Generation on radial transmission lines 1

[Mbrooke]

Is there anything that I need to consider in terms of protective relaying when protecting radial lines with very large amounts of generation on the other side, ie in this case a 1,500MW+ plant on a 30 mile radial 345kv? I'm guessing I could have step distance under-reach for faults out on the line, but remain unsure.

 

[crshears]

Depending on circuit capacity: generation run-back to respect thermal limits? Or, during planned circuit outages, if the combined path of the remaining circuits is skinny enough, generation rejection for instability?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

But nothing where the typical MHO elements under reach? And nothing special in the realm of out-of-step blocking?

 

[crshears]

I don't have the expertise to speak to those concerns, sorry.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

No sweat. But I did find your prior posts helpful, so nothing wasted

 

[cranky108]

Be careful with reclosing.

 

[Mbrooke]

I'm leaning toward not reclosing since its radial, but if I have two parellel lines, I'm all ears.

 

[crshears]

My utility's historic practice has been to not provide any reclosing on radial generation circuits, although depending on switchyard configuration at the generation end, that could be changing, particularly where the units and the circuits don't share breakers...but where they do, the N minus one issues that could arise, even for such short duration as would be required to manually return the circuits in question to service, might prove problematic for our IESO / balancing authority due to system security considerations.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

I could see reclosing to re-power station service transformers once the generator trips during the open period.

 

[crshears]

Might or might not be necessary, depending on station topology / connectivity; a 1.5 GW plant's units could reasonably expected to have a unit's station service fed from the isolated phase bus with the unit in service, these [often, in my experience] being provided with auto-transfer to what we called reserve station service supply upon unit trip. As such schemes can have profound effects on unit survivability and safety, the end [as-constructed] disposition of these elements often hinges upon the type of generation under consideration, i.e., whether fossil or nuclear.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

This is fossil, but what do you know about unit surviavability? My understanding is that once the line opens and no outlet exists for the generator's power, it trips immediately and station service comes from outside.

 

[crshears]

Various scenarios come to mind...unit trips and line trips / auto-reclosures are generally very different animals...

Picture a breaker-and-a-half type switchyard in use for a three unit 1500 MW fossil station and three export circuits, with one 500 MW unit and one export circuit per diameter. Reserve station service transformers are connected to the "upper" and "lower" busses. A 1.5 breaker scheme is also used at the remote end of the export circuits.

Say station is at full production and one export circuit sustains an automatic trip with reclosure, due to, say, a lightning strike; provided the remaining export paths can absorb the full station output in the short term without developing excessive load angle and therefore instability, there is no reason to expect them not to survive. If the generating units are equipped with quick-acting AVRs or TSECs [ transient stability excitation controllers ] there is no reason not to expect them to survive the event as well. Based on the normal time delays applied to the typical reclosure schemes I'm familiar with, the circuit is placed on potential via its under voltage plus time breaker at the remote terminal, the companion remote breaker also recloses under synchro-check supervision, then the local breakers also reclose under synchro-check supervision. Mint!

Since circuit restoration has occurred rapidly enough, no generation runback occurs, as the time delay setting in the run-back scheme has not elapsed before everything is back to normal, therefore no thermal limits have been encroached...

Planned outages to one circuit would alter this scenario considerably...and unit trips are another story entirely.

Hope I'm not being too rudimentary...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

No, just fine, and well explained Run-back would be reducing the generator's output?

 

[crshears]

That's correct; since the highest thermal limits are [again, in my experience] 5- or 15-minute LTRs [ limited time ratings ], and circuit reclosure schemes commonly complete their full operating process/event evolution within 30 seconds or less, a one minute delay before generator governor run-back begins is generally considered acceptable. The unloading rate is set to some compromise ramp that will meet both the thermal limit criteria and unit stability; only where absolutely necessary are pulverizer or burner trips employed to hasten the runback rate, as these greatly increase the risk of the entire boiler tripping off.

Depending on if the 1500 MW of generation is on an isolated grid or even if it's part of an interconnection, the recognized effects of large unit trips may be such that they are the MSSC [ most severe single contingency ] for that grid, and load/generation rejection schemes may be required.

At the risk of being obvious, nuclear units commonly have much more stringent station service power supply requirements than do fossil units..."cross-pollinating" reserve station service supplies and unit outputs is also practiced, meaning that unit outputs and their reserve station service supplies are [where possible] not tied to a common circuit breaker, since if said breaker goes into breaker fail both the unit and its reserve supply would go down at the same time.

 

[Mbrooke]

Makes sense and much appreciated.

 

[crshears]

My pleasure.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

...anybody have input for the question MBrooke posed in the OP?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

Hey MBrooke, here's another real-life scenario I used to encounter...

In the eight-unit 300 MW per unit thermal plant where I worked many years ago, the 230 kV switchyard used a ring bus configuration that only ran closed when there were less than four units on line; any greater number than that necessitated ring bus splits to respect fault current infeed limits and breaker interrupting capability.

The actual layout looked like this, with Xs representing breakers:

- X - G1 - X - Line 1 - X - G2 - X - G3 - X - Line 2 - X - G4 - X - G5 < truncated > - X - Line 4 - X - G8 -
| |
| |
------------------------------------------------------------------------------------------------------------

As a consequence it sometimes happened that a unit [or two] might be feeding its/their output through a single circuit breaker, such that for breaker operations associated with a trip of that circuit a generating unit would lose an export path...but since much of the unit's station service load was supplied from its own isolated phase bus, and the unit was external to the actual faulted zone, the unit would remain running but carrying only some station service load. In such situations the unit governors would limit the turbine speed rise to < 1% . . . but the unit operators would be out of their chairs toot sweet, quickly reducing the boiler's firing rate and making every effort to have the unit survive the event.

If the line contingency was due to lightning, that circuit was usually switched back into service manually within five minutes; if not, the IESO would lose little time making changes to the bus split configuration to provide a fresh export path for the surviving units. The upshot was that if the unit survived it was generally re-synchronized fairly rapidly and loaded back up...which was important, as failure to promptly re-load the unit unit could and usually would lead to steam turbine differential expansion problems.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

@Crshears: Thanks and big Thanks

So basically with station service load placed on the generator it can continue spinning even after load is dropped, and then that generator can be re-synchronized once an export path is found?

 

[Mbrooke]

And I tried to give your new reply another star, but the thread just took it from your first one. :/