Research On The Key Issues Of The Three-level Active Power Filter

The active power filter (APF), as a dynamic harmonic suppression and reactivepower compensation apparatus, has gained more and more research attention due toits advantages such as anti-perturbation ability to the network parameters and fastdynamic response compared with the traditional passive power filters. Three-levelneutral-point-clamped inverters applied to APF not only increased the installedcapacity greatly but also improved the harmonic compensation performance andsystem reliability. This paper investigates the key techniques of the three-levelneutral-point-clamed inverter applied to APF.As for the APF double closed-loop space vector control, the observation of thespace vector orientation angle is a critical point. Whether the observation of theorientation angle is rapid and accurate directly affects the final harmoniccompensation performance of APF. For the traditional phase lock loop, the parameterdesign for detecting the voltage orientation angle is relatively complicated. And animproved observation method of detecting the virtual flux angle of the APF grid sideis proposed in the paper, which adopts a novel identification algorithm composed ofhigh-pass filters and novel coordinate transformation in place of the pure integrator inthe traditional grid virtual-flux observation, thus eliminating the virtual-fluxobservation errors brought about by the integral initial value and DC offsets. Theimproved method can effectively avoid the impact of grid high-frequency harmonicsand improve the accuracy of the observed grid virtual-flux angle. Simulation andexperiments demonstrate that the novel identification algorithm applied to flux angleis feasible and superior.Compensation current detection is one of the critical techniques for APF andwhether the detection is accurate and fast directly affects the compensation precision.An improved FBD harmonic detecting method is proposed in this paper, whichemploys the virtual-flux observation method and reconstructs the three-phase voltageof the unit amplitude by the relations between the grid virtual-flux angle and thethree-phase voltage actual angle, thus eliminating the phase lock loop. And besides, inthe improved FBD harmonic detecting method, two low-pass filters are adopted toreplace the pure integrator, which guarantees the amplitude and the phase angle of nodeviation.APF is characteristic of multi-input, multi-output, strong coupling and nonlinearity. Considering the disturbance attenuation performance and modeluncertainty as a whole, a novel H∞robust control strategy is proposed in this paper.According to the mathematical model of APF, state equations of the generalizedcontrolled target are established. And taking parameter perturbation and externaldisturbance into consideration, the particle swam optimization (PSO) algorithm isemployed for parameter tuning and optimization. By solving the Riccati inequality,the solution to the designed controller is obtained. Simulation and experimentsdemonstrate that the proposed controller meets the requirements of system robuststability and disturbance attenuation simultaneously.APF demands the inner current control loop of rapidity, robustness andstability. So a novel sliding mode controller is also proposed in the paper. Someimprovements are made to the exponential reaching law put forward by AcademicianGao Weibing and therefore a new variable-exponent reaching law is derived, whichmakes the system state variables approach to the sliding surface at the variable speedand the exponential speed in the beginning, and when the status point is close to thesliding surface, the value of the exponential component gets near to zero, and then thevariable speed component plays a key role. Chattering can be inhibited effectivelywhen state variables are infinitely close to zero in the process of system stabilizing.Simulation and experiments have demonstrated the effectiveness, simplealgorithm, easy engineering implementation of the proposed controller, and the moresuperior invariance to robust control aforementioned.In the devising stage of the practical system, to seek a solution to theelectromagnetic compatibility problem of the main power circuit is the most critical. Aflat-panel design idea based on finite element analysis is presented in the paper. Bymeans of appropriate modeling and optimization, the ideal low inductance flat-panelbus can be achieved. Both the simulation and field measurement demonstrate thevalidity of the method.