General Function Block Features; Frequency Measurement; Measurement Modes - ABB RELION SMU615 Technical Manual

1MRS758407 C
9

General function block features

9.1

Frequency measurement

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 × F
measurement range always starts from 0.6 × F
limits, 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 × U
• 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 or zero (depending on the
Def frequency Sel setting) 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 OPERATE
outputs are blocked until the frequency restores to a valid range.
9.2

Measurement modes

RMS
The RMS measurement principle is selected with the
using the value "RMS". RMS consists of both AC and DC components. The AC
component is the effective mean value of the positive and negative peak values.
RMS is used in applications where the effect of the DC component must be taken
into account.
RMS is calculated according to the formula:
I
RMS
SMU615
Technical Manual
n
1
2
∑
I
=
i
n
=
i 1
General function block features
. When the frequency exceeds these
n
at F .
n n
Measurement mode setting
. The df/dt
n
(Equation 15)
247