Summary
Besides offshore-wind connections, modular multilevel converters for high voltage direct current transmission (MMC-HVDC) are considered additionally for onshore embedded interconnector structures. Those interconnectors provide the possibility of using di fferent transmission solutions. Hereby, transmission grid planning flexibility is increased substantially as either DC cables or overhead lines can be utilized. In case of single DC pole to ground faults, which are associated with the most severe transient overvoltages, comparisons and conclusions on air clearance calculation between different realizations need to be addressed for feasible system design considerations. In case of symmetrical monopolar MMC-HVDC settings, the non-faulted pole is shifted during a short circuit contingency towards twice its rated voltage, which is limited only by the chosen protective level of the used surge arresters to approximately 1.6-1.8 per unit (pu). Given the fact that the voltage level remains constant after both converter stations are blocked, further investigations on insulation coordination aspects are mandatory. While considerations for cable operational aspects are addressed in recent publications, the impacts on air clearance calculation need to be broached as well. Insulation coordination is demanding for DC systems as standard voltage ratings are, compared to high voltage alternating current (HVAC) systems, not yet existing. This implies that established insulation coordination standards known for AC applications are not directly applicable for HVDC technology. Especially standard insulation levels cease to be valid, as occurring overvoltages depend on several system design aspects. This yields to the effect, as proposed in IEC 60071-5, that project-specific system analysis is used for the determination of representative voltages and overvoltages. Based on these results, withstand values are assigned and may be used for air clearance calculation. For this purpose, transient voltages occurring in a symmetrical monopolar MMC-HVDC system with a rated DC voltage of ± 320 kV and hal f-bridge submodules are investigated. Results obtained in EMTDC software PSCAD provide initial data to estimate applicable switching impulse test levels and are evaluated for an embedded interconnector structure using cable and overhead line transmission technology, respectively. Investigated fault scenarios are discussed with respect to withstand voltage assignment and ai r clearance calculation, as the disruptive strength of ai r is among others dependent on the voltagetime behavior of the impulse.
Additional informations
| Publication type | ISH Collection |
|---|---|
| Reference | ISH2017_122 |
| Publication year | |
| Publisher | ISH |
| File size | 698 KB |
| Pages number | 6 |
| Price for non member | Free |
| Price for member | Free |
Authors
PROSR, CHVOJKA, Andersson, FANTANA
