Distance Relay Fundamentals

Distance functions have been in use for many years and have progressed from the original electromechanical types through analog types and now up to digital types of functions.

Simple MHO Function

A simple mho distance function, with a reach of Z ohms, is shown in Figure 1. This diagram is exactly equal to an R-X diagram except that all of the impedance vectors have been operated on by the current I. The mho function uses the current and voltage measured at the relay to determine if the apparent impedance plots within the mho characteristic.

If the angle is less than or equal to 90°, then the fault impedance Zf plots within the characteristic, and the function will produce an output.

If the angle is greater than 90°, then Zf falls outside of the characteristic and no output will be produced. Assume that the angle of maximum reach (q) and the angle of ZL (f) are equal. On that basis, the conditions shown in Figure 2 will be obtained. The key point to note in this phasor analysis (a convenient way to view relay performance) is the magnitude of the IZ – V (Vop) phasor and its relationship to the V (Vpol) phasor.

Operation will occur whenever Vop and Vpol phasors are within 90° of each other and provided both Vop and Vpol are greater than the minimum values established by the sensitivity of the relay design.

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