Research On A New Series Hybrid Active Power Filter And Some Related Problems

With the wide application of power electronic devices and other nonlinear loads, harmonic pollution on power systems has become a serious problem in recent years. Hybrid active power filter (HAPF) has both the advantages of passive power filter (PPF) and active power filter (APF), such as low cost, high reliability, flexible compensation and so on. Therefore, HAPF has broad application on high-voltage high-power harmonic suppression projects. In this dissertation, a new series hybrid active power filter (SHAPF) with small self-inductance series transformer (ST) is proposed, and three related problems about it are investigated in detail.(1) By two types of modeling methods, the mathematical model of the existing SHAPF based on fundamental magnetic flux compensation (FMFC) is established. Then, the filtering characteristic of the filter is analyzed in detail. Two measures to improving the filtering effects, including harmonic disturbance voltage feed-forward control and in-phase harmonic injection method are presented. The validity of the proposed improvements is verified by the well experimental results.(2) On the basis of the work of part (1), a new series hybrid active power filter with small self-inductance ST is proposed. Different from conventional SHAPF based on FMFC, the self-inductance of the ST in this filtering system is even smaller about 0.01～0.10 in per unit, and the series active filter is controlled as a harmonic current source instead of a fundamental current source. The main advantages of this new SHAPF includes:The capacities of the inverter and secondary winding of ST are reduced, the protection of the active part is easy, the system power supply reliability is high, and the short-circuit current of supply system is limited. The excellent filtering effects of the filter are verified by both the simulation and experiment results.(3) Currently, HAPFs are mostly applied to three-phase three-wire medium-voltage or high-voltage system. Phase-deficiency operation characteristic of three-phase three-wire HAPF is investigated here. The source harmonic currents are calculated on four different situations, including:active part is out of running, three phases of active part are running, two phases of active part are running and only one phase of active part is running. Two types of comparisons are clarified:The harmonic current comparison between different phases on one situation, and the comparison between certain phase harmonic currents on different situations. Theoretical analysis, simulation and experimental results all indicate the below conclusions:only two phases of active part are obligatory to get satisfying filtering effects, in other words, augmenting the equivalent harmonic impedance of any two-phase source branches, all the three phase harmonic currents of load will be forced to flow into the passive power filter branches. For harmonic components of the source voltage, also only two phases of active part are needed to suppress the superfluous harmonic currents generate.(4) Unified fixed point coordinate transformation (UFP-CT), and a novel zero steady-state error current control method for active power filter based on UFP-CT are proposed. UFP-CT can convert the three-phase current commands of APF in a-b-c stationary frames, which contain multiple sinusoidal components, into constant DC quantities inδ-ξUFP frames. In addition, the state space average models of APF in a-b-c stationary frames andδ-ξUFP frames are different. Compared with the former, the latter has additional self disturbance and coupling disturbance. In order to realize zero steady-state error current control of APF, not only PI controller and source voltage feed-forward control should be used inδ-ξUFP frames, but also self disturbance and coupling disturbance feed-forward controls are required to cancel out the influences of these two types of disturbances. Simulation results demonstrate that the proposed current control method has zero steady-state error and good dynamic response.The current commands of three-phase three-wire system are linearly dependent. Therefore, the UFP-CT in this case is 2-dimensional. Control systems can be clarified as multi-input multi-output systems and single-input single-output systems, and multi-input multi-output systems can decompose into multiple single-input single-output systems. Therefore, researching on 1-dimensional UFP-CT has important values. This dissertation analyzes the expression of 1-dimensional UFP-CT. To solve the problem of the command cannot pass zero, a virtual 2-dimensional UFP-CT is presented, and a zero steady-state error current control method based on virtual 2-dimensional UFP-CT is proposed to apply to single-input single-output systems. Finally, taking single phase active power filter as an example, the state space average model is established, and the validity of the present solution is validated by simulation results.(5) In most high-voltage high-power APFs or HAPFs, multi-winding transformer is used to match the voltage and current grades between two different electric systems. By multi-winding transformer, two types of SHAPFs investigated in this dissertation both can realize higher voltage or power levels. Therefore, modeling and impedance parameter design for the multi-winding transformer is very important. To solve this problem, an efficient and universal approach based on Ansys/APDL and Matlab/PSB is proposed in this dissertation. According to the abecedarian transformer structure and by magnetic field energy method, a special 3-D static field analysis program based on Ansys parametric design language (APDL) is redeveloped to calculate the multi-winding inductance matrix. On the basis of the calculated inductance matrix, the linear simulation model of multi-winding transformer based on Matlab/PSB is established and related simulation research is carried out. According the feedback simulation results, the transformer structure could be modified until it satisfies requirements. By the presented method, the current distribution among parallel multi-branch is obtained with high precision. Moreover, the calculation results of composite short-circuit impedance on different short-circuit conditions also satisfy the engineering requirements.