Designing optimized control strategy for DRU topology used in offshore transmission to ensure stable grid connection
I. Nigam (TU Delft - Electrical Engineering, Mathematics and Computer Science)
M. Cvetkovic – Mentor (TU Delft - Electrical Engineering, Mathematics and Computer Science)
A. Lekić – Mentor (TU Delft - Electrical Engineering, Mathematics and Computer Science)
Joris Van Doorn – Mentor (TenneT TSO B.V.)
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Abstract
With the continued growth of offshore wind energy, it is well established that HVDC is the technically and economically preferred way to bring this power onshore. The cost of the offshore HVDC converter station, however, remains high. This thesis explores a simpler and more robust way to rectify offshore wind power, the diode rectifier unit (DRU), which lowers this cost but comes at the expense of controllability: unlike a voltage-source converter, a diode rectifier cannot impose or regulate voltage, so the offshore AC collector grid must be established and stabilized entirely by the wind turbine converters operating in grid-forming (GFM) mode. This work sets out to address the resulting technical barriers one by one, and is, at its core, an attempt to make the uncontrollable DRU work as a deliberate engineering process.
The study is carried out within TenneT’s 2GW, ±525kV bipolar offshore connection system, using a detailed PSCAD/EMTDC model. It is first shown analytically that the conventional virtual synchronous machine (P--f/Q--V) structure is structurally incompatible with DRU commutation behavior. A P--V/Q--ω droop architecture, derived directly from the commutation physics, is established as the physically correct GFM structure, with the grid-side converter forming the AC voltage and the machine-side converter regulating the DC link. A cascaded control design for the grid-side converter is developed for this architecture, and its operation is demonstrated in simulation. The study then extends to a mixed wind farm, investigating whether grid-following (GFL) turbines can be added alongside grid-forming units and identifying the issues arising from their coexistence.
Rather than presenting the DRU as a solved technology, this thesis resolves several of the barriers the uncontrollable diode rectifier imposes on grid-forming control, while making explicit those that remain.