Boosterless Inverters - The Pitfall?
Manufacturers of photovoltaic components face fierce competition. They invest heavily in R&D to release the next model of solar modules or inverter with better efficiency and optimized costs. Every tenth of a percent gained is a new success! However, it is a shame to encounter solar plants made up of the best components but never reaching the expected performance due to a simple sizing error.
I regularly observe a scenario involving a length of module strings that is not compatible with the use of high-efficiency boosterless inverters.
What is a Boosterless Inverter?
From a functional standpoint, an inverter is a device that converts direct current into alternating current, typically three-phase. In reality, a solar inverter often includes an input stage called a booster, which increases the input DC voltage before it is converted into alternating current by the inverter stage itself. A booster is somewhat like a transformer that converts a DC voltage to a higher voltage level. Thanks to the booster, an inverter can operate with a very wide input voltage range, for example, 250 - 800 V. The downside, however, is the efficiency losses caused by the booster, as well as the impact on the complexity and thus the price of the inverter. A boosterless inverter generally achieves better efficiency at lower costs, albeit at the expense of reduced sizing flexibility.
No Problem, Just Read the Datasheet and Respect the Mentioned Minimum Input Voltage!
Indeed, the minimum input voltage is always mentioned in an inverter's datasheet. The pitfall, however, lies in the footnotes and the reality on the ground, which can sometimes differ considerably from theoretical values. The minimum input voltage of a boosterless inverter is generally mentioned for the nominal grid voltage (for example, 230 V). And the nominal voltage of a solar module is mentioned for a cell temperature of 25°C. Actual conditions, however, can be quite different.
How Does Grid Voltage Play a Role?
To understand how grid voltage influences the minimum input voltage of an inverter, let's observe how an inverter functions. A photovoltaic inverter for residential or industrial plants operates based on an output voltage that is generally three-phase with an RMS voltage of . This corresponds to a peak voltage of . The nominal grid voltage can be different for large plants equipped with dedicated transformers, but we will not address this type of plant here.
Three-phase system at 50 Hz with a nominal voltage of 230 V.
Let's now focus on the upper and lower envelopes of our three-phase system. This envelope corresponds to the voltage levels that the inverter must be able to generate from the DC voltage.
Upper and lower envelopes for a three-phase sinusoidal system.
A boosterless inverter can only generate an alternating voltage whose difference between the upper and lower envelopes is always less than the DC input voltage. The three-phase alternating voltage will be "cut" from the DC voltage, as if we were cutting the envelope of the sinusoidal voltages from a strip of paper. The theoretical minimum DC voltage is therefore:
Difference between lower and upper envelopes, highlighting a minimum DC voltage of 580 V.
In practice, a slightly higher DC voltage is required. It is thus not uncommon for a minimum input voltage of 580 V to be mentioned in datasheets for a nominal grid voltage of 230 V.
Summer Reality
Now that we understand the relationship between grid voltage and DC input voltage, we can focus on the sizing required to ensure optimal operation of a boosterless inverter. Let's consider the most unfavorable situation: a summer day. Two phenomena then have a decisive impact to consider.
- Increase in grid voltage: On a summer day, it is not uncommon for significant photovoltaic electricity production to lead to a local increase in grid voltage. A phase voltage exceeding 240 V is then quite possible.
- Decrease in the voltage of solar modules: On the same summer day, the temperature of the solar modules can easily rise to 60-70°C. The nominal voltage of the solar modules can consequently drop by about 15% due to the relationship between temperature and voltage of the solar modules.
It is not uncommon to observe boosterless inverters that require an input voltage exceeding 600 V in the summer due to the grid voltage, even though a minimum voltage of 580 V is mentioned in the datasheet.
A nominal voltage of the module strings of 660 V, for example, may initially seem sufficient if the minimum input voltage of the inverter is 580 V. In the summer, however, the MPP voltage of such a system can quickly fall below the actual minimum input voltage of the inverter due to the combined effect of the increase in module temperature and grid voltage. This situation can lead to considerable losses, as shown in the real example below. This example highlights a correctly sized inverter (Inv. 1) compared to a boosterless inverter whose input voltage is not sufficiently matched with the length of the module strings (Inv. 2).
Summer losses caused by inappropriate sizing of a photovoltaic plant involving a boosterless inverter.
Ideal Sizing
To avoid unnecessary energy losses, it is crucial to understand the relationship between module temperature, grid voltage, and the behavior of the chosen inverter. A photovoltaic plant should be sized so that the MPP voltage of the module strings is always higher than the minimum input voltage of the inverter. It is then important to consider the maximum module temperature, the maximum grid voltage, and to include a margin that will cover other factors that can also influence the MPP voltage at the inverter input (DC cable losses, long-term degradation phenomena of the modules, mismatch, local shading, etc.).
Do you need to further develop your skills in the field of photovoltaics? Would you like to obtain tools for detailed monitoring of the performance of your plants? Do not hesitate to contact me!