Purpose of ANSI 21 Distance Relay?

[peebee]

On an existing generator-utility full-time paralleling installation, there is an ANSI 21 Distance Relay monitoring the incoming utility MV feeder. This was specified to be provided for this customer-owned installation by the utility as a condition of operation of the system.

Per the Red Book, a 21 distance relay "is a relay that functions when the circuit admittance, impedance, or reactance increases or decreases beyond a predetermined value".

Where does the word "distance" come in to play here? nothing in the above description mentions distance.

What is the point of this relay -- what does it do that a 27 undervoltage, 50/51/G overcurrent, or 59G overvoltage (ground-fault) relay wouldn't pick up? As a practical matter, every single time that the 21 has tripped, the 59G has also tripped. Are the 21 and 59G more-or-less redundant to each other, or is the 21 truely providing some additional level of protection? Is the 21 more accurate, or maybe faster?

How do these things actually work? Are they really monitoring source system impedance as the description seems to imply? Or perhaps, the transmission line impedance? How do they do that?

 

[buzzp]

I guess how the relay responds is not really known. I think the old eletromechanicals may act as peebee suggested but don't think the new ones would respond in that matter unless set up that way. The other question is if it responds to one phase being low or if the average of the three phases is used to determine a trip.