background image
www.GEMultilin.com
www.GEMultilin.com
www.GEMultilin.com
141
www.GEMultilin.com
141
Transformer Principles
T
ransformer Pr
ot
ection
Three-Phase Transformers
As most power is distributed in the form of three-phase AC, we will
focus on the various types of three-phase transformers that
support this type of power system. By nature, three phase
transformers are designed with three primary windings and three
secondary windings mounted on a three-legged core.
The windings within a three-phase transformer may be connected
in several ways. The primaries may be connected in a delta
configuration and the secondaries in a wye configuration or visa
versa.
Delta vs. Wye transformers
Most distribution systems utilize a wye connected system as it
offers greater advantages to the over-all system performance. As
such, most power and distribution transformers are constructed
using the same connections.
In a wye connection, one wire from each coil is connected together
to form the neutral. In most power systems, the neutral is grounded,
providing a low resistance connection to earth and an electrical
reference point at zero points. By having this reference point,
safety, voltage stability and protection system design are
improved.
When determining and comparing which connection type is most
suitable for a specific application, the following criteria could be
used:
RatioofkVAoutputtothekVAinternalratingofthebank
Degreeofvoltagesymmetrywithunbalancedphaseloads
Voltageandcurrentharmonics
Transformergroundavailability
Systemfault-currentlevel
Switchingandsystemfaultandtransientvoltages
Advantages of a grounded-wye connected transformer
LoweroperatingvoltageasequipmentisconnectedLine-to-
Neutral (L-N) as opposed to the higher voltage required by Line-
to-Line connections (L-L)
Smallertransformersizerequiredasequipmentisconnectedat
a lower voltage
Easiertodetectline-to-groundfaultsasanyfaultcurrent
generated, either as an overcurrent or a short-circuit would
result in a significant of current on the neutral
Advantages of a Delta connected transformer
Polarityendofonewindingisconnectedtothenon-polarityend
of the next, allowing for an isolated ground
Onetransformercanberemoved(whenconnectedinabank)
while the remaining units deliver three-phase power at 58% of
the original output
Easysystemtokeepbalancedelectricalloadswiththe
transformer being electrically grounded or ungrounded, making
it more versatile.
Since a 3-phase transformer can have its primary and secondary
windings connected the same (delta-delta or wye-wye), or differently
(delta-wye or wye-delta). It is important to note that the secondary
voltage waveforms will be in phase with the primary waveforms
when the primary and secondary windings are connected the same
way.
In a situation where the primary and secondary windings are
connected differently, the secondary voltage waveforms will differ
from the corresponding primary voltage waveforms by 30 electrical
degrees.Thisiscalleda30phaseshift.
The danger with this phase shift is, if two transformers are connected
in parallel, and their phase shifts are not identical, a short circuit will
occur when the transformers are energized.
Sizing and Selecting a Transformer
Transformer size is determined by the kVA of the load. To determine
the size of the transformer you need to determine the following
requirements:
LoadVoltage
LoadCurrent/Amps
LineVoltage
Equation (three-phase transformers):
(volts x amps x 1.732) / 1000 = kVA
The kVA rating of your transformer should be greater than or equal
to the kVA resulting from the above equation.