Jan Lykkegaard

Overvoltage Protection of Large Power Transformers—A Real-Life Study Case

This paper demonstrates the results from a detailed study of the overvoltage protection of a particular 400/150-kV 400-MVA power transformer. The work presented here is based on a real-life power system substation design and data and initiated by Danish TSO Energinet.dk as a consequence of serious transformer overvoltage damage. A simulation model for the entire system consisting of overhead line, transformer, surge arrester, and earth grid has been created in PSCAD/EMTDC. The main focus has been put on the earth grid, which has been submodeled in detail in MATLAB using an electromagnetic transient approach based on the thin-wire program made by J. H. Richmond for NASA in 1974. The earth grid model is verified with excellent agreement compared to already published results. The overvoltage performance of the particular case is analyzed, and it shows that the transformers LIWL have probably been exceeded. It is clearly illustrated that the transient performance of the earth grid plays an important role in the overall overvoltage protection system design.

Hybrid time/frequency domain modelling of nonlinear components

This paper presents a novel, three-phase hybrid time/frequency methodology for modelling of nonlinear components. The algorithm has been implemented in the DIgSILENT Power Factory software using the DIgSILENT Programming Language (DPL), as a part of the work described in [1]. Modified HVDO benchmark model is used as a basis for its implementation. First, the linear network part is replaced with an ideal voltage source and a time domain (EMT) simulation is performed. During the initial oscillations, harmonic content of the converter currents is calculated at every period by a fast Fourier transform and the periodic steady state is identified. Obtained harmonic currents are assigned to current sources and used in the frequency domain calculation in the linear network. The obtained three-phase bus voltage is then inverse Fourier transformed and assigned to the voltage source and the time domain simulation is performed again. This process is repeated until the change in the magnitudes and phase angles of the fundamental and low order characteristic harmonics of the bus voltage is smaller then predefined precision indexes. The method is verified against precise time domain simulation. The convergence properties of the method are shown. The method gives precise results for harmonic frequencies up to the 50 th, but after adjustments can be used for harmonic frequencies above the 50 th, time-varying and inter-harmonic frequencies.

Validation techniques of network harmonic models based on switching of a series linear component and measuring resultant harmonic increments

In this paper two methods of validation of transmission network harmonic models are introduced. The methods were developed as a result of the work presented in W. Wiechowski (2006). The first method allows calculating the transfer harmonic impedance between two nodes of a network. Switching a linear, series network component that links the nodes causes a variation of the background harmonic voltage distortion at the nodes. Harmonic current in the series element is measured together with the harmonic voltages at both nodes, before and after the switching. Obtained incremental values of harmonic voltages and the current are used for calculation of the transfer harmonic impedance between the nodes. The determined transfer harmonic impedance can be used to validate a computer model of the network. The second method is an extension of the first one. It allows switching a series element that contains a shunt branch, as for example a transmission line. Both methods require that harmonic measurements performed at two ends of the disconnected element are precisely synchronized.

Transformer Core Nonlinearities Modeled in Harmonic Domain

This paper demonstrates the results of implementation and verification of an already existing algorithm that allows for calculating saturation characteristics of single-phase power transformers. The algorithm was described for the first time in 1993. Now this algorithm has been implemented using the DIgSILENT programming language (DPL) as an external script in the harmonic domain calculations of a power system analysis tool PowerFactory. The algorithm is verified by harmonic measurements on a single-phase power transformer. A theoretical analysis of the core nonlinearities phenomena in single and three-phase transformers is also presented. This analysis leads to the conclusion that the method can be applied for modelling nonlinearities of three-phase autotransformers