MIL-HDBK-1004/5
Section 6:
PASSIVE-ELEMENT FILTERS
6.1
Requirement. Passive-element filters are installed to reduce
equipment- and system-generated harmonics.
6.2
Harmonic Distortion. Harmonic distortion is an undesired change in
a wave form. Total harmonic distortion (THD) provides an indication of the
harmonic content of an alternating-current wave. It is expressed as a percent
of the fundamental or:
THD = 100 (Eh/Ef)1/2
where,
Ef = Square of the amplitude of the fundamental
6.3
Equipment Providing Unacceptable THD. MIL-STD-704 requires that the
THD of the wave form supplying the aircraft shall not exceed 5 percent. The
analyses indicate that only nonlinear loads, such as large Avionics Test
Equipment (ATE) full-wave rectifier bridge loads, provide distortion exceeding
the 5-percent limitation.
6.4
Harmonic Distortion Reduction. Usually, filters of three elements
or less can reduce the harmonic distortion level to criteria limits when the
filters are located at or near the nonlinear loads. Three filter sections
will usually reduce the distortion sufficiently. More filtering may further
reduce the distortion factor, but the reduction may not be cost-effective.
A three-section passive-element filter that has been used for this
purpose has the parameters indicated in Figure A-40.
6.5
Resonant Frequency Impacts. For the filter on Figure A-40, voltage
will peak at frequencies where series resonance occurs. Damaging voltage may
result when the resonant frequency is equal to or close to a harmonic
frequency. Therefore, when passive-element filters are introduced as part of
the system, a thorough study must be made to ensure that resonant frequencies
of the passive-element filter do not fall on a harmonic of the power frequency
that will be present in an amplitude significant enough to produce a damaging
voltage.
6.6
Resonant Frequency Analysis. The resonant frequencies depend on the
connected system elements and their values. A computer analysis can be made
to determine the effects of the important parameters, such as the following:
a)
The
magnitude and power factor of the load;
b)
The
setting of the line drop compensator;
c)
The
impedance of the utilization service assembly transformer;
d)
The
amount of filter sections used;
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