Purpose of ANSI 21 Distance Relay?
Purpose of ANSI 21 Distance Relay?
(OP)
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?
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?






RE: Purpose of ANSI 21 Distance Relay?
A numerical relay with basic device-21 {mho-distance} capabilities is described at
www.selinc.com/datasheets/pg10.pdf
www.selinc.com/techpprs/6010.pdf
www.selinc.com/techpprs/6022.pdf
Also www.geindustrial.com/products/applications/ger3966...
www.geindustrial.com/pm/notes/artsci/art14.pdf
RE: Purpose of ANSI 21 Distance Relay?
RE: Purpose of ANSI 21 Distance Relay?
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).
RE: Purpose of ANSI 21 Distance Relay?
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
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?).
RE: Purpose of ANSI 21 Distance Relay?
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?
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
RE: Purpose of ANSI 21 Distance Relay?
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.
RE: Purpose of ANSI 21 Distance Relay?
Also, it appears that the correct term is "broken delta", not "open delta".
RE: Purpose of ANSI 21 Distance Relay?
Bung
Life is non-linear...
RE: Purpose of ANSI 21 Distance Relay?
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...
RE: Purpose of ANSI 21 Distance Relay?