| 1. |
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| 2. |
- Klionsky, Daniel J., et al.
(författare)
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Guidelines for the use and interpretation of assays for monitoring autophagy
- 2012
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Ingår i: Autophagy. - : Informa UK Limited. - 1554-8635 .- 1554-8627. ; 8:4, s. 445-544
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Forskningsöversikt (refereegranskat)abstract
- In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.
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| 3. |
- Björk, Joakim, 1989-
(författare)
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Fundamental Control Performance Limitations for Interarea Oscillation Damping and Frequency Stability
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the transition towards renewable energy and the deregulation of the electricity markets, the power system is changing. Growing electricity demand and more intermittent power production increase the need for transfer capacity. Lower inertia levels due to a higher share of renewables increase the need for fast frequency reserves (FFR). In this thesis, we study fundamental control limitations for improving the damping of interarea oscillations and frequency stability.The first part of the thesis considers the damping of oscillatory interarea modes. These system-wide modes involve power oscillating between groups of generators and are sometimes hard to control due to their scale and complexity. We consider limitations of decentralized control schemes based on local measurements, as well as centralized control schemes with limitations associated to actuator dynamics and network topology. It is shown that the stability of asynchronous grids can be improved by modulating the active power of a single interconnecting high-voltage direct current (HVDC) link. One challenge with modulating HVDC active power is that the interaction between interarea modes of the two grids may have a negative impact on system stability. By studying the controllability Gramian, we show that it is possible to improve the damping in both grids as long as the frequencies of their interarea modes are not too close. It is demonstrated how the controllability, and therefore the achievable damping, deteriorates as the frequency difference becomes small. With a modal frequency difference of 5%, the damping can be improved by around 2 percentage points whereas a modal frequency difference of 20% allows for around 8 percentage points damping improvement. The results are validated by simulating two HVDC-interconnected 32-bus power system models. We also consider the coordinated control of two and more HVDC links. For some network configurations, it is shown that the interaction between troublesome interarea modes can be avoided. The second part considers the coordination of frequency containment reserves (FCR) in low-inertia power systems. A case study is performed in a 5-machine model of the Nordic synchronous grid. We consider a low-inertia test case where FCR are provided by hydro power. The non-minimum phase characteristic of the waterways limits the achievable bandwidth of the FCR control. It is shown that a consequence of this is that hydro-FCR fails at keeping the frequency nadir above the 49.0 Hz safety limit following the loss of a HVDC link that imports 1400 MW. To improve the dynamic frequency stability, FFR from wind power is considered. For this, a new wind turbine model is developed. The turbine is controlled at variable-speed, enabling FFR by temporarily borrowing energy from the rotating turbine. The nonlinear wind turbine dynamics are linearized to facilitate a control design that coordinate FFR from the wind with slow FCR from hydropower. Complementary wind resources with a total rating of 2000 MW, operating at 70–90% rated wind speeds, is shown to be more than enough to fulfill the frequency stability requirements. The nadir is kept above 49.0 Hz without the need to install battery storage or to waste wind energy by curtailing the wind turbines.
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| 4. |
- Chow, Joe H., et al.
(författare)
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Guidelines for Siting Phasor Measurement Units : Version 8, June 15, 2011 North American SynchroPhasor Initiative (NASPI) Research Initiative Task Team (RITT) Report
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Synchrophasor technology is relatively new to many utility companies as they plan for its introduction as part of their smart grid infrastructure investment. A first question that arises in planning for synchrophasor technology use is where should we place the phasor measurement units (PMUs)? Because synchrophasor data can be used in a variety of applications, the placement of PMUs should be driven by the intended applications, which may have varying data needs and differing PMU siting considerations. This white paper intends to provide information on and be a resource for such PMU placement issues, using recently developed PMU placement documents from various power grid operators and from applications described in technical papers. This white paper offers some guidelines for designers and users of the synchrophasor systems to decide where to place new PMUs. The paper addresses the information requirements of power system applications under all conditions (normal, alert, emergency, restorative) and across all time horizons (monitoring, protection, control, operation, planning). As many PMU placement documents are already available, the main sections of this document will be quite brief. Some expanded details on the guidelines can be found in the appendices.
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| 5. |
- Chow, Joe. H., et al.
(författare)
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Preliminary synchronized phasor data analysis of disturbance events in the US Eastern Interconnection
- 2009
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Ingår i: Power Systems Conference and Exposition, 2009. PSCE ’09. IEEE/PES. - : IEEE. - 9781424438105 ; , s. 136-143
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Konferensbidrag (refereegranskat)abstract
- This paper presents analysis results of synchronized phasor data from 10 disturbance events recorded in the US Eastern Interconnection (EI). The phasor data covers a wide region in the EI, allowing for the study of disturbance propagation, interarea modes, and oscillations in voltages and currents. The analysis is not straightforward because the EI is a meshed system with adequate interarea mode damping. Disturbances involving tripping a single large generator unit produce very short interarea swing responses. Islanding events involving regions at the perimeter, however, provide more prominent responses for analysis.
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| 6. |
- Dotta, Daniel, et al.
(författare)
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A MATLAB based PMU Simulator
- 2013
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Ingår i: 2013 IEEE Power and Energy Society General Meeting (PES). - : IEEE. - 9781479913039 ; , s. 6672629-
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Konferensbidrag (refereegranskat)abstract
- The use of Phasor Measurement Unit (PMU) data in power system operation is of practical importance. These data are currently used for real-time operation monitoring and off- line analysis. Understanding of the mechanisms involved in the phasor estimation process is necessary for correct phasor data interpretation and analysis, as well as for design of advanced control and protection schemes. In this paper the main phenom- ena involved in the phasor measurement process are illustrated using a MAT L A B based PMU simulator.
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| 7. |
- Li, Wei, et al.
(författare)
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Pseudo-Dynamic Network Modeling for PMU-Based State Estimation of Hybrid AC/DC Grids
- 2018
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Ingår i: IEEE Access. - : Institute of Electrical and Electronics Engineers (IEEE). - 2169-3536. ; 6, s. 4006-4016
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a PMU-based state estimation (SE) algorithm that uses a pseudo-dynamic network modeling approach. The pseudo-dynamic network model combines different equations with static network equations. Then it applies the weighted least squares algorithm to solve an over-determined least squares estimation problem. The proposed method can improve SE accuracy during both steady state and transient conditions without increasing the computational burden. In addition, the proposed modeling approach is applied to networks containing both a STATCOM and a voltage source converter-HVdc to demonstrate how to develop and apply a pseudo-dynamic SE model. Case studies aim to illustrate and verify the performance of the proposed method under steady state and transient conditions.
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| 8. |
- Parniani, Mostafa, et al.
(författare)
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Voltage Stability Analysis of a Multiple-Infeed Load Center Using Phasor Measurement Data
- 2006
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Ingår i: Power Systems Conference and Exposition, 2006. PSCE ’06. 2006 IEEE PES. - : IEEE. ; , s. 1299-1305
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Konferensbidrag (refereegranskat)abstract
- Voltage stability is a security concern for modern power systems. It can be analyzed using detailed or equivalent models. In this paper a new approach is presented for voltage stability analysis using synchronized phasor measurement data. Simple equivalent models of the interconnected system and load side at a measurement point are estimated from the data, and then used for calculating PV curves and predicting the stability limit. Two different models are proposed, and compared based on the analyses performed on the event recordings from US western power system. Minimal modeling and formulation makes the method suitable for online calculations. The models are continuously updated to reflect the effects of different system components and changes
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| 9. |
- Shereen, Ezzeldin
(författare)
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Security of Time Synchronization for PMU-based Power System State Estimation: Vulnerabilities and Countermeasures
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phasor Measurement Units (PMUs) constitute an emerging technology that is essential for various smart grid applications such as phase angle monitoring, power oscillation damping, fault localization, and linear state estimation. To obtain precise PMU measurements of voltage and current phasors, time synchronization in the order of 1 microsecond is typically required. Nevertheless, time synchronization sources for PMUs, such as GPS satellites and Precision Time Protocol (PTP), are vulnerable to Time Synchronization Attacks (TSAs). A TSA can disrupt time synchronization, resulting in malicious phase angle measurements, potentially leading to serious consequences to the stability of the power grid. Moreover, sophisticated attackers may be able to develop undetectable TSAs that would lead to incorrect but credible estimates of the system state, which will bypass traditional Bad Data Detection (BDD) algorithms employed in the grid. Therefore, the detection and mitigation of such undetectable TSAs is of utmost importance for power system operators.The first part of this thesis explores the threat of undetectable TSAs by investigating their practical feasibility. We provide necessary and sufficient conditions for a set of PMUs to be vulnerable to undetectable TSAs and provide an efficient algorithm to compute attacks against any number of vulnerable PMUs. Furthermore, we show that the set of undetectable TSAs forms a continuum if at least three vulnerable PMUs are targeted by the attack. This fact can be exploited by an attacker to develop low-rate attacks that would adapt to the clock servo that controls the PMU clock, and would bypass typical change detection-based security solutions. The feasibility of computing undetectable TSAs was demonstrated using realistic PMU data and a widely-used clock servo implementation.The second part of this thesis considers the detection of TSAs. To this end, we proposed three detection approaches focusing on various aspects of PMU and power grid operations. The first proposed approach is decentralized, and attempts to detect TSAs at every PMU individually by leveraging the dependence between the PMU clock state and the measured phasor. The approach is based on the observation that a TSA changes the correlation between the PMU clock frequency adjustments and the change in the measured phase angle. We proposed model-based and data-driven machine learning-based TSA detectors exploiting the change in correlation. Using extensive simulations and realistic PMU clock models, the proposed detectors were shown to perform well even for relatively low-rate attacks. The second proposed approach is centralized and is based on performing state estimation using the complete three-phase model instead of the simpler and more widely-used direct-sequence equivalent model. Our analytical results and extensive simulations showed that three-phase state estimators are significantly more resilient to TSAs compared to single-phase state estimators in unbalanced three phase systems. The third proposed approach is based on the framework of Graph Signal Processing (GSP) in power systems. We showed that by regarding the system state as a graph signal, the low-dimensional structure of the PMU measurements and the system state can be exploited for TSA detection. Based on GSP, we proposed a high-pass graph filter as well as machine learning classifiers utilizing GSP features, both showing superior performance not only in detecting the presence of a TSA, but also in localizing the attacked PMUs.The third and final part of the thesis considers the mitigation of TSAs, with special focus on PTP networks. In this regard, we investigated recently standardized authentication schemes in PTPv2.1 and their effect on both the synchronization accuracy and network latency in an experimental testbed. The results showed that the authentication schemes pose no significant overhead on the synchronization accuracy or the network latency. Moreover, the cost considerations of PTP authentication were investigated by considering the partial application of the authentication schemes to a PTP network only in the parts that are vulnerable to undetectable TSAs, thus combining TSA mitigation and detection. We showed that the problem of mitigating undetectable TSAs at minimum cost is NP-hard. We formulated the problem as an integer linear program and proposed two approximation algorithms based on linear relaxation and a greedy heuristic. Through extensive simulations on both synthetic graphs and realistic IEEE benchmark power system graphs, we showed that our proposed algorithms, combining both state estimation and PTP authentication, can dramatically reduce the cost of mitigating TSAs.The TSA detection and mitigation approaches presented in this thesis constitute a step towards secure and reliable time synchronization for PMU applications and a more resilient smart grid infrastructure.
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| 10. |
- Vanfretti, Luigi, et al.
(författare)
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A framework for estimation of power systems based on synchronized phasor measurement data
- 2009
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Ingår i: 2009 IEEE POWER & ENERGY SOCIETY GENERAL MEETING, VOLS 1-8. - : IEEE. - 9781424442409 ; , s. 3271-3276
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Konferensbidrag (refereegranskat)abstract
- This paper presents a new approach for synchronized phasor measurement-based state estimation, which starts from the buses with synchronized phasor data and enables highly accurate estimation of portions of the power system with connectivity to these phasor data buses. Observability analysis is used to assemble the buses for this Phasor State Estimator (PSE). The state estimation is formulated as an iterative least-squares problem, with bus voltage and line current magnitudes and phases as the estimated variables. An important aspect of this formulation is its ability to correct for constant or random phase biases that may exist in some phasor measurement equipment. Beside being a standalone state estimator, the PSE can enhance the reliability of a conventional state estimator. The PSE methodology is demonstrated for a power transfer path in the New York power system.
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