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?

 

[busbar]

A numerical relay with basic device-21 {mho-distance} capabilities is described at

http://www.selinc.com/datasheets/pg10.pdf

http://www.selinc.com/techpprs/6010.pdf

http://www.selinc.com/techpprs/6022.pdf

Also

http://www.geindustrial.com/products/applications/ger3966.pdf

http://www.geindustrial.com/pm/notes/artsci/art14.pdf

 

[scottf]

It comes from being able to sense where a fault occurs on a line, or the distance away that the fault is physically. Based on the line parameters, by knowing the impedance, you know the distance.

 

[buzzp]

A 21 device is an impedance device (I know it as a mho distance relay) and is used for zone protection. A 51 or similar device does not care where the fault is, whereas a 21 device can discriminate between a fault that can be cleared downstream vs. one that should be cleared locally. Example - a generator is supplying power to some switchyard, the 51 device would clear a fault anywhere on the line even though some device downstream (a breaker) could clear the fault without taking down the generator. The 21 device would know where the fault is and whether it can be cleared (out of reach of the 21) by a downstream breaker or if it needs to be cleared with the generator breaker (fault is within the plant, for example, before another downstream breaker).
I am not sure why your 59 device is tripping when the 21 does. Are you sure its not your 27 thats tripping? If it is your 59, then it could be (just a guess here) be set up with a too quick a trip time (1-4 cycles)so when the fault is cleared, the momentary rise in voltage is tripping the 59.
BTW, the distance comes into play because the distance matters when setting up the impedance trip level (distance is directly proportional to impedance).

 

[stevenal]

A distance relay uses monitored voltage and current to determine distance to a fault on a transmission line. Trip time is Dependant on distance. A close in fault has low voltage and high current at the relay (low impedance) and is generally set to trip instantly. More distant faults are delayed to coordinate with distant protection.

 

[peebee]

Thanks all. I have not yet gone thru busbars links, but I sure will.

Re "I am not sure why your 59[G] device is tripping when the 21 does. Are you sure its not your 27 thats tripping?" The 59G is also monitoring the utility side. The point of both relays is to isolate our generators from the utility system in the event of a utility fault so that we do not energize their fault and/or electrocute their electricians.

I'm still a little puzzled about why the 21's were required by the utility. The utility relay package trips us out regardless of whether we faulted or they faulted. Either way, they want us off their line, and understandably so. But given that, and given that the point of the 21 is to discriminate between who (us or utility) faulted, well, then why are they there? Maybe the only reason is that the utility was specifying the relays, but we (rather, my client) was paying for them?

Not that any of this is a big deal, or even much of a concern of mine. Can't be TOO much cost for one extra relay. And I've certainly had AHJ's and utilities ask us to provide much sillier things in the past.

 

[stevenal]

Fast responses. Three came in while I was composing mine.
They're looking for that delayed zone 2 protection(described as more distant in my post above) to back up their protection. A fault beyond the remote bus that fails to clear with their breaker will be cleared by your's.

 

[peebee]

Hmmm... But wouldn't 51 and 27 and 59G already do that? Or is this a belt-n-suspenders approach?

 

[stevenal]

The only one of the three that might reliably detect a distant fault would be the 51. The time curve of a 51 makes it difficult to predict operation speed. A distant fault will already be at the slow end of the curve. Some fault impedance will delay it even more. The distance relay is simpler to coordinate, especially when other distance relays are involved. Any fault within the zone will cause tripping to occur at a defined time.

Maybe system stability is an issue and backup clearing times are critical. Or maybe they simply always pair up another distance relay when given the choice.

 

[alehman]

Your generator will interfere to some degree with the utility's relays ability to see a distant (from them) fault. They want your generator off the line so their relaying can see the fault quickly.

A straight 51 may not be reliable if your generator doesn't produce high fault current for whatever reason. A 21 doesn't care if your generator is functioning correctly or not. It may not be possible to set it to cover the utility's entire line either, however.

Another possibility - is the 21 is looking into your generator instead of out on the line? It can be used that way as backup generator protection.

 

[powercam]

a distance relay is also called an impedance relay.yeh you are right about the word distance. in 1968 ( when engineers first started to introduce these relays they were purely used to detect faults on lines and somehow the relay manages to measure the disance to the fault. by doing this it constantly monitors the voltage and current. since ohms law states that v=I*Z any change in V or I will will affect the impedance of the line while the Z in the relay is fixed. this protection was later started to be used in all types of protection schemes and by having a good communication sheme can be made unit protection. there are many different characteristics ie, quadrilateral. trapizonal, mho. micro-mho etc. depending on what type of reach you require you as the engineer will select the best suited. i hope this helps with your question. chow.

 

[busbar]

The ‘59G’ relay sounds like it may be misapplied, particularly if it provides tripping of the same equipment as the 21. Its pickup is normally <0.1 p-u line voltage, and cannot differentiate ground- fault direction—much the opposite of a distance [or directional-overcurrent] relay, and is usually associated with grounded-wye—broken-delta potential transformers.

 

[buzzp]

We use 21's to protect from the generator out to the switch yard. Otherwise, the only choice is a 51/50 device and these can't tell if the fault is after the switchyard or before so you may end up taking a generator offline unnecessarily. Most of our plants feed multiple lines and some use ring busses so keeping the generator online is very important.

Still don't think your 59 should be tripping at the same time. Perhaps it is tripping after the 21 takes something offline. Not sure but something just does not sound right. Again, are you sure it is not the 27 tripping (is the 27 and 59 in the same relay?).

 

[peebee]

They are all discrete relays.

I cannot positively confirm that it was indeed the 59G that tripped, that's all 2nd-hand reporting, but everyone who I've spoken too seems to say that yes, it's the 59G and 21 that trip every time.

Buzzp -- why are you surprised that they trip together? Why wouldn't a ground fault trip both relays?

 

[mc5w]

One very good reason why distance relays are popular is that they can sense where a fault is without having a communication line between relays. This allows the distance relay to act quickly for faults that it is supposed to disconnect.

Distance relays also help locate a fault so that you can find a bad insulator or a rogue tree that is growing like HE double toothpicks. Sometimes clearance to a distribution line crossing underneath is too small because of old age sagging or the distribution line was built later and the transmission line was not built high enough.

With directional power relays and differential current relays you have to have a communication line and logic that allows the relays to decide whether a fault is in between them resulting in a trip or beyond their zone resulting in a decision to not trip. If the distance is very short such as at a power plant bus or a few hundred meters or thousand yards from a 345 to 138 KV transformer and them to a 138 to 69 KV transformer then distance relays do not work.

 

[buzzp]

A 59 is an over voltage relay. With a ground fault, I would expect the voltage to drop, not rise. However, if the load was lost (presumably so) then a corresponding rise in voltage might be seen after the load has tripped (if near a generation source and you are). This should not be a cause for an OV trip and you might consider changing the trip time on the 59 relay.

 

[peebee]

The "overvoltage" in 59G is almost a misnomer.

The PT's to a 59G are wired with grounded Y primary and an open delta secondary (that's grounded Y on the PT's -- the distribution system itself is an ungrounded system, or more accurately stated, high-impedance grounded through the PT's and cable capacitance to ground).

Normally, all primary phase voltages are equal. With the open delta, that means that the vector sum around the open delta should be zero.

However, if one of the phases faults to ground, then the voltage on one phase of the primary Y side will drop to some lower value or potentially to zero. When looking at the secondary side, the reduction in secondary voltage will distort the vector sum such that the voltage around the open delta will no longer be zero. The relay sees an overvoltage (some voltage greater than zero). Hence, the term "overvoltage".

At least I think that's where the term overvoltage is coming from. Alternately, it might be that if one phase grounded, the other two phases would have a higher potential to ground than what they normally would.

 

[peebee]

See

http://basler.com/downloads/1714_990F.pdf

(page 1-1, under "UNGROUNDED SYSTEMS") for a better description of the 59G than my description above (Basler calls it a 59N).

Also, it appears that the correct term is "broken delta", not "open delta".

 

[Bung]

I would hazard agues that this "distance" relay is not actually being used as such. It is more as a directional voltage dependent overcurrent (50) relay in this application. Logically, that is all that an impedance relay is. (Impedance -> length if you know the ohms / km of a protected line). Z=V/I, so the higher the voltage the higher the pickup setting of the "50".


Bung
Life is non-linear...

 

[Bung]

PS to above - a "21" also makes a really good overcurrent relay with load encroachment blocking. I would say that is one reason why they are popular on MV distribution feeders in Germany.

I'm willing to bet that the load encraoachment blocking feature in the SEL351A is based on the SEL311 distance algorithm, especially as the two relays only differ in one having "21" elements and the other not...

Bung
Life is non-linear...