Solar inverter - Page 1
Forum section: electronics / general

author: Poe
author_rank: Frequent Contributor
timestamp: June 30, 2013, 12:24:35 pm
content: So I was thinking about the different types of power inverters for solar applications. Square wave, modified square wave, multilevel, pure sine, etc. Specifically how efficient they are.An inverter driving a resistive load with a pure sine wave would deliver power as a sine wave with peak power higher than average. Do inverters typically have a large bank of capacitors to compensate for the peak/valley or do they only deliver some fraction of the panel supplied power?

author: Harvs
author_rank: Super Contributor
timestamp: June 30, 2013, 01:17:19 pm
content: Typically they have a boost converter facing the PV array (to allow a large variation in input voltage) which has a maximum power point tracker of some sort feeding a DC link capacitor bank. Then whatever the load happens to be hangs off that, be it a grid connected inverter or battery bank charger in a standalone system.

author: Poe
author_rank: Frequent Contributor
timestamp: June 30, 2013, 04:46:59 pm
content: Quote from: Harvs on June 30, 2013, 01:17:19 pmAhh. So there is a capacitor bank. Must be pretty huge to maintain the kilowatts of peak power.Any idea what chemistry and ratings? I'm guessing something like 600V and a dozen or so Farads?With such an expensive component(s) and very little chance of a customer knowing if they're not there, I could see cheaper inverter manufacturers just eliminating them and outputting ~70% of input power.

author: SeanB
author_rank: Super Contributor
timestamp: June 30, 2013, 05:35:45 pm
content: Would not be big, more like 10 000uF 100V or 2200uF 450V so that the loop can respond in a period of 2-3 seconds. That would be for a grid tie unit. For a battery charger you can use a lot less capacitance and just place enough to not have too much ripple at the switching rate of the MPPT preregulator. The capacitor bank does not store significant power, it only provides a smoothing short term for variations like a bird flying over the panel.

author: Poe
author_rank: Frequent Contributor
timestamp: July 01, 2013, 03:13:42 am
content: Quote from: SeanB on June 30, 2013, 05:35:45 pm>2KW for 2-3 seconds would be a massive amount of stored energy IMO. I'm just talking about the peak/valley power output during each cycle (~4mS). For example, during the peak portion of the sine wave when the inverter is sourcing more power than the average power it gets from the solar panels.450V makes sense considering the input can exceed 300V.I guess the milliFarad range makes sense since it's only milliseconds, and the voltage level is doing most of the heavy lifting.But still, these make up a 'big' capacitor bank:http://www.i-businesslogistics.com/ebayimages/IBL-BCDEP2400uF450VDCCapacitor_DisplayPicture.jpgSome inverter pics I found are too small to house anything like that. Maybe that's a good way to tell the quality of your inverter. Not enough input capacitance and you're loosing 30% of generated power?

author: Poe
author_rank: Frequent Contributor
timestamp: July 01, 2013, 07:20:43 pm
content: Quote from: Poe on July 01, 2013, 03:13:42 amEDN did an SMA Sunny Boy tear down and showed a relatively small amount of capacitance.The more I read, the more this appears to be the case. i.e. Less than 70% of thepotential powerdeveloped by the solar array can get to house without energy storage due to the discontinuous power delivery of an AC source! That's like only using ten of a fourteen panel array!Is there something I'm missing? I find this surprising considering the money people pay for these inverters/panels.

author: Harvs
author_rank: Super Contributor
timestamp: July 02, 2013, 12:12:41 am
content: No, as I said you have a boost converter facing the array controlled by a Maximum Power Point Tracker. While it's not entirely the case, its a reasonable simplification that the boost converter acts as a constant power source under constant PV power production (at the maximum level the PV array can produce.)Your DC-link then has a significant current at 2xline frequency (i.e. 100Hz in Oz) which it needs to supply. The capacitor bank is charged to a voltage that taking into account the 100Hz ripple is still not close to the peak voltage of the output waveform. Lets take an example, if you're in oz the line is at 240Vrms, so that's ~340Vpeak. If you took 450V rated caps and held them at 400VDC, that will give you a modulation of 0.85. This will have a 100Hz component on it, but it's not going to be anywhere near the level which is required to run out of head-room.You're limiting factor is now the 100Hz current vs life expectancy of the capacitor bank. If you go take a look at any of the capacitors designed for DC-links in inverters or VFDs (which is actually the much bigger market compared to inverters), the datasheet is structured around giving you the information you need to predict how long the cap is going to live with a certain ripple current impressed on it.

author: jnissen
author_rank: Regular Contributor
timestamp: July 02, 2013, 01:56:27 am
content: I have Enphase micro-inverters employed in a system of mine. They are very efficient at close to 96% efficient.http://enphase.com/products/m215/This is a grid tie type inverter and can communicate over the AC power lines. What I really like about this is that each panel reports in via web interface and I can get statistics and performance data on a per panel basis. Partial shading can be observed and troubled panels are easily identified. Most of the newer grid tie systems being installed here in the States are moving to this type of inverter.Example of the whole system:https://enlighten.enphaseenergy.com/public/systems/f9CW28621

author: Poe
author_rank: Frequent Contributor
timestamp: July 02, 2013, 04:53:12 pm
content: Quote from: Harvs on July 02, 2013, 12:12:41 amI get the feeling like no one is understanding what I'm talking about. Sorry. I'm not much better in person.Thepotentiallysteady stream of power is supplied to the inverter where it outputs adiscontinuousstream of power. This discontinuous stream of output power looks like a 'rectified' sine wave, wherepeak output islargerthan the average. If average input power equals average output power, the inverter needs to outputmore power than is suppliedtwice each cycle. Energy storage is needed for this.If it doesn't have adequate energy storage, it must limit peak output power to the average input power. If this is done, practically zero input capacitance is needed to completely smooth out input ripple because demand simply 'dips' from what the PV array can supply.Using your example, lets say we have an ideal PV array, ideal inverter, in ideal conditions capable of producing a continuous 1kW all day long. We also have amassivecapacitor bank on the inverter input so it can easily supply greater than 1kW for at least a second. Theaveragepower output could then be 1kW since that's what the PV array is capable of supplying and we are assuming there are no losses.Scenario #2, assume we remove that energy storage. Our inverter can only ever supply 1kW peak so it only can pull 1kW peak. Since it must supply power in a discontinuous manner, our output is now limited to ~707W average. PV is supplying power in lumps even though the sun's intensity is continuous. Without energy storage the average supply is reduced.In your example, the 400V capacitor would need to be 'large' in order to supply the energy required for scenario #1.If inverter manufacturers went with scenario #2, I couldn't see how anyone could know the difference. How would jnissen know if their panel should be producing 700W or 1kW? Sure it can convert 96% of the powerit doespull from the array to power the grid, but how can you know theaveragepower it pulls is equal to the maximum average power the array is capable of producing for those conditions?Hopefully that better conveys what's in my head.

author: mtdoc
author_rank: Super Contributor
timestamp: July 02, 2013, 05:58:13 pm
content: I'm not sure I completely understand your question but as far as storage vs no storage I think you may be missing something about basic solar system design.Residential solar installations come in 3 general varieties and none of them rely on large capacitors for energy storage:1. Strict Grid Tied - No storage needed - the grid acts in effect as an extremely large storage pool. PV production is limited only by thepanel efficiency and solar insolation. Power produced that exceeds local loads is sent into the grid.2. Off Grid. Batteries are used for storage. The charge controller - typically using a 3 stage charging method - limits PV powerdelivered to batteries based on need/charging stage. The inverter is not connected directly to PV but pulls power from batteries todeliver VA to loads on demand.3. Grid Tied with Battery Back Up. (aka Hybrid or Grid interactive system). Same basic design as off grid with PV power productioncontrolled by charge controller but now with an inverter that can deliver power (via batteries) to local loads and send excess powerinto to the grid.

author: Poe
author_rank: Frequent Contributor
timestamp: July 02, 2013, 08:30:08 pm
content: Quote from: mtdoc on July 02, 2013, 05:58:13 pmYou're probably correct about me missing something. I just wish I knew what.Consider an off-grid system driving a simple resistive heating element. Instantaneous power output near zero-cross is nil. Shortly after zero-cross it's still mostly nothing. 90° later it's at maximum. If there's no capacitor for energy storage, this maximum instantaneous power output is equal to or less than the peak PV supply power. i.e. Power out can not exceed Power in. So all of that other time (0° to 89°, 91° to 179°, etc) it's outputting less than what the PV array could supply?Assume we have a grid-tie system. We also assume peak output can never be greater than peak input. Now one way output power could be maintained when voltage is not at peak is by increasing current. Although the inverter's maximum output current is not so huge that this is realistic. Additionally I'll bet there's some kind of regulation preventing a solar inverter trying to force the grid into a square-wave.

author: Poe
author_rank: Frequent Contributor
timestamp: July 03, 2013, 11:43:50 am
content: Took a while to find information from a knowledgeable source, but it appears this IS an issue...http://www.eetimes.com/document.asp?doc_id=1278778Spoke to an installer and he said input capacitance appears to have dropped significantly when warranties increased from five to twenty years. Makes me wonder if their solution was to just eliminate the failure point all together and decrease output to compensate. hmm.Anyone know of someone who designs these things or has torn them apart?

author: Harvs
author_rank: Super Contributor
timestamp: July 03, 2013, 11:44:28 pm
content: Quote from: Poe on July 02, 2013, 04:53:12 pmNo I understand what you're talking about. I've tried to explain it how it is designed, in the frequency domain, because it's much easier analysed that way.QuoteYes, that's the 100Hz ripple (on a 50Hz line frequency) I was talking about.QuoteOk, but here is where we differ considerably. If I designed any type of inverter and had the DC link be able to hold up to a voltage greater than the peak AC output voltage for 1 sec I would be absolutely mad. There is no point. The purpose of the DC link capacitance is to supply all the current needed for the 100Hz AC component, and the ripple at whatever the switching frequency is and it's harmonics (or be it twice the switching frequency for uni-polar switching.) All the DC component is supplied by the boost converter which is supplied by the PV array.QuoteWell it needs to be adequately sized. Whether you call that large depends on your standpoint in the world.I think the easiest way to explain this is going to be to do some maths, however you need to appreciate that it's done in the frequency domain because that's the only reasonable way to do it.Refer to the first attachment for calculation of the ripple current the capacitor bank must support. Next step in sizing the bank is to choose the capacitors, if we go to CDE's website and look at inverter grade caps specifically designed for DC-links. For the sake of this quick calculation we'll take the CDE 381LX,http://www.cde.com/catalogs/381-383.pdf.As for life, lets say we want our inverter to last 10yrs at 6hrs a day of max rating. That's 365*10*6~22kHrs. Also we'll assume the operating temp is 75 degrees C. If you look at page 3, you can get the ripple current multiplier for that combination, it's off the chart so you can take 1.5 (the largest on the chart.)Now if you scroll down to the end of the datasheet to the 420V caps. If we wanted low profile caps for our inverter, the largest we can get for our inverter is the 220uF in 1" high. If you look at the 120Hz ripple current rating it's 1.1A, which when multiplied by 1.5 is 1.65Arms.Earlier I just did a geometric addition of the switching and 2nd harmonic current, these actually should be corrected to 120Hz using the tables in the datasheet, but for the sake of this I'm not going to waste the time.To cover the 5A ripple current, we need at least 3 of these caps, so lets say 4. With 4 caps that's 880uF, which at 100Hz gives an impedance of 1.8ohms. So that gives a peak to peak ripple second harmonic voltage of 2*sqrt(2)*1.8*3.54~18V.So I hope you can see, that even with what I would describe as a fairly weak DC-link, we are still no-where near dipping from 400Vdc to the 340Vpeak of a 240Vrms sinewave, and it is the capacitor life that is the limiting factor.

author: johnwa
author_rank: Frequent Contributor
timestamp: July 04, 2013, 06:43:52 am
content: Quote from: Poe on July 02, 2013, 08:30:08 pmYes, this should work, at least in theory. Although you would not be able to fully absorb the input power (e.g. when the grid is near zero volts), it would be possible to convert most of it, at the expense of a somewhat uprated output stage. I suppose the savings in cost and unreliability of a capacitor bank might even pay for a few extra MOSFETS.However, I don't really think this approach is to be encouraged. The power factor wold be horrible, and I expect it would also spew large quantities of RFI.