From Protection of Electricity Distribution Networks, 2nd Edition

5.10 Use of Digital Logic in Numerical Relaying

5.10.1 General

When using numerical relays it is necessary to provide a suitable method for handling the relay logic capabilities, which should include blocks with control inputs, virtual outputs, and hardware outputs. A group of logic equations defining the function of the multifunction relay is called a logic scheme.

Numerical relays can be configured to suit a particular specification by defining the operating settings (pick-up thresholds and time delays) of the individual protection and control functions. Operating settings and logic settings are interdependent, but separately programmed functions. Changing logic settings is similar to rewiring a panel, and is used for managing the input, output, protection, control, monitoring and reporting capabilities of multifunction protection relay systems. Each relay system has multiple, normally-contained function blocks that have all of the inputs and outputs of its discrete component counterpart. Each independent function block interacts with control inputs, virtual outputs, and hardware outputs based on logic variables defined in equation form with relay logic. Relay logic equations entered and saved in the relay system's nonvolatile memory integrate the selected or enabled protection in order to provide the operating settings that control the relay pick-up threshold and time delay values.

5.10.2 Principles of Digital Logic

Digital systems are constructed by using three basic logic gates. These gates are designated AND, OR and NOT. There also exist other logical gates, such as the NAND and EOR gates. The basic operations and logic gate symbols are summarised in Figure...

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Programmable Logic Controllers (PLC)
Programmable logic controllers (PLC) are the control hubs for a wide variety of automated systems and processes. They use a wide variety of inputs and outputs and provide network capability.
Field-Programmable Gate Arrays (FPGA)
Field-programmable gate arrays (FPGAs) have a different architecture than SPLDs and CPLDs, and typically offer higher capacities. FPGAs are also known as logic cell arrays (LCA) and programmable ASIC (pASIC).
Logic Decoders and Demultiplexers
Logic decoders and logic demultiplexers move data between inputs and outputs. In the case of digital decoders, the coded information is translated into familiar or uncoded formats, while digital multiplexers transmit data from one input through to several output lines. 
Simple Programmable Logic Devices (SPLD)
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Topics of Interest

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