Harris Platinum Z2 CD, Manual

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2-34 888-2406-002
WARNING: Disconnect primary power prior to servicing.
Installation & Initial Turn-On
2.12.3 The Causes of Line Unbalance
How does a line unbalance occur? It is a rare case in which a large commercial
power producer would generate unbalanced voltage, so we must look elsewhere in
the system. When you have large single phase power users on a power line this can
cause uneven distribution of the line currents in the system. Uneven currents
through balanced impedances will result in line-to-line voltage unbalance.
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.
2.12.4 Three Phase Delta Distribution Transformers
Figure 2-21 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 2-21
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.