Disconnecting CT, PT & SA from Hot System

[Bronzeado]

Hi folks,

In the past the maintenance people of my company used to disconnect CTs, PTs and SAs (Surge Arrester) with the system energized. Today this is not allowed.

I wonder if somebody in this forum has experience in this kind of work or can guide me to get information obout it.

What kind of problems could happen? Is there any risk of equipment explosion?

Best regards,

Herivelto S. Bronzeado
Chesf, Recife, Brazil

 

[davidbeach]

What do you mean by disconnect? Disconnect the primary from the system, disconnect the secondary from the instruments, or something else? CTs aren't something be "disconnected" under load anyway.

 

[Bronzeado]

Sorry, davidbeach for being not clear.
Thank you for your reply.

What i need to discuss is about how disconnect the CT primary from the system (energized).

I am talking about maintenance procedures with hot line in HV system. Of course, it will be necessary to bypass the CT primary before disconnecting it. However, there still some electric arc when the cable iof the primary circuit (bypassed) is openned.

My question is: Is this procedure will damage the CT? Does it may cause an equipment explosion?

Best regards,

Herivelto S. Bronzeado
Chesf, Recife, Brazil

 

[davidbeach]

Never heard of such a thing and there is no way that I could ever support trying to take a CT out of an energized circuit.

 

[PHovnanian]

What sort of 'system' are we talking about here? Voltage? Overhead (cross-arms, open conductors, metering type CTs mounted on an arm? Or cables through window type CTs?

Some installations will be easier to work with (hot) than others. With some, it may be impossible to remove the equipment (particularly the CTs) unless some means of bypassing/disconnecting them has already been incorporated.

I don't see how there would be a significant arc when the CT primary is opened. Assuming it has been bypassed and the bypassed circuit length is on the order of a few feet, the charging current will be minute. Of course, the CT secondaries will be shorted, right?

The question of what is/is not allowed is one of the applicable working rules where you are. They vary by government jurisdiction and companies' differing working rules. If this was something your company used to do but no longer can, I'm assuming a change in such rules or laws. How was it done in the past and why is that no longer allowed?

 

[Bronzeado]

Davidbeach,

We have done this work in the past and had no problems.
We have disconnected not only CTs but also PTs and SAs. However, due to questions raised about personal security we stop in doing that.

What I wamt to know is if this procedure is generally done worldwide or not.

Best regards,

Herivelto S. Bronzeado
Chesf, Recife, Brazil

 

[stevenal]

Bypassing the primary of a CT before disconnection is done all the time when the CTs in question are BCTs in a circuit breaker that is taken out of service. Nothing damaging about such an operation done properly.

 

[marks1080]

If I understand what you are saying correctly, and we are talking about the same equipment, it is not possible to disconnect bushing CTs from live equpipment on the primary side and there would be extremely high risk of death and equipment failure. That's putting it lightly. Do you mean disconnecting secondary equipment? Remember, the primary of a CT is the bus/line/whaterver itself.

 

[rcw retired EE]

40 years ago, when I worked summers at a USA utility, the linemen would bypass CT's hot in open air switchyards to make repairs and replacements. I don't recall it ever being done above 13.8 kV.

The design of the old substations and distribution required de-energizing a large portion of the town or working the equipment hot. Those substations were gone last time I was in the area. I believe the utililty upgraded the system to allow off-line access to the equipment without dropping customers.

While a good, well-trained, well-equipped, line crew could do the work hot at lower voltages, I would not recommend it. The risk is too great.

 

[marks1080]

rcwilson: I think I understand better now. With the bypass I guess it would be possible.

 

[Bronzeado]

Dear friends,

I am talking about external HV CTs (PTs and SAs) at 230kV, 500kV), which is connected through cables from a HV bus bar or transmission line.

When we have problems in a CT and we want to put it out of operation (or replaced it) we used to work with the power system energized to not interrupting the load.

So, we first bypass the CT primary and then disconnect the CT by taking the cable out from its terminals (after loosing the cable from the conector) with a isolated "pole". This procedure always causes a electric arc which stopped after the cable depart from the CT terminals.

In the case of PTs and SAs, the electric arc is smal as the current in the equipment terminals is small.

Is this procedure common in your company? There is any procedure in taking out an equipment like tose with the system energized? What is the "normal" procedure? We should interrup the load if there is not another supply?

Best regards,

Herivelto S. Bronzeado
Chesf, Recife, Brazil