Technical description
1 Introduction
1.2 Principles of numerical protection
techniques
VM259.EN007
VAMP 24h support phone +358 (0)20 753 3264
47
1.2.
Principles of numerical protection
techniques
The device is fully designed 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 device is primarily based
on an adapted Fast Fourier Transformation (FFT). In FFT the
number of calculations (multiplications and additions), which
are required to filter out the measuring quantities, remains
reasonable.
By using synchronized sampling of the measured signal
(voltage or current) and a sample rate according to the 2
n
series, the FFT technique leads to a solution, which can be
realized with just a 16 bit micro controller, without using a
separate DSP (Digital Signal Processor).
The synchronized sampling means an even number of 2
n
samples per period (e.g. 32 samples per a period). This means
that the frequency must be measured and the number of the
samples per period must be controlled accordingly so that the
number of the samples per period remains constant if the
frequency changes. 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.
However, if this is not possible then the frequency must be
parameterised to the device.
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 principle block diagram of a numerical
device. The main components are the energizing inputs, digital
input elements, output relays, A/D converters and the micro
controller including memory circuits. Further, a device contains
a power supply unit and a human-machine interface (HMI).
Figure 1.2-2 shows the heart of the numerical technology. That
is the main block diagram for calculated functions.
Figure 1.2-3 shows a principle diagram of a single-phase
overvoltage or overcurrent function.