Instrument Transformer Data; Motor Protection - GE 469 Instruction Manual

Application Example
Instrument Transformer
Data

Motor Protection

GE Multilin
Use the above data to set the output relays to achieve breaker control; to set digital
inputs for breaker status, remote operations, remote status, and alarm indication.
Assume that the communications between the station and the master control center
will be done by the RTU. Alarms, status indication, and breaker commands will be
hard-wired from the relay to the RTU. Similar information could be exchanged
between the RTU and the relay via an RS485 or RS422 Serial Link using the Modbus
RTU protocol. Refer to GE Publication GEK-106491: 469 Communications Guide for
additional information.
• Voltage Transformers
– 2 × Open Delta connected, ratio = 600:120 V
– Motor System Voltage = 575 V
• Phase CTs
The phase CTs should be chosen such that the FLC is 50% to 100% of CT
primary. Since the FLC is 347.5A a 350:5, or 400:5 CT may be chosen; 400:5 is
a standard available size and so would probably be selected.
• Ground CT
For high resistive grounded systems, sensitive ground detection is possible with
the 50:0.025 CT. Use a 1 A or 5 A secondary CT on solidly grounded or low
resistive grounded systems where the fault current is much higher. If a residual
connection is chosen, pickup levels and timers must be set with respect to the
acceleration time. The chosen zero-sequence CT must be able to handle all
potential fault levels without saturating. In this example, 50:5A CT is selected.
• Motor FLC
Set the motor full load current to 348 A, as specified by the data sheets.
Use the above data to set the relay system parameters, such as CT and VT
connections, VT secondary voltage, and CT and VT primary to secondary ratios.
• Overload Pickup
The overload pickup is set to the maximum allowed by the service factor of the
motor. Since this motor has RTDs and the relay will be using the RTD bias
feature for enhanced protection, set the overload pickup to the highest setting
of 1.25 x FLC for the motor service factor of 1.15. If service factor is unknown,
assume 1.0.
• Overload Curve
Select the standard overload curve to be just below the cold thermal limit to
give maximum process uptime, without compromising protection.
The best fitting curve is curve 7 (see figure below)
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469
Motor Management Relay
1–19