Alternator Protection
6
Page 1 of 6 01 JANUARY 27, 2011 TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY 2011-2011 / 3 RD L. VALCOS Alternator Protection Alternator Protection Also known as Generator It is the heart of electrical power system as it converts mechanical energy into its electrical equivalent. Three main types of prime movers 1. Steam turbines 2. Gas turbines 3. Water turbines 4. Diesel turbines Small and medium sized alternators are connected direct to the distribution system. Large sized alternators are connected to the EHV grid via transformer. System Faults Electrical Mechanical Stator insulation failure Overload Overvoltage Unbalanced load Rotor faults Loss of excitation Loss of synchronism Failure of prime mover Low condenser vacuum Lubrication oil failure Loss of boiler firing Over speeding Rotor distortion Excessive vibration
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Transcript of Alternator Protection
Page 1 of 6
01JANUARY 27, 2011
TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY
FORM OVPAA-002A
2011-2011 / 3RD L. VALCOSAlternator ProtectionPROGRAM CHAIR/ CLUSTER COORDINATOR
Alternator Protection
Also known as Generator
It is the heart of electrical power system as it converts mechanical energy into its electrical equivalent.
Three main types of prime movers
1. Steam turbines2. Gas turbines3. Water turbines4. Diesel turbines
Small and medium sized alternators are connected direct to the distribution system.
Large sized alternators are connected to the EHV grid via transformer.
System Faults
Electrical MechanicalStator insulation failureOverloadOvervoltageUnbalanced loadRotor faultsLoss of excitationLoss of synchronism
Failure of prime moverLow condenser vacuumLubrication oil failureLoss of boiler firingOver speedingRotor distortionExcessive vibration
Page 2 of 6
01JANUARY 27, 2011
TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY
FORM OVPAA-002A
2011-2011 / 3RD L. VALCOSAlternator ProtectionPROGRAM CHAIR/ CLUSTER COORDINATOR
Stator Protections against Electrical Faults
Earthing and Earth Faults
The neutral of the generator stator winding is normally earthed for protection. Impedance is generally used to limit earth fault current. This is when Generator is connected direct to the
distribution network. A setting of 10% of the maximum earth fault current is the safest for the connecting instantaneous relay. Time delay setting of 5% of the maximum earth fault current is applied for the connecting relay. Earth fault protection can be applied by using a transformer and relay to measure the secondary current.
Earth fault protection using a relay in parallel with a loading resistor to measure secondary voltage.
Page 3 of 6
01JANUARY 27, 2011
TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY
FORM OVPAA-002A
2011-2011 / 3RD L. VALCOSAlternator ProtectionPROGRAM CHAIR/ CLUSTER COORDINATOR
Overload Protection
The amount of power that can be delivered by the prime mover may cause overloading of the generator.This is then continuously monitored by its Governor and Regulator.
Page 4 of 6
01JANUARY 27, 2011
TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY
FORM OVPAA-002A
2011-2011 / 3RD L. VALCOSAlternator ProtectionPROGRAM CHAIR/ CLUSTER COORDINATOR
A Thermistor or Thermocouple embedded in the stator winding checks the rotor winding by measuringthe resistance of the field winding.
Overcurrent Protection
An Electromechanical IDMTL relay is used to operate only under fault condition.
Page 5 of 6
01JANUARY 27, 2011
TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY
FORM OVPAA-002A
2011-2011 / 3RD L. VALCOSAlternator ProtectionPROGRAM CHAIR/ CLUSTER COORDINATOR
Overvoltage Protection
Overvoltage occurs at either as a high-speed transient or as a sustained condition at system frequency. Overvoltage is a result of the following
Defective voltage regulatorManual control error resulted to sudden variation of loadSudden loss of load due to nearby circuit tripping
Surge arresters at strategic points on the system or at generatorterminals protects the network against overvoltage.
Rotor Protections against Electrical Faults
Potentiometer Method
The field winding is connected with a resistance having center tap. The tap point is connected to the earth through a sensitive relay R.
AC Injection Method
An auxiliary supply is injected to the field circuit via a coupling capacitance. The capacitor prevents entry of high current passing through the transformer.
Page 6 of 6
01JANUARY 27, 2011
TOPIC TITLE SY / TERM OF EFFECTIVITY PREPARED BY
FORM OVPAA-002A
2011-2011 / 3RD L. VALCOSAlternator ProtectionPROGRAM CHAIR/ CLUSTER COORDINATOR
DC Injection Method
A rectified voltage is injected to the field circuit, hence eliminating the flow of capacitance currents. The auxiliary voltage is used to bias the field voltage to be negative with respect to the earth.
Alternator Protections against Mechanical Faults
Reverse Power
More applied when generators run in parallel to protect the system againstthe failure of prime mover.
Loss of Excitation
A mho-type impedance relay is used to detect the condition of excitationloss on the primary side.
Loss of Synchronization
When the generator is synchronized, the frequency of the system willchange depending on load and the average characteristics of all thegenerating units connected to the grid. Large changes in system frequency cancause the generator to fall out of synchronism with the system. Protectivedevices on the generator will operate to disconnect it automatically.
Field Suppression
When a machine develop a fault, the field should be suppresses as quicklyas possible.
This can be done by using an automatic air circuit breaker with blow contactsfor small to medium sized machines. On larger sets say above 5MVA, a fielddischarge resistor is used.
