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In this paper, a different control strategy of a small wind generator, including a permanent magnet synchronous machine (PMSM), has been studied. The objective is to analyze the produced power quality of small wind turbines connected to weak AC grids. The extraction of the electrical energy from the wind turbine is based on a maximum power point tracking (MPPT) algorithm to control a pulse width modulation (PWM) rectifier. The grid connection is realized by means of a PWM voltage source inverter via a filter. This inverter is controlled by three different methods based on current or active and reactive power control. These methods are applicable for different power ranges. The obtained results demonstrate the efficiency of the system and the energetic contribution, mainly for rural weak AC grids. [DOI]: 10.1115/1.2748813] Keywords: wind energy conversion, variable speed, direct-drive system, current control, power control, maximum power point tracking

The structure of a compact air-cored high-voltage spiral strip pulse transformer is relatively simple, but considerable attention is needed to prevent breakdown between transformer windings. Since thickness of the winding conductors in the spiral strip type is in the order of submillimeter, field enhancement at the edges of the winding conductor is very high. It is, therefore, important to find proper winding parameters to reduce the field enhancement and, thus, to make the system compact. An insulator structure, which has multilayered dielectric materials with different dielectric constants, has been considered and numerically simulated. The field enhancement factor has been defined and obtained as a function of the conductor edge shape, dielectric constants of the insulation layer, and the thickness of multilayered dielectric material. Optimum design parameters are obtained to reduce the field enhancement at the edge of the windings. Index Terms--Field enhancement, high voltage, numerical analysis, pulse transformer.

Locating ground faults is a difficult and challenging problem for low-voltage power systems that are ungrounded or have high-impedance grounding. Recent work in pilot signals has renewed efforts in developing fault location methodologies. This paper presents a method for directional ground-fault indication that utilizes the fundamental frequency voltages and currents. Although the ground-fault current is small and usually less than the load currents, the fault has zero-sequence components that distinguish it from the load. Signal processing techniques are used to identify and compare the fault signals to determine the fault direction. The process takes advantage of the currents flowing from the distributed grounding capacitance. An experimental microprocessor-based directional indicator unit is tested in an industrial power distribution system. Directional indication of ground faults is applied near tap-off branch circuit connections. Promising results from field test conducted in a harmonic-noisy setting are presented. Directional indicator units simplify the search process on large networks, thus reducing the time and effort necessary to locate and remove the fault, and thereby significantly reduces the probability of a second ground fault with its destructive currents. Index Terms--Directional relay, fault location, high-resistance grounding, ungrounded systems.

In this paper, a new predictive current controller for a permanent magnet synchronous motor (PMSM) considering delays is presented. In a full digital current control system for a PMSM, there are inevitable delays in calculating and applying the inverter output voltages to the motor terminals. A predictive current controller implemented in a full digital system has serious problems such as the oscillation and large overshoot. A discussion of compensation methods to cope with the nonlinearities of the real system is also presented. The proposed current controller has been analyzed, and the experimental results are shown to prove the feasibility and effectiveness of the proposed predictive current controller using a prototype 750 W PMSM servo drive system. Index Terms--Digital current control system, oscillation, PMSM, stator currents.

This paper explores predictive digital current programmed control for valley, peak or average current. The control laws are derived for the three basic converters: buck, boost, and buck--boost. It is found that for each variable of interest (valley, peak or average current) there is a choice of the appropriate pulse-width modulation method to achieve predictive digital current control without oscillation problems. The proposed digital control techniques can be used in a range of power conversion applications, including rectifiers with power factor correction (PFC). Very low current distortion meeting strict avionics requirements (400-800 Hz line frequency) is experimentally demonstrated on a digitally controlled boost PFC employing predictive average current programmed control. Index Terms--Current-programmed control, digital control, switching power converters.

In this paper, we present temperature stability investigations of different overvoltage protection components employed in low-voltage applications. The following components have been examined: transient suppresser diodes, metal-oxide varistors (MOVs), gas-filled surge arresters (GFSAs), and polycarbonate capacitors. The aim of the research presented here is to identify the effectiveness of the overvoltage protection as a function of temperature. Experimental results show that GFSAs have rather stabile operational characteristics in a wide temperature range (from--90[degrees]C to +210[degrees]C). As for MOVs, they exhibit certain improvement of their protective characteristics at higher temperatures. Experimental results are backed up by theoretical explanations. Index Terms--Gas-filled surge arresters, metal-oxide varistors, polycarbonate capacitors, temperature stability, transient suppresser diodes.

One technique for raising the voltage in a fast pulse involves the use of a tapered transmission line transformer, whose transit time is large compared to the risetime of the pulse. For continuous variation of the characteristic impedance, the high-frequency/early-time voltage gain is given by the square root of the impedance ratio of each end of tapered line. An undesirable product of the tapered or transmission line transformer is the droop of the pulse after the initial step rise at the transmission line output. A formulation based on the voltage/current variables and renormalized wave variables has been used to examine the pulse droop with the idea of minimizing it. In the analysis, when the dominant term in expansion is optimized, the resulting geometry is that of the exponentially tapered line. The case of the exponentially tapered transmission line is discussed in detail and is shown to have an optimal transfer function in terms of early time voltage gain and improved droop characteristics. Index Terms--Impedance transformation, nonuniform transmission lines, pulse droop, tapered transmission lines transformers.

This paper presents a simple way to evaluate the distortion level of the input current for single-phase switch-mode rectifiers (SMRs). Based on the possible conducting paths of various topologies of SMRs, slopes of the input and command currents are defined to form a slope map. By comparing the defined current slopes, the causes of current distortion are examined. Distorted waveforms of the input currents under hysteresis control are reviewed and classified into three types. Design criteria for the circuit parameters are also studied to reduce the current distortion level. Simulations and experiments are performed and the results show strong conformity to the theoretical analysis. Index Terms--Current slope map, distortion analysis, single-phase switch-mode rectifiers.

The matrix converter (MC) is a direct frequency conversion device with high input power quality and regeneration capability. As a device without energy storage elements, it has higher power density than pulse-width modulation (PWM) inverter drives. However, for the same reason, the ac line side disturbances can degrade its performance and reliability. In this paper, the behavior of the MC drive under abnormal input line voltage conditions has been investigated. A technique to eliminate the input current distortion due to the input voltage unbalance has been developed and its feasibility proven via computer simulations and laboratory experiments. The power line failure behavior has also been investigated and the rapid restarting capability of the MC drive has been demonstrated via laboratory experiments. Index Terms--Current harmonics, matrix converter, power failure, PWM, voltage unbalance.

Minimization of the power required to sustain weakly ionized plasmas can be achieved if the energy of ionizing electrons is high, from tens to thousands of electronvolts. These electrons spend about a half of their energy on ionization cascades, in contrast to low-energy (1-3 eV) electrons in conventional discharges that dissipate most of their energy in nonionizing inelastic collisions. High-energy electrons can be injected into the gas as beams. Alternatively, they can be created in situ by applying a very strong electric field for a short time, with a repetition rate matching the rate of recombination. Analytical calculations show that the power budget in the high-voltage, repetitive pulse mode can be significantly lower than in the dc regime, but still much higher than in the case of electron beam ionization. For each pulse length, there exists an optimum electric field that minimizes the power budget. A detailed modeling of spatio-temporal dynamics of pulsed discharges reveal that voltage displacement into the cathode sheath plays a critical role. Fully coupled modeling of nonlocal kinetics of high- and low-energy electrons, ionization processes, charge particle transport, and electrodynamics was performed for a high-voltage pulse developing with a substantial pre-existing plasma. The kinetic modeling shows formation of a large group of high-energy electrons in high-voltage nanosecond pulse and dramatic increase in ionization rate. New effects are indicated, such as field reversal, 'two-cathode' effect, and interpulse ionization. Index Terms--High voltage, ionization, ultrashort pulse, weakly ionized plasmas.

Two new temperature compensated current sources implemented from bipolar transistors have been inspected. They are both deduced from the schema of the cascode current mirror and necessitate a total of six or seven transistors. Their performance have been compared to an earlier introduced current source with four transistors. Simulation and experimental results are reported. They indicate that for 5 V supply the new current sources are both less temperature sensitive than the preceding one over the range -55 [degrees] C to +125 [degrees] C. One of them (circuit C) was also found particularly suited to design current sources with a low supply voltage approaching 2 [V.sub.BE] Index Terms - Bipolar transistors, current mirrors, current sources, low voltage, temperature compensation.

It's that time again, when we present our biennial Best of the Best in field instrumentation. Over the past two years, we have had the opportunity to work with some [...]