The relay measuring element in high impedance restricted earth fault (in electromagnetic type) is an attracted armature unit of simple and robust construction which usually supplied from a bridge rectifier. Attraction relays can be supplied by AC or DC, and operate by the movement of a piece of metal when it is attracted by the magnetic field produced by a coil. There are two main types of relay in this class. The attracted armature relay consists of a bar or plate of metal which pivots when it is attracted towards the coil. The armature carries the moving part of the contact, which is closed or opened according to the design when the armature is attracted to the coil. The other type is the piston or solenoid relay in which a bar or piston is attracted axially within the field of the solenoid. In this case, the piston also carries the operating contacts. The current transformers used in high impedance circulating current differential protection systems must be equal turns ratio and have reasonably low secondary winding resistance. The knee-point voltage is defined as the point on the magnetization curve at which a 10% increase in excitation voltage produces a 50% increase in excitation current.
Relay measuring elements whose functionality is based on the comparison of two independent quantities are essentially either amplitude or phase comparators. For the impedance elements of a distance relay, the quantities being compared are the voltage and current measured by the relay. There are numerous techniques available for performing the comparison, depending on the technology used. They vary from balanced-beam (amplitude comparison) and induction cup (phase comparison) electromagnetic relays, through diode and operational amplifier comparators in static-type distance relays, to digital sequence comparators in digital relays and to algorithms used in numerical relays.
The induction relay works only with alternating current. It consists of an electromagnetic system which operates on a moving conductor, generally in the form of a disc or cup, and functions through the interaction of electromagnetic fluxes with the parasitic Fault currents which are induced in the rotor by these fluxes. These two fluxes, which are mutually displaced both in angle and in position, produce a torque.
Any type of impedance characteristic obtainable with one comparator is also obtainable with the other. The addition and subtraction of the signals for one type of comparator produces the required signals to obtain a similar characteristic using the other type. For example, comparing V and I in an amplitude comparator results in a circular impedance characteristic centered at the origin of the R/X diagram. If the sum and difference of V and I are applied to the phase comparator the result is a similar characteristic.