| In the offshore wind power system, high power direct-drive wind turbine with a unit capacity of more than 5MW has gradually become the mainstream model. Owe to the advantages such as low loss, high efficiency, small volume and ease of installation & operation, the medium-voltage technical route is the the optimal choice for this wind generator. Based on the Subjects of Supported Program of the Ministry of Science and Technology during the 12th Five-year Plan Period (No.2012BAA01B04), this paper makes a deep study on the correlative key technologies of medium-voltage three-level full power wind power converter used in medium-voltage direct-drive wind power generation system, comprising the pulse-width modulation stategy, filter optimal design, modular parallel technology, fault ride through control strategy, several questions of power unit and the engineering practice & experimental study. The main contents and innovations of this paper are as follows:1) Aiming at the difficulty of harmonic filtering and poor control performance in the grid-side converter (GSC) for the reason of low switching frequency, DPWM strategy is introduced to increase the carrier frequency. Six kinds of DPWM are comparatively analyzed. DPWMA with the best comprehensive performance is selected. And then, the carrier-based (CB) method is given and NPP fluctuation is discussed. Aiming at the common-mode voltage (CMV) ansd NP banlance requirement in motor-side converter (MSC), LMZVM stategy with no samll vector is selected, and a NP balance method adding adjacent small vector with low CMV and the carrier-based method is studied. Futher more,A CB generalized DPWM with-flexible NP control and optimal losses is proposed, which can minimise the switching losses on the premise of putting NPP into a given range.2)The high-order filters of GSC are summarized and contrastively analyzed. Aiming at the output harmonic characteristics of imporved DPWMA, an LLCL filter is adopted to enhance the attenuation of high-order harmonic and reduce the filter size. Aiming at the stability of LLCL filter, two simple passive damping methods are compared and an improved damping scheme is proposed, which can effectively improve the carrier frequency harmonic attenuation, ensure the resonance suppression and significantly reduce the damping resistance losses. Based on the bridge-side current ripple analysis of DPWMA, The parameter design principle and process of the filter are given. In addtion, the parameter design principles of dc capacitor and LRC filter are also given.3)The topology and control scheme of 3L wind power conveter are classified and compared. Therewith, zero-sequence circulating current (ZSCC) for 3L back-to-back converter are analyzed and the equivalent model is built. According to the distributed parallel mode, a low-frequecy ZSCC eliminating method based on PR control with immediate sampling is adopted and the control stragey for two dc-link paralle modes is given. Then, the problem of use in medium-voltage high-power system is discussed. According to the centralized parallel mode, a control scheme based on EtherCAT technology is proposed, which can greatly improve the transmission rate and reliability of the communication and precisely guarantee the synchronization of carrier signal, and thus enhance the scalability of modular parallel converter. Furthermore, the oversampling control technology is introduced to improve the control bandwidth of the system and ensure that the system has a good stability margin.4)Based on the analysis of dc overvoltage, GSC controllability and NPP fluctuation in FRT, this paper has researched the method of dc overvoltage suppression, the dc volatege self-adaptive control and the NP balance control based on zero-level elimination. Then, the control scheme of FRT under the whole grid voltage change is proposed, which effectively improve the FRT ability of medium-voltage 3L converter.5) The problems in 3L SPWM digital implementation such as narrow pulse, pulse lose and start and stop logic are analyzed and solved. Aiming at the voltage unbalance between the inner and outer devices, the mechanisms of this phenomenon for 3L-NPC converter in various conditions are comprehensively analyzed. Based on this, a simple and reliable solution is proposed, which can effectively suppress the over-voltage of the inner device and protect the safety of switching devices.6) Three kinds of experimental platform under diverse voltage ansd power level are constructed. Based on these platforms, detailed experimental studies are also carried out. The feasibility and validity of the theories and design scheme described in this paper are verified. Moreover, a load test based on a 5MW/3300V PMSG traction systems for the medium-voltage 3L converter is carried out, which further verify the performance of designed hardware and software, thereby establishing the foundation for the further trial running in wind farm. |
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