Reactance Relay
These devices measure only reactance, and are not affected by variation in circuit resistance, their characteristics is a straight line parallel to to the resistance axis, with some pre-determined setting value of x. the relay will measure any value of reactance below the setting.
Like impedance relay, it is not directional and when current is flowing towards the bus bars at the end where protection is installed, it will even operate for values of x well in excess of the setting, including the reactance presented by the load.
As the zone characteristics are parallel to the R-axis, therefore any increase due to arc resistance has no effect on the performance of the relay. The relay is least effected by the arc resistance.
When a power system is operating in steady state all rotor or machine angles are constant. whenever there are sudden and large changes of power in th system, the rotor angle undergo oscillations till the system reaches a new stable state. This phenomenon is known as power swing.
During power swing the apparent impedance enters the trip region of the relay slowly, whereas during the fault it enters suddenly. The effect of power swing on reactance relays is maximum.
The relay is inherently non-directional. it needs directional element. This relay is suitable for short length lines.
MHO Relays
The term MHO is derived from the fact that he relay characteristics when plotted on R-X diagram cuts the intersection of the x and R axis. With such a characteristics the relay measures the distance in one direction only. The characteristics of MHO relay on admittance diagram is a straight line.
A mho unit therefore combines directional action with ohmic measurement and thus combines both functions in one unit.
The characteristics are shown on the next slide. Mho relays are also known as admittance relays. The 45 degree setting is suitable for HV distribution lines 33KV or 11KV, where line angle vary between 45 and55 degree. The 60 degree setting is suitable for 66 or 132KV lines. For 220KV and above a setting of 75 degree characteristic for the line is used.
Effect of high arc resistance result in under reaching of the relay. A fault in zone 1 with high resistance may shift it i the next zone increasing the relay operation time and hence the fault clearance time. the effect is severe if the fault occurs in second half of the zone.
During power swing the apparent impedance enters the trip region of the relay slowly, whereas during the fault it enters suddenly. The effect of power swing on MHO relays is least.
The relay is inherently directional. it needs no directional element ad this relay is suitable for long transmission lines.
