Myth 1 - "It is possible for generators connected to the grid to operate in different frequencies (small of large) for the Entire Operation Period/Sustained Periods of time "

Fact - "NO, generators connected to the grid CANNOT OPERATE AT DIFFERENT FREQUENCIES for sustained periods of time "

Explanation - There will be small frequency deviation around a particular value. AT ONE TIME INSTANT OR A SNAPSHOT it will look like the generators are operating in different frequencies BUT the generators or group of generators hunt "AROUND" a frequency value. This swinging of generators in one area with generators of the other area is called "low frequency oscillation" or "Interarea oscillation". The Magnitude of frequency hunting may vary from 0.1 - 0.5 hz (at any one given time instant). The power flow change will depend on the gird size, the generator group, the load state, etc. typically this low frequency load hunting can range from 50MW to 200 MW. It is possible for Inter-area oscillations to continue for sustained periods of time, but this DOES NOT MEAN THAT THE group of generators are out of sync with each other. The article explains below how interarea oscillation occur and how they are controlled with PSS.

Paper on interarea occilations - http://www.2shared.com/document/Eya2tNK0/interarea_occilations.html

 

Myth 2 - "When a generator is connected to teh grid and grid is stable, POLESLIPPING is caused due to overspeed."

Fact - "NO, Pole slipping is not caused due to overspeeding it is caused due to "EXCESSIVE LOADING" of the generator beyond its stability limit."

Explanation -

what is meant by pole slipping?
When the generator load angle exceeds beyond 180 deg, pole slipping is said to have taken place.

What is the steady state load angle for a generator?
The steady state load angle for a generator WILL be designed in such a way that it will be within 50-55 deg AT MAX GENERATOR OUTPUT, at RATED TERMINAL VOLTAGE at the MIN allowable EXCITATION VOLTAGE at full load. min allowable excitation voltage is the negative var limiter voltage of the AVR.

What is the steady state load angle limit?
The steady state load angle limit for a pure inductive circuit is 90 deg. but in practical cases depending on the system resistance it will be around 80 deg.

so why is pole slipping is said to happen at 180 when steady state load angle limit is only 90 deg? should the machine not be tripped at 90 deg instead?

During transient condition, like a fault, it is possible for the generator to go beyond 90 deg and swing back to less than 90 deg when the fault is cleared. 90 deg is the onset of instability, at 180 deg is when stability is lost forever. The time of clearing the fault from fault time to the time when the generator just has enough energy to swing back is called "critical clearing time." This means that if the fault is cleared before critical clearing time, the machine will remain in synchronism. but if the fault is cleared after this time time, the machine will lose its synchronism.

What are the factors which affect this critical clearing time?
1. Initial loading of the generator ( higher the loading , lower the critical clearing time )

2. Type of fault/proximity to fault (closer to fault, lower the critical clearing time)

3. Max power of the generator (higher the generator power output, larger the critical clearing time)

4. Excitation of the generator

5. Terminal voltage drop during fault etc ....

6. Generator Inertia
Note - *no. of poles of the machine has nothing to do with the critical clearing time or with the speed excursions.

so what is the principle cause of overspeed in turbines/generators?

The principle cause for overspeed in generators/turbines are load throw offs not loading of generators.. (* loading here means increasing the electrical load connected to the generator). Most; if not all overspeed accidents/trips occur during a load throw off. Imaging you are connected to the grid supplying a 200 MW load, ( in a fully condensing steam turbine with a average efficiency that is roughly 600-700 T/hr of steam input). Normally for a no load operation the steam input for running at full speed no load is about 10-30 T/hr. So when the grid breaker opens the governor must be fact acting to reduce the input from 600-700 T/hr to 10-30 T/hr. if this does not happen, the turbine will accelerte to overspeed and trip, or worse damage the whole station.

Matlab code for critical clearing time - http://www.2shared.com/file/7F_AcZDW/swingeq_rk4.html

This is a matlab code written by me some time back to calculate critical clearing time for a given system. rangakutta 4 numerical integration is used here. The input is the fault clearing time. I have not included any damping in the system. the code is heavily commented and probably anyone with matlab experience can use it. change the clearing time and you will see that it will go out of sync when the clearing time is goes high. In the numerical example already given there, the clearing time is 0.08 sec. so if you give 0.05 you will see that the load angle swings and returns back. But for value like 0.1, it just goes off and does not return back indicating loss in synchronism. I am using matlab as its very easy to plot graphs in matlab. i apologize that i cannot give a binary executable, but i believe that students who read this thread will find it useful.

I will also upload in a few days matlab code for multi-machine stability. But if you need to see more advanced featured plots you will have to use software like PSCAD, ETAP for transient analysis. I have proficient in ETAP and anyone who needs guidance in that can contact me here.

Text book on power system stability - Prabha kundur http://www.mediafire.com/?boodv5jhjwcvqfl ( best textbook ever written on power system stability , and co incidentally the author seems to have come from this part of the world )

 

Myth 3 - "Generation follows load instantaneously"

Fact - "NO, generation lags behind load. load changes first and then the generation catches up. this is the reason for the frequency change in the grid."

Explanation -
The governors cannot act instantaneously to the load change. the degree of variation depends on the type of unit. hydro and gas can respond quickly to the load change and thus can speedily resolve the frequency deviation (if connected to a AGC). but thermal units are sluggish to respond. typical response time for a thermal system will be two - four minutes. This is not my opinion, but its a fact well known and published in journals. the IEEE paper below takes in depth about it have a look.

Paper on time averaging regulation service - http://www.2shared.com/document/geouTgmR/time_average_regulation.html

 

Myth 4 - "All prime movers (that includes generators) in the system do not recognize what is load. They can only recognize speed. Sound strange right? But it true. So their responses are based on frequency deviation. "

Fact - "This statement or myth if you prefer is TRUE , the "final reference" setpoint for any prime mover steam , gas or hydro is speed (speed reference).

Explanation
When connected to the grid the governor can be put in only two modes of operation
a. Droop mode
b. Load control mode

if put in the droop mode , the concept is clear , speed reduces there exists a difference between the droop reference and actual speed. The fuel input increases, and the power output increases. The reverse happens when speed of the grid increases

It is slightly more tricky in load control mode, here the initial reference is "load" the final reference is always speed. If the load setpoint is increases, what actually increases is the speed reference setpoint. this in turn causes the fuel/steam/water increase and increases the power output.

I am uploading my GT governor control modes manual for clearer perception of this concept. In this doc you can see GE code for GT control , you can see clearly that only in droop or in Load control. the fuel input and thus ultimately the load is controlled only by the difference between TNR (the speed reference setpoint) and TNH the actual speed of the machine.

Governor control modes - http://www.2shared.com/document/Uidq-hB_/Governor_Modes_of_Operation.html

 

Myth 5 - " Namatimangan08 and Process Value are fortunate to live in a part of the world that experiences many transients and to have such other-worldly experiences to share. If only they would recognize them as transients. - CSA"

(* i cannot speak for namatimangan08, who i do not know if he is from India, but i am from INDIA )

What i understood when i saw it "I live in a part of the world (INDIA) that experiences transients. and other-worldly things happen in India and Indian engineers like myself cannot differentiate between transients and steady state operation because we operate the grid in a perpetual transient condition "

Personal reply to CSA- It pains me to see venerated and experienced person like you making such a comment. I usually laugh most of the things off (many of my posts have a " ha ha ha " in the beginning or the end), but there comes a time to make a stand and i am choosing to make it now.

first of all a stable grid with extraordinarily frequency regulation takes place in only a few developed countries. Few developed countries i am referring to USA , Canada , UK , eurozone and some middle east countries. but In most of asia, africa, south america, middle east there has always been a power deficit. the grids in these countries are not what you call stable with "excellent regulated" frequency. But this unstable grid does not mean that the people working in these "unstable grids where nominal frequency is a dream " are "TECHNICALLY incapable or incompetent." on the contrary they have more experience dealing with emergencies and have good knowledge about steady state and transient conditions.

The systems are "unstable" or "unregulated" because of a variety of reasons. First and foremost is lack of generation capacity; then comes transmission bottlenecks; then there is the humongous distribution network; then there is the inter-rivalry between the states..etc, etc... the list can go on. But i do not think that you truly understand or appreciate the work done by power engineers in countries like India, where like the southern grid the generation defect at peak load is in the order of 2000-2500 MW. every contingency analysis is a failure and a nightmare. We cannot build power plants in a day, but we need to manage and supply power daily, with what is given. we make the best out of it.

Yes , we are UNFORTUNATE to have a power deficit. We are UNFORTUNATE to have bottlenecks. We are UNFORTUNATE to have "poor frequency regulation." But I am FORTUNATE to work with engineers who are the best at what they do, who know how to make the best out of any given situation. I am FORTUNATE to receive training of the highest order from them. I am FORTUNATE to see and experience and know how to deal with "other worldly things/transients" which i can assure you happens all around the world even in the most stable of grids.

I MAY NOT AGREE with all the things posted by Namatimangan08 as you would have come to know from reading my other posts in this thread. but to pick on a person based on the fact that he wrote 428 rpm for a hydro machine when the actual theoretical speed is 428.57. (most of the text books and industry manuals use 428 for a 14 pole machine. if you have a doubt please see the link below, or ask anyone in hydro industry what is the no of poles of a 428 rpm machine. no one will ever say its a 14.01869 pole machine), and to insinuate that he does not know about 120f/p is plain CRUEL.

Hydro plants design ( go to page 29) http://www.2shared.com/document/cx6xXbnC/ESHA_Guide_on_how_to_develop_a.html
if you have a power plant engineering text book you can look into that too.

People respect and admire you for the contribution you have done to control.com. no one can ever deny that, but comments like these do not bring you any laurels. and i will conclude my personal reply with a quote from this part of the world (INDIA) which roughly translates in english as "words spoken and arrows fired can never be taken back". good day to you sir.

 

> Myth 1 - "It is possible for generators connected to the grid to operate in different frequencies (small of large)
> for the Entire Operation Period/Sustained Periods of time "

> Fact - "NO, generators connected to the grid CANNOT OPERATE AT DIFFERENT FREQUENCIES for sustained periods of time "

Obviously it can't. This statement does not mean there is physical limitation that prevents it from happening.

> Explanation - There will be small frequency deviation around a particular value. AT ONE TIME INSTANT OR A SNAPSHOT
---- snip ----

> Paper on interarea oscillations -
> http://www.2shared.com/document/Eya2tNK0/interarea_occilations.html

The title of the paper is about inter area oscillation. Not specifically about parallel generators.

Even that so it mentions that it is possible to have low frequency oscillation 0.1-3Hz. To me this is absolutely normal. Very unlikely any generator will go out of synchronism.

Let us see paper that specifically talk about generator loss of synchronism.

Goggle generator loss of synchronism. You will find dozens with complete mathematical equations to explain it.

This is from answer.com
*************************************************
Q: What if the generator loss excitation?

Answer:

A loss of excitation will cause the generator to start drawing more and more reactive power over time. Over the first few seconds, and until about thirty seconds after the LOE (depending on the load on the generator before the loss of excitation occurred), active power will stay relatively constant, but reactive power will continue to be absorbed from the rest of the system, and voltage levels will drop. Eventually, the magnetic field between the stator and rotor degrades too much, pole slipping and loss of synchronism will occur. At this point, catastrophic damage will likely have been done to the generator.

Note: There are comments associated with this question. See the discussion page to add to the conversation.

Read more: http://wiki.answers.com/Q/What_if_the_generator_loss_excitation#ixzz1nXitQcEL
**************************************************

 

> Myth 2 - "When a generator is connected to teh grid and grid is stable,
> POLESLIPPING is caused due to overspeed."

Is it not similar to what has happened to my client's hydro turbine generator that I have mentioned?

But pole slipping can also happen due to under speed! Remember out of phase can be +180 degree or -180 degree.

In the hydro turbine generator that I have mentioned in other posts its generator tends to go over speed because of high terminal voltage and poor flow regulation. I removed one of the factors. The t/generator could last for a week before it tripped off again.

If the generator badly under excite to a point that even the rotating magnetic field of the system cannot drag it to move at equal speed, then it will go out of synchronism by -180 degree.

> so what is the principle cause of overspeed in turbines/generators?
>
> The principle cause for overspeed in generators/turbines are load throw offs not loading of
> generators.. (* loading here means increasing the electrical load connected to the generator). Most; if not all
> overspeed accidents/trips occur during a load throw off. Imaging you are connected to the grid supplying a 200 MW
> load, ( in a fully condensing steam turbine with a average efficiency that is roughly 600-700 T/hr of steam input).
> Normally for a no load operation the steam input for running at full speed no load is about 10-30 T/hr. So when the
> grid breaker opens the governor must be fact acting to reduce the input from 600-700 T/hr to 10-30 T/hr. if this does
> not happen, the turbine will accelerate to overspeed and trip, or worse damage the whole station.

If the machine breaker opens, the ST has already tripped off. So why bother about over speed trip?

In this case, concern is more towards by passing the steam to condenser. The reason is to prevent the ST to go to runaway speed. For good design of a turbine generator, it has consider minimum period for a t/generator to operate under runaway speed so that the building and the supporting structures do not collapse due to resonant frequency.

 

Dear Process Value,
we are lucky to have ppl like you in this forum.

I had one query regarding LFO. Is this inter area oscillation or LFO not responsible for Load angle change of alternators connected with grid? or is there any relation between them? plz explain
I request other resourceful persons also to throw some light on it.