Cell Bypass; Fast Bypass; Fast Bypass Process - Siemens 6SR41 series Product User Manual

6.14

Cell Bypass

6.14.1

Fast Bypass

Up time is an important factor in many processes. A Medium Voltage drive is often a critical
part of the process and even small interruptions in output torque of a Medium Voltage drive
can cause the process to stop. This can result in lost material and production time.
Fortunately, in general, a process can ride through an interruption of ½ second or less. The
NXG control has a feature that is designed to limit the interruption of torque to the process by
less than ½ second if a cell fault is detected. This feature is called Fast Bypass, which
utilizes the capabilities of the NXG Control system and the cell bypass functionality. The
conditions that must exist for the drive to meet this ½ second maximum interruption are
described below.
Possible abnormal operating conditions are detected in cell control board hardware. This
hardware is designed to quickly shut down the drive to minimize damage. Once this
happens, NXG control is then signaled. The control can then quickly determine which cell
faulted and the bypass process can be started. When the drive trips and stops delivering
torque to the motor, the motor acts like a generator and produces a voltage on the drive
output terminals. This voltage decays over time, but can be near the drive rated output
voltage for a few seconds. If a cell is bypassed, the remaining cells may not be able to
support this voltage and damage can occur. To prevent this damage, a check is done in the
control to verify that the motor output voltage can be supported before a cell is bypassed. If
sufficient output voltage is available, the cell can be bypassed and torque can be delivered to
the motor in under ½ second from the time the cell fault occurred. If the motor voltage is too
high, the control will delay the bypass process to allow the back EMF voltage to decay. To
guarantee that the drive will bypass a cell fault in under ½ second, the drive needs to be
operating at an output voltage that N-1 cells can supply. One way is for the drive to be sized
so that it has more than the minimum number of cells required to provide the voltage
needed, defined as N+X redundancy (where X is typically 1 or 3), following cell bypass.
Another solution is to limit the maximum speed following cell bypass. These issues must be
studied and resolved before the drive is installed.
Note
Fast bypass is dependent upon the available output voltage.
Note
Acceleration following cell bypass is dependent upon parameter ramp rates and available
torque current.
6.14.2

Fast Bypass Process

The Fast Bypass process is summarized as follows, where control mode must be OLVC,
CLVC, Fast Bypass must be enabled.
1. VFD output enabled, output phase locked loop valid
2. Any cell fault occurs, output phase lock loop continues to track motor
3. VFD output disabled (all power cells disabled by NXG hardware)
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
Application and Operation
6.14 Cell Bypass
141