Control of three-phase grid-connected voltage source converter under unbalanced grid faults greatly depends on the active and reactive powers processed by the converter. The instantaneous active power theory with sequence decomposition is employed to analyse the instantaneous power components, especially the second-order oscillation power. Study shows that the second-order oscillation power comprises two quadrature components, the cosine and sine terms, which are contributed by the average active power and the average reactive power, respectively. This finding sheds insight on the regulation of oscillation power under unbalanced grid conditions. Based on this observation, the authors propose a positive and negative sequence conductance and susceptance control scheme, which enables simple regulation of the active power or reactive power oscillation with the average active power and reactive power control. In addition, the authors investigate the relationship between the positive/negative sequence conductance and susceptance distribution factors with power oscillation and peak current. A maximum current limitation scheme is embedded into the current reference generation block for overcurrent protection. Numerical simulations and prototype measurements verify the accuracy of the analysis and the effectiveness of the control scheme.
Permanent magnets are interesting for the use in magnetic drug targeting devices. The magnetic fields and forces with distances from magnets have limited the depth of targeting. Producing greater forces at deep depth by optimally designed magnet arrays would allow treatment of a wider class of patients. In this study, we present a design of a permanent magnet array for deep magnetic capture of super paramagnetic iron oxide nano-particles, which consists of an array of 3 individual bar permanent magnet positioned to achieve a reasonably magnitude magnetic field and its gradient within a deeply region. These configurations were simulated with two-dimensional finite-element methods. The super paramagnetic iron oxide nano-particles were adopted Fe 3 O 4 particles with diameter 40 nm by chemical co-precipitation method. Performance factors were defined to relate magnetic field force with mass. The field strength and gradient were measured by a Hall probe and agreed well with the simulations.
Injection of both positive and negative sequence currents is desired for three-phase voltage source converters (VSC) under unbalanced grid fault condition. However, the second-order oscillation exists in the positive and negative sequence synchronous reference frame. Several approaches have been reported to cope with this second-order oscillation. Here, a simple direct proportional plus integral current control method in double synchronous reference frame is proposed. The proposed method does not require the additional filter or decoupling circuit, and does not require signal reconstruction for the current reference. Theoretical analysis and experimental study show that the proposed current control not only achieves zero steady-state error, but also exhibits good transient response.
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