Digital Overcurrent Relay
The overcurrent relay is the most commonly used for a type of protection relay protection of distribution systems.
Digital relays are extensively used in the power industry operation. They operate when the circuit current exceeds the predetermined value. The microprocessor uses multiplexer for sensing the fault currents in a number of circuits and sub-circuits. The microprocessor accepts signals in the voltage farms, the CT fault current derived is first converted into proportionate voltage signals, ‘and then fed to the rectifier, multiplexer, analog to digital converter and the microprocessor.
Block diagram of Digital Overcurrent Relay
The output of the rectified voltage signals is fed into the multiplexer. The microprocessor (microcomputer) then sends command for switching ON the desired channel of the multiplexer in order to obtain the rectified voltage in the particular circuit.
Since the microprocessor needs digital signals, the output of the multiplexer is fed into the analog to digital converter (A/D). Again, the microprocessor sends a signal to the A/D for starting the conversion and reads the end of the conversion signal to examine whether the conversion is over and compares the signal with the pre-determined pick-up value. A programme flowchart for an overcurrent relay is shown in figure 5.14.
Flow Chart Of Digital Overcurrent Relay
Benefits of Digital Overcurrent Relay :
(a) Lower cost and the ability to provide a full range of characteristics in one product.
(b) The required characteristics can be selected using the relay front paneL
(c) Only one current input is required per relay.