ROTOR EARTH FAULT

VISHAL YADAV & SANDEEP KHUDANIA

ROTOR EARTH FAULT PROTECTION The rotor is one of the important part of Generator and it

should be well protected against Earth faults or Inter turn faults these are caused by thermal and mechanical stresses.

The field system is normally ungrounded so when a single fault between field winding and rotor body exist does not give rise to any fault current.

But when second earth fault exist it short circuit the rotor winding and therefore produce an unsymmetrical field system and unbalance force on the rotor. This causes vibration of the rotor and damage to the bearings. So rotor earth fault protection is provided to restrict the fault spreading.

• Insulation failure at a single point :− No fault current, therefore no danger− Increase chance of second fault occurring• Insulation failure at a second point :− Shorts out part of field winding− Heating (burning of conductor)− Flux distortion causing violent vibration of rotor

F=Attracting force at the rotor surface.A= AreaB=Flux density.

NEGATIVE SEQUENCE CURRENT

Because of faults , unbalance in three phase stator currents exists. As unbalance three phase currents have a Negative Sequence Component, it rotates in a opposite direction at synchronous speed giving rise to double frequency currents. This results in overheating of the rotor and other damage to the rotor. So rotor temperature indicators are used for detecting the rotor overheating.

NEGATIVE SEQUENCE CURRENT

ROTOR WINDING COOLING

METHODS FOR CALCULATING ROTOR RESISTANCE.

A.C. injection method

D.C. injection method

Potentiometer method

AC/DC Injection Method

It works with a low frequency alternating test voltage (1 Hz, +/- 24V) which is connected via slip rings to the main exciter winding and to rotor earth

MEASURING PRINCIPLE

MFR1L RELAY BLOCK DIAGRAM

Different measured values may be displayed by pressing the "Menu" button

• The "U1=" display indicates the amplitude of the measuring voltage generated in the unit. If the ampli-tude falls below 10.0 V, the unit is not ready for operation anymore.

• The "Rx>" display indicates the currently measured ground resistance. The measured value should be above 100 kΩ if the generator is stopped.

POTENTIOMETER METHOD

Potentiometer method:

In this method a high resistance is connected across the rotor circuit & its mid point is grounded through a sensitive relay. This relay detects the earth fault for whole circuit except the rotor center point.

(EFREM) Earth Fault Resistance

Quantitative values of fault severity (continuous resistance

measurements) – trend your high accuracy data readings!

Continuous monitoring for faults (always operational, while

rotating and when off line) Fault location indicator for ease of

diagnosis and repair Field excitation voltage level

monitoring Alarm relay contact outputs for

multiple resistance limits (for instance, an early warning alarm and a

machine trip).

Current Trends for Rotor EF detection

THANK YOU

VERY MUCH…