Technical description
1 Introduction
1.2 Principles of numerical
protection techniques
VM59.EN001
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41
Flexible communication option concept available to support
different media requirements (serial interfaces, optical
fibres, Ethernet etc),
Built-in, self-regulating ac/dc converter for auxiliary power
supply from any source within the range from 40 to 265 Vdc
or Vac. The alternative power supply is for 18 to 36 Vdc.
Built-in disturbance recorder for evaluating all the analogue
and digital signals.
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 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, some current has to be injected
to the current input I
L1
to adapt the network frequency for the
device. 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 current unbalanced stage is based on the use of
the negative phase sequence component of the current.
Figure 1-1 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 shows the heart of the numerical technology. That is
the main block diagram for calculated functions.
