Multi-frequency stability of converter-based modern power systems

Nowadays, the transformation of power systems from being dominated by classical synchronous-machine-based generation to being characterised by renewable converter-based generation is accelerating. Conventional power systems typically exhibit high natural damping, fault current infeed and natural inertia. However, future power systems are expected to be characterized by limited damping, short-circuit current contribution and natural inertia from rotating masses. Systems with high penetration of power electronic (PE) equipment can enhance power system functionality and performance. However high penetration of converter-based equipment may potentially trigger unstable operation, if not investigated carefully and if the converter control is not coordinated with grid resonances and between different PE systems.

Moreover, the electrical infrastructure is becoming more complex due to the introduction of long high voltage alternating current (HVAC) cables, high voltage direct current (HVDC) connections, widespread penetration of renewable energy sources, e.g. photovoltaic (PV) plants, wind power plants (PPs), and offshore grid development. This power system transformation may pose instability risks locally within the harmonic frequency range as well as in a wider power system area within the frequency range close to the fundamental component.

The complexity of power system behaviour due to increasing application of power converter systems, creates challenges regarding stability. That requires multi-timescale analysis of multi-converter systems because control feedback loops in grid-connected converter control are characterized by multiple time constants and the plurality of PE equipment may trigger undesirable oscillatory behaviour. However, the use of converter devices provides a wide range of power system performance and stability enhancement solutions.

As the number of PE-based power generating units (PGUs) is rapidly increasing and the power system infrastructure is becoming more complex, there is a need for careful investigation of the system stability to assure robust and reliable operation. Even though there are several publications addressing instability terminology and classification of analysis methods, development of theory and mitigation techniques are ongoing for the analysis of potential stability problems in converter-based systems within the subsynchronous and harmonic (or supersynchronous) frequency ranges.

Better understanding of instability phenomena, applicable stability analysis methods, and instability mitigation measures are needed. Moreover, recommendations, guidelines and best practice for academia as well as industry are required to allow for gradual power system transformation from conventional generation to renewable-based generation relying on power converters. Power system operators, operators of renewable PPs, transmission solution developers, renewable generation developers, academic units and original equipment manufacturers expect coordinated effort to understand how to identify potential instability risks and how to resolve them.