9/15/06
888-2586-001
2-43
WARNING: Disconnect primary power prior to servicing.
Section 2 Installation & Initial Turn-On
Platinum Z16HD+
Another likely source of this problem can come from unbalanced impedances in the
power distribution system. Unbalanced impedance will always be seen when an “open”
delta three phase distribution system is used. Transformer design textbooks clearly
show that the voltage regulation of an unbalanced system is poor.
2.11.4 3 Phase Delta Distribution Transformers
Figure 2-19 shows open and closed delta systems. The closed delta impedance looking
into each terminal (A, B & C) is exactly the same; but this is not the case in the open
delta configuration. Depending on the impedances of the transformers in the open delta
circuit, line voltage unbalance sufficient to impair satisfactory operation of the overall
transmitter may result. For this reason, along with their inherent susceptibility to
transients, Harris does not recommend the use of open delta systems.
Figure 2-19 3 Phase Closed Delta vs. Open Delta
The only advantage of the open delta is lower initial cost, and this is partially offset by
the fact that when only two transformers are used, they must be larger than the three
transformers in a closed delta system.
Difficulties have often been experienced with open delta systems; but when a third
transformer was added to close the delta, the problems disappeared.
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.
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