A question about variable speed (frequency) wind generators
A question about variable speed (frequency) wind generators
(OP)
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 ?
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
www.edison.co.in





RE: A question about variable speed (frequency) wind generators
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.
RE: A question about variable speed (frequency) wind generators
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.
RE: A question about variable speed (frequency) wind generators
Muthu
www.edison.co.in
RE: A question about variable speed (frequency) wind generators
RE: A question about variable speed (frequency) wind generators
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
RE: A question about variable speed (frequency) wind generators
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
www.3rps.com
RE: A question about variable speed (frequency) wind generators
Automotive alternators take care of this issue with a voltage regulator circuit. I imagine that wind generators do as well
RE: A question about variable speed (frequency) wind generators
Curt Wilson
Delta Tau Data Systems
RE: A question about variable speed (frequency) wind generators
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
RE: A question about variable speed (frequency) wind generators
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
RE: A question about variable speed (frequency) wind generators
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.
RE: A question about variable speed (frequency) wind generators
Muthu
www.edison.co.in
RE: A question about variable speed (frequency) wind generators
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
RE: A question about variable speed (frequency) wind generators
RE: A question about variable speed (frequency) wind generators
This double system allows the generator waveforms to be asynchronous to the grid.
RE: A question about variable speed (frequency) wind generators
Muthu
www.edison.co.in
RE: A question about variable speed (frequency) wind generators
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.
RE: A question about variable speed (frequency) wind generators
Welcome back, Muthu:
Why don't you increase the excitation current proportional with decrease of speed?
Wolf
www.hydropower-consult.com
RE: A question about variable speed (frequency) wind generators
Muthu
www.edison.co.in
RE: A question about variable speed (frequency) wind generators
Muthu:
I always forget that I'm a synchronous man.
Wolf
www.hydropower-consult.com