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The open delta three phase service

Several months ago, I drove up to an FM transmitter site, looked up at the utility pole and saw this:

Three Phase open delta transformer bank

Three Phase open delta transformer bank

Three phase open delta is a bad hombre.  Most, if not all, transmitter manufactures will void the warranty of any transmitter connected to a service like this.  What is perplexing is it appears that all three phases are available on the primary side, why would this be necessary? Perhaps it was not always so at this location.  Regardless, this was the source of power for 20 KW FM transmitters since 1958 until we moved it to a new building last month.

According to a GE publication on transformers, open delta 3 phase power is undesirable because:

Although this connection delivers three-phase currents which are approximately symmetrical to a three-phase symetrical load, the currents flowing in the high voltage circuit are not equal nor are they 120 degrees apart.  The maximum safe output of the bank operating in this manner is 58% of a 3 pot Wye/Delta bank. The system is grossly unbalanced, both electrostatically and electromagnetically.

Schematically, it looks like this:

3 Phase open delta power

3 Phase open delta power

Regular 3 phase delta looks like this:

3 phase delta power

3 phase delta power

Most utility companies will not hook up 3 phase delta on the customer side anymore because the the “high” or “wild” leg, which as shown in the diagram runs a good deal higher than 120 volts to neutral.  Hook up a high leg to a single phase 120 volt piece of equipment and wait for the power supply to blow up.  Also true with 277 volt lighting circuits, as my assistant once found out with the Coke Machine in the break room.  New 3 phase service will almost invariably be 208 wye unless there is some very compelling reason, which is fine.

There are many ways to get around three phase open delta, perhaps the best is a rotary phase converter.  This piece of equipment will take a 240 volt split phase and add a third leg.  These legs will not be 120 degrees apart, as they would be in a true three phase, however, they will be close enough that 3 phase motors and transformers will be happy.

Rotary Phase converter

Rotary Phase converter

This leads to an unbalanced voltage/current condition which needs to be accounted for in the design of the unit.  The second way to do this is to power a three phase generator with a split phase motor.  This will completely isolate the 3 phase equipment from the utility service and provide for true three phase power.

The downside to any motor/generator or rotary converter is moving parts and conversion inefficiencies.  At any transmitter site that uses this type of equipment, either a backup power converter or a lower power split phase backup transmitter should be installed.  With all mechanical things, eventually this will need to be repaired and it would suck to be off air while that is happening.

Regardless of any of that, this particular service is about to be disconnected permanently.  Good riddance.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

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~Benjamin Franklin

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~Rudyard Kipling

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~Alan Weiner

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