Research On The Practical Key Issues Of Three-level Neutral Point Clamped Shunt Active Power Filter

In recent years,the comprehensive harmonic current,reactive power current and unbalanced current power quality problems become more and more attractive with the widely use of various power electronic equipment in power system.As an effective method to solve the comprehensive power quality problems,shunt active power filter(SAPF)causes many scholars devoting to research in depth.The neutral point clamped(NPC)type SAPF is preferred by people as it gives low output voltage change rate,small current ripple and low requirements of switching devices.Considering the existing problems of SAPF in practical applications,this paper studies the related practical key issues of NPC-SAPF in three-phase three-wire and four-wire distribution networks based on the existing research results of domestic and foreign scholars.The major research contents and related achievements are presented as follows:(1)The unified mathematical model of three-phase three-wire and four-wire NPC-SAPF in distribution networks is established.And on the basis of mathematical model,the influence of phase lock loop,output filter,reference current detection control method on the operation of NPC-SAPF,which provides the reality basis for the subsequent research;(2)In order to solve the inaccurate detection of synchronous rotating frame phase lock loop(SRF-PLL)under three phase unbalance voltage,harmonic voltage and variable frequency conditions,this paper proposes a frequency adaptive pre-filtering dual loop SRF-PLL.The improved SRF-PLL reduces moving average filter(MAF)filtering error caused by the non-integer sample point through second order Lagrange interpolation method,which ensures the separation accuracy of fundamental positive sequence component.Meanwhile,the phase detecting accuracy is assured by adjusting reference angular frequency through the secondary loop.The improved SRF-PLL not only inherits the outstanding characteristics of many existing SRF-type PLLs under unbalance voltage and harmonic voltage conditions but also overcomes the phase detecting error under variable frequency condition;(3)Considering the contradiction between system stability and high frequency switching harmonics suppression performance of traditional LCL-type output filter,a universal output filter-DRLCL for two-and three-level SAPFs is proposed based on two kinds of inverters’ current characteristics.DRLCL suppresses haploid and double switching harmonics through two series resonant circuits,and restrains resonant poles by grid-side resistor and series RC circuit,which eliminates the negative influence on the system of LCL-type filter poles.Moreover,a high frequency attenuation circuit is adding to improve the filtering performance beyond half switching frequency.After the step-by-step graphical design scheme,DRLCL obtains obvious advantage in unstable pole suppression and filtering performance;(4)This paper proposes the broadband compound control scheme based on frequency division concept aiming at the output current bandwidth limitation of the classical closed-loop control for SAPF.The proposed scheme regulates the low frequency harmonics and high frequency harmonics by delay compensation-based proportional resonant controller and Lyapunov stability control method,respectively.Combing with reference current comprehensive extraction scheme and DC neutral point voltage stable control,two NPC-SAPFs output 1st-49th harmonics stably and gives good compensation performance;(5)The corresponding MATLAB verifications are conducted in each chapter and the physical experiment platform of two NPC-SAPFs for compensating harmonic current,reactive power current and unbalanced current is established.Furthermore,the corresponding dynamic and steady-state experimental research is carried out aiming at the aforementioned theoretical research.