| In order to simultaneously realize the consumption of new energy,efficient use as well as flexible control of electric energy,the power system has been developed into a electronization power system.However,the inherent nonlinear characteristics of power electronic devices,together with the start-stop process of large equipment,impact load,distributed generation(DG),electrical equipment containing arcs and ferromagnetics can cause serious harmonic pollution to the power system.So far,a variety of flexible AC transmission systems(FACTS)can be implemented on the basis of adjustable impedance,or even work directly in a special state of adjustable impedance.While the magnetic-fluxcompensation(MFC)type adjustable impedance can achieve stepless regulation in four quadrant with negligible harmonic pollution through simple control strategies,it is of great necessity to study the unified principle of various MFC type adjustable impedance,so as to obtain the general construction methods of various MFC-based FACTS.A MFC-based selected harmonic elimination active harmonic isolator(SHEAHI)with high performance and low capacity is built and three key points in the implementation of proposed SHEAHI,which are suppression of ultra-high harmonic voltage across series transformer,current controller and selected harmonic current detection,are studied in this paper as follows:With the combination of transformer dual-port network model and the idea of fundamental-harmonic magnetic flux hybrid control,the unified principle of MFC type adjustable impedance with a combination topology of series transformer and inverter is concluded,and a total of 30 different impedance expressions under four different control modes are derived.Then range of magnetic flux control coefficients of the MFC type adjustable impedance is acquired according to Routh stability criterion.At last,a new principle of MFC-based SHEAHI is proposed,of which the topology and harmonic isolation characteristics are analyzed in depth.The adoption of L reactive power filter is unable to effectively isolator the ultra-high switching harmonic voltage introduced by PWM control,which is then almost fully coupled to the grid through series transformer,leading to a significant deterioration of voltage quality.So the LCL filter is adopted at the AC side of voltage source inverter(VSI)in this paper to achieve the suppression of ultra-high switching harmonic voltage.The PR controller implemented in stationary coordinate system can realize zero-steady-error tracking of sinusoidal signals,so a multi-resonance current controller is adopted in this paper according to adaptive PR controller.The influence mechanism of digital control delay and sampling on both the active damping of LCL filter and the phase lag of PR controller is analyzed,and parameters of LCL filter and multi-resonance current controller are designed in discrete domain.The simplified principle of single-phase harmonic current detection on the basis of αβ-dq coordinate transformation without construction of virtual quadrature component or adoption of PLL is derived,then a delay cancellation method is proposed to improve the steady-state accuracy and dynamic performance of harmonic current detection.The proposed delay cancellation harmonic current detection method significantly increases the lowest harmonic frequency in dq coordinate,leading to the employment of a first-order low-pass filter with relatively higher cutoff frequency.At last,the influence mechanism of grid frequency offset on the proposed delay cancellation harmonic current detection method is deduced and verified through simulation.The principle and design methods proposed in this paper are simulated and analyzed by MATLAB toolbox,then a 0.2kW single-phase MFC-based SHEAHI prototype is built for experimentation.Related experimental results verifies the effectiveness of proposed MFCbased SHEAHI and its implementation methods. |
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