RTD and DCT for Generator

[shkim2000]

I was told that problem happens to generator terminal box. The junction box is fitted on the generator and is used for connection for PT-100 Ohm RTD(Winding) and Diffenrential CT. RTD is connected to Monitoring Module while Differential CT(DCT) is connected to Protection relay in switchboard.
When my staff had look at inside of junction box, wires for DCT has been burt and insulation of wires for RTD is melt and shorted. For refenrece the resistance of CT is 0.9 Ohm.
When going thru the above status, does anybody tell me possible reasons I can assume causes problems ?

 

[itsmoked]

Size of the generator?
Does the generator still function correctly?

 

[shkim2000]

The size of generator is 2800kW and generator is still O.K. In addition to the above, several cards in the monitoring module has been burt and destroyed. Is it possible to damage cards through RTD wires ?

 

[ScottyUK]

What was the source of the heat? A diff. CT - presumably one of a group of CTs? - should not be running that hot unless it's either the wrong CT or there is something wrong with the installation wiring. Is the CT itself ok? Has it suffered an insulation failure from primary to secondary? What is the wiring size for the CT?

One possibility if the CT secondary normally carries current - i.e. it is not a core balance type - is that a loose terminal in the circuit has caused a high resistance and after that Ohm's Law does the rest through I²R heating.




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If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

How about the CT was open circuited the voltage went waaay high and insulation failed?

 

[edison123]

Do you have any space heaters inside the generator terminal box ? Some do to keep the generator terminations (a source of low IR values) warm. If so, may be the heaters are the culprits.

Normally, it is a good engineering practice to have a separate metering/measuring box

RTD's by themselves are of very low power/current consumption and the leads are of high temp rated PTFE insulated.

 

[Sargardani]

From the info given above ... the CTs could be the cause. I would check for loose connectiones on the CT wiring and also check the insulation & turn ration of the CTs

Sarg

 

[Sargardani]

From the info given above ... the CTs could be the cause. I would check for loose connectiones on the CT wiring and also check the insulation & turn ratio of the CTs

Sarg

 

[rbulsara]

sounds like CT leads opened up in the junction box..this will create very high voltage, can ignite adjacent things such as poor RTD wires..

I have seen a sustained blue arc on a 480V, 1200A open circuited CT circuit, which burned a hole thru a fiber backboard.

 

[ScottyUK]

rbulsara,

Open-circuit CTs are scary aren't they? I disturbed a bad termination on the secondary of an 18000/5 bus-duct differential CT on a generator auxiliary transformer. The arc was long and powerful. I didn't remember leaping over the handrail of the platform 8' up in the air in my haste to get away from it - my colleagues told abut that afterwards. The terminal rail was severely burnt once we got the generator shut down to take a look.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[rbulsara]

Scotty:

Yes they are..the arc I referred to was also established (or reestablished) when I disturbed the wiring looking for cause for 'false' ground fault trips.

Oh yes, I am glad you survived the jump!