Open Star-Delta Configuration

[opmgr1]

I have a situation at my utility where a customer applied for capacity for 25HP motor. There is an existing 100kVA transformer at the location supplying existing single-phase load.
What are the disadvantages of banking a 25kVA transformer with the existing 100kVA transformer to provide a 3-phase open delta supply to the motor. The 100kVA transformer is currently supplying 66kVA.

Regards.

 

[davidbeach]

Voltage drop, phase angle stability.

 

[lansford]

Voltage balance at the load will be worse than if three idnetical tranformers are used. But for NEMA motors, there has been some issues with warranty in the past when motors are applied to this transformer configuration.
JIM

 

[opmgr1]

Thanks davidbeach and lansford for the quick response. I am more interested in the power distribution among the two transformers. Now each transformer capacity is reduced by approx 13% and while this can easily supply the 25HP motor, is there any danger that loading the larger transformer (with single-phase loads) to above say 3 times the working capacity of the smaller transformer will cause the smaller transformer to burn if the three-phase load remain constant?

Regards,
Craig.

 

[waross]

In full Delta, the current in the motor is 1.73 times the current in each transformer. In open delta, the current in each transformer is the same as the current in the motor.
With only the motor connected, the current in the 25 KVA transformer will equal the motor current.
In the 100 KVA transformer the current drawn by the motor will not be in phase with the current drawn by your single phase loads. The total current on the 100 KVA transformer will be less than the sum of the single phase loads plus the motor load.
Issues;
With a firm supply and low impedance transformers with good regulation open delta may work well.
Possible issues;
1> Supply-
You will be drawing primary current from two phases and the neutral. Voltage drops may be an issue. A voltage drop in the neutral will result in a displaced neutral which causes phase angle errors. The unequal loading may cause unequal voltage drops in the primary phases.
These factors can result in unequal voltages at the motor terminals which cause disproportionate motor heating.
2> Transformer regulation-
Again, unequal PU loading may result in unequal voltage at the motor terminals.
What does this mean in real lfe?
These issues may be present in a closed delta or wye system also.
If the primary supply has reasonably balanced voltages and little neutral current and is able to accept your loading without causing a noticeable voltage drop on the neutral you are probably safe.
If your primary voltages are unequal, usually due to unequal loading but possibly due to a lack of transitions, you will have issues even with a three transformer bank. The issues will be worse with open delta.
If you have three single phase voltage regulators upstream, you may already have phase angle issues.
There are issues with voltage drop, phase angle errors and neutral displacement that may be present in a long, often rural, distribution line. These will result in motor heating.
Adding a third transformer may improve but will not always eliminate these issues.
Now let's look at the motor. The motor inherently acts as a generator. It is generating back EMF. The Back EMF phase angles and voltages are dependent on the spacing and the number of turns in the windings. It is inherently balanced three phase. Should the supply not be balanced three phase, the result will be disproportionately high currents and motor heating.
If you have a solid primary system open delta should work.
If you have a weak primary system with unequal loading and relatively high impedance, you may have issues with either an open delta or full three phase. The motor heating may vary depending on variations in loading and voltage drops on the primary system.
It is often impractical to correct the primary issues.
If the motor is lightly loaded it is often able to accept the unbalanced voltages without exceeding the temperature rise. If the motor is fully loaded there is no thermal capacity left to allow for extra heating.
The bottom line. If there are problems with an open delta installation, the answer may be to use an oversized motor so as to have excess thermal capacity.
If you go full delta, be aware of the issues with wye delta banks. I prefer wye/wye rather than wye/delta.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[opmgr1]

Thanks Bill,
This info really helps. I was struggling with some rule of thumbs established at the utility about not banking transformers together where the capacity of the larger transformer is more than 3 times the capacity of the smaller transformer. I have also looked at the ABB transformer hand book and suspect how the rule of thumb came about.
Again Thanks.

Regards,
Craig.

 

[apowerengr]

Open-delta banks are usually reserved for predominately single-phase loads with light three phase load. The connection should be made "leading" where the lighter tub is connected to the leading phase and the power (or kicker) tub connected to the lagging phase. This connection minimizes collapse of the "open" phase for lagging power factor loads. Most utilities have the customer sign some release for operation of three phase loads on open banks.

If you have three phase near the site I'd close the bank for a 25HP motor. As stated the closed connection will reduce or remove the imbalanced load from the system neutral and allows more load-sharing between the three transformers (1/3 each for three phase load, 2/3 single phase load on the lighter tub, 1/3 single phase load on each of the two power (kicker) tubs). See ABB or GE transformer manuals for a better discussion on load sharing for closed-delta power banks.

 

[opmgr1]

The background to the problem is as follows:
At the utility the customer is required to pay for 3-phase transformation. Now at the location there is a 100kVA transformer that have a spare capacity of approx. 34kVA. A least cost proposal came from an engineer to bank the existing 100kVA transformer with a new 25kVA transformer, so in essence the customer pays for only the 25kVA transformer. An engineer indicated that he read some where that as a rule of thumb when banking transformers open-delta the larger transformer should not be more than 3 times the size of the smaller transformer as, if the 3-phase load remains constant and the single phase load is increased, the smaller transformer will burn. I have searched for this info in leading standards and handbooks and cant seem to find it anywhere. This rule of thumb is what I have some issues with and want to see if it holds true.

 

[waross]

That rule of thumb probably applies to a three transformer bank, not an open delta connection.
In a closed delta transformer bank, The current fr a single single phase load on "A" transformer will be supplied equally by transformer "A" and by transformers "B" and "C" together.
The current through "B" and "C" will be in phase with "A" and so will be 60 degrees out of phase with "B" and "C". Thus the single phase current through "B" and "C" can not be added arithmetically to any three phase load currents. Generally the KVA rating of a three transformer three phase delta bank should be based on the smallest transformer in the bank.
You are considering an open delta bank.
The teaser transformer must be capable of supplying the full motor current. The motor current and the single phase current through the transformer supplying the single phase loads will not be in phase. A simple check is to add the motor current and the single phase current arithmetically.
The actual current will be less, so if the transformer can safely supply the arithmetic sum of the currents, it has adequate capacity.
There is no circuit to allow the teaser transformer to share the single phase loads.
Be aware that if the primary phase supplying the single phase loads is lost while the motor is running, the motor will act as an induction generator and back feed to transformer to supply the single phase loads. This will grossly over load both the motor and the teaser transformer. Be sure that you have good single phase protection on the motor controls. On an open delta you may have to set any unbalance protection to accept a slightly higher current unbalance. However, phase unbalance protection is important to protect against the gross unbalances associated with single phasing.

This proposal depends to a great extent on the motor, the motor load, and the firmness and voltage balance of the primary circuit.
Given one combination of factors it may work well. With another set of factors it may be a source of continual issues.
Bottom line: I would probably go with the 100 KVA plus the 25 KVA. I would make the customer aware that if problems develop the solution may be a larger motor. But, with a poor primary feed, a larger motor may be required in any event.
There is not enough information available to give a better quality recommendation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

is there any danger that loading the larger transformer (with single-phase loads) to above say 3 times the working capacity of the smaller transformer will cause the smaller transformer to burn if the three-phase load remain constant?

Not for open delta.

In case of confusion, I refer to the transformer that is added, to develop three phase, as the "Teaser" transformer.


Bill
--------------------
"Why not the best?"
Jimmy Carter