Abstract
To evaluate and optimise insulation coordination concepts for state of the art high-voltage direct current (HVDC) transmission systems, appropriate test voltage shapes are required for laboratory imitation of occurring stresses. While especially transient voltages in the monopolar modular multilevel converter (MMC)–HVDC links show an extensive deviation from commonly applied switching impulse shapes, this study focusses on the analysis of over-voltages subsequent to direct current pole to ground faults. Additionally, novel methods for synthetic laboratory test voltage generation are proposed. Based on simulated transients occurring during fault scenarios in different symmetrical monopolar ±320 kV MMC–HVDC schemes, curve fitting, and related analysis techniques are used in order to compare simulated over-voltages with standard test voltage shapes. Moreover, these techniques further allow the identification of novel relevant impulse characteristics. Subsequently, design considerations for the generation of non-standard impulses based on single-stage circuits are derived and discussed. Those synthetically generated voltages may, later on, provide the basis for future investigations on related dielectric effects caused by those non-normative over-voltages.
Highlights
High-voltage direct current (HVDC) transmission is currently a vital part of the area of electric energy transmission technology
Time to peak significantly increases with increasing cable length. o Time to half is the shortest for overhead line (OHL) transmission technology and in the case of cable transmission larger, whereas time to half is less affected by cable length but by chosen curve fitting horizon. o Voltage time areas are roughly similar if evaluation time is limited to teval = tflt + 40 ms
The analysis of obtained simulation results shows that normative test voltage waveforms such as switching impulse (SI) only allow a poor approximation of the occurring over-voltages
Summary
High-voltage direct current (HVDC) transmission is currently a vital part of the area of electric energy transmission technology. Applicable switching impulse (SI) test voltage ratings for MMC–HVDC monopolar configurations are not yet fully standardised compared with high-voltage alternating current [2, 3] and line commutated converter-HVDC schemes [4]. While the latter research activities are mostly related to cable stresses, associated impacts on air clearance calculation [10] at the converter direct current (DC) busbar are still rare. [11]) based on normative standard impulses, such as lightning impulses and SIs, provide first steps towards insulation strategies, no general standard exists for voltage source converters [12]. To evaluate associated consequences for insulation coordination concepts, transient voltage stresses at the converter. DC busbar in MMC–HVDC schemes need to be evaluated, analysed and compared with normative impulses in greater detail. Besides theoretical influence analysis based on accessible test data results, novel proposals to generate nonstandard laboratory test voltage waveforms need to be derived
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