Arc Guard Equipment or Transformer Differential Protection

[davva]

We have calculated some pretty high incident energy levels on our 440V main switchboards.

Looking to mitigate the arc flash hazard by either implementing arc guard equipment (light and current sensing) or alternatively considering fitting transformer differential protection on the MV/LV transformer.

What are the typical trip times that can be expected with each of these systems?

 

[rbulsara]

Transformer differential is not an alternative to arc flash mitigating measures.

Transformer differential protection is for faults internal to the transformer. Faults on the secondary switchboards that which are the arc flash concern, will not be seen by the diff. protection.


Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

Your easiest solution is to retrofit your main breaker with a trip unit that has an arc flash reduction switch (Maintenence settings) to reduce your Ei on the switchgear and all the feeder breakers.

The transformer secondary up to the main breaker is harder, we usually out a mini VCB in the MV switch where the fuses usually go with relays from the secondary side of the transformer.

Of couse most plants have just gone to remote racking and switching so they can operate and rack all thier breakers wirelessly, much easier and cheaper solution if you have several substations.

I handle all 3 of these solutions everyday, mitigation is my job.

-Scott (Zog)

http://www.cbsnuclear.com

 

[davva]

Sorry I should have stated that arc flash incident energy level on the line side of the switchgear is high. We want to use transformer diff protection (including cables up to incoming breaker).

If we adopt this method for the rest of the switchboard we intend to fit maintenance switch to reduce protection settings.

Alternative would be to use arc guard for the whole switchboard.

 

[rbulsara]

My previous statement still stands. Even if you include cable and breakers in the xfmr diff. zone, it only covers the transformer feeder. The switchboard would have other breakers too.

Do not confuse the transformer diff protection with arc guard equipment or even bus differential protection.

If you are really concerned with personal safety, design the system with a bypass or redundancy so that you can shut down a gear for maintenance without interrupting service to loads. Second choice may be the arc guard.



Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

If your transformer zone is bounded by the CTs on the bus side of the main breaker, that differential will significantly reduce your arc flash energy in the main breaker section of the gear. Transformer diff will probably have a relay time of 2-3 cycles. If that's not quite fast enough, the optical systems can produce a trip in less than 1/2 cycle, but you still need to include breaker time. Combine the transformer diff with bus diff and you have everything covered up to the load bushings of the breakers. You need optical or a low set instantaneous to provide coverage on the cable terminations.

 

[dpc]

I agree with David - a combination of transformer diff and bus diff will generally be fast enough to achieve a reasonable arc-flash energy level. Of course, the transformer 87 zone has to include the main breaker. 2-3 cycles is pretty conservative - it should generally be a little faster. Also, if there is a lockout relay, its operate time may also need to be include, although these are generally less than a cycle.

The optical systems promise faster response, but there are possible issues with retrofit of optical sensors on existing equipment.

 

[davva]

Thanks all.

 

[Maak]

dpc wrote: "The optical systems promise faster response, but there are possible issues with retrofit of optical sensors on existing equipment."

I would like to hear what kind of issues there are. As far as I know, retrofit of optical sensors is rather easy. Optical systems (often combined with instantaneous overcurrent) are de facto standard in some countries.

 

[dpc]

The issue is having confidence that any arc in any location will be detected by your sensors and having someone willing to accept the liability for it not working when needed.

When provided as part of new equipment, the manufacturer presumably has tested the installation and is assuming responsibility for the location and quantity of sensors. In a retrofit, it isn't as clear cut.