15 kV Bus Grounding

[Zifkwong]

I came across an odd situation today that does not seem legit. Is it acceptable to ground a 15 kV bus through a wound type current transformer and safely work on it? My concern was that the CT's primary could become energized, to theoretically infinite voltage, should a current flow in the secondary.

What do you guys think?

 

[realolman]

Re: 15 kV Bus Grounding

Could you explain that a little more. I don't understand.

 

[Zifkwong]

Re: 15 kV Bus Grounding

There are mechanics performing work on a section of overhead tubular bus. In between the point where the workers are working and the closedground switch there is a wound type current transformer.


Ground --- bus --- current transformer primary --- bus --- workers

 

[don_resqcapt19]

Re: 15 kV Bus Grounding

The secondary of a CT is not normally connected to a source of power.
Don

 

[rcwilson]

Re: 15 kV Bus Grounding

I don't think the CT can induce enough voltage from the secondary to be a hazard.

A CT acts like a voltage transformer when its primary is open circuit. Say it is a 600:5 CT and someone puts 600 V across the secondary leads. Only 5 volts to ground (600/5) is induced on the primary, not enough to be a problem. (Assume that this is a C800 CT that won't saturate until about 800 V is across the secondary.) Any higher voltage on the secondary and the CT will saturate or fail.

The CT primary impedance looks very high on the secondary. It changes by the square of the ratio. For the 600:5 CT the ratio is 120/1, so 1 ohm across the primary
looks like 14,400 ohms on the secondary. That large reflected impedance prevents much current flow in the secondary so there is not much current flow in the primary. (Unless the primary is short circuited by a very low impedance. But in that case there is no voltage build up through the CT and the bus remains near ground potential. V=I x R and R is very low.)

So voltage is low and getting any current is difficult. I don't think there is a hazard.

This explanation is not completely technically accurate, it neglects saturation and other things. But my experience in working with CT's is that they can be treated as a bus bar.

But I would like to hear from some substation electricians who know more about this.

 

[realolman]

Re: 15 kV Bus Grounding

Are you talking about an instrumentation transformer for an ampmeter or something else?

 

[Zifkwong]

Re: 15 kV Bus Grounding

The particular current transformer that I am referring to is supplying current to a SVAR control system.


Under normal conditions there should be no current flowing in the secondary when the primary has no current flowing through it. However, I was concerned with the odd ball situation where for some unknown/unexpected reason a current does flow in the secondary.

I know that if you have a CT with current flowing in the primary and the secondary open you will get an abnormally high voltage across the secondary terminals. This is attributed to the CT's high impedance and the fact that there is current flowing through it. Does the same hold true for a current flowing in the secondary of an open circuited primary CT?

 

[rbalex]

Re: 15 kV Bus Grounding

Theoretically, it does. It isn't immediately obvious, but ALL transformers are both voltage and current transformers.

An energized secondary of a CT will indeed induce a voltage on the primary ?winding;? however, the ?open-circuit? voltage on the primary will turn out to be exceptionally small by Lenz?s Law. Ignoring hysteresis effects, the primary voltage will be secondary voltage DIVIDED by the secondary/primary turns ratio. Remember the primary ?turns? for most CTs is 1.

Edit Add: I should also note something that is not immediately obvious. Under "normal" operating conditions, the ?primary voltage? on the CTs you are discussing IS NOT 15kV. Rather it is the small voltage drop induced on the conductor caused by the presence of the CT load.

[ February 10, 2006, 03:17 PM: Message edited by: rbalex ]

 

[realolman]

Re: 15 kV Bus Grounding

I know I am dense, but I would like to understand what is being discussed here.

The current transformers I am familiar with only have two terminals... to connect to an ampmeter or some kind of instrumentation.

Is this a regular step down transformer as you would use for anything else?

When you say it's supplying "current" for a SVAR Control system ( I don't know what that is either ) do you mean it is supplying power, or is this SVAR doing something based on the magnitude of currrent in the 15kV buss?

Are you talking about the The CT inducing a voltage in the buss or is there a connection of some sort?

Again... I'm sorry I'm so dense, but I would like to understand.

 

[Zifkwong]

Re: 15 kV Bus Grounding

I think I may be confusing people.


This CT is a wound type ct, has multiple primary turns, not a window, one primary turn, and it's terminals are at line potential. It is in series with the bus that is feeding the load from the source.

 

[ramsy]

Re: 15 kV Bus Grounding

Could there be a kwh meter used with this current transformer?

My understanding of kWh meters for use with CT's permit smaller wires, but these doughnut shaped CT's typically have only one winding, the secondary.

Caution applies to the CT terminals, secondary, which must always be short-circuited while any current is flowing in the (primary/load wire running thru the middle).

The meter usually acts as the short, but without the meter, these CT's needs a short-circuiting bar across its 2ndary terminals.

Touching the terminals of a CT that isn't short-circuited, or the open meter socket, could find a dangerous voltage of many thousand volts.

 

[realolman]

Re: 15 kV Bus Grounding

Zifkwong kindly sent me a PM rather than highlight my ignorance in public. I do that quite well myself, thank you.

What he's talking about is a system for correcting power factor. Perhaps he would be kind enough to explain it further in this thread. I imagine many of us would be interested.

re: the original question: I can't imagine a CT being able to induce much power onto a buss bar; particularly if the buss was grounded, but, if the thought bothered me, I'd short it out.

[ February 12, 2006, 08:26 AM: Message edited by: realolman ]