WYE-DELTA CLOSED THREE (3) PHASE BANKING OF SINGLE PHASE TRANSFORMER TUTORIALS

WYE - DELTA CLOSED TRANSFORMER BANKING TUTORIALS
A Tutorial On Transformer Banking (Wye -  Delta Closed)


WYE-DELTA CLOSED
YΔ CLOSED / NEUTRAL = PRIM NO-SEC NO


DIAGRAM

WHERE USED
To supply three-phase loads. No excessive circulating currents when transformers of unequal impedance and ratio are banked. No problem from third harmonic over-voltage or telephone interference. If a ground is required, it may be placed on either an X1 or an X2 bushing as shown.


WYE-DELTA FOR POWER
Often it is desirable to increase the voltage of a circuit from 2400 to 4160 volts to increase its potential capacity. This diagram shows such a system after it has been changed to 4160 volts. The previously delta-connected distribution transformer primaries are now connected from line to neutral so that no major change in equipment is necessary. The primary neutral should not be grounded or tied into the system neutral since a single-phase ground fault may result in extensive blowing of fuses throughout the system.


BANK RATING
Maximum safe bank rating for balanced three-phase loads (when transformer kva's are unequal) is three times the kva of the smallest unit. A disabled transformer renders the bank inoperative.

IMPEDANCE & GROUNDING
The wye-delta connection is one of the most popular connections used today. Transformers are often connected from delta-delta to wye-delta to take advantage of 1.732 times the delta transmission voltage.
In this connection, it is not necessary that the impedance of the three transformers be the same.
This connection should not be used with CSP single-phase transformers since when one breaker opens serious unbalanced secondary voltages may appear.

The wye of this system should not be grounded because then the bank serves as a grounding bank and will supply ground-fault current for a phase-to-ground fault on the primary system. Also for unbalanced three-phase loads on the primary system, the secondary acts as a balance coil; therefore, circulating current may result in an overload.


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.

FERRORESONANCE
Negative effects of ferroresonance are potentially present on non-grounded primary wye connections. There is more danger at 14,400/24.900 VAC and higher. There is more danger with smaller transformers.


A rule-of-thumb concerning negative ferroresonance effects is that transformers 25 KVA and smaller at 14,400/24,900 are susceptible to damage. 30 KVA and larger transformers are relatively safe from adverse ferroresonance effects at 14,400/24,900. Higher voltages than 14,400/24,900 would necessitate larger transformers than 30 KVA to be considered inherently safe from adverse ferroresonance effects.

On a floating Y-Δ connection, temporarily ground the primary neutral when closing or opening primary fuses to avoid adverse ferroresonance effects. A “chain ground” (a fourth or neutral cutout) should be installed and closed while closing or opening the power cutouts and then re-opened after all of the power cutouts are closed.

Configurations used to avoid ferroresonance are an open Y-Δ with a solidly grounded primary Y or a Y-Y with a solidly grounded primary and secondary Y connection.