Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 40)

5-22 DESCRIPTION OF OPERATION
1557-UM050F-EN-P – June 2013
Line Converter Protection
Except for the dc link overcurrent and overvoltage detection, the line converter protection is implemented
entirely in software. Adjustable parameters specifying the trip level and time delay are provided for each
fault (see Chapter 6, Parameter and Variables Description). The dc link current and line voltage feedback
are obtained as described in the section on line converter feedback. The current feedback for ground
fault detection is obtained from a zero sequence current transformer installed in the drive input (only on
drives with AC line reactor). The voltage across the CT burden resistor is measured through a differential
analog input (AI2) on the line converter Drive Control Board.
For all faults except line converter overload, if the measured voltage or current exceeds the specified trip
level for the specified time interval, the corresponding fault bit is set in one of the fault words and the
master fault bit is set in Logic Status 1. For the detection of overload faults, the dc link current is first
converted to its ac equivalent by multiplying by 1.10 and then squared. The amount by which the square
of the current exceeds a threshold level is then integrated with time. An overload fault is declared if the
output of the integrator exceeds the trip level calculated from the specified overload trip level and time.
An overload warning is issued when the I
2
t level reaches half the trip level.
The response to a line converter fault falls into three categories, depending on the type of fault. For class
1 faults, such as line overcurrent, line overvoltage and undervoltage, and dc link overcurrent and
overvoltage, the line converter is immediately phased back to retard limit until the current drops to zero.
The gating is then disabled and the input and output contactors are opened. For class 2 faults, such as
ground fault and overload, the motor is brought to a normal stop before the drive is shut down and the
contactors opened. For class 3 faults (warnings), such as a pending overload, no action is taken.
The dc link overcurrent and overvoltage are special cases in that the fault detection is performed by
hardware but the time delay is performed by software. The detection of these two faults is implemented
in hardware because a very fast response is required. The hardware fault detection responds to
instantaneous values rather than average values as the software fault detection does. The response to
dc link overcurrent faults is different from all other faults because it freezes the GTO gating until the dc
link current has dropped to zero. This action is taken to prevent the GTO's from being destroyed by
attempting to commutate a current above their safe rating. Line converter current feedback is used for
this function instead of motor current feedback because the motor current includes the current supplied
by the filter capacitor, which does not flow through the GTO bridge.