A modified sequence domain impedance definition and its equivalence to the dq-domain impedance definition for the stability analysis of AC power electronic systems

Representations of AC power systems by frequency dependent impedance equivalents is an emerging technique in the dynamic analysis of power systems including power electronic converters. The technique has been applied for decades in DC-power systems, and it was recently adopted to map the impedances in AC systems. Most of the work on AC systems can be categorized in two approaches. One is the analysis of the system in the \textit{dq}-domain, whereas the other applies harmonic linearization in the phase domain through symmetric components. Impedance models based on analytical calculations, numerical simulation and experimental studies have been previously developed and verified in both domains independently. The authors of previous studies discuss the advantages and disadvantages of each domain separately, but neither a rigorous comparison nor an attempt to bridge them has been conducted. The present paper attempts to close this gap by deriving the mathematical formulation that shows the equivalence between the \textit{dq}-domain and the sequence domain impedances. A modified form of the sequence domain impedance matrix is proposed, and with this definition the stability estimates obtained with the Generalized Nyquist Criterion (GNC) become equivalent in both domains. The second contribution of the paper is the definition of a \textit{Mirror Frequency Decoupled} (MFD) system. The analysis of MFD systems is less complex than that of non-MFD systems because the positive and negative sequences are decoupled. This paper shows that if a system is incorrectly assumed to be MFD, this will lead to an erroneous or ambiguous estimation of the equivalent impedance

System-Wide Harmonic Mitigation in a Diesel Electric Ship by Model Predictive Control

Email Print Request Permissions This paper proposes a system-oriented approach for mitigating harmonic distortions by utilizing a single Active Power Filter (APF) in an electrical grid with multiple buses. Common practice for control of APFs is to locally compensate the load current harmonics or to mitigate voltage harmonics at a single bus. However, the operation of an APF in a multi-bus system will influence the voltages at neighboring buses. It is therefore possible to optimize the APF operation from a system perspective instead of considering only conventional local filtering strategies. For such purposes, Model Predictive Control (MPC) is proposed in this paper as a framework for generating APF current references that will minimize the harmonic distortions of the overall system within a given APF rating. A diesel-electric ship, with two buses supplying separate harmonic loads, with an APF located at one of the buses, is used as study case. The operation with on-line MPCbased optimization of the APF current references is compared to two benchmark methods based on conventional approaches for APF control. The results demonstrate that the MPC generates current references that better utilize the APF current capability for system-wide harmonic mitigation.2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other work

Oral health-related quality of life in 4-16-year-olds with and without juvenile idiopathic arthritis

Background: Few studies have investigated oral health-related quality of life (OHRQoL) in young individuals with juvenile idiopathic arthritis (JIA). Aims were to investigate whether OHRQoL differs between children and adolescents with JIA compared to controls without JIA, while adjusting for socio-demographic-, behavioral- and oral health-related covariates. Furthermore, to explore whether socio-behavioral and oral health-related covariates of OHRQoL vary according to group affiliation and finally, specifically for individuals with JIA, to investigate whether disease-specific features associate with OHRQoL. We hypothesized that participants with JIA have poorer OHRQoL compared to participants without JIA. Methods: In this comparative cross-sectional study participants with JIA (n = 224) were matched to controls without JIA (n = 224). OHRQoL was assessed according to Early Childhood Oral Health Impact Scale (ECOHIS) (4–11-years-olds) and the child version of Oral Impacts on Daily Performances (Child-OIDP) (12–16-years-olds). JIA-specific characteristics were assessed by pediatric rheumatologists and socio-demographic, behavioral and self-reported oral health information collected by questionnaires. Index teeth were examined for caries by calibrated dentists. Multiple variable analyses were performed using logistic regression, reporting odds ratio (OR) and 95% confidence interval (CI). Two-way interactions were tested between group affiliation and the socio-behavioral- and oral health-related variables on the respective outcome variables. Results: In total, 96 participants with JIA and 98 controls were evaluated according to ECOHIS, corresponding numbers for Child-OIDP was 125 and 124. Group affiliation was not associated with impaired ECOHIS or Child-OIDP in adjusted analyses (OR = 1.95, 95% CI 0.94–4.04 and OR = 0.99, 95% CI 0.46–2.17, respectively). Female adolescents with JIA were more likely than males to report oral impacts according to Child-OIDP. Continued activity or flare was found to adversely affect Child-OIDP, also self-reported outcome measures in JIA associated with Child-OIDP. Conclusions: This study did not provide consistent evidence to confirm the hypothesis that children and adolescents with JIA are more likely to have impaired OHRQoL compared to their peers without JIA. However, female adolescents with JIA were more likely than males to report impacts on OHRQoL. Furthermore, within the JIA group, adolescents with continued disease activity, flare or reporting pain, physical disability, had higher risk than their counterparts of impaired OHRQoL.publishedVersio

Feasibility Studies on Integrating Offshore Wind Power with Oil Platforms

This thesis is centered around the possibilities of integrating offshore wind power together with oil and gas platforms. The motivation behind this topic is to reduce the emissions of CO2 and other pollutive gases from conventional offshore power plants. The electrical systems on oil platforms are weaker than an onshore grid, so measures should be taken to let the wind power integration contribute to a more stable operation on the platform. To explore existing and future technologies that can achieve this is an important part of this work. Two different power systems are presented, denoted System 1 and System 2. Their schematics are shown in Fig. 1. Corresponding simulation models are built from fundamental blocks in the software PSCAD/EMTDC. Aggregated models are utilized in order to save computational time.System 1 consists of an offshore wind farm and an oil platform connected together in islanded operation. The oil platform contains an synchronous generator with an associated gas turbine and can adjust both active and reactive power quickly. The load consists of fixed-speed induction motors. The wind farm contains a back-to-back Voltage Source Converter (VSC) which is used for variable speed operation of the turbine. The converter is also used for voltage support to the system, and this functionality reduces voltage oscillations during disturbances. The most critical scenario investigated is to start a large induction motor. It is shown that the transmission cable may contribute to an increasing risk of voltage collapse during the start-up. Another critical event is when the wind power is suddenly disconnected, and the gas turbine has to adjust the power output quickly in order to avoid too large frequency deviations. The simulations show that a disconnection of the whole wind farm does not lead to critical operation or possible instability. This would not be the case in a system with slower control systems in the gas turbine and synchronous generator.System 2 consists of a VSC-HVDC connection between Kollsnes and the platform Troll A. This existing configuration is powering a gas compressor on the platform through a variable speed synchronous motor. This thesis proposes to integrate wind power on the DC-side of the Troll A VSC-terminal. The challenge is to inject wind power in a way such that the operation of the gas compressor is not disturbed. The proposed control system is working as intended, and the selected simulation cases show that the compressor system is not affected by the wind power. The DC voltage control system is able to maintain a constant voltage at the Troll terminal during normal operation. The wind farm reduces the losses in the HVDC-cable, and surplus wind power is sent to the land grid during low-load operation. The DC-voltage drops to 73 % of the rated value for 10 ms when the wind power is suddenly disconnected, but the duration is so short that it does not affect the gas compressor operation.The simulation results indicate that both configurations are feasible. However, the results are heavily dependent on the parameter data, and further research should put more efforts into gaining as correct values as possible. A sensitivity analysis is performed to System 1 as a guideline to which parameters that are most decisive, and therefore should be modeled most accurately. In addition, new simulation cases might reveal challenges that this thesis does not concern