high leg load

[jim dungar]

I'm not following this statement.

If we ignore parasitic secondary and primary impedances, but include in our transformer model the primary inductance that creates the magnetic flux in the iron core, my understanding is that the magnetic flux is constant, independent of the load currents. The primary magnetizing current creates the magnetic field in the iron core; that's end of the story when the secondary is open circuit. If we add a secondary load, current flowing in the secondary creates a magnetic flux, which will be precisely canceled by the magnetic flux from the referred current in the primary.

So what's wrong with saying that since the secondary current is equal and opposite in the two half of the split secondary coil, the magnetic fluxes from those two halves will cancel each other, and the referred current in the primary is zero?

The magnetic flux and current are dependent. In a closed delta primary there is always some current flowing so there will be flux in a specific direction, as well as the magnetizing current you mentioned. This is part of the reason many utilities do not like to use 3-legged cores, and instead ask for tri-plex construction which are effectively three single phase transformers in a common metallic tank. Many of these concerns go away with an open delta bank of two separate transformers.

 

[wwhitney]

The magnetic flux and current are dependent.

So, I'm a bit new to the magnetic side of things, but for idealized transformers, that's not what I'm reading as far as the load current. E.g.

Why we use constant value of magnetic flux in transformers

I have a confusion regarding the formulas of the transformer. I have seen in many places people using a constant value of the flux B in the transformer equation: turns per volt=1/(1.44 x Bmax x A ...

electronics.stackexchange.com

For the transformer model below (excerpted from above), the magnetic flux would depend on I_M, and not the load currents I_L or I_O.

For a real transformer, I gather there's leakage flux, which would depend on total current I_P and hence the load currents, and voltage drop on the primary supply, which would likewise depend on total current, but I believe those are second order effects.

Cheers, Wayne

 

[wwhitney]

Which is why it is rare to see a closed delta that allows for any significant neutral loading.

If your 3W delta to 4W delta transformer is constructed so that the primary coils are identical, and the secondary coils are also identical (except for the center tap), then there will be a maximum allowable current for each secondary coil. E.g. for a 75 kVA transformer with a 240V/120V 3P4W secondary, it would be 75 kVA / 0.24 kV / sqrt(3) = 180A.

For any unbalanced loading, isn't it sufficient to just determine all the secondary coil currents, and check that none exceed the maximum allowable current? E.g. if you wanted to use that 75 kVA transformer for a single large 120V load, you could supply up to 180A, or 120V * 180A = 21.6 kVA, as long it supplies nothing else. You'd just be using the transformer at less than 1/3 of its potential capacity, as you're only using 1/6 of the secondary coils.

In other words, if you need to supply X kVA of balanced 3 phase loads, and Y kVA of unbalanced 120V loads (no L-L single phase or balanced 120V loads, just for simplicity), then can't you use any transformer whose rating is at least X + 2 * sqrt(3) * Y kVA?

Thanks,
Wayne

 

[jim dungar]

If your 3W delta to 4W delta transformer is constructed so that the primary coils are identical, and the secondary coils are also identical (except for the center tap), then there will be a maximum allowable current for each secondary coil. E.g. for a 75 kVA transformer with a 240V/120V 3P4W secondary, it would be 75 kVA / 0.24 kV / sqrt(3) = 180A.

For any unbalanced loading, isn't it sufficient to just determine all the secondary coil currents, and check that none exceed the maximum allowable current? E.g. if you wanted to use that 75 kVA transformer for a single large 120V load, you could supply up to 180A, or 120V * 180A = 21.6 kVA, as long it supplies nothing else. You'd just be using the transformer at less than 1/3 of its potential capacity, as you're only using 1/6 of the secondary coils.

In other words, if you need to supply X kVA of balanced 3 phase loads, and Y kVA of unbalanced 120V loads (no L-L single phase or balanced 120V loads, just for simplicity), then can't you use any transformer whose rating is at least X + 2 * sqrt(3) * Y kVA?

Thanks,
Wayne

You are talking about (3) individual 25kVA transformers being connected into a 75kVA delta configuration.

Each winding can only carry its single phase rating of 104.16A at 240V.

 

[wwhitney]

Each winding can only carry its single phase rating of 104.16A at 240V.

Yes, sorry sqrt(3) error. As you say, 104A per coil, which makes the transformer sizing X kVA (3 phase balanced load) + 6 Y kVA (120V unbalanced load). Which makes a lot more sense, since the unbalanced 120V is only using 1/6 of the coils.

Do they make high leg delta secondary transformers in which the split secondary coil, and corresponding primary coil, is made with a higher allowable current than the other legs?

Otherwise I see your point now, the penalty factor of 6 for the unbalanced 120V load is quite large.

Cheers, Wayne

 

[jim dungar]

It is extremely common for these transformer banks to be made with unequal size transformers. One of which is sized for the neutral load.

 

[topgone]

Yes, sorry sqrt(3) error. As you say, 104A per coil, which makes the transformer sizing X kVA (3 phase balanced load) + 6 Y kVA (120V unbalanced load). Which makes a lot more sense, since the unbalanced 120V is only using 1/6 of the coils.

Do they make high leg delta secondary transformers in which the split secondary coil, and corresponding primary coil, is made with a higher allowable current than the other legs?

Otherwise I see your point now, the penalty factor of 6 for the unbalanced 120V load is quite large.

Cheers, Wayne

In my utility days, we used to configure open-delta banks for customers who applied for 3-phase power upgrades from their old single-phase connections. The center-tapped single-phase transformer carries all the single-phase loads while the smaller single-phase transformer is added and connected to the center-tapped transformer (open-delta) to cater for the 3-phase portion of the additional 3-phase load. I guess, that is what Jim was talking about.

 

[jim dungar]

In my utility days, we used to configure open-delta banks for customers who applied for 3-phase power upgrades from their old single-phase connections. The center-tapped single-phase transformer carries all the single-phase loads while the smaller single-phase transformer is added and connected to the center-tapped transformer (open-delta) to cater for the 3-phase portion of the additional 3-phase load. I guess, that is what Jim was talking about.

Yes, utilities like to use open-delta partly because of the issues associated with closed deltas and unbalanced neutral loading.

 

[tortuga]

As we discussed in this thread the most common 'mistake' I see with a open delta service is loading B-C legs with a large 240V single phase load.
I have actually seen two separate transformer failures due to this.
I seriously doubt an electrician would connect a 208V B-N load to a 240V delta or open delta system however it has been discussed frequently on here so who knows?

Yes, utilities like to use open-delta partly because of the issues associated with closed deltas snd unbalanced neutral loading.

I have never had an issue with a closed delta. Only an open delta.
I dont understand why a full delta bank would be of any concern?
However in my experience all the open delta facilities that had failures, once had lots of three phase motor loads and later were renovated to have large 240V single phase (server, HVAC and lighting) loads.
Converting them to closed delta fixed the issues.

 

[LarryFine]

In my utility days, we used to configure open-delta banks for customers who applied for 3-phase power upgrades from their old single-phase connections.

And added a delta breaker for the new 3ph load.

 

[jim dungar]

I have never had an issue with a closed delta. Only an open delta.
I dont understand why a full delta bank would be of any concern?
However in my experience all the open delta facilities that had failures, once had lots of three phase motor loads and later were renovated to have large 240V single phase (server, HVAC and lighting) loads.
Converting them to closed delta fixed the issues.

The use of (3) individual transformers is also a positive factor as opposed to the common core construction found in non-utility installations.

 

[wwhitney]

The use of (3) individual transformers is also a positive factor as opposed to the common core construction found in non-utility installations.

For circuit analysis, with idealized transformers, does that make a difference? Or is the benefit only related to factors like leakage flux that don't show up in idealized transformer models?

Thanks,
Wayne