Articles de revista
http://hdl.handle.net/2117/3669
20170426T14:06:33Z

Energy dissipation in the presence of subharmonic excitation in dynamic atomic force microscopy
http://hdl.handle.net/2117/103298
Energy dissipation in the presence of subharmonic excitation in dynamic atomic force microscopy
Chiesa, Matteo; Gadelrab, Karim Raafat; Verdaguer, Albert; Segura, Juan José; Barcons Xixons, Víctor; Thomson, Neil H.; Phillips, M.A.; Stefancich, M.; Santos Hernández, Sergio
Amplitude modulation atomic force microscopy allows quantifying energy dissipation in the nanoscale with great accuracy with the use of analytical expressions that account for the fundamental frequency and higher harmonics. Here, we focus on the effects of subharmonic excitation on energy dissipation and its quantification. While there might be several mechanisms inducing subharmonics, a general analytical expression to quantify energy dissipation whenever subharmonics are excited is provided. The expression is a generalization of previous findings. We validate the expression via numerical integration by considering capillary forces and provide experimental evidence of subharmonic excitation for a range of operational parameters.
20170404T14:10:15Z
Chiesa, Matteo
Gadelrab, Karim Raafat
Verdaguer, Albert
Segura, Juan José
Barcons Xixons, Víctor
Thomson, Neil H.
Phillips, M.A.
Stefancich, M.
Santos Hernández, Sergio
Amplitude modulation atomic force microscopy allows quantifying energy dissipation in the nanoscale with great accuracy with the use of analytical expressions that account for the fundamental frequency and higher harmonics. Here, we focus on the effects of subharmonic excitation on energy dissipation and its quantification. While there might be several mechanisms inducing subharmonics, a general analytical expression to quantify energy dissipation whenever subharmonics are excited is provided. The expression is a generalization of previous findings. We validate the expression via numerical integration by considering capillary forces and provide experimental evidence of subharmonic excitation for a range of operational parameters.

Cantilever dynamics in amplitude modulation AFM: continuous and discontinuous transitions
http://hdl.handle.net/2117/102923
Cantilever dynamics in amplitude modulation AFM: continuous and discontinuous transitions
Santos Hernandez, Sergio; Barcons Xixons, Víctor; Font Teixidó, Josep; Thomson, Neil H.
Transitions between the attractive and the repulsive force regimes for amplitude modulation atomic force microscopy (AFM) can be either discontinuous, with a corresponding jump in amplitude and phase, or continuous and smooth. During the transitions, peak repulsive and average forces can be up to an order of magnitude higher when these are discrete. Under certain circumstances, for example, when the tip radius is relatively large (e.g. R > 20–30 nm) and for high cantilever free amplitudes (e.g. A0 > 40–50 nm), the L state can be reached with relatively low setpoints only (e.g. Asp/A0 < 0.30). We find that these cases do not generally lead to higher resolution but increase the background noise instead. This is despite the fact that the imaging can be noncontact under these conditions. The appearance of background noise is linked to increasing cantilever mean deflection and tip–surface proximity with increasing free amplitude in the L state.
Cantilever dynamics in amplitude modulation AFM: Continuous and discontinuous transitions (PDF Download Available). Available from: https://www.researchgate.net/publication/231025693_Cantilever_dynamics_in_amplitude_modulation_AFM_Continuous_and_discontinuous_transitions [accessed Mar 27, 2017].
20170327T15:56:43Z
Santos Hernandez, Sergio
Barcons Xixons, Víctor
Font Teixidó, Josep
Thomson, Neil H.
Transitions between the attractive and the repulsive force regimes for amplitude modulation atomic force microscopy (AFM) can be either discontinuous, with a corresponding jump in amplitude and phase, or continuous and smooth. During the transitions, peak repulsive and average forces can be up to an order of magnitude higher when these are discrete. Under certain circumstances, for example, when the tip radius is relatively large (e.g. R > 20–30 nm) and for high cantilever free amplitudes (e.g. A0 > 40–50 nm), the L state can be reached with relatively low setpoints only (e.g. Asp/A0 < 0.30). We find that these cases do not generally lead to higher resolution but increase the background noise instead. This is despite the fact that the imaging can be noncontact under these conditions. The appearance of background noise is linked to increasing cantilever mean deflection and tip–surface proximity with increasing free amplitude in the L state.
Cantilever dynamics in amplitude modulation AFM: Continuous and discontinuous transitions (PDF Download Available). Available from: https://www.researchgate.net/publication/231025693_Cantilever_dynamics_in_amplitude_modulation_AFM_Continuous_and_discontinuous_transitions [accessed Mar 27, 2017].

Performance evaluation of secondary control policies with respect to digital communications properties in inverterbased islanded microgrids
http://hdl.handle.net/2117/102634
Performance evaluation of secondary control policies with respect to digital communications properties in inverterbased islanded microgrids
Martí Colom, Pau; Velasco García, Manel; Martín Rull, Enric Xavier; García de Vicuña Muñoz de la Nava, José Luis; Miret Tomàs, Jaume; Castilla Fernández, Miguel
A key challenge for invertedbased microgrids working in islanded mode is to maintain their own frequency and voltage to a certain reference values while regulating the active and reactive power among distributed generators and loads. The implementation of frequency and voltage restoration control policies often requires the use of a digital communication network for realtime data exchange (tertiary control covers the coordi nated operation of the microgrid and the host grid). Whenever a digital network is placed within the loop, the operation of the secondary control may be affected by the inherent properties of the communication technology. This paper analyses the effect that properties like transmission intervals and message dropouts have for four existing representative approaches to secondary control in a scalable islanded microgrid. The simulated results reveals pros and cons for each approach, and identifies threats that properly avoided or handled in advance can prevent failures that otherwise would occur. Selected experimental results on a low scale laboratory microgrid corroborate the conclusions extracted from the simulation study.
20170320T08:14:59Z
Martí Colom, Pau
Velasco García, Manel
Martín Rull, Enric Xavier
García de Vicuña Muñoz de la Nava, José Luis
Miret Tomàs, Jaume
Castilla Fernández, Miguel
A key challenge for invertedbased microgrids working in islanded mode is to maintain their own frequency and voltage to a certain reference values while regulating the active and reactive power among distributed generators and loads. The implementation of frequency and voltage restoration control policies often requires the use of a digital communication network for realtime data exchange (tertiary control covers the coordi nated operation of the microgrid and the host grid). Whenever a digital network is placed within the loop, the operation of the secondary control may be affected by the inherent properties of the communication technology. This paper analyses the effect that properties like transmission intervals and message dropouts have for four existing representative approaches to secondary control in a scalable islanded microgrid. The simulated results reveals pros and cons for each approach, and identifies threats that properly avoided or handled in advance can prevent failures that otherwise would occur. Selected experimental results on a low scale laboratory microgrid corroborate the conclusions extracted from the simulation study.

Modelbased active damping control for threephase voltage source inverters with LCL filter
http://hdl.handle.net/2117/91268
Modelbased active damping control for threephase voltage source inverters with LCL filter
Guzmán Solà, Ramon; García de Vicuña Muñoz de la Nava, José Luis; Morales López, Javier; Castilla Fernández, Miguel; Miret Tomàs, Jaume
This paper presents a robust modelbased active
damping control in natural frame for a threephase voltage
source inverter with LCL filter. The presence of the LCL
filter complicates the design of the control scheme, particularly
when system parameters deviations are considered. The proposed
control method is addressed to overcome such difficulties and
uses a modified converter model in an state observer. In this
proposal, the converter model is modified by introducing a virtual
damping resistor. Then, a Kalman filter makes use of this model
to estimate the system statespace variables. Although the state
estimates do not obviously match the real world system variables,
they permit designing three current slidingmode controllers that
provide the following features to the closed loop system: a) robust
ande active damping capability like in the case of using a physical
damping resistor, b) robustness because the control specifications
are met independently of variation in the system parameters, c)
noise immunity due to the application of the Kalman filter, and
d) power loss minimization because the system losses caused
by the physical damping resistor are avoided. An interesting
side effect of the proposed control scheme is that the sliding
surfaces for each controller are independent. This decoupling
property for the three controllers allows using a fixed switching
frequency algorithm that ensures perfect current control. To
complete the control scheme, a theoretical stability analysis is
developed. Finally, selected experimental results validate the
proposed control strategy and permit illustrating all its appealing
features.
(c) 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 works.
20161031T10:50:17Z
Guzmán Solà, Ramon
García de Vicuña Muñoz de la Nava, José Luis
Morales López, Javier
Castilla Fernández, Miguel
Miret Tomàs, Jaume
This paper presents a robust modelbased active
damping control in natural frame for a threephase voltage
source inverter with LCL filter. The presence of the LCL
filter complicates the design of the control scheme, particularly
when system parameters deviations are considered. The proposed
control method is addressed to overcome such difficulties and
uses a modified converter model in an state observer. In this
proposal, the converter model is modified by introducing a virtual
damping resistor. Then, a Kalman filter makes use of this model
to estimate the system statespace variables. Although the state
estimates do not obviously match the real world system variables,
they permit designing three current slidingmode controllers that
provide the following features to the closed loop system: a) robust
ande active damping capability like in the case of using a physical
damping resistor, b) robustness because the control specifications
are met independently of variation in the system parameters, c)
noise immunity due to the application of the Kalman filter, and
d) power loss minimization because the system losses caused
by the physical damping resistor are avoided. An interesting
side effect of the proposed control scheme is that the sliding
surfaces for each controller are independent. This decoupling
property for the three controllers allows using a fixed switching
frequency algorithm that ensures perfect current control. To
complete the control scheme, a theoretical stability analysis is
developed. Finally, selected experimental results validate the
proposed control strategy and permit illustrating all its appealing
features.

Evaluating the 2014 retroactive regulatory framework applied to the grid connected PV systems in Spain
http://hdl.handle.net/2117/90837
Evaluating the 2014 retroactive regulatory framework applied to the grid connected PV systems in Spain
Hoz Casas, Jordi de la; Martín Cañadas, María Elena; Miret Tomàs, Jaume; Castilla Fernández, Miguel; Guzmán Solà, Ramon
© 2016 Elsevier Ltd. The RD 413/2014 new economic and regulatory framework for producers of electricity from renewable energy sources in Spain has been here analysed, putting the focus on its impact on the economic results of the existing grid connected PV systems. A complete formulation of the new remuneration scheme is first presented, making evident its high complexity and the great number of regulatory parameters involved. Then, in order to facilitate the discernment of its operating mechanism a simplified model is proposed. This approach has proven to be a useful tool to identify the most influential regulatory parameters and also which could be the plausible future regulatory actions in order to contain the cost to the Spanish Electricity System. Finally, the NPV and the IRR among other indicators are calculated for a representative case study facility, evidencing a substantial profitability reduction in the transition from the former economic and regulatory framework to the RD 413/2014 new remuneration scheme.
20161018T10:31:08Z
Hoz Casas, Jordi de la
Martín Cañadas, María Elena
Miret Tomàs, Jaume
Castilla Fernández, Miguel
Guzmán Solà, Ramon
© 2016 Elsevier Ltd. The RD 413/2014 new economic and regulatory framework for producers of electricity from renewable energy sources in Spain has been here analysed, putting the focus on its impact on the economic results of the existing grid connected PV systems. A complete formulation of the new remuneration scheme is first presented, making evident its high complexity and the great number of regulatory parameters involved. Then, in order to facilitate the discernment of its operating mechanism a simplified model is proposed. This approach has proven to be a useful tool to identify the most influential regulatory parameters and also which could be the plausible future regulatory actions in order to contain the cost to the Spanish Electricity System. Finally, the NPV and the IRR among other indicators are calculated for a representative case study facility, evidencing a substantial profitability reduction in the transition from the former economic and regulatory framework to the RD 413/2014 new remuneration scheme.

Comparative study of reactive power control methods for photovoltaic inverters in lowvoltage grids
http://hdl.handle.net/2117/86884
Comparative study of reactive power control methods for photovoltaic inverters in lowvoltage grids
Momeneh, Arash; Castilla Fernández, Miguel; Miret Tomàs, Jaume; Martí Colom, Pau; Velasco García, Manel
The increasing installation of photovoltaic panels in lowvoltage grids causes an over voltage problem, especially during high production and low consumption periods. Generally the over voltage problem is overcome by implementing reactive power control methods. The advances in networked control systems theory and practice create new scenarios where reactive power control methods can offer additional features and benefits. To explore these new capabilities, this study presents two new reactive power control methods that exploit the networked approach. These two methods are evaluated in a comparative reference framework that also includes the baseline case where no reactive control method is applied, the conventional droop method approach, and a solution based on a nearoptimal location of a high power STATCOM derived from one of the new proposed networked methods. The main merit factors used to compare the control methods are the maximum voltage across the distribution grid, the power factor in the point of common coupling, and the total power losses and economic cost of the installation. With these merit factors, the advantages and limitations of the new and existing control methods are revealed and discussed. A useful discussion for selecting the best control solution is also reported.
20160510T14:45:57Z
Momeneh, Arash
Castilla Fernández, Miguel
Miret Tomàs, Jaume
Martí Colom, Pau
Velasco García, Manel
The increasing installation of photovoltaic panels in lowvoltage grids causes an over voltage problem, especially during high production and low consumption periods. Generally the over voltage problem is overcome by implementing reactive power control methods. The advances in networked control systems theory and practice create new scenarios where reactive power control methods can offer additional features and benefits. To explore these new capabilities, this study presents two new reactive power control methods that exploit the networked approach. These two methods are evaluated in a comparative reference framework that also includes the baseline case where no reactive control method is applied, the conventional droop method approach, and a solution based on a nearoptimal location of a high power STATCOM derived from one of the new proposed networked methods. The main merit factors used to compare the control methods are the maximum voltage across the distribution grid, the power factor in the point of common coupling, and the total power losses and economic cost of the installation. With these merit factors, the advantages and limitations of the new and existing control methods are revealed and discussed. A useful discussion for selecting the best control solution is also reported.

Modelbased control for a threephase shunt active power filter
http://hdl.handle.net/2117/84364
Modelbased control for a threephase shunt active power filter
Guzmán Solà, Ramon; García de Vicuña Muñoz de la Nava, José Luis; Morales López, Javier; Castilla Fernández, Miguel; Miret Tomàs, Jaume
This paper presents a robust modelbased control
in natural frame for a threephase shunt active power filter.
For the proposed control method a linear converter model is
deduced. Then, this model is used in a Kalman filter in order
to estimate the system statespace variables. Even though the
states estimation do not match the variables of the real system,
it has allowed to design three sliding mode controllers providing
the following features to the closed loop system: a) robustness
due to the fact that control specifications are met independently
of any variation in the system parameters; b) noise immunity,
since a Kalman filter is applied; c) a lower THD of the current
delivered by the grid compared with the standard solution using
measured variables; d) the fundamental component of the voltage
at point of common coupling is estimated even in the case of
a distorted grid; and e) a reduction in the number of sensors.
Thanks to this solution the sliding surfaces for each controller are
independent. This decoupling property of the three controllers
allows using a fixed switching frequency algorithm that ensures
a perfect current control. Finally, experimental results validate
the propos
20160315T11:07:33Z
Guzmán Solà, Ramon
García de Vicuña Muñoz de la Nava, José Luis
Morales López, Javier
Castilla Fernández, Miguel
Miret Tomàs, Jaume
This paper presents a robust modelbased control
in natural frame for a threephase shunt active power filter.
For the proposed control method a linear converter model is
deduced. Then, this model is used in a Kalman filter in order
to estimate the system statespace variables. Even though the
states estimation do not match the variables of the real system,
it has allowed to design three sliding mode controllers providing
the following features to the closed loop system: a) robustness
due to the fact that control specifications are met independently
of any variation in the system parameters; b) noise immunity,
since a Kalman filter is applied; c) a lower THD of the current
delivered by the grid compared with the standard solution using
measured variables; d) the fundamental component of the voltage
at point of common coupling is estimated even in the case of
a distorted grid; and e) a reduction in the number of sensors.
Thanks to this solution the sliding surfaces for each controller are
independent. This decoupling property of the three controllers
allows using a fixed switching frequency algorithm that ensures
a perfect current control. Finally, experimental results validate
the propos

Voltage security in AC microgrids: A power flowbased approach considering droopcontrolled inverters
http://hdl.handle.net/2117/84059
Voltage security in AC microgrids: A power flowbased approach considering droopcontrolled inverters
Souza, Antonio Carlos Zambroni de; Santos, Marcos; Castilla Fernández, Miguel; Miret Tomàs, Jaume; García de Vicuña Muñoz de la Nava, José Luis; Marujo, Diogo
This study deals with the problem of voltage security in microgrids. In general, voltage security is an issue for power systems, where the lack of reactive power is a concern. For microgrids a complexity is imposed, since frequency and voltage level deviations may take place by the operation of droopcontrolled inverters. This study incorporates the load margin calculation into a microgrid structure. For this purpose, a special power flow program is developed in order to consider the effects of the droopcontrolled inverters. Simulation results are obtained with the help of an islanded distribution system with only generators coupled by droopcontrolled inverters, so the proposed methodology may be tested and discussed.
20160309T13:43:14Z
Souza, Antonio Carlos Zambroni de
Santos, Marcos
Castilla Fernández, Miguel
Miret Tomàs, Jaume
García de Vicuña Muñoz de la Nava, José Luis
Marujo, Diogo
This study deals with the problem of voltage security in microgrids. In general, voltage security is an issue for power systems, where the lack of reactive power is a concern. For microgrids a complexity is imposed, since frequency and voltage level deviations may take place by the operation of droopcontrolled inverters. This study incorporates the load margin calculation into a microgrid structure. For this purpose, a special power flow program is developed in order to consider the effects of the droopcontrolled inverters. Simulation results are obtained with the help of an islanded distribution system with only generators coupled by droopcontrolled inverters, so the proposed methodology may be tested and discussed.

Control strategy to maximize the power capability of PV threephase inverters during voltage sags
http://hdl.handle.net/2117/81655
Control strategy to maximize the power capability of PV threephase inverters during voltage sags
Sosa, Jorge Luis; Castilla Fernández, Miguel; Miret Tomàs, Jaume; Matas Alcalá, José; AlTurki, Y.A.
Under voltage sags, gridtied photovoltaic inverters should remain connected to the grid according to lowvoltage ridethrough requirements. During such perturbations, it is interesting to exploit completely the distributed power provisions to contribute to the stability and reliability of the grid. In this sense, this paper proposes a lowvoltage ridethrough control strategy that maximizes the inverter power capability by injecting the maximumrated current during the sag. To achieve this objective, two possible active power situations have been considered, i.e., high and lowpower production scenarios. In the first case, if the source is unable to deliver the whole generated power to the grid, the controller applies active power curtailment to guarantee that the maximum rated current is not surpassed. In the second case, the maximum allowed current is not reached, thus, the control strategy determined the amount of reactive power that can be injected up to reach it. The control objective can be fulfilled by means of a flexible current injection strategy that combines a proper balance between positive and negativecurrent sequences, which limits the inverter output current to the maximum rated value and avoid active power oscillations. Selected experimental and simulation results are reported in order to validate the effectiveness of the proposed control strategy.
20160119T09:24:18Z
Sosa, Jorge Luis
Castilla Fernández, Miguel
Miret Tomàs, Jaume
Matas Alcalá, José
AlTurki, Y.A.
Under voltage sags, gridtied photovoltaic inverters should remain connected to the grid according to lowvoltage ridethrough requirements. During such perturbations, it is interesting to exploit completely the distributed power provisions to contribute to the stability and reliability of the grid. In this sense, this paper proposes a lowvoltage ridethrough control strategy that maximizes the inverter power capability by injecting the maximumrated current during the sag. To achieve this objective, two possible active power situations have been considered, i.e., high and lowpower production scenarios. In the first case, if the source is unable to deliver the whole generated power to the grid, the controller applies active power curtailment to guarantee that the maximum rated current is not surpassed. In the second case, the maximum allowed current is not reached, thus, the control strategy determined the amount of reactive power that can be injected up to reach it. The control objective can be fulfilled by means of a flexible current injection strategy that combines a proper balance between positive and negativecurrent sequences, which limits the inverter output current to the maximum rated value and avoid active power oscillations. Selected experimental and simulation results are reported in order to validate the effectiveness of the proposed control strategy.

Contribution to the PVtoinverter sizing ratio determination using a custom flexible experimental setup
http://hdl.handle.net/2117/78227
Contribution to the PVtoinverter sizing ratio determination using a custom flexible experimental setup
Camps, Xavier; Velasco Quesada, Guillermo; Hoz Casas, Jordi de la; Martín Cañadas, María Elena
This work presents a novel approach to the experimental validation of the optimal PVtoinverter sizing ratio value for the energy yield maximization of a GCPVS by means the implementation of a custom workbench using a solar array simulator which has allowed to replicate a wide variety of technical configurations and environmental data. The compliance between the experimental setup and the mathematical model developed to simulate the optimal PVtoinverter sizing ratio value was demonstrated by the specific tests carried out on its two main subsystems (the PV generator and the inverter), thus the subsequent simulations were made on a firm basis. Likewise, the evaluation of the overall system also showed a good agreement between the experimental and the simulated energy yield and optimal PVtoinverter sizing ratio results, rendering relative errors below 3% for both magnitudes. (C) 2015 Elsevier Ltd. All rights reserved.
20151026T12:10:10Z
Camps, Xavier
Velasco Quesada, Guillermo
Hoz Casas, Jordi de la
Martín Cañadas, María Elena
This work presents a novel approach to the experimental validation of the optimal PVtoinverter sizing ratio value for the energy yield maximization of a GCPVS by means the implementation of a custom workbench using a solar array simulator which has allowed to replicate a wide variety of technical configurations and environmental data. The compliance between the experimental setup and the mathematical model developed to simulate the optimal PVtoinverter sizing ratio value was demonstrated by the specific tests carried out on its two main subsystems (the PV generator and the inverter), thus the subsequent simulations were made on a firm basis. Likewise, the evaluation of the overall system also showed a good agreement between the experimental and the simulated energy yield and optimal PVtoinverter sizing ratio results, rendering relative errors below 3% for both magnitudes. (C) 2015 Elsevier Ltd. All rights reserved.