Mini Hydro Scheme 2

[jasonf987]

Is there any limitation in the size motor which can be used as a generator other than physical size? I understand that by driving a motor above synchronous speed it will start acting as a generator, providing it has a magnetic field set up via some reactive energy, but can you use this approach with any motor, for example up to about 250kW?

 

[waross]

Yes. Size is not a limit.

Bill
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"Why not the best?"
Jimmy Carter

 

[cranky108]

However, the type of motor is a limit. Many smaller motors, like the universal motor, may not be usable in this application.
I think what you are asking about is the induction motor, so know the type.

 

[EChristenson]

Have you ever wondered how they make all that electricity at Niagra falls or the hoover dam? Those AC generators run like 20KA and 20KV. They are also 20 feet high and 40 feet in diameter.

The controls are simplest if you have a brushed or brushless permanent magnet DC motor. Just look inside a toyota prius or a honda civic. But yes, there are some types of AC motors that are very difficult to get to generate.

 

[cranky108]

If you have a very small size, just use a dc generator, and a solar inverter. It will save you a bunch of interface devices.

 

[ScottyUK]

Efficiency, system stability, capability to produce reactive power all tend to favour the synchronous machine for large scale generation. Induction generators larger than a few MW are unusual.


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If we learn from our mistakes I'm getting a great education!

 

[jasonf987]

yes, i am looking at an induction motor. I actually want to see if i can't use a pump as a turbine and then couple up the motor to run as a generator. This is on a system where we have effluent running away from our mill. It drops a total of roughly 60 meters and has a flow rate of in the region of 50 Mega litres a day. a potential energy resource on a small scale for nothing more than the cost of the kit and it's installation etc. I'm in the early days of the design and just want to try and consider all of the available options.

 

[cranky108]

If I calculate correctly that's about 195 feet.
I don't know about that, as most of our drops are over 1000 feet (high head), which we have to break down the pressure anyway.

There are others who probally know better than me about low head applications.

However, our smallest sync. machine is 1MW, but it is quite old. Our smaller machines are induction machines.

 

[rasevskii]

for jasonf987

Yes in principle you can use a centrifugal pump as a turbine but it will not run at the same speed as it did as a pump. About one half or one third I have heard. So a direct drive to the same motor is not possible, but a speed increaser will be needed. Obviously in the other direction also.

Depending where you are in the world, it might be better to look up some of the many suppliers of small hydro units available as engineered assemblies. There is a lot of info on the net on "small hydro".

Also, is the site on your own company premesis or not-- there are all kinds of legal, bureaucratic, and environmental problems involved which may make the affair impossible, very delayed, or uninteresting.

Also what happens when the unit becomes disconnected from the grid, is overspeed, so some forms of protection are needed as well as backup supplies to the controls, etc.

The companies doing small hydro know about all this and can offer good solutions, but at a price...

regards, rasevskii

 

[burnt2x]

You'd probably get 200kW max with your head and flow specs (60% turbine efficiency, below 10% pipe friction loss and about 90% generator efficiency).
I recommend you try "cross-flow" turbine instead of using a centrifugal pump as turbine. As rasevskii pointed out, you should limit your runaway speed to prevent damage to your equipment should your generator loose its load. Mini-hydro plants of that size require a better overspeed protection compared to micro-hydros which make use of dummy loads to take the extra power should some loads are dropped.
Try surfing the web, search keys: "Banki", or "cross-flow"

 

[waross]

Many manufacturers use the same frame for 1200, 1800 and 3600 RPM motors in the smaller sizes. Try to check with the motor manufacturer. Your motor may be capable of 3600 RPM with no problems in the event of a load loss.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

Agree with burnt2x.

A crossflow seems to be the correct choice for this site.

That means usually the generator driven over a gearbox from the turbine. A horizontal arrangement. Engineered by the supplier including all hydraulic and plc based control. Fail safe by a weight driven lever to close the inlet to the turbine on shutdown and also for load regulation.

See Ossberger as a reference, for built examples.

By that size of unit a synchronous generator is indicated, meaning synchronizing equipment, AVR, etc. It is getting expensive already and it is not even built yet...

It depends on what you already have for a system in your mill, is it off/grid, is there other generation or not..etc...What about short-circuit levels, existing switchgear, etc...Voltage levels, MV or HV ?? New transformer?

Not to mention all the other legalities, politics and economics involved. What is your feed in tariff. What is the load factor, how much water is available all the time
what is the flow history over time.

I studied with a partner small hydro some years ago for sites in Europe, it is a millionaires project usually.

Most sites fail the test. Yours might win, depending on where you are and what are the conditions.

Tell us more.

rasevskii

 

[jasonf987]

Thank you all for your input, it def does give one quite a bit to think about. I will try and put some answers to the questions posed.

I agree, my calcs put me in the region of roughly 200kW aswell, the installation would be on our company premises and due to the fact that we already have permission from the gov. to discharge the effluent, there would be no problems with the bureaucratic "issues" which tend to crop up w.r.t licences or environmental concerns.

we have a young mech engineer who believes he can re-engineer a pump to allow us to operate it in the fashion that i was originally thinking of, personally i am a bit sceptical but he seems keen to prove me wrong so i have left him to try.

in terms of load, i am thinking of obviously controlling the input flow to the "pump"/or now with the advice maybe cross flow turbine. if there is a problem then there will be a spillway which will divert the flow away from the installation back to its original path. this flow history over the past 10 years has remained largely unchanged which is good for this installation.

I am based in South Africa, and there seem to be very few suppliers in this region w.r.t Mini or micro hydro schemes. I have seen that there are a number of resources across the rest of the world, hence im trying to get as much info together as possible.

We are connected into the main grid, we have two 10MW generators which we use to supplement are energy requirements from the national supplier, in terms of control, we are in the process of placing the generators under static excitation and i have identified the opportunity to include my little project on the funds required to make the excitation changes, along with some un-used equipment i will be able to salvage most of what i need to get my project operational under a temporary installation (of course we all know that a temp installation will become a permanent one if management has their way) so i will try and get it done the right way the 1st time.

We do have water available at all times which is what makes this installation viable, but the real kicker is that our utility provider is increasing the power price by 35% per year, for the next 3 years! To supplement the building of new power stations which they failed to identify a need for over the past 10 years. (this is what happens when politics gets involved with engineering) so as the next 3 years go this project becomes far more attractive with an excellent ROI.

As far as protection/switchgear/transformers go, yes we will def need this but as stated earlier, i have some equipment available but i need to complete the design first, im still just feeling the water so to speak to get my head around this.

But thank you all for your input thus far, it really is an interesting topic and its great to here the opinions of other professionals.

 

[waross]

I would try an installation with the equipment at hand. Based on that performance, consider changing the pump out for a cross flow turbine. Controls may be quite simple. Open the waste water valve and connect the generator/motor to the line when it is slightly above (a few RPM) synchronous speed. Overload protection and a basic differential protection scheme.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rasevskii]

Hello Jasonf987

Well, that sounds like a good basis for a project. The power level available makes it worthwhile.

But I think that the crossflow from an established manufacturer will be be the best option. I assume you have looked at some of the existing plants done elsewhere on the web.

Crossflow turbines are made by a number of manufacturers but the quality, choice of materials, and design varies widely. So we are told, anyway.

Also a rather long penstock will likely be required. The closing time of the turbine inlet gate will be determined by the permissable pressure rise in the penstock. Likely a flywheel will be required on the generator shaft to reduce the temporary speed rise on 100% load rejection.

The coordination between the turbine design, penstock, and other civil works has to be done and designed by a qualified engineering company to make a safe installation.

Burst penstocks and full gate turbine runaways are not unknown in the hydro field. But you will not read about these events in any company sales literature...

As said a syn generator is the best option. But an induction machine is also possible but the magnetizing inrush will be an issue as well as the low P.F. A soft starter can mitigate the inrush but that may bring other issues.

There are many experts on this forum on Soft Starters, I am not one of them.

I assume you will be dealing with a 415 V. system. Since the current would be rather high at that voltage, you may need a transformer at the new hydro. I suppose the mill has a 6.6 kV system...

Normally such a unit is controlled by forebay level which regulates the turbine inlet flow. This is standard available from the turbine supplier. Proportional control parameters, after the unit is paralelled.

A possible issue is erosion if the water contains tailings..

Keep us informed, it sounds interesting.

regards, rasevskii

 

[rasevskii]

for jasonf987

In addition it is not absolutely necessary to have a gearbox between the turbine and generator. A direct drive should be possible at that head and flow. I am assuming a crossflow. This is also in the range of a small Francis turbine.

Get some quotations from suppliers for the head and flow available. The speed could be 750 or 600 /syn speed/probably not advisable to go to 1000 rpm as balancing and vibration would be problems. A flywheel likely needed, that means another bearing at least.

The lower the speed the more expensive as more material is involved, but longer life. The lower the speed the less dangerous is the overspeed on 100% load rejection.

Perhaps you are thinking of using an existing induction motor that you have around.

The penstock is another issue. Many dangers in this. You will need a Civil Engineer to get involved.

regards, rasevskii

 

[nento]

Halllo jasonf987
I am new in the group but the following information might be useful
The Head is 60 m and the flow .578 m3/sec. A well designed plant should produce 220 Kw.
The pump: Pumps speed is limitted to the allowable stress of the impeller. For good grade steel impeller the maximum peripherial speed is about 32 m/sec. But when it is used as turbine the speed is dictated by the head. In your case the head produced gravitatioan velocity of Sqrt(2*H*g)= 2*60*9.82=34.3 m/sec. Most designers take about 46-48% i.e say 47%*34.3=16.1 m/sec for turbine peripherial speed. If your pump has 300 mm impeller, the optimum effeciency is at 1025 RPM., likewise 200 mm impeller, will be 1570 RPM. If your system is 50Hz, than you will need 6 pole motor for the big and 4 pole for the 200mm impeller. Increraing/decreasing the RPM will drop the effeciency drastically. Forcing the 300mm impeller to 1500 RPM will generate only about 170KW or less.
Pump outlet (turbine inlet) has to accomodate the designed flow. This flow will ruquired an area of Q/V =0.578/34.3= 16 850mm2. 6” pump outlet size will be enough. Using 4” size will restrict the flow into half. If you stict to directly couple with 1500 rpm motor/generator then use 2 pumps, each has at least 4” discharge outlet.
Penstock. Depending on the length of the pipe the size will be at least 600mm in diamater or more. Recommended max friction loss is 4% to bring your net head to 57.6 m only. If your mechanical engineer can make it 2% then you have 58.8m.
Grid connection. In normal operation the grid control the frequency. But if for some reason the grid fails, yout control system will open the breaker, to shut down the system. This will logically automatically closes your fail sava inlet valve. But in microhydro the “fail save closing valve” inside the power house may read “fail severe pipe bursting” outside. To avoid this the inlet valve should not be closed very quickly. The closing time from full open to full close is between 10-to 20 second, depending on the thickness calculations in terms of surge pressure, friction loss, length of pipe etc.
But during 100% load rejection, slow closing the vlave will overspeed the turbine and generator as well. The cheapest braking system is using a dummy load. When the main breeker to the mains closes, the electronicaly controlled device diverts the load to the water immersed heater. In off grid sytem, in addition to overspeed braking, this device also keeps the frequency steady even at severe load fluctations at end users,
Small cross flow turbine up to 500 KW is very simple to fabricate. If your mill has standard maintenance equipments such as welding, cutting torch, 14” cut off grinder then you can fabricate at your workshop. The shaft may be machined outside if you do not have a lathe. Schedule40 6” pipe for blades so you do not need a bending machine for making curved blades. The ruuner diamater will be 463mm. running optimally at 667 RPM.
Speed increaser. Again the cheap and most efficient one is to use flat belt pulley, just like a belt conveyor head pulley. Further the set of pulleys aspecially at turbine end also serve as flywheel. When the weight is not enough to reach your calculated flywheel effect then put additional thick plate stiffner inside
Regards