What are the benefits and drawbacks of using DC vs AC switchgear motor voltage?

[electrical429]

I always specify DC, since in the event of AC supply failure you can still operate the switchgear for 6-12h until the battery runs down. Are there any other points that should be taken into account?

 

[che12345]

Dear Mr. electrical429 (Electrical)
"...I always specify DC, since in the event of AC supply failure you can still operate the switchgear for 6-12h until the battery runs down. Are there any other points that should be taken into account?.."
Che. 1. Sir, what is the [voltage] level (<1kV) or (>1kV) and what is the [wattage] level (<1W), (<100W), (<1 kW), (<100 kW), (<1 MW) etc., are you talking/concerned about?
2. AC can be in single-phase or three-phase with (voltage) being easily [transformed up/down] very efficiently. But not DC. AC Single-phase or three-phase motors are very robust and efficient, which DC motors impossible to match; unless under special condition/requirement.
3. There are transmission lines with three-phase voltage (>400kVac). There are also EHV DC (>400kVdc)transmission lines but eventually invert back to AC; for ease of voltage transformation.
4. Domestic, commercial and industry electrical equipments with 99.99% operate on AC. But EV, forklift , torch-light etc., operate on DC.
Che Kuan Yau (Singapore)

 

[Coco_HueHueHue]

Are you talking about DC vs AC power or DC vs AC controls?

For MCC, i often see AC controls (from control transformers) so in a loss of power or short-circuit with an upstream protection operating, the motors shut down immediatly and it is fail safe.
I have often seen DC controls in switchgear because you don't want to lose those controls and protection relays while you had a short-circuit somewhere and an upstream breaker opens as you will need those controls turned ON to re-power safely all your equipment.

 

[electrical429]

Hi All

I am referring to the spring charge motor spec for 11 to 132 kV switchgear. So I need to specify the following:

1. Control voltage - always DC because control of circuit breaker needs to remain under AC power loss (using DC batteries);

2. Spring charge motor voltage - can be DC or AC. As per my original question I always specify DC, but curious of any advantages of specifying AC motors;

3. Heating - usually 240VAC.

 

[davidbeach]

If you might ever close the breaker to energize the station go with DC. IF the station must always be energized first, using AC close and charge can save on the size of the battery plant with limited impact to the station. Transmission, where nearly all connections can be sources, go with DC. Distribution, where all the feeders are radial, load serving, there's a certain economic advantage to AC close/charge.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Sn00ze]

Why not specify motors that can operate with both AC and DC?