| 1. |
- Aaditya, V. B., et al.
(författare)
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Study of dielectric properties of polypyrrole/titanium dioxide and polypyrrole/titanium dioxide-MWCNT nano composites
- 2018
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Ingår i: Journal of materials science. Materials in electronics. - : Springer. - 0957-4522 .- 1573-482X. ; 29:4, s. 2848-2859
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Tidskriftsartikel (refereegranskat)abstract
- The polypyrrole/titanium dioxide nano composites and polypyrrole/titanium dioxide-MWCNT nano composites were synthesized by chemical polymerization technique in the presence of an ammonium persulphate (oxidizing agent). Different concentrations viz. 15, 30, 45 and 60 wt% of titanium dioxide (TiO2) as well as mixture of TiO2-MWCNT in polypyrrole (PPy) respectively were used in the present study. The nano composites have almost spherical type shaped particles which have cluster formation as confirmed from SEM photos. The XRD graphs reveal that the PPy/TiO2 (PT) nano composites have shown the semi-crystalline nature and also, the graphs indicate the changeover of the structure of PPy/TiO2-MWCNT (PTM) nano composites from amorphous to semi-crystalline nature. From the FTIR figures, shift in wavenumber towards lower side is noticed in the case of PT and PTM nano composites when compared to PPy. The dielectric properties such as dielectric constant, dielectric loss and tangent loss have shown good behavior. This reveals that, the TiO2 as well as mixture of TiO2-MWCNT particles have shown strong dependence on PPy and helps to form good composites. So, the nano composites are good dielectric materials.
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| 2. |
- Abid, Fahim, et al.
(författare)
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Intermodulation due to interaction of photovoltaic inverter and electric vehicle at supraharmonic range
- 2016
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Ingår i: 2016 17th International Conference on Harmonics and Quality of Power. - Piscataway. NJ. - 9781509037926 ; , s. 685-690
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Konferensbidrag (refereegranskat)abstract
- Advent of power electronic switching is introducing more and more non-linear loads in the low voltage grid. Besides harmonic current generation in the frequency range below 2 kHz, these non-linear loads are also responsible for current emission in the range of 2 kHz to 150 kHz, commonly known as supraharmonic emission. Supraharmonic currents mainly flow between nearby appliances and heavily influence the overall emission of neighboring devices. This paper presents an analysis of supraharmonic interaction between a photovoltaic inverter and an electric vehicle. It has been noticed that intermodulation distortion arises as a result of interaction between different switching frequencies used by the devices. Later, additional household equipment were added to photovoltaic and electric vehicle to observe their effect on intermodulation distortion. All the measurements were conducted in a controlled laboratory environment imitating a domestic customer.
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| 3. |
- Abrahamsson, Lars, et al.
(författare)
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AC cables strengthening railway low frequency AC power supplysystems
- 2017
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Ingår i: ASME/IEEE 2017 Joint Rail Conference. - : ASME Press. - 9780791850718
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Konferensbidrag (refereegranskat)abstract
- In present-day railway power supply systems using an AC frequency lower than the one in the public power system of 50/60 Hz, high voltage overhead transmission lines are used as one measure of strengthening the railway power supply system grids. This option may be economically beneficial, compared to strengthening the grid purely by increasing the density of converter stations or increasing the cross section areas of the overhead catenary wires. High voltage AC transmission lines in the railway power supply system allow larger distances between converter stations than would otherwise be possible for a given amount of train traffic. Moreover, the introduction of AC transmission lines implies reduced line losses and reduced voltage level fluctuations at the catenary for a given amount of train traffic. However, due to the increased public and government resistance for additional overhead high voltage AC transmission lines in general, different alternatives will be needed for the future improvements and strengthening of railway power systems. For a more sustainable transport sector, the share and amount of railway traffic needs to increase, in which case such a strengthening becomes inevitable. Earlier, usage of VSC-HVDC transmission cables has been proposed as one alternative to overhead AC transmission lines. One of the main benefits with VSC-HVDC transmission is that control of power flows in the railway power systems is easier and that less converter capacity may be needed. Technically, VSC-HVDC transmission for railway power systems is a competitive solution as it offers a large variety of control options. However, there might be other more economical alternatives reducing the overall impedance in the railway power system. In public power systems with the frequency of 50/60 Hz, an excess of reactive power production in lowly utilized cables imposes an obstacle in replacing overhead transmission lines with cables. In low frequency AC railway power system, the capacitive properties are less significant allowing longer cables compared to 50/60 Hz power systems. Moreover, in converter-fed railways, some kind of reactive compensation will automatically be applied during low-load. At each converter station, voltage control is already present following the railway operation tradition. Therefore, in this paper, we propose AC cables as a measure of strengthening low-frequency AC railway power systems. The paper compares the electrical performances of two alternative reinforcement cable solutions with the base case of no reinforcement. The options of disconnecting or toggling the cables at low load as well as the automatic reactive compensation by converter voltage control are considered. Losses and voltage levels are compared for the different solutions. Investment costs and other relevant issues are discussed.
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| 4. |
- Abrahamsson, Lars, 1979-, et al.
(författare)
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Evaluating a constant-current load model through comparative transient stability case-studies of a synchronous-synchronous rotary frequency converter-fed railway
- 2019
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Ingår i: 2019 Joint Rail Conference. - : ASME Press.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper continues the pursuit of getting a deeper understanding regarding the transient stability of low-frequency AC railway power systems operated at 16 2/3 Hz synchronously to the public grid. The focus is set on the impact of different load models. A simple constant-current load model is proposed and compared to a previously proposed and studied load model in which the train’s active power is regulated.The study and comparison is made on exactly the same cases as and grid as with the already proposed and more advanced load model. The railway grid is equipped with a low-frequency AC high-voltage transmission line which is subjected to a fault. The study is limited to railways being fed by different distributions of RFC (Rotary Frequency Converter) types. Both AT (auto transformer) and BT (booster transformer) catenaries are considered.The RFC dynamic models are essentially Anderson-Fouad models of two synchronous machines coupled mechanically by their rotors being connected to the same shaft.The differences in load behaviour between the proposed constant-current load model and the previously proposed and studied voltage-dependent active power load model are analyzed and described in the paper.
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| 5. |
- Abrahamsson, Lars, 1979-, et al.
(författare)
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Further studies on the transient stability of synchronous-synchronous rotary frequency converter fed railways with low-frequency AC high-voltage transmission
- 2018
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Ingår i: International Journal of Energy Production and Management. - : WIT Press. - 2056-3272 .- 2056-3280. ; 3:4, s. 266-276
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Tidskriftsartikel (refereegranskat)abstract
- This paper continues the pursuit of getting a deeper understanding regarding the transient stability of low-frequency AC railway power systems operated at 16 2/3 Hz that are synchronously connected to the public grid. Here, the focus is set on such grids with a low-frequency AC high-voltage transmission line subject to a fault. The study here is limited to railways being fed by different distributions of Rotary Frequency Converter (RFC) types. Both auto transformer (AT) and booster transformer (BT) catenaries are considered. No mixed model configurations in the converter stations (CSs) are considered in this study. Therefore, only interactions between RFCs in different CSs and between RFCs, the fault, and the load can take place in this study. The RFC dynamic models are essentially two Anderson-Fouad models of synchronous machines coupled mechanically by their rotors being connected to the same mechani- cal shaft. Besides the new cases studied, also a new voltage-dependent active power load model is presented and used in this study.
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| 6. |
- Ackeby, Susanne, et al.
(författare)
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Prosumer with demand-response:Distribution network impact and mitigation
- 2013
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- This report is the result from a project funded by ELFORSK done by STRI. Theproject is studying the effects the introduction of so called “prosumers”(customers with own production) and electrical vehicles will have on differenttypes of networks. Four different cases are studied: covering urban and ruralareas with different types of customers.In the urban areas the power through the transformer will be the limitingfactor. The major impact in the cases studied is from the introduction ofproduction from photovoltaics at the customer-side of the meter. This willresult in an introduction of surplus due to production which in one case led toan increase of the absolute power through the transformer with more than30%, which resulted in transformer overloading.In the rural areas the voltage drop or rise will be the limiting factor. The casesstudied had already high voltage drops even in the base cases. In the casestudies it was seen that the voltage drop could be slightly reduced whenintroducing more local production, but the production also led to that voltagerise could appear. As a result the interval of the voltage variations wasincreased, which in turn leads to difficulties with designing the network suchthat neither overvoltage nor undervoltage occurs.Introducing control algorithms had a very positive effect on reducing the netproduction from the photovoltaics. Using both hard and soft curtailment madeit possible to remove all overcurrents or overvoltages. Using hard curtailment,where all production is turned off during overcurrent or overvoltage, leadshowever to a large reduction in energy from renewable energy sources.Therefore soft curtailment should as much as possible be used.The control algorithms studied for reducing the net consumption had a morelimited effect and even resulted in an increase of the maximum netconsumption. When trying to reduce the net consumption during an overload,the reason of the overload could only be moved in time and not removed as inthe case of reducing the net production. And since often the period duringwhich the power exceeds the limit is longer than the number of hours possibleto move the energy, sometimes moving the energy had an adverse effect.The model used for controlling the net consumption needs furtherdevelopment, but it is still possible to draw the conclusion that this type ofcontrol offers only limited possibilities for mitigating overload or undervoltage.The effects of introducing prosumers and more electrical vehicles as defined inthe selected cases did not show any alarming results in this study. However,studies to learn more about the possible consequences of changes atcustomer-side are important to be able to handle the impact of such changeson the network.Further future studies needed: database with load and production data;improved control algorithms; demonstration project; experience from othercountries and studies on reactive power compensation.
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| 7. |
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| 8. |
- Ahmed, Kazi Main Uddin, 1989-, et al.
(författare)
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A Novel Reliability Index to Assess the Computational Resource Adequacy in Data Centers
- 2021
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Ingår i: IEEE Access. - NY : IEEE. - 2169-3536. ; 9, s. 54530-54541
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Tidskriftsartikel (refereegranskat)abstract
- The energy demand of data centers is increasing globally with the increasing demand for computational resources to ensure the quality of services. It is important to quantify the required resources to comply with the computational workloads at the rack-level. In this paper, a novel reliability index called loss of workload probability is presented to quantify the rack-level computational resource adequacy. The index defines the right-sizing of the rack-level computational resources that comply with the computational workloads, and the desired reliability level of the data center investor. The outage probability of the power supply units and the workload duration curve of servers are analyzed to define the loss of workload probability. The workload duration curve of the rack, hence, the power consumption of the servers is modeled as a function of server workloads. The server workloads are taken from a publicly available data set published by Google. The power consumption models of the major components of the internal power supply system are also presented which shows the power loss of the power distribution unit is the highest compared to the other components in the internal power supply system. The proposed reliability index and the power loss analysis could be used for rack-level computational resources expansion planning and ensures energy-efficient operation of the data center.
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| 9. |
- Ahmed, Kazi Main Uddin, 1989-, et al.
(författare)
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A Review of Data Centers Energy Consumption And Reliability Modeling
- 2021
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Ingår i: IEEE Access. - : IEEE. - 2169-3536. ; 9
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Forskningsöversikt (refereegranskat)abstract
- Enhancing the efficiency and the reliability of the data center are the technical challenges for maintaining the quality of services for the end-users in the data center operation. The energy consumption models of the data center components are pivotal for ensuring the optimal design of the internal facilities and limiting the energy consumption of the data center. The reliability modeling of the data center is also important since the end-user’s satisfaction depends on the availability of the data center services. In this review, the state-of-the-art and the research gaps of data center energy consumption and reliability modeling are identified, which could be beneficial for future research on data center design, planning, and operation. The energy consumption models of the data center components in major load sections i.e., information technology (IT), internal power conditioning system (IPCS), and cooling load section are systematically reviewed and classified, which reveals the advantages and disadvantages of the models for different applications. Based on this analysis and related findings it is concluded that the availability of the model parameters and variables are more important than the accuracy, and the energy consumption models are often necessary for data center reliability studies. Additionally, the lack of research on the IPCS consumption modeling is identified, while the IPCS power losses could cause reliability issues and should be considered with importance for designing the data center. The absence of a review on data center reliability analysis is identified that leads this paper to review the data center reliability assessment aspects, which is needed for ensuring the adaptation of new technologies and equipment in the data center. The state-of-the-art of the reliability indices, reliability models, and methodologies are systematically reviewed in this paper for the first time, where the methodologies are divided into two groups i.e., analytical and simulation-based approaches. There is a lack of research on the data center cooling section reliability analysis and the data center components’ failure data, which are identified as research gaps. In addition, the dependency of different load sections for reliability analysis of the data center is also included that shows the service reliability of the data center is impacted by the IPCS and the cooling section.
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| 10. |
- Ahmed, Kazi Main Uddin, 1989-, et al.
(författare)
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Characterizing Failure and Repair Time of Servers in a Hyper-scale Data Center
- 2020
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Ingår i: Proceedings of 2020 IEEE PES Innovative Smart Grid Technologies Europe (ISGT-Europe) 26-28 October, 2020. - : IEEE. ; , s. 660-664
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Konferensbidrag (refereegranskat)abstract
- Hyper-scale data centers are used to host cloud computing interfaces to support the increasing demand for storage and computational resources. For achieving specific service level agreements (SLA), this infrastructure demands highly available cloud computing systems. It is necessary to analyze the server failure incidents to determine the way of improving the reliability of the system since the computational interruption causes financial losses for the data center owners. Regarding the reliability analysis, it is important to characterize the time to failure and time to repair of the servers. In this paper, a publicly available data set from Google cloud-cluster data center will be analyzed to find the distribution function for the time to failure and the time to repair for the servers in a cloud based data centers.
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