Regulation On Account Of Mpp Voltage Range

[Oguzhan]

Hello everyone,

 

I would like to ask a question about the losses about regulation on account of MPP Voltage Range.

Actually I work on effect of MPP range on total energy yield and now I use trial version of PVSOL to check whether the software gives expected results or not. 

Minutely simulations are supposed to seize the instantaneous voltage drops on PV arrays due to momentary low irradiance values that exist quite frequently in cloudy weathers. So inverters whose MPP range extends towards lower voltages can keep operating with relatively high efficiencies compared to the inverters with narrow MPPT range in low irradiations. 

In order to test this situation, I made a simulation. Simulation parameters are given as follows:

- Location: Berlin/Germany

- "Simulate the irradiance with synthesized minute values" option is checked.

- PV Modules: ASUN250-M (A-sun by Aton Energy), 400 Modules (20 Strings, each string has 20 series modules), 100kW PV Nominal power

- Inclination is 30 degree, azimuth is 0 degree.

- Installation type is roof parallel.

- Inverter is PVS800-57-0100kW-A (100kW Central Inverter of ABB), its MPP voltage range is defined as 450V-825V.
- Other options are set as default.

 

After this simulation, a virtual inverter is defined. Parameters of this virtual inverter is exactly same with PVS800-57-0100kW-A but its MPP range is specified set as 100V-825V. And same simulation is repeated with this new virtual inverter.

 

Comparison of these simulation results are given as a PNG file as attached [Comparison of MPP Range.png]. Total difference between the grid feed-in values is only 38.1kWh that seems too low to me. There is no loss due to MPP voltage range in second simulation as expected. What surprises me is that losses due to the regulation on account of MPP voltage range is only 26,02kWh which corresponds to only 0,02% of all losses in first simulation. Isn't it too low?

 

After this result I repeated the first simulation with hourly taken irradiance data and losses due to the regulation on account of MPP voltage range is found as 21,04kWh that is just 5kWh less than the result of simulation where we used minutely taken irradiance data as input. 

I believe there is something wrong (maybe I did not do something correctly) as irradiance data in minute steps is expected to seize the instantaneous voltage drops on PV modules due to momentary low irradiances. However simulation results show that the difference between minutely taken irradiance data and hourly taken irradiance data is just 5kWh on losses due to the MPP range at the end of a year which is just neglicible for a 100kW nominal DC power system.

So am I doing something wrong? Maybe someone can give me a reasonable explanation here

Thanks

Oğuzhan

[developer_mh]

Dear Oguzhan,

 

thanks for your interesting question and the detailed simulation information. The losses due to the limits of the MPP voltage range depend highly on the setup. In your setup the strings have a MPP voltage that is well within the MPP tracking range of the inverter. See the first bar of the configuration check:

 

 

That means, the MPP voltage of your modules will very rarely drop below the minimal MPP voltage of the tracker (425 V).

 

 

I simulated your PV plant more in detail with our analytic tools to show you how the MPP voltage of the strings behaves. The simulation was conducted for the first two months only, i.e. winter time, where in your case the effect is strongest. The following diagram shows the MPP voltages that occur at given irradiance values. As you can see, most of the time the voltage is between 500 and 700 V.

 

 

The following diagram shows the percentage of the irradiating energy on the module surface. Here you can see more clearly that MPP voltage values of 425V and below are energetically irrelevant.

 

 

So, the reason why the losses are so low in your example, is that the tracking voltage range of the inverter is large enough to cover all relevant MPP voltages of your strings. If you would narrow down the tracking voltage range on, say, 550 - 600 V you would see clear effects.

 

Some years ago there was an interesting investigation on the influences of the voltage range on yield losses. The poster is in German, unfortunately, but I think you can get the point when looking at the diagrams:

http://www.mike-zehner.de/publications/symp12-DeltaU.pdf

 

Hope that helps, kind regards,

Martin Hofmann

[Oguzhan]

Dear Mr.Hofmann,

I appreciate for your detailed answer and clear diagrams. I have also investigated the poster that shows the influences of the MPP voltage range on yield losses. Thanks for your effort.

Best Regards