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WARNING: Disconnect primary power prior to servicing.
Installation & Initial Turn-On
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.
2.10.4 Three 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 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.