| 1. |
- Wärmefjord, Kristina, 1976, et al.
(författare)
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Welding of non-nominal geometries : physical tests
- 2016
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Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271 .- 2212-8271. ; 43, s. 136-141
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Tidskriftsartikel (refereegranskat)abstract
- The geometrical quality of a welded assembly is to some extent depending part positions before welding. Here, a design of experiment is set up in order to investigate this relation using physical tests in a controlled environment. Based on the experimental results it can be concluded that the influence of part position before welding is significant for geometrical deviation after welding. Furthermore, a working procedure for a completely virtual geometry assurance process for welded assemblies is outlined. In this process, part variations, assembly fixture variations and welding induced variations are important inputs when predicting the capability of the final assembly.
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| 2. |
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| 3. |
- Johansson, Lars-Olof, et al.
(författare)
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A Boundary Practice Perspective on Co-creation of ICT Innovations
- 2016
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Ingår i: Nordic Contributions in IS Research. - Cham : Springer Publishing Company. - 9783319435961 - 9783319435978 ; , s. 100-115
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Konferensbidrag (refereegranskat)abstract
- Research has shown that collaboration and co-creation among different groups of stakeholders add complexity and challenges to the innovation process. In this paper a study of co-creation in a multi-stakeholder innovation process is presented. The co-creation is explored and described from a boundary practice perspective. The empirical data presented in the study is based on a user-centric innovation project, Free2Ride, where researchers, developers and members of two equestrian clubs co-created a piece of ICT safety equipment consisting of a transmitter (on the horse) and a receiver (application on a smartphone) to be used by equestrian club members during their everyday riding activities. Three episodes were extracted from the empirical data and presented in the paper. From these episodes the researchers have identified four characteristics of the spanning of boundaries in co-creation from a boundary practice perspective. One of the contributions in the papers is a description of boundary practice-spanning. The research approach adopted in the study is the action case approach.
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| 4. |
- Karatsivos, Evripidis, et al.
(författare)
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Disturbance Management in a DC Grid using a Hierarchical Control Structure
- 2015
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Ingår i: Lund Symposium 27/28 May, 2015.
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Konferensbidrag (refereegranskat)abstract
- DC grids will play a key role in the future, interconnecting generation and load centers as well as energy markets. Large multi-terminal VSC-HVDC systems of high capacity will require automated coordination for several operation scenarios especially when operating under disturbances. With this in mind, and considering the dynamics of a DC system, communication loss events are considered severe disturbances in the operation of the multi-terminal system along with major electrical faults such as 3-phase faults on the AC sides of the converters that require considerable coordination efforts. This work aims at enabling the multi-terminal system with communication loss ride-through and automatic post-fault rescheduling capabilities. This is achieved within a hierarchical control structure whose features are briefly described. A 3-terminal VSC-HVDC system is modelled for the needs of demonstration under different operation scenarios. Communication loss ride-through capability is demonstrated by comparing the behavior of the 3-terminal system during a normal operation scenario and a worst-case scenario where communications with all-three terminals fail. Its behavior does not change but since communications fail, the operation is uncoordinated greatly reducing the security of the system regarding upcoming events. During 3-phase faults DC droop performs the primary control stabilizing the system. Secondary control is performed by the overall control after the system state is identified. Automatic, post-fault rescheduling takes place in an effort to follow the power flow schedule where possible without exceeding substation limits.
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| 5. |
- Safari, Reza, et al.
(författare)
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Impedance Matching for VSC-HVDC Damping Controller Gain Selection
- 2018
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Ingår i: IEEE Transactions on Power Systems. - 0885-8950.
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Tidskriftsartikel (refereegranskat)abstract
- The impedance matching concept is employed in this paper to select the optimum gain of a VSC-HVDC damping controller. The damping controller of the DC link is based on control of active power in proportion to the difference in local frequency at the VSC-HVDC converter stations. To explain impedance matching, the small-disturbance electro-mechanical dynamics of a power system is transformed to an equivalent LC-circuit. The VSC-HVDC damping controller corresponds to introducing a resistor in the circuit model. Using impedance matching to select resistor value in the circuit model is equivalent to selecting damping controller gain that gives maximum damping ratio. An analytical derivation is conducted for a linearized model of a generic two-area power system, including an expression for the optimum gain. It is also shown how the concept may be employed without a circuit model and that the optimum value is hardly affected by changes in power flow. Then the performance of the proposed approach in multi-mode test systems and with nonlinearities is evaluated using dynamic simulations in DIgSILENT PowerFactory. The results show that impedance matching maximizes the targeted mode damping ratio while non-targeted modes are not negatively impacted.
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| 6. |
- Safari Tirtashi, Mohammad Reza, et al.
(författare)
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Impedance Matching for VSC-HVDC and Energy Storage Damping Controllers
- 2018
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Ingår i: IEEE Transactions on Power Delivery. - 0885-8977. ; 33:2, s. 1016-1017
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Tidskriftsartikel (refereegranskat)abstract
- The small-disturbance electro-mechanical dynamics of a power system can be translated to an equivalent circuit model with inductances and capacitances. Damping control of active power in proportion to local frequency as with a VSC-HVDC link or an energy storage then corresponds to introducing a resistor in the circuit model. This letter shows that impedance matching can be used to select resistor value, and equivalently damping controller gain, that gives maximum damping ratio. It is also shown how the concept is employed without a circuit model.
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| 7. |
- Tirtashi, M. Reza Safari, et al.
(författare)
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Dynamic and static analysis of the shunt capacitors control effect on the long-term voltage instability
- 2016
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Ingår i: 2016 IEEE Power and Energy Society General Meeting, PESGM 2016. - 9781509041688 ; 2016-November
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Konferensbidrag (refereegranskat)abstract
- This paper concerns the dynamic and static analysis of the effect of shunt capacitor control strategies on the long-term voltage instability. For this purpose a simplified test system called N3area, reflecting the key voltage instability characteristics of NORDIC32 is considered. Two different control strategies for shunt capacitors including the local and neighboring schemes are applied to improve the voltage control in the system. First the dynamic simulation results for a specified long-term voltage instability scenario are explained and discussed then the static investigation is conducted based on the PV curves and it is shown that the neighboring scheme injects more reactive power to the system. Afterward, based on the modal analysis technique and V-Q sensitivity analysis, it is demonstrated that the possibility of involving the most critical buses from voltage stability perspective in the capacitors connection decisions are much higher in the neighboring scheme compared to the local one. The two strategies are explained and compared in both dynamic and static analysis and it is shown that control using the voltage at neighboring buses gives better performance.
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| 8. |
- Tirtashi, M. Reza Safari, et al.
(författare)
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VSC-HVDC application to improve the long-term voltage stability
- 2017
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Ingår i: 2017 IEEE Manchester PowerTech, Powertech 2017. - 9781509042371
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Konferensbidrag (refereegranskat)abstract
- This paper concerns the VSC-HVDC application to improve the long-term voltage stability. For this purpose the NORDIC32 test system is considered. All necessary dynamics to study the long-term voltage stability are included in the test system. An appropriate control strategy for VSC-HVDC is proposed in the paper. The goal is to reach the best condition from long-term voltage stability perspective with respect to system configuration and also the VSC-HVDC capability curve. To show the capability of the proposed control methodology, two long-term voltage instability scenarios are considered. For the first scenario, VSC-HVDC can lead to a stable condition with its initial active-reactive power set points. In the second voltage instability scenario which is more severe, the proper power flow change in the system using VSC-HVDC is needed to avoid the voltage collapse. Otherwise the VSC-HVDC could buy some time and lead to later collapse which is still important. The simulation results are explained and thoroughly discussed. It is demonstrated that with the proper control of VSC-HVDC, long-term voltage stability could improve dramatically.
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