19 research outputs found

    SCFSAP controls organ size by targeting PPD proteins for degradation in Arabidopsis thaliana

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    Control of organ size by cell proliferation and growth is a fundamental process, but the mechanisms that determine the final size of organs are largely elusive in plants. We have previously revealed that the ubiquitin receptor DA1 regulates organ size by repressing cell proliferation in Arabidopsis. Here we report that a mutant allele of STERILE APETALA (SAP) suppresses the da1-1 mutant phenotype. We show that SAP is an F-box protein that forms part of a SKP1/Cullin/F-box E3 ubiquitin ligase complex and controls organ size by promoting the proliferation of meristemoid cells. Genetic analyses suggest that SAP may act in the same pathway with PEAPOD1 and PEAPOD2, which are negative regulators of meristemoid proliferation, to control organ size, but does so independently of DA1. Further results reveal that SAP physically associates with PEAPOD1 and PEAPOD2, and targets them for degradation. These findings define a molecular mechanism by which SAP and PEAPOD control organ size

    Successful fault current interruption on DC circuit breaker

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    This study focus on the interruption capability of the DC circuit breaker employing a current commutation approach and evaluates the two main factors that determine the success rate for breaker current interruption, namely the current slope di/dt before current zero and the rate of rise of the transient recovery voltage dv/dt across the mechanical breaker contacts after current zero. A vacuum circuit breaker is used to evaluate DC breaker characteristics. Detailed mathematical and graphical analysis are presented for the proposed circuit operation used in analysing the circuit breaker properties, with simulation and experimental results at fault current levels up to 330 A

    Cascaded commutation circuit for a hybrid DC breaker with dynamic control on fault current and DC breaker voltage

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    This paper proposed a cascaded commutation circuit based on current commutation approach for low-to-medium voltage DC fault current interruption, without snubber circuits, which slows the fault current di/dt prior to current-zero and the rate of rise of the transient recovery voltage dv/dt across the mechanical breaker contacts after current zero. The proposed dynamic control of the fault current di/dt and circuit breaker voltage dVVCB/dt increase the fault current interruption capabilityat the first and second current-zeros. Detailed mathematical equations are presented to evaluate the operational waveform profile and the validity of the cascaded commutation principle is confirmed by simulation and experimental results at 600Vdc, 110A and 330A

    Direct current hybrid vacuum breaker

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    Hybrid DC circuit breaker switching techniques have been proposed for use in multi-terminal HVDC networks, to create an efficient, reliable and controllable system. There are two DC hybrid switching techniques. The first is arc interruption based on current oscillation. The second is interruption without an arc, like with a voltage commutation approach. The main difference is whether or not the interruption mechanism deals with the arc. Interrupting the arc not only causes erosion of the electrode surface, therefore reducing useful lifetime, but also introduces uncertainty to the process of successful interruption. This thesis therefore focuses on determination of the best switching technique for HVDC applications. To highlight the advantages of interruption without an arc, this thesis uses a vacuum circuit breaker (VCB) as the main breaker to investigate arc effects on interruption performance, when employing a forced current commutation method. The vacuum arc characteristics are detailed, thereby giving the VCB operation understanding needed. The VCB opening time is reduced from approximately 50ms to 13ms by employing a coil DC activation method. This allows the coil counter-EMF to be observed and utilised to optimally trigger the auxiliary commutation circuit. An active commutation test circuit is proposed, and in simulation and experimentally, the VCB interruption properties are investigated in terms of varied interruption current, di/dt, dvVCB/dt, and gap distance. Experimental results elicit that the key parameters determining arc interruption probability is di/dt, where even if dvVCB/dt is low, successful interruption become impossible if di/dt is above a certain level. By analysing the post-arc current based on its electrical behaviour, the reason why di/dt and dvVCB/dt dominate successful interruption probability is explained. A cascaded commutation circuit is proposed, and its validity is confirmed by simulation and experimentally, showing the interruption probability is improved compared to the test circuit, even with small circuit inductance.Hybrid DC circuit breaker switching techniques have been proposed for use in multi-terminal HVDC networks, to create an efficient, reliable and controllable system. There are two DC hybrid switching techniques. The first is arc interruption based on current oscillation. The second is interruption without an arc, like with a voltage commutation approach. The main difference is whether or not the interruption mechanism deals with the arc. Interrupting the arc not only causes erosion of the electrode surface, therefore reducing useful lifetime, but also introduces uncertainty to the process of successful interruption. This thesis therefore focuses on determination of the best switching technique for HVDC applications. To highlight the advantages of interruption without an arc, this thesis uses a vacuum circuit breaker (VCB) as the main breaker to investigate arc effects on interruption performance, when employing a forced current commutation method. The vacuum arc characteristics are detailed, thereby giving the VCB operation understanding needed. The VCB opening time is reduced from approximately 50ms to 13ms by employing a coil DC activation method. This allows the coil counter-EMF to be observed and utilised to optimally trigger the auxiliary commutation circuit. An active commutation test circuit is proposed, and in simulation and experimentally, the VCB interruption properties are investigated in terms of varied interruption current, di/dt, dvVCB/dt, and gap distance. Experimental results elicit that the key parameters determining arc interruption probability is di/dt, where even if dvVCB/dt is low, successful interruption become impossible if di/dt is above a certain level. By analysing the post-arc current based on its electrical behaviour, the reason why di/dt and dvVCB/dt dominate successful interruption probability is explained. A cascaded commutation circuit is proposed, and its validity is confirmed by simulation and experimentally, showing the interruption probability is improved compared to the test circuit, even with small circuit inductance
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