MLO MV Switchgear

[rockman7892]

On several projects recently I have come across Main Lug Only (MLO)MV switchgear located on the secondary of transformers in unit substations. I'm assuming that the omission of a main breaker in many of these cases is due to cost and perhaps in some cases available switchgear real-estate.

Are there any major issues or things to consider with the omission of the main breaker in these MV switchgear lineups? I have also seen similar applications on Medium Voltage Control lineups.

Most of these applications did have CT's in the incoming section of switchgear that were part of a transformer differential scheme or had both CT's and a relay in incoming section that transfer tripped an upstream breaker on transformer primary. I guess these secondary CT's in both of these cases act as a "virtual main"?

 

[davidbeach]

Lowest first cost. Depends on your financial model as to whether or not that approach provides the lowest life cycle cost.

We used to omit the low-side main, years and years, but have determined that the breaker cost up front lowers the life cycle cost. Your mileage may vary.

 

[rockman7892]

Thanks daidbeach

I'm curious to hear what you have learned over the years how the breaker cost up front lowers the life cycle cost?

 

[davidbeach]

As a utility the treatment of capital vs. O&M costs are different than they might be for other businesses. A reduction in O&M costs provides more benefit than it might for others.

The low-side main breaker provides a higher degree of safety; most locations where we don’t have a low-side main we’re relying on a low-set instantaneous “maintenance mode” setting for crew arc-flash protection when someone is working in the gear. That instantaneous element is responsive to feeder faults.

With a low-side main we can have a transformer differential and a separate bus differential zone. Better safety, easier maintenance, and we’re less likely to trip the transformer. If the transformer is tripped, there’s a good chance it is going to be tested; that takes time and expense. Saving a transformer test over the life of the gear may be enough to pay for the breaker. The increased crew safety helps too but may be a less tangible benefit.

 

[rockman7892]

davidbeach

Thank you for your insight that is helpful.

When you refer to a low-set instantaneous "maintenance mode" I'm assuming you are referring to implementing this maintenance mode setting on the primary relay to see through the transformer for secondary faults?

Another application I see a lot of is having CT's in the switchgear incoming section and using these CT's for differential across the transformer as well as setting up overcurrent & INST settings on these secondary CT's to transfer trip to upstream breaker. I have also seen maintenance mode settings applied with these secondary CT's to provide fast INST response on the secondary of these CT's in order to transfer trip to upstream breaker. I have seen this application a lot on LV switchgear too where these secondary CT's act as a "virtual main" for required phase and ground fault protection.

With LV switchgear there are more NEC requirements than with MV switchgear however as long as you can provide these requirements without a main breaker or providing CT's as a "virtual main" I don't really see any issues with omitting main breakers on LV Switchgear either?

 

[davidbeach]

If you have low-side CTs without a breaker you can gain much of the benefit of a low-side main by implementing a transformer differential and using the low-side CTs for the maintenance mode. Transformer differential plus bus differential goes even further.

If there's the option of a tie to the low-side of another transformer (or more) the low-side main makes that switching easier. Industrial installations are more likely to approach that in a Main-Tie-Main assembly in a single line-up. I'm more used to a separate switchgear per transformer with ties between, the main makes that easier. In the industrial world the lack of a main often suggests that there isn't another readily available alternate source.