MICROPROCESSOR BASED IMPEDANCE RELAY

[attachment=16760]

INTRODUCTION

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.

OPERRATING PRICIPLE OF THE IMPEDANCE RELAY

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’’)


DISADVANTAGES OF ELECTROMECHANICAL IMPEDANCE RELAYS

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.

MICROPROCESSOR BASED IMPEDANCE RELAY

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.

ADVANTAGES OF MICROPROCESSOR BASED RELAYS

Flexibility
Highly reliable
Fast operation

IMPEDANCE RELAY

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.