Control Principle And Application Research On Four-switch Type Active Power Filter

Four-switch-type Active Power Filters (F-APF) could be used as fault tolerant circuit topology of traditional Six-switch-type Active Power Filters (S-APF). It has the potential to restore part of the system performance rapidly when the S-APF goes wrong. In addition, as two switching devices have been reduced, F-APF has the advantages of lower cost, lower loss, smaller size, simpler driving circuit and higher reliability. Therefore, F-APF has significant research value. Based on the financial support of The National High Technology Research and Development of China (863Program) and The National Natural Science Foundation of China, this paper emphasizes on the control method and engineering application of F-APF. The working principle, space vector pulse width modulation (SVPWM) algorithm, space vector discrete current control method and dc side capacitor voltage control method of Three-phase Four-switch Shunt Active Power Filter (TFSSAPF) are firstly discussed, then according to the characteristics of micro-grid inverters and the power quality of a110kV substation, the problems related to the application of TFSSAPFs in low voltage micro-grid and high voltage distribution network are studied in detail. The emphasis and achievement of the paper mainly manifests in the following aspects:(1) The working principle of TFSSAPF is analyzed and a SVPWM algorithm is proposed, which is able to greatly optimize the total computational amount and accuracy of the trigger pulse production method.After analyzing the topology of TFSSAPF in detail, the conditions to control TFSSAPF to output three-phase balanced positive sequence(or negative sequence) voltage are revealed, which lays the foundation to eliminate harmonic currents, compensate reactive power and suppress negative sequence currents. An improved coordinate transformation is proposed which can make the basic output voltage space vector locate right on the axes of the αβ coordinate system. The sectors of reference voltage vectors and action time of fundamental voltage vectors can be acquired by simple sign judgment and arithmetic operation, thus the calculation amount and accuracy are optimized significantly. The pair of basic voltage vectors which are of the same amplitude and opposite direction are controlled to act the same time in a switching cycle to obtain zero vectors. Furthermore, a five-segment SVPWM algorithm is proposed and the realization process is also presented. Simulations and experimental results have validated the correctness of theory analysis and the effectiveness of the proposed algorithm.(2)The constraint condition of the equivalent voltage error vector of the space vector current control method is explained and a dual-hysteresis space-vector discrete current control method for TFSSAPF is proposed, which has the advantage of increasing the tracing speed of instruction currents and decreasing the operation frequency of the power switches.The constraint condition of the equivalent voltage error vector is analyzed when the space vector current control method is used for TFSSAPF. This condition ensures that the magnitude of the current error is reduced and the equivalent voltage error vector is within the output capacity of TFSSAPF. The optimal direction of the equivalent voltage error vector is pointed out. In this direction, the equivalent error voltage vector with unit amplitude can produce the greatest attenuation of the current error. Then, the discretization method is used to describe the output voltage expression to realize real-time tracking of the instruction currents. A dual-hysteresis space-vector discrete current control method is proposed, which uses the vector synthesis method when the current error is large and the optimal vector selection method when the current error is small, thus to reduce the switching frequency and meet the requirements of the output current total harmonic distortion. The performance of the proposed method, the optimal voltage vector hysteresis control method and the precise output of output voltage vector method is compared and analyzed in simulation and experiment. The simulation and experimental results have verified the superiority of the proposed method on improving the reference current tracking speed and reducing the switching frequency.(3)The basic mathematical model of TFSSAPF is constructed. The control objectives of dc capacitor-voltage are described, and a dc capacitor-voltage balance control method for TFSSAPF is presented, which ensures the stability of dc-side capacitor voltages.In order to balance the energy storage of dc capacitors and equalize their voltages, energy exchange principle of TFSSAPF is analyzed and a method of controlling the active component of output current is used. The relevance between the output voltage of phase C and the differential value of dc capacitor average voltages is established, and a method to balance the dc capacitor voltages by adding voltage balance component to the reference current of phase C is proposed. A SPWM current modulation algorithm which treats the phase currents as command currents is proposed, which has diminished many drawbacks of the traditional line current modulation method. Combining the SPWM current modulation algorithm based on phase current, the mean value stable method of total voltage of DC capacitor and the balance method of DC side two-capacitor voltage, a method of the DC side capacitor voltage control of TFSSAPF is proposed and the design method of the key parameter are analyzed. The experimental results have verified the effectiveness of the proposed method.(4)The influence of load harmonic currents to inverters in micro grids under the existence of virtual impedance is analyzed. A coordination control method for the TFSSAPF is proposed to regulate the angle frequency of the bus voltage and suppress the load harmonics, which not only eliminates the influence of harmonics, but also sufficiently uses the reactive power capacity of the TFSSAPF.The equivalent model of the micro grid inverter system is established and the deviation of the detected power and the actual power brought by load harmonics is analyzed. By introducing the virtual harmonic power, the deviation is quantified. The influence of load harmonics to the performance of DG inverters is described using the small signal method. In order to reduce the influence mentioned above and increase the capacity utilization ratio of TFSSAPF, a coordination control method is proposed. A reactive current component is added into the reference current to regulate the angle frequency of the bus voltage. The simulation results show that the proposed method could decrease the angle frequency of the bus voltage and suppress the load harmonics, thus the stability of DG inverters is increased.(5) The power quality problems of an110kV substation are analyzed and a power quality compensator FSI-HAPF is presented. The hardware and software of FSI-HAPF is designed in detail, which realizes large capacity reactive power compensation and dynamic harmonic suppression on35kV bus.According to the GB standard of power quality, the power quality problems of110kV HURUN Substation of Guangxi Power Grid is analyzed, which illustrate serious harmonic pollution and inadequate capacity of reactive power existed in segment I of35kV bus. A high-voltage hybrid compensation system FSI-HAPF based on TFSSAPF is proposed, which consists of TFSSAPF and resonance injection branch group. FSI-HAPF has lower cost and higher reliability than the traditional IHAPF. The structure and filtering principle of the FSI-HAPF is analyzed and the parameter design method of both the passive and active part of FSI-HAPF is provided as well as the design and realization method for the hardware and software of FSI-HAPF. The application results show that FSI-HAPF is able to meet the needs of the industrial requirement. After the application of FSI-HAPF, the main indexs of power quality at segment I of35kV bus is restrained within the national standard (GB) limited values, which indicates that FSI-HAPF has achieved harmonic dynamic management in high-voltage system and large capacity reactive power compensation with low cost.