IHS-1T

The IHS 1T devlce is intended for directional earth-fault protection of networks with isolated, compensated, low resistance earthed and directly earthed neutral.
Information on homopolar current can be obtained from either a parallel connection of three current measuring transformers or from a cable transformer. The use of a cable transformer offers more sensitive current setting of the directional earth-fault protection.

The directional earth-fault relay receives information on voltage from a voltage measuring transformer while information on current is received from a current measuring transformer.
According to the block diagram in fig. 1, information on current goes to the current comparator I > in which the required pick-up current setting is effected (in the range of 0,0 1 A to 0,6 A) .
Information on current direction is formed through observing both voltage and current zero crossings. The zero crossing of voltage triggers a monostable multivibrator circuit which generates a square wave pulse at its output of duration b, adjustable from 0,55 ms to 10 ms. The falling edge of the square wave pulse having duration b triggers the next monostable multivibrator circuit which generates a square wave pulse of duration h= 10 ms.
If the instant of zero crossing of current falls within the interval h, that means that the current Iaggs behind the voltage for the time equal to at least b. Considering that the Iagging time corresponds to phase shift, the duration b is calibrated in degrees directly (and can be set from 10° to 180°). A graphical interpretation is presented in fig. 2. With voltage and current measuring transformers connected as in fig. 1, the angle b can be defined as the angle existing between voltage and the "nearer" limit of the relay pick-up zone (having the "width" of 10 ms, resp. 180°). It is therefore possible, by selecting b, to select an arbitrary position of the relay pick-up semi-plane in the plane Uo, Io and thus to match the relay optimally to network conditons.
In the example on fig. 2, it is visible that fault current Io1 falls within the relay pick-up zone.

The IHS-1T relay is made with high quality and reliable semiconductor components. Static construction has offered high reliability, accuracy and sensitivity. The sensitivity does not depend on the angle between current and voltage. The relay operation is normal already at 0, 1% Un and 10 mA earth-fault current.
The choice of pick-up current settings and of pick-up zone positions of the relay is possible within wide limits by mepns of the setting elements on the relay front panel. The recommended values for b angle settings are:

-in network with isolated neutral. b = 10° to 50° .
-in network with compensated neutral b= 50° to 80°