Alstom MiCOM P846, Technical Manual

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Firmware Design P846/EN FD/D
MiCOM P846
(FD) 9-7
VT
CT
VT
CT
Buffer
16-bit
ADC
Optical
isolator
Optical
isolator
Noise
filter
Noise
filter
Buffer
Serial
interface
Sample
control
16:1
Multiplexer
Anti-alias filters
Transformer board
Input board
8
4
4
4
Up to 5 current inputs
3/4 voltage inputs
8 digital inputs
Parallel bus
Serial sample
data bus
Trigger from processor
board
Calibration
E PROM
Parallel bus
Up to 5
Up to 5 Up to 5
Diffn
to
single
Diffn
to
single
Diffn
to
single
Diffn
to
single
Low
pass
filter
Low
pass
filter
Low
pass
filter
Low
pass
filter
8
2
P0127ENa
Figure 2: Main input board
The signal multiplexing arrangement provides for 16 analog channels to be sampled. The
P846 relay provides up to 9 current inputs and 4 voltage inputs. 3 spare channels are used
to sample 3 different reference voltages for the purpose of continually checking the operation
of the multiplexer and the accuracy of the A - D converter. The sample rate is maintained
at 32 samples per 60Hz cycle or at 40 samples per 50Hz cycle of the power waveform by a
logic control circuit that is driven by the frequency tracking function on the main processor
board. The calibration E
2
PROM holds the calibration coefficients that are used by the
processor board to correct for any amplitude or phase errors introduced by the transformers
and analog circuitry.
FD
The other function of the input board is to read the state of the signals present on the digital
inputs and present this to the parallel data bus for processing. The input board holds 8
optical isolators for the connection of up to eight digital input signals. The opto-isolators are
used with the digital signals for the same reason as the transformers with the analog signals;
to isolate the relay’s electronics from the power system environment. The input board
provides some hardware filtering of the digital signals to remove unwanted noise before
buffering the signals for reading on the parallel data bus.
1.3.4.3 Universal opto isolated logic inputs
The P846 relay is fitted with universal opto isolated logic inputs that can be programmed for
the nominal battery voltage of the circuit of which they are a part. The inputs can be
programmed with a pick-up/drop-off characteristic selectable as the standard 60% - 80%
value or an optional characteristic of 50% - 70%. This implies, that they nominally provide a
Logic 1 or On value for Voltages
≥
80% or 70% of the set lower nominal voltage and a Logic
0 or Off value for the voltages
≤
60% or 50% of the set higher nominal voltage. This lower
value eliminates fleeting pickups that may occur during a battery earth fault, when stray
capacitance may present up to 50% of battery voltage across an input. Each input also has
a selectable filter of ½ cycle which renders the input immune to induced noise on the wiring;
although this method is secure it can be slow, particularly for intertripping and back-tripping.
This can be improved by switching off the ½ cycle filter in which case one of the following
methods to reduce ac noise should be considered. The first method is to use double pole
switching on the input, the second is to use screened twisted cable on the input circuit.