Research On Ripple Voltage Suppression Strategy Of Active Power Filter DC Side

The Active Power Filter(APF)has been considered the most prospective power quality regulator since its inception,enabling real-time tracking with good dynamic performance.Typically,the main harmonic components present in a three-phase system are 5th and 7th harmonics,and in a single-phase system the main harmonic components are 3rd harmonics.When the load is unbalanced or the load abruptly changes,the third harmonic and the negative sequence component will be generated in the three-phase system.When the APF compensates for these harmonics,the grid and the APF exchange energy,and the negative sequence of the base wave and the third harmonic will generate a large number of second ripple voltage in the DC side of the APF capacitor,and superimposed with the sixth ripple to generate large voltage fluctuations,affecting the compensation performance of the APF and even the breakdown of the DC side capacitor.The study of APF DC side ripple voltage suppression strategy is of great significance for the safe and reliable operation of APF.In order to address the above problems,two suppression strategies are proposed:(1)a high and low frequency control strategy;(2)a ripple voltage suppression strategy based on an improved Time-Domain Transformation Algorithm(TTA).The high and low frequency suppression strategy is realized by setting two controllers for the high frequency component and the base-wave negative sequence component respectively;the TTA-based suppression strategy is realized by increasing the corresponding weight of the base-wave negative sequence component and superimposing it on the instruction current.The main research of the paper is as follows.(1)In response to the problem that the Fourier detection algorithm has a large detection error at dynamic load and the instantaneous reactive power detection algorithm cannot extract specific sub-harmonics,an improved time-domain detection algorithm is proposed that can both guarantee detection accuracy at dynamic load and extract specific sub-harmonics,and provides a detection algorithm for a second suppression strategy.The results indicate the correctness of this detection algorithm.(2)The ripple voltage calculation method of three-phase and single-phase APF is studied,and the ripples caused by each subharmonic are analyzed by the superposition principle and the equation between harmonics and ripple voltage is summed;the reason for the increase of ripple voltage fluctuation on the DC side of APF during load imbalance is analyzed;for the DC side voltage control,a recurrent probabilistic fuzzy neural network with an asymmetric mmbership fnction(RPFNN-AMF)control is proposed,and the network structure,online learning algorithm and convergence analysis of this algorithm are introduced in detail.(3)A ripple voltage high and low frequency suppression strategy and a TTA-based suppression strategy are proposed,and the main structural block diagrams are derived from theory and simulated separately.Firstly,we analyzed the compensation performance and DC side voltage waveform during unbalanced load steady-state operation,and the results showed that the high and low frequency suppression strategy has higher compensation accuracy,but the second suppression strategy has obvious effect on ripple suppression;then we simulated the unbalanced load mutation situation,and we can see that both methods have better compensation effect and can quickly track to the set value when the capacitor voltage drops.