Sizing at the limits: BlueSolar/SmartSolar MPPT trackers for PV modules with high module voltage
Hello Victron Community,
I am currently planning my PV system and, due to the varying shading patterns and orientations of the flat roofs, I am leaning towards Victron, and specifically towards BlueSolar/SmartSolar MPPT trackers.
Conditions: Three flat roofs with different orientations and pitches. The modules will be mounted at angles between 5 and 20 degrees, depending on the flat roof.
To achieve the maximum output, I am considering the following modules:
AIKO NEOSTAR 3P+54 and AIKO NEOSTAR 3P+60
The AIKO NEOSTAR 3P+60 modules have a maximum rating of 550 W in their most powerful version, with a Voc of 45.80 V, 14.92 A, a V temperature coefficient of -0.220 and an I temperature coefficient of 0.050.
I always want to connect three modules to one solar MPPT.
I live in postcode 01XXX.
Now to my question…
Three modules in series produce the following values at different design temperatures:
-15 degrees C with the STC values: exactly 150 V
-20 degrees C with the STC values: exactly 151 V
If the specified module tolerance of +3% for the output is also added to the voltage, the figures are as follows:
-15 degrees C with the STC values: exactly 153.96 V
-20 degrees C with the STC values: exactly 155.52 V
What has been your experience with such borderline cases? Do these values actually occur, and could they destroy the solar chargers, or is this more of a theoretical consideration in winter when the modules are not optimally tilted and are NOT (amended after the first reply) oriented towards the south?
I have observed series connections of two Jasolar jam60d41 modules on the SmartSolar 100/50, as well as three of these modules on a 150/45. This was on a larger system whose charge controllers really do start limiting output in the morning and display the open-circuit voltage. The modules do have a slightly lower Voc, but a higher temperature coefficient of -0.25%/K. In winter, they remained well below the 100 V and 150 V limits, and also below 145 V. Even though it is often said (including by me) that the open-circuit voltage is almost independent of irradiance, that is a relative statement referring to the approximately 45 V. But we have to consider the difference from the “limit”. Every volt counts there. And a module can certainly provide that when you look at the absolute values. With lower irradiance, the voltage is somewhat lower, as you can see in the diagram:
For a long time, I tried to come up with conditions that could cause the voltage to become too high. The first prerequisite would be an off-grid system or a grid-connected system during a power outage. Either the irradiance is not strong enough, the air temperature is too high, or the controller is not in a state where it is limiting output. I always had a system in mind located on a mountain, with its modules oriented almost vertically towards the east-southeast. There, even into April, you can have low temperatures, wind and sufficiently strong irradiance in the early morning with good orientation. But despite the wind, the modules would still warm up considerably. I was unable, even in theory, to construct a scenario in which the sun “switches on” within a few seconds without the modules having time to warm up. There could, however, be a risk if the voltage were already too high at around +2°C. A hail shower in early April, after the passage of a polar front, could then create the necessary conditions as high-pressure conditions build. The modules would be cooled to approximately 0°C around midday, and a few seconds after the shower subsided, the sun would shine out of a bright blue sky, surrounded by dazzling white clouds. Irradiance above 1,000 W/m² would then be possible. Dangerously high voltages could be generated even with power output at just 25% of the modules’ rated power.
The 150/35 does not start if 150 V is exceeded at “start of operation”. It displays an error message in that case. It will not switch on again until the voltage drops below 145 V.
Many users think, “I’ll just add a small resistor.” While we’re at it, it’s worth taking another look at the diagram to calculate a load resistor that would reduce the voltage by around 3 V per module: at an irradiance of 1,000 W/m², the resistor would have to dissipate around 370 W for each module!! It would have to draw 8 A at 41 V and have a resistance of 5.1 Ω. For the series connection, that would mean 15.3 Ω/1,110 W
danke R.L. for your explanation. What do you mean by “significantly”? The modules you mentioned have, considering the worst-case scenario, around 147.5 V open-circuit voltage at −20 °C when using three modules, and are therefore below the voltage limit specified by Victron.
Does anyone have any further experience?
My planned PV system consists of nine MPPT 150/35s and two MPPT 250/60s. I don’t want them all to blow up at the first sign of trouble.
There is still a residual risk. The voltage that destroys the electronics is not the PV voltage. Chopping the PV current generates unavoidable voltage spikes, which are added to the PV voltage. I would assume that the controllers measure the voltage first and then decide whether they should start operating. That’s how I would programme them myself, and I would also trust Victron on this, because the necessary hardware is available. I don’t think it harms the controllers if, for example, 155 V is present and the controller is not operating. The controllers perform a scan every 10 minutes, briefly bringing the PV current down to zero. This allows you to view the highest PV voltage in VictronConnect and check whether it is actually becoming too high.
How likely is it that −20°C will actually be reached at your location?
For example, how often has that happened over the last 10 or 20 years?
There’s also the fact that the MPPT always tries to find the MPP, which isn’t at Voc. In other words, as long as the MPPT can charge — i.e. it doesn’t have to curtail the power — it will keep the voltage below Voc.
The question is whether you can always guarantee that. What if there’s a power cut while the battery is full and it’s cold at the same time…? Do you really want to take that risk?
As for the MPPT in general: at the PV input, it has electronic switches (MOSFETs), which have a certain voltage withstand rating. In this case, we’re talking about 150 V. If the PV voltage exceeds this value, such a MOSFET can break down and would be irreparably damaged. The component naturally has a certain tolerance as well — in other words, if 150 V is guaranteed, it will probably be a 160 V ±5 V component — but we simply don’t know for certain.
I would therefore base the decision more on how likely such low temperatures are.
Seventeen years ago, it reached -20°C once; within the last 10 years, it reached -16°C twice, and otherwise it has been above -14°C.
However, the official figures from the weather service only list daily values, not hourly ones. That means I can’t tell whether these occurred during the day or at night.
Alternatively, a normally open contactor could be installed between the PV modules and the MPPTs, which drops out when the SoC is >95% and only picks up again when the SoC is <90%. The only thing still needed would be some way of adding a condition that no power is currently being drawn from the system.
Or a 150 V surge protector/surge arrester in front of each MPPT.
But the risk is probably actually lower, so I could go with the 150/35 or 250/60.
Also, a DC isolator before the MPPT makes sense anyway, for maintenance or similar.
That would solve a situation like this, but it’s purely manual and depends on you.
A surge protector can never be set that precisely. These components usually have fairly wide tolerances and wouldn’t help you at all.
At 151 V max. at −20°C, I’d probably be willing to go along with it, as I consider that very unlikely. Anything above that, I wouldn’t do—I’d redesign it.
Although it can be switched off, the PV voltage is still present, and the charger’s input circuitry isn’t disconnected in any way (there’s no relay or anything like that). So if the PV voltage is too high, the MPPT will still be damaged.
The input MOSFETs are directly connected to the PV voltage. All that gets switched off is the DC/DC conversion.
That’s why a 2-pole DC isolator isn’t just sensible; in some cases, it’s even mandatory.
I have too little experience with input voltages this close to the limit. I’ll switch to PV modules that, when connected in series, remain below the maximum voltage of 150 or 250 V.
