transitory stability
transitory stability
(OP)
Hi,
I am performing a simple work to evaluate the transitory stability of the system shown attached when a three-phase fault happens in the substation S. Osorio. Due to this fault three-lines shown in the picture have been cut off from the system. Now the system lost its synchronism, I need to find a way to make it come back to synchronism again. I was trying to cut off the generation that is attached to the S. Osorio bus, but according to the simulation I'm doing that is not the solution of the problem because the machine's angle grows forever to different directions. I would like to know if you have some tip on what to do to keep this system operating without the 3 line transmissions.
Thanks.
I am performing a simple work to evaluate the transitory stability of the system shown attached when a three-phase fault happens in the substation S. Osorio. Due to this fault three-lines shown in the picture have been cut off from the system. Now the system lost its synchronism, I need to find a way to make it come back to synchronism again. I was trying to cut off the generation that is attached to the S. Osorio bus, but according to the simulation I'm doing that is not the solution of the problem because the machine's angle grows forever to different directions. I would like to know if you have some tip on what to do to keep this system operating without the 3 line transmissions.
Thanks.






RE: transitory stability
But from your question I gather the problem is that when the breakers trip on those 3 lines the resulting phase angle then becomes to great to reclose? Or the generator looses rotor angle stability? Is this what you mean by loss of syncronism? When you say 3 phase fault in the S Osorio, this would be a bus fault, correct? If so a bus fault (assuming a straight bus) would clear more than just those 3 lines. Thus I am assuming the issue is the loss of the two lines between S Osioro and C Mourao and the line between S Osioro and Areia?
Apologies if I am misunderstanding.
RE: transitory stability
RE: transitory stability
I was thinking of that generation shedding due to the loss of that 3 lines. That'll be an over generation in the station Osorio and I need to reduce that generation or find a way to drain out that energy. I was thinking of putting a bank of resistors there. But I don't know if it is something usual to do.
I'm sending a picture of the generation rotor angle for different machines of the benchmark when I put a bank of resistors. Its capacity was chosen to be the line fluxes sum before the breakers trip.
RE: transitory stability
Another possibility is to just live with the risk. In a well maintained substation bus faults tend to be fairly rare; and there are many, many cases around the world (even the US) where loosing load and/or generation is considered acceptable on the account its a low probability event. If you can continue supplying critical loads via the remaining bus section it becomes even more tolerable.
And while I am not an expert in RAS (remedial action schemes) and GRB (generator run back), someone else on here who is might be able to determine if thats a valid option.
But I am curious- In your case once the 3 lines and generator trip, what is the per unit voltage on the remaining bus? Does the (faulted) bus section in question also supply load?
RE: transitory stability
RE: transitory stability
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: transitory stability
RE: transitory stability
RE: transitory stability
RE: transitory stability
First: As QBplanner pointed out, if the generators are Hydro Units I can reduce the generation in substation S.Osorio until there is a match between load and generation.
Second: This is more complex. In this scenario the generator unit may not be dispatchable, so I need to cut it off from the system when I lose those three lines. The voltage from the remaining unfaulted buses drops to 0.7 p.u (P. Branco, Xanxere, C. Mourao, Maringa and Apucar.) So I need to cut off those loads too, I guess.
The graph attached shows the voltage at different substations when the generator is unplugged from the system when the fault is cleared 150 ms after the fault. Also the loads are unplugged!
I would like to know whether those solutions are doable.
RE: transitory stability
RE: transitory stability
I did not see the 0.7p.u. voltage profiles in the drawing. The voltage profile seems too high at two buses all beyond 1.1pu.
Hydro units is easy to shed within cycles without causing damages.
RE: transitory stability
RE: transitory stability
RE: transitory stability
Under voltage load shedding on none critical distribution/sub-transmission feeders to bring the voltage back up to stable limits before voltage collapse can take place. Most SEL "utility" class relays have under voltage trip options.
Just be careful how you set it though, to much load being dropped through out the entire system for major regional disturbances can back fire.
What do you mean by the loads being "unplugged"?
RE: transitory stability
I know I am nit picking, and technically you are right by which way you look at it, but I politely beg to differ. Utilties frequently invest into preventing a system from going into N-3 and over. Typical example I can think would be separating a triple circuit tower into 3 single circuits, eliminating circuit cross overs, upgrading straight buss substations to breaker-and-a-half, replicating station relaying, and the like. Yes its often not cost practical for most utilities to design a system to lose 3 elements without dropping any load, but in this case a bus fault would count as a single individual element contingency.
RE: transitory stability
No problem, even rudely beg to differ is fine for me. What I imply is for an "existing system", if there is a bus fault causing three elements out, typical utility way nowadays is to save money by either trip generation or load other than re-configure the system like what you proposed. But for a new system to be designed, your proposal flies.
There is no exception here in North America, I agree with you if one fault tripping out three elements, then in the old NERC TPL standard, it is treated a multiple contingency and allow load shedding. However, in the new NERC TPL standard001-04, NERC keeps silent on the number of elements out but more focus on single contingency or multiple contingencies. The scenario you described belongs to P2 -bus section fault taking out three elements, if the bus section is EHV system then non-consequential load loss is not allowed and utility has to spend money to fix the problem such as what you proposed.
Cheers!
RE: transitory stability
RE: transitory stability
you said about too many load drops can back fire the whole system, I think for this N-3 scenario there's not much to be done tbough. The two unfaulted substations (P. Branco and Xanxere) are fed by the Generator tied to S.Sorio bus, so if I don't drop those loads the voltage level will drop too, the same happens to substation C. Mourao, the voltage drops to 0.7 pu due to generator disconnection. I know there are other options to restore the voltage level such as a bank of capacitors there, but they do not exist in the substation when fault happens, so I cannot account with them, maybe the substation does not have physical space to shelter those bank of capacitors.
RE: transitory stability
Honestly, now that I think about it its more if my bias speaking then all of reality. You are correct. Around here the tendency is to reconfigure existing substations rather than shed load (even if just HV) when a bus fault of breaker failure results in a certain amount of load or generation being lost. It is not so much a NERC standard but I guess one from the local ISO.
So I do hold guilt in speaking as though local standards apply when they do not everywhere else.
Well- if the entire system (hundreds of substations) has load shedding implemented through out. For just this substation I honestly can not think of a major problem.