Center Tapped Delta Delta Transformers

[jwilson3]

Several manufacturers recommend that a three phase delta-delta transformer, with the 240V secondary center tapped to serve lighting or other 120V loads, should have the 120 volt load limited to only 5% of the kva rating of the transformer because of the resulting loading caused by circulating currents.
I understand the circulating currents caused by impedance unbalance. I also recognize the limits of serving 240V single phase loads from a 240V delta winding, but that limit is as high as 50% of rating if no three phase load is being served.
I've tried to analize this arrangement to establish the amount of circulating current, but have not been successful.
If anyone can direct me to a good resource that covers this topic, it would be appreciated.

 

[mc5w]

DO NOT use the center tap of the secondary of these transformers. Eventually, someone will connect enough 120 volt load to destroy the transformer. The better ways to get 120/240 volts 4-wire delta are as follows:

1. Two single phase transformers connected in the 3-wire T-primary 4-wire T-secondary configuration. This requires conventional primary and secondary protection.

2. Three single phase transformers in the 3-wire wye primary 4-wire delta secondary configuration. You need both conventional primary and secondary protection plus 5 supplemental overload relays which could be motor overload relays connected to cut the primary power in the event of an overload. 3 of the supplemental overload relays need to be connected on the primary wires. The other 2 supplemental relays need to be connected in the 2 halves of the center tapped secondary winding.

 

[itsmoked]

mc5w; Did you mean 4-wire star secondary? (solution 2)

 

[jwilson3]

mc5w,
What you're proposing may be correct, but that's not what I asked for. This is not something I'm planning to do, it's something that's already been done and I'm analizing it.

 

[mc5w]

Solution 2 is a 3-wire star primary with the primary neutral point floating. The secondary can be 3-wire or 4-wire delta. This is the usual configuration for delta secondary transformers on U.S. wye primary utility distribution.

If what you have is an existing transformer a 3rd solution is to ignore the secondary center tap and to connect a balance coil to derive a neutral. A balance coil is essentially a single phase version of a zig-zag set that is used to refit a 3-wire 3-phase system so that is can be used as a 4-wire wye system. In the case of a balance coil you connect a 240 volt primary 240 volt secondary transformer to form a 240/480 volt autotransformer. You need 2 overload relays for the 2 halves arranged to interrupt the primary power in the case of an overload. You also need to pass the 2 hot leads of the balance coil in opposite directions through the window of a current transformer to detect internal faults. An overload relay in the current transformer secondary is set to trip when the primary current imbalance is 50% of full load for the balance coil. The overload relay in the CT secondary interrupts the control circuit for the primary contactor or undervoltage trip just like the 2 overload relays for protecting the balance coil against overload.

However, to meet 2002 and later National Electrical Code the overload relays for the balance coil need to be supplemented with instantaneous magnetic or electronic relays for sensing short circuits. The additional relays would be set to trip at 500% to 2000% of the thermal overload relays. The instantaneous relay for the current transformer secondary needs to be set high enough to pass the balance coil inrush current.

 

[alehman]

This is a very common configuraiton in rural areas where only a small amount of 120/240 single phase is needed (e.g. remote pumping stations). Sometimes it's a termed 'high-leg' or 'wild-leg' system.

Sorry I'm not able to provide input on the circulating current question. I've never run across that before.

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