Open circuit CT's (again) 1

[bsmotorsport]

Hi, Ive read all the posts on open circuit CT's already but they all refer to what happens when primary current is applied to them. My quick question is what happens in a power frequency withstand test, with the secondary open circuit, i.e. 11kV AC applied to primary conductor. Anyone?

 

[scottf]

Generally, the voltage is applied to both primary terminals, hence, there is no current flow in the primary.

Most folks short and ground the secondary winding during this test.

 

[bsmotorsport]

thanks, as a rule I always do short and ground them, im just curious what happens if you dont. Like you say there would be no current flow but would the CT start to behave like a VT and generate a huge secondary voltage?

 

[davidbeach]

Secondary voltage would be proportional to the primary voltage - the voltage across the primary "winding" of the CT - not the total voltage to ground. No current flow, no voltage on the primary, no voltage on the secondary.

 

[scottf]

If you don't ground the secondary winding, the winding will float up to (or close to) the primary winding potential, which could be a dangerous situation.

In other words, open-circuit voltage is not a concern....voltage rise of the entire secondary is.

 

[gunax]

When you applied primary voltage, there will be current flow and built up the secondary voltage. If you can't control your primary voltage and primary current...you will get uncontrolled voltage at secondary?

Just currious, you don't need to do this..for further how to test the CT, chek megger tester guidance

 

[bsmotorsport]

Many thanks for that.
Gunax, I wasnt testing the CT itself I was pressure testing the HV switchgear, theres no way of doing this without passing HT through the CT on this particular gear. We got talking as to the do's and donts and I said I wasnt sure what would happen if the CT was open circuit in this particular test, as above I normally ensure the shorting bar is in and earth link is in.

 

[bsmotorsport]

just to confirm, what are the chances of damage occuring as a result of doing this?

 

[stevenal]

Damage? Why speak of damage and chances when injury is a possible outcome? Especially when it it so easily mitigated.

I don't really understand the problem here. You go from instruments connected with secondary neutral grounded to CT secondary short circuited and grounded. There is no step where the ground needs to be or should be removed.

 

[bsmotorsport]

I know it shouldnt be removed but what if the person you are working with has taken the earth link up to megger it then forgot to put it down, then without knowing you pressure test the switchgear. So as for not needing to remove the ground, under commissioning you do to prove it (and if your new to the job you leave it up evidently).
What Davidbeach has wrote seems to make perfect sense to me, which brought me to the question, would damage be caused by just applying HT to the primary?

 

[stevenal]

Lotsa what ifs in electrical work. Do a proper tailboard, proceed step by step, and verify each other's work.

David is correct regarding the voltage across the winding. Scottf is correct about voltage to ground.

Damage to equipment? I suppose it depends on the equipment configuration. You have a capacitive voltage divider, with one side pulling the secondary voltage up, the other side pulling it down. How this divides out would not be easily calculated.

 

[gunax]

Damage?

If you apply on the primary/source side of the switchgear, you're probably doing voltage withstand normal frequency/insulation test, no current will be flowing through the bus, so no current/voltage will be built up at secondary side of the CT.

When you apply, current test to the source side of the switchgear/lets assume mommentary current test flowing through the switchgear, you would get high voltage at secondary side of the CT, if you failed to short & ground it, which may damage the CT/change its characteristic/blow out.