Topics: Relay, Analog-to-digital converter, Flowchart Pages: 8 (2012 words) Published: December 26, 2012


With growing complexity of modern power systems, faster, more accurate and reliable than existing protection schemes have become essential. Microprocessor based protective schemes are the latest development in this area. These micro processor based schemes generally deliver better performance at relatively lower cost and with simpler construction because the operation of the scheme depends largely on programming the micro processor and little on the actual hardware connections. In this paper the implementation of an impedance relay using 8085 microprocessor is described. That kit used for this purpose is Vinytics VMC 8506 which has an inbuilt ADC interface based on ADC0809 chip and also some relays which can be turned on or off by providing simple 8085 instructions. The relay is operated in three zones with the required delay based on impedance.


In some applications it is necessary that the relay protecting a part of the power system operate for faults within a certain distance of the location on any one of the lines. The protecting scheme accordingly uses distance relays and is divided into three zones. The zones are classified based on the impedance seen by the relay and the relays are hence called impedance relays.


The operation of an impedance relay can best be understood by examining the complex plane impedance locus which is shown in figs.1 If the fault impedance is Z then the relay operates instantaneously when | Z | < |Z 1| that is if it lies in the zone 1. If |Z 1| < | Z | < | Z 2|, then the fault is in second zone and thus the relay operates after some delay. For | Z | lying between | Z 2 | and | Z 3 | a greater delay is introduced before the operation of the relay because the fault is in the third zone of operation. If | Z | exceeds | Z 3 | then the relay will not operate as the fault impedance is outside the operating zone of the impedance relay.

TORQUE PRODUCED IN AN ELECTROMECHANICAL IMPEDANCE RELAY In an impedance relay, the torque produced by a current element is balanced against the torque of a voltage element. The current element produces positive (pick up) torque proportional to I2 whereas voltage element produces negative torque proportional to V2. the torque equation is T=K’I2 - K’’ V2 + K’’’

Where K’ and K’’ are torque constants and K’’’ is spring constant and is generally neglected. At balance point T=0, from this equation we get impedance V/I = Sq. root of (K’/K’’)


➢ It has poor mechanical stability.
➢ Operates rather slowly.
➢ Possibility of incorrect operation because of the mechanical constraints. ➢ Very tough to change the zones of protection.


The disadvantages of a conventional impedance relay arte overcome by using microprocessors for realizing the operation of the relays. Microprocessor based relays perform very well and their cost is relatively low.


❖ Flexibility
❖ Highly reliable
❖ Fast operation


To realize an impedance relay, the voltage and current are supplied to the microprocessor via an A/D converter which supplies the corresponding digital values to the processor. The microprocessor then finds the fault impedance by dividing the voltage count with the current count. Based on this fault impedance the microprocessor decides the zone in which the relay has to be operated and sets the delay time accordingly.



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