Papers by Francisco Freijedo
IEEE Transactions on Industrial Electronics, 2000
Multiphase converters are being applied to an increasing number of industrial applications in rec... more Multiphase converters are being applied to an increasing number of industrial applications in recent years. On the other hand, multilevel converters have become a mature technology mainly in medium and high power applications.
IEEE Transactions on Power Electronics, 2015
Rigorous analysis and design of the current control loop in voltage source converters (VSCs) requ... more Rigorous analysis and design of the current control loop in voltage source converters (VSCs) requires an accurate modeling. The loop behavior can be significantly influenced by the VSC working conditions. To consider such effect, converter losses should be included in the model, which can be done by means of an equivalent series resistance. This paper proposes a method to identify the VSC equivalent loss resistance for the proper tuning of the current control loop. It is based on analysis of the closed-loop transient response provided by a synchronous proportional-integral current controller, according to the internal model principle. The method gives a set of loss resistance values linked to working conditions, which can be used to improve the tuning of the current controllers, either by online adaptation of the controller gains or by open-loop adaptive adjustment of them according to prestored data. The developed identification procedure is tested in the laboratory at different specifications of power level and switching frequency.
IEEE Transactions on Industrial Electronics, 2000
The increasing number of distributed power generation systems (DPGSs) is changing the traditional... more The increasing number of distributed power generation systems (DPGSs) is changing the traditional organization of the electrical network. An important part of these DPGSs is based on renewable energy sources. In order to guarantee an efficient integration of renewable-based generation units, grid codes must be fulfilled. Their most demanding requirements, such as low-voltage ride-through and grid support, need a really fast transient response of the power electronics devices. In this manner, the current controller speed is a key point. This paper proposes a methodology to assess and optimize the transient response of proportional-resonant current controllers. The proposed methodology is based on the study of the error signal transfer function roots by means of pole-zero plots. Optimal gains are set to achieve fast and nonoscillating transient responses, i.e., to optimize the settling time. It is proved that optimal gain selection results from a tradeoff between transients caused by reference changes and transients caused by changes at the point of common coupling. Experimental results obtained by means of a three-phase voltage source converter prototype validate the approach. Short transient times are achieved even when tests emulate very demanding realistic conditions: a +90 • phase-angle jump in the current reference and a "type C" voltage sag at the point of common coupling.
2009 35th Annual Conference of IEEE Industrial Electronics, 2009
The actual lines have many operation problems like power flow distribution, oscillations... A bad... more The actual lines have many operation problems like power flow distribution, oscillations... A bad power flow distribution on the network lines appears for example during a fault, in this case is posible that some lines of the network would be overloaded and other lines under their thermal value. A posible solution of the problem is using series compensators to redistribute the power flow, the converters are bypassed until the fault appears and after this, it start to compensate. Evolutive algorithms can solve the problem of the ubication in the network of these converters and to chose the power of it too, minimizing the total installed power to make that power flow redistribution.
2015 IEEE 13th Brazilian Power Electronics Conference and 1st Southern Power Electronics Conference (COBEP/SPEC), 2015
Real time grid-synchronization techniques are studied in this dissertation. The synchronization b... more Real time grid-synchronization techniques are studied in this dissertation. The synchronization block is a crucial part of the controller in grid-connected power converters. The phase-angle of the voltage/current vector fundamental component at the point of common coupling (PCC) should be tracked online in order to control the energy transfer between the power converter and the ac mains. Synchronization algorithms have been evolving since the first analog zero-cross detectors to current high performance digital implementations. An in-depth review of the state-of-art in grid synchronization is contributed in the Chapter 2 of this PhD dissertation. In some ways, it could be said that the grid synchronization state-of-art has been evolving to avoid malfunctions due to power quality phenomena. The Ainsworth proposal of using a voltage controlled oscillator (VCO) inside the control loop of a High Voltage Direct Current (HVDC) successfully dealt with the novel, at that time, harmonic inst...
IEEE Transactions on Power Electronics, 2016
Ieee Transactions on Power Electronics, 2016
Precise knowledge of the plant time constant L/R is essential to perform a thorough analysis and ... more Precise knowledge of the plant time constant L/R is essential to perform a thorough analysis and design of the current control loop in voltage source converters (VSCs). From the perspective of the current controller dynamics in the low frequency range, such plant time constant is also suitable for most cases in which an LCL filter is used. As the loop behavior can be significantly influenced by the VSC working conditions, the effects associated to converter losses should be included in the model, through an equivalent series resistance. In addition, the plant inductance may also present important uncertainties with respect to the value of the VSC L/LCL interface filter measured at rated conditions. Thus, in this work, a method is presented to estimate both parameters of the plant time constant, i.e., the equivalent inductance and resistance in the plant model of current-controlled VSCs. The proposed technique is based on the evaluation of the closed-loop transient responses of both axes of the synchronous reference frame when a proportional-integral current controller is implemented. The method gives a set of resistance and inductance values that should be employed for a rigorous design of the current controllers. Experimental results validate the approach.
IEEE Transactions on Power Electronics, 2015
IEEE Transactions on Power Electronics, 2015
Generally speaking, designing single-phase phaselocked loops (PLLs) is more complicated than thre... more Generally speaking, designing single-phase phaselocked loops (PLLs) is more complicated than three-phase ones, as their implementation often involves the generation of a fictitious orthogonal signal for the frame transformation. In recent years, many approaches to generate the orthogonal signal have been proposed, the simplest perhaps being the transfer delay based method. In the transfer delay based PLL (TD-PLL), the orthogonal signal is generated by delaying the original singlephase signal by T /4 (one-quarter of a period). The phase shift caused by the transfer delay block, however, will not be exactly 90 • under off-nominal grid frequencies, which results in errors in the estimated quantities by the TD-PLL. To alleviate this issue, an improved version of TD-PLL, called the non-frequency dependent TD-PLL (NTD-PLL), has recently been proposed. The NTD-PLL uses another T /4 delay unit in its feedback path to make the PLL immune to grid frequency variations. To the best of the authors' knowledge, the accurate small-signal modeling of the TD-PLL and NTD-PLL has not yet been carried out, and no detailed analysis of their performance has been presented. The main aim of this paper is to address these issues and explore new methods to enhance their performance. The stability analysis, control design guidelines and performance comparison with the state-of-the-art PLLs are presented as well.
En este artículo se presenta una solución a la ecualización de corrientes en un convertidor reduc... more En este artículo se presenta una solución a la ecualización de corrientes en un convertidor reductor multifase. La solución propuesta se basa en la comparación de la corriente de pico instantánea en el MOSFET principal de cada fase con la media dinámica de las corrientes de pico de todos los módulos CC-CC. Esta comparativa es suficiente para igualar las corrientes medias de todas las fases variando consecuentemente el ciclo de trabajo de cada una de ellas. Con ello se logra igualar la potencia entregada por fase y aumentar la fiabilidad de todo el conjunto, ya que no se sobrecargan unos módulos más que otros. La implementación de la etapa de potencia se ha realizado empleando tecnología SMD y todo el control se ha integrado en una FPGA.
Para poder desarrollar mejor las competencias de comunicación escrita y oral de los estudiantes d... more Para poder desarrollar mejor las competencias de comunicación escrita y oral de los estudiantes de ingeniería industrial, de la orientación instalaciones y construcción, se emplea un trabajo que consiste en la elaboración de un informe técnico consistente en tres documentos: un informe, un juego de transparencias y un guión para las transparencias. Aspectos complejos de los equipos electrónicos relacionados con la compatibilidad electromagnética, la fiabilidad y la seguridad son abordados y asimilados; buscando y leyendo información, y también mediante la creación del informe. La tutorización del informe ayuda al alumnado a descubrir y entender cómo la información puede ser clasificada y ordenada para, más tarde, obtener de ella una presentación adecuada para los diferentes tipos de audiencias técnicas.
IEEE Transactions on Power Electronics, 2014
The grid voltage phase and frequency are crucial information in control of most grid connected po... more The grid voltage phase and frequency are crucial information in control of most grid connected power electronic based equipment. Most often, a phase-locked loop (PLL) is employed for this purpose. A PLL is a closed-loop feedback control system that the phase of its output signal is related to the phase of its input signal. Arguably, the simplest PLL is a type-1 PLL. The type-1 PLLs are characterized by having only one integrator in their control loop and therefore having a high stability margin. However, they suffer from a serious drawback: they cannot achieve zero average steady-state phase-error in the presence of frequency drifts. To overcome this drawback of type-1 PLLs, and at the same time, to achieve a fast dynamic response and high filtering capability, a modified PLL structure is proposed in this letter. The proposed PLL has a similar structure to a type-1 PLL, but from the control point of view is a type-2 control system. For this reason, it is called the quasi-type-1 PLL (QT1-PLL). The effectiveness of the proposed PLL is confirmed through simulation and experimental results and comparison with standard PLLs.
IEEE Transactions on Power Electronics, 2015
The phase, frequency, and amplitude of the fundamental-frequency positive-sequence component of t... more The phase, frequency, and amplitude of the fundamental-frequency positive-sequence component of the grid voltage are crucial information in control of most grid-connected power electronic based equipment. Often, a standard phaselocked loop (PLL) with a prefiltering stage is employed for the extraction of them. Inspired by the concept of delayed signal cancellation (DSC), the generalized DSC (GDSC) operator has recently been introduced as an interesting option for the PLL prefiltering stage. In its typical structure, the GDSC operator extracts the grid fundamental component and feeds it to a conventional synchronous reference frame PLL (SRF-PLL). The frequency estimated by the SRF-PLL is then fed back to the GDSC operator to make it frequency adaptive. This structure, however, suffers from two main drawbacks: 1) the system is highly nonlinear and, therefore, it is very difficult to ensure its stability under all circumstances; 2) adapting the GDSC to grid frequency variations increases the implementation complexity and computational effort, particularly when the interpolation techniques are used for this purpose. To avoid these problems while maintaining high accuracy in the extraction of grid voltage quantities, an efficient and low-cost implementation of the GDSC-PLL is suggested in this paper. The proposed structure, which is called the enhanced GDSC-PLL (EGDSC-PLL), uses a nonadaptive GDSC operator as its prefiltering stage, and compensates the phase-shift and amplitude scaling caused by this operator by using two units, called the phase-error compensator and amplitude-error compensator. The effectiveness of the EGDSC-PLL is confirmed through simulation and experimental results.
IEEE Transactions on Power Electronics, 2015
Precise knowledge of the plant time constant L/R is essential to perform a thorough analysis and ... more Precise knowledge of the plant time constant L/R is essential to perform a thorough analysis and design of the current control loop in voltage source converters (VSCs). From the perspective of the current controller dynamics in the low frequency range, such plant time constant is also suitable for most cases in which an LCL filter is used. As the loop behavior can be significantly influenced by the VSC working conditions, the effects associated to converter losses should be included in the model, through an equivalent series resistance. In addition, the plant inductance may also present important uncertainties with respect to the value of the VSC L/LCL interface filter measured at rated conditions. Thus, in this work, a method is presented to estimate both parameters of the plant time constant, i.e., the equivalent inductance and resistance in the plant model of current-controlled VSCs. The proposed technique is based on the evaluation of the closed-loop transient responses of both axes of the synchronous reference frame when a proportional-integral current controller is implemented. The method gives a set of resistance and inductance values that should be employed for a rigorous design of the current controllers. Experimental results validate the approach.
IECON 2010 - 36th Annual Conference on IEEE Industrial Electronics Society, 2010
Non-sinusoidal back electromagnetic forces cause torque ripple in permanent magnet synchronous mo... more Non-sinusoidal back electromagnetic forces cause torque ripple in permanent magnet synchronous motors. These harmonics should be reduced by means of an appropriate current control to improve performance. Good results have been obtained with linear regulators such as repetitive controllers in synchronous reference frame (SRF) and resonant controllers in stationary frame. Repetitive controllers present drawbacks such as difficult frequency adaptation, due to its requirement for variable sampling frequency. On the other hand, previous approaches that employ resonant controllers in stationary frame pose several problems. These implementations are quite resourceconsuming, since they need the online computation of trigonometric functions, and when the compensation is not limited to just low frequencies, instability occurs due to the system delay. In this paper, it is proposed to use resonant controllers in SRF for torque ripple minimization in surface-mounted permanent magnet machines, so that it is possible to reduce their number by four. Additionally, cosine terms calculation is optimized by Taylor series, and a frequency adaptive phase lead is included in each resonant controller to compensate the plant delay. In this manner, an effective torque ripple minimization is achieved with optimum resource-consumption and good stability margins, even if high and variable frequency components are compensated.
IEEE Transactions on Industrial Electronics, 2015
Current controller performance is key in gridconnected power converters for renewable energy appl... more Current controller performance is key in gridconnected power converters for renewable energy applications. In this context, a challenging scenario is arising in multi-MW wind turbines, where sampling and switching frequencies tend to be lower and lower as power ratings increase. This strongly affects achievable control time constant. With this perspective, this paper presents a systematic procedure for accurate dynamics assessment and tuning of synchronous-frame PI current controllers, which is based on linear control for multiple input multiple output (MIMO) systems. The dominant eigenvalues of the system are calculated with explicit consideration of time-delay and cross-coupling terms, two factors which clearly impair the system dynamics when considering a low sampling frequency. The proposed methodology is summarized as follows. Firstly, the plant and controller matrices are modeled in state-space. Subsequently, the characteristic polynomial of the closed-loop system is obtained and a computer-aided parametric analysis is performed to calculate the MIMO root locus as a function of the control gain. By its inspection, it is possible to identify the gain which minimizes the current closed-loop time constant. This tuning is suitable for wind turbine applications, taking into consideration cascaded-control structures and grid-code requirements. The validity and accuracy of the analysis is fully supported by experimental verification.
Apec 2007: Twenty-Second Annual Ieee Applied Power Electronics Conference and Exposition, Vols 1 and 2, 2007
For low-power grid connected applications a single phase converter can be used. In PV application... more For low-power grid connected applications a single phase converter can be used. In PV applications it is possible to remove the transformer in the inverter in order to reduce losses, costs and size. Galvanic connection of the grid and the DC sources in transformerless systems can introduce additional leakage currents due to the earth parasitic capacitance. This currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid and losses. Amplitude and spectrum of leakage current depends on the converter topology, on the switching strategy and on the resonant circuit formed by the ground capacitance, the converter, the AC filter and the grid. In this paper, the leakage current in a 1.5 kW PV installation is measured under different conditions and used to build simulation model. The installation includes a string of sixteen PV panel, a full bridge inverter and a LCL filter. This model allows studying the influence of the harmonics injected by the inverter on the leakage current.
2008 34th Annual Conference of IEEE Industrial Electronics, 2008
AbstractPhase locked loop (PLL) algorithms for grid syn-chronization are a very important part o... more AbstractPhase locked loop (PLL) algorithms for grid syn-chronization are a very important part of the control in most of the grid-connected power converters applications. The perfor-mance of the PLL should not be much affected under distorted grid conditions: the presence of ...
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Papers by Francisco Freijedo