Cell Bypass; Fast Bypass (U11) - Siemens SINAMICS PERFECT HARMONY GH180 Function Manual

Operating the Control

7.4 Cell Bypass

7.4 Cell Bypass
7.4.1

Fast Bypass (U11)

Fast bypass is a feature that limits the interruption of torque to a process by less than ½ second
if a cell failure is detected. This helps to prevent operational down-time as a small interruption
in output torque of a medium voltage drive can cause a process to stop. Most processes can ride
through an interruption of ½ second or less.
In fast bypass the drive will start to deliver torque to the motor in ½ second after a fault occurs.
It may take longer for the drive to get back up to the setpoint speed based on load inertia, and
the loss of speed when torque is interrupted.
Fast bypass does not prevent a drive fault from occurring. It provides a means of isolating the
faulted cell, and quickly resetting the drive back into the run state. A drive fault still occurs, and
is logged by the system.
The drive can meet this ½ second maximum interruption under the following conditions:
Cell fault detection
All cell failures are detected in hardware. The hardware is designed to quickly shut down the
drive so that additional damage will not occur. The control is notified in the event of a cell failure;
it quickly determines which cell failed and starts the bypass process. A cell fault is always a drive
fault. Fast bypass issues an automatic reset to the drive after the cell has successfully bypassed.
Drive trip
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.
A check in the control serves to prevent this damage. The control verifies if the motor output
voltage can be supported before it bypasses a cell and restarts the drive. If the check passes, the
cell is bypassed and torque is delivered to the drive in under ½ second from the time the fault
occurred. If the motor voltage is too high, cell bypass is delayed to allow the voltage to decay to
a safe level.
Number of cells
To guarantee that the drive will bypass a cell fault in under ½ second the drive needs to be
running at an output voltage that can be supported by one less than the existing number of cells
per phase.
• One option is to size the drive so that it has more than the minimum number of cells required
to provide the voltage needed.
• Another option is to limit the maximum speed.
These issues will have been studied and resolved before the drive is installed.
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NXGPro+ Control Manual
Operating Manual, A5E50491925A