Measurement-based analysis of harmonic instabilities of single-phase photovoltaic inverters in public low voltage networks

The transition of power systems from central energy generation to decentralized energy generation leads to a growth of renewable energy generators with power electronic devices in public low voltage networks.
In practice, the interaction of a power electronic device and the power grid, i.e. the low voltage network, challenge the device operation, the operation of other grid-connected devices and the network equipment.
For a manufacturer, the network characteristics where the device is installed, are typically unknown, e.g. especially for devices in the low power range up to some kW, where the number of manufactured devices is large. The characteristics vary largely between different low voltage networks and even between measurement points within the same low voltage network.
For the network operators, the installed devices are unknown, i.e. a black-box, since the manufacturers keep the detailed device designs including the device parameters a manufacturer's secret.
With the aim to pursue the climate goals, renewable energy generators become more important. One of the main renewable energy generators are low power photovoltaic systems. These photovoltaic systems are connected to the grid via single-phase in-verters. The stable operation of the inverter is consequently relevant for the reliability of power systems. One of the phenomena that challenge the stable operation of inverters are harmonic instabilities. The harmonic stability analysis identifies an instable opera-tion of an inverter based on the interaction of the inverter control as well as the AC-side filter circuit and the network impedance in the harmonic frequency range, i.e. above 50 Hz up to 2 kHz.

In this dissertation, the currently known theory is extended to enable measurement-based assessments of the harmonic stability of unknown single-phase inverters for photovoltaic applications. The studied single-phase inverters are commercially available while in addition simulation models are developed. Advancements of the measure-ment-based model identification and the harmonic stability analysis are presented and validated. Next to theoretic test cases, impedance characteristics of real low voltage networks are also included to assess the harmonic stability of the inverters. As a con-clusive result, device design recommendations are derived from the findings of the assessment and limitations of the harmonic stability with regard to the overall stable operation of the inverter are presented.