Flexible Network Model to Study the Impact of Future Changes in Transmission Systems on Harmonic Levels and Impedance

CIGRE Session 2024

Location

Paris, France

Links

Preprint (PDF)

Keywords

Modeling, Harmonic Network Impedance, Harmonic Propagation, Transmission System, Inverter-Based Resources, Transmission Line Modeling

Abstract

The vastly growth of power electronic installations, like HVDC converter stations, wind parks, solar farms, FACTs, etc., has significantly increased the interest of harmonic studies in transmission systems. Harmonic studies in transmission systems have considerable challenges, mainly due to the complexity of the network (meshed network topology, realistic modelling of transmission lines and assets), the lack of information about parameters of network elements as well as the harmonic emission and impedance of connected customers or downstream networks, especially in case of unbalanced conditions. In order to study different aspects related to harmonics, like the impact of the downstream networks on the harmonic levels in transmission systems, the impact of the transition to inverter-based resources, the influence of transmission line modelling detail or the validation of methods for assessing the harmonic emission of customers, a test network has been developed and continuously improved over the last five years. The test network consists of an extra high voltage network, which is formed by three interconnected regions with a nominal voltage of 380kV and 220kV. Two downstream 110kV networks (one in a 380kV region and one in a 220kV region) are modelled in detail, to allow studies of the interaction between distribution and transmission systems, e.g. in terms of propagation or allocation of emission limits. The test network is developed in cooperation with the German Transmission System operators and is parametrized in order to represent the frequency-dependent impedance of all network components as realistic as possible. The test network represents typical characteristics of meshed transmission networks in central Europe. The first part of the article introduces and describes the test network in detail. The second part presents two examples of how the test network has been used to study practical research questions. The first example discusses the impact of inverter-based resources on harmonic impedance and propagation. The results show that inverter-based resources has a high impact on zero sequence impedances and harmonic influence coefficients, which quantify the propagation between network nodes. This is because inverter-based resources have not only a different impedance curve compared to conventional synchronous generators, but can also considerably change grounding conditions of the network. The second example discusses the impact of transmission line characteristics on harmonic impedance and propagation. The results show that the tower geometry and transposition configuration have a considerable impact on harmonic impedance and harmonic influence coefficients. The impact is higher for zero sequence impedances. The study also shows that in case of asymmetrical geometry conditions, a coupling exists between the impedance sequences. This means that balanced current harmonic injections can result in unbalanced voltage harmonics in the network. The paper concludes by discussing the potential of the test network to study other factors that affect harmonic propagation. Future work will focus on studying the impact of the characteristics of transformers, downstream distribution networks, customer installations, and power electronic based generation on harmonic impedances and propagation.