BECKWITH ELECTRIC M-0420, Инструкция

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A–2
M-0420 Instruction Book
VTs & CTs
2K byte
Dual-Ported
RAM
Digital Signal
Processor
(DSP)
TMS 32015
8K byte
ROM
12-bit
Analog-to-Digital
Converter
(ADC)
i
v
a
b
c
n
a
b
c
n
Programmable
Gain Amplifier
DSP Processor Board
Input Board
Host Processor Board
MUX
Address/Data Bus
128K byte Flash-
Programmable
ROM
Host Processor
10 MHz 64180
Front Panel Board
LED
Targets
Front-Panel
Controls
RS 232
Serial I/O
Analog Multiplexer
Anti-Aliasing Low-Pass Filters (LPF)
Output Board
Relay
Outputs
Contact
Inputs
2-Line by 24-Character
Liquid Crystal Display
32K byte RAM
EEPROM
RAM, Clock
10 Year
Lithium Battery
Power Supply Board
Power Supply
i
i
i
v
v
v
i'
a
b
c
i'
i'
Opto-Isolation Opto-Isolation
Temperature
Sensor
COM1 COM2
Figure A-1 System Block Diagram
A.2 System Block Diagram
As shown in Figure A-1, the inputs to the relay,
filtered to remove higher order harmonics, are
multiplexed (MUX) and then passed through an
analog-to-digital converter (ADC) to the digital signal
processor, which performs a discrete fourier trans-
form (DFT) 16 times per cycle for each of the
eleven inputs. The host processor manages I/O
functions (including the front panel controls, the
LCD display, and the two RS-232C ports), up-
dates the memories for setpoint values, records
trip information, and ultimately analyzes the data
from the DSP to determine the need for a trip
command.
The voltage and current signals are scaled and
isolated using voltage transformers (VT) and cur-
rent transformers (CT). These isolated analog sig-
nals must be filtered to remove unwanted harmonic
content above 480 Hz to accommodate a 960 Hz
sampling rate, according to the Nyquist criterion,
to avoid aliasing or the incorrect interpretation of
frequency components by the system. Anti-aliasing
low-pass filters (LPF) are applied individually to
the eight inputs for this purpose.
To this point, the circuit is duplicated in hard-
ware for each voltage or current input, but it now
becomes efficient to multiplex or time-share those
values for the digital portion of the design. The
MUX performs a very high-speed electronic switch
function, sequentially accepting scaled voltage
signals from the filters and routing those signals
to the programmable gain amplifier (PGA).
Because of the wide dynamic range of possible
input current values, it is difficult to maintain
accuracy for both low and high pickup values.
Additionally, the gain of the voltage channels
should be independent of the phase current pickup
setting, and the gain of the neutral current channel
may differ from that of the phase current channels.
While electromechanical and static relay designs
address this problem by providing coil taps on
the input current transformer to adjust the pickup
current, the multifunction relay uses the PGA.
Noting that the relay needs to measure only up
to 20 times the pickup value, the gain of the
PGA can be adjusted accordingly for each input.