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Controlling the rotational speed of a generator
2

Controlling the rotational speed of a generator

Controlling the rotational speed of a generator

(OP)
Hi,

I am working on a project that would insert a generator/expander system to a pressurized stream of gas in order to generate electricity.

I would like to control the rotational speed of the generator in order to adjust the gas stream flow rate.

Would it be possible to control the rotational speed of a generator through some sort of applied resistance? I am thinking something like an eddy brake.

Additionally, would large amount of resistance applied to the generator result in more power generated?

I appreciate anyone's input.

RE: Controlling the rotational speed of a generator

Any sort of "resistance" implies throwing away some of the hard work that your prime mover (engine/turbine) did in the first place, by throwing away some of the electricity that you just generated (and turning it into heat), so that's not a preferred choice.

What is the nature of the load that you are connecting the generator to? Is it a single light bulb (resistive load), or is it a motor (electromotive load - at which point you also have to ask about the nature of the load that said motor is driving), or is it a battery charger (fixed, or almost-fixed, voltage), or is it the local electrical grid (essentially your generator is a little pee going into an ocean, and it's essentially powerless to do anything about the voltage or frequency), or something else?

What is the nature of load/output-torque control of your prime mover?

Normally in a turbine application, you would use some sort of variable inlet guide vane to redirect the gas flow and/or change the effective area of the inlet nozzle and/or some means of varying the upstream amount of pressurised gas being generated (e.g. by varying the amount of coal you throw on the fire), and let the generator generate what it wants to generate. In other words, backwards to your way of thinking. Use varying upstream pressure/flow by some suitable means to vary the amount that the generator generates, as opposed to somehow trying to work it backwards.

RE: Controlling the rotational speed of a generator

Brian has covered it fairly well.
It may help you to understand if I say somewhat the same things with different words and examples.
A classic example is a recovery boiler and turbo generator.
Recovery boilers run at very high temperatures and pressures.
The steam pressure is too high to be used directly so it is sent to a two stage turbo generator.
In the first stage, some of the steam exits at about 125 PSIG and supplies some of the steam needs of the mill.
In the second stage the steam is dropped to about 30 PSIG and used in the drier heaters.

The generator is tied to the grid and the grid sets the speed and/or frequency.
The output of the turbo generator depends on the amount of steam needed.
As the steam demand varies, the Watts output of the generator varies.
The speed stays the same.

I suggest that you look for ways to control the flow of gas apart from the generator, and use the generator to regain the available energy.

But, that said:
If your project is quite small, and controlling gas flow with variable loading on a turbine is an accepted technique in your field,
this may work:
Connect a load bank permanently to the generator.
Use a PID controller and a suitable transducer to vary the excitation of the generator.

At this point at least one of my friends is pulling his hair out, so I will say what he wants to say:

In place of the generator use an induction motor running as an induction generator.
Connect this to the grid with a fully regenerative VFD.

Vary the speed of the generator by varying the set point of the VFD.
THe induction motor/generator, when over-driven by gas flow will stay within a few RPM of the set point.

You will be able to control the speed quite precisely and recover almost all of the energy available.
(Have I made any silly typos, Keith?) grin

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

RE: Controlling the rotational speed of a generator

Negatory! Right-on! Power to da people!


I'd also say that Dawsonh4 is set up to make a large amount of expensive silly errors from that vague description.

Dawsonh4; You should fill us in a great deal more so we can zero in on a cost effective solution.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Controlling the rotational speed of a generator

(OP)
Thanks for the reply.

Currently this is just a theory that I am trying to test the efficacy of. I have more of a chemical/mechanical background than electrical - I am out of my element. Any and all input is welcome.

The load would be the grid. Selling electricity to the grid much like a back pressure turbine you see at paper mills.

I have researched a few different ways to control the feed going into the turbine/expander, i.e. guide vains, regulators, JT valves, etc. However, all those ways result in energy lost through the mechanical throttle. I was hoping to find a way to waste less energy through the control of the rotational speed of the generator/turbine system. I do understand there will always be a need for control valves.

Waross - thanks for the input. Could you elaborate on the major differences between using a controller to vary the excitation level of the generator vs using the induction generator with a regenerative VFD?

RE: Controlling the rotational speed of a generator

That would be synchronous generator versus induction generator.
Cost and availability.
Depending on the size, the VFD and Induction motor/generator solution is available off the shelf.
The VFD and Induction motor/generator solution easily lends itself to 4-20 ma control.
Pole slip may be an issue with a synchronous generator at low loading.
The induction generator depends on pole slip for re-generation.

The speed of the synchronous generator will be locked to the grid.
The output of the synchronous generator will depend on the energy supplied to the turbine.
Hence some added flow control devices which you do not wish to use.

What size of machine do you anticipate?
As you get into sizes at which synchronous motors are readily available, the cost advantage of the VFD becomes less.

One thing that I do not know, and there may not be enough information given for anyone to answer:
Will a turbo-expander do what you want to do?
What will be the operating limits of the turbo-expander?

Size matters.

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

RE: Controlling the rotational speed of a generator

Quote (OP)

Could you elaborate on the major differences between using a controller to vary the excitation level of the generator vs using the induction generator with a regenerative VFD?
"vary the excitation level of the generator and a resistor bank"
Not easily compatible with the grid.
"the induction generator with a regenerative VFD?"
Close to "Plug and Play".

There was a scheme back in the late 70s, early 80s in the early days of the energy crisis.
Part of the advantage of this scheme was the tax advantages allowed for an energy saving system.
Wound rotor induction motors driving blowers were speed controlled to vary the output volume of the blowers.
Rather than using resistors in the rotor circuit to control the speed, the current in the rotor circuit was controlled by rectifying it to DC and then inverting it back to AC at the proper voltage, frequency, and phase angle to return energy to the grid.
HPs varied from 60 HP to 400 HP.

The concept worked, but the implementation was the stuff that the "Failures and Disasters" forum is made of.
The designer was on site.
A wise old electrician foreman (No, not me. I wasn't that old then. This became part of my education.) suggested to the designer that his choice of a main contactor may be undersized.
The designer scorned and chastised the foreman somewhat brutally, citing his PHD in electrical engineering and the testing that they had done.
("You should know your place and stay in your place.")
Ahh, hubris and too little field experience.

When the contactor in question failed catastrophically, the arcs destroyed the control panel.
The coupling joining the motor to the fan exploded, and parts were found all over the machine room.
The motor shaft was bent.
The fan shaft was bent.
The old foreman (and his crew) got his payback in actual cash, working through a weekend to repair the damage, (at double time wages).

The point of this?
The system was sound and went into service with an appropriate sized contactor.
There is more than one way to harvest energy.
You could spend a lot of money (more than you are thinking) to hire engineers to design electronics for a special case,
or
You can use a regenerative VFD which does all of that, out of the box.

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

RE: Controlling the rotational speed of a generator

What are you expanding?
How much of it are you expanding?
What kind of pressure are you starting with?
Is this expansion to atmosphere or must it remain contained?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Controlling the rotational speed of a generator

(OP)
Thanks again for everyone's help.

It seems to me that there are no clear advantages with a synchronous generator when compared to and induction.

A turbo expander would likely be best for the application. It would have to be appropriately sized to the pressure, flow, and generator variable.

The expansion would be a hydrocarbon gas stream. The volume and pressure can vary greatly lets says an average of 14,000 MMSCFD at 7,000 psi.

The expansion must be contained and knocked down to approximately 800 psi.

RE: Controlling the rotational speed of a generator

What determines the flow?
What is the maximum expected flow?
As I remember the pulp mill turbo expanders, there was a flow control valve integral to the turbine that varied the flow to provide a relatively constant outlet pressure.
How much energy is released when that volume is dropped from 7,000 psi to 800 psi?
If the energy to be harvested is in the range of MegaWatts or tens of MegaWatts, a pulp mill type of turbo expander with a synchronous generator may be the way to go.
Your application may be beyond the capability of off the shelf VFDs.

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

RE: Controlling the rotational speed of a generator

(OP)
Flow and pressure is determined by the underground reservoir. It starts high and begins to decline.

I will work out the math on the energy released.

Would you be able to point me in the direction of a company that manufactures these pulp mill turboexpander generators?

RE: Controlling the rotational speed of a generator

Thanks for the help, Strong.

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

RE: Controlling the rotational speed of a generator

14 BILLION SCFD????

Please check your units.

And pressure.

This sounds like you're trying to use the energy fron a well head??

I think you'll find out soon why no one does it this way.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Controlling the rotational speed of a generator

(OP)
14 MMSCFD

@Stron. Thanks! I spoke to LAturbine, but their expanders have issues with anything over 2.5% water.

@LittleInch. Yes from a well head as stated above. Care to elaborate why it won't work?

RE: Controlling the rotational speed of a generator

Note on the induction generator. The use of a VFD is an unnecessary complication.

If a motor is driven so it's rotation speed = line frequency + slip, electric power to the mains supply will equal motor rating. As the motor is excited from the mains loss of mains supply will result in rapid voltage decay.

However loss of mains supply creates a problem - now the turbine is still powering the motor shaft, but the mains are not absorbing any energy. The result is a very rapid increase of speed.

Even if your arrangement does not need to a governor to regulate generator speed / frequency, it still needs something to monitor rotational speed, and must be able to rapidly close the turbine steam supply valve. Failure to close the steam supply valve will result in rapid disassembly of the system, likely with parts exiting via holes in the roof. Don't ignore the need for an overspeed trip.

https://www.nrc.gov/docs/ML0635/ML063560418.pdf
https://www.aig.com/content/dam/aig/america-canada...

RE: Controlling the rotational speed of a generator

Dawsonh4,

My other fellow posters are much more informed about the generator end of this.

My interest is in the driver end. Your desire to use this energy is laudable, but given that people have been drilling gas wells for 100 years and no one does it this way, kind of tells you there is an issue, mainly it costs too much.

Well fluids are a mixture of gas, saturated water vapour, free water and liquid hydrocarbons. These do not go through high speed axial turbines very well.

Is your 7,000 psi a flowing well head pressure or reservoir pressure or what?
Either way, there is a practical limit on pressure ratio to get to your 800 psi, i.e. too much expansion of the gas. I'm not sure about axial turbines for drives, but in compressors (reverse of turbines), there is a max pressure ratio of about 2.1 So 7000 to 800 in one go is just too much.

Well head equipment is there to control and reduce pressures to that which makes the rest of the system much cheaper to build. Your device may work when flowing, but not when the well is shut in / starting up / controlling flow or pressure, so anything else is extra cost to the equipment already there. High pressure equipment costs a lot of money.

The amount of pressure energy in a gas well is actually quite small, so the generation and money you can get from this generation won't pay for the equipment, especially when, as you say, the pressure declines over a relatively short time.

But the key issue is the extra fluids which come up the well pipe wrecking your turbine. Also as you drop pressure, you drop temperature and even more water and liquids drop out. Turbo expanders are used in CPFs to lower temperature, drop out liquids and then the end of the expander connects to a compressor which pumps the gas pressure back up, just not to the same level or it needs a bit of help from a motor. But this is downstream of filters, separators etc which remove all free sand, dirt and liquids. If you have to stick a separator onto each well head before the turbine that's even more money.

Lots of people have tried to do this and not succeeded.

This lot http://www.energyrecovery.com/isoboost-isogen/ make a good unit for liquid pipelines, but no one so far has succeeded in getting sufficiently large amounts of power out of the hundreds if not thousands of gas pressure let down stations across the world where high pressure gas in transmission lines is let down to lower distribution pressures to make it worthwhile to install the equipment.

Especially at well heads, you can't mess around with the equipment - failure leads to catastrophic damage.

Whilst doing a quick search I cam accross this form 2009 - https://www.theguardian.com/environment/2009/jan/0...

A key paragraph is below and then silence - never happened.
"The idea is not completely new. US companies experimented with turbo expanders in the 1980s and Mercer said a handful of similar efforts have already been set up in Europe. "But this isn't a cheap way to generate electricity. The reason it hasn't really taken off is that it's expensive." "

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Controlling the rotational speed of a generator

(OP)
Thanks FracEngr. If there is no VFD could you explain in simple terms how the rotational speed would be controlled?

RE: Controlling the rotational speed of a generator

In this application, the generator, whether synchronous or induction will be tied to the grid.
The grid will lock the speed of the generator and turbine.
This will reclaim the let-down energy from an electrical point of view (subject to turbine suitability).
This will not control the flow.
The greater the flow, the more energy will be reclaimed.
You must still either let the system run full open or control the flow externally.

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

RE: Controlling the rotational speed of a generator

(OP)
LittleInch.

I am in the same boat as you. Very familiar with the mechanical end - less so with the electrical.

There are people that have tried to do this, but you are correct. Most have not succeeded. Calnetix is a company that may be able to accomplish it.

As you state there are a few issues from a mechanical and cost stand point that need to be overcome:

1.) There are liquids and debris in the flow stream that will need to be dealt with
2.) Pressure limits and knock down ability
3.) Energy harvesting and ROI

Here are my thoughts on the above.

1.) Using an Expander rather than a Turbine would help some of the concerns. Also, still using a JT valve to knock down some of the pressure and a filter/liquid drop system before hand.
2.) The 7,000 psi would be the casing pressure. There are expanders out there that can do a 5 to 1 ratio. If more is needed you can run in series.
3.) I think that a solution to this would be a "skid" that went to wells for the first few months of flow. This is when there is the largest amount of pressure drop and energy to harvest. I believe pay back could be less than two years if power lines are in close proximity.

RE: Controlling the rotational speed of a generator

The steam turbines in pulp mills are multistage to handle higher let-down ratios.

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

RE: Controlling the rotational speed of a generator

Since the question was asked - how do you control the speed of an induction generator, i might get a bit wordy here.

Synchronous generators only need to be speed controlled for two cases
Generator Island-ed
Generator large enough to control the frequency of the network.
in all other cases speed regulation of individual generators is not necessary. Frequency regulation is performed at the system level by sending all of the generators a increase / decrease signal that impacts their throttles.

Induction generators require VARS from a external source to induce an a real power output, so if an induction generator is islanded it looses excitation, and the real power output rapidly decays. Once excitation is lost frequency will drift off of mains frequency.

As voltage and frequency are set by the mains excitation to an induction generator, a change in throttle position results in a change in real power output equal to the change in power input.

How this impacts rotational speed is
When an induction machine is acting as a motor, and a motor is producing torque, shaft speed is slower than synchronous speed, at nameplate speed (usually 2% or so slower than synchronous) the motor will generate full torque and consume nameplate rating amps. When the induction machine is acting as a generator, shaft speed is faster than synchronous, Torque is fed into the machine, and real power is provided to the mains.

Adding a VFD to this mix is only useful for situations where is is not possible for the ideal operating speed to be matched to mains frequency. The most common examples of this are run of stream water turbines and windmills where ideal speed is a function of wind or water velocity.

Your application - capturing a high pressure geological gas source and extraction energy sounds similar to geothermal steam power generation. You might want to look at the turbine service experience of that community.

Fred

RE: Controlling the rotational speed of a generator

It's actually nothing like steam generation as the only energy in the fluid is pressure, not heat.

The OP is looking, I think, to see if there is a way to dial in extra resistance to the expander/turbine and generator set up so that the pressure drop will increase across the expander as the flow rate increase and power goes up.

Personally I think that flow of the gas is your primary control as you indicate.

There are many other issues in trying to do this from the driver end and whilst it CAN work, nobody has yet made a commercial version tat generates sufficient power to make it worthwhile buying the machine. Plus the gas will get VERY cold as it exits the machine.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Controlling the rotational speed of a generator

"It's actually nothing like steam generation as the only energy in the fluid is pressure, not heat."

I feel the need to nit-pick that statement. The work extracted is due to delta(PV). The role of heat is only to increase the pressure and volume of the steam compared to water. The difference between steam and water in energy content is the far greater volume of steam. I'm sure there is a more elegant way to say this.

RE: Controlling the rotational speed of a generator

If you care to nit pick, both turbines are First Law energy conversion devices.
If you feed a relatively cold gas stream into a positive displacement motor - like is used in an air tool, the gas stream exits the device only a bit cooler.
If you use an expander, more work is extracted, but the exiting gas stream is much colder. Significant risk of ice buildup as geologic wells almost always produce saturated gas streams.
The need to keep the gas above condensing temperature is a good reason to heat the gas before it is expanded with energy extraction = unless the goal is refrigeration or making snow.

This is a bit off course for a discussion on turbine speed control.

Fred


RE: Controlling the rotational speed of a generator

A grid connected synchronous generator speed is controlled by the grid frequency.

The frequency of a grid connected generator is controlled by the grid frequency.

Different case, using a turbine generator to control the flow of the driving gas.
The frequency of the induction generator is controlled by the regenerative VFD output frequency.

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

RE: Controlling the rotational speed of a generator

Fred,

I knew I was missing something before - the issue here is the motive fluid for the turbine (A gas well) and how to control it.

The "snow" you refer to is gas hydrate and if you don't de hydrate the gas before you stick it through an expander, you're going to get rather a lot of it very fast. Hence another reason why it's not done.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Controlling the rotational speed of a generator

My first experience with making snow was with wet air, freezing weather, and air tools. The result is about 2 minutes run time, then a trip to the heater to thaw the tool out (and the fingers). It is an experience that you don't forget.

Fred

RE: Controlling the rotational speed of a generator

(OP)
All,

Thank you for the helpful back and forth. I think the project is now much better understood than I had originally posted. The primary goal is still to control a variable inlet pressure gas stream.

The examples of the geothermal steam turbine is applicable. As Compoitepro stated - it is a fluid dynamics problem and not so much a thermo problem. Admittedly there are thermo issues as play here as the gas will get very cold due to increased potential energy (JT effect) and there is some moisture content.

The way Fred explained things is very helpful. It sounds like the best way to do this is to start with a induction generator and a VFD. As he said - this is similar to a wind turbine where you can really control input power.

I was reading that wind turbines also might do an AC-DC-AC conversation, but I get a little lost as to the reasons behind that.

RE: Controlling the rotational speed of a generator

I tried to explain, but I will try again.
Your goal is not to control the speed of a generator, your goal is to use a turbine generator to let down gas pressure.
You don't care what speed the generator works at as long as the system works.
Just use an induction motor.
Start to slowly admit gas to the turbine so that the motor accelerates slowly.
When the motor is close to or at synchronous speed, close the switch and go DOL. (Direct-On-Line)
Or, in other words, connect the motor to the grid.
The grid will now control the speed of the motor (induction generator).
Now the more gas flows through the turbine, the more energy will be delivered to the grid.
Your utility has strict rules and requirements for exporting power.
You may find it easier to use a scheme that supplies your own energy requirements but avoids exporting energy.
Thinking out of the box, you may use electric heaters to use some of the generated energy to re-heat the gas stream.
Disclaimer: My expertise is the electrical side. Others have and will comment on the do-ability of your project on the turbine side.

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

RE: Controlling the rotational speed of a generator

Bill's right, definitely try to use the power locally instead of exporting it. Especially in your case where the energy source will dry-up in a short period.

Use it to pump oil, or compress processed gas, or even to make valuable ice.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Controlling the rotational speed of a generator

I think we got a two-fer going here Keith.
As well as electric energy we have negative energy in the form of very cold gas, that can be used to increase the efficiency of the ice machines.
Think pre-cooling the water and cooling the refrigeration condensers.

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

RE: Controlling the rotational speed of a generator

Study gas hydrates a bit more before you go much further. Otherwise your system will run for about 5 minutes then block up.

Very cold methane and free water is a very bad combination.

Gas wells have been around for the best part of a century. If it was possible to do this then it would be done by now.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

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