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MPPT wind Master Thesis sepic harmonic mitigation
By domwild, Section Controls Posted on Thu Jun 26th, 2008 at 03:34:52 AM MST
"Max. Power Point Tracking For Wind Turbines inc Harmonic Mitigation"

Just in case our members with a black belt in electrickery have not seen this article, here it is:
http://www.ewec2006proceedings.info/allfiles2/846_Ewec2006fullpaper.pdf
Briefly: Brazilian uni, reference to master thesis by Mr Adegas is listed, 2006, 770 kb 10 page pdf doc.
Even briefer:
V, Amp are measured of one phase, a first-order low-pass filter gets DC part of signal (am not clear on that! - suspect clever averaging), RPM is measured and is entered into look-up table for max. electrical power, output is active power for one phase. Signals are subtracted, error signal drives PWM control of sepic buck/boost converter.
Master thesis gives formulae for inductors, caps, etc. Do not have that. One formula for duty cycle and inductor values is given. 27% more power at 12 m/sec is claimed plus better power factor.
The schematic of the sepic looks like even I could build it, however, from my reading of the wind MPPT holy grail i know it is extremely complex. Being more of a software person i can see a high-end Picaxe could do the entire Volt, Amp, RPM, PWM bit, even the look-up table in ePROM. The Picaxe V, A, RPM + diversion switching schematic and code is given in the excellent forum of www.thebackshed.com by "gizmo".
From further reading i remember reading the operation of a sepic converter is not entirely understood by academia.
But i am probably VERY wrong suggesting an ease of building it otherwise there would be cheap commercial wind (not solar/hydro) MPPT units around.
Best to leave it to our forum members to evaluate and criticise.

MPPT wind Master Thesis sepic harmonic mitigation | 10 comments (10 topical, 0 editorial)

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#1)
by Flux on Thu Jun 26th, 2008 at 12:11:05 AM MST
(User Info)

I have tried this in boost mode but I found no improvement with the low harmonic rectifier. If the input inductors are increased to the point where conduction becomes continuous the thing works as well.

This may be a peculiarity of the air gap ironless alternators. I expected an improved efficiency with a reduction in the harmonics of the rectifier but I couldn't measure it. perhaps with an iron cored alternator it would show a better result.

As I said I used it in boost mode only below the main machine cut in. Under these conditions you still have significant losses from the line unless you use very large cables.

For my buck mode experiments I have used a conventional rectifier feeding a buck converter. It may be that the low distortion rectifier could have some benefit but the experience in boost mode made me think that at least with the air gap alternators the improvement would be small and I preferred to keep the rectifier simple. The improvements from matching the wind power curve and from the reduction in line loss with raising voltage with load are considerable and would probably swamp any gain from a pfc rectifier.

Can't comment on the MPPT bit, this sort of thing is beyond me and best left to the young ones. I can match the load by analogue methods near enough from alternator speed.

Flux

Re: MPPT wind... (3.00 / 0) (#2)
by joestue on Thu Jun 26th, 2008 at 06:02:39 AM MST
(User Info)

Iron cored machines act like a very lossy inductor at switching frequencies.
the entire point of the low pass filter is to store that energy in a low loss ferrite or exotic strip wound iron core.

A trade off occurs between the frequency at which leakage inductance becomes dominant, and machine inductance, which is very lossy above 500hz. for this reason GTO's can be used at 300-500 hz for a first harmonic of 30-60 in large ship dive systems as an "electric transmission" but igbt's at 5khz need carefully designed output and input filters.
This is one of the biggest reasons why 98% efficient grid tie inverters cost so much.

On the extreme side, 500khz converters utilizing the line inductance between the motor and the inverter as part of the system hav been at least experimented with.

Air cored alternators have a particularly high third harmonic, so a conventional rectifier with a large inductor takes advantage of it. whereas to decrease iron loss an iron cored machine is as close to a single harmonic as possible, so when you draw 3rd harmonics form it, a percentage is converted into heat.

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#3)
by herbnz on Thu Jun 26th, 2008 at 11:57:57 PM MST
(User Info)

Hi
I have started to digest the article still long way to go but I have problems with the continuous referance to Power factor. Also I notice that inductors are shown in the equivalent circuit.
The inductance of the generator windings cannot affect the power factor.
The reason is that the only flux that can flow in a generator is rotor flux the current in the windings does produce mmf that opposes this rotor flux but for obvious reasons can not overcome it. The rotor flux induces an emf and if a load is applied current will flow the power factor of this current depends on the load entirly.
The rectifiers and batteries do mean current will not flow until later in the cycle but also at the end of the cycle will stop early. The current and voltage are in step.
however distortion is introducted causing harmonics. Their SEPIC cct is helping here i have no doubt but to refer to pfc is not correct .
Their graphs show V & I in phase to

The article is useful in it gets the brain working.
Herb

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#4)
by Flux on Fri Jun 27th, 2008 at 01:56:36 AM MST
(User Info)

As I see it, those inductors are not the winding inductance of the alternator. For the thing to work as a pfc rectifier, the switching mosfet ( IGBT) need to run in discontinuous mode and the winding inductance even of an ironless alternator is far too large. The capacitors isolate the high frequency switching from the alternator inductance and the small extra inductors are added for the high frequency stage following the capacitors.

When I tried this, I used E core ferrite inductors with about 5 turns on them. With one core off, the inductance let the rectifier work as shown and the waveforms were much as shown for the pfc rectifier. If I fitted the second E, even with a fair air gap, the inductance was sufficient to cause continuous conduction in the mosfet and the rectifier waveforms reverted to that of a conventional rectifier as shown in the other diagram.

All this discussion of power factor is not about the alternator winding inductance, it is about the fact that a conventional rectifier works at about .95 pf lagging, not because of phase shift but because of waveform distortion. With pure sine waves the power factor is caused by a phase displacement between current and voltage as we normally consider it ( cos phi). With chopped waveforms there can be a power factor less than unity even if the current and voltage are in phase. We need then to take the basic definition of power factor as power/VA.

For large installations the reduction of harmonics in the waveforms may be very important when grid connected and elaborate pwm rectifier schemes such as the Vienna rectifier and pwm converter bridges may have to be used to maintain quality of grid supply. For battery charging there is no advantage unless the thing results in lower losses within the alternator. Raising pf from .95 lag to unity will have little effect.

From my experiments I saw no improvement, in fact I think it worked better with the second E core in place. It also worked perfectly without the capacitors, just using the winding inductance, but I was not happy to send chopped waveforms radiating RF all the way up to the alternator via many feet of cable. At least the star capacitors with inductors on the chopped side will reduce the radiated noise, but if you are a radio Ham it may still pose a lot of problems.

Flux

[ Parent ]

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#5)
by herbnz on Fri Jun 27th, 2008 at 09:00:06 AM MST
(User Info)

Hi
Thanks for That Flux. Well explained, I can see that electonic dedevives can cause delay in current hence some PF but as you say slight @ .95. Their sepic cct if giving improvements would be as they title it mitagation of distortion or harmonics.
At some stage I would like to open another related topic to brainstorm what is actually happening when capacitors are placed across the windings but better not highjack this topic.
Herb

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#6)
by Ungrounded Lightning Rod on Fri Jun 27th, 2008 at 04:20:03 PM MST
(User Info)

Here's how I read what they're doing.

First: The capacitors on the output of the genny are to keep the high-frequency switching waveforms out of the alternator. They bypass the high frequency component and the alternator sees the average current. On the other side the switching regulator (composed of the inductors after the caps, the diodes, and the switching transistor) sees three stable voltages that slowly change with the cycling of the mill's generated waveform.

Second: The switching regulator adjusts the load to control the current drained from the generator to track the maximum power point. But the maximum power point tracking is done on the average current over a cycle of the generator's wave. The switching regulator has finer control than that. It can adjust the current in various PARTS of the cycle - producing a leading, lagging, or near-resistive load. So...

Third: The regulator is set up to also control this where-within-the-cycle loading, producing current corresponding to a near-resistive load, which corresponds to a high power factor as seen by the generator. This minimizes the current for a given amount of power pulled, thus minimizing the resistive losses in the generator (which are proportional to the square of the actual current, which (to a sinusoidal first approximation) is the vector sum of the in-phase (real) and 90-degrees-out-of-phase (imaginary) current components.

Just as with ordinary power generation and transmission equipment, running at a high power factor means higher efficiency, achieved by lowering or eliminating I-squared-R losses from out-of-phase currents.

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#7)
by Ungrounded Lightning Rod on Fri Jun 27th, 2008 at 04:24:36 PM MST
(User Info)

Of course this means that the capacitors must be of a particular ballpark value to perform well. Too small and they let enough of the switching waveform into the genny, resulting in rapid current peaks and a resulting increase in I2R loss. Too large and they result in a strongly capacitive load, producing reactive currents that increase I2R loss.

The size of the capacitors depends on the frequencies of the basic generation, the frequency of the switching regulator (also a design parameter), and the voltage/current tradeoff (impedance) of the system. Much of the thesis is about how to calculate this.

[ Parent ]

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#8)
by domwild (domwild at hotmail dot com) on Sun Jun 29th, 2008 at 12:09:48 AM MST
(User Info)

Thanks for the comments. Ungrounded and flux: I understood your explanations of what they are doing.

Sampling is done of V and A within the wild AC. Then the instantenious power is calculated. This is then filtered. Wikipeadia told me what a first-order low-pass filter is doing and even gives me an explanation how it can be digitally simulated:

alpha = dt/ (RC + dt)

for i = 1 to n
y(i) = y(i - 1) + alpha * (x(i) - y(i-1))

where

x(i) are the input samples in watts,
y(i) are the output samples,
dt or deltat is the time difference for samples taken, and
RC is the time constant of the filter.

Questions:

I take it the sample frequency depends on the wild AC frequency. Does Nyquist rear his ugly head here in respect of how often should be sampled?
What is the required RC time constant of this filter once the min. and max. frequency is known??

Thanks.

dom There is one thing money cannot buy: POVERTY!
[ Parent ]

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#9)
by Ungrounded Lightning Rod on Sun Jun 29th, 2008 at 01:20:25 PM MST
(User Info)

Nyquist does show up here. You need to sample more than twice the frequency to figure out the amplitude and phase. Much more is better - to push the aliasing fold up to where the attenuation of the harmonics times the rolloff of the low-pass filter stomps them down well below the signal you're after. Synchronized with the switcher may also be good - to avoid systematic distortion from a beat between the sampling rate and the clock of the switching waveform making it back through the inductors and caps and biasing your measurements.

That's off the top of my head. Don't have time to examine the thesis for how it's dealt with there.

= = = =

Which is how you get the measurement. But what you do with it is a tad more complicated.

Adding load to the genny raises the current and lowers the voltage. But the higher current adds load to the blades that slows them. That changes the working TSR, which changes the angle of attack on the blades, which changes the available power, ...

The max power point will be at a particular TSR for pretty much all wind speeds. This means the RMP (and the GENERATED voltage, before resistive drops in the coils and transmission wiring) at max power will go up with the wind speed. But the available power goes with the cube of the wind speed. So the torque (and the current) will go up with the wind speed squared.

Some max power point trackers work by "hunting" for and tracking the max power point. They modulate the load slightly, at a frequency below the response time of the blade speed, measure whether there's more power at the high or low end of the modulation, and use this as feedback to move the average of the load to chase the max power point as the wind speed changes. That works well when the wind is reasonably steady. But it is problematic when the wind is gusty or the mill is furling.

This one (according to the introductin) uses an apriori approach: It tries to keep the relationship between voltage and current right (current proportional to square of GENERATED voltage) to get the the most out of the mill. That means it has tuning parameters (probably one or two of 'em) that form an estimate of the size and impedence of the mill (diameter, TSR, turn count, field strength) and the resistance of the coils and wiring. This works even when things are gusty. But if the tuning parameters aren't set right it tracks at some offset to the actual max power point and you don't get the most out of the mill. Also: If it overcorrects for the wiring resistance you effectively have a negative resistance, so the system will oscillate. So it must be set either to undercorrect or not correct at all, missing the target somewhat.

[ Parent ]

Re: MPPT wind Master Thesis sepic harmonic mitiga (3.00 / 0) (#10)
by Ungrounded Lightning Rod on Mon Jun 30th, 2008 at 09:56:48 AM MST
(User Info)

This means the RMP (and the GENERATED voltage, before resistive drops in the coils and transmission wiring) at max power will go up with the wind speed.

Of course that should have been "RPM", not "RMP".

[ Parent ]

MPPT wind Master Thesis sepic harmonic mitigation | 10 comments (10 topical, 0 editorial)

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