Induction generators for steam turbine application

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Thread Starter

SolarforChange

Hello,
I am designing a solar trough power plant that would output 1.5MW to the grid. I am thinking of using an asynchronous(induction) generator and was hoping someone would explain to me some of the advantages an induction generator has in comparison to a synchronous generator.

I am familiar with the basic power generation process of the induction generator but I am not sure how a person would be able to assure a 60Hz frequency from the generator. Also, I have little knowledge on grid hookup and was hoping someone would be able to explain to me what components would be needed to assure correct hookup and safety of both my side of the plant and the grid's side.

Thank you very much
SolarforChange
 
SolarforChange... the frequency is determined by the "source" to which the Ind-Gen is connected. In your case, 60 Hz!

The speed of the steam-turbine driving the Ind-Gen is maintained at a rotational speed/frequency slightly higher than 60 Hz! An example follows:

If the Ind-Gen, as a motor, had a full-load slip of, say 3%, then as an Ind-Gen it must be driven at about 3% above 60 Hz or 61.8 Hz!

Major Advantage:
The Ind-Gen does not need an exciter, nor slip-rings, nor a voltage regulator. Furthermore, it will certainly reduce electrical power cost for your facility.

Major Disadvantage:
The "grid" to which it is connected must be able to supply the Ind-Gen's magnetic kVAr!

I have purposely excluded details covering connection with the utility. They can be horrendous depending on the utility's requirements. In most cases, you will paid a fraction of the price you normally pay for the consumption.

In closing, you will literally find many topics on the web!

Regards, Phil Corso
 
Phil Corso,

I was counting the number of exclamation points until I choked on this tidbit:

>The "grid" to which it is connected
>must be able to supply the Ind-Gen's
>magnetic kVAr!

<bSupply</b> VArs!?!?!?!?!!!!!???!!??!?!

That means VArs must be produced!!!!!!!!!!!!

HAH!!!!

Vindication--at last.

It's a magnificent day in the neighborhood.
 
P

Process Value

Solar Power

well, you are planning to use induction generator for a steam turbine, interesting concept usually induction generators are used when the prime mover is a variable speed one, like a wind turbine. there are two classes of induction generators available

a. single fed induction generator - here this takes reactive power from the grid for magnetization and gives real power as output. in order to compensate for the low power factor , you need to have a capacitor bank installed. the major disadvantage of the machine is not capable of supplying reactive power to the gird.

b. Doubly fed induction generator - here the rotor is connected to the grid supply with the ac/ac converter and both the stator and rotor are connected to the grid (hence the name doubly fed). here the controls are complex. you need a DTC to control the rotor. this is the trend these days in wind turbine industry as the generator can supply reactive power to the grid and has better integration with the grid.

In both the cased you need not worry about the frequency , even though the rotor is running at a slip speed (speed higher than the grid frequency) the stator which transfers the active power will have the same frequency as the gird. you can contact a wind turbine installation and have a look of how they integrated to the grid. you can then emulate what they have done in your project. the main disadvantage is that these machines are not mass produced , the cost will be higher. much much higher. but i see potential problem areas when you use steam turbine in conjuction with a indution generator

a. as the speed of the induction generator needs to be greater than the sync speed you might be putting your steam turbine in overspeed/critical speed conditions. it mainly depends on the rated speed of the turbine and the generator speed. to avoid this you will have to carefully select the generator poles and also perhaps suitably design a reduction gear.
b. and most of the speed turbine governors are designed for sync generator application. you will have to perhaps design new control scheme for the induction generator.

As you are using a steam turbine , a sync generator will do a good job , the main advantage is the proven technology and the cost will be very very low. you have unmpteen no of Diesel generator set makers who make sync generators of 1.5 MW capacity. you can contact them and order just the generator of your spec. as it is a mass produced product you will have a huge price margin. they will supply the generator control panel with the excitation system also. you need not have to worry about it. you just have to couple the steam turbine (with or without a reduction gear depending on the speed) to the generator and you have a working power plant.

In my opinion, using a sync gen with a steam turbine is the most stable, economic and viable option, induction generators do not seem to offer any advantage in conjunction with a steam turbine. steam turbines are designed to work at a rated speed and with a narrow margin. this lends itself beautifully to the sync gen operation. It is of course possible to attach a induction gen but you will have a job cut out to control it.
 
CSA... in addition to your Spell-check and counting skills, you certainly have the tenacity of a hound dog!!!

Regarding your comment about VAr... of course they must come from the supply! Do you believe the Ind-Gen produces its own VAr to satisfy its magnetic needs? And, if not connected to a "grid" then its VAr comes from connected Capacitors, thus, the Ind-Gen is considered "self-excited!"

Now, let's hear how you think an Ind-Gen works!

Me thinks you have mis-counted your chickens!

Phil Corso
 
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SolarforChange

Thank you very much. This information was really helpful. I was able to find companies who supply controls for induction generators that are used for steam applications. Hopefully they will be able to come up with something and spare me from having to come up with the controls myself... hah.

I just have a question about what comes between the generator and the grid. I know that the utility services have a lot of rules and regulations for hooking up to the grid but I'm wondering what sort of components would be needed in order to assure that I supply proper power to the grid (Voltage & current through transmission lined)? and what will I need to make sure that if some fault would occur my side (plant & generator) would be safe?

Basically.. I am hoping someone would explain what comes between the generator and the grid.

Thank you,
 
Phil Corso,

You keep saying there's no such thing as reactive power, but you acknowledge that VArs are required for the ind-gen to work, and that they must come from the grid.

So, VArs are 'consumed' and 'produced', just like watts.

And, you're not the first person to accuse me of not letting go of a bone. Nor will you be the last.

Again, I absolutely do not want to get into any kind of protracted exchange with you, on- or off-line. I just want to point out what appears to be an inconsistency. With the rest of the world's view of AC electrical power production and consumption (watts and VArs).

As for the ind-gen, instead of producing torque from amps it's an induction machine that produces amps from torque.

You don't ascribe to that theory, either.

I'm going to go back to my old ways, which are the same as most every text and reference I've ever had the pleasure (or displeasure in many cases) to read.
 
SolarForchange,

The specific answer depends a lot on the location of the generator and the utility and their requirements and any regulatory agency requirements.

Also, depending on the voltage of the generator, you may or may not need a transformer, but I would pretty much say you will need some kind of transformer as I understand they are also fault-current limiting devices of sorts.

You will also likely need some protective relaying and metering. Reverse power relays and differential relays at a minimum, probably several more, as well, that would be specific to induction generators; power metering (for reporting and monitoring) including Watts and VArs and possibly power factor; synchronization equipment (synch and synch-check relays); a generator breaker of sufficient size and rating; a DC power supply for the breaker close and trip circuit (or a compressor for instrument air for the breaker operator, depending on the type of breaker); some heavy duty bus bars of sufficient rating with the proper separation. You'll probably also need some kind of basic interface with the utility to tell them the status of breakers and generator and for them to possibly be able to open a breaker should they see fit to do so.

That's a basic list, by no means complete. If you are going to pursue this further, I suggest you might want to find a student of power transmission and distribution who might throw in with you and could perhaps help with the details. Or, obtain the services of a reputable power system engineering firm, one with experience with utility in the are where you are planning to connect it to the grid.
 
CSA... you had to bring up "Reactive Power vs VAr".

What I said was, "reactive power" (as a noun) is an oxy-moron. I suggested, nay, recommended, using "VAr" instead! I also said (but you keep stating you hadn’t read it, yet) and I quote, "Note, this recommendation does not preclude anyone from using terms with which they are familiar!"

Phil C
 
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