Hardware and Functional Development of Protection Devices and Systems - Digital Signal Processing in Power System Protection and Control - page 23

Multiplexer

Sampled
analog signals

Converter

Digital
signals

D

n

A

f s

Analog
memory

Fig. 3.11 A/D converter with analog memory and multiplexer

The A/D converters are characterized by two basic parameters:

• number of bits m (called also the word length of A/D converter, m = 8…16),
• converter range M.

The converter resolution is a derivative of the parameters m and M. The grain of
conversion is defined as:

M
2 m 1 :

d ¼ ð 3 : 8 Þ

From ( 3.8 ) results that higher resolution (lower grain) is obtained for higher
number of bits m. When the signal changes in wider range the resolution for the
same number of bits gets lower. That is why the A/D converters for current signals
must have higher number of bits (m = 12…16), since fault currents are signifi-
cantly higher than normal operation (load) levels.
If one wants to maintain the relative error of A/D conversion e (related to the
smallest expected signal value X min ) at given level, then the minimum required
number of bits should obey the relationship:

þ 1 :

0 : 5M
eX min

m log 2 ð 3 : 9 Þ

3.2.3 Digital Signal Processing

The protection operation related to determination of state of the protected plant
(faulty or healthy) and issuing the final decision is based on digital processing of
sampled input signals, with the aim of calculation of the respective criteria values.
A great number of protection criteria measurement algorithms have been
developed so far, that can be roughly grouped into four families, as illustrated in
Fig. 3.12 . One can see that the first step of signal processing is usually filtering out
of signal components that are hoped to bear the information about the protected
plant state. The other components are treated as noise and should be rejected or at