Wye, Too (not to be confused with Y2K)
Richard P. Bingham, Dranetz-BMI
In the previous article, we reviewed some of the differences between a wye and
delta wiring configuration. Now we'll take a look at the effect of the monitored
values, depending on what side of the transformer you are connected to, as well
as some other effects from different configurations.
The most common type of power quality disturbance is the sag, a temporary
reduction in the voltage, with a duration ranging from one cycle to a minute. On
the electrical distribution system, a common cause of this is single line to ground
(SLTG) faults. This means that there is an unintentional connection between one
of the phase conductors and earth. It may be a low or high impedance path, but
the result is that excessive current is flowing in the conductor. This current
produces a larger voltage drop in the wiring, which leaves less voltage remaining
for the load, hence, the sag.
Monitoring is often done on the secondary side of the transformers where the
voltage levels are within the input range of the monitor without having to add
extra potential transformers. Therefore, the data that will be recorded will be
affected by the transformer type, as shown in the table below.
TRANSFORMER SECONDARY VOLTAGES (pu or per unit)
WITH Single-Line-to-Ground FAULTS
TRANSFORMER CONNECTION
Phase-to-Phase
Phase-to-Neutral
(primary - secondary)
Va
Vb
Vc
Van
Vbn
Vcn
Grounded wye - grounded wye
0.58 1.00 0.58
0.0
1.0
1.0
Grounded wye - ungrounded wye
0.58 1.00 0.58
0.0
1.0
1.0
Ungrounded wye - ungrounded wye
0.58 1.00 0.58
0.33
0.88
0.88
Ungrounded wye - grounded wye
0.58 1.00 0.58
0.33
0.88
0.88
Delta - delta
0.58 1.00 0.58
---------------Ungrounded wye - delta
0.33 0.88 0.88
---------------Grounded wye - delta
1.00
0.5
0.5
---------------Delta - grounded wye
0.88 0.88 0.33
0.58
1.00
0.58
Delta - ungrounded wye
0.88 0.88 0.33
0.58
1.00
0.58
Table 4 from an IEEE paper on "The Impact of Voltage Sags of Industrial Plant Loads" by Charles Smith,
Jeff Lamoree, Paul Vinett, Tom Duffy, and Mike Klein. The table shows what the transformer secondary
voltages would be with SLTG faults and the tradeoffs on the impact to the phase voltages that occur, based
on the wiring configuration of the transformer.
For example, where there are wye-wye or delta-delta connections, two phase-tophase voltages will drop to 58% of nominal, while the other phase-to-phase is
unaffected. However, for delta-wye and wye-delta connections, one phase-tophase voltage will be as low as 33% of nominal, while the other two voltages will
be 88% of nominal. It is the circulating fault current in the delta secondary
Richard P. Bingham is the manager of technology and products for Dranetz-BMI.
© copyright January 2000 Electrical Contractor Magazine
www.ecmag.com
windings that results in a voltage on each winding. An example of this can be
seen in the below figure, where the faulted phase (C) is approxinately. 33% of its
pre-fault value, and the other two phases are approximately 88% .
Richard P. Bingham is the manager of technology and products for Dranetz-BMI.
© copyright January 2000 Electrical Contractor Magazine
www.ecmag.com