Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - Classic Control - Manual (Page 21)

1-4 Overview of Drive
7000-UM150I-EN-P – June 2013 7000 “B” Frame
Sample line current and voltage are also shown in Figure 1.3. The
THD of line current is approximately 5.6%, while the THD of line
voltage (line-to-line) is approximately 2.0%. (THD of line voltage is
a function of system impedance.) The 18-pulse rectifier consists of
one master bridge and two slave bridges and will always have a total
of 18 SCR switching devices.
Figure 1.3 – 18-pulse Rectifier and its input waveforms
a) Line current
b) Line-to-line voltage at PCC
Active Front End (PWM Rectifier)
An active front end (AFE rectifier) suitable for the PowerFlex 7000
topology is also commonly 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. (See
Specification 80001-004, Requirements for Medium Voltage Motors
Without Common Mode Voltage Elimination). 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 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.
a)
b)