To determine the impact of shifting existing 400 Hz loads to the existing
ship service 60 Hz, the 400 Hz loads and 60 Hz electrical systems of two
classes of ships were examined. The assumptions of the study were:
(1) All electronic loads are rectified using six-pulse
(2) The electrical plant is operated split plant
(3) The electrical plant must maintain 5% or less total harmonic voltage
distortion at the user interface.
With these assumptions, calculations showed both the DD 963 and DDG 47 to
exceed 5% harmonic voltage distortion at approximately 15% of the rated
capacity of the generator.
Imposing a 3% amplitude limit on the harmonic currents produced by
six-pulse rectification increased the utilization of the generator capacity to
between 53% and 57%. The total DD 963 load could be transferred to 60 Hz under
these conditions and remain within 5% harmonic voltage distortion. However,
this was not true of the DDG 47. It would have to operate generators in
parallel to remain within 5%.
The assumption of 12-pulse rectification, instead of 6-pulse rectification,
did not reduce the harmonic voltage distortion below 5% with split plant
operation. The DD 963 could be brought within this limit by parallel operation
of the generators but the DDG 47 would still be out of specification even with
the generators paralleled.
CONCLUSION OF THE STUDIES
The conclusion of the various studies performed is that while limiting
harmonic currents to a 3% amplitude is barely adequate for today's combat
loads, it is also barely within what is practical with the technology
generally available to Navy contractors. Three percent may not be adequate for
future combat loads. However, improved technology may be available to meet the
future requirements if there is sufficient R&D seeking solutions.
Alternately, the use of DC input power for electronic systems may eliminate
the problem. This is being given serious consideration and a draft standard
has been prepared and distributed for comment (Reference