Research On Key Techniques Of Digital Controlled Large Capacity Shunt APF

With the progress of China’s energy saving and pollutant reduction, the grid harmonic control issue as a major part of green energy movement has gained increasingly more attention recently. Active Power Filters (APF) is proved to be one of the most effective ways to deal with the grid harmonic pollution. Low voltage and large capacity APF plays an important role in industrial users’harmonic elimination. Comparing to the industrial developed countries, there still exist gaps in terms of single equipment capacity, harmonic tracking precision and adaptability, etc. The high current slew rate, low switching frequency and severe application field are typical chanllenges that impeded the application of large capacity APF. Due to the advantages of flexibility, repeatability, and reliability, digital control is gradually replacing analog one and becomes the main-stream control techique in APF control. To address these concerns, this dissertation has presented an in-depth research on the issues of digital control, high compensation precision and adaptability of large capacity APF under the background of380V/400A APF prototype developing.Firstly, based on the three-phase and three-wire APF mathematic model, this thesis derived the control objects of the current control loop and DC voltage control loop in s-domain. Then, the effect of the factors such as, anti-aliasing filtering, zero-order hold and one-beat lag control during were analyzed and modeled. To cancel the negative effect of those factors on the stability of the closed-loop, the stability margin of the closed-loop with the controller parameters which were designed in s-domain is checked in z-domain and the controller parameters were redesigned accordingly. A novel control scheme for the DC voltage impact suppression during the start-up processes of the large capacity APF was proposed, and its efficiency has been demonstrated through experimental results.Secondly, because of the unsatisfactory compensation accuracy of conventional digital PI controlled APF, Internal-Mode based high tracking precision controllers were introduced into the current control loop, including both Generalized Integrators (GIs) and Repetitive Controllers (RCs). Design criteria, tracking performance, and stability determination of both controllers were analyzed and deducted in detail. Since the tracking precision of both controllers is sensitive to the grid frequency, this thesis proposed one control-period-regulation based frequency adaptive scheme. Experimental results show that both controllers maintained high performance in presence of line frequency variation. Thirdly, in order to enhance the flexibility and adaptability of the APF, the selective harmonic compensation technique was studied. This selective function of APF can be obtained by either harmonic detection or control ways. Concerning harmonic detection, this thesis proposed a simple band pass filter method derived from the GIs. Experimental results show that it consumes smaller DSP memory and shorter computing time, comparing to traditional Recursive Discrete Fourier Transform (RDFT) or Multi-Harmonic Synchronous Rotating Frame (MHSRF) methods, and has been applied to the400A industrial field prototype. Concerning current control, this thesis proposed a current control strategy with an array of GIs, one for the fundamental and the others for each harmonic. Experimental results demonstrated the selective function and high steady-state compensation accuracy.Finally, since the real industrial electromagnetic environment is often very severe, a common digital Phase Locked-Loop (PLL) based on zero crossing detection may not be competent for this situation. Based on two different oscillators’mechanism, this thesis derived two corresponding mathematic models for two software PLL methods, including one with fixed control period and the other one with regulated control period. Pole-placement was adopted as the controller gains tuning method in design of PI controller parameters. Experimental results verify the correctness of the developed model. In addition, this thesis proposed one software phase-sequence identification method based on the synchronous rotation transform and the phase-sequence adaptive control of APF is realized.