A few words about the digital protection relay

History of the protection relay
Around the 1980s, the digital relay entered the market. Compared with solid-state relay, digital protection relay takes advantage of the advantages of the development of microprocessors and microcontrollers. Instead of using analog signals, the digital relay converts all measured analog quantities into digital signals.
Digital protection relays are a revolutionary step in the evolution of Relay technology.
In digital relays, microprocessors and microcontrollers replace the analog circuits used in solid-state relays to implement relay functions. Digital protection relays were introduced in 1980.
However, this technology will be completely replaced in the next five years by digital relays.
By the mid-90s, the solid-state and electromechanical relay had been largely replaced by the digital relay in new construction. In distribution applications, the replacement by the digital relay proceeded somewhat slower.
While the vast majority of power relays in newer current applications being digital, the solid-state relay still has applications where the simplicity of the application allows the relays to be simplified and the complexity to be avoided. digital relays.

Principles of measurement
Compared to solid-state relays, digital relays introduce analog-to-digital converter ( A / D conversion ) of all measured analog quantities and uses a microprocessor to implement the protection algorithm.
The microprocessor can use a counting technique or use the Discrete Fourier Transform (DFT) to implement the algorithm.
The microprocessors used in digital relays have limited processing capacity and memory compared to those provided by digital relays.

Digital Protection Relay
Digital Protection Relay

Relay function
Functionality, therefore, tends to be limited and largely limited to the protection function itself. Additional functionality to those provided by an electromechanical or solid-state relay is generally available. They generally take the form of a larger and more precise range of parameters.
A communication link with a remote computer can also be provided.

The limited power of the microprocessors used in digital relays limits the number of waveform samples that can be measured per cycle. This, in turn, limits the operating speed of the relay in some applications. Therefore, a digital relay for a particular protection function may have a longer operating time than the equivalent solid-state relay.
However, the additional time is not significant in terms of overall trip time and possible effects of power system stability.

Relay operation
The digital relay includes:
Analog input subsystem,
Digital input subsystem,
Digital output subsystem,
A processor with RAM ( notebook ),
main memory ( historical data file ) and
Power source
Digital relay involves the digital processing of one or more analog signals in three steps:
Converting an analog signal to digital form
Digital form processing
Boolean decision to trip or not.

Advantages of digital relay
High level of functionality integration.
Additional monitoring functions.
Functional flexibility
Capable of working in a wide temperature range.
They can implement more complex functions and are generally more precise
Self-checking and self-adaptability.
Capable of communicating with other digital equipment (pear to pear).
Less sensitive to temperature, aging
Economical because it can be produced in volume
More precise.
plane for remote relaying is possible
Signal storage is possible

Digital relay limits
Short lifespan due to the continuous development of new technologies.
Devices quickly become obsolete.
Sensitivity to power system transients.
As digital systems become more and more complex, they need specially trained personnel for operations.
Appropriate maintenance of parameters and monitoring data.

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