Platinum Z5 CD, Manual

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WARNING: Disconnect primary power prior to servicing.
Installation & Initial Turn-On
2.10 Power Distribution for Optimum Transmitter
Performance
This section is applicable to the three phase supplies only, as well as other three
phase equipment. For many years HARRIS engineers have recommended that the
three phase power distribution system should be either a closed delta or WYE
configuration to provide better radio and television transmitter performance by
helping prevent line unbalance. Operation with substantial voltage unbalance from
line to line results in higher than normal signal-to-noise ratio in the transmitter
output signal, increased three phase transformer heating, and hot three phase
motors.
2.10.1 Overheating from Line Unbalance
Even a device as simple as a three phase motor should be operated from a power
line in which the voltage is balanced within 1%. It takes only a 3.5% line unbalance
to produce a 25% increase above normal temperature. A 5% unbalance will cause
destructive temperature rises of 50% greater than normal!
Similar characteristics can be expected in the windings of a three phase power
transformer down inside the cabinet of your transmitter. Transformers and motors
can be designed with extra safety features where thermal rise is limited to
acceptable levels; however, in this case, other transmitter parameters cannot be
made acceptable at a reasonable cost.
2.10.2 Transmitter Noise Performance
The most difficult parameter to meet with power line unbalance is transmitter noise
performance. Most large transmitters use six-phase or twelve-phase high voltage
power supplies. The energy storage capacitors are expensive to install and large
stored energies make destructive faults inevitable. A good design will have
sufficient energy storage capacitors to meet the specified signal-to-noise but not
much more. When the equipment is then operated from an unbalanced line, the
power supply ripple frequency will be twice the line frequency instead of six to
twelve times. It becomes obvious that it would take three times as much energy
storage to achieve the original performance goal.
2.10.3 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.