Islanding detection in grid-connected power converters using harmonics due to the non-ideal behavior of the inverter

This paper analyzes the use of the voltage distortions in PWM voltage-source-inverters (VSIs) caused by the non-ideal behavior of the inverter for islanding detection purposes. The non-ideal characteristic of the inverters, mainly due to the dead-time needed to have safe commutations, produces fundamental frequency dependent harmonics (-5th, 7th...) in the output voltage. Although these harmonics are in principle an unwanted effect, since they reduce the power quality, they can potentially be used for islanding detection purposes. The physical principles of the method would be the same as for high frequency signal injection methods that have already been proposed but without the need of injecting a high frequency signa

Sensorless control of doubly-fed induction generators based on stator high frequency signal injection

High frequency signal injection based methods have been widely investigated for sensorless position/speed control of induction machines (IMs), permanent magnet synchronous machines (PMSMs) and more recently for doubly fed induction generators (DFIGs). When used with IMs and PMSMs, the high frequency signal is injected in the stator windings, an asymmetric (salient) rotor being required for this case. Contrary to this, both stator and rotor terminals are accessible and sensored in DFIGs, being therefore possible to inject the high frequency signal either in the stator or the rotor terminals. As consequence of this, the method can be used even if the machine is non-salient. In the implementation of the method with DFIGs, the high frequency voltage signal is typically injected in the rotor, the high frequency components (voltages of currents) induced in the stator being used for rotor position estimation. A drawback of this alternative is that the method is sensitive to the grid impedance in the stator side, which will be affected by the grid configuration, and is normally unknown. This paper proposes the sensorless control a DFIG injecting the high frequency voltage in the stator side, and using a high frequency current cancellation strategy in the rotor side. The main advantage of the proposed strategy is that the estimated position is independent of the grid characteristic

Islanding detection in three-phase and single-phase systems using pulsating high frequency signal injection

This paper analyzes the use of pulsating high frequency signal injection for islanding detection purposes. Active islanding detection using high frequency signal injection is an appealing option due to its reduced non-detection zone, reduced cost and ease of implementation. The use of a rotating high frequency signal has been reported and analyzed. However, this method can only be applied to three-phase systems. In this paper, the use of a pulsating high frequency signal injection is proposed. While it uses the same principles as rotating signal injection, it can be applied to both threephase and single-phase system

Coordinated operation of parallel-connected inverters for active islanding detection using high frequency signal injection

The high frequency impedance measured at the terminals of inverters connected in a microgrid by means of the injection of a small magnitude, high frequency voltage, has been shown to be a reliable metric to detect islanding. While the implementation of this method is simple when only an inverter injects the high frequency signal, this case is of limited applicability. On the other hand, several concerns arise when multiple inverters work in parallel, primarily due to risk interference among inverters. Islanding detection using high frequency signal injection in microgrids with multiple parallel-connected inverters is studied in this paper. A strategy for the coordinated operation of the inverters, without the need of communications or pre-established roles is proposed. Simulation and experimental results will be provided to demonstrate the viability of the concep

Accurate rotor speed estimation for low-power wind turbines

Small grid-tied wind turbines based on permanent magnet generators often use a cost-effective power converter topology consisting of a passive rectifier, a boost converter, and an H-bridge inverter. Speed or position sensors are rarely used due to cost issues. Model-based estimators relying on electrical magnitudes are used instead. However, such estimators are parameter sensitive, which limit their accuracy. Further concerns arise if these parameters change with the operating condition of the machine, mainly due to temperature. Speed sensorless control using the rectifier voltage ripple is analyzed in this paper. This technique provides good dynamic response and does not depend on machine parameters. Simulations are provided for speed and power tracking comparison with an accurate model-based speed estimation method operating at non-rated parameters. The speed accuracy and power tracking capability of the proposed method are similar to that provided by a speed sensor. This is translated into a 0.9% power increase when the model-based speed estimator shows 9% of error. Experimental results are carried out to test the effect of current and temperature in the estimation, showing temperature insensitivity and some distortion due to fast current transients. A speed estimation accuracy of zero mean error and 1.7% standard error is experimentally obtained in the regular operation of the wind turbin

Polarización y riesgo de automatización del empleo en el mercado laboral español

El presente artículo analiza las transformaciones que ha sufrido el mercado laboral español como consecuencia del proceso de polarización del empleo, estudiando, además, cómo puede evolucionar la estructura ocupacional según el riesgo de automatización de cada grupo. Los resultados muestran que la polarización del mercado laboral español se correlaciona, por un lado, de manera positiva con la automatización de tareas de contenido rutinario y, por otro, de forma negativa con los niveles educativo y salarial

Mujeres cirujanas en la historia de la medicina

Throughout the history of medicine, the number of women who have been able to practice as surgeons has always been considered as limited or irrelevant. In many occasions it has been due to the social role granted to women in relation to medicine with special dedication to other occupations. In many cases it has also been due to the lack of women who exercised this activity, to the impossibility due to the difficulties created by the members of society themselves and especially by those of the male sex as dominant. However, there have been times when women have had their role and even great contributions. It has been possible to detect its presence in ancient cultures although its references have not been sufficiently documented. On the other hand, in more recent times, there have been women who exercised surgery in some cases disguised as men having knowledge of very curious cases and had a great social impact at the time they occurred and are especially those of Elena Céspedes, Enriqueta Chaves and James Barry among others. At the present time, the number of women who perform surgery is very relevant, reaching outnumber men, in many specialties.  A lo largo de la historia de la medicina, siempre se ha considerado como limitado o irrelevante el número de mujeres que han podido ejercer como cirujanas. En muchas ocasiones se ha debido al papel social otorgado a las mujeres con relación a la medicina con dedicaciones en especial a otras ocupaciones. En muchos casos también se ha debido la inexistencia de mujeres que ejercieran esta actividad, a la imposibilidad por las dificultades creadas por los propios integrantes de la sociedad y en especial por los del sexo masculino como dominante. Sin embargo, han existido épocas donde la mujer ha tenido su protagonismo e incluso grandes aportaciones. Se ha podido detectar su presencia en culturas antiguas, aunque sus referencias no han sido suficientemente documentadas. Por otro lado, en épocas más recientes, han existido mujeres que ejercieron la cirugía en algunos casos disfrazadas como hombres teniendo conocimiento de casos muy curiosos y que tuvieron una gran repercusión social en la época en que se produjeron y son especialmente los de Elena Céspedes, Enriqueta Chaves y James Barry entre otros. En el momento actual el número de mujeres que ejerce la cirugía es muy relevante, llegando a sobrepasar en número al de varones, en muchas especialidades &nbsp

Mujeres cirujanas en la historia de la medicina

A lo largo de la historia de la medicina, siempre se ha considerado como limitado o irrelevante el número de mujeres que han podido ejercer como cirujanas. En muchas ocasiones se ha debido al papel social otorgado a las mujeres con relación a la medicina con dedicaciones en especial a otras ocupaciones. En muchos casos también se ha debido la inexistencia de mujeres que ejercieran esta actividad, a la imposibilidad por las dificultades creadas por los propios integrantes de la sociedad y en especial por los del sexo masculino como dominante. Sin embargo, han existido épocas donde la mujer ha tenido su protagonismo e incluso grandes aportaciones. Se ha podido detectar su presencia en culturas antiguas, aunque sus referencias no han sido suficientemente documentadas. Por otro lado, en épocas más recientes, han existido mujeres que ejercieron la cirugía en algunos casos disfrazadas como hombres teniendo conocimiento de casos muy curiosos y que tuvieron una gran repercusión social en la época en que se produjeron y son especialmente los de Elena Céspedes, Enriqueta Chaves y James Barry entre otros. En el momento actual el número de mujeres que ejerce la cirugía es muy relevante, llegando a sobrepasar en número al de varones, en muchas especialidades

Polarización y riesgo de automatización del empleo en el mercado laboral español

The current document examines the transformations of the Spanish labour market -as a consequence of the employment polarisation process- . It also analyses how the occupational structure based on the risk of automation of each group can evolve. The results show that the polarisation of the Spanish labour market correlates, on the one hand, positively with the automation of routine content tasks and, on the other, negatively with educational and salary levels.El presente artículo analiza las transformaciones que ha sufrido el mercado laboral español como consecuencia del proceso de polarización del empleo, estudiando, además, cómo puede evolucionar la estructura ocupacional según el riesgo de automatización de cada grupo. Los resultados muestran que la polarización del mercado laboral español se correlaciona, por un lado, de manera positiva con la automatización de tareas de contenido rutinario y, por otro, de forma negativa con los niveles educativo y salarial

Diagnosis of pre-sarcopenia from a single selectional crosscut at C3 region, using CT scans before radiotherapy

PURPOSE: the main purpose of this study was to diagnose pre-sarcopenia in cancer patients who had lack of computed tomography (CT) abdominal images, with a newly discovered method based on cervical images. MATERIAL AND METHODS: a sample of 37 patients with either lung cancer or a cancer that affected the upper digestive system underwent radiotherapy computed simulation which included measurements at C3 and L3 regions. Skeletal muscle mass (SMM) and skeletal muscle index (SMI) were determined by Hounsfield units and compared in both regions. Pre-sarcopenia was identified according to the cut-points currently established: ≤ 41 cm2/m2 in females, ≤ 43 cm2/m2 in males with a BMI ≤ 25 kg/m2, and ≤ 53 cm2/m2 in males with a BMI > 25 kg/m2. RESULTS: the correlation of SMM and SMI between the C3 and L3 regions was R2 = 0.876 and R2 = 0.805, respectively. Moreover, there was a positive association (86.49%) in terms of the diagnosis of pre-sarcopenia according to both regions. In total, eleven pre-sarcopenic patients (29.37%) were identified; three of them being overweight (27.27%) and two of them being obese (18.18%). CONCLUSION: a single sectional cross at the level of C3 can be used for the diagnosis of pre-sarcopenia. This new method avoids unnecessary irradiation, saves hospital costs and detects malnutrition before starting radiotherapy treatment in cancer patients who have lack of CT abdominal imaging