Direct grid connection and low voltage ride-through for a slip synchronous-permanent magnet wind turbine generator

[摘要] ENGLISH ABSTRACT:The slip synchronous-permanent magnet generator (SS-PMG) is a direct-driven, direct-to-grid generatorfor wind turbine applications. This investigation focuses on achieving automated grid connection andlow voltage ride-through for a small-scale SS-PMG. To reduce cost and complexity, components suchas blade pitch controllers and frequency converters are avoided. Instead, electromagnetic braking isemployed to control turbine speed prior to grid synchronisation and compensation resistances are usedto facilitate grid fault ride-through.The conditions under which the SS-PMG can be successfully synchronised with the grid are determined,indicating a need for speed control. An evaluation of electromagnetic braking strategies revealsthat satisfactory speed control performance can be achieved when employing back-to-back thyristors toswitch in the braking load. Simulations show that controlled synchronisation can be executed successfullyunder turbulent wind conditions. All controllable parameters are held within safe limits, but theSS-PMG terminal voltage drop is higher than desired.Compensation is developed to allow the SS-PMG to ride through the voltage dip profile specified bythe Irish distribution code. It is found that a combination of series and shunt resistances is necessary toshield the SS-PMG from the voltage dip, while balancing active power transfer. The flexibility offered bythyristor switching of the shunt braking load is instrumental in coping with turbulent wind conditionsand unbalanced dips. The South African voltage dip profile is also managed with conditional success.Following on from the theoretical design, the grid connection controller is implemented for practicaltesting purposes. Protection functions are developed to ensure safe operation under various contingencies.Before testing, problems with the operation of the thyristors are overcome.Practical testing shows that grid synchronisation can be undertaken safely by obeying the theoreticallydetermined conditions. The speed control mechanism is also shown to achieve acceptable dynamicperformance. Finally, the SS-PMG is incorporated into a functioning wind turbine system and automatedgrid connection is demonstrated under turbulent wind conditions.Future investigations may be focused on optimal control strategies, alternative solid-state switchingschemes, and reactive power control. Low voltage ride-through should also be optimised for the SouthAfrican dip profile and validated experimentally.

[发布机构] Stellenbosch University