Control For Distributed HESG-MC Variable Speed Generation System

Variable speed constant frequency (VSCF) generation technology becomes a main trend indistributed generating application such as micro-turbines generation, wind energy conversion, smallhydroelectric generation etc. Generator and power converter are two indispensable componentsapplied in VSCF generation system. Hybrid Excitation Synchronous Generator (HESG) combinespermanent-magnet and electrical excitation sources, and possesses the merits of high efficiency andcontrollable air-gap magnetic field. Thereby, HESG is a preferred candidate in VSCF generationsystem for a wide-speed range operation. Matrix Converter (MC) is a kind of AC/AC converter withthe features such as the compact size, high power density and the absence of the intermediate DC linkfor energy storage. It achieves sinusoidal input and output waveforms simultaneously. As a result, theintegration of the HESG and the MC is a favorable choice for VSCF generation system.This paper focuses on the control technologies of HESG-MC based variable speed distributedgeneration system. Several methods, such as theoretical analysis, digital simulation and experimentverification, are adopted in a series of studies on MC modulation scheme, grid connected control,active damping control and closed-loop control method for the HESG-MC generation system.The modulation scheme is the core of MC control strategies. In this paper, the double line-to-linevoltage synthesis (DLLVS) modulation scheme which is based on the instantaneous value of the inputline-to-line voltage is fully studied. As there are various combinations of input and output sectors inMC, a novel sector division regular is proposed to simplify the digital realization of DLLVS method.By introducing a parameter which indicates the pole of input determinate phase voltage, the inputsector division is interrelated with output sector division. A novel output sector division regular isproposed to reduce the output sectors from6to3, and also reduce number of sector combinationsfrom36to18.The grid-connected control for matrix converter is the premise for distributed variable speedgeneration. This paper studies the DLLVS modulation strategy and grid voltage oriented currentcontrol method for operation of grid-connected matrix converter. The problem of narrow pulses inMC is unavoidable because of existing modulation algorithm performance. Narrow pulses may affectcommutation safety and cause non-linearity distortion especially when MCs are applied as gridconnected converters. The problem of narrow pulse was analyzed in this paper. Based on the normaolpulse state and narrow pulse state, a dual side and single side hybrid chopping pattern is explored to compensate the narrow pluses, depressing the distortion in grid connected current effectively. Thismethod chooses dual side chopping pattern for normal pulses, and single side chopping pattern fornarrow pluses, largely reduced the numbers of narrow pluses which may cause commutation problems.For narrow pluses which still can not fininsh a whole four step commutation, a compenstation methodis adopted. The compensation guarantee the average value of output voltage do not change by updatedduty ratio.To obtain a good performance of the MC, it is necessary to design an input LC filter to smooththe input currents. The supply source of MC is a generator in this proposed generation system, so thearmature windings of the generator are used as the filter inductors. In this way the volume and weightof LC filters can be reduced. Additionally, the stability of MC system is concerned, by establishing thesmall signal molde for LC filter, matrix converter and inductive loads, the Lyapunov stability theory isutilitized to analyze the relation of parameter variation and system stable region. Based on thestability analysis, an active damping control with proportion and derivative fed forward algorithm isalso investigated for better performance and stability.In HESG-MC variable speed generation system, excitation current and modulation ratio are twocontrollable variables. Based on these variables, a two-stage closed loop control scheme is proposed.The preceding stage is a generator terminal voltage closed loop control, which is realized byregulating excitation current, reduced the influence of speed fluctuation on MC performance. Thebackward stage is an instantaneous value closed loop control of MC, and MC can choose currentclosed loop control or voltage closed loop control, whch based on the operation state of distributedgeneration system. The backward control can achieve fast output voltage and current adjustment andgood dynamic performance of generation system.Finally, the experimental platform of the HESG-MC variable speed distributed generation systemis constructed, and the software control core is realized in digital signal processor (DSP) and complexprogrammable logic device (CPLD). The proposed VSCF generating system topology and itsfeasibility are verified on the prototype. The platform also provides a reliable basis for future research.