Buck-Boost

[charlie b]

There is an existing space, a small lab that has mostly computer-type equipment (120V). I am designing a change to its power source. It has a few oddities that I need to deal with. Let me present two questions now, and I may have others later.

Situation: There is a 120/208V, three phase panel, from which a 50 amp, two pole breaker presently supplies ?normal? power to the lab. Immediately above the 120/208V panel, there is a buck/boost transformer. I do not understand exactly how such things are wired, nor how they operate. There is a backup generator (120/240V), a transfer switch, a UPS, a bypass switch, and some other stuff, all of which we will ignore for now. All of this stuff provides power to a 120/240V, single phase panel, and all lab loads are fed from there.

We all know that upstream of the buck/boost, the voltages will be:
? L1-N at 120V,
? L2-N at 120V, and
? L1-L2 at 208V.

Question 1: I believe that downstream of the buck/boost you will have L1-L2 at 240V. But do you still have 120V line to neutral on both lines?

Question 2: Upstream, L1 and L2 are separated from each other by 120 degrees. Will the buck/boost cause a further 60 degree phase shift, so that from that point downstream it looks just like any standard 120/240V single phase distribution system?

 

[Mike01]

buck boost

buck boost

it all depends how its wired..if you need a three phase load only you could wire two in an open delta configuration and get a 240 or 230V-3phase secondary but no neutral if you need a neutral you will need three transformers to carry the neutral thru. just small 24x12 or 16x32 step down transformers with the insulation beefed up to 600v for the applications wired similar to an autotransformer with a common winding . http://www.schneider-electric.us/pr...age-transformers/buck-and-boost-transformers/

http://forums.mikeholt.com/showthread.php?t=118337

 

[charlie b]

They do not have, or need, three phase in the lab. I think that all they have is 120 volt loads, although there is a rumor of their supplying 208 (or is it 240?) to a rack-mounted power supply. My present concern (one of several, but the only one under discussion) is

(1) whether a receptacle connected to their 120/240V power panel as a Line 1 to neutral load is actually getting 120 volts, and

(2) same concern for a receptacle connected Line 2 to neutral.

 

[LarryFine]

Charlie, if you only have line-to-neutral 120v loads, line-to-line 208v loads that do not need a neutral, and line-to-neutral-to-line 120/208v loads that need a neutral and are okay with 208v line-to-line, you're also okay without one.

If you have any 240v line-to-line loads that do not need a neutral, one buck-boost is enough. But, if you have any 120/240v loads that do need a neutral, you will need either two buck-boosts, or a 208v-to-120/240v transformer and a GES.



I think.

 

[hillbilly1]

........... But, if you have any 120/240v loads that do need a neutral, you will need either two buck-boosts,...........I think.

If you need 120/240 one or even two buck/boosts will not work for what you are doing, only the 208 to 120/240 transformer that you mention later in this post will work. The line to neutral voltage also gets boosted when you use the buck/boost transformer 136 with the two transformers, or 120/152 using one transformer. Most computer rack power supplies that require 240 input, create their own neutral for the 120 volt loads using an isolation transformer built into the power supply.

 

[scott thompson]

Charlie,

If there is only One Single-Phase Transformer, and the Secondary Voltage is listed as 16/32V, chances are the output Voltage desired is 240V.
As many Servers are setup for 240V 1 Phase input, this may be true in your case.

With a 208V 1 Ph. 2 Wire input to 240V 1 Ph. 2 Wire output, the Voltage to Ground would be somewhere in the 136V range.

If the Secondary Voltage is 12/24V, the L-L output Voltage could be 220V or 232V - all depends on the way the Secondary Windings are connected.

Is the Transformer KVA rating in the 2.0 KVA range?

Scott

 

[gar]

100402-2150 EST

Roger:

Looking at your posts 1 and 3 I suggest that you make some voltage measurements if it is not easy to circuit trace the wiring.

At the three phase Y source what are the line to neutral voltages and the line to line voltage (before the two pole breaker)?

I believe from your description that from the 3 phase panel there are four wires to the 120/240 panel. These are L1, L2, N, and EGC and I am guessing are terminated on the bus bars at the 120 panel.

This buck-boost transformer is on the 120 panel. At the moment I do not know how it is wired. If the voltages on the bus bars are essentially the same as at the three phase panel, then the buck-boost has no effect on the bus voltages and the 120 circuits.

If the buck-boost transformer output precedes the bus bars in the 120 panel, then these 120 v panel bus voltages will be different than at the 3 phase panel. I doubt this is the case.

If the 240 V primary of the buck-boost is supplied line to line with 208 and its output with 240 in is 32 V, then this is reduced to 32*208/240 = 32*0.866 = 27.7 V. Adding 27 to 208 produces 235. This summation is valid because the waveforms are in phase. If the primary was supplied from one of the 120 line to neutral voltages, then the summation would be less.

It should be possible to put the boost secondary in series with the 208 and supply the primary of the boost with this summation (208 + 32), and thus get 240 output. Therefore, some of the energy to supply the primary of the boost is coming from the secondary of the boost. This will lower the available output current to the external load.

.

 

[charlie b]

Many thanks for the assistance. But as it turns out, not only was I not clear about the nature of my question, I was also wrong about the existing configuration. I have since learned that the two power sources, including the "normal" source from 2-poles of a 120/208 panel, the one that goes through a buck/boost, do not bring a neutral along for the ride at all. The neutral to the lab's panel is being derived either from the UPS (normal configuration) or from a small local transformer (UPS bypass configuration). So the line-to-neutral voltage values that I had been worried about (i.e., upstream of the UPS) are not relevant after all.

Just for the sake of its potential entertainment value, I am attaching a sketch of the existing power distribution system. I see a few problems. Do you? :grin:

 

[LarryFine]

I see a few problems. Do you?

Two things:

1. Without knowing, is the 10kva transformer's 120/240v secondary neutral bonded and electroded?

2. You're feeding 208-to-240v boosted, and 240v from the genny, to two things expecting 208v.

 

[hillbilly1]

1. The UPS output would not be 120/208, It would be 120/240.
2. The neutral is probably not bonded at the generator, so with it being capped at the t-switch also, a fault to ground would not trip the O.C.P on the generator.

 

[charlie b]

I do not recall seeing evidence of a grounding electrode conductor running from either the transformer or the panel it serves. I wasn't specifically looking for one either, since that wasn't related to the task at hand, the last time I was there. But I have some photos of the area, and they do not reveal a GEC. So the absence of one is among my collection of concerns here.

Any comments about secondary protection? That is the essence of my concerns.

 

[charlie b]

2. You're feeding 208-to-240v boosted, and 240v from the genny, to two things expecting 208v.

The downstream panel is rated 120/240. The only loads I know of are either 120 volts or are rated for the range of 208 to 240.

 

[LarryFine]

The downstream panel is rated 120/240. The only loads I know of are either 120 volts or are rated for the range of 208 to 240.

I was referring to the 10kva transformer and the UPS, both of which you show with 208v inputs, yet both receiving 240v.

 

[charlie b]

OK, Larry, I see what you meant. I have a problem with that as well. For one thing, I don't know what the UPS will do with a 240 volt input. For another, if you put 240 into a transformer rated for a 208 input and a 120/240 output, I infer that you will get 140/280 volts out. That can't be good for any downstream circuit, and might even exceed the rating of the 120/240V panel.

Just FYI, I have formally notified the client of my concerns, and cited the specific NEC articles that I suspect might be violated by the existing configuration.

 

[hillbilly1]

....... For one thing, I don't know what the UPS will do with a 240 volt input. For another, if you put 240 into a transformer rated for a 208 input and a 120/240 output, I infer that you will get 140/280 volts out. That can't be good for any downstream circuit, and might even exceed the rating of the 120/240V panel.

Some UPS units can have the input voltage configured electronically for either 208 or 240, others have to be ordered with the correct input voltage, especially if there is a bypass module. As far as the output goes though, it will regulate it at 120/240 if it's doing its job.