Shipboard and domestic model APC UPS Difference

[BillP]

Hi,

The following link tells the difference between shipboard and domestics UPS.

Frequently Asked Questions

The following explanation is not clear for me;

"As a result, it is possible that 180Vrms may be

passed to the load when switching to battery (120Vac from the UPS inverter on

one phase and 60 volts from the ship on the second phase)."

I can understand 120Vac from UPS inverter on one phase. But, what is 60Vac which comes from the ship? How?

[BillP]

From what I understand, it is because ships especially large ones uses Delta Power instead of the regular residential outlet configuration.

[voidstar_apc]

Residential power has one hot leg (120V) and two grounded (0V) legs (neutral and ground). When switched to battery, residential UPSes produce 120V on the hot leg, and pass neutral through for safety reasons. That means the load sees 120V between hot and neutral.

Delta power systems have two hot legs (60V each) and one ground (0V). Loads still see 120V between hot and neutral in this system. When a residential UPS switches to battery, it produces 120V on the hot leg (not the 60V used in this system), and passes the neutral (possibly 60V from the ship) directly to the load. So under this condition the total can be as high as 120V+60V = 180V between hot and neutral.

[voidstar_apc]

That's a good idea -- a properly configured transformer powering the UPS sounds reasonable to me. I came across this from Powerstar.

[BillP]

"So under this condition the total can be as high as 120V+60V = 180V between hot and neutral."

Is this a regular 180 VAC, pure sin wave voltage or not?

Assume that all the systems that supplied by UPS can accept 100-240 VAC power. In this case, can the following be declared easyly?

"domestics UPS can also be used in ships if the system supplied by UPS are in 100-240 VAC power input range?

The reason to ask such a question is to find whether domestics UPS can also be used as shipboard UPS in case of maintainance issues or unsuccessfull procurement phase.

[voidstar_apc]

erdemk wrote:

"So under this condition the total can be as high as 120V+60V = 180V between hot and neutral."

Is this a regular 180 VAC, pure sin wave voltage or not?

Yes, providing you're using a pure-sine wave SmartUPS and ship power is not drifting in frequency.

Assume that all the systems that supplied by UPS can accept 100-240 VAC power. In this case, can the following be declared easyly?
"domestics UPS can also be used in ships if the system supplied by UPS are in 100-240 VAC power input range?

You could probably get away with it, but according to the FAQ, APC doesn't support that configuration.

One reason I'd be wary is the phase and frequency of the ship power and the UPS inverter will match when transferring to battery, but will drift thereafter. When the ship power and the inverter are completely out of phase (ie, completely in-phase with the "neutral" hot leg), the output will be 120V - 60V = 60V which is below the 100VAC input range. Maybe you're never on battery long enough for that to be an issue, but if it's sensitive enough to be on a UPS, then why risk it?

Edit: Just wanted to add a picture of frequency drift.

Here's what the voltage looks like at the load when frequency of the UPS and ship power drift by 0.3 Hz, which is a lot, but makes a good picture:

The two voltage sources in the graph are 60 Hz and 60.3 Hz. At a timescale of tens of milliseconds, the waveform appears as a clean 60 Hz sine wave. On a longer time scale, the amplitude of the voltage appears to cycle between 120+60 = 180VAC, and 120-60 = 60VAC. Cycling occurs at the beat frequency... 60.3 Hz - 60Hz = 0.3 Hz which is how fast the two voltage sources go into phase and subtract from each other and come back out of phase and add to each other.

So to me it seems pretty easy to go below the 100VAC limit if ship power is available during a transfer to battery. But of course the time it takes for this to occur depends on amount of drift.

[wayne_i_apc · Accepted solution]

As a "plan B" to a shipboard UPS model, do you think a carefully configured isolation transformer between the ship power and the UPS input could be a solution to the "missing" neutral of the ship power? When I say "carefully configured" I mean "floating" (not ground referenced) primary and "bonded" (connected to ground) neutral secondary, and of course, properly sized for KVA (power) capacity. Isolation transformers are NOT cheap, but might be a work-around as Erdemk suggests for maintenance, etc. Clearly, a domestic UPS directly connected to shipboard power is asking for trouble.