Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C-Frame) - ForGe Control - Manual (Page 18)

18 Rockwell Automation Publication 7000L-UM301D-EN-P - June 2014
Chapter 1 Overview of Drive
Rectifier Designs There are two offered designs for the rectifier of the PowerFlex 7000L drive.
18 Pulse Rectifier
An 18 Pulse phase controlled rectifier is shown in Figure 2 . In an 18 Pulse
configuration, the IEEE 519-1992 requirements are met in the majority of cases
without the need for passive filters; however, a multi-winding isolation
transformer is required to mitigate the low order harmonics by phase shifting
principles. The 18 Pulse solution is superior to 6- or 12-pulse offerings in terms of
lowering line side harmonics.
Isolation transformers are available in indoor dry type, cast coil, and outdoor oil-
filled designs for maximum flexibility in dealing with floor space, installation
costs, and control room air conditioner loading. (Refer to Specification 80001-
005, Rectifier Duty Transformers, for more details on transformer requirements
and features.)
Sample line current and voltage are also shown in Figure 2 . 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 2 - 18 Pulse Rectifier and its input waveforms
Active Front End (AFE Rectifier)
An active front end (AFE rectifier) suitable for the PowerFlex 7000L topology is
also commonly called a PWM rectifier. This is particularly attractive since it does
not require an isolation transformer to meet IEEE 519-1992. Most available
technologies in today’s MV market require a multi-winding transformer to
mitigate the unwanted harmonics 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.
Line
current
Line-to-line
voltage at PCC