BECKWITH ELECTRIC M-0420, Instruction Book

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A–1
Theory of Operation – Appendix A
A
Theory of Operation
A.1 Introduction ................................................................................ A–1
A.2 System Block Diagram ............................................................. A–2
A.3 Discrete Fourier Transform ....................................................... A–3
A.4 Acknowledgments ..................................................................... A–7
A.5 References ................................................................................. A–7
A.1 Introduction
One early presumption in the design of the M-0420
Multifunction Relay was that the pursued approach
would involve very sophisticated analytical pro-
cedures in order to efficiently accommodate mul-
tiple functions in one package. In fact, it was
realized that the design would require digital sig-
nal processing (DSP) techniques in order to make
the extremely rapid computations necessary to
accommodate all functions in real time. At the
same time, the inherent limits on accuracy for
many analog components would not be a factor
in the new relay.
The approach followed in this project replaces
the entire analog signal-processing hardware with
a Digital Signal Processor (DSP). If the relay
were designed with an analog “front end,” it would
require analog circuitry for each channel to cal-
culate the magnitude and phase angle, and other
analog circuits for the calculation of negative
sequence current, real and reactive power, and
other functions.
While DSPs are highly effective for signal-pro-
cessing applications, they are not very efficient
for general purpose applications and have limited
memory space. Therefore, the multifunction re-
lay uses a dual-processor architecture: the DSP
executes all the signal-processing algorithms, while
a general-purpose processor (denoted as the host
processor) is used mainly for input/output (I/O)
processing, including the relay logic.