Do any MPPT for wind turbines have PFC  - Page 1
Forum section: electronics / projects

author: qwaarjet
author_rank: Regular Contributor
timestamp: April 19, 2021, 04:47:47 am
content: So I've started looking into building a wind turbine setup as part of building hybrid micro grid( 60Kwh, wind, solar, diesel) in a remote location. Two main problems one the market is saturated with cheap Chinese junk so finding good quality is pain (any recommendations?)The other is, as far as I can tell no max power point trackers (MPPT) below the 100KW range seem to have any power factor correction (PFC) for the turbine, pretty much everyone just uses a basic 3 phase diode rectifier and then draws the switching converter off the wild DC bus. This seems like a fairy inefficient way to do things especially for small systems where every watt counts. I would just through a PFC correction module in, but most are optimized for mains voltage and frequency.So what am I missing? Are common MPPTS way smarter than I give them credit, or has the small turbines industry just ignored this problem for cost savings, or is this just a case of "I know just enough to be dangerous"?For refence I'm looking for turbines in the 2-5kw range for relatively low but consistent wind into the 4m/s range

author: Berni
author_rank: Super Contributor
timestamp: April 19, 2021, 05:30:34 am
content: The whole "a diode rectifier feeding a switching converter" is how PFC actually works. All of the magic is in adjusting the switchmode converters power draw shape to roughly match the input waveform.That being said in 3 phase power a rectifier without using a large storage capacitor actually has a reasonably good power factor. The theoretical ideal PFC would be drawing power from the entire 180 degree half cycle of a sine wave. The 3 phase rectifier draws power from 90 degrees of those 180 while about 2/3 of the sinewaves area exists in those 90 degrees. Since power goes up by the square of the voltage this actually puts significantly more then 2/3 of the total power in that 90 degree region.Also the power that falls outside of this 90 degree sweet spot is not thrown away, it simply gets redistributed inside the 90 degree region, causing a slightly higher current on the phase that causes slightly more resistive losses. Besides the output of a BLDC motor is often not quite a perfect sine wave to begin with.You can also squeeze more power out of it by having a more efficient switching converter that turns less of the power into heat along the way. But as you chase more and more efficiency the gains become smaller and smaller. So if you need the extra power, at some point it gets more cost effective to simply get a 10% larger windmill then improve the efficiency by 10%. In both cases you get 10% more power.

author: KT88
author_rank: Frequent Contributor
timestamp: April 19, 2021, 09:15:52 am
content: Quote from: Berni on April 19, 2021, 05:30:34 amYou unfortunately confused current ripple with voltage ripple.The current ripple causes excessive copper losses not the voltage ripple. Larger capacitors increase the current ripple while reducing the charging angle. The voltage ripple decreases though...While the grid poses a relatively low impedance, I would expect a higher impedance (inductance) from a generator in the lower kW range. This means that the (wind-) generator would actually help to reduce the current pipple thus the I2R losses in the system.The effort is at least questionable as a vienna PFChttps://en.wikipedia.org/wiki/Vienna_rectifiercomes at a cost and not without losses.That said, in case of a small wind turbine an active PFC may not be viable in most cases.As a workaround in case you still measure some higher peak current, a passive PFC in front of the inverter might be a viable option...CheersAndreas

author: 2N3055
author_rank: Super Contributor
timestamp: April 19, 2021, 09:42:11 am
content: You're not gonna like this: just don't do small wind generators.4 m/s won't even spin the rotor on most generators, and even if it does, you'll get 200W from a 2kW turbine...They are also quite expensive, so you are better off by spending all money for as much solar as you can get and good storage.You need clean air, 4-8m/s average wind, install turbine at 10-15 m height and have a rotor of at least 5-6 meters diameter...Then wind starts to be really interesting. Anything smaller than that, you will generate something, but won't be worth the investment.Small turbines are for special applications and energy scavenging, aka you have no other choice, so you take what you can.

author: Berni
author_rank: Super Contributor
timestamp: April 19, 2021, 10:08:38 am
content: Quote from: KT88 on April 19, 2021, 09:15:52 amYes the current ripple is what is important for PFC. And the point of having PFC is reducing resistive losses.This is the reasoning behind not using large input capacitors. Even without a capacitor at all, the ripple on rectified 3 phase is about 50%, while a switching converter can easily handle such a large input voltage ripple at low frequency. It still needs a little bit of capacitance for the operation of the switching converter, but on the low frequency 3 phase side this small capacitance is irrelevant, it acts as if there is no cap. So the current is nicely spread out trough the two 90 degree sections of a phase where the rectifier stage is passing current. While outside those 90 degree regions the phase voltage is so low that there is very little power available to harness from it (Since good power factor means you are only allowed to draw small currents when voltage is small).So my point is that a properly designed 3 phase rectifier+switchmode design already has a pretty good power factor, so you are unlikely to gain much from more advanced PFC methods.

author: Wolfram
author_rank: Frequent Contributor
timestamp: April 19, 2021, 11:19:03 am
content: An uncontrolled 3-phase rectifier has a peak-peak output ripple of around 13 % of the amplitude (1 - sqrt(3)/2). When loaded with a resistive load, the power factor is around .956, so from an efficiency standpoint it's pretty close to a pure resistive phase-phase load. The main motivation for using more advanced PFC schemes is to keep the line current harmonics under regulatory limits. If power is being taken from a generator and you don't care about voltage THD (caused by current harmonics), there's little reason to go with anything fancier than a three-phase rectifier followed by a boost that approximates a resistive load.

author: KT88
author_rank: Frequent Contributor
timestamp: April 19, 2021, 04:35:47 pm
content: @Berni: Apologies, I missed the WITHOUT (a large capacitor) in your reply. You are 100% correct with your statement!

author: qwaarjet
author_rank: Regular Contributor
timestamp: April 20, 2021, 01:27:08 am
content: Yea that is what I'm starting to think, add more panels maybe do tracking on them, would be a better bang for the buck. And the power electronics are a lot simpler

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 20, 2021, 08:34:20 am
content: Quote from: qwaarjet on April 20, 2021, 01:27:08 amPannels are so cheap now, that it is more reasonable to have two fixed arrays at a different orientation. If you have space for it.

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2021, 03:31:25 pm
content: The poor power factor results in larger resistive losses between the source and rectified output, but if the losses are already low, then this increase is small and the power factor correction stage could result in greater losses.

author: boz
author_rank: Regular Contributor
timestamp: April 20, 2021, 08:04:26 pm
content: Quote from: KT88 on April 19, 2021, 09:15:52 amWow, summed up pretty closely a whole chapter in power electronics

author: qwaarjet
author_rank: Regular Contributor
timestamp: April 23, 2021, 03:32:45 am
content: Yea it's funny how with a few years of experience you can sum up a lot of collage classes. Personally I get a kick out of melting interns brains.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 23, 2021, 05:03:22 am
content: Quote from: Berni on April 19, 2021, 10:08:38 amActually, there's a trick in that the generator windings doubles as the inductor!https://techno-fandom.org/~hobbit/cars/boost-hack/

author: Berni
author_rank: Super Contributor
timestamp: April 23, 2021, 05:34:07 am
content: Yeah the teaching methods are sometimes backwards.They throw a mountain of theory at you with no context what so ever, then explain what it is for at the end when you already developed a hate for the whole thing, or forget to even say that and just throw a test at you.In my opinion a good example of this was control theory. They give you more math than you can shake a stick at, but at no point explain what any of it really does in real life. They have you calculate the angular speed of a control system in radians per second, you get a number out the end, alright done. Yet it was never mentioned to me that this is basically just the bandwidth of the thing in units that make less sense than Hertz and tells you how fast it reacts... yet how fast a regulation loop reacts is a pretty significant thing to know. The whole thing looks really complicated but in reality implementing a PID controller is really easy and involves elementary school level of math, them turning them to be stable are also simple cookbook recopies that involve similarly simple math. But i only figured that out by actually trying to make a PID controller on my own.Another example is that i never really understood how FFT works. I memorized the numeric process, got a number out and passed the test, then forgot it all within a week. Then i saw a few minute long youtube video that nicely graphically explains how FFT works with graphs and animations while pointing out what part of the math does that part to give it context. Suddenly it all made sense and it looked like a really simple and easy thing.Teaching is an art in itself and unfortunately most people suck at it.Quote from: NiHaoMike on April 23, 2021, 05:03:22 amAh nice one.Another example is the motor driver in the Reault Zoe car where they use the motor as an inductor of a switching converter during charging. Allowing them to reuse a lot of the electronics to create a high power onboard charger without much extra weight and cost.

author: 2N3055
author_rank: Super Contributor
timestamp: April 23, 2021, 09:57:30 am
content: Quote from: Berni on April 23, 2021, 05:34:07 amYeah, apparently we went to same schools...I have a problem learning things I don't know why. My whole life I kept asking my teacher and professors for reason and how is that useful to me... Very few knew, and even rarer where ones that liked I asked...Good thing is that I like experimenting, so I connected the dots myself eventually. But it would be easier to start with application, show some cool robot and then when you are interested, say : " but in order to create this robot, we need to know how to calculate this.... "..But no, it's backwards..

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 23, 2021, 11:02:39 am
content: Quote from: 2N3055 on April 23, 2021, 09:57:30 amTo exercise your memory ... was the answer. Is it good or bad