
1.2 Principles of numerical
protection techniques
1 Introduction
Technical description
44
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VM265.EN011
1.2.
Principles of numerical protection
techniques
The manager is using numerical technology. This means that
all the signal filtering, protection and control functions are
implemented through digital processing.
The numerical technique used in the manager is primarily
based on an adapted Fast Fourier Transformation (FFT)
algorithm. Synchronized sampling of the measured voltage and
current signals is used. The sample rate is 32 samples/cycle
within the frequency range 45 Hz ... 65 Hz. The frequency is
measured from the voltage signals and used to synchronize the
sampling rate. Therefore secondary testing of a brand new
device should be started with voltage protection functions and
voltage injection to let the relay learn the local frequency. The
learned frequency is used for sampling rate synchronization
when no voltage is present. The local network frequency can
also be manually given for the relay.
Apart from the FFT calculations, some protection functions also
require the symmetrical components to be calculated for
obtaining the positive, negative and zero phase sequence
components of the measured quantity. For example, the
function of the unbalanced load protection stage is based on the
use of the negative phase sequence component of the current.
Figure 1.2-1 shows a hardware block diagram of the relay. The
main components are the current and voltage inputs, digital
input elements, output relays, A/D converters and the
microcomputer and a power supply unit.
Figure 1.2-2 shows the inputs and outputs of a general
protection function. The FFT block is calculating the
fundamental frequency phasors and also harmonics used by
some protection functions. The block matrix is used for simple
interlocking. (More complex interlocking is done with the
user's programmable logic.) The output matrix is used to
connect the pick-up and trip signals from protection blocks to
the output relays and indicators.
Figure 1.2-3 shows a block diagram of a basic overcurrent or
overvoltage function with definite and inverse operation time.
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