For all control master, currently i have problem in load sharing new turbine. We build a new power plant for cement plant, 2x35 MW coal fired by using Siemens Turbine. Existing plant operates Steam Turbine Generator, STG A & B, while new turbine STG C & D will be controlled by new plant. The grid to cement plant is only 1, and it comes from existing plant, which currently run manually since from information, operators need to adjust the frequency in response of load changing. We try making load sharing in DCS for new turbine C & D only without involved STG A & B? Let say normal operation load to 100 MW, where 60 MW will be handled by new turbine STG C & D, and the rest by existing turbine. If anytime the load decrease to let say 80 MW, then we will maintain the power from existing at 40 MW, means it's new plant will adjust its power down to 40 MW. If the load continue decrease to 50, then both new STG will contribute only for 10 MW. But in case the required power is above the normal load to peak load, it's existing plant need to adjust to cover the requirement. So, when the load is over the max normal load, i.e : 120, then it's existing STG which will adjust, means STG C & D will be maintain at 60 MW, while STG A & B will be up to 60 MW. If the load is decrease but still above the max normal load, i.e: 110, then still operator will need adjust STG A & B only to 40 MW.

The questions are :

1. Is it possible to do such operation and load sharing? Siemens insist that such thing is impossible. According to them, operation will only work when load sharing involve whole 4 STG.

2. Is it dangerous to do above operation, especially to new Siemens turbine ?

Sorry if my question looked so silly since i'm just newbie and still zero in a power plant.

Really appreciated to any response.

 

Sounds to me like you need to implement an automatic generation control (AGC) system that will set unit loading.

 

> Sounds to me like you need to implement an automatic generation control (AGC) system that will set unit loading. <

So, it's safe for operation isn't it?
Siemens seems afraid that partial load sharing may cause trip to their new turbine (STG C & D)

 

Unfortunately life is not as simple as you are putting it.

Your new 2x35MW coal fired units definitely have a minimum recommended load below which you cannot operate these units continuously. So your assumption that the new STG units will generate just 10MW together when required is wrong, and that is why Siemens insisted that such thing is impossible.

As a rule of thumb coal fired units have a turn down ratio of just 50%, i.e. the minimum you can get out of your new 2x35MW units (when both are operating) is just 35MW. Reasons for this limitation vary from boiler flame stability to steam turbine exhaust cooling requirements.

I would recommend that your load sharing in DCS handles all your four STG units the same way, i.e. the load is always shared evenly between the four units. Alternatively you can opt to load more the more efficient units but always keeping in mind that the load of any unit that is on line does not fall below 50% of its nominal rating. Mind you some designs of boiler/steam turbine units allow lower loads but this is normally not lower than 40% of nominal.

 

>> Sounds to me like you need to implement an automatic generation control (AGC) system that will set unit loading. <<

> So, it's safe for operation isn't it? Siemens seems afraid that partial load sharing may cause trip to their new turbine (STG C & D) <

this is going to be a overly long post (as usual)

why exactly do you need to do the above load sharing ?? you might have your reasons but this is the first time i am hearing such a proposal. i am just curious to know the reasons behind it. is it for optimization ? or for something else. please so explain why such a load sharing is sought.

AGC is a grid level instrument for automatic scheduling and frequency control. It is primarily used for regulating the grid frequency to a nominal value and maintain interchange power flow between areas. It is also used for maintaining select units at scheduled power , an optimization tool to reduce losses. It costs a lot of money and it not implemented in its full form in small grids.

AGC's sister variant used in small islands is called iso load sharing scheme , and they do NOT work well. i know a 3-4 of sites who tried it and ultimately gave up and put the machines in droop. if you ask me , it is a waste of money and time.

may sites have a load sharing which is a mixture of load control and droop. It is predominately used in sites which have a mixture of gas turbine and steam turbine units. part load efficiency of the gas turbine is lower and operating it on part load is a criminal waste of money. the solution is the gas turbine units are put in load control and operated at near base load conditions. the steam turbine is put in droop. this in-effect is analogs to a gas turbine connected to a " grid" made of steam turbines. any load change is taken up by the steam turbine. let us say for eg that a island has 4 turbines 2 GT of 20MW capacity and 2 ST of 30 MW. let the plant load be say 80 MW. now the two gas turbines are kept at load control of 20 MW. thus the steam turbine will take up the extra 40 MW load in 20 MW each. now in case there is a drop in the load , say by 10 MW , the steam turbine will reduce its load by 5 mw each . as the gas turbine is put in load control it will be at 20 MW. thus a new load of 70 MW is shared as 20-20 by the GT and 15-15 by the steam turbine. the system frequency will rise and it is up to the plant operator to reduce the frequency to nominal value. the reverse happens in case there is a load increase. steam turbine takes up the load with a small reduction in frequency. This scheme works , my parent refinery uses the same. the driving factor here is to use the GT close to full load. thus behind every load sharing scheme there is a definite reason which drives the solution not the other way round.

now coming to the problem at hand

" We try making load sharing in DCS for new turbine C & D only without involved STG A & B? Let say normal operation load to 100 MW, where 60 MW will be handled by new turbine STG C & D, and the rest by existing turbine. If anytime the load decrease to let say 80 MW, then we will maintain the power from existing at 40 MW, means it's new plant will adjust its power down to 40 MW. If the load continue decrease to 50, then both new STG will contribute only for 10 MW. But in case the required power is above the normal load to peak load, it's existing plant need to adjust to cover the requirement. So, when the load is over the max normal load, i.e : 120, then it's existing STG which will adjust, means STG C & D will be maintain at 60 MW, while STG A & B will be up to 60 MW. If the load is decrease but still above the max normal load, i.e: 110, then still operator will need adjust STG A & B only to 40 MW. "

Theoretical solution to the above problem with least expense (ie no AGC or other software controlling your load , i despise them )

1. Put the two stg A and B in load control and keep set point at 20. this will keep the load in STG A and B fixed at 40 MW

2. keep the other stg stg C and D at droop mode. this will adjust to the plant load. if it increases the stg load will rise , if it reduces both the stg c and d load will reduce

3. once the sum of stg c and D reach a desired max value (60 in this case) , send a signal to the other two stg's A and B to come out of load select to droop . now you will have 4 machines in droop which will equally take part in any change. your STG will have a similar signal to come out of load control once grid connection is broken, you just have to OR the signal to include this.

practical solution

1. don't do it. put all the machines in droop in your island. over a period of years it is seen that machines in droop achieve the most stable performance in island conditions , iso and iso load sharing do not have a good track record. "If optimization and cost reduction" is not a factor then this is the best solution.

2. If you really want to go for load sharing , then "it must involve all the generators" , as a isolated system with a 4 generators going for load sharing in two and not in other will result in load hunting. But again i do not know the Electrical SLD in your system or a normal operating configuration. but i am guessing that once online all machines will be parallel with one another. If i am wrong about your operating conditions , please post with a sld and normal operating condition.

3. if you really really want to implement the load sharing as you have said in your post , you can go with the theoretical solution and hope that the system load will not come below 40 MW , because if it does you are in for a big blackout.

As a ending note i would like to point this out , i have seen that people spend a lot of money for iso and iso load sharing because they are obsessed with maintaining 50Hz frequency at all times. i do not understand why this is so. a reduction in frequency by 0.2-0.4 hz will is not going to affect your plant performance or your downstream plant performance. as far as i know it has no effect at all. there is a reason why for multi machine operation droop is preferred. even in grids the AGC will produce 10-40 MW load swings as a min , but as the grid is vast such load swings is not a problem , and it does not affect the frequency much. in a small grid , even a 0.5-1 Mw load hunting is not good even the most sophisticated iso load sharing scheme will produce such swings. this is one of the reason why it is not popular. The last time i heard a successful load sharing scheme implemented was involving aspen tech's suite which optimized set points of generators, but in that case all the machines were in parallel to the grid and the suite just provided a external set point to the governors. in case of a isolation from the grid , all the machines came to droop and maintained the last references.

 

AGC by definition is "Automatic generation control". Now many people have different ideas of what AGC does--but I think it's safe to say that AGC is an outside control system that divides the load up among plants according to limits programmed into it. It was done in the old days by humans.

Here is a good article that may be useful: http://www.powergenworldwide.com/in...tion-of-a-complex-industrial-power-plant.html

To me it sounds like a system is needed to "dispatch" the units to certain MW output while following a strict set of limits on max and min loading. Another goal is maximizing efficiency of units by running them in the most economical way possible.

DCS is designed to run the plant. AGC is designed to distribute load among various units. I would use the right tool for the job. AGC isn't inherently complex or expensive--it can be PLC based. The units will run in droop mode and the DCS will do the complex management of functions inside the plant. An outside AGC system will follow it's program and assign load to each generator as needed.

 

I think "impossible" is too "rude" in this context. I will suggest possible but not advisable. If I get your problem correctly, let me suggest why.

All your 4 generators are in parallel operation. They are in synchronism. That means all their synchronism torque angles of the rotating magnetic fields deviate less than 90 degree between each other. If one of your generators has its synchronism torque angle deviates by >90 degree with any of the remaining rotating magnetic fields your system will no longer in sychronism. Eventually the system will collapse. The one that loss the sychronism tends to go over frequency. Its generator simply doesn't take up load.

When you load a generator you are actually accelerating the rotating magnetic field of your generator by accelerating rotation of its turbine shaft. If you are loading up two generators you are accelerating rotating magnetic fields of that two generators. What happen to the other two that you are not loading up? They try to maintain their current rotating frequencies due to inertia. What does it suppose to mean? Two rotating magnetic fields that are being accelerated try to drag the other two that want to remain as they are. Thus there are net forces that try to deviate synchronism torque angles of the rotating magnetic fields. As long as the two accelerating magnetic field can drag the other two to accelerate together and their synchronism torque angles stay within 90 then everything should be ok. This is likely the case if your system is connected to a large grid.

Unfortunately your system is not grid connected. The demand load might move up and down very rapidly probably within minutes. The chance is those 4 rotating magnetic fields can depart by greater than 90degree. This leads to loss of sycrhronism phenomenon, follows by load hunting. Eventually your system collapse.

If you share the load change evenly and the speed droops for all your turbines are well set, the "dragging" phenomenon quite unlikely to occur. Or at least you can keep it at the minimum. For example, if system frequency goes down all generators try to accelerate their rotating magnetic fields. Similarly if system frequency goes up all generators try to decelerate the rotating magnetic. At least they move towards the same direction. Thus, reducing net force to "tear them apart". No rotating magnetic field is trying to drag one into another by that much.

All that I mentioned above is related to primary response of prime movers in system operations. Off course you need a generator, at most, to regulate your system frequency to the scheduled frequency. This task is called frequency control (secondary response). You can do it slowly and gradually. As long as your droops (primary response) are able to bring frequency rate of change to zero every time supply and demand are mismatched, everything should be ok.

Siemens may or may not have similar reason like I do.

For further info "google": Loss of synchronism

 

You are right. I like to add...

AGC has many horizons, such as 10 secs horizon, 10 minutes, 45 minutes, 4 hours and so on. For a 10 second horizon or shorter it is called automatic frequency control & regulation. Although optimization is still can be considered while deciding which plants have to be loaded by AGC system, however the ultimate objective is to reset system frequency to the scheduled frequency. This task can't be achieved via speed droops.