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).
