WIRING SECONDARIES OF TWO TRANSFORMERS IN PARALLEL

[ALEVIN]

ARE THERE ANY POTENTIAL PROBLEMS OR HAZARDS IN WIRING THE SECONDARIES OF TWO TRANSFORMERS IN PARALLEL.BOTH TRANSFORMERS HAVE THE SAME CHARACTERISTICS AND SECONDARY FEEDERS ARE INSTALLED OF EQUAL LENGTHS AND PROPERELY PHASED (A, B, & C) OUT TO LOAD. BOTH TANSFORMERS SHARE THE LOAD EQUALLY AND THE ACTUAL LOAD IS HALF THE RATING OF THE TRANSFORMERS. THE LOAD DOES NOT REQUIRE A NEUTRAL FROM THE TRANSFORMERS. IF ONE OR TWO SECONDARY FUSES OPEN IN ONE OF THE TRANSFORMERS, THIS WILL RESULT IN AN IMBALANCED CONDITION FOR BOTH TRANSFORMERS. SINCE BOTH TRANSFORMERS DO NOT HAVE A NEUTRAL CONDUCTOR TO TAKE THE IMBALANCED LOAD, WOULD THE EQUIPMENT GROUND CONDUCTOR FROM ONE TRANSFORMER SEE A VERY HIGH CURRENT FLOW WHICH WILL CIRCULATE BACK TO THE OTHER TRANSFORMER VIA ITS GROUND CONNECTION?

 

One potencial problem with wiring two T/F's in parallel is that if an Engineer in your case. or even worse a user (if it was a standard mains plugable item) were to disconect one of the transformers primary, They would wrongly expect the primary to be isolated from the supply.
This exact problem has occured within the toy industry when a boy wired two AC plug top PSU's in parallel.
The accident was reported from the authorities in the Netherlands.

 

[ALEVIN]

Ian,
I APPRECIATE YOUR RESPONSE TO MY CONCERN. BUT I AM INTERESTED IN KNOWING IF THE IMBALANCED LOAD WOULD CAUSE THE EQUIPMENT GROUND CONDUCTOR FROM ONE TRANSFORMER TO SEE A VERY HIGH CURRENT FLOW WHICH WILL CIRCULATE BACK TO THE OTHER TRANSFORMER VIA ITS GROUND CONNECTION?

ARTSAND88

 

[jbartos]

Suggestion: The parallel transformers should have designed their operation, protection and grounding such a way that their operation, including various fault conditions or occurrences, is safe for the transformer as well as the downstream loads. It can be done and it is done. However, the protection and grounding are more elaborate. See Beeman D. "Industrial Power Systems Handbook," McGraw-Hill, 1955.

 

[rhatcher]

You will see current through a ground conductor between the transformers if they have wye connected neutral grounded secondaries with no neutral connection to the common load. You are describing a special case (ie. parallel transformers) which should be carefully researched to ensure that proper protection is applied. Obviously if a transformer blows a fuse, there could be an issue in terms of heating due to unbalanced currents and unintentional (ground) current flow. More importantly, if the system described is truly redundant, or in other words if you have twice the system capacity required by the load ("BOTH TANSFORMERS SHARE THE LOAD EQUALLY AND THE ACTUAL LOAD IS HALF THE RATING OF THE TRANSFORMERS&quot then you need to take into consideration the "short circuit interrupting rating" of the equipment downstream. Specifically, dual transformers will provide twice the short circuit current to a fault. The OC protection for each transformer will be rated to protect the transformer, but the protection for the load downstream may not be able to interrupt the combined short circuit current of both transformers.