Research On Control Technology For Permanent-Magnet Direct-Driven Tidal Generation Electricity Convertors

With the development of society and economy，the demand for electric energy growsrapidly. Not only traditional power generation pollutes the environment, but also the fossilenergy resources are limited, such as coal, oil and natural gas. Tidal current energy hasreceived increasing attention, as pollution-free, renewable and rich resource. Convertor isone of the key equipments，and its performance directly affects the performance of theentire power generation device.The installation environment of the tidal current energy generation device is harsh andthe rotor position sensor is hard to installed, expensive and easy to damage. The sensorlesspermanent magnet synchronous generator control technology was studied in this paper. Themodel of the permanent magnet synchronous generator was constructed and analyzed. Therotor position estimation methods based on phase locked loop and state observer werestudied, and two rotor control strategies based on the two estimation methods wereintroduced. A novel rotor back electromotive force method was introduced.With the increase of tidal current energy generation device capacity, convertor paralleltechnology is an effective way to increase the capacity and reliability, to reduce thedevelopment cost. There are two kinds of convertor parallel topological structures, the firstone is the common dc bus parallel topology and the second one is the independent dc busparallel topology. The average value models of the two topology structures were constructed.The zero sequence circuit current reduces efficiency, causes current distortion and affectsthe stability of the system. The zero sequence circuit current average models wereconstructed by the analysis of the convertor parallel models. The effects of the dead time,switch pipe voltage drop and PWM method on the zero sequence voltage were analyzed. Anovel zero sequence circuit current suppression method based on virtual impedance wasintroduced, by the analysis of the convertor zero sequence impedance. The methodincreases the zero sequence impedance to suppress the zero sequence circuit current, andincreases some harmonic impedance to improve the performance, because of that the zerosequence voltage is periodic signal. The closed control model of the zero sequence circuitcurrent was constructed and analyzed. The zero sequence voltage of the convertor is themain disturb. A novel rapid repetitive control method was proposed, the response speed ofwhich is third times faster than the traditional repetitive control method. When tidal current generation device works in island mode, the convertor need tosupply three-phase four-wire power, and the three-phase four-leg convertor has the ability. Anovel three-phase four-leg PWM modulation method was proposed in this paper, and hasthe advantages of high dc voltage utilization ratio and small calculation volume. By themethod, reference voltage was divided into two components, the AC component and thecommon component. The AC component was used for abc-phase legs SVPWM; thedifference between common component of abc-phase modulation voltage and the commoncomponent of reference voltage was used for the fourth leg modulation. It was testified thatthe fourth leg output three-phase average voltage is same with the reference voltage, by thetheoretical analysis. The average model of the three-phase four-leg convertor wasconstructed and analyzed, which was in synchronous rotate coordinate. The voltage andcurrent feedforwards was added in the control loop to decouple the d-axis loop and theq-axis loop. The three-phase four-leg convertor is same to three dc buck convertors. A novelcontrol strategy that the PI controller was parallel with the resonant controller was proposed,by the analysis of the load current.The use of LCL filter in convertor can reduce the weight and cost of the device, so thedesign method and resonance suppression methods of LCL filter were studied. The LCLfilter design limited conditions of voltage drop, harmonic suppression, reactive power, lossand response speed is studied. The relationship between the limited conditions and thecomponents parameters was studied, and an LCL filter design method was introduced. TheLCL filter is third order under-damped, the large gain at the resonant frequency increasesthe design difficulty and reduces system stability. The resonant problem was analyzed andsome resonant suppression methods were introduced.The research results can be applied to UPS, active power filter, wind generator andsolar generator, et al.