may be required as a backup protection function to cater for Governor or throttle control failure following loss of load or during unsynchronised running.

Over frequency-causes

Over Shedding Loss of Load

Damages :

Over Frequency - Causes

Rotor Earth fault Protection

Reverse Power Protection

Detects active power flow in to the Generator. A Time Delay (typically 5 sec) should be prevent operation of the protection during some system fault conditions and power system swings.

Prime Mover Motoring power Possible Damage

Diesel Engine 5 % to 25 % Risk of fire or Explosion

Gas turbine 10 % to 15 % Mechanical

Hydro turbine 0. 2 % to 2% Blade &Runner Cavitations

Steam Turbine 0. 5% to 3 % Thermal Stress Damage

Reverse Power protection

Low Forward Power protection

Operates when the forward power falls below the set value.

Negative Phase sequence Protection

Protects the rotor of a Generator from damage resulting from the heating effects of negative phase sequence currents.

Protection must be Time graded to allow downstream protection to clear an unbalance fault.

It provides backup protection for uncleared asymmetric faults.Models the cooling characteristic of the Generator ,following

exposure to negative phase sequence.The NPS protection function is provided for applications

where a Generator is particularly susceptible to rotor thermal damage.

Negative Phase sequence protection

Contd..In the event of the current supplied to the Power system

becoming unbalanced.The degree of susceptibility will depend on the generator

Rotor design.(cylindrical,or salient construction), methods of forced cooling employed and the presence of any ancillary metallic rotor components.

The I22t for Thermal 30

Hydel 40And continuous negative sequence current(I2) is 8%.

Field Failure Protection

Monitors the Generators terminal impedance in order to detect the failures in the excitation system.Uses a circular ,offset ,mho ,impedance characteristic .The diameter of the impedance characteristic is based on the direct synchronous reactance of the Generator.The Offset of the impedance characteristic based on the direct axis transient reactance of the generator.

An associated definite time delay prevents the operation of the protection during stable power swings.Can be interlocked with the under voltage protection element to prevent the operation during the power swings.A delay on drop off timer can be used to detect cyclic operation . This could result during pole slip.


This protection measures the impedance at the terminals of a Generator that is run in parallel with another source to detect failure of the Generator excitation.


Pole Slip Protection

A Generator might pole slip or fall out of step with other power system sources in the event of failed or abnormally weak excitation or as a result of delayed system fault clearance especially when there is a weak Transmission link between the Generator and rest of the power system.In case of Generators connected to a dense, interconnected system,pole slipping protection may not be required.

Pole slip Protection required:1.in case of remote generation weak Transmission link to the

load center.2.For small Generators running in parallel with strong public

sources.Pole slipping Causes

Prolonged fault clearing Under excited Operation Low System Voltage Line Switching operations


The Pole slipping Relay designed to protect synchronous Generators against the possibility of machine running in the unstable region of the power angle curve which would result in pole slip.The Relay consists of one directional relay and one blinder relay operation in conjunction with 40-80 m sec Timer.The Timer is incorporated so that the discrimination can be made between a power swing and pole slip.


If the fault never reaches the operate region of the blinder or moves between the directional relay and blinder characteristics in time less the timer setting , no operation will occur.

Consequences High Currents, Voltage Swings

Stator Winding Stress Pulsating Torques Transients in the Step up Transformers


Under Voltage Protection

Operates when the 3 phase voltages fall below the common set point.Can be interlocked with the field failure protection to prevent its operating during stable power swings.The pickup level should be set to less than the voltage seen for a three phase fault at the remote end of any connected feeder.The Time delay should be set to allow the appropriate feeder protection to operate first to clear the fault ,and also to prevent operation of the protection during transient voltage dips.

A dedicated input is provided to block the operation of under voltage and under frequency protection during run up or run down of the generator.This input can be driven from an auxiliary contact (NC) in the circuit breaker.Under voltage protection can be used to detect abnormal operating conditions or an uncleared power system fault that may not be detected by any other generator protection.


In case of large Thermal power plant Generators ,a prolonged under voltage condition could adversely affect the performance of auxiliaries (BFPs,Mills,PA fans etc.,).This would ultimately have an effect on the plant performance. If such situation is envisaged, the application of time delayed under voltage protection to trip the generator might be a consideration . The Time setting of under voltage protection can be set longer than the time required for backup feeder protection to clear remote end feeder faults.The delay should preferably longer than the time required for the generator back up over current protection to respond such fault. The required time delay would be typically in excess of 3 S –5S


Over Voltage Protection

Operates when 3 Phase voltages are above the common set point.Protects against damage to the insulation and that of any connected plant Recommended for Hydro generators which may suffer from Load rejection.Time Delayed protection should be set with pickup voltage of 100 –200 % of the nominal Voltage and Time delay sufficient to overcome operation during transient over voltages.Instantaneous protection with setting of 130 % -150% of the nominal Voltage can be implemented.Typical Time delay would is 1 s –3 s.

Voltage Balance Protection

Detects VT Fuse failure.Supplied from the secondaries of two VTs or two separately fused secondary circuits of a single VT.Used to raise an alarm and block voltage sensitive protection if necessary.

Dead Machine Protection

If a Dead machine is energised from a live power system , rotor currents will be induced and machine will accelerate as an Induction motor .The induced currents in the rotor body and windings would be very high with the initially at standstill and could rapidly result in Thermal damage unless the machine is designed for direct –on-line run up as an induction motor.The un expected shaft rotation could also result in rapid mechanical damage if lubrication systems are not running or if a steam turbo –alternator is on turning gear.

The Under Voltage and Over current protection could respond to the condition and can be interlocked with manual tripping logic to protect the machine against the inadvertent energisation of a Dead Machine.

Dead Machine Protection

Date post:	29-May-2017
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