WYE - DELTA TRANSFORMER BANKING TUTORIALS
Wye - Delta Primary No Secondary Yes Banking Tutorials
WHERE USED
For supplying three-phase, 240 VAC loads with small amounts of 120/240 VAC, single-phase loads. No excessive circulating currents when transformers of unequal impedance and ratio are banked. No problem from third harmonic overvoltage or telephone interference.
WYE-DELTA FOR LIGHT & POWER
This diagram shows the connections for the Y-Delta bank to supply both light and power. This connection is similar to the delta-delta bank with only the primary connections changed. 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. The single-phase load reduces the available three-phase capacity. This connection requires special watt-hour metering.
The transformer with the midtap carries 2/3 of the 120/240-volt, single-phase load and 1/3 of the 240-volt, three-phase load. The other two units each carry 1/3 of both the 120/240 and 240-volt loads.
CAUTION
The secondary neutral bushing can be grounded only on one of the three transformers.
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.
HIGH-LEG MARKING
NEC 2002: 110.15 High-Leg Marking.
On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded to supply lighting and similar loads, the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.
NEC 2002 Handbook:
Added for the 2002 Code, this section now contains a requirement that appeared in 384-3(e) of the 1999 NEC. This requirement was moved to Article 110, where the application becomes a more general requirement.
The high leg is common on a 240/120-volt 3-phase, 4-wire delta system. It is typically designated as “B phase.” The high-leg marking is required to be the color orange or other similar effective means and is intended to prevent problems due to the lack of complete standardization where metered and non-metered equipment are installed in the same installation. Electricians should always test each phase relative to ground with suitable equipment to determine exactly where the high leg is located in the system.
ARRANGEMENT OF BUSBARS AND CONDUCTORS
NEC 2002: 408.3 / Support and Arrangement of Busbars and Conductors / (E) Phase Arrangement The phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard or panelboard. The Bphase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. Other busbar arrangements shall be permitted for additions to existing installations and shall be marked.
Exception: Equipment within the same single section or multisection switchboard or panelboard as the meter on 3-phase, 4-wire, delta-connected systems shall be permitted to have the same phase configuration as the metering equipment.
FPN: See 110.15 for requirements on marking the busbar or phase conductor having the higher voltage to ground where supplied from a 4-wire, delta-connected system.
NEC 2002 Handbook:
The high leg is common on a 240/120-volt, 3-phase, 4-wire delta system. It is typically designated as “B phase.” Section 110.15 requires the high-leg marking to be the color orange or other similar effective means of identification. Electricians should always test each phase to ground with suitable equipment in order to
know exactly where this high leg is located in the system.
The exception to 408.3(E) permits the phase leg having the higher voltage to ground to be located at the right hand position (C phase), making it unnecessary to transpose the panelboard or switchboard busbar arrangement ahead of and beyond a metering compartment. The exception recognizes the fact that metering compartments have been standardized with the high leg at the right position (C phase) rather than in the center on B phase.
See also 110.15, 215.8, and 230.56 for further information on identifying conductors with the higher voltage to ground. Other busbar arrangements for making additions to existing installations are permitted by 408.3(E).
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