ImaStabil – Impedance Analysis of PV Power Plants to Ensure Stable and Reliable Operation on the Grid

The project addresses the assurance of a high quality of supply with a strong expansion of photovoltaics. With differential impedance analysis, the consortium plans to test and establish a new method for evaluating the stability and harmonic behavior of PV power plants in the field for the first time. This method takes into account the impedance at the grid connection point as well as the input impedance of the inverters. In addition to the Fraunhofer Institute, the project consortium consists of Helmut Schmidt University and the two companies morEnergy GmbH and greentech GmbH. 

In contrast to conventional large-scale power plants, photovoltaic (PV) power plants are made up of a large number of generation units. The electrical characteristics at the grid connection point result primarily from the interaction of the inverters used. Furthermore, cabling and transformers within the power plant play a decisive role. Up to now, there are no adequate solutions for the stability and harmonic analysis of such complex plants. This is reflected, among other things, by undesired resonance effects or high harmonic levels (OS levels), which are increasingly occurring despite extensive grid connection procedures. With the method of impedance spectroscopy of inverters, a method for determining the effective impedance curve and the internal OS sources of inverters has already been developed in the completed research project »StarStrop«, with which the frequency-dependent behavior of individual units can be well described. In order to be able to use this method for complex renewable power plants and grids, Fraunhofer ISE is developing methods for measuring the frequency-dependent impedance and the internal harmonic sources of equipment in PV power plants in the laboratory in order to derive generic models for simulating the harmonic emission and analyzing the harmonic stability. Using these models, methods were developed for modeling power plants and the corresponding grid connection points, which can then be used to simulate the harmonic emission and analyze the harmonic stability of the plants and grid sections. These methods are validated by measurements of the grid impedance with a mobile measurement container of the Helmut-Schmidt-University as well as the harmonic emission in three different PV power plants. In addition, morEnergy GmbH developed a mobile plant impedance measuring device in order to be able to determine the devices and plant impedance even for existing devices in the PV power plant. Together with greentech GmbH, a health monitoring method based on the plant impedance was also tested.

Results:

The project successfully implemented the impedance-based stability criterion in practice; this method now makes it possible to minimise incorrect assessments of grid connection points, thereby preventing yield losses and damage to equipment in PV parks.

Two PV parks were surveyed in detail. In the laboratory, impedance spectra of system components such as cables, inverters and transformers were recorded. These were used to simulate the two PV parks and verify the stability-enhancing methods.

The findings are already being incorporated into grid connection procedures, and future measurement methods are to be standardised. Inverter impedances can be specifically shaped (so-called impedance shaping), and inverters can provide additional grid services as ‘dampers’.