Is 200/1 Amp CT equivalent to 100/0.5 Amp CT? 1

[edison123]

I have a failed 100/0.5 A CT used in an AVR circuit of a diesel generator. Since I am unable to source such a CT immediately, can I use a 200/1 A CT instead? The generator current being measured varies from 20 to 55 A max. Will the CT accuracy be affected by using such s higher primary current rating?

Muthu

http://www.edison.co.in

 

[ScottyUK]

If this is the quadrature compensation CT then I don't think it needs to have remarkably good accuracy. The ratio matches, so providing you're using a metering CT rather than a protection CT then it should be fine.

No option to re-wind the original CT core?

 

[edison123]

Thanks Scotty. You nailed it. Yes, it's for quadrature compensation. The whole generator was burnt to cinders. I was able to rebuild everything but got stuck in this tiny CT which was also burnt. It's a German make generator and CT OEM is out of business. You don't think a 200/1 A CT will be a problem for the AVR?

Muthu

http://www.edison.co.in

 

[ScottyUK]

It's a measuring CT so it doesn't have any real burden on it, unlike the boost CT's which you occasionally see used to help a generator ride through a fault. I don't see it as a problem if the accuracy is out a little, and the secondary current will be transformed in the correct ratio so I'd be happy to go for it. 20A - 10% of rating - is still well into the linear range for the CT.

 

[edison123]

Thanks Scotty. Attached the photo of the original CT. The generator middle phase lead goes through donut hole.

It looks like I might be able to buy a 100/1 A donut CT with 10-15 VA. Wonder how to convert that to 100/0.5 A CT.

Muthu

http://www.edison.co.in

 

[waross]

Wonder how to convert that to 100/0.5 A CT.

Easy with a window type CT. Two turns of the primary conductor through the window.
I did a project that required the CT ratios to be such that 150% of rated motor current would equal 5 Amps secondary current.
The designer wanted to use identical CTs and the same SCADA program for all motors.
100:5 CTs were used for all motors.
The motor currents ranged from about 6 Amps to over 50 amps.
By adding primary turns and secondary turns we were able to change the ratios.
In some cases the secondary turns were wound counter to the the existing secondary turns.
Some windows were very full of turns.
The circuits were all tested by primary current injection and all were found to be within specifications.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xnuke]

Bill, your ampere-turn math doesn't seem to be correct with two turns of the primary through the window to create a 100/0.5 A CT out of a 100/1 CT. A 100/1 CT has 1 primary turn and 100 secondary turns. You'd need to have a 1/2 turn in the primary with 100 A to get 0.5 A on the secondary; needless to say, that can't be done.

I think using a 200/1 CT as edison123 originally suggested is correct - with 100 A in the primary, 0.5 A will flow in the secondary. There's no need to wrap any primary or secondary conductors through the window with a 200/1 CT.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

The senior's moments come more frequently.
The quadrature circuits that I remember develop a bias voltage across a resistor to bias the AVR input voltage.
The bias resistor may be internal or external.
In some designs the bias resistor is adjustable.
Changing the bias resistor may be another solution if the correct CT is not available.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

Thanks xnuke. Agree doubling the turn will halve the ratio.

Thanks Bill. The innards of AVR are not available to me to tweak with.

I am leaning towards 200/1 A CT.

Muthu

http://www.edison.co.in