Mini Hydro - Synchronous or Asynchronous 1

[ccdubs]

Hi All,

I am designing the electrical components for a 200 kW hydro scheme. I have done a cost comparison using a synchronous (genset) generator and an induction generator (with caps) and the two prices are very comparable.

My preference is to go with synchronous as this is what I am familiar with and it is a very flexible topology with respect to voltage control.

My research on other schemes of about the same size seems to show that induction machines are typically used. I was wondering if anyone knew why this is.

The turbine is a low head device using a crossflow design. Power will be controlled by a governing valve.

Any info is appreciated.

 

[jraef]

While this may be better suited for the Energy Conversion Engineering forum forum626 , I'll say what little I know (and remember) from having worked on low-head irrigation canal hydro's 30 years ago.

You don't mention whether or not you will be in island mode, but let's assume so. Induction machines are typically better used when connected to an existing grid because they are not self exciting. I can infer that you already know this from your mention of the capacitors. The caps make use of residual magentism in the rotor to ceate excitation current, but that is not necessarilly as reliable as separate excitation (as in synchronous gens), depending on the rotor design and how long it was off line. Speed control is also more critical as well, making them less suitable for wind turbines, but with a good flow control system you may be able to make it work well. You also must ensure that there is neve a load on it prior to going super-synchronous or it will never self excite.

For these reasons, sometimes it is better to go with synchronous generators in island mode operations. If the cost is the same, I'd go with synchronous.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

I agree completely with jraef;
I will add that if the utility goes off-line your frequency is oncontrolled. I understand that there have been improvements in induction generator technology and it may be possible to run in island mode. It may be possible to use a modified VFD that is energised by the induction generator and outputs a steady voltage and frequency in island mode.
However the induction generator would then lose it's advantages of low cost and simplicity.
I favour synchronous if the price is close.
respectfully

 

[ccdubs]

Thank you both for your info.

I can clarify that this is a grid connected station and that at this time there is no need for islanding. I am 90% confident that we will be going synchronous for this project.

jraef: FYI, you mentioned synchronous generators and wind turbines. Refer to

http://www.windflow.co.nz

which is the company I work for. Note: the gearbox technology that we base our design off isn't being used in this hydro scheme.

 

[Skogsgurra]

If not islanding, the typical way to go in the small (30 - 400 kW) hydro-electric plants I have worked with is induction machines. You don't need capacitors for magnetization, but perhaps for P.F. (there is a slight difference - the former is a physical necessity in islanding while the latter is a dollar/cent thing when connected to the grid).

Synchronous machines have a more complicated excitation system. They cannot usually be connected to the grid without having the right speed, phase and voltage. Induction machines can.

Synchronous machines are more sensitive to transients (lightning in rural areas). Induction machines are not.

Synchronous machines usually have a better efficiency (if you forget about excitation losses). But induction machines can also have a good efficiency if you have o low slip machine. For small systems (I think below 50 kW) the excitation losses make a synchronous machine less efficient than an induction machine.

I am surprised to hear that the price difference is small. Induction machines usually come out a lot cheaper. What if you do not calculate the capacitors and associated switchgear?



Gunnar Englund

http://www.gke.org

 

[ccdubs]

To quantify the cost issue, the synchronous machine is 15% more expensive all up. This includes the auto-sync unit. That equates to about NZ$2000 more expensive. But in rural areas there is a growing need, in NZ anyway, for reactive compensation, which some lines companies are now compensating for.

The $2000 extra for the asset will easily be recouped if/when compensation becomes available in the area this plant is connected. Also the immediate load behind the meter for this plant is induction motors for irregation pumps, any upgrades here mean we could also use the sync machine for PF correction. All info I neglected to include earlier as I was after a generic answer.

From the research I have done, I believe that it is a misnomer that sync is significantly more expensive than induction, at least in the 150-500kW range. I think the reason for this is that these machines are now mass produced for the deisel genset market and power electronics are cheap.

I will admit that a sync machine design and set-up is more complex but once understood I beleive the benefits of sync outway async on flexibility alone.

 

[waross]

Thanks for the feed back ccdubs
respectfully

 

[jraef]

I think the "generic" answer would have been asynchronous for a grid connected system, but certainly the reactive compensation issue pushes it over the edge towards the synchronous version in this case. You may be right about the "once understood" issue, but for the average user synchronous systems can be intimidating at first. I have often tried to sell users on connecting large low RPM continuous operation machines to synchronous motors as synchronous condensers, but have had little success because of that intimidation factor, warranted or not.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[niallnz]

Just a wee note of caution, Hydro machines are subject to high transient overspeeds if they are disconnected from the load. The synchronous machines made for diesel generators are not be designed for the kind of overspeeds that your little hydro unit could experience, as diesel engines have governors, and a fuel system that you can shut off in an instant.

If this is installed on a little back blocks farm, there is every posibility that the maintenance will not be done as rigorously as it should, and at some time the machine might experience a full runaway speed fault. Which would have your generator spining at aprox 165%-180% speed for an indeterminate period of time. The overspeed will depend on the chacateristics of your cross flow turbine.

Just some thing to have a think about, good luck.
Niallnz

 

[ccdubs]

Thanks niallnz, this is something I was concerned about. The guy doing the crossflow and mechanical design is installing a brake (and was going to do so regardless of generator type). The brake may be necessary as there is a pump that is also being piggy backed on this system.