Generation transfer-tripping

[stevenal]

A radial line feeds an industrial site that has generation partially offsetting their load. Synchronizing equipment is at the generator breaker only. It is decided the sourcing substation should send a transfer-trip signal to the generator breaker when conditions dictate. Would you source this signal from a protective trip only, or include manual tripping as well?

Argument for including manual tripping: Any open might cause an island and possible out sych situation when the breaker is eventually manually closed.

Arguments for not including manual tripping: A sustained island is unlikely since load exceeds generation. The operator, knowing that generation exists down line, will be sure to contact the industrial site prior to closing. Fewer transfer-trip sources decreases the chance of an inadvertent generation trip.

Have I missed any arguments? What do the members here do? Thanks.

 

[Overvoltage]

If their island is not going to be able to / expect to run on their own generation alone, then I would use the status of the breaker to trigger DTT. Anytime it is open, you should be sending a trip signal to the generator. Obviously, you can manually disable that if needed, but that would take an active choice by a technician/operator to do so.

I wouldn't rely on the operator making the right decision to avoid equipment damage. I don't like to put that kind of responsibility to remember unique characteristics for specific customers on them. They're busy enough already. And if you use the breaker status to trigger DTT, you're still only using one source. The only reason you wouldn't want it to send DTT on a trip would be during testing, and DTT could be disabled as part of the test procedure for that.

But that's just what I would do at first glance with no other information than what you've provided. Obviously I take no responsibility or liability for your design/project, etc.

 

[bacon4life]

I would not rely on generation magically tripping. A few arguments:
1) Loads change over time. I doubt that folks are going to regularly check on the ratio load versus gen as they determine how to operate the plant. This applies both to near-term decisions made by operations staff and to longer term decisions made by managers/accountants.
2) Transients can cause loads to trip. What happens if just the right portion of the industrial load trips to leave load matching generation? As a brand new engineer, I had been assured by experienced folks that a particular generation plant would always trip if islanded, so DTT was not needed. However, I could never get an answer as to exactly what protective element guaranteed the trip. A few years later a relay misoperation islanded the generator with several substations, and the plant stayed islanded for many minutes until an operator manually tripped the plant.
3) Inadvertent generator tripping has been a contributing factor to many blackouts. Many of the recent changes to generator protection standards have prioritized the need to keep generators online during unusual grid events. I assume generator ride-through requirements will continue becoming more comprehensive in the future. Even generators that historically easily tripped offline once the protection settings are adjusted to comply with the latest standards.
4) Even if the generator protection issues a trip relatively quickly after forming an island, relying on under frequency relaying will expose the generator to transient conditions. Steam turbines have a cumulative lifetime limit to off-nominal frequency, so repeated islanding could result in excessive turbine damage.
5)Depending on your industrial process, it may be better to have an instantaneous shutdown rather than having individual components shut down over a period of seconds or minutes during an islanding event.

 

[stevenal]

Just to be clear there are two entities here. The plant is responsible for keeping the plant load/generation mix acceptable. The loss of the small generator will not lead to any blackout, since the serving utility can serve the entire load. TT is intended to allow automatic reclosing in the event of a temporary fault, without the need for synchronizing at the utility side. Breaker status is not part of the scheme, only the trip signal. The signal from the manual switch was the surprise.

 

[jghrist]

It is necessary for generation to be disconnected from the distribution line any time that the utility source is open. This prevents possible voltage problems and the possibility of lines being energized while utility maintenance is being done. It is usually possible to have anti-islanding protection perform the generator tripping locally to avoid the cost of communications for transfer trip. Transfer tripping is more reliable, and if the communications is already in place, I believe that it should be used to trip the generation regardless of the reason for opening the substation breaker.

 

[Amit Bajpayee]

@Jghrist it very well explained.

https://agnisolar.com/