WYE - DELTA OPEN TRANSFORMER BANKING BASICS AND TUTORIALS

WYE - DELTA OPEN TRANSFORMER BANKING BASIC INFORMATION
How To Bank Wye-Delta Open Transformer?


WHERE USED
To supply large single-phase, 240-volt loads with small amounts of three-phase loads. (Usually transformers of different kva sizes are used.) Also used for emergency operation when one unit of a four-wire primary, wye-delta bank is disabled. If a ground is required, it may be placed on an X1 or an X2 bushing as shown.

OPEN Y-DELTA
When operating Y-delta and one phase is disabled, service may be maintained at reduced load as shown. The neutral in this case must be connected to the neutral of the step-up bank though a copper conductor.

The system is unbalanced, electro-statically and electro-magnetically, so that telephone interference may be expected if the neutral is connected to ground. The useful capacity of the open Y-delta bank is 87 percent of the capacity of the installed transformers when the two units are identical. The capacity is 57 percent of a three transformer bank.

BANK RATING
This connection is relatively inefficient where three-phase loads predominate since it has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7 % of the three-phase rating of a closed delta-delta bank of three units.

STATIC DISCHARGE
Potentially present on a non-grounded primary wye connection. A high, excessive voltage results on a 3-phase Y-Δ connection on the secondary line to ground when one leg of the primary is open.

The voltage present is static with no power and bleeds off when taken to ground. This static can damage a volt-ohm meter. The static is greater when the secondary feeder is short and lesser when the secondary feeder is long.

The static problem is resolved by grounding one phase or the center tap of one transformer on the secondary side, but this usually requires special KWH metering. This static condition is present only when a primary line is open, not the secondary.

This static condition can occur on an open (2-transformers) or closed (3-transformers) bank. This static condition can occur with any primary voltage.