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Monitoring of AC analog measurements
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The design of the alternating quantities measuring function follows the design of all
REx 5xx-series protection, control, and monitoring terminals that have distributed func-
tionality, where the decision levels are placed as closely as possible to the process.
The measuring function uses the same input current and voltage signals as other protec-
tion and monitoring functions within the terminals. The number of input current and
voltage transformers depends on the type of terminal and options included. The maxi-
mum possible configuration comprises five current and five voltage input channels.
Measured input currents and voltages are first filtered in analog filters and then convert-
ed to numerical information by an A/D converter, which operates with a sampling fre-
quency of 2 kHz.
The numerical information on measured currents and voltages continues over a serial
link to one of the built-in digital signal processors (DSP). An additional Fourier filter
numerically filters the received information, and the DSP calculates the corresponding
values for the following quantities:
•
Five input measured voltages (U1, U2, U3, U4, U5), RMS values
•
Five input measured currents (I1, I2, I3, I4, I5), RMS Values
•
Mean RMS phase value, U, calculated from the first two phase-to-earth voltages U1
and U2 (two phase system) or the voltage from U1 (single phase system)
•
Mean RMS value, I, of the first two measured RMS values I1 and I2 (two phase sys-
tem) or the first measured RMS value I1 (single phase system)
•
Total active power, P, related to the first two measured currents and voltages (I1,
U1, I2, U2, two phase system) or related to I1 and U1 (single phase system)
•
Total reactive power, Q, related to the first two measured currents and voltages (I1,
U1, I2, U2, two phase system) or related to I1 and U1 (single phase system)
•
Total apparent power, S, related to the first two measured currents and voltages (I1,
U1, I2, U2, two phase system) or related to I1 and U1 (single phase system)
•
Mean value of frequencies, f, as measured with voltages U1 and U2 or just U1
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