STATIC OVER CURRENT RELAY

The over-current relays, even though simplest of all types of electro-mechanical relays, are the most difficult static relays. Static over current relays are of two types:

(i) Instantaneous over-current relays and

(ii) Time over-current relay.

Static instantaneous Over-Current Relay

The block diagram of an instantaneous over-current relay is shown in fig 21. The same construction may be used for under-voltage, over-voltage and earth fault relays too.

The secondaries of the line CT’s are connected to a summation circuit (not shown in the fig). The output of this summation CT is fed to an auxiliary CT, whose output is rectified smoothened and supplied to the measuring unit (level detector). The measuring unit determines whether the quantity has attained the threshold value (set value) or not. When the input to measuring unit is less than the threshold value, the output of the level detector is zero.

For an over-current relay

For I _input < I _threshold; I_output = 0

For I _input > I _threshold; I_output = Present

In an actual relay I _threshold can be adjusted.

After operation of the measuring unit, the amplifier amplifies the output. Amplified output is given to the output circuit to cause trip/alarm. If time-delay is desired, a timing circuit is introduced before the level detector. Smoothing circuit and filters are introduced in the output of the bridge rectifier. Static over-current relay is made in the form of a single unit in which diodes, transistors, resistors, capacitors etc., are arranged on printed board and are bolted with epoxy resin.

Static Over-Current time Relay

The block diagram of static over current time relay is shown in fig 22.

The current from the line CT is reduced to 1/1000 th by an auxiliary CT, the auxiliary has taps on the primary for selecting the desired pick-up and current range and its rectified output is supplied to level detector I (over-load level detector) and an R-C timing circuit. When the voltage on the timing capacitor Vc attains the threshold value of the level detector II, tripping occurs. Time delay given by the timing circuit shown in fig 22 b is given as Tc = RC log _e E/(E – V_T) .

Where V_T is the threshold value of the level detector II. By varying values of R and C the time can be varied without difficulties

Written by John on March 17th, 2009 with 3 comments.
Read more articles on classification of relays and Power System and Relays and Switchgear&Protection.

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