Influence of local, frequency-dependent grid and earthing impedances on the current distribution in the earth and earthing system

• Applicant: Professor Dr.-Ing. Uwe Schmidt
• Subject Area: Electrical Energy Systems, Power Management, Power Electronics, Electrical Machines and Drives
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521916793

The neutral point treatment in connection with the design of earthing systems is an essential part of the measures to comply with personal safety in the operation and planning of electrotechnical systems and networks. It has an effect on the dimensioning of medium and high-voltage networks in particular. Compliance with the normative requirements in the event of earth fault currents and the resulting touch voltages is obligatory. The dominant neutral point treatment in power systems with nominal voltages up to and including 110 kV is resonant neutral point earthing (RESPE). The advantage of this neutral point treatment is very low (residual) earth fault currents and the associated low touch voltages in the event of a fault. Measurements in recent years show that high proportions of harmonic currents often dominate the residual earth fault current. This leads to controversial discussions about the influence of harmonic currents on the design of earthing systems. The measures of the current normative standard always refer to rms values and are formulated in relation to the nominal grid frequency (50 Hz). The differentiated handling of components of higher frequency is not defined. Taking these into account requires additional measures and leads to the expectation of impermissible touch voltages. If the frequency-dependent current distribution in the earthing system is known, it may be possible to neglect harmonic components of the potential-raising current. This significantly reduces the effort required to comply with the touch voltage. Within the scope of the research project, the frequency-dependent current distribution in the earthing system is investigated. Based on measurements in distribution networks up to and including 110 kV, a model is developed to describe the locally distributed, frequency-dependent impedances and the effective current loops in the event of an earth fault. The frequency-dependent current distribution in the earth and earthing system is fundamentally described. By developing mathematical procedures for mapping this current distribution, resulting reduction factors for the earth current and earth voltages are calculated. A worst-case consideration for any distribution network configurations is derived. The University of Applied Sciences Zittau/Görlitz has been involved in the technical work of the committees in a leading role for many years. The Chair of Electrical Energy Systems has many years of experience in the research field. Numerous publications prove the necessary expertise. The investigations provide valuable results for the dedicated consideration of harmonics in the earth fault current, especially in the residual earth fault current at RESPE, and the description of the potential-raising effect of these on earthed equipment in different network constellations. On a scientific basis, quantitative criteria are provided for the evaluation of existing earthing measures and areal earthing systems.

• DFG Programme: Research Grants