However open delta has some drawbacks for 3-phase loads, as stated below.
A.C. POWER DISTRIBUTION FOR OPTIMUM TRANSMITTER PERFORMANCE
by J. B. Pickard
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
overly hot three phase motors.
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
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.
Transmitter Noise Performance
The most difficult parameter to meet with power line unbalance is
transmitter AM 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.
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.
Three Phase Delta Distribution Transformers
Figures 1 and 2 show closed and open 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.
The only advantage of the open delta is lower cost, and this is partially
offset by the fact that two transformers capable of 0.577 the total kVA are
required instead of three 0.333 kVA transformers. Harris customers have
experienced difficulties with open delta systems -- but when a third
transformer was added to close the delta, the problems disappeared.
There is another problem that 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 AC transmission line
which is highly susceptible to transient phenomena. Devices such as Metal
Oxide Varistors are inexpensive and very effective in reducing overvoltage
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. Several Harris customers, upon installation of a third transformer
and transient protection devices, have eliminated power line difficulties.
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 3). 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 that are
generated due to rectification of the secondary voltages.
Today, many transformers are supplied with all of the primary terminals
available so that either a delta or WYE connection can be made. Table 1
shows the different line-to-line voltages that are available with this
In summary, both symmetrical power distribution systems are satisfactory
because of their balanced impedances. Use either a closed delta or a four
wire WYE system for maximum transmitter performance. Never use an open
delta system just to cut costs. It could cost you dearly in the long run.
J. B. Pickard was AM Product Development Manager at Harris Broadcast
Products Division (retired - 1994).