I am looking to build a small, 25kW, hydro generator and need help understanding the electrical part of the installation. I want to use an induction motor as the generator. This is to be a grid tied system. The grid at this location is single phase.

If I understand correctly, this can be done so that in the event of power outage, the generator will automatically stop generating. Is this true? Islanding is not allowed. Will any dump load be required?

If power goes down I can figure out how to shut down the turbine. Restarting it would be a manual process. Syncing to the grid becomes a question, one that freaks out any electrical person I have talked with, most swearing that it will take elaborate, expensive equipment to accomplish. IF this is the case, my plan to restart the generator would involve starting it as a motor (no water flowing yet) and once it is up to speed opening the jets on the turbine to push current the other direction. This process should sync the system to the grid, right? Will the generator/motor act as governor for the system?

Motors large enough to use as the generator (30 or 40 hp) are invariably 3 phase but I only have single phase on the grid. I need an efficient way to change the 3 phase output of my generator to the single phase on the grid. How can this be done?

Lastly, if I have ~25kW of mechanical energy on the turbine, how big of a motor should I be looking for.

Finding a competent EE for this job has been a chore and one not completed to satisfaction. The power company isn’t much help either. Any recommendations for consultants? Just FYI, I am an ME by trade.

 

A lot of the answers to your questions have to come from the utility you will be connecting to. Many have very restrictive rules and regulations about any generation that's connected to their grid. They will likely require one or more protective relays of one type or another.

As for synchronization, you won't need to have any sophisticated equipment for that. You will start the electric machine as a motor, as you have stated, and it is then "synchronized" to the grid. When more torque is being applied to the electric machine than it is producing (by opening the jets) it will mysteriously and automatically become a generator.

I would venture to say that the cost of engineering a solution that meets the rules and regulations required for connection to the utility grid will far outweigh the income you would derive from the utility. Some even require bonds or insurance in the amount of millions of US Dollars to connect any generator to their grid.

You might find a EE (PE, would probably be best) who would take the job on as kind of an exercise, but you will likely be doing a lot of the work to find someone in the utility who can say for sure what the rules and regulations and requirements are for connecting generators to their grid.

Many friends and colleagues of mine have installed battery banks (usually constructed of used tow-motor (lift truck; fork lift) batteries to store the electricity for use on some circuits in their home or small business, this to avoid the cost of connecting to the grid (protective relays; bond/insurance; etc.).

Please write back to let us know how you fare with your endeavour.

 

I am not to sure that an induction motor is the right way to go. If I remember correctly there have to be capacitors added to all three phases of the three-phase motor in order to operate it as a generator. A single phase induction motor can be converted to generator in the same fashion. To try and find a single phase motor of the size needed to produce 25KW when operated as a generator would be very difficult at best.

I am not sure but I don’t think you can run the motor as a motor while the added capacitors are in the circuit. I am sure you could rig up some sort of contactor to cut the capacitors in and out of the circuit but I am not sure about the motor staying synced to the grid during that process. The other draw back to using an induction motor as a generator is that it is very limited as to the size of an induction motor that it can start. A normal rule of thumb is that a 1 hp induction motor when operated as a generator only has enough capacity to start a one 1/10 hp induction motor.

If I were trying to do what you want to do I would start looking for a gas powered generator/welder with a trashed motor. It should already be set-up for single phase 220 volt AC power. It is designed to handle heavy loads and sudden load changes. It also has some protective relaying in the form of a circuit breaker. Most have a volt and amp meters already wired in as well.

As CSA stated there will have to be some protective relaying installed. At the very least you will have to install an under-voltage relay to protect the generator during power outages. That relay can be part of the over-speed protection as well. The problem you are going to have is protecting the generator from voltage spikes such as from near by lightning strikes. The good news is that there are a lot of sites that have converted from the old style electro-mechanical protective relaying to digital protective relaying so it shouldn’t hard to find a lot of the old style relaying.

Syncing to the grid is not that difficult. A simple set up with two 110 volt lights is a crude but effective way of doing it. If you use the search feature on this website you can find a number of threads concerning syncing.

Thanks
Mark Allen

 

umm... at the risk of raining on your parade, I see a lot of problems with this.

You are right to think of starting the generator as a motor, as that wouldn't require any special synchronization. The process of making an induction motor into an induction generator is just like CSA said, add more torque to the rotor while it is spinning and the terminals are connected to a source. All you need is to get the rotor to spin faster than synchronous speed and current will start to flow out of the machine. And because the induction machine doesn't source it's own reactive power, the moment voltage is lost, the generator stops generating, no load dump required.

Theoretically, undervoltage protection would need to be addressed, but that's not your biggest problem...

Making a single phase motor out of a three phase motor is the main deal breaker. You would have to somehow phase shift the single phase into three phases that are 120 degree apart because of the physical properties of the motor and how the stator is wound. I can't say that its impossible, but I have no idea how that would work.

Getting the rotor to spin faster than synchronous speed is probably another deal breaker. Unless you have some fancy gearing mechanism or transmission system, this is going to be tough. I think most motors are going to be rated at 1800 rpm. That means you need to get the machine rotor to spin faster than 1800 rpm in order to generate power. Large scale hydro turbines are synchronous machines with many poles, so their mechanical speed can be much slower than a traditional 4 pole, 1800 rpm machine. Most small scale hydro, and the majority of wind power, rely heavily on power electronics to eventually convert the generated power to a form suitable to be feed into the grid.

And all this ignores the regulations your local electricity provider may have.

Again, I can't say it's impossible, but I will say good luck, you're going to need it.

cheers,
nic

 

Nic, You and CSA are correct.

I had assumed (I know...it is always a bad thing to do.) that the fellow would want to be able to operate as an island if he had an extended power outage. That is why I mentioned adding the capacitors in order to operate the induction motor as a stand alone generator.

Thanks
Mark Allen

 

nic raises an excellent point about the rating. 25kw is approximately 34 horsepower; that would be a <b>lot</b> for a single phase motor. I think that's what MarkAllen was trying to allude to.

 

Mark,

Even with capacitors, there would be no source of magnetizing current without the terminals connected to a voltage source. He could rig up some sort of doubly fed induction generator (DFIG) like the ones popular in wind farms today, but that gets back to the whole power electronics challenge.

All that said, if he were to hook up a single phase of the three phase motor and then start pushing a lot of water through a turbine connected to the rotor, I'm not really sure what would happen. I have a couple guesses, none of which end triumphantly.

DG1953,
I didn't mention this in my last post, but thought it might help you understand why synch check is unnecessary when using induction generators.

Induction machines operate on the principle of slip, meaning the rotor circuit is electrically and mechanically slipping past the stator circuit because they are operating at different frequencies. Unlike a synchronous machine where a direct current source excites the rotor field circuit, the rotor circuit of an induction machine is not energized by any external source, and instead its rotor windings are shorted together (except type3 and 4 machines (aka DFIGs, as mentioned above), mainly used in wind farms, but I won't get into that right now).

As the current from the source rotates around the stator winding, flux passes out of the stator circuit, through the airgap and cuts through the rotor windings. This induces a current in the rotor. This creates a flux linkage between the stator and rotor and causes the rotor to start turning because it is being pulled around by the flux linkage between the stator and rotor. As long as the source connected to the stator continues to rotate around the stator faster than the rotor is rotating (so its flux lines cut through the rotor winding) the rotor will continue to rotate. We call this "slip" - the stator field is slipping past the rotor, and in doing so the flux lines are cutting though the rotor windings to create a flux linkage between the stator and rotor.

This is why there is no such thing as synchronism in induction machines. If the induction machine rotor ever rotated at the same speed as the rotating stator field, the flux lines cannot cut through the rotor windings to induce a current in the rotor (because they are rotating at the same speed, they are stationary with respect to each other), and without the rotor current the flux linkage between the stator and rotor would cease to exist, and the rotor would slow down because nothing is pulling it around. And the moment the the rotor slows down, we start to get slip, and that causes the stator flux lines to cut through the rotor windings, which creates the flux linkage and starts to pull the rotor... So this is a long way of saying induction machines don't need to be synchronized to a source.

Not sure if that helped or made things worse, but I thought I would give it a try.

Cheers,
nic

 

Nic you might find this interesting. I began reading about doing this a long time ago when I was thinking about micro hydros.
(http://praveshkafle.webs.com/apps/blog/show/5141619-how-do-you-convert-induction-motor-to-generator-)

There are a number of articles and how-to sites on the web that discuss it. My neighbor had a home-made generator built from a 1/2 hp induction well pump motor that was designed to run at 3600 and an old Briggs and Stratton engine. The only problem was that while it worked great for lighting loads or motors with brushes but it was very limited when used to start/run induction motors.

 

That is interesting. They are relying on the rotor being magnetized from previous operation. It's like a permanent magnet induction generator! Very clever. And that does explain why it can't support more inductive loads.

Cheers for that, Mark!

 

I have been running small hydro plants for years and what you want is not a big problem at all.

Can you please let me know what kind of turbine you intend to use and what the head is?

What you basically do is drive an asynchronous motor at over synchronous speed as mentioned before. With a Francis this usually means that you find an 8 pole and drive that with a flat belt.

Forget about a single phase motor. You will not find one for that speed, but a 3 phase motor will be easy to find and you can convert three phase to single phase with a transformer for that purpose.

The electrical system is really easy. The only thing you really need is a relay between the motor and the grid that falls off when the power fails.

The only problem I see is how you are going to feed back 25 kW through a single phase, or do you want to run it covering your own power consumption?

If the main power fails the generator will stop producing power and the turbine will speed up.
This normally is not a problem, because efficiency decreases with speed and becomes 0% at approx. 1,6 x nominal speed. In most cases that is no problem.

Cheers,
Hubert