Rockwell Automation 7000A PowerFlex Medium Voltage AC Drive - Air-Cooled (A Frame) - Manual (Page 20)

1-4 Overview of Drive
7000A-RM001A-EN-P – January 2011 7000 “A” Frame
Rectifier Designs
Active Front-End (AFE) Rectifier
An Active Front-End rectifier is particularly attractive since it does
not require an isolation transformer to meet IEEE 519-1992.
Many competing technologies in today’s MV market require a multi-
winding transformer to mitigate the unwanted harmonics through
cancellation by phase shifting the transformer secondary windings.
Depending on the topology, the transformer can have up to 15 sets of
secondary windings.
Elimination of the isolation transformer reduces capital and
installation costs, saves on valuable floor space, reduces
operating costs and increases overall system efficiency.
The AFE rectifier requires a switching pattern that complies with
similar rules as the inverter. The pattern used for the example shown
in Figure 1.3 is a 42-pulse selective harmonic elimination (SHE)
pattern, which eliminates the 5
th
, 7
th
and 11
th
harmonics. The integral
input capacitors are designed to reduce the current harmonics of the
higher order.
The filter resonant frequency is placed below 300 Hz (for a 60 Hz
system) where no residual harmonics exist. This prevents the
excitation of system harmonic frequencies. Other factors that are
considered when designing the filter are the input power factor and
the requirement on Total Harmonic Distortion (THD) of input
current and voltage waveforms.
The small integral AC line reactor (see Figure 1.2) provides
additional filtering and current limiting features to a line side short
circuit fault. The line current and voltage waveforms are also shown
in Figure 1.2. The line current THD is approximately 4.5%, while
line-to-line voltage THD is approximately 1.5%. (THD of line
voltage is a function of system impedance.) Input power factor with
the AFE rectifier is near unity throughout a typical operating speed
range for variable torque loads.