# Distance Relay



## mdbennett (Nov 2, 2007)

Does anyone know a good reference for distance relays?


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## Dark Knight (Nov 2, 2007)

mdbennett said:


> Does anyone know a good reference for distance relays?


Check for Blackburn's book. It is the best of the best. Another good book is the one by P.M. Anderson(not sure about the initials but it is the right last name.


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## mudpuppy (Nov 2, 2007)

Yes, Blackburn's book is widely regarded as the ultimate authority for relaying in general. This book should be more than adequate for the PE exam. If you're looking for something a little more advanced about distance relays (digital relays in particular), these two papers provide a wealth of information, but they're way beyond what you should expect on the exam: Distance Relay Element Design and Z = V/I Does not Make a Distance Relay


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## mdbennett (Nov 3, 2007)

mudpuppy said:


> Yes, Blackburn's book is widely regarded as the ultimate authority for relaying in general. This book should be more than adequate for the PE exam. If you're looking for something a little more advanced about distance relays (digital relays in particular), these two papers provide a wealth of information, but they're way beyond what you should expect on the exam: Distance Relay Element Design and Z = V/I Does not Make a Distance Relay




Thank you. Talk about getting a drink out of a fire hose. Can you distill down to a sentence or two what a distance relay does and how it does it?


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## Dark Knight (Nov 3, 2007)

mdbennett said:


> Thank you. Talk about getting a drink out of a fire hose. Can you distill down to a sentence or two what a distance relay does and how it does it?


Without going into details a distance relay applies the most simple concept of Electrical Engineering: Ohms Law.

Using values of current and the impedance(Z) of a transmission line the relay constantly monitors the voltage and in case the voltage collapses, the common aftermath of a fault, will send an order to the breaker protecting the line to trip.

Some people also call the distance relay Zone Relay. That is because you can protect the line using different settings and the relay can be set to protect Zone 1(usually 85% of the length of the line) Zone 2(110% as a typical value) or sometimes up to Zone 3. Of course, that depends on what kind of transmission line you have(Short or Long)

Oh boy. I am full of it and probably did not explain it well. Please feel free to to post again if that did not was good enough.


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## mudpuppy (Nov 3, 2007)

BringItOn said:


> Without going into details a distance relay applies the most simple concept of Electrical Engineering: Ohms Law.


Yep. A distance relay measures "apparent" impedance, Z = V/I, by measuring the voltage and current on one terminal of a line. As the voltage goes down and/or the current goes up, the apparent impedance decreases. If this apparent impedances goes too low, it can be assumed there is a fault on the line. For instance, say a 10 mile long line has an impedance of 10 ohms at 85 degrees. If the relay's measured apparent impedance drops to 8 ohms, then there must be a fault on the line (or a severe overload.) Assuming the line is homogenous (this is, the impedance per mile does not change over the length of the line) the location of that fault can be pinpointed--thus the name distance relay. In this example, the fault would be approximately 8 miles from the relay. Note that this is not exact because the fault may have impedance in it, and there could be differences in angles, but you get the idea.

Note that distance relays are mostly used for phase faults, but there are also ground distance relays that use the same concept except they measure zero-sequence voltage and current.



> Some people also call the distance relay Zone Relay.


I've always called them impedance relays because that seems more straightforward to me.


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## Wolverine (Nov 5, 2007)

Good explanations all. I'm a visual learner though and think a picture is worth a thousand words, so here's my contribution:

Distance Relays for Dummies: Hope it loads okay and adds to the discussion.


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## rcurras (Nov 5, 2007)

Wolverine said:


> Good explanations all. I'm a visual learner though and think a picture is worth a thousand words, so here's my contributionistance Relays for Dummies: Hope it loads okay and adds to the discussion.


Excellent…This is an excellent example of the huge importance of this forum…Thanks a lot Wolverine.

RC


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## BamaBino (Mar 3, 2011)

Wolverine said:


> Good explanations all. I'm a visual learner though and think a picture is worth a thousand words, so here's my contributionistance Relays for Dummies: Hope it loads okay and adds to the discussion.


Wolverine,

Could you provide the picture again?

Thanks


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