Digital Protective Relay

In early days, there used to be isolated thermal plants for each locality known as urban system. However, with the growth in the demand of electricity for different purposes such as industrial, agricultural, public places and domestic consumers, the need arises to have both the thermal as well as hydroelectric plants intercon­nected to supply common loads. Such systems are known as grid systems which have both thermal plants operating as base load plant and hydel plant operating as peak load plant for supplying electric power at normal operating condition to the complex load system. For such complex power system, the protection scheme required must be highly sensitive, efficient and reliable in operation. The protection scheme includes both the Digital protective relay as well as the switching circuit, i.e. the circuit breaker. The Digital protective relay that works as a brain is an essential element. In the early days, there were electromechanical relays, particularly of induction disk-type. But, later the disk was changed by the inverted cup that is hollow cylinder, and the new relay is known as induction cylinder relay or induction cup relay. This protective relay which is in use even now possesses several important features such as higher speed, higher torque for a given power input and more uniform torque.

However, with the advent of electronic tubes, electronic Digital protective relays were developed during 1940s which had several distinct features. With the advent of solid state devices during 1950s, static digital protective relays using these solid state components with numerous advantages were developed. The digital computer was developed during the 1950s and the first planning studies on the digital computer were completed by the year 1957.

The use of digital computers and microprocessors for protective relaying purposes has been engaging the attention of research and practicing engineers since late 1960s. The first serious proposals for using digital computers came from Rockefeller. The algorithms proposed till now can be classified into two categories; distance relay algorithms and traveling wave relay algorithms. The distance relay algorithms involve the determination of the fundamental-frequency impedance to fault from the fundamental components of voltages and currents which are ex­tracted from the complex post-fault waveforms by analog and/or digital filters.

Questionnaire:

• Explain more about  digital relays.