SUPPLYING POWER FROM A POTENTIAL TRANSFORMER

[ssantoshr]

This is a medium voltage switchgear application and my question is "the potential transformer (PT) supplying power to a breaker control circuit".

Potential Transformer details:
Catalog# PTG5-2-110-1322FF
Primary: 13200V
Secondary: 120V
Click on the link below for the cutsheets:

http://pm.geindustrial.com/ITI/sheets/40661.pdf

This switchgear is supplied to one of the utilities and they want to power the main breaker control circuit from the PT. The PT secondary is protected with a 10A fuse and the breaker control control circuit is protected with a 30A fuse.

My question is
1. Does PTs have the capability to supply power to a breaker control circuit?
2. If PTs could generate enough current for the breaker control circuit, how much current could be generated with the above mentioned PT?

Please let me know if you need anymore information to answer my question.

Appreciate your valuable time.

 

[dpc]

The cutsheet says it has a thermal rating of 1000 VA at 55 deg C.

You can calculate the maximum current available based on that. For short-term demand, it can probably produce a lot more than that, but at the cost of increased voltage drop and increased error.

Normally, larger control power transformers are used for breaker control/auxiliary power.

David Castor

http://www.cvoes.com

 

[PHovnanian]

What sort of protection functions is this PT supporting? And how will those functions be affected by the voltage drop and phase shift introduced by the PT's impedance and the connected load?

 

[ssantoshr]

Hi dpc, Thank you for your response

The secondary current would come to 1000VA/120V = 8.33A
But..PTs are used to measure and monitor voltages. I have not known about supplying power to any circuits until this project i am doing right now.

If PTs could generate current, then what could be the advantage of powering from PT rather than from a CPT.

Also in your response, you said "For short-term demand, it can probably produce a lot more than that". My application will be for a long-term demand and the continuous current it will need is atleast 10 amps.

Responding to Phovnanian, the breaker control circuit consists of close coil, trip coil, charge motor, SR750 relay (GE Multilin), lockout relay (Electroswitch which will need control power of atleast 10 amp.

After reading the info provided above, is it recommended to connect the breaker control power to a PT?

 

[dpc]

The trip and close coils and the lockout relay only draw current for very short periods of time. the SR750 will have modest power requirements. I would not expect anywhere near 10 A continuous.

I would not recommend using PT for breaker control power, especially if the PT is used for protection (undervoltage, overvoltage, or polarization of directional relay),

The advantage of using the PTs for control power is that it saves the expense and space needed for CPTs. Except for that, it's a bad idea.





David Castor

http://www.cvoes.com

 

[rbulsara]

Bad ideas have no advantages, just excuses.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

If this transformer is supplying protection circuits, don't connect any other load to it. You will degrade the accuracy and introduce added failure mechanisms. For instance, a shorted breaker coil may blow the primary fuses and interrupt protection reference voltage.
If this transformer is to be used soley to power the breaker it may be a cost effective measure.
BUT you need a more complete knowledge of the loads than>---

the breaker control control circuit is protected with a 30A fuse.

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

 

[cranky108]

I've seen a set of 3 CPT powering a battery charger, RTU, transducers, and MOAB's. It worked fine until someone thought the batteries need heaters for the winter.
Some how that became the only place in the system with low transmission voltage. Removing the heaters fixed the problem.

So here is an issue of why you should not do this. Not because you won't put to much load on the CPT, but because someone else might.

 

[pie314]

Years ago GE used to make "Power Potential Transformers" These transformers were larger than the standard potential transformers, but could supply a 2000 VA load and still meet accuracy standards. Maybe someone could still provide this.

 

[burnt2x]

From a control reliability point of view, I think it would be wiser not to introduce additional failure point unto the control equipment. Just keep it stupidly simple.

 

[slavag]

Hi.
I think is very bad idea.
From my point of view it must be separate circiuits: voltage and power supply.

Best Regards.
Slava

 

[ssantoshr]

Thank you all for your valuable advice.