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Power electronic control in electrical systems 53
where I L is the line current and I ph is the phase current. With wye connection they are
one and the same current. The dotted line in Figure 2.20 shows the possibility of a
connection between the neutral point of the supply and the star point of the load.
This connection may be used to stabilize the potential of the star point where there is
an excess of triplen harmonics in the current or voltage waveforms of the load.
The current and voltage in Figures 2.19 and 2.20 are displaced in phase by the
power factor angle f. Figure 2.21 shows a complete phasor diagram for a balanced
wye-connected load with a lagging power factor.
An alternative connection of the three phases is the delta connection shown in
Figure 2.22 together with the construction of the phasor diagram under balanced
conditions with a lagging power factor.
For the delta connection,
p
V LL V ph and I L 3I ph (2:26)
The delta connection is used to provide a path for triplen harmonic currents. For
example, when transformers operate at higher than normal voltage the magnetizing
Fig. 2.21 Phasor diagram for balanced wye-connected load.
Fig. 2.22 Three-phase connection with delta-connected load and phasor diagram.
