Hardware and Functional Development of Protection Devices and Systems - Digital Signal Processing in Power System Protection and Control - page 17

Instrument
transformers

Analog
conditioning

A/D
conversion

Digital Signal
Processing

Logic
& Comm.

Binary
I/O

Relay
Output

Internal
Bus

T A L

V&I

B
I/O

A/D DSP

R/O

C

Trip

Power
supply

LON

Processing of
analog signals

Digital
algorithms

Output
blocks

Fig. 3.4 Structure of a digital relay of the 4th generation

Within this philosophy the system-wide protection and control has emerged,
with the following (exemplary) functions:

• adaptive protection system with adjustment to various network topologies,
• wide-area differential protection embracing network objects over a number of
voltage levels,
• global (centralized) decision-making based on exchange of information,
• short-circuit detection and location at the relaying center,
• application of adaptive protection settings,
• additional functions (monitoring of system stability, connection with substation
automation systems, etc.).

An example of such a structure for system stability monitoring and control is
shown in Fig. 3.5 . The information from Phasor Measurement Units installed at
selected places in the network is processed to evaluate the safety margin. In case of
low level of this margin certain control actions can be performed, with the aim of
reversing the process and maintaining system stability [ 2 , 11 ].
All the advantages of 3rd and 4th generation of protective devices require
efficient digital processing of the input signals. Both theoretical basics and specific
algorithms for signal conditioning, calculation of criteria values and decision-
making are provided in the following sections of this topic.

3.2 Functional Blocks of a Digital Protection Device

3.2.1 Analog Antialiasing Filtering

Before sampling the input signals go through the antialiasing low-pass filters that
are used to pass all the components with frequencies lower than f k (highest