Increasing DG Capacity of Existing Networks through Reactive Power Control and Curtailment

Renewable energy sources (RES), especially wind turbines, have become more important during the last years. An increasing number of distributed generation (DG) units are connected to weak medium voltage distribution networks in rural areas where they have a large influence on the voltage and the line losses. Voltage rise is in this case often a limiting factor for the maximum amount of DG capacity. Already current wind turbines with a capacity of 2 MW can often not easily be connected to existing 10 kV feeders. To increase the DG capacity of existing networks without reinforcement DG units can be controlled. This paper proposes abandoning unity power factor used today and letting the converters used as network interface of many new wind turbine generators absorb reactive power to reduce the voltage level. Since reactive power has great influence on losses in the network the use of reactive power is limited. Line losses due to the transfer of reactive power are taken into account in this study. Furthermore the use of curtailment is analysed. Simulations of voltage change and line losses when using reactive power control by the connected wind turbines and curtailment in a simple test system are presented. Without reinforcement of the network it was possible to increase the DG capacity from 2;7MW to more than 4MW in the test network without violating voltage limits. Line losses increase but to a reasonable extent and lost energy due to curtailment is insignificant

Fatigue in stroke survivors: a 5-year follow-up of the Fall study of Gothenburg

Longer term knowledge of post-stroke fatigue (PSF) is limited. Our aim was to describe the prevalence of, and to identify baseline predictors associated with, PSF 5 years after stroke. We undertook a follow-up of stroke survivors from the 504 consecutively recruited participants in the observational "The Fall Study of Gothenburg", conducted between 2014 and 2016. The dependent variable, PSF, was assessed using the Swedish version of the Fatigue Assessment Scale (S-FAS) and defined as having a S-FAS score ≥ 24. The S-FAS questionnaire was mailed to potential participants in August 2020. The independent variables, previously obtained from medical records, included age; sex; comorbidities; stroke severity; hospital length of stay; body mass index (BMI); number of medications and lifestyle factors at index stroke. To identify predictors of PSF, univariable and multivariable logistic regression analyses were performed. Of the 305 eligible participants, 119 (39%) responded with complete S-FAS. Mean age at index stroke was 71 (SD 10.4) years and 41% were female. After a mean of 4.9 years after stroke, the prevalence of PSF was 52%. Among those with PSF, almost two thirds were classified as having both physical and mental PSF. In the multivariable analysis, only high BMI predicted PSF with an odds ratio of 1.25 (95% CI 1.11-1.41, p < 0.01). In conclusion, half of the participants reported PSF 5 years after index stroke and higher body mass index was identified as a predictor. The findings from this study are important for healthcare professionals, for planning health-related efforts and rehabilitation of stroke survivors.ClinicalTrials.gov, Identifier NCT02264470

Modulation of microRNA processing by 5‐lipoxygenase

The miRNA biogenesis is tightly regulated to avoid dysfunction and consequent disease development. Here, we describe modulation of miRNA processing as a novel noncanonical function of the 5‐lipoxygenase (5‐LO) enzyme in monocytic cells. In differentiated Mono Mac 6 (MM6) cells, we found an in situ interaction of 5‐LO with Dicer, a key enzyme in miRNA biogenesis. RNA sequencing of small noncoding RNAs revealed a functional impact, knockout of 5‐LO altered the expression profile of several miRNAs. Effects of 5‐LO could be observed at two levels. qPCR analyses thus indicated that (a) 5‐LO promotes the transcription of the evolutionarily conserved miR‐99b/let‐7e/miR‐125a cluster and (b) the 5‐LO‐Dicer interaction downregulates the processing of pre‐let‐7e, resulting in an increase in miR‐125a and miR‐99b levels by 5‐LO without concomitant changes in let‐7e levels in differentiated MM6 cells. Our observations suggest that 5‐LO regulates the miRNA profile by modulating the Dicer‐mediated processing of distinct pre‐miRNAs. 5‐LO inhibits the formation of let‐7e which is a well‐known inducer of cell differentiation, but promotes the generation of miR‐99b and miR‐125a known to induce cell proliferation and the maintenance of leukemic stem cell functions

Load Modulation for Damping of Electro-Mechanical Oscillations

Today it is feasible for utilities to control selected customer loads in a nondisturbing fashion. This is comparable to load shedding, but has a wider area of application. This paper focuses on direct load control or modulation for power system damping and is based on a field test of on-off control and modal analysis of differential-algebraic equation models of three test systems. Real power load controlled by bus frequency is found to be robust to changes in network topology. It is shown that on-off modulation is more effective than sinusoidal modulation for damping of large oscillations, but not for small oscillations. It proves difficult to give a general measure of the required amount of controlled load, since modulation of real power loads has an additive rather than multiplicative effect on power flows. Different structures of load control systems are listed. If the system is centralized, it is suggested that the distribution company owns and operates the system and that damping is sold as ancillary servic

Power System Damping - Structural Aspects of Controlling Active Power

Environmental and economical aspects make it difficult to build new power lines and to reinforce existing ones. The continued growth in demand for electric power must therefore to a great extent be met by increased loading of available lines. A consequence is that power system damping is reduced, leading to a risk of poorly damped power oscillations between the generators. This thesis proposes the use of controlled active loads to increase damping of such electro-mechanical oscillations. The focus is on structural aspects of controller interaction and of sensor and actuator placement. On-off control based on machine frequency in a single machine infinite bus system is analysed using energy function analysis and phase plane plots. An on-off controller with estimated machine frequency as input has been implemented. At a field test it damped oscillations of a 0.9 MW hydro power generator by controlling a 20 kW load. The linear analysis uses two power system models with three and twenty-three machines respectively. Each damper has active power as output and local bus frequency or machine frequency as input. The power system simulator EUROSTAG is used both for generation of the linearized models and for time simulations. Measures of active power mode controllability and phase angle mode observability are obtained from the eigenvectors of the differential-algebraic models. The geographical variation in the network of these quantities is illustrated using the resemblance to bending modes of flexible mechanical structures. Eigenvalue sensitivities are used to determine suitable damper locations. A spring-mass equivalent to an inter-area mode provides analytical expressions, that together with the concept of impedance matching explain the structural behaviour of the power systems. For large gains this is investigated using root locus plots. The effect of using two dampers is studied. For the three machine system this is done for all combinations of the two gains in a certain range. In the twenty-three machine case one gain takes only two values as the other is varied

Load Modulation at Two Locations for Damping of Electro-Mechanical Oscillations in a Multimachine System

Modulation of customer loads for increased power system damping is suggested as an alternative to control of HVDC links and FACTS devices. The loads are modeled as real power controlled by local bus frequency, which is equivalent to viscous damping in a mechanical system. The qualitative aspects of the interaction between two load modulation controllers are studied using linearized models of two test systems with three and twenty-three machines respectively. Root locus plots show that transfer function zeros limit damping. Tuning a controller for maximum relative damping is equivalent to impedance matching. This is used to explain the zero dynamics and why a second controller may not be able to contribute to added damping. Time simulations of a nonlinear system model verifies the final design of the controllers for the large test syste