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Saturday, 14 February 2015

Plug setting and Time setting in Induction type Relay

Electromagneticinduction relays, one of the most widely used relays for protective relaying purposes, operate on the principle of electromagnetic induction and therefore can be used only on AC circuits. 

Setting in Induction type Relay:

"In induction disc relay there is a facility for selecting the plug setting and the time setting such that the same relay can be used for a wide range of current, and time characteristics." 

The minimum torque required for the movement of the disc is fixed for a particular design, i.e. the ampere-turns required for the disc movement are fixed. 

Often it is desirable to adjust the pick-up current to any desired value. This adjustment is known as current setting and is normally achieved by the use of tapings on the relay operating coil (primary coil). Therefore, for different pick-up current settings, number of turns is changed effectively so as to keep the same ampere-turns. 

The value assigned to each tap are expressed in terms of percentage full-load rating of the Current Transformer (CT) with which the relay is associated and represents the value above which the relay disc starts to rotate and finally closes the trip circuit.

Plug Setting Multiplier (PSM):

Selection of the required current setting is done by means of a plug setting multiplier plug. While the plug is withdrawn for adjusting it to a different current setting during on-load condition, the maximum current tap is automatically connected and thus the risk of open circuiting the secondary of the CT is avoided.

The ratio of fault current in the relay coil to the pick-up current is known as plug setting multiplier (PSM). Hence,

PSM = (Fault current in relay coil / Pick-up current),

Since, Fault current in the relay coil = (Fault current / CT ratio)
PSM = (Fault current / (Pick-up current x CT ratio))  

Time Setting Multiplier (TSM):

The operating time of the relay depends upon the distance between the moving contact and the fixed contact of the relay. The distance between the contacts is adjusted by the movement of the disc backstop which is controlled by rotating a knurled moulded wheel at the base of a graduated time multiplier scale. This is known as time setting multiplier

The higher the time multiplier setting, the greater is the operating time. The time setting multiplier is marked from 0 to 1 in steps of 0.05. If the relay takes a certain time, say ‘S’ seconds with time multiplier setting as 1, then the same relay will take a time equal to 0.5 S seconds for a time multiplier setting of 0.5.