Revenue declined about 15 percent
In a solar power plant in southwest Germany, transformerless central inverters are used for grid connection. In some central inverters, EMC filters failed. Measurements on site revealed a strong superimposition of the DC voltage with AC components in almost every second inverter used. The DC PV power was reduced by approx. 30%. morEnergy GmbH was commissioned to find the cause of the reduced power and to solve it.
Grid impedance and power quality
Measurements were carried out on site to determine the frequency-dependent grid impedance and the power quality for an inverter without significant AC components in the DC voltage and an inverter with high AC components in the DC voltage during feed-in operation. There were no high AC components, so that a problem with the regulation and control of the inverter could be excluded. During the measurement of the impedance it was noticeable that the parallel resonances showed a different quality over a wide frequency band. These deviations could only be explained by differences in the filters, which were then examined during further.
Filters
Considerations have led to the assumption that the AC components on the DC side couple into the DC side through filter capacitances above ground. The measurement results showed that the capacitance between two external conductors is similar for all inverters. However, the capacitance between the outer conductor and earth differed by a factor of 1000 from problematic to nonproblematic inverters. This explained the different grid impedance curves. From these different earth capacitances an explanation was found why some inverters can be operated without problems and others cannot – their age.
Inverters and their characteristics
Inverters are electrical circuits that can turn direct current into alternating current. However, this alternating current is strongly distorted compared to the 50 Hz alternating current in the grid. This reduces the voltage quality of the grid, which is why filters are used in every inverter. State of the art are passive filters, which consist of an electrical network with capacitors and coils. These components are subject to an ageing process. If, for example, capacitors age, in most cases the capacity decreases until there is no more capacity and the capacitors have a completely insulating effect. These aging processes can be dangerous for installations, as over the years the filter parameters shift and as a result the installations are no longer properly protected.
The morEnergy solution
To prevent AC components from coupling to the DC side of inverters, the AC main filter and an AC auxiliary filter should be modified or replaced with new filters. It should be noted that filters were used which have no or only very low capacitance to earth. A short-term immediate measure was to disconnect one filter. This reduced the capacitive earth currents and thus the increased coupling of EMC interference from the AC side to the DC side. However, the earth capacitances of the AC main filter also had to be removed in order to completely eliminate the problem.
According to morEnergy’s recommendations the solar park was able to operate at 100% power again, which resulted in a significant increase in revenues.
morEnergy offers its customers preventive measurements within obligatory accident prevention measurments (DGUV V3) or other control measurements to detect faulty components. This can be done easily even during the power plant is operating with the ONIS technology developed by morEnergy and is currently unique worldwide.