GE 869 Instruction Manual page 209

CHAPTER 4: SETPOINTS
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
However, this filter when used for the VFD motor application running at low frequency
results in a very long Trip/Alarm times delay. For example: if setpoint Motor Load Filter
Interval is set 10 cycles and motor is running at 20Hz (tracking frequency), then the Trip/
Alarm delay is increased by 0.5 sec.
In order to avoid long Trip/Alarm delay, especially when the Motor is running at low
frequencies, the Trip/Alarm delay is limited internally to the number of cycles equal to the
minimum of the maximum delay @Nominal Frequency and average filter delay @Tracking
Frequency, per relation as follows:
Where
This adjustment to the filter length to avoid large Trip/Alarm delay is only applicable when
VFD Function is enabled and Bypass Switch operand is not asserted. This adjustment is not
applicable when this filter is applied to motor application with reciprocating load.Following
examples shows the Trip/Alarm times delay calculation with the above mentioned
adjustment to the filter length when the average filter of length "Motor load Filter Interval"
is applied for the VFD applications:
Example 1:
• Setpoint "Motor Load Filter Interval", N = 20 cycles
• Tracking frequency, F = 40Hz
• Setpoint "Nominal Frequency", F
• Maximum time delay @ Nominal Frequency = (1/F
Actual time delay @ or tracking Frequency = (1/F) x 20 cycles x 1000 = ~500 msec
The Trip/Alarm time delay is then = Min(270, 500) = 270msec
Example 2:
• Setpoint "Motor Load Filter Interval", N = 4 cycles
• Tracking frequency, F = 70 Hz
• "Nominal Frequency", F
• Maximum time delay @ Nominal Frequency = (1/F
Actual time delay @ Actual or tracking Frequency = (1/F) x 4 cycles x 1000 = ~60 msec
The Trip/Alarm time delay is then = Min(270, 60) = 60 msec
For the VFD motor application, setpoint Motor Load Filter Interval (under Setpoint/System
Setup/Motor) can be determined from the captured load waveforms obtained from the
Datalogger or Oscillography features by following the steps below:
1. Capture the pre-filtered load analog value from the Datalogger/Oscillography. This
analog value is defined as "Motor Load" in the Thermal Model analog values. Motor
Load is the average of three RMS input currents and is applied at the input of motor
load averaging filter.
2. By analyzing the captured waveform in Step 1, estimated the length of oscillation that
repeats itself at regular intervals. Estimation of length must be done in nominal power
cycles.
3. Set the Motor Load Filter Interval equal to the value estimated in Step 2 plus a
recommended margin of 1 cycle.
4. Capture the "Fltd Motor Load" analog value from the Datalogger/Oscillography. "Fltd
Motor Load" is the motor load current after filtration of "oscillations" due to VFD.
= 60Hz
n
= 60 Hz
n
) x 16 cycles x 1000 = ~270 msec
n
) x 16 cycles x 1000 = ~270 msec
n
SYSTEM
Eq. 1
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