Effects of virtual resistor in power electronic converter on subsynchronous oscillation damping

Virtual impedance concept provides the impedance effects without need to put real physically impedance in the circuit. In power system, oscillation will not occur as long as the impedance seen by turbine‐generator set is ‘passive’. In order to ensure the impedance is seen as ‘passive’ by the turbine‐generator set, a series resistor can be placed in the system. This paper proposes implementation of virtual resistor concept in grid connected power electronic converter to replaced the aforementioned series resistor, then analyze its impact on subsynchronous oscillation phenomena. It can be shown that virtual resistor can improve the negative region of converter input admittance. Moreover, it also gives us better damping in system involving grid connected inverter. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Wind velocity and rotor position sensorless maximum power point tracking control for wind generation system

In order to perform maximum power point tracking control of wind generation system, it is necessary to drive windmill at an optimal rotor speed. For that purpose, a rotor position and a wind velocity sensors become indispensable. However, from the aspect of reliability and increase in cost, rotor position sensor and wind velocity sensor are not usually preferred. Hence, wind velocity and position sensorless operating method for wind generation system using observer is proposed in this paper. Moreover, improving the efficiency of the permanent magnet synchronous generator is also performed by optimizing d-axis current using the Powell method.     

Estimated impedance islanding detection method for grids with high motor penetration

In the dispersed generation (DG) field, islanding is known as a situation in which those generators are disconnected from the utility feeders and a bunch of local loads being powered by them is isolated. Accordingly, anti‐islanding schemes are used to immediately detect the phenomenon in facilitating the detachment of the DG or adaptively controlling the autonomous operation. Negative‐sequence impedance measurement is considered as a novel method of detecting electrical islands in DG networks. However, in the case of high induction motor penetration, this method might malfunction. Using two‐round impedance measurement, this paper deals with that problem while making typical assessments on how the impedance tolerance values are selected and how these values vary with network configuration. The simulation in PSCAD/EMTDC and experiments validate the methodology. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Impact of a virtual resistor on the input admittance of a grid‐connected inverter

A grid‐connected inverter offers good operation and controllability; however, there is a risk of local instabilities due to particular control schemes. As long as the input admittance is positive at a particular resonance frequency, the risk of local instability can be suppressed. In this paper, an attempt to improve the input admittance of the grid‐connected inverter (GCI)[vnsn1] using a virtual resistor is proposed. The impact of the virtual resistor on the inverter's passivity is analyzed by observing the input admittance of the inverter. Numerical analysis shows that the employment of the virtual resistor gives better input admittance to the GCI, particularly in the low‐frequency regions. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Nicotine given intracerebroventricularly does not inhibit the preovulatory surge of LH and PRL secretion in female rats

Twenty-three cases of Little Leaguer's shoulder were reviewed including the history and physical examination findings, as well as bilateral internal and external rotation anteroposterior comparison radiographs of the proximal humerus. The average follow-up was 9.6 months (range, 1.5 to 54), and all patients were observed until they had either returned to baseball or their symptoms had resolved. The average age of the patients in this series was 14 years. The chief complaint in all patients was pain localizing to the proximal humerus during the act of throwing. The average duration of symptoms was 7.7 months. Nineteen patients (83%) were pitchers. Physical examination revealed tenderness to palpation over the proximal humerus in 20 patients (87%), with 16 (70%) demonstrating specific tenderness over the lateral aspect of the proximal humerus. Swelling, weakness, atrophy, and loss of motion were uncommon findings. All 23 patients demonstrated radiographic widening of the proximal humeral physis of the throwing arm on internal and external rotation comparison anteroposterior radiographs of the shoulder. All patients were treated with rest from baseball throwing for an average of 3 months. Twenty-one of the 23 patients (91%) returned to playing baseball and were asymptomatic. The classic radiographic finding of widening of the proximal humeral physis can easily be seen on bilateral anteroposterior internal and external rotation radiographs of the proximal humerus. Rest from throwing for at least 3 months is recommended, followed by a gradual return to throwing in an asymptomatic shoulder.     

An adaptive dead-time compensation strategy for voltage source inverter fed motor drives

This paper presents an adaptive dead-time compensation strategy to obtain fundamental phase voltage for inverter fed vector controlled permanent magnet synchronous motor drives. The amplitude of phase dead-time compensation voltage (DTCV) to compensate disturbance voltage due to undesirable characteristics of inverter, such as dead-time, turn-on/off time of switching devices, and on-voltages of switching devices and diodes is adaptively determined according to a dead-time compensation time (DTCT). DTCT is identified on-line with using a /spl delta/-axis disturbance voltage in the current reference frame that is synchronized with current vector. The /spl delta/-axis disturbance voltage is estimated by a disturbance observer. The accuracy of identified DTCT is experimentally confirmed by calculating the mean absolute percentage error (MAPE) between a calculated active power and a measured one. MAPE for adaptive DTCT is almost within 5% at any operating point.     

Position control for ultrasonic motors using backstepping control and dead‐zone compensation with fuzzy inference

The ultrasonic motor has comparatively high nonlinearity which varies with driving conditions, and possesses a variable dead zone in control input associated with the applied load torque. The dead zone is a problem for industrial applications and it is important to eliminate the dead zone in order to improve control performance. This paper proposes a new position control scheme for ultrasonic motors to overcome the dead zone. The dead zone is compensated by fuzzy inference, and backstepping control performs accurate position control. Compared with model reference adaptive control, which uses an augmented error, backstepping control can analyze a transient response. Mathematical models are formulated and experimental results show the effectiveness of the proposed position control scheme. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(1): 69–77, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20016     

Load‐following control for dispersed generators using a PI controller

Recently, there has been growing interest in utilizing dispersed generation systems, which are grouped into micro turbine systems, co‐generation systems, and so on, as a substitute for fuel oil energy and a technology to prevent global warming. Since start‐up time of dispersed generation systems is short, it is possible to operate systems to supply load power corresponding to a demand. Moreover, PPSs (Power Producers and Suppliers) can participate in a power retail sales company, since deregulation of electric utilities was instituted in March 2000. However, PPSs have to keep instantaneous generating power commissioning rule, to maintain supply‐and‐demand balance between customer and supplier. Therefore, in this paper, we examine instantaneous generating power commissioning for dispersed generators where start‐up time is short and it is possible to operate systems to supply a power load corresponding to a demand. We adopt a PI controller as a controller. The system is composed of double control loop in inner loop and in outer loop. In inner loop electric power is controlled and in outer loop electric energy is controlled. The controller parameters are designed using the pole‐placement technique. The effectiveness of the proposed control system is confirmed by simulations. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(2): 58–66, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10200     

A simplified fault currents analysis method considering transientof synchronous machine

The transient fault currents of a synchronous machine fault were mathematically analyzed by using the two-reaction theory, Clarke's components, etc. However, the methods were extremely complicated and the transient fault analysis of synchronous machine with a load was not carried out except for three-phase faults. This paper proposes a simplified fault currents analysis method considering synchronous machine transients. The method is based on the symmetrical components scheme. The advantages of the method are that the transient analysis is able to be accomplished more easily than the conventional method when the fault point is a synchronous machine terminal or a transmission line. The accuracy of the method is confirmed by comparing the analysis results with Electro-Magnetic Transient Program (EMTP) digital simulations     

Terminal voltage and power factor control of induction generators for wind power generation system

This paper deals with two types of control method for induction generators used in wind power generating stations. One is aimed at maintaining the voltage of the station node and the other is aimed at maintaining a constant power factor through the branch connecting to the station. Both can be in the controllable area by adjusting the generator reactive power with compensating capacitors. First, theoretical equations for the terminal voltage and power factor of an induction generator are derived, and then the compensating capacitance corresponding to the wind velocity variation is determined quantitatively. By using the proposed control method, it is shown to be possible to maintain the terminal voltage and operating power factor of a wind power generation system at the target values, even when abrupt deviations are caused by wind power disturbances. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(3): 19– 26, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20298