THREE-PHASE CONNECTIONS OF SINGLE-PHASE TRANSFORMERS BASIC INFORMATION AND TUTORIALS

Single-phase transformers can be connected to form three-phase transformer banks for stepping voltages up or down in three-phase systems. Four common configurations for connecting transformers in three-phase systems are delta–delta, wye–wye, wye–delta, and delta–wye.

The first three are shown in Fig. 3-9. The delta–wye is not shown because it is simply the reverse of the wye–delta connection.

Delta–delta connection
The delta–delta connection, shown in Fig. 3-9a, is widely used for moderate voltages. This connection has the advantage of remaining operational in what is known as the open delta or V connection if one transformer is damaged or taken out of service, leaving the remaining two functional.

If it is operated this way, the bank still delivers three-phase currents and voltages in their correct phase relationships. However, the capacity of the bank is reduced to 57.7 percent of the value obtained with all three transformers in service.

Wye–wye connection
In the wye–wye connection, shown in Fig. 3-9b, only 57.7 percent (or 1/1.73) of the line voltage is applied to each winding, but full line current flows in each transformer winding. The drawback to this connection is that power circuits supplied from a wye–wye bank generate serious electromagnetic interference, which could interrupt nearby communications circuits.

Because of this and other disadvantages, the wye–wye connection is seldom used. However, the wye–wye connection can be used to interconnect two delta systems and provide suitable neutrals for grounding both of them.

Delta–wye and wye–delta connections 
The delta–wye connection (not shown) is suitable for stepping up voltages because the voltage is increased by the transformer ratio multiplied by a factor of 1.73. Similarly, the wye–delta connection, shown in Fig. 3-9c, is used for stepping down voltages.

The high-voltage windings of most transformers operating at more than 100 kV are wye-connected. To match the polarities correctly in a wye connection, the H and X markings must be connected symmetrically.

In other words, if an H1 or X1 terminal is connected to the neutral, then all of the H1 or X1 terminals must be connected to the neutral and the remaining H2 or X2 terminals must be brought out as the line connections, as shown in Fig. 3-9b.

By contrast, in a delta connection, H1 must always be connected to H2 and X1 to X2, and the line connections must be made at these junctions, as shown in Fig. 3-9a.

When a large number of single-phase loads are to be served from a three-phase transformer bank, the wye connected low-voltage winding is recommended because the single-phase loads can be balanced evenly on all phases.

DELTA – DELTA (ΔΔ) CLOSED / NEUTRAL = PRIM NO-SEC YES TRANSFORMER CONNECTION TUTORIALS AND BASIC INFORMATION

WHERE USED

For supplying three-phase, 240-volt loads with small amounts of 120/240-volt, single-phase load. No problem from third harmonic overvoltage or telephone interference. With a disabled unit, bank can be reconnected in open-delta for emergency service.

DELTA-DELTA FOR LIGHTING AND POWER

This connection is often used to supply a small single-phase lighting load and three-phase power load simultaneously. As shown is diagram, the mid-tap of the secondary of one transformer is grounded.

Thus, the small lighting load is connected across the transformer with the mid-tap and the ground wire common to both 120 volt circuits. The single-phase lighting load reduces the available three-phase capacity. This connection requires special watt-hour metering and is not available from all utilities.

DIAGRAM

BANK RATING

The transformer with the mid-tap 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

High circulating currents will result unless all units are connected on same regulating taps and have same voltage ratios. Bank rating is reduced unless matching impedance transformers are used. The secondary neutral bushing can be grounded on only one of the three transformers.

IMPEDANCE

When three transformers are operated in a closed-delta bank, care should be taken to make certain the impedances of the three units are practically the same. Transformers having more than 10% difference in impedance rating should not be operated together in a closed-delta bank unless a reactor is used to increase the impedance of the unit having the lower impedance rating to a value equal to the other units.

If the voltage ratio of all three of the transformers is not the same, there will be a voltage tending to circulate current inside the delta. The current will be limited by the impedance of the three transformers considered as a series circuit.

It is a good practice, before applying voltage to three transformers in closed delta, to insert a fuse wire between the leads coming from the high-voltage bushings of two transformers closing the delta bank. The fuse wire should be of sufficient size to carry the exciting current of the transformers.

The use of this fuse wire offers a very simple means of making certain the transformers have the proper polarity.

This connection should not be used with CSP transformers if used to supply a combined three-phase and three-wire single-phase load due to unequal voltage division of single-phase load when the tapped transformer breaker is opened.

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 B phase 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).

DELTA, STAR, AND OPEN DELTA TRANSFORMER CONNECTION COMPARISON

The choice between the methods would be governed largely by the service requirements. When the three transformers are delta-connected, one can be removed without interrupting the performance of the circuit, the two remaining transformers in a manner acting in series to carry the load of the missing transformer.

The desire to obtain immunity from a shutdown due to the disabling of one transformer has led to the extensive use of the delta connection of transformers, especially on the low-potential delivery side. It is to be noted that if one transformer is crippled, the other two will be subjected to greatly increased losses.

Thus, if three delta-connected transformers are equally loaded until each carries 100 A, there will be 173 A in each external circuit wire. If one transformer is now removed and 173 A continues to be supplied to each external circuit wire, each of the remaining transformers must carry 173 A, since it is now in series with an external circuit.

Therefore, each transformer must now show 3 times as much copper loss as when all three transformers were active, or the total copper loss is now increased to a value of 6 relative to its former value of 3. An open-delta installation is made frequently when considerable future increase in load is expected.

The increase can be accommodated by adding the third transformer to the bank at a later date and thus increasing the capacity of the load that can be carried by about 75 percent.

A change from delta to Y in the secondary circuit alters the ratio of the transmission emf to the receiver emf from 1 to 1.73 .

On account of this fact, when the emf of the transmission circuit is so high that the successful insulation of transformer coils becomes of constructive and pecuniary importance, the three-phase line sides of the transformers are connected in “star” and the neutral is grounded.

The windings of most transformers operating on systems of 100,000 V or more are star-connected.


Comparative cost of transformers for different grouping for three-phase service. 
The accompanying table shows the costs of the single-phase transformers, of proper capacities for either a delta or an open-delta grouping, and of a three-phase transformer to serve a 75-kVA installation. The relative costs will be the same for the present date.

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.