Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C Frame) - Classic Control - Manual (Page 24)

1-6 Overview of Drive
7000L-UM300I-EN-P – June 2013 7000 “C” Frame
PWM Rectifier (Active Front-End)
An active front-end rectifier suitable for the PowerFlex 7000L “C”
Frame topology is called a PWM rectifier. This is particularly
attractive for applications with new motors since it does not require
an isolation transformer to meet IEEE 519-1992. (See Specification
80001-004, Stator Insulation Requirements for MV Motors Applied
to MV Drives without isolation transformers). Most available
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, 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 1.4 is a 42-pulse selective harmonic elimination (SHE)
pattern, which eliminates the 5
th , 7th 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. The filter transfer function
technique is used to place the filter break frequency in a region where
no harmonics are present. 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 1.3) provides additional
filtering and current limiting features to a line side short circuit fault.
The rectifier input current, the rectifier terminal voltage and the line
current and voltage waveforms are shown in Figure 1.3. 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.
The PWM rectifier can be used in conjunction with a rectifier duty
isolation transformer or with an AC line reactor (as shown in Figure
1.3).