888-2406-002 2-35
WARNING: Disconnect primary power prior to servicing.
Installation & Initial Turn-On
There is another problem which can occur with an open delta system, and that is
caused by lightning and switching transients. When lightning strikes or heavy loads
are switched on a power distribution system, high voltage transients are propagated
throughout the system. Unbalanced impedances will enhance these transients and
can cause transmitter damage, particularly to solid state rectifiers.
Many transmitters are located at the end of a long transmission line which is highly
susceptible to transient phenomena. Devices such as Metal Oxide Varistors are
inexpensive and very effective in reducing over voltage spikes. These units are
limited in the amount of energy that can be dissipated, but will handle, if designed
properly, very large currents. You can’t take a direct lightning hit and still operate,
but not many things will. It has been reported by engineers that installation of a
third transformer and transient protection devices, have eliminated the difficulty.
2.12.5 Three Phase Wye Distribution Transformers
The WYE connected system is also considered a symmetrical form of three phase
power distribution. All impedances are balanced as seen from each terminal, see
Figure 2-22. It is important when using a WYE connected system that the fourth
wire (neutral) is connected to the mid-point of the system as shown in the diagram.
When this connection is made it provides a path for the zero sequence currents as
well as any harmonic currents which are generated due to the rectification of the
secondary voltages.
Figure 2-22 3 Phase 4 wire WYE Configuration
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