Modelling and stability analysing of fractional frequency offshore wind power system with modular multilevel matrix converter

Lin, Kai (2023). Modelling and stability analysing of fractional frequency offshore wind power system with modular multilevel matrix converter. University of Birmingham. Ph.D.

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Abstract

Offshore wind energy contributes significantly to the reduction of carbon emissions and the dependability on fossil fuels. Current transmission systems have been challenged by the growth trend of offshore wind power system establishments. The maximum transmission distance of the traditional high voltage AC (HVAC) transmission is hampered by the reactive current charging requirement. In contrast, the fractional frequency transmission system (FFTS) is a promising technology to solve this issue. The offshore grid is designed with a fractional frequency equal to around one-third of the system frequency, which significantly reduce the charging current of the transmission system. The frequency converter is the central component of the FFTS, and the modular multilevel matrix converter (M^3 C) is regarded as the recognised new AC/AC converter technology for offshore wind power integration due to its advantages of simple scalability, full controllability, and high effectiveness. The system structure, working principle, and control system of the M^3 C are presented initially in this study in Chapter 3 with the electromagnetic (EMT) simulation model developed in MATLAB/Simulink software. It has been demonstrated that M^3 C can serve as the frequency converter for FFTS.
The small signal stability is an enduring topic of power system study. The M^3 C based FFTS is shown to be a competent contender for offshore wind power system integration, therefore, a small signal model is required to examine its damping ability. In Chapter 4, the small signal model of FFOWPS contains the doubly fed induction generator (DFIG) based wind power system and the M^3 C is proposed, taking into account the dynamics of DFIG system, the M^3 C and the interaction between the two power electronic systems. In addition, an eigenvalue based small signal stability analysis is implemented to examine the impacts of controller parameters and circuit elements on the damping ability of the system.
It is proven that the proposed DFIG-M^3 C based FFOWPS has numerous technical advantages, nevertheless, its economic viability must be evaluated in order to determine its industrial applicability. A techno-economic cost comparison between M^3 C and back-to-back modular multilevel converter (BtB-MMC) is presented in Chapter 5. The two frequency conversion technologies utilised in FFOWPS are studied and compared in terms of technical performance. The performance of the two technologies is validated. For further comparison, an economic analysis model incorporating the capital costs, unavailability costs, operation and maintenance (O&M) costs, and power loss costs for FFOWPS components is developed. Cost components comprising a significant portion of the total cost are highlighted in the two versions of FFOWPS and it is indicated the wind turbine consist of up to 50% of total cost of FFOWPS. To further pursue the economic advantage of the M^3 C based FFOWPS, the economical transmission distance of the M^3 C based FFOWPS and BtB-MMC based HVDC system is investigated, and hence the advantageous transmission range of the two technologies is identified while the M^3 C based FFOWPS is a more cost effectiveness choice for medium transmission distances around 280 km.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):