Rockwell Automation 7000A PowerFlex Medium Voltage Drive (A-Frame) - Classic Control - Manual (Page 23)

Overview of Drive 1-5
7000 “A” Frame 7000A-UM150F-EN-P – June 2013
PWM Rectifier (Active Front-End)
An active front-end rectifier suitable for the PowerFlex 7000 “A”
Frame topology is called a PWM rectifier. This 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 PWM rectifier requires a switching pattern that complies with
similar rules as the inverter. The pattern used for the example shown
in Figure 1.4 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 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 Fig. 1.4) provides additional
filtering and current limiting features to a line side short circuit fault.
The line current and voltage waveforms are shown in Figure 1.4. 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 from 30-100% speed when applied to variable torque
loads.