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Power Generation Riddle No.7 - Upgrading the old Francis turbine
I own an old watermill in France.  I am in the process of upgrading the old Francis turbine.  I am thinking of installing a new Single Regulated Kaplan Turbine (variable wicket gate, fixed pitch runner (propeller)).

Looking at the efficiency (hill) graphs for similar turbines it seems that there would be considerable gain in efficiency by operating the turbine at variable speed to accommodate the variable water flow and the variable head.  Head is nominally 2.5 m (min 2.0 m, max 3.0m); water flow is up to 10 cubic meters per second.

Grid tied inverters (400 V, 50 Hz) are available to take the variable frequency and variable voltage AC generated by Wind Turbines.   I am thinking of using a Permanent magnet Generator (PMG) directly connected to the turbine - no gears or belt drive, less loss, less noise, more stable turbine operation etc.

I have two main questions:

(a) are there links on the web to theoretical and empirical studies of the benefits of using variable speed generators for Single Regulated Kaplan turbines on small hydro projects?

(b) is it possible to obtain inverters that operate with hydro turbines?

To operate the turbine at the optimum speed for the desired water flow it must be necessary to vary the torque at the generator shaft.  To vary the torque it must be necessary to vary the electrical load applied by the inverter.  How is this done?

Is it possible to program the inverter to optimize the speed by continuously hunting for the optimum speed for maximum electrical output?  Is this software algorithm available online?

Thank you in advance for any pointers.

Moulin De Aillevans   (Latitude, Longitude)       (47.586256,  6.431401)
Author : Tony Murtagh - From: Irland - living in France
Sun, March 27th, 2011 - 02:04
Generally there are some small or micro hydro elelectric system type as following:

Off-Grid Battery-Based Microhydro-Electric Systems

Most small off-grid hydro systems are battery-based. Battery systems have great flexibility and can be combined with other energy sources, such as wind generators and solar-electric arrays, if your stream is seasonal. Because stream flow is usually consistent, battery charging is as well, and it´s often possible to use a relatively small battery bank. Instantaneous demand (watts) will be limited not by the water potential or turbine, but by the size of the inverter.

Off-Grid Batteryless Microhydro-Electric Systems
If your stream has enough potential, you may decide to go with an AC-direct system. This consists of a turbine generator that produces AC output at 120 or 240 volts, which can be sent directly to standard household loads. The system is controlled by diverting energy in excess of load requirements to dump loads, such as water- or air-heating elements. This technique keeps the total load on the generator constant. A limitation of these systems is that the peak or surge loads cannot exceed the output of the generator, which is determined by the stream´s available head and flow. This type of system needs to be large to meet peak electrical loads, so it can often generate enough energy for all household needs, including water and space heating.

Grid-Tied Batteryless Microhydro-Electric Systems
Systems of this type use a turbine and controls to produce electricity that can be fed directly into utility lines. These can use either AC or DC generators. AC systems will use AC generators to sync directly with the grid. An approved interface device is needed to prevent the system from energizing the grid when the grid is out of action and under repair. DC systems will use a specific inverter to convert the output of a DC hydro turbine to grid-synchronous AC. The biggest drawback of batteryless systems is that when the utility is down, your electricity will be out too. When the grid fails, these systems are designed to automatically shut down.

But about your technical question :

1-A dump load is used for electrical energy producing and consumption balancing. it is an electrical resistance heater that must be sized to handle the full generating capacity of the microhydro turbine. Dump loads can be air or water heaters, and are activated by the charge controller whenever the batteries or the grid cannot accept the energy being produced, to prevent damage to the system. Excess energy is "shunted" to the dump load when necessary.
2- Inverters transform the DC electricity stored in your battery bank into AC electricity for powering household appliances. Grid-tied inverters synchronize the system´s output with the utility´s AC electricity, allowing the system to feed hydro-electricity to the utility grid. Battery-based inverters for off-grid or grid-tied systems often include a battery charger, which is capable of charging a battery bank from either the grid or a backup generator if your creek isn´t flowing or your system is down for maintenance.
In rare cases, an inverter and battery bank are used with larger, off-grid AC-direct systems to increase power availability. The inverter uses the AC to charge the batteries, and synchronizes with the hydro-electric AC supply to supplement it when demand is greater than the output of the hydro generator.
The function of a charge controller in a hydro system is equivalent to turning on a load to absorb excess energy. Battery-based microhydro systems require charge controllers to prevent overcharging the batteries. Controllers generally send excess energy to a secondary (dump) load, such as an air or water heater. Unlike a solar-electric controller, a microhydro system controller does not disconnect the turbine from the batteries. This could create voltages that are higher than some components can withstand, or cause the turbine to overspeed, which could result in dangerous and damaging overvoltages.
Off-grid, batteryless AC-direct microhydro systems need controls too. A load-control governor monitors the voltage or frequency of the system, and keeps the generator correctly loaded, turning dump-load capacity on and off as the load pattern changes, or mechanically deflects water away from the runner. Grid-tied batteryless AC and DC systems also need controls to protect the system if the utility grid fails.
Author : Hamid - From: Iran
Tue, April 19th, 2011 - 00:21
I have come across some variable frequency motor controllers that have regenerative braking.

In regenerative braking mode they can act as variable torque inverters to connect to the electrical grid (400V, 50Hz).

The challenge is programming the motor controllers to vary the torque on the generator and thus on the hydro turbine to optimize the power output.

Does anyone have experience of programming a 200 kW motor controller in regenerative braking mode ?

What is the best control algorithm ?  I was thinking of continually changing the torque up and down very slightly to try to find the top of the power curve for a given water flow (wicket gate setting) through th hydro turbine.

  Does this correspond to the point of optimal turbin efficiency for a Kaplan turbine ? 
Author : Tony Murtagh - From: Ireland
Tue, April 19th, 2011 - 14:55
Your concerned subject is similar to wind turbine generator as you can see in following figure. An optimum wind power generation system with a cage-type machine and two-sided PWM voltage-fed converter is shown in figure below. The machine excitation is supplied by the PWM rectifier, where the excitation current ids maintains the flux constant. Currents ids and iqs are controlled by vector control within the speed control loop, and the speed is programmed with the wind velocity Vw to extract the maximum output power. This means that the optimum operating speeds are ωr1, ωr2, ωr3, and ωr4 for wind velocities Vw1, Vw2, Vw3, and Vw4, respectively, as shown in the figure. Of course, the wind velocity requires monitoring for the control. The line-side converter is responsible to maintain the dc link voltage Vd constant as shown. The line phase voltage waves va, vb, and vc are sensed, and the corresponding co-phasal line current commands ia*, ib*, ic* are generated by multiplying them by the output of the Vd control loop as shown. The phase currents are then controlled by HB PWM current control. When the turbine output power tends to increase the dc link voltage, the line currents tend to increase so that a balance is maintained between the line output power and turbine power.

Author : Hamid - From: Iran
Tue, April 26th, 2011 - 02:52
There are similarities with a wind turbine.   The wind velocity is monitored and the torque and turbine speed is obtained from a previously defined look up table.

I would like to use an adaptive algorithm for the hydro turbine.  

Perhaps every 5 minutes alter the torque and hence the turbine speed very slightly to see if the generated power increases or decreases.  If the speed is optimum at the top of the efficiency curve the slight increase and decrease in speed should both cause a slight drop in output power.  Otherwise the speed needs to be increased or decreased to achieve the optimum and the test performed again.  In essence a finite differentiation is performed to determine the maximum.

Has anyone experience of using such an algorithm with a variable speed hydro turbine ?

Author : Tony Murtagh - From: Ireland
Tue, April 26th, 2011 - 16:41
Graph of Power versus Speed for a reaction turbine (Francis and Kaplan)

It would appear that optimizing the electrical power generated for any particular gate opening (water flow) would also optimize the efficiency.

If this is true it would greatly simplify the controls - no need to know the exact position of the gate opening, the water flow through the turbine or the head.  

The firmware in the inverter could simply periodically optimize the torque on the generator shaft to optimize electrical power generated.

This may only be possible on a hydro turbine because of the relatively  slow response time of the turbine and the slowly changing river conditions.  A wind turbine must respond much more quickly to changes in wind speed and would not have time to optimize the torque.  A predetermined  lookup table of turbine performance would be used.

Would such an optimizing scheme work for a Kaplan hydro turbine ?

Has anyone got experience of optimizing the power from a variable speed hydro turbine ?
Author : Tony Murtagh - From: Ireland
Thu, April 28th, 2011 - 17:52
I can give our references of the variable speed to hydro application: we manufacture both generators and static converters.

I understood that your moulin is rated near 200 kW
we did application with similar application, 300kW, 600kW with permanent magnet generator water cooled, driven by inverter and coupled to the national grid by an IGBT active front end converter.
the "hill" curve is Q(m3/s) vs H(m), water flow versus head; you can try to find the best efficiency by variating one of them.
The algoritm should be fixed by the turbine manufacturer, who knows the "hill" curve of its runner profile.
You supplied a feedback of speed (or torque) to the inverter.
The inverter "brakes" the generator in order to match the speed.
The active front end convert to the grid the power produced at the nominal voltage (400V, 690V, MV...)
The applications are with Pelton, with variable head (500-600 rpm), S-Kaplan variable pitch with variable head (180-230 rpm),. ..

I remain
Author : Agostino Valentini - From: Italy
Sat, April 30th, 2011 - 11:46
Thank you for the update Agostino.

Getting an accurate efficiency graph ("Hill curve") from a hydro turbine manufacturer is not easy.

It may be necessary to determine the efficiency graph experimentally on site after the turbine is installed.

This also allows for site variations such as the design of the Draft Tube for the water exit from the turbine.

I would like, if possible, to have the inverter automatically adapt to the operating conditions, i.e. vary the torque on the generator and hence on the turbine to find the optimum operating turbine speed.

For a fixed pitch Kaplan hydro turbine (propeller turbine) I believe that the point of maximum power production is very close to the point of maximum turbine efficiency - for a given gate opening.    Is this true ?

Has anyone used an inverter to vary the speed of a Kaplan turbine to determine this ?
Author : Tony Murtagh - From: Ireland - living in France
Sun, May 8th, 2011 - 17:26
Some Motor Controllers can provide braking by means of Regenerative Braking instead of dissipating energy in a Braking Resistor.

An inverter in the motor controller feeds electrical power back into the electrical grid.

Has anyone got experience of programming a Motor Controller in regenerative braking  mode ?

I am particularly interested in varying the braking torque to vary the speed of the generator driven by the hydro turbine (Kaplan).

Can anyone recommend a 200 kW motor controller with regenerative braking and a programmable front end to control the torque on the motor / generator ?

Supplementary question: how do motor controllers with regenerative braking handle  network failure while braking  ?  Do they have anti-islanding capabilities to protect network workers ?  - or is external anti-islanding equipment required ?  If so, can anyone recommend such equipment ?

Author : Tony Murtagh - From: Ireland - living in France.
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