Undervoltage block in the underfrequency protection 9

[slavag]

Dear All.
What is your practic of undervoltage setting/limit for the underfrequency protection?
Thank you in advance.
Best Regards.
Slava

 

[DanDel]

This is typically used for an application where you want the generator voltage to be above a certain value before the underfrequency trip becomes active. This way, when a generator is coming up to speed, the UF won't trip.

 

[slavag]

Thank you DanDel.
Best Regards.
Slava.
P.S. What about load shedding systems? Some difference way?

 

[Bronzeado]

Hi Slava,

In general, we use 60-70% of the nominal voltage.

Best regards,

Herivelto Bronzeado

 

[REM76]

Usually 30%

 

[slavag]

Hello.
Herivelto, REM76.
Thanks a lot for your information.
Both of settings are used in lot of projects.
I try understand: When and why?
Are you have some documents or link for more information?
Best Regards.
Slava

 

[SteveFehr]

Sudden loading of an unloaded generator will cause a momentary undervoltage/underfrequency as the governor adjusts the throttle. You need to ensure your settings aren't such that the underfrequency relay in the breaker trips open. By the same token, you don't want to set it too low, or it will fail to trip for an actual underfrequency event. Depending on the circumstances, additional relays may be required.

I believe +/-5% frequency tolerance is standard for most facilities, but it really depends on what your load is, and how much tolerance it has.

 

[GTstartup]

We use 40%. It prevents nuisance trips during start up and shut down due to generator residual magnetism and resulting low voltage during start. If you didn't have the low voltage cut off the relay could sense low frequency and trip. This is especially imoprtant when starting gas turbines because often they are started by motoring the generator

 

[slavag]

Hi.
Thank you SteveFehr and GTstartup for the valible information.
It's exactly reason of my Q.
range from 0.3 up to 0.6/0.7 Unom.
O.K., I see some logic in the setting, continue learn.
BTW, GTstartup, from time to time for avoid unwanted trip,
we used CB position status for block underfrequency protection.
Thanks again to all.
Best Regards.
Slava

 

[GTstartup]

Thanks Slav.

Blocking underfrequency protection by using the generator won't normally work for a GT. There is still a concern about protecting the GT against surge at off design frequency even if not connected to the grid. In this case the GT must be tripped regardless of the position of the breaker, for instance in island mode or just at full speed no load

 

[ScottyUK]

Tripping the GT is a poor response to a falling grid frequency. Load dumping at the gen breaker will put much less thermal shock onto the machine than a trip, thus preserving hot parts life. After a load dump the machine should remain at sync idle ready to resume load, or go into a controlled shutdown.


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If we learn from our mistakes I'm getting a great education!

 

[JJHorak]

I think the undervoltage block at 75% voltage would be more appropriate. Some of the logic for higher settings:

a) The largest use of frequency elements is for load shedding. If frequency deviates from nominal, you have an overloaded system. When a transmission line fault causes a substation to be isolated (blackout), and there is a high induction motor content in the substation load, the motors backfeed the substation with relatively low voltage and rapidly decaying frequency. You need to keep 81 elements used for load shed from tripping, so that when the transmission line is restored, the operators do not also need to reset load shed relays. The motor backfeed can be notable for a couple cycles, and the load shed relays are typically very fast, so you need to set the undervoltage setting as close to nominal as you feel comfortable with.

b) In a utility environment, voltage below 75% is not a normal operating point and is indicative of a fault. You could likely use as much as the low 80's. In an industrial environment, maybe larger dips are more normal, though.

c) Faults tend to cause changes to the zero cross point that most relays use for frequency measurements. The fault tends to tell the relay for a cycle or two that the frequency has changed. If your 81 had time delays of 3 cycles or less, you need the undervoltage block to help stop a relay operation during a fault.

 

[JJHorak]

I mispoke on item a). The 81 relays reset automatically, but they trip a breaker that is not reclosed automatically. Further, in systems where the SCADA system does not give clear indication that it was the 81 that tripped they breaker, the operators have to guess on what occurred before they can justify reclosing the breaker. Not a good situation.

 

[GTstartup]

Scotty,

I wasn't proposing tripping the GT in response to failing grid frequency. But if you disconnect from the grid and the GT still has low frequency for some other reason, this is wnen the GT should be tripped. Therefore using generator breaker position to disable the underfrequency relay will not work.

The sequence should be:

Stage 1 - Alarm
Stage 2 - Trip generator breaker
Stage 3 - Trip GT

All set at prgressively lower values

 

[slavag]

Hello All.
Thanks again to you all.
Very intresting topic for me and hope to others.
GTstartup, I checked now few application.
For GT never used block of underfrequency by CB position.
For the steam turbine few times ( and reverse power too)used, requested by few customers.
Intresting also block of underfrequency function in the generator protection by 60 function ( fuse blown) in additional to undervoltage block.
For DG; some mix of requests, no rules.
Timesabroom; good points.
Best Regards.
Slava
Have a nice weekend, guys

 

[ScottyUK]

Gtstartup,

I knew you knew that.

Slava,

I've never seen GT protection for under-speed through a generator relay: usually it is performed within the turbine's control system and / or via an external supervisory underspeed / overspeed protection unit. It's an interesting idea though.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi Scotty.
I think it's standard solution of GE and Siemens GT with newer relay, like to : G60, 7UM6xx.
Regards.
Slava

 

[ScottyUK]

Interesting - thanks Slava.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
Scotty.
I asked today person are responsible for the generator protection setting in our local utilities about underfrequency protection.
Answer:
Installed in all types of GT ( GE, Siemens), in part of small GT used only one stage with trip signal to LOR.
setting to turbine trip 47Hz 1sec.
Best Regards.
Slava