Starting Heavy Load

Some numbers, Boss?

Hi Crabtree,
I would like a general idea on how you proceed to make sure that when you start a heavy consumer, the frequency drop will be ok, I have 3 generators Apparent power, active power of them, base load, starting current of heavy consumer starting, its active power.

Thanks RAM.

Absolutely.

Does that mean that this one is reportable as a Duplicate Thread?

I didnt find any thread related specifically to the frequency start issue, in case we already have it could you please post here the link ?
thanks in advance

You need to hire someone who knows what s/he is doing.

CrabTree , i am looking for a direction because i couldnt find a good basis information on the books i have, the help doesnt necessary needs to be an easy and direct answer, maybe an direction on material or book would be welcome. If you dont want to help just try not to ridicularize who is trying to learn.

People come to cr4 to get others' insights with a view to helping with a problem. Without numbers to define it, any help will be of a lower quality than were some measurements available, and will take longer to arrive. Your ball, Mildred.

I agree-of course-with RAMconsult. In order to understand the complexity of the problem you may consult-for instance-ch.8 Stability studies from IEEE Std 399-1997

IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis mainly the subchapter 8.4 System disturbances that can cause instability:

"The most common disturbances that produce instability in industrial power systems are (not

necessarily in order of probability)":

d) Starting a motor that is large relative to a system generating capacity

Thanks,
I will look for this lecture :)

Really, the only way to plan such things is always assuming a stopped load. That will happen, even if only occasionally.

If is still spinning, that is simply a small plus!! Smaller starting load....

It also makes the calculation simpler....

Best of luck!

(assuming a DC power system) So you have already done a soft start, and are now up to two times the speed you need, is your question then "do I have enough reserve to slam in a load equal to half my capacity?" This is a fair question if you are trying to start, say, an F-104. If you are trying to start a load with an AC generator set, I am not sure if speed is the criterion... the loss of frequency when you slam the load to it will be dependent upon the sheer mass of the gen set. A 707 aircraft (for instance) would not allow the gen set to even come on line until the frequency was within five hz.

Sorry, thats all I got. Maybe I don't understand the question. Maybe you are not using the same kind of gen sets I used to maintain.

Starting a heavy load basically gives us voltage dips and when the load is of large inertia, the voltage dip will remain in the system for longer duration.

When a heavy load is to be started on islanded generation, it is common practice to specify the generator MVA to be much higher than the Engine / GT rating. This way, the generator (with its larger excitation system and associated higher VAR capability) can supply the required VARs to the motor (under start) and thus ensure voltage dips are within limits.

In your case, it is an existing system and it should be possible to estimate whether you need to run two machines from the spinning reserve or one is sufficient by performing simple voltage dip calculations.

There is unlikely to be any issue of frequency dip.

Morgan_fer,

Using the energy stored in a "spinning reserve" to help start a heavy (large) motor load was probably much more common a few decades ago than it is now. I recall reading about the massive pumps that were lifting the water in the California Aqueduct up to the crest of the mountains. They had to parallel at least two separate utility primary feeders to start the first pump; after this, they could use the spinning reserve of that pump to help start the next, and so on until all the pumps were started.

In that case the inertia of each of the motor/pump combinations was about the same. I suspect that Crabtree's sensible questions about details are (in part) trying to find out if the inertia of the spinning reserve is similar in magnitude to that of the load you are wanting to start. That would be a small part of the overall information needed for a reliable answer to your question.

--JMM

Hi Jmuller,
and thanks all
So the system is:
3 gas turbine generators of 12w / 19.6 MVA1 steam turbine generator of 12w / 15MVABase load of 41Wand the load to be started is 5MW/ Starting current of 1200A
I think this gives an general idea

...and the reason for your concern is...?

My concern is how can i make sure that the frequency drop will not be so much that can possibly trip the generator

As others have repeatedly stated, you have not provided enough information about the system, especially the load, for any meaningful response. In general, general information yields general results which, generally speaking, aren't going to satisfy you.

There's a world of difference between starting an unloaded centrifugal pump and a loaded rock crusher.

I think he did a fair job in post 17.

Yusef,

Not at all, his information is no different than telling us he has '69 Mustang and then asking how fast it will go in the quarter mile. Without information like is it a 4, 6, 8 cylinder, what displacement, auto, manual, number of speeds, gearing, rear end ratio, stock, modified, fuel, carb, fi,etc., all you can do is quote a range that varies by 4 to 1.

Same thing here, no info on voltages, ratings, GD², load type, transformers, cabling, protective relaying, speed torque curves, etc.; dynamics is much more complicated then statics.

RAM

Hi RAM,
I never dealed in details with making this dynamic study completely, so its difficult for me to pass all the data you need, if i dont know what is the whole necessary data. For this beggining i will try start with the material provided by our colleagues.
Thanks all for cooperation

Morgan, I'm not trying to be harsh, just protective. Some people are going to tell you that as long as you have 10x the load as generation you'll be ok but I've seen that fail as well. As long as your management doesn't mind your experiment then great, and please report back your results for the further edification of CR4.

good answer

Our company have done this dynamic study , so based on ther results we could have the idea if it was posible the starting of the load, before we didnt have it, so now it made the things clear.
many thanks!

Well,at least it worked out. I feel I failed a bit here though, but I am stoked that your company allowed you to do the dynamic study. So...is ten times capacity for load enough?