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






RE: Undervoltage block in the underfrequency protection
RE: Undervoltage block in the underfrequency protection
Best Regards.
Slava.
P.S. What about load shedding systems? Some difference way?
RE: Undervoltage block in the underfrequency protection
In general, we use 60-70% of the nominal voltage.
Best regards,
Herivelto Bronzeado
RE: Undervoltage block in the underfrequency protection
RE: Undervoltage block in the underfrequency protection
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
RE: Undervoltage block in the underfrequency protection
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.
RE: Undervoltage block in the underfrequency protection
RE: Undervoltage block in the underfrequency protection
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
RE: Undervoltage block in the underfrequency protection
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
RE: Undervoltage block in the underfrequency protection
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If we learn from our mistakes I'm getting a great education!
RE: Undervoltage block in the underfrequency protection
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.
RE: Undervoltage block in the underfrequency protection
RE: Undervoltage block in the underfrequency protection
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
RE: Undervoltage block in the underfrequency protection
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
RE: Undervoltage block in the underfrequency protection
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!
RE: Undervoltage block in the underfrequency protection
I think it's standard solution of GE and Siemens GT with newer relay, like to : G60, 7UM6xx.
Regards.
Slava
RE: Undervoltage block in the underfrequency protection
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If we learn from our mistakes I'm getting a great education!
RE: Undervoltage block in the underfrequency protection
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