Rockwell Automation 1557 MEDIUM VOLTAGE AC DRIVE - Manual (Page 213)

COMMISSIONING 9-13
1557-UM050F-EN-P – June 2013
Line Reactor versus Isolation Transformer
Displaced Neutral Voltage
Current source inverters (CSI) create a displaced neutral voltage. The CSI drive uses a large DC link inductor,
and as a result, there is a phase shift in the neutral voltage between the rectifier and the inverter. The effect is
that a drive which is operated from a grounded supply system, without a isolation transformer, will produce a
high neutral to ground voltage and therefore higher phase to ground voltage on the drive output. The Motor
winding insulation must be increased to handle the increased voltage stress.
In an existing system, where the motor could not handle this increased voltage stress, an ungrounded isolation
transformer can be used on the line side of the drive and a grounding network added to the output side of the
drive. This would transfer the higher phase to ground voltage to the primary side of the drive and onto the
isolation transformer winding. It is simpler to design the transformer for the increased voltage stress. This
solution is the only method for retrofit applications. where a drive is added to control an existing motor.
Isolation Transformer Applications:
The drive’s output grounding network must be connected This will transfer the higher phase to ground voltage
to the primary side of the drive and onto the isolation transformer winding. The drive’s output grounding
network clamps the motor’s wye point to near ground potential and a ground fault will displace this neutral
voltage. The neutral voltage impressed upon the motor is typically used to detect a ground fault on a drive
equipped with an isolation transformer. A steady state neutral voltage of over 0.20 p.u. almost certainly
signifies that a ground fau
lt exists somewhere in the drive or in the equipment connected to the drive’s output.
Note:
The drive parameters used to set up ground fault protection on a Isolation transformer application are
“Gnd Flt Overvoltage Trip” and “Gnd Flt Overvoltage Delay”. The parameters are located in the “P
Mach Protect” parameters group.
Note: The drive’s output neutral voltage can be monitored by observing the variable ”V Neutral Motor”, the
variable is located under the Display Group “V Motor Model” motor model variables group. Verify that
the running “V Neutral Motor” variable is under 0.20 p.u
Line Reactor Applications
In a line reactor application, the bus feeding the input to the drive must have a reference to ground. This may
be in the form of a grounded wye point of the feeder transformer or a ground resistor network on the feeder
bus. The zero sequence C.T. located on the drive input will detect any current flowing to ground during a
ground fault condition in the drive or at the motor.
Note:
The drive’s internal output grounding network must be disconnected! If the drive’s output grounding
network is left connected, it will result in approximately 1 to 2 amps of ground current. The default
value for the “Ground Fault Overcurrent Trip” has been lowered to 0.5 amps with a 1.0 amp maximum.
If the output-grounding network is left connected it will nuisance trip the drive and may also trip the
customer’s upstream protection relay.
Note: The parameters used to set-up ground fault protection on a Line Reactor applic ation are “Gnd Flt
Overcurrent Trip” and “Gnd Flt Overcurrent Delay”. The parameters are located under the P Line
Protect parameters group.
Note: The neutral voltage on a line reactor application varies from approximately 0.3 to 0.6 p.u. You will
have to
increase the “Gnd Flt Overvoltage” setting to 0.7 or greater to prevent nuisance trips.
Note: Drive ground fault current can be monitored by observing the variable ”Gnd Flt Current”. The variable
is located under the Analog I / O Variable group. Monitor
the variable “V Motor Neutral” and verify that
the variable is running at a value greater than 0.3 p.u. This will confirm that the customer has their
input feed referenced to ground. Monitor the variable “Gnd Flt Current” and verify that with the drive’s
output grounding network disconnected, the Gnd Flt Current is not greater than 0.5 amps