Island Mode Operation

Hello,

I have a question regarding our steam turbine generator operating in island mode (Grid is disconnected) and the excitation system is controlling the terminal voltage (6.6 kV).

When a sudden load is added, which of the following will be affected first?
  1. The generator frequency (speed) then the turbine will readjust the speed again automatically.
Or
  1. The generator terminal voltage then the excitation system will readjust the voltage again automatically.

Thanks :)
 
Atteya,

We would have to know a great deal more about the steam turbine control system, number of controlled extractions, and type of actuators, AND about the excitation system (brushless; brushed; static; etc.). We would also need to know the nature of the load(s)--inductive, resistive, capacitive--and how the load ”settles out” (how much inrush current is drawn by the load; does it take 3 seconds or 7 seconds of inrush current before stabilizing), as well as the percentage of rated steam turbine load is being added at once. Also, if the percentage of added losd is high relative to the steam turbine rated load, can the steam supply system keep up with the sudden demand?

The type of load and the nature of the load pretty important, as is the type of excitation.

A SWAG (Scientific Wild-Arsed Guess)--the speed (frequency) is going to drop first, and if the excitation is properly designed for the expected load increase the voltage will drop a little and recover relatively quickly. This presumes the percentage of load increase versus the steam turbine nameplate rating is about 20% or less, and the steam supply can increase quick enough to help hold speed and load. A properly designed control system (for the expected load changes) should be able to control both frequency and voltage without large changes or oscillations.

If you're experiencing something different, you should describe it here; maybe someone can offer help or suggestions.

Lastly, the above presumes the steam turbine-generstor is the only unit supplying the load(s) AND the steam turbine control system is operating in Isochronous Speed Control mode (NOT Isochronous Lo) AND there is NO external load control trying to maintain frequency by sending signals to the steam turbine control system--and possibly the excitation system. Any of these things can change anything and everything.
 
Atteya,

We would have to know a great deal more about the steam turbine control system, number of controlled extractions (No Extractions), and type of actuators (Hydraulic), AND about the excitation system (brushless; brushed; static; etc.) (Brushless - Siemens RG3 S7). We would also need to know the nature of the load(s)--inductive, resistive, capacitive--and how the load ”settles out” (how much inrush current is drawn by the load; does it take 3 seconds or 7 seconds of inrush current before stabilizing) (Load is a motor 770 kW/6.6 kV/79 A/4.6s startup time/6.5 starting current ratio), as well as the percentage of rated steam turbine load is being added at once. Also, if the percentage of added losd is high relative to the steam turbine rated load, can the steam supply system keep up with the sudden demand? The generator rated output is 13750 kVA and the steam turbine steam supply has no issues.

The type of load and the nature of the load pretty important, as is the type of excitation.

A SWAG (Scientific Wild-Arsed Guess)--the speed (frequency) is going to drop first, and if the excitation is properly designed for the expected load increase the voltage will drop a little and recover relatively quickly. This presumes the percentage of load increase versus the steam turbine nameplate rating is about 20% or less, and the steam supply can increase quick enough to help hold speed and load. A properly designed control system (for the expected load changes) should be able to control both frequency and voltage without large changes or oscillations.

If you're experiencing something different, you should describe it here; maybe someone can offer help or suggestions.

The problem that occurred in our system is as follows:

  1. In normal operation, the steam generator (6.6 kV) is working in parallel with external grid to feed the plant load in a PF (power factor) control mode.
  2. A short circuit took place in the external grid, so the External grid CB tripped in 110 msec
  3. The generator became in island mode and the excitation system changed from PF mode to voltage regulation mode, with 90% setpoint (5.95 kV)
  1. After 64 seconds of stable operation, within 0.5 seconds we found that the voltage suddenly decreased to 52% Un causing Undervoltage stage 2 tripping (Setting: 79% Un-3 sec, 64.5%Un-0.5 sec). The turbine speed did not fluctuate.
  2. On the excitation system, we didn't find any alarms or warnings, i.e. no excitation limiters or protection have been activated.
  3. The generator has started again and synchronized without any problems. So, the root cause of the problem is still unknown.


Lastly, the above presumes the steam turbine-generstor is the only unit supplying the load(s) (Yes) AND the steam turbine control system is operating in Isochronous Speed Control mode (NOT Isochronous Lo) (Yes) AND there is NO external load control trying to maintain frequency by sending signals to the steam turbine control system (Yes) --and possibly the excitation system. Any of these things can change anything and everything.
Thanks for your quick response. I really appreciate it.
I added my comments in red above.
 
Thanks for your quick response. I really appreciate it.
I added my comments in red above.
Thanks for your quick response. I really appreciate it.
I added my comments in red above.
Good day all,

Atteya,

According to your description of this event:

-Did you check AVR undervoltage /underexcitation limiters/thresholds??
-Are they in accordance (protection selectivity) with IED protection relay??

Is there a opportunity for sharing trends on the event that you described??

Also , is that (Load is a motor 770 kW/6.6 kV/79 A/4.6s startup time/6.5 starting current ratio), is the only one load...


Without be able to check the trends of this event, I cannot add more comments.

Looking to hear back from you soon,
Controls Guy25.
 
Good day all,

Atteya,

According to your description of this event:

-Did you check AVR undervoltage /underexcitation limiters/thresholds??
-Are they in accordance (protection selectivity) with IED protection relay??

Is there a opportunity for sharing trends on the event that you described??

Also , is that (Load is a motor 770 kW/6.6 kV/79 A/4.6s startup time/6.5 starting current ratio), is the only one load...


Without be able to check the trends of this event, I cannot add more comments.

Looking to hear back from you soon,
Controls Guy25.
Controls Guy25,

Thanks for your response. I've attached the AVR and the protection relay settings for your reference.

The generator is feeding the whole plant with a lot of different loads. Actually, at the moment of the trip, we could not identify any load disturbance from major loads. However, since we do not know the reason behind the trip, I'm assuming that one large load was starting without being detected by the DCS system (Very low probability). So, I chose one of the large loads in our plant that can impact the network and I'm trying to figure out its impact on the generator on island mode.

Thanks for your support. If required, I'll try to get the trends and upload it.
 

Attachments

Controls Guy25,

Thanks for your response. I've attached the AVR and the protection relay settings for your reference.

The generator is feeding the whole plant with a lot of different loads. Actually, at the moment of the trip, we could not identify any load disturbance from major loads. However, since we do not know the reason behind the trip, I'm assuming that one large load was starting without being detected by the DCS system (Very low probability). So, I chose one of the large loads in our plant that can impact the network and I'm trying to figure out its impact on the generator on island mode.

Thanks for your support. If required, I'll try to get the trends and upload it.
Atteya,

Thanks for your reply and clarifications.

After first reviewing the FILE of IED protection relay, looks like some protection functions are disabled such as 27/59 which is undervolatge/overload protections ?? Do you confirm ? Also there is no Overfluxing protection shown ... can you clarify?

For the Avr file it is looking correct expecting that there is no mention of Overfluxing limiters/thresholds?? Thats ok I just found it, it is titled "overexcitation" which is same But Min Frequency is adjusted to 45HZ do you know the reason why ?

Setting of Undervoltage limiters have been adjusted on site ( during commissioning ) did it take account of grid failure??

It would be great if we can retrack the event from turbine controls system trends viewer , as There is some shadows behind the issue that you are describing.

That could be mal function like :
- some Transfer bus/BREAKER CLOSURE
-overcurrent protection misoperation
- AVR non correct BEHAVIOR
- MVAR load fluctuations/swinging ( see generator capability curve for OEL UEL limits ...)
-Underfequency interlocking trip relays operation.....w th load shedding scheme not ok.....
- And many others cause can be possible .

Did you ever been face to an "external grid failure" before and what was the scenario??

Again i would ask for some trends to go further on investigation....

Hope this can help,
Controls Guy25.
 
"...Actually, at the moment of the trip, we could not identify any load disturbance from major loads. However, since we do not know the reason behind the trip, I'm assuming that one large load was starting without being detected by the DCS system (Very low probability). So, I chose one of the large loads in our plant that can impact the network and I'm trying to figure out its impact on the generator on island mode."

Yah gotta love it.

Not.

(Emphasis was mine.)

Actually, this is most likely my last post to Control.com. Between the unsolicited PMs/DMs demanding help without even asking a single question and these fake events (plural!), my patience has run out after more than a decade.

All things come to an end. This is me, cutting my losses, turning and exiting. I just ordered a Kreg Pocket Hole Jig and am looking forward to building some nice things for my entertainment and hobby pleasure. I enjoyed helping people with real problems, and not receiving unsolicited and demanding PMs/DMs without a choice in the matter. Now, it's made-up problems and demanding, unsolicited PMs/DMs.

Adios, amigos! It was fun while it lasted--but the fun has ended. Abruptly.

Thanks for the memories. Be safe--and stay healthy, everyone!!!

(Lumber in the USA has gone up 100% per board foot since the pandemic started. Guess I kind of picked the wrong time for woodworking as a hobby--but I gotta give this one up (Control.com), and nothing's free in this world, eh?)
 
Dear CSA,

Apologies for any misunderstanding. I did not mean to upset you.

My original question was asking about whether adding a load to an STG in island mode will affect the speed (freq) or the voltage. This information is helpful to me to understand/troubleshoot a blackout that occurred in our plant. We suddenly had a generator undervoltage without a prior speed disturbance so I was trying to understand if it is because of a load started that was not picked by the DCS or is it an AVR misfuction or other reason.

Anyway, sorry again and thanks for your help so far. I really appreciate it.
 
Atteya,

You're in good hands with ControlsGuy25. He's very thorough.

Yours was just the straw that broke the camel's back. I've accomplished more than I thought possible at times here on Control.com.

Best of luck with your investigation.
 
Atteya,

You're in good hands with ControlsGuy25. He's very thorough.

Yours was just the straw that broke the camel's back. I've accomplished more than I thought possible at times here on Control.com.

Best of luck with your investigation.
Good day all,

CSA,
Thank you for your above comments, I appreciate it!

I wish you all the best !

Atteya,
We will surely get that issue clarified and even solved soonly!

ControlsGuy25.
 
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