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How it protects the Alternator ?
1. The Faults on Synchronous Generator Field Winding
Synchronous generator excitation system is insulated DC electrical system. Rotor of generator, like a rotating part, has higher probability of single ground fault occurrence than stator. Rotor winding damage is frequently, mainly on turbo generators where rotor winding insulation is fairly stressed effected by centrifugal forces
2. Ground fault of field winding.
The first rotor ground fault causes just a little fault current, although it stresses the insulation in portions of the winding where the fault is. The real danger is the second ground fault which can cause significant forces. During dual ground fault can draw large currents which may cause extensive damage to rotor winding and armature.
It is important to detect the first ground fault, thereby preventing a serious failure.
The first ground fault is signalizing only. Generator is tripping and damaged winding is repairing. In this way it can prevent large damages which could be during the second ground fault. But there is probability of occurrence of the second ground fault which presents short circuit with very large currents, dynamic and thermal stress of filed winding and rotor armature.
PROTECTION
The Rotor Protection relay is used in synchronous motors and generators to identify the presence of an earth fault in the rotor winding. While the winding in the rotor is insulated from the ground during normal operation, the Rotor is subjected to stresses due to vibration, heat, etc. These stresses can cause the winding to give way in a particular place and the winding can get earthed.
While a single earthing in the winding is not immediately damaging. It sets the stage for damage if a second failure should occur. The second earthing can cause a short-circuit through the rotor causing extensive damage to the rotor and the winding.
The currents produced during a rotor earth fault can cause excessive vibration and disturb the magnetic balance inside the alternator. These forces can cause the rotor shaft to become eccentric and in extreme cases cause bearing failure.
Hence, it is necessary that any earthing in the rotor is detected at the earliest.
In slip ring rotors, carbon deposits on the slip rings may compromise the insulation resistance of the rotor. Hence, the slip rings need to be inspected for any deposits.
The Rotor Earth Fault Protection Device consists of a current injection device which applies an AC voltage to the rotor winding by means of a slip ring fitted on the rotor. The current is applied to the rotor through a coupling capacitor. In the normal condition, the system is floating and the current flowing through the device is zero as the resistance is high. When a fault occurs, the current increases causing the relay to operate. The relay can be configured for alarm or trip depending on the practicality.
A single ground fault does not affect the rotor and the generator but increases the stress to the ground in field when stator transients induce extra voltage & increases occurrence of second ground fault. This causes unbalance air gap fluxes which causes vibration and rotor producing eccentricity
This is why rotor earth fault protection is necessary; there are 3 different types of rotor earth fault protection.
1. Potentiometer method: A resistance is connected with centre tapping b/w the field winding and excitor grounded via a voltage sensitive relay. Unable to detect abnormalities at he centre of the field winding, but can be adjusted by changing the tapping of the resistor, voltage will appear in relay and it gets operated
2. AC injection method: A voltage sensitive relay is attached b/w the excitor and the field windings with other terminal connected to a capacitor and then secondary of an auxiliary transformer, when during a fault the really gets closed via the earthed path and secondary voltage appears on the terminal. Main disadvantage is introduction of leakage current due to capacitor in ckt and requirement of an auxiliary supply for operation of relay
3. DC injection method: Due to introduction of leakage currents, this method is used. DC voltage sensitive relay is used, connected with the positive terminal of the excitor & negative terminal of a bridge rectifier connected to the primary of an auxiliary transformer. In case of field or excitor fault the positive potential of the relay will appear at the relay and it will operate.
In Alternator Field side JB we generally found Terminal
1) J & K = Alternator Excitor Field Terminal.
2) SR1 & SR2 = Slip-ring1 &2, Generally SR1 is the +Ve of Alternator Field winding/Rotor & SR2 is the -Ve of Alternator Field winding/Rotor.
To identyfy Rotor condition, when TG is stopped Take Magger ( Volt) of SR1 to Ground or SR2 to Ground, the value must be above1.5 M-Ohm- M-Ohm. In normal running condition it is arround Mega-ohm or more.If say0.5 Mega ohm, Insulation got weakend or, not at all advisable to run.
We generally take SR2 & Ground connected with Rotor Earth Fault Relay through2 numbers of Resistance-box.Ist Resistance R-1 say =- K-Ohm( For Alarm),2nd Resistance R-2 Say =8-5 K-Ohm( Trip).
Rotor Earth Fault Tripping is Class A Tripping, means it will trip the entire Unit/Entire Power Plant.
To Check Rotor Earth Fault Relay without any Testing kit. Simply make short SR-2 & Ground through a short-link. Ist stage will operate first then2nd Stage will operate, Give trip command thT.
