Benomyl Effects on Plant Productivity through Arbuscular Mycorrhiza Restriction in a Greek Upland Grassland

Interactions between plants and microbes are important for plant community structure. Many plants establish symbioses with arbuscular mycorrhizal (AM) fungi, which play a central role in soil fertility, plant nutrition and the maintenance of stability and biodiversity within plant communities by improving uptake of nutrients and water. AM fungi can improve growth/performance in a variety of plant species by influencing intra- and interspecific competition of neighbouring plants and thus regulate coexistence and diversity in mixed communities. The aim was to study AMF effects on plant productivity and diversity in Greek upland grasslands

Analysis of DC-Link capacitor losses in three-level neutral point clamped and cascaded H-Bridge voltage source inverters

Loss estimation is a critical aspect of inverter design.The present work investigates the losses occurring in the DC-link capacitors of the three-phase three-level neutral point clamped and cascaded H-Bridge inverter topologies, by performing a harmonic analysis of the capacitor currents.Results are verified by simulations. Their analysis reveals the advantage of the NPC inverter

Hybrid modulation strategies for eliminating low-frequency neutral-point voltage oscillations in the neutral-point-clamped converter

Nearest vector (NV) modulation strategies for the neutral-point-clamped converter are known to generate low-frequency neutral point (NP) voltage oscillations. Non-NV strategies can eliminate these oscillations, but at the expense of higher switching losses and output voltage harmonic distortion. This letter proposes a simple way of creating hybrid strategies, as combinations of NV and non-NV strategies, which are also able to eliminate NP voltage oscillations. The approach minimizes the participation of non-NV strategies and hence their drawbacks, while it can be applied to any type of load (nonlinear and/or unbalanced). Simulations in MATLAB-Simulink are used to illustrate its operation.<br/

An extended boost three-phase transformerless PV inverter for common-mode leakage current reduction

A known issue with transformerless photovoltaic (PV) inverters is the generation of common-mode (CM) ground leakage currents. Single-phase transformerless topologies have been proposed that can achieve CM current suppression, but the relevant concepts do not perform equally well when applied to three-phase topologies. Existing three-phase transformerless topologies also have increased component count and losses, while their modified modulation strategies typically reduce output voltage quality and DC-bus voltage utilization. This paper presents a new three-phase PV inverter topology which achieves CM current suppression by incorporating a modification, the addition of a diode, in the boost converter that precedes the three-phase inverter stage. The boost converter switching is coordinated with that of the inverter, which operates based on an adapted modulation strategy that practically eliminates the CM current when the PV array voltage is lower than a certain threshold. For higher PV array voltages, conventional modulation strategies can be applied, which are also shown to reduce the generated CM current by up to 90%. All the above strategies retain a high output voltage/current quality, while the added diode losses have a minor impact of approximately 0.3% on the converter efficiency. The effectiveness of the topology with respect to CM current suppression is demonstrated through simulations in MATLAB/Simulink and experimental results from a comprehensive laboratory setup

Current balancing of scalar-controlled induction motors with long imbalanced cables for artificial lift systems

Induction motor current imbalance increases losses, torque ripple and vibrations. Current imbalance is known to appear in artificial lift systems, where motors are driven over long imbalanced cables. Power hardware modifications, namely transposition of cable phases in the wellbore, adjustment of the step-up transformer taps, and addition of balancing inductors have so far been proposed to suppress the imbalance. However, these solutions compromise the system's reliability or involve costly additional equipment, which must be customized according to the cable characteristics. This paper proposes a control method for current balancing of induction motors driven by scalar-controlled variable speed drives. In the proposed method, Second-Order Generalized Integrators (SOGIs) are used to extract the negative-sequence component of the motor currents, which is then suppressed by a Synchronous Reference Frame (SRF) current controller. The frequency and angle information required by the SOGIs and the SRF controller are obtained directly from the scalar algorithm, without needing a position sensor or observer, thus offering a novel, simple, robust and computationally effective implementation, which is also independent of the cable characteristics. The paper presents MATLAB/Simulink simulation results to illustrate the method's operating principles and performance in a variety of transient conditions. Experimental results obtained using full-scale equipment are also provided to demonstrate its effectiveness

Combined positive-sequence flux estimation and current balancing for sensorless motor control under imbalanced conditions

Sensorless motor control under imbalanced conditions, commonly caused by the use of an imbalanced cable, poses a number of challenges relating to stability and power quality. With reference to stability, concerns arise because the angle and frequency estimates of conventional PLLs and observers deteriorate in the presence of imbalance, which in turn degrades the response of synchronous-reference-frame current controllers. Power quality is also degraded due to the asymmetry of the currents supplied to the motor, which results in torque ripple and increased motor losses. In this paper, an adaptive positive-sequence flux estimator based on Second-Order Generalized Integrators (SOGI) is presented to solve these problems. The balanced fluxes generated by the estimator are suitable for PLL-based sensorless control of a motor over an imbalanced cable. With negligible additional computational effort, the flux estimator also provides negative-sequence current estimates, which are then controlled to balance the motor currents. The paper presents the results of simulation and experiments with a permanent magnet synchronous motor (PMSM) supplied by a commercial motor drive via a flat long Electrical Submersible Pumping (ESP) cable. It is shown that the proposed method can prevent instabilities that occur when using conventional flux estimation methods and reduce current imbalance by approximately 10 to 20 times, to less than 1%