A question about variable speed (frequency) wind generators 1

[edison123]

The set-up for a variable speed (variable frequency) wind generator is

Wind generator - Active rectifier - Inverter - Line Filter - Trafo - Grid.

Question - Since the generator speed is varying, the generator output voltage also varies. How does the inverter then ensure that output voltage is constant, given that an inverter output voltage cannot exceed its input voltage ? Or is my last theory about inverters wrong ?

Muthu

http://www.edison.co.in

 

[FreddyNurk]

I'll have a guess (and call it that) others can shoot my theory down if required.

As I understand it, the reason on why an inverter cannot produce more than the input voltage is that unless you use a different topology to that normally used in 3 phase inverters, you can't switch the DC bus higher than what you'd find at the input, as the DC bus level is set by rectification of the input 3 phase.

I assume in all of the above, that they don't use some sort of synchronised boost converters to boost up the DC to something else.

That said, theres also the assumption inherent that the wind generator operates at the line voltage. Theres every chance its a 690V generator for a 415V L-L system or similar. Theres also the chance that its some other strange voltage and frequency as well.

Whilst I've been involved in an installation that included an Enercon turbine, I'd never managed to get that involved in the detail.

 

[LionelHutz]

It's not a controlled rectifier, it's a 3-phase inverter section.

Think about a standard typical low voltage VFD and specifically the operation of the inverter section between the DC bus and the motor. This inverter section is capable of producing any AC voltage between 0V and the DC bus voltage divided by sqrt(2). This inverter section can also produce any frequency between 0Hz and it's maximum operating frequency. This inverter section is also capable of running with the motor in regen, where the motor is acting as a generator and pushing power back into the capacitors.

So, the inverter section between the wind generator and the DC bus works similar to any other VFD inverter section. The generator is running in regen mode. The inverter section tracks the frequency and voltage while loading the generator and moving power into the DC bus.

 

[edison123]

Thanks Lionel. My point was that at low speeds, the generator voltage comes down. So the dc bus voltage comes down and correspondingly the inverter ac output voltage. But we need a constant rated voltage to synch with the grid. So, how does the inverter get that constant rated voltage when the generator speed is low ?

Muthu

http://www.edison.co.in

 

[OperaHouse]

I saw one wind generator where they fed AC into the rotating field. After a very minimum rpm it could create any frequency and voltage out. Think of it as a rotating transformer under microprocessor control.

 

[cswilson]

A couple of months ago, we had a question on this forum as to how a braking electric vehicle could continue to charge the battery pack as it slowed down and the back EMF generator voltage reduced. It's fundamentally the same question, because it's the same circuit concept. (Last year, we were able to use an electric vehicle controller as the prototype controller for a wind turbine generator.)

The key to understanding what is going on is to realize that the back EMF is not the only source of voltage. With a switching (e.g. PWM) inverter stage, when a power transistor turns off, the winding inductance can push the motor terminal voltage above the DC bus voltage for a fraction of a cycle, allowing current to flow through the flyback diode from the terminal to the bus.

It's a concept that's not intuitive to most people. When I got into the field, I had to sit down with a circuit diagram and timing table and work through step by step what happened in a PWM cycle before I really understood what was going on.

Curt Wilson
Delta Tau Data Systems

 

[mrspcs]

Don't know whether this will help you but it helped me to visualize what's happening in windgen and your question in particular (variable rpm by constant voltage up to the grid). I'm am a ChemE but have done work with VFDs in connection with Process Control applications. From that experience, this is how I understood the windgen case.
Think of an active-front end VFD (one that can feed energy back to the grid): Grid / Trafo / Inverter-DC link-Inverter / Motor. With the motor in 'motoring' mode (e.g. driving the load) you have constant Voltage fed to the VFD but you may have different motor speeds (e.g. different output frequency from the Inverter at the motor side).
In windgen, with the motor in 'generation' mode (e.g. load driving the motor) the principle is just the same but viewed backwards: different motor speeds (e.g. different frequencies) but constant voltage out of the inverter on the line side.
Again, this was just a way for me to view it to facilitate understanding the devices. I never required going deeper into the circuits and electric principles involved.

All the best



MS

http://www.3rps.com

 

[Compositepro]

"Since the generator speed is varying, the generator output voltage also varies."

Automotive alternators take care of this issue with a voltage regulator circuit. I imagine that wind generators do as well

 

[cswilson]

It is probably helpful to understand that the windings of a motor/generator attached to an inverter stage are not fixed to any absolute voltage. Depending on which of the power devices are on (closed) and which are off (open), they can be working around the +DC bus or the -DC bus and can switch between the two levels every switching cycle.

Curt Wilson
Delta Tau Data Systems

 

[rbulsara]

Not an WTG expert, but if the inverter is connected to a grid, its voltage and frequency will be fixed by the grid. So if the WTG is not producing forward power, it will shut off.

As for the variable speed of the WTG, they produce useful power over very narrow range of "sustained" wind speed. Within that range the actual alternator speed is controlled (maintained) by varying the pitch of the turbine blades and their direction and, I presume, some gearing. Generator is taken off line, if wind is outside the range of operating speed (both low and high limits).

For off-grid inverters, they would need storage batteries and voltage regulator.


Rafiq Bulsara

http://www.srengineersct.com

 

[cswilson]

Rafiq -- The whole point of an inverter in a wind-turbine generator is to electrically separate the generator from the grid, permitting it to operate at independent voltage, frequency, and phase. This provides a lot of advantages:

It allows the production of useful power over a much wider range of sustained wind speeds, even permitting generation when generator terminal voltages are less than grid voltages (the non-intuitive aspect that was responsible for the original question).

For wind transients (gusts and lulls), it means that hard grid synchronization is not required, so the response can instead be optimized for maximum power and minimum mechanical stress.

Since you don't need even on average to produce 50 or 60 Hz from the generator, you are starting to see designs without a gearbox between turbine and generator, eliminating the biggest reliability issue.

The issues you mention are for the older inverterless designs. The justification for adding expensive inverters is to eliminate these issues.

Curt Wilson
Delta Tau Data Systems

 

[sibeen]

The active rectifier takes care of all your problems.

Hopefully I' not talking out of my arse here and 'active rectifier' means a fully controlled PWM controlled IGBT (or similar) style rectifier rather than the older line commutated thyristor models.

A PWM rectifier can boost the incoming AC voltage to a required DC link voltage. So within certain input tolerances the DC link voltage will remain constant.

 

[edison123]

Thanks sibeen. Any informative article on how the active rectifier works, especially the voltage boosting part ?

Muthu

http://www.edison.co.in

 

[rbulsara]

cwilson:
We are talking about two different inverters in the system then. One that physically connects to grid has to be in sync.

Rafiq Bulsara

http://www.srengineersct.com

 

[sibeen]

edison, I hope this somewhat simplistic explanation helps get the general idea across.

 

[cswilson]

Rafiq -- Yes, in a sense it is a double inverter with the two sides (one to the generator, one to the grid) tied together through the DC link. In a purely generating system, some will call the side tied to the generator an "active rectifier, but it is fundamentally the same as an inverter if it does more than simply turn whatever AC is present into DC of the same level.

This double system allows the generator waveforms to be asynchronous to the grid.

 

[edison123]

Thanks again sibeen. I think I am getting the hang of it.

Muthu

http://www.edison.co.in

 

[LionelHutz]

I think there are 2 different things being discussed here.

The active wind inverter chain goes something like this.

Generator -> Inverter1 -> DC Bus Caps -> Inverter2 -> Grid

The pdf sibeen linked describes how the inverter could boost the DC voltage so it's slightly higher than the AC incoming voltage. I'm not an expert on wind inverters but I don't believe this is what is occurring at all. These wind double inverters do not start operating by using inverter1 to charge the caps.

Say you're starting from a dead state with the turbine brake on. The inverter2 will basically charge the DC Caps to sqrt(2) x the grid RMS voltage. At this point, the unit could run if there is wind.

The generator will most likely be a permanent magnet type. Due to this, it will produce a "wild" 3-phase AC which varies linearly in both frequency and voltage with speed. This generator is designed so that at maximum turbine speed it will produce a voltage < the grid RMS voltage. It will also produce a frequency < the inverter1 maximum operating frequency.

So, the brakes are released and the turbine begins to spin. The generator begins to produce an output voltage and frequency. The inverter1 will track this output and it in turn will output (towards the generator) a slightly lower voltage and frequency. By doing so, inverter1 attempts to slow down the generator. In turn, this loads the wind turbine and energy is transferred from the generator to the DC bus.

So, the simple fact that inverter1 can simulate any voltage and frequency on the generator side is the way it loads the generator. Inverter1 puts the generator into regen mode the exact same way a motor on any modern VFD regens when you use the VFD to slow down the motor. Inverter1 is not boosting the generator voltage to the DC bus voltage level. I's just say inverter1 simply causes a net current flow towards the DC bus caps and this charges the capacitors. Remember, this is an AC circuit and you don't necessarily need a positive voltage to cause a positive current flow.

The DC bus voltage level is set by inverter2 since inverter2 is the line connected inverter. Inverter2 is used to hold the DC bus stable by moving energy to the power grid, or, when required, using grid power to charge the DC bus capacitors.

 

[wolf39]

Welcome back, Muthu:

Why don't you increase the excitation current proportional with decrease of speed?

Wolf

http://www.hydropower-consult.com

 

[edison123]

Thanks wolf. This is an induction generator.

Muthu

http://www.edison.co.in