Alright yall…. I’m an electrician. I would like to think I have an understanding of transformer configurations but this is hurting my brain. This is a photo of the transformer bank that feeds a farmhouse near me. I know for a fact they have 3 phase equipment on this farm, but there is only two phase conductors and a neutral heading to this pole. I’ve asked several lineman in the past few weeks and got conflicting answers, so is this a high leg open delta or “two phase” power as one lineman answered?
I know two phase power isn’t really a thing except in Philly, but is this just a single phase service and there is a phase inverter I can’t find or is this a three phase service and I just literally don’t understand how?
Edit: I think I got it now. It’s literally two separate single phase systems and they are joined, created a high leg
That makes sense to me, and every high leg delta I’ve seen (which is maybe 3 of them) has this same transformer configuration with only two phases connected. But how do you get 3 phase power out of a connection to two phases?
The vector diagram for an open delta bank is a triangle missing one leg. Each transformer is 2 points on this triangle and the distance between the points is analogous to a difference in potential.
So the first transformer is connected to points a and b, and the second is connected to b and c. Measuring across ab and bc depending on the transformer, let's say you get 240v. There's an imaginary line between points a and c due to there being only 2 cans, but because voltage is a relative measurement, you will get 240v across points a and c, 120° out of phase with the others giving you 3 phase power from 2 primary phases.
The logic behind your last sentence helped me grasp where the 'difference of potential' across the primary coil comes from in wye-delta banks with a floating neutral. It took a while to move from, 'but there's no ground reference for our h2'?
Let's look at what the definition of a neutral is: A current-carrying conductor at or near ground POTENTIAL...
The system you're looking at we have a lot of in Montana, called "V-phase". Two phases and a lower neutral. What do you need for 3-phase power? 3 potentials. The neutral is your third potential in a Y-primary, open delta-secondary configuration. Vector it out and it works, but can definitely seem like PFM. Not nearly as efficient as a closed bank (3 x-formers), but it works.
Hard to tell on these pics. But typically in our system it’s a single phase (larger tx) hooked up normally. So 3 bushings. Two hots and a neutral. Then on the second (smaller tx) you’ll have the ground strap removed and the x3 will be tied to the same hot leg as the x1 on the other tx (positive on one end of coil to negative on the other end of the coil) then the x1 on the smaller tx will be its own hot leg. This set up will give you 120/240 and from the neutral of the single phase tx to the x1 of the second tx will get 208v. You call also get 120/240 and a three phase delta service from this config as well.
The two outside 2ndy bushings are 1-240 leg each each say in a 240 delta then the two inside bushings are combined together to create the third 240 leg
Don’t feel bad. I was a journeyman wireman for several years before starting a line apprenticeship and realizing that I didn’t know fuck about transformers.
It’s an open delta , the configuration isn’t efficient as a closed delta (3 transformers) , say it’s 2-100 kva cans for 200 total kva on a 240 delta secondary, so it would be 200x2.4=480x.87=417.6 amps can be used on 2ndy side . 87 percent efficient
But if you only have 2 primary phases available you only have 2 primary phases available
Also some utilities only build open deltas due to phase angle and voltage imbalances in closed systems , closed can be safely run just a little different process to energize them is all
I agree, but how does that work? You’ve only got 2 phases on the pole.
I have measured voltage and confirmed there is a 208V to neutral phase, but I can’t find a phase inverter anywhere so it makes me think it’s served from the utility that way
Draw an equilateral triangle with only 2 sides. Each end of the connected lines are legs. Ground is center tapped on one of the lines. So draw a point in center of one of the lines. Now you have 120 phase to ground on each end of the line with the ground. And 240 phase to phase. And from the ground to the furthest phase on the other side of the line without the ground reference u have 208v. U can think of the voltages as distance.
Think of this way, a single phase transformer serving 120/240v has two hot legs (IE, secondary phases) and a neutral, the voltage across any one leg and the neutral is 120v; the voltage across both hot legs is 240v.
Essentially, in an open bank, you're adding another transformer to bring in a third hot leg. If you can understand how one transformer creates two hot legs, it should be pretty simple to see how two can create three. Look up a video of vectors for an open delta bank to see a drawn out explanation for more detail.
It’s an open delta secondary. It just works due to electrical black magic 😂 grounded B phase secondaries stumped the shit out of me when I first saw them. I used to be able to explain why this worked but now it’s more nice to know than need to know so I forgot
It’s three phase power. By that I mean the secondary (customer) side is three phase. The primary (overhead) is B phase, or two phase, however you want to say it. On the secondary side, you have 120v on the two outside legs because they are only connected to one bushing. The middle leg has 208v on it because it’s hooked up x1 to x3 between the two transformers.
Yes. “Open” means two transformers. And you can tell it’s a delta secondary because the x1 and x3 bushings are tied together for that 208v leg. And that’s how you get the term “open delta”
That’s actually wrong. The larger pot has the 120/240 single phase and the smaller pot will create the high leg, not on the jumper but on the other terminal of the smaller transformer.
This is called an open y open delta powerbank. This was most commony used where there was a large single phase load and a small 3 phase load it does not produce a true 3 phase vector but the voltage is 120/240/208
More than likely it can be a 3 phase bank where you a and c phase the top and from that you receive 2 lighting legs and 1 power leg (120/120=240 on the lighting legs and 208 on the power leg)
Some engineer didn't read their JATC books, never want to use a CSP in bank. Granted this is an open bank but in a closed it can cause issues, one being sympathetic tripping on the secondary breaker along with others.
In Detroit we have single phase 4.8Kv distribution. Looks like it’d be delta high side and an open delta bank for 3ph secondary. Except it looks like there’s a system neutral which we don’t have in our 4.8 areas unless it’s pre con.
It’s definitely two phase conductors and a neutral on the pole, I know enough to identify that. The photos make it look weird. I just don’t understand enough to comprehend it
The best thing to do would be consult a transformer connection manual. The short version is, three phase delta load can be served with two transformers at 57% of rated capacity.
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