Differential protection relay CT wiring question

[d-decherd]

We just had a problem with a transformer.
The 69 kV primary side multi tap CT's were wired so the connections to the 100:5 tap would go to the relay.
One side was grounded as recommended and ALL unused taps were also jumpered and grounded.

The percent differential relay tripped at 30% like it was programmed to trip.
The secondary CT's operated fine, they were reading current in the differential meter.

The W1 primary CT's were NOT reading current.
When the differential relay tripped off line the event recorder recorded current for W1, a,b&c,
but all other events had "0" current recorded on the W1 on the high side ct's.

We resolved the problem by relocating the ground wires and unshorting the unused terminals and the primary ct's read correctly. W1 current matches W2 winding current and shows up in the meter.

No one can tell me why open circuiting the unused CT's on a multi tap ct will cause the CT to transfer current to the relay and having the unused taps shorted will not pass current.

Any ideas?
I hear theories but no one has paperwork to back it up.
"They say it is done all of the time."
Everything I have read states that the CT secondary must be shorted to prevent high voltages from being created across an open terminal when current is flowing. I have seen open circuited CT's start a fire in switchgear.

 

[templdl]

Re: Differential protection relay CT wiring question

Can you get an internal wiring diagram of the CT from the CT manufacturer?
A common CT w/o taps has a single continuous secondary winging which must be either connected to a device of shorted when in service.
I'm not familiar with a CT with taps but if the CT in questions has a single winding that is taped at various ratios and you short the unused taps out, wouldn't that change the current that you are supposed to be getting at the connection used if shorted incorrectly? I would like to think that the current would be lower than one would expect to be getting.
If the taps were intended for the convenience of having one CT that would make numerous rations available on one product I would understand that only two connection points could be used at any one time. The unused part of the CT winding above the ration being used would be shorted within that portion of the winding.
Again, it would be nice to see a wiring diagram of the device as compared to what you're trying to accomplish.

 

[69boss302]

Re: Differential protection relay CT wiring question

OK I'm going to try this just shooting from the hip. The various taps on a CT are for tapping of the winding at different points. There is one winding with different connecting points. Depending on the amount of possible current in the line and the meter you are using will determine the taps you want to use. If you short out the unused taps you are shorting out part of the turns of the transformer whereby changing the turn to turn ratio which will change the output of the transformer.


Now a CT on it's own with out being tapped or shorted out can cause a large voltage to be induced into the CT and can and I will say usually will burn up. I think of it this way, You can not leave a CT open or it will find a way to short itself out. It's the same concept as an old coil for a car. This sounds over simplified but a current transformer will produce the voltage it needs to have the rated output to push current in secondary in the proportion it is rated for depending on what the primary current is.

Edit: I figured somebody would respond quicker if I rambled to much.

[ June 29, 2005, 10:13 AM: Message edited by: 69boss302 ]

 

[rbalex]

Re: Differential protection relay CT wiring question

New in the 2002 NEC:

110.23 Current Transformers. Unused current transformers associated with potentially energized circuits shall be short-circuited.

The code requirement is for unused CTs not unused taps.

I'm going along with 69boss302 on this one. Without actually seeing what you did the most likely thing is you shorted out the CT completely.

The danger is when the entire CT secondary is open-circuited. If the CT circuit is properly "burdened" between any two terminals, then the over-voltage will not develop and the other taps should be left alone.

[ June 29, 2005, 12:43 PM: Message edited by: rbalex ]

 

[rcwilson]

Re: Differential protection relay CT wiring question

Shorting the unused taps steals the secondary current from the tap connected to the relay. No measurable current will go to the relay.

A multi-tapped CT provides different secondary ratios with a single primary winding. The single secondary winding is tapped to provide the various ratios. All of the taps are on the same winding, they just include a different number of turns. Connecting a relay to a tap or shorting out any tap, provides a path for secondary current to flow.

The CT is designed to generate voltage on the secondary until enough current flows in the winding to make secondary amp turns = primary amp turns. When you open circuit a winding, the CT tries to make current flow by increasing voltage; but there is an infinite resistance so Ohm's law says the CT needs an infinite voltage. The CT tries to do that until it lets the smoke out.

Take a 1200:5 CT with a 600:5 tap (600:5 = 120 turns). At 600 amps primary there is 600 amp turns in the core, so the 600:5 tap will develop voltage until 5 amps flow to equal the 600 amp turns (5 x 120).

But if the 1200:5 tap (240:1) is also shorted, that circuit will have less resistance and the CT gets the 600 amp turns it needs by forcing 2.5 amps through the short. There is no energy left to push any current through the relay on the 600:5 tap.

Never open circuit a winding. That is not the same as open circuiting a tap on a single winding. You can open circuit the taps as long as one of the taps has a secondary circuit.

Bottom line:
Never open circuit a CT winding.
Always short unused CT?s.
Do not short out part of a CT winding, if you are using another part of the same winding.
Always ground the CT circuits.

Remember these are not voltage transformers, they are current transformers. You can connect loads to different taps on the same voltage transformer, but not on a CT .

 

[69boss302]

Re: Differential protection relay CT wiring question

Originally posted by rcwilson:
but there is an infinite resistance so Ohm's law says the CT needs an infinite voltage. The CT tries to do that until it lets the smoke out.

I agree completely but let's please not start the discussion on the existance of infinite resistance and infinite voltage.

 

[d-decherd]

Re: Differential protection relay CT wiring question

The original installation had a 600:5 multitap ct in the circuit. I attached the W1 100:5 taps to the differential relay. I grounded the non polarized tap X2. The X1 polarized tap went to the relay. The remaining taps X3, X4, X5 were grounded to X2 and ground. There was no current flow in the secondary of the CT of this "Winding 1" 69kv primary circuit. Eventually the 10 MVA transformer tripped on percent differential, as it should have. I missed the fact that the 13.8kV secondary winding Ct's did not have their secondary multi tap unused terminals shorted. They only had the two used terminals connected and the non polarized terminal grounded. Consequently there was current flow in the 13.8kV secondary winding CT's and the protection relay did trip. It is intuitively obvious that the multitap CT must have the unused taps not connected in order to function. ABB told me that the CT is wound in a straight line with taps at various locations to get the desired ratio. I am still not clear as to why the unused taps do not build up voltage. Standard C35.13 states that a CT is to be "built to withstand 3,500 volts for 1 minute on an open secondary circuit". Do the unused taps build up voltage? Since they have been in service without a smoke escape alarm being emitted I am guessing that they do not. Do they build up voltage? To what extent does this happen?

 

[rcwilson]

Re: Differential protection relay CT wiring question

The unused taps do not build up voltage by themselves. The complete secondary winding builds up voltage until enough current flows in any part of the winding to match the primary amp turns.

The voltage will be distributed across the winding in proportion to the CT turns in each tap.

Assume your relay winding and secondary wiring resistance totals 2 ohms. With a 100 amp primary load you need 5 amps secondary current (100:5 ratio). The 100:5 tap (X1-X2) will generate 10 volts (5 amps x 2 ohms) to make 5 amps flow to the relay. If you put a voltmeter on the 600:5 tap (X5-X2) you would see 60 volts (10 V x 600/100) due to the transformer action of the CT turns.

The 400:5 tap will have 40 volts, the 300:5 tap 30 volts, etc.

The CT doesn't need to build up any more than 60 volts on X2-X5 because the 100 amp-turns in the primary circuit (100 amps times one turn) is in balance with the 100 amp-turns in the 100:5 amp tap. (100:5 = 20 turns. 20 turns x 5 amps = 100 amp-turns = 1 turn x 100 amps.)

The dangerous voltages only occur when a high impedance circuit makes the CT generate a lot of volts to get enough current flow to balance the amp-turns. An open circuit is very high impedance so you get very high voltages.

I hope this helps you understand CT circuits. I get a little wordy at times.

If it is any consolation, I shut down a portion of a refinery and cost my employer many $100K's by shorting out unused CT taps on a differential CT circuit. My brief stint as summer help on a utility's protective relay crew taught me that open-circuit CT's are dangerous, so I shorted every unused tap I found. The plant tripped from the incorrect CT signals. (You know what they say about summer help, "Summer help, some are not.")

I hope I helped this time.

Bob W.

 

[templdl]

Re: Differential protection relay CT wiring question

It is of my understanding that the CT would be loaded from the point where it is grounded to the point where it is connected to a tap. However, shouldn't there be a concern over a voltage that would be induced from that point of the tap to the unused tap that is the next highest ratio?
I would like to think that it would be imparitive to short from the point of the tap being used to the highest tap not in use. For example, if you had a 400/800:5CT and you where using the 400:5 tap would the CT winding between the 400:5 tap to the 800:5 connection point develop a voltage unless there was a jumper installed between these two points?

 

[rcwilson]

Re: Differential protection relay CT wiring question

Using your 400/800/5 CT as an example and assuming that it is a single winding CT with two taps:

There will be a voltage developed on the 800/5 CT winding, but it will be within the CT rating and will not be destructive.

If the 800:5 taps are shorted, no current will flow in the 400:5 winding and relay during a fault or under load.

A typical CT circuit has about 2 ohms impedance. With a 400:5 CT, the system might have a fault level of about 12,000A. With a perfect CT there would be 300V across the 400:5 taps during a fault (2 ohms x 12000A x 5/400 = 300V). The 800:5 amp tap would see 600V, which is well within the typical 1000V insulation rating of the CT.

The CT probably has a C100 rating, meaning it saturates around 100 Volts on the 400:5 secondary. That would keep the 800:5 tap around 200 V, again well within the momentary insulation rating of the CT.

As long as one of the taps in a multi-ratio CT has a secondary circuit for current to flow there should not be any destructive voltage on the other taps.

 

[templdl]

Re: Differential protection relay CT wiring question

Good discussion. Makes sense.
So what you are saying is if you are using the 400/5 tap that and potential developed between the 400/5 tap and the 800 connection would not be and issue because it's within the CT rating. In other words you would not have to jumper between the 400 and 800a connection points.
For example in you had a 400/800:5 CT where the XO in grounded and X1 in the 400:5 tap and X2 was the 800:5 tap that if you were using the 400:5 XO-X1connection that any potential between the X1 and X2 would not be relevant and would not have to be shorted..
Then jumpering a simple 400:5CT is not necessary either.

 

[rcwilson]

Re: Differential protection relay CT wiring question

Originally posted by templdl:

Then jumpering a simple 400:5CT is not necessary either.

Not quite. A simple 400:5 CT does not need to be jumpered if it has a circuit connected for the current to flow. If there is no connection on the simple 400:5 CT, the internal voltages developed will be destructive when primary current flows.

Short unused CT windings. Do not short unused taps if another tap is in use.

Sorry for the confusion.

 

[rbalex]

Re: Differential protection relay CT wiring question

Bob [Wilson] has been doing such an excellent job here, I haven?t made any more comments. Without going very deeply into the theory of properties of magnetically induced voltages by Lenz?s Law, it?s difficult to explain why leaving the other taps alone is not a problem.

Bob?s statement,? There will be a voltage developed on the 800/5 CT winding, but it will be within the CT rating and will not be destructive? is correct; however you would have a difficult time measuring it accurately with any common instruments. Virtually the entire ?voltage drop? created by induce currents in the secondary is across the burden and the instant there is any attempt to measure any of the other ?open? taps the instrument changes the CT burden.

What is not immediately obvious is that all transformers are both current AND voltage transformers where they have common-core windings. The difference is primarily one of application. We just rarely think about the fact that actual primary voltage of a CT is NOT the rated primary system voltage but the slight voltage drop across the (usually single winding) primary conductor induced on it by the magnetic coupling.

As long as it is not driven excessively beyond saturation, the induced secondary current will oppose the over-voltage over the entire CT secondary ? no matter which set of taps are selected.

 

[templdl]

Re: Differential protection relay CT wiring question

Thanks guys. It's back to the basics which we seem to need reminding of. Again, a good discussion.