Modular multilevel matrix converter for fractional frequency transmission in offshore wind power integration

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Luo, Jiajie (2019). Modular multilevel matrix converter for fractional frequency transmission in offshore wind power integration. University of Birmingham. Ph.D.

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Abstract

Offshore wind power plays an important role in reducing carbon emission and the reliability on fossil fuel. The traditional high voltage AC transmission suffers from the disadvantage of high charging reactive current and therefore the maximum transmission distance is very limited. Fractional frequency transmission (FFT) is proposed to overcome this difficulty. With the offshore side operating at one third of the onshore grid frequency, the reactive current demand of FFT is greatly decreased. Modular multilevel matrix converter (M3C) is the core component of the FFT. With the advantages of easy scalability, excellent controllability and low loss, M3C is considered to be the future-generation AC/AC converter for offshore wind transmission and other high power applications. To start with, the operating principle and the control strategy of the M3C are presented. It is proven that M3C is capable as the frequency changer for FFT.
Although M3C FFT has various technical advantages, it is necessary to look into economic aspects to evaluate its feasibility in offshore wind power industry. A cost analysis of M3C FFT as well as a cost comparison with modular multilevel converter (MMC) high voltage DC (HVDC) transmission is presented. A cost model is developed considering capital cost and time-related costs including unavailability cost, operation and maintenance cost and power loss cost for different components in offshore wind power systems. Cost elements taking up large proportions of the total cost are identified and the cost-effective transmission distance ranges for the two technologies are calculated. Also, sensitivity analysis is conducted to investigate the influence of various parameters on the cost comparison.
It is shown that M3C FFT is a competitive candidate for offshore wind power transmission particularly at medium distance. Hence, there is need of a model to study its small signal stability and its influence on existing power systems. A small signal model of M3C is proposed for FFT system which considers the dynamics of AC currents from both sides, the sub-module capacitor voltage with DC and ripple components and the control system. In addition, a small signal analysis is presented to investigate the effect of controller parameters and sub-module capacitance on the small signal stability of the system. The small signal model is easy to implement and benefits system stability study and controller design.
Harmonic analysis is crucial from the stability point of view. In M3C, two frequencies from both AC sides couple and produce a complex harmonic profile due to the interactions between the arm quantities and the fast-switching sub-modules. A harmonic analysis method is proposed to determine how the harmonic components at various frequencies are generated in M3C. The harmonic components are quantified and factors that have large impact on the harmonic magnitude are discussed. The influences of different types of harmonics on the converter and external AC systems are analysed. Analyses are conducted on sub-module capacitors and arm inductors to give a guideline on their value selection to limit the harmonic level. A zero-sequence current mitigation controller is implemented and tested for M3C.
Time domain simulations are carried out in the electromagnetic simulation tool Real Time Digital Simulator (RTDS) in order to validate the correctness and accuracy of the M3C control strategy, the proposed small signal model and the harmonic analysis.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):