# Generator Loss of Excitation Protection (ANSI 40)

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#### niazbibiyana

Hello,

I know modern generator relays detect field loss of excitation from generator terminal voltage and current. I checked that, in relay, its somehow calculated from P-Q diagram and convert to R-X diagram (For MHO type relay). I want to know, how this conversion take place?

second question, why this type of relay is called MHO relay? as, its calculated on R-X, it should be called impedance relay. But at many place, i noticed, they write it as MHO relay.

Thanx

#### ControlsGuy25

Hello,

I know modern generator relays detect field loss of excitation from generator terminal voltage and current. I checked that, in relay, its somehow calculated from P-Q diagram and convert to R-X diagram (For MHO type relay). I want to know, how this conversion take place?

second question, why this type of relay is called MHO relay? as, its calculated on R-X, it should be called impedance relay. But at many place, i noticed, they write it as MHO relay.

Thanx
Niazbibiyana,

You can have, significant and precious informations, on LOE protection scheme, by reading this powerpoint:
https://fr.slideshare.net/mmuralintpc/loss-of-excitation.

I am sure that it can give you better overview, of the LOE Protection designing and implementation for Generator protection .

Controlsguy25.

#### PhilCorso

Niazbibiyana...
You're showing your age! No sweat, I'm probably decades older. However I've graduated from being "old" to being an "Elder"!
I'm sure you know that for an L.O.E. condition, the synch-gen'r becomes an induction-gen'r and the reactive energy flowing toward the load, reverses and flows toward the gen'r ! The smart relay guys said "we can't differentiate direction ! What should we do ? They decided to invert the equation related to Z=R+jX having Ohm's or O-H-M's as measuring-units, to equivalent M-H-O's having conductance and susceptance as measuring-units with which direction could be determined ! Q.E.D.
Regards, Phil Corso
Ps1: an update... Mho's are now called "Siemens" ! Ps2: Similar problem for Transmission-Line relaying !