ABB RELION 620 Series Technical Manual page 881

1MAC504801-IB E
12.4
12.5
620 series ANSI
Technical Manual
Frequency measurement and protection
All the function blocks that use frequency quantity as their input signal share the common
features related to the frequency measurement algorithm. The frequency estimation is
done from one phase (phase-to-phase or phase voltage) or from the positive phase
sequence (PPS). The voltage groups with three-phase inputs use PPS as the source. The
frequency measurement range is 0.6...1.5 xF
it is regarded as out of range and a minimum or maximum value is held as the measured
value respectively with appropriate quality information. The frequency estimation
requires 160 ms to stabilize after a bad quality signal. Therefore, a delay of 160 ms is added
to the transition from the bad quality. The bad quality of the signal can be due to
restrictions like:
• The source voltage is below 0.02 xVn at f
• The source voltage waveform is discontinuous.
• The source voltage frequency rate of change exceeds 15 Hz/s (including stepwise
frequency changes).
When the bad signal quality is obtained, the nominal frequency value is shown with
appropriate quality information in the measurement view. The frequency protection
functions are blocked when the quality is bad, thus the timers and the function outputs are
reset. When the frequency is out of the function block's setting range but within the
measurement range, the protection blocks are running. However, the TRIP outputs are
blocked until the frequency restores to a valid range.
Measurement modes
In many current or voltage dependent function blocks, there are various alternative
measuring principles.
• RMS
• DFT which is a numerically calculated fundamental component of the signal
• Peak-to-peak
• Peak-to-peak with peak backup
Consequently, the measurement mode can be selected according to the application.
In extreme cases, for example with high overcurrent or harmonic content, the
measurement modes function in a slightly different way. The operation accuracy is
defined with the frequency range of f/fn=0.95...1.05. In peak-to-peak and RMS
measurement modes, the harmonics of the phase currents are not suppressed, whereas in
General function block features
. When the frequency exceeds these limits,
n
.
n
Section 12
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