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Chapter 2 Faults and Troubleshooting Innovation Series Medium Voltage GP Type - G Drives GEH-6385
No. Name Type Description
135 AC line transient Alarm
The AC line transient alarm occurs as a result of significant phase lock loop
error or significant phase imbalance.
A phase imbalance signal is calculated by subtracting a control calculated
threshold from a filtered signal which is formed by filtering the sum of two
signals. One of these signals is the phase lock loop error and the other is the
error between the demodulated real component of line voltage and the
measured magnitude of the line.
The calculated threshold phase imbalance level which is computed by the
control is based on the magnitude of the input line voltage. This calculated
phase imbalance threshold represents a phase imbalance of about 18% or a
phase lock loop error of about 6.7 degrees. The phase imbalance signal
which is a result of the previously mentioned subtraction is equal to about
18% imbalance when it becomes positive.
The phase imbalance signal feeds an integrator designed to cause the AC line
transient alarm when the threshold has been exceeded for a very short time.
That amount of time is dependent upon the amount of the phase imbalance,
but the alarm will occur eventually if the imbalance signal remains positive.
Primary causes:
AC line disturbances.
Transient phase imbalances.
Weak control of frequency on diesel generator sets or gas turbine generator
sets.
Very fast voltage magnitude changes.
Damaged reactor or transformer
Possible board failures:
VATF-SRC
FOSA
BICM
DSPX