3 Phase Transformers
Rory Typert-Morrison
1. How many transformers are needed to make an open-delta connection?
2
2. Two transformers rated at 100 kVA each are connected in an open-delta connection. What is
the total output power that can be supplied by this bank?
200,000 (0.866) = 173,200kVA
3. How does the NEC specify that the high leg of a four-wire delta connection be marked?
It must be connected to an orange wire or by tagging it at any point where a connection is made
4. An open-delta three-phase transformer system has one transformer center-tapped to provide
a neutral for single-phase voltages. If the voltage from line to center tap is 277 V, what is the
high-leg voltage?
277 x 1.732 = 479.76V
5. A three-phase transformer connection has a delta-connected secondary, and one of the
transformers has been center-tapped to form a neutral conductor. The phase-to-neutral value of
the center-tapped secondary winding is 120 V. If the high leg is connected to a single-phase
load, how much voltage will be applied to that load?
120 x 1.732 = 207.84V
6. A three-phase transformer connection has a delta-connected primary and a wye-connected
secondary. The center tap of the wye is used as a neutral conductor. If the line-to-line voltage is
480 V, what is the voltage between any one phase conductor and the neutral conductor?
Ep = 480V / 1.73 = 277.13V x 1.732 = 480V
7 A three-phase transformer bank has the secondary connected in a wye configuration. The
center tap is used as a neutral conductor. If the voltage across any phase conductor and neutral
is 120 V, how much voltage would be applied to a three-phase load connected to the secondary
of this transformer bank?
El = Ep x 1.73 = 207.85V
8 A three-phase transformer bank is connected wye–delta. The primary voltage is 12,470 V, and
the secondary voltage is 480 V. The total capacity of the transformer bank is 450 kVA. One of
the three transformers that form the three-phase bank develops a shorted primary winding and
becomes unusable. A suggestion is made to reconnect the bank for operation as an open-delta.
Can the two remaining transformers be connected open-delta? Explain your answer as to why
they can or why they cannot be connected as an open-delta. If they can be reconnected opendelta, what would be the output capacity of the two remaining transformers?
The open-delta transformer connection can only be made with two transformers instead of
three, so in order to connect the transformers in an open-delta would require you to only use
two of them anyways. Should this be the case you will now have a total capacity of about 60%
150kVA x 2 x 0.866 = 259.8kVA
9 A three phase transformer has a Primary connected in WYE with a LINE voltage of 7200v,
Secondary Delta configuration with a line voltage of 240 V configuration and a WYE connected
load with 4 Ohms per phase
Calculate for
Primary
Secondary
Load
E-line:
7200V
240V
240V
E-phase:
4156.9V
240V
138.56V
I line:
1.155A
34.64A
34.64A
I phase:
1.155A
20A
34.64A
VA:
14400VA
14400VA
14400VA
Ratio: 17.32:1