Microprocessor based numeric relay
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Date
2010
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Abstract
Relaying voltages and currents at are passed through Isolation Transformers. Since
analog to digital conversion is usually performed on voltages, the current signals are converted to representative voltages by passing them through a known resistance. All the signals are then filtered using simple analog Anti aliasing Filters. Since ADCs are
expensive it is common to use only one in a numeric relay. Our relay utilizes built-in
ADC of PIC 16F877A Microcontroller. Software level Multiplexer is used, under
microprocessor control, sequentially to select the required signal into ADC. Since an
ADC takes a finite conversion time usually 25usec, it is necessary to hold the incoming
signal for the duration of conversion. This is achieved by a Sample and Hold amplifier.
Having been manipulated by the ADC the signal can be fed to Microprocessor which in
our case is PIC 16F877A. There may be one or more processors, one for running algorithm ad another for scheme logic. The relaying program will be located in ROM and a RAM will be used for storing sampled quantities and intermediate products in relaying
algorithm. Relay settings can be stored in EEPROM or they can be defined in program as
in our case for fixed application. A Power Supply is also incorporated to supply a
regulated and constant power to relay electronics. These are typically 5V or 12V.
Switched mode supplies are generally used as they are more efficient. Voltage and
current values of all three phases are continuously being displayed and updated on LCD separately. LCD also shows particular operation of relay in case of fault.
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Keywords
Microprocessor, Relaying