PV string voltage double or split in half - is there such a product? - Page 1
Forum section: electronics / general

author: marekm
author_rank: Contributor
timestamp: May 05, 2023, 01:18:06 pm
content: Some background - due to high electricity costs, but favorable net-metering (can store energy in the grid for later use within one year, with 80% effficiency) I need to expand a rooftop PV installation. The roof is mostly free of shades, but has many small areas in different direction (20+ years ago it was not designed with PV in mind).This means a few short PV strings connected in parallel, but then the voltage is a little too low (and current too high) for the MPPT range of many typical (cheap and NC RfG certified) 10 kW on-grid inverters.So, I have an idea - make a synchronous buck (or boost if power flows in reverse direction) DC/DC converter with fixed 50% duty cycle - that would double the voltage and halve the current.To reduce the power levels that need to go through this converter, two PV strings could be connected in series (instead of parallel) and the middle point forced by the converter to half the total voltage.Roughly +/-10A at 200V output from 400V input, still 2kW is too much power for a switched capacitor charge pump (like the good old 7660 scaled to 10000x higher power) - but perhaps two IGBT half-bridges driven out of phase (50% duty minus some dead time), with two output chokes to share the power and reduce ripple, would be a workable design?The fixed duty cycle (no control loop at all) means it should have no effect on any MPPT algorithms in the inverter, would work just like AC transformer but for DC.Before I try to roll my own - has anyone heard of an existing similar product already made in China, cheaper than I can do it myself?A variation on this - it could be made isolated (with some loss of efficiency), then transformerless inverters could be used with grounded PV strings in case they are affected by PID.Again, no need for any complex control loops - just a fixed duty cycle, possibly with some overload current limit (but PV modules are already current-limited) and thermal shutdown.Has anyone heard of such a product? With almost no shading, per-module optimizers or micro-inverters seem overkill.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 05, 2023, 01:31:51 pm
content: Quote from: marekm on May 05, 2023, 01:18:06 pmNo no no, it does not. Where did you get this idea of paralleling PV strings? If you do that (with added diodes), complete strings go to zero when partially shaded.QuoteVoltage is not the problem. Parallel connection in itself is. Just make longer series strings and use the inverters as designed, so that single panel shade affects only that single panel.QuoteThe key to success with partial shading is the opposite of what you are doing - MPPT per panel. Similar idea, but something which actually works, has been invented already: it's calledoptimizer. Another take is a microinverter.Although, if the partial shading is not heavy, good old string inverter does nearly as well (shaded panels go to zero production instead ofsome, during shading conditions; but for yearly average, this is not too many %).Modern string inverters have several inputs with separate MPPTs and voltage ranges start from 150V or so, just 5-6 panels per input, eating into the partial shading market segment classically held by microinverters / optimizers.

author: marekm
author_rank: Contributor
timestamp: May 05, 2023, 02:15:08 pm
content: Shading can be ignored in this case (fortunately, simply no high trees nearby), happens only in the mornings and evenings when produced power is very low anyway (not worth the effort to optimize).I couldn't find a reasonably priced 10kW inverter with more than two MPPT inputs. The one I have has one input rated Imp_max 26A and the other 13A (Isc_max 40A/20A respectively), both rated Voc_max 1000V. I'm limited by the currents - could double the voltages and still stay within the limits, but don't have the space for such long strings (the roof has a complex shape with many small triangles facing east/south/west). Right now it's 26*380W total: 3 parallel strings of 7 modules each (1 east, 2 west) and 1 string of 5 modules (south). Want to add another 12 modules: 5 (east) and 7 (west) parallel to the existing strings of the same length, but this will exceed the 13A string Imp rating. It's not a complete new installation, work on the roof is expensive and risky, so I'd like to avoid making any changes to what is already there (optimizers would have to be added to all modules, not just the 12 new ones but also the 26 existing ones). With these constraints, it should be clear why I'm looking for a different solution. Parallel connection of strings is not as bad as it sounds, as the MPP voltage is almost constant in practice (it rises a little with more sunlight, but drops a little with higher temperature, and the two effects cancel each other).

author: Siwastaja
author_rank: Super Contributor
timestamp: May 05, 2023, 02:45:50 pm
content: Easiest and probably cheapest way to expand a system is to keep the existing system as is without touching it and adding a new, independent system. 5 + 7 panels to two different directions sounds like a job for the cheapest 2-MPPT 3-4kW string inverter with attention paid to minimum MPPT voltage.

author: marekm
author_rank: Contributor
timestamp: May 05, 2023, 04:03:37 pm
content: The existing inverter can handle 10kW just fine (nominally 26*380=9.88kW peak, but real peak is about 6kW at noon on a sunny day like today, due to non-optimal east/west directions - on the other hand, power production starts earlier and stops later compared to the optimal south-only direction). A non-regulated non-isolated DC/DC converter should be much simpler than the second complete on-grid inverter - and cheaper, if someone made such a product in larger quantities. Like a power board from some inverter, controlled by a simple MCU programmed for PWM with some protection functions. I'm actually thinking about something like EVAL-M1-IM828-A + Raspberry Pi Pico (RP2040) here, but the magnetics and EMC issues at such high power levels may be a challenge.

author: nctnico
author_rank: Super Contributor
timestamp: May 05, 2023, 04:55:27 pm
content: Nowadays you can buy 3 to 4kW inverters from Growatt for around 500 euro and these have super wide MPPT input voltages. A good DIY solution will cost you more. I'm with Siwastaja here: just add an extra inverter for the extra panels.

author: marekm
author_rank: Contributor
timestamp: May 05, 2023, 06:50:05 pm
content: I agree DIY will cost more when done just once (especially if magic smoke goes out, which sometimes happens when prototyping high power circuits), that's why I'm first looking for an existing product (or something easily modified to do what I need). If successful, it could be repeated to help others expand their PV. More than one inverter means I'd have to control total power sent to the grid to not exceed 10 kW (above this, the regulations become less favorable).Actually I'm thinking about setting the PWM duty cycle to 5/12 and connecting 5+7 modules in series, this is about 400V DC total which is closer to optimal inverter efficiency (even if it could work down to 150V), and leaves the second MPPT still free in case I find a cheap wind turbine somewhere... I have the control part already running (RP2040 + Arduino + simple C program to make 6 PWM signals for 3-phase interleaved buck with dead time, confirmed working with oscilloscope) but I'm a bit nervous about the high power part (years ago I have already worked with some DC/DC converters including manually wound magnetics, even with no smoke at all, but only low power like 30W or so).

author: nctnico
author_rank: Super Contributor
timestamp: May 05, 2023, 11:23:58 pm
content: How can your total power ever exceed 10kW if your panels are in a sub-optimal orientation?A more basic question is: how is your solar power system rated as seen by the grid company? Over here it is the Wattpeak (Wp) number that counts and they don't care about the orientation. If your situation is the same and if you are going to add extra panels, you are already exceeding the 10kWp limit.OTOH: if the actual peak output power ability counts, you can easely extrapolate the extra maximum power possible based on data from the existing solar array.Alternatively, solar inverters allow to set an export limit (at least the modern ones). You can use this feature to make sure to stay below 10kW output to the grid.

author: marekm
author_rank: Contributor
timestamp: May 07, 2023, 10:16:36 pm
content: After expanding the PV system, the grid company should be notified within 14 days. They care about total power of the modules, and type of the inverter (must be certified). They don't care where the modules are (sub-optimal on the roof, optimal all south, or even mechanically rotated to always point at the sun to make almost twice as much energy), but their smart meter will detect if I ever exceed the rated power (monitored over GSM, I can see hourly statistics after logging in on their website, internally they probably have 15-minute granularity) or send power to the grid when voltage is too high (a certified inverter is not allowed to do that, unless set for wrong country which is a big no-no). So it is possible to have a larger system technically (legally it's a gray area) as long as one is careful to not do any bad things that could draw their attention.Expanding PV systems has become a popular thing here as energy costs rise dramatically - first 2 MWh/year for households is still reasonably priced ("frozen" as it was before), but above that the price goes up by about 200%. Unfortunately I have electric heating (using most energy during formerly cheaper off-peak hours 13-16/22-7) which suddenly has become prohibitively expensive so I don't want to be "frozen" literally.A single inverter is limited to its rated power, and can be set lower if necessary. (BTW, looking at the prices of some 4-5-6-8-10 kW inverters I'm wondering if it isn't actually the same hardware, just limited in software so it can be sold at different power levels at slightly different prices.) Limiting total power summed from more than one inverter becomes more tricky if each of them produces maximum power at different times of the day. On the other hand, it would make it possible to take into account the local consumption as well.Voltages and currents are within the range of an input PFC stage of some big power supply. But perhaps a full bridge with a transformer is better than a buck or boost with a big inductor - at close to max duty cycle (square wave through the transformer) the output bridge rectifier should not need much filtering. It is easy to buy off-the-shelf 1:1 AC 50/60 Hz transformers of any size from small to big, wish it could be the same with 1:1 DC/DC converters.I've found a similar question discussed here -https://www.eevblog.com/forum/projects/1kw-200400v-dcdc-converter-design-tip/- so I'm not the only one with such crazy ideas