PHOTON INVERETER TEST
Our goal: Helping system operators select the right inverters
PHOTON Lab has been carrying out inverter tests successfully since 2007, informing PHOTON readers whether or not a device is up snuff. The lab assigns grades ranging from A++ to E, which correspond to an overall efficiency defined by PHOTON.
Since the beginning of 2007, PHOTON Lab has employed its own inverter test methodology, and regularly published results from those tests. To make the- se results easier to comprehend, the editorial staff, drawing its inspiration from school report cards, launched a grading system with its own testing certificates: the grades range from A to E. The highest grade (»A«) has three different levels: an A grade, an A+ or an A++. This was deemed necessary since one of the de- vices tested performed so much better than its competitors that it required a more subtle gradation. An E grade is as- signed to an inverter with an efficiency so poor that it‘s essentially not worth the money paid for it – put simply, so much of the produced solar electricity is either converted into heat or unabsorbed that the resulting loss in feed-in tariffs is so high that it exceeds the device‘s cost. In this sense, devices like these are too expensive to even give away.
To assign a grade, we first need to determine the efficiency to which the grade refers. Both peak efficiency and European efficiency aren‘t well-suited for this purpose. This is why PHOTON has decided to define its own efficiency value, the explanatory value of which will far exceed conventional efficiency data. Furthermore, the goal of this grading system is to enable better comparisons of individual devices. The grades provided in the survey can essentially be associated with the devices‘ overall utility, which is often difficult to determine from the pro- duct datasheets provided by manufacturers, as many of them omit important information on operating parameters.
Currently, above all, technical data from manufacturers often omits information on the efficiency‘s dependence on input voltage. Also, it‘s uncommon to find data on an inverter‘s suggested MPP operating point. And information on the influence of input current limitation on the operating point, and the relation between temperature and conversion efficiency, is often nowhere to be found. To give readers of our inverter test an immediate sense of a device‘s value, we assign a single grade for an efficiency determined by PHOTON to each inverter that takes into account all of the above-mentioned factors. No other individual scores have an influence on the grade. The parameters reflected in the grade are reviewed on an annual basis and are be discussed with manufacturers in advance.
Efficiency: Explanations of measurements and diagrams
The diagrams for MPPT efficiency, conversion efficiency and overall efficiency demonstrate the dependence of these values on input voltage VMPP and input power PDC. The MPP voltage range is divided into 20 steps and the DC power range into 24 steps. The result is 480 different solar generator curves and every curve has a fill factor of 75 percent.
The 480 individual measurements form the basis of the three-dimensional diagrams. The third dimension in the diagrams is color, which shows all efficiencies achieved at different VMPP and PDC levels. The color spectrum and its correlation to measurements are pictured next to the diagram. While the input voltage VMPP (in the range specified by the manufacturer) is provided in absolute numbers on the y-axis, the specified power PMPP is shown on the x-axis in relative values. This is standardized according to the inverter’s nominal input power PDCNom and given in percent of PMPP nominal power. Just how far this range stretches beyond the 100-percent mark depends on manufacturer specifications.
If the maximum MPP voltage specified by the manufacturer is close to the maximum DC voltage, hatched areas show limitations on the inverter when it’s used with crystalline modules, and below that another area with hatching in the opposite direction that shows limitations when used with thin-film modules.
MPPT adjustment efficiency is calculated comparing the available DC power (PMPP) with the DC power absorbed by the inverter. It provides insight into the inverter’s static MPP tracking – so how well the solar generator absorbs the inverter’s predefined PMPP power.
Conversion efficiency is the relationship between the AC power PAC supplied by the inverter and the power absorbed on the inverter’s DC side PDC. Both above and to the right of the diagram are cross-sections that are pictured in the three-dimensional color diagram. These show the dependency of efficiency on standardized power, and efficiency on voltage VMPP. At the top right, the inverter’s operating range is shown in relation to the MPP voltage range and the MPP power.
The overall efficiency is calculated as a product of the conversion efficiency and the MPPT adjustment efficiency for all 480 measurements. The diagram is arranged in a manner similar to that of conversion efficiency.
The diagram showing weighted conversion efficiency shows the measured efficiency level for medium irradiation (European efficiency) and for high irradiation (Californian efficiency), based on the California Energy Commission’s (CEC) definition, over the entire MPP voltage range.
The graph displaying efficiencies at different VMPP voltages shows the course of efficiency at nominal power PMPP for minimum and maximum MPP voltage (VMPPmin and VMPPmax), as well as for the lowest and highest MPP voltage value at which the inverter’s maximum efficiency is achieved (VMPPηSumMaxMin and VMPPηSumMaxMax). The maximum values (ηSumMax) for each of these levels are noted in the diagram. In the event that the courses of the VMPPηSumMaxMin and VMPPmin or VMPPηSumMaxMax and VMPPmax are identical, only one plot will be shown in the graph with the corresponding values (VMPPmin and VMPPmax).
The average overall efficiency gradient is shown in the same diagram and its highest value is noted, too (ηAvgSumMax). Average overall efficiency is attained by averaging all overall efficiencies at every level of the MPP nominal power range over the entire MPP voltage range outlined by the manufacturer. The average gradient is formed for power levels between 5 and 100 percent of nominal power. If the figures for medium (ηPmed) and high irradiation (ηPmax) are weighted, the PHOTON efficiency is determined. This value is also stated in the diagram.
The total grade is based on two criteria: the assessment of the efficiency determined by PHOTON and the temperature-related reduction of efficiency. The grade for this efficiency is assigned without any differentiation according to the suitability of the inverters use with a particular solar generator. The best device is the one with the highest efficiency independent of whether or not it has potential separation, is exclusively designed for use indoor or outdoors, or is a broad voltage device. In the meantime, there are now suitable transformerless inverter topologies for all known module types. Only the conversion efficiency‘s temperature interdependency has a relevant influence on the grade. Furthermore, we provide information on the inverter‘s efficiency at 25 °C and the maximum temperature before any power reduction is detected. Both values are subtracted from one another. If the resulting efficiency reduction reaches or exceeds the difference from the next, lower grade (for example, there‘s a 1.5 gap between »B« and »C«), the device receives the lower grade (i.e. »C«).
The question of whether an inverter is well-suited for use with a particular module type is best answered by the manufacturer, but our tests should provide some guidance. For instance, the connections of some thin-film module types cannot be charged with negative potential against the ground. A few crystalline high-power modules require a high-impedance ground at the DC connection to avoid polarization effects. PHOTON requests approval from the manufacturers of these module types for the inverter under examination. As a matter of principle, one always needs to know the inverter input‘s potential in relation to the ground. Naturally, PHOTON Lab also measures the efficiency and the MPPT adjustment efficiency, both based on the specified PMPP power – the product of which is the overall efficiency. This is then applied across all the measured input voltages to establish the average at each power level. This average is then weighted according to European and Californian efficiencies, and included in the evaluation. The overall efficiency is based on Heinrich Häberlin‘s definition of »total efficiency,« which is described in his book on the efficiency of PV inverters published in 2005. The PHOTON efficiency for medium and high irradiation levels is an artificial value that represents an image of the voltage and power interdependencies of an inverter‘s efficiency. The European and Californian weighting system reveals the dependence of the average overall efficiency from the geographic latitude at which the PV system is installed. This dependency is expressed with different weighting factors that result from the inclusion of meteorological data. This data allows one to make frequency distributions for certain solar irradiation values, which in turn provide weighting factors for particular power levels. The innovative part of the calculations used to establish PHOTON‘s efficiency is that it includes all measured input voltages as specified in the manufacturer‘s description of the device‘s input voltage range – even if the device cannot perform as required in all parts of this range, in which case the efficiency is then listed as 0 percent. This reflects the conditions of a real PV system: after all, if an inverter had to face these conditions, it would cease operating properly.
The graphical representation shows these areas. For instance, the color diagram included in our inverter tests shows the inverter‘s efficiency, the MPPT adjustment efficiency, and the overall efficiency. The diagram is colored black if the maximum MPP voltage isn‘t adequately distanced from the inverter‘s maximum DC voltage, and if it doesn‘t have an active overload limit according to the manufacturer, which means no measurements can be conducted in this range, since the MPP tracker won‘t operate properly. The diagram also reflects the DC current limitation range. These black areas, which reflect a value of zero, are used to calculate an average in the new PHOTON grading system, and therefore have a strong influence on the grade. The resulting effect is desired, and a consequence of considerations about the inverter‘s actual, useable MPP range: an inverter will only get a good grade in the test if it actually can operate without limitations in the voltage range specified by the manufacturer. Finally, manufacturers who change their product data to reflect a more sensible MPP range will receive an improvement in their grade.
The color diagram also includes white, hatched areas. These represent areas in the MPP voltage range that are considered critical when designing a PV system. They are located in the upper end of the MPP range. There are two types of hatching marks. The diagonal upward lines represent an MPP range in which the VMPPmax is generally absent for PV systems with crystalline modules. The hatching marks in the other direction (i.e. sloping diagonally downward) also represent the MPP range in which the VMPPmax is generally absent for PV systems with thin-film modules. The exact definition of these limits can be established when designing a system with actual modules. Hatching can also be seen in the lower portion of the MPP range. This denotes the area in which the activation of the DC current limitation prevents the inverter from feeding 100 percent of available DC power PMPP into the grid. A PV system‘s VMPP shouldn‘t be located in this range either, since that would result in a yield loss.
The result of all of this is an efficiency number that is generally lower than the European efficiency, since this is usually measured at the »best« voltage levels, and does not take into account mismatching and unreliable operating ranges. That means that PHOTON‘s efficiency can make an inverter look like it will fair worse than its true performance in a real PV system, since it takes into consideration the entire input voltage range specified by the manufacturer – regardless of whether that range will actually be exploited by a particular PV system. Hence PHOTON‘s efficiency tells us something about the least you can expect from an inverter – and gives information for all system configurations that operate within the input voltage range specified by the manufacturer.