Table. 5.2.4 - 43. Frequency tracking effect (FF changes from 6 Hz to 75 Hz).
The measurement error with a fixed 50 Hz sampling frequency
when the frequency changes. The constant current is 5 A, the
frequency sweep is from 6 Hz to 75 Hz.
The measurement error with frequency tracking when the
frequency changes. The constant current is 5 A, the
frequency sweep is from 6 Hz to 75 Hz.
As the figures above show, the sampling frequency has a major effect on the relay's measurement
accuracy. If the sampling is not tracked to the system frequency, for example a 10 Hz difference
between the measured and the set system frequency can give a measurement error of over 5 %. The
figures also show that when the frequency is tracked and the sampling is adjusted according to the
detected system frequency, the measurement accuracy has an approximate error of 0.1...- 0.2 % error
in the whole frequency range.
AQ -200 series devices have a measurement accuracy that is independent of the system frequency.
This has been achieved by adjusting the sample rate of the measurement channels according to the
measured system frequency; this way the FFT calculation always has a whole power cycle in the buffer.
The measurement accuracy is further improved by Arcteq's patented calibration algorithms
that calibrate the analog channels against eight (8) system frequency points for both magnitude and
angle. This frequency-dependent correction compensates the frequency dependencies in the used,
non-linear measurement hardware and improves the measurement accuracy significantly. Combined,
these two methods give an accurate measurement result that is independent of the system frequency.
Troubleshooting
When the measured current, voltage or frequency values differ from the expected values, the following
table offers possible solutions for the problems.
Problem
Check / Resolution
The measured current or
voltage amplitude is lower
than it should be./
The values are
"jumping" and are not
stable.
The set system frequency may be wrong. Please check that the frequency settings match the
local system frequency, or change the measurement mode to "Tracking" (
Measurement
→
Frequency
→
"Smpl mode") so the relay adjusts the frequency itself.
The frequency readings
are wrong.
In Tracking mode the relay may interpret the frequency incorrectly if no current is injected into the
CT (or voltage into the VT). Please check the frequency measurement settings (
Measurement
→
Frequency).
Settings
Table. 5.2.4 - 44. Settings of the frequency tracking.
Name
Range
Step
Default
Description
Sampling
mode
0: Fixed
1: Tracking
-
0: Fixed
Defines which measurement sampling mode is in use: the
fixed user-defined frequency, or the tracked system frequency.
A
AQ
Q-M257
-M257
Instruction manual
Version: 2.07
50
© Arcteq Relays Ltd
IM00021
Summary of Contents for AQ-M257
Page 1: ...AQ M257 Motor protection IED Instruction manual...
Page 2: ......
