WE HAVE INSTALLED 30 MW CAPATIVE PO

WE HAVE INSTALLED 30 MW CAPATIVE POWER UNIT FOR OUR STEEL PLANT.
WE DO HAVE 20 MW GAS TURBINE GENERAOR AND 10 MW STG.
WHILE WE WERE WORKING WITH GENERATOR ROTOR EARTH FAULT RELAY
MAKE : SIEMENS , TYPE :7UR2202-1AA05/CB.
RELAY INPUT : 110 VOLT AC
RELAY SETTING : ALARM : 80 KILO OHMS
TRIP:5K OHMS
MY QUESTION IS :
WHILE WE WERE CHECKING VOLTAGE AT MEASURING CKT WITH MULTIMETER
IN DC MODE , THE MULTIMETER WAS SHOWING SOME VOLTAGE.
AS SOON AS WE CHANGE THE MULTIMETER TO AC MODE , MACHINE WAS
IMMEDIATELY TRIPS ON ROTOR EARTH FAULT. ( ROTOR WAS HEALTHY ENOUGH)
WHAT COULLD BE REASON FOR TRIPPING?
PLEASE GIVE YOUR SUGGETION
REGARDS
NIKHIL GAJJAR
30 MW CPP
OPERATION
ESSAR STEEL LTD
SURAT. INDIA

 

[jbartos]

Suggestions: The following may be considered:
1. Check the multimeter internal impedance |Z|, which shall be less than 5333 Ohms in order to avoid tripping, since when you connect in parallel 5333 ohms with 80000 ohms (it may be higher, this is the assumed worst case; else the alarm would be on) the result is 5000 ohms at which the relay trips.
2. The multimeter dc internal resistance may be much higher. It may also be measured.
3. If you measure those voltages by an oscilloscope, for example, you will get the voltage waveforms and high input resistance/impedance of the oscilloscope. Then, the relay shall not trip.

 

[peterb]

Two thoughts -
1. Did you by any chance switch the multimeter to AC while still connected to the measuring circuit? If so, did you switch through an ammeter range on the multimeter? If this happened, you would have created a low-resistance path to earth (assuming that you were measuring from one lead to earth), which would have constituted an earth fault to the relay.
2. I suggest that you check with the generator supplier as to whether they absolutely require that a single rotor earth fault needs to trip the unit. A single rotor earth fault doesn't present a problem for unit operation - a second fault will short out a section of the winding and will result in high rotor current and magnetic unbalance. Most operators provide an urgent alarm, which allows the unit to be taken offline in a controlled manner, avoiding disruptions to the plant supply.