Alejandro Gómez Yepes | Universidade de Vigo (original) (raw)

Papers by Alejandro Gómez Yepes

2016 IEEE Energy Conversion Congress and Exposition (ECCE), 2016

Multiphase machines (MPMs) have become serious contenders in several applications, such as offsho... more Multiphase machines (MPMs) have become serious contenders in several applications, such as offshore wind energy and electric vehicles. Low-order current harmonics arise in actual drives due to converter and machine nonlinearities, thus producing losses and torque ripple. In comparison to three-phase machines, in MPMs this effect is aggravated because of the existence of low-impedance subspaces. To cancel these harmonics, a multiple resonant controller (RC) (MRC) structure has recently been proposed for MPMs, which combines RCs and synchronous frames (SFs). The MRC scheme allows a significant computational saving in comparison to the multiple SF (MSF) strategy, which includes a proportional-integral controller in an SF per each harmonic. However, such MRC method is only suitable for MPMs with symmetrical winding arrangement (SWA), while asymmetrical winding arrangement (AWA) is also a common choice. In this paper, the MRC strategy is extended to MPMs with AWA. Different neutral configurations, whose effect on the harmonic mapping is more complicated than for SWAs and has hardly been studied so far, are considered. The optimum combinations of frequencies at which the RCs and the SFs should be tuned for AWAs are assessed. Simulation results are provided.

IEEE Transactions on Power Electronics, 2022

Two common defects in induction machines (IMs) are eccentricity and interturn faults, which shoul... more Two common defects in induction machines (IMs) are eccentricity and interturn faults, which should be diagnosed to prevent performance degradation and further damage. A popular fault-detection approach is the current signature analysis (CSA), because of its simplicity and non-intrusiveness. Under closed-loop control, it is combined with analogous voltage-reference (VR) signature analysis (VRSA). However, by using these methods in three-phase IMs it is difficult to discriminate between these faults, which cause similar symptoms. Multiphase machines provide remarkable advantages such as inherent tolerance to open-phase faults. Six-phase IMs are particularly attractive since they allow adopting three-phase converters. Among them, those with symmetrical spatial arrangement of the stator phases offer superior fault tolerance. Nonetheless, the distinction between eccentricity and interturn failures in these IMs has not been addressed so far. This paper studies the discrimination between eccentricity and interturn faults in symmetrical six-phase (S6) IMs by CSA or VRSA. It is shown that, conversely to three-phase IMs and most other multiphase IMs, in S6 ones these two types of failures can be easily distinguished: interturn faults considerably alter the currents or VRs in the so-called x-y plane, whereas eccentricity leads to current/voltage symptoms only in the alpha1-beta1 plane. Experimental results confirm the theory.

IEEE Transactions on Power Electronics, 2020

In three-phase converters it is frequently important to control current components other than the... more In three-phase converters it is frequently important to control current components other than the positive-sequence fundamental. Adequate voltage saturation and antiwindup (VSA) should also be included to prevent malfunction under output-voltage saturation due to, e.g., large voltage disturbances or dc-link-voltage reduction. A multifrequency VSA technique was recently proposed, based on modifying the current references so that the control output voltage is brought back to within the hexagonal boundary. Thus, it avoids the additional distortion that would arise if the control output surpassed (even partially) the hexagon. However, no current limitation was considered. If the current references demanded by the VSA are saturated, windup of the integrator contained in said VSA scheme may occur. Furthermore, current saturation also causes steady-state overmodulation; then, the actual current differs from that fed back to the control (due to the internal-model-control structure included in the VSA), and hence it is not effectively limited. This paper adds current-saturation functionality to the multifrequency VSA by several relevant modifications. Windup of the integrator of the VSA when current saturation occurs is prevented. The actual current is effectively limited to the desired value also when there is steady-state overmodulation. Experimental results with a three-phase permanent-magnet machine are provided.

IEEE Transactions on Power Electronics, 2018

Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating ... more Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating and enhanced fault tolerance. For multiphase machines, multifrequency current control (MCC) is often required, e.g., for harmonic cancellation or injection. In certain situations, converter output voltage (OV) saturation occurs, which can cause windup and additional OV distortion. For MCC, obtaining antiwindup and distortion-free OV saturation, with full dc-bus exploitation, entails extra complexity. Recent publications address this problem in three-phase and dual three-phase (treated as two independent three-phase) systems with one and two isolated neutrals, respectively. However, extending such solutions to drives of any phase number n, winding arrangement (symmetrical or asymmetrical) and neutral configuration is not straightforward. This paper proposes a general MCC scheme, including antiwindup and distortion-free OV saturation with full dc-link utilization, for n-phase machines with different winding arrangements (symmetrical or asymmetrical) and neutral configurations. The computational burden required by the proposed antiwindup and saturation strategy is studied; it is concluded that, although it is significant in comparison with the resources needed by the rest of the MCC, the total computational load is acceptable in most cases. Experimental results with two multiphase machines are provided, including a comparison with other approaches.

IEEE Transactions on Industrial Electronics, 2021

Five-phase induction machines are attractive due to inherent benefits such as lower current ratin... more Five-phase induction machines are attractive due to inherent benefits such as lower current rating than threephase ones. On the other hand, ac motor drives often need to operate in the overmodulation (OVM) region, e.g., to increase the maximum speed or to work with reduced dc-link voltage. Most of the existing OVM strategies for five-phase drives are based on injecting low-order harmonics in the no-torque x-y plane while avoiding low-order α-β harmonics (at least up to a modulation index of 1.2311 p.u.), so as to prevent the torque ripple associated with the α-β components. However, in practice the x-y impedance is very small, and hence this approach can easily lead to overcurrent. This paper proposes an OVM method for five-phase induction motors that adaptively modifies the injected components so that at each moment the addition of low-order x-y harmonics is favored over α-β ones but without surpassing the maximum current rms of the drive. In case the total stator copper loss (SCL) tends to exceed its rating during OVM in a given scenario, the proposed scheme automatically reduces the x-y injection to keep the SCL at its rated value. Experimental results verify the theory.

IEEE Transactions on Power Electronics, 2019

Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating ... more Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating and enhanced fault tolerance. For multiphase machines, multifrequency current control (MCC) is often required, e.g., for harmonic cancellation or injection. In certain situations, converter output voltage (OV) saturation occurs, which can cause windup and additional OV distortion. For MCC, obtaining antiwindup and distortion-free OV saturation, with full dc-bus exploitation, entails extra complexity. Recent publications address this problem in three-phase and dual three-phase (treated as two independent three-phase) systems with one and two isolated neutrals, respectively. However, extending such solutions to drives of any phase number n, winding arrangement (symmetrical or asymmetrical) and neutral configuration is not straightforward. This paper proposes a general MCC scheme, including antiwindup and distortion-free OV saturation with full dc-link utilization, for n-phase machines with different winding arrangements (symmetrical or asymmetrical) and neutral configurations. The computational burden required by the proposed antiwindup and saturation strategy is studied; it is concluded that, although it is significant in comparison with the resources needed by the rest of the MCC, the total computational load is acceptable in most cases. Experimental results with two multiphase machines are provided, including a comparison with other approaches.

IEEE Transactions on Power Electronics, 2017

Multiphase (i.e., with more than three phases) induction machines (MIMs) are particularly attract... more Multiphase (i.e., with more than three phases) induction machines (MIMs) are particularly attractive for safety-critical applications, because they are able to operate under open-phase fault. On the other hand, algorithms for sensorless speed estimation are of great interest, since, e.g., they improve reliability and decrease cost and maintenance. Among these methods, the approach based on rotor slot harmonics (RSHs) is especially popular, due to advantages such as its independence from time-varying parameters (e.g., resistances and inductances). In healthy machines, usually the fundamental or any given harmonic component has ideally equal magnitude in all stator phases. The previous RSH-based techniques rely on this characteristic. Conversely, under open-phase fault there is current imbalance. Moreover, due to the postfault strategies for optimum current reference generation, such imbalance varies with the load. Thus, the existing RSH-based methods are not suitable for open-phase fault, and alternative ones should be developed. This paper addresses the sensorless speed estimation based on RSHs for MIMs under open-phase fault. The main RSHs obtained in faulty situation are studied. Then, accordingly, a speed estimation technique for MIMs under open-phase fault is developed, able to work with good accuracy despite the variable current imbalance. Finite-element and experimental results are provided.

IEEE Transactions on Power Electronics, 2018

Dual three-phase machines (D3PMs) are particularly attractive due to their fault-tolerance capabi... more Dual three-phase machines (D3PMs) are particularly attractive due to their fault-tolerance capability. In general, under open-phase (OP) faults, the current references are adapted following a minimum-loss strategy (MLS) or a maximum-torque strategy (MTS). The MLS offers lower copper loss than the MTS for any given torque, but a smaller torque operation range. The full-range MLS (FRMLS) has recently been proposed, providing minimum copper loss, for each torque value, in the whole torque operation range (the MTS torque operation range); however, only one OP was considered. Furthermore, neither the torque operation range of the MLS nor the copper loss of the MLS or MTS have been assessed with two OPs. Consequently, it is difficult to infer a priori the relevance that the FRLMS may have for two OPs. This letter evaluates the extension of the FRMLS to D3PMs with two OPs. First, the procedure to carry out this extension is described. Then, the convenience of the FRMLS for two OPs is studied for different situations. It is concluded that for two OPs in most cases the FRMLS is substantially better than the MTS and MLS in terms of copper loss and torque operation range. Experimental results verify the theory. Index Terms-Dual three-phase machine (D3PM), fault tolerant control, multiphase drives, variable speed drives. ACRONYMS D3PM Dual three-phase machine. FRMLS Full-range minimum-loss strategy. OP Open phase. MTS Maximum-torque strategy. MLS Minimum-loss strategy I. INTRODUCTION D UAL three-phase machines (D3PMs), a.k.a. asymmetrical six-phase machines, are an attractive alternative to three-Manuscript

IEEE Transactions on Power Electronics, 2016

In three-phase applications, the synchronous- reference frame phase-locked loop (SRF-PLL) is a st... more In three-phase applications, the synchronous- reference frame phase-locked loop (SRF-PLL) is a standard PLL, which benefits from a simple structure and satisfactory perfor- mance under symmetrical and undistorted grid conditions. Under unbalanced and harmonically distorted conditions, however, it suffers from a very poor performance in the detection of grid voltage parameters. To deal with this challenge, incorporating different filters inside its control loop or before its input has been proposed. Recently, using the moving average filter (MAF) as the SRF-PLL prefiltering stage has been suggested in several works. The MAF is a linear-phase filter that can behave like an ideal low- pass filter under certain conditions. The main aim of this letter is to derive the small-signal model of the SRF-PLL with MAF-based prefiltering stage (briefly called the PMAF-PLL), which has not been presented before. This model enables the designer to simply analyze the stability condition and dynamic behavior of the PMAF-PLL. After developing the model, a simple modification to enhance the PMAF-PLL performance under frequency varying environments is presented. Finally, the equivalence of PMAF- PLL and the space-vector Fourier Transform based PLL (SVFT- PLL), which is a well-known PLL in three-phase applications, is proved. This equivalence implies that the small-signal model of the PMAF-PLL and the method presented to enhance its performance are valid for the SVFT-PLL

IEEE Transactions on Power Electronics, 2017

Low-order current harmonics arise in ac drives due to nonlinearities, producing torque ripple and... more Low-order current harmonics arise in ac drives due to nonlinearities, producing torque ripple and extra losses. In multiphase machines, which offer advantages over three-phase ones, the latter is aggravated because some harmonics map in low-impedance no-torque subspaces. A multiple-resonant-controller (MRC) structure, combining resonant controllers and synchronous frames, was proposed for harmonic cancellation. It permits substantial computational saving over the multiple-synchronous-frame (MSF) strategy, which includes proportional-integral control in one synchronous frame per harmonic. However, such MRC method is only suitable for symmetrical winding arrangements (SWAs), while asymmetrical winding arrangements (AWAs) are also widespread. Adapting the MRC for AWAs is not straightforward, since the harmonic mapping differs significantly from SWAs, and the effect of neutral configurations on it is more complicated and has hardly been studied. In this paper, an MRC strategy for multiphase machines with AWA is developed. Different neutral configurations are considered; particularly, it is shown and taken into account that for a single isolated neutral, unlike with SWAs, certain subspaces are coupled and unbalanced. The optimum frequencies of the resonant controllers and synchronous frames are assessed. The computational burden of the MRC and MSF schemes is compared, and the differences with SWAs are established. Experimental results are provided.

—Dual three-phase machines are attractive due to advantages such as inherent fault tolerance. Sev... more —Dual three-phase machines are attractive due to advantages such as inherent fault tolerance. Several strategies for current reference generation have been proposed to improve the postfault performance under open-phase fault. However, for the development and analysis of these strategies, only the stator winding losses were considered, but not the converter ones. In fact, there are no studies so far evaluating the converter losses during postfault operation. Aiming to fill this gap, this letter addresses this topic. Namely, it compares the main postfault control strategies in terms of converter losses for dual three-phase machines with sinusoidally distributed windings under single open-phase fault.

IEEE Transactions on Power Electronics, 2016

Traditionally, the current control of grid-tied converters with LCL filter is based on proportion... more Traditionally, the current control of grid-tied converters with LCL filter is based on proportional-resonant or proportional-integral controllers, which often need an additional active damping method to achieve stability. These solutions do not permit to place the closed-loop poles in convenient locations when dealing with such high-order plants. This constraint results in degraded reference-tracking and disturbance-rejection responses. On the other hand, the existing methods based on direct pole placement or other modern control strategies, do not control with zero steady-state error both positive and negative sequences of the grid current, but only the positive one. This limitation is undesirable under unbalanced grid conditions. This article presents a current controller for grid-tied converters with LCL filters based on direct discrete-time pole placement. The proposed controller makes it possible to control both positive and negative sequences of the grid-side current with zero steady-state error. Contrarily to the classical resonant controllers, the closed-loop poles can be placed in convenient locations, yielding a fast response with negligible overshoot and low controller effort. Moreover, no additional damping methods of the resonance are necessary to achieve stable operation, regardless of the switching frequency and LCL filter used. Simulation and experimental results that validate the proposal are presented.

Fault tolerance is an advantageous characteristic of multiphase machines when compared with three... more Fault tolerance is an advantageous characteristic of multiphase machines when compared with three-phase ones. During open-phase fault, the current references need to be adapted to provide ripple-free torque. As a consequence of this modification, the post-fault phase currents might be larger than the rated current. Such situation leads to overheating, and to preserve the integrity of the system, some limits are set to the post-fault phase currents. Two main strategies have been proposed for the post-fault situation: maximum torque (MT) and minimum losses (ML). The MT strategy allows to obtain the widest torque operation range (TOR) in the post-fault situation but does not minimize the stator winding losses; conversely, the ML strategy provides the minimum stator winding losses for each torque value, at the expense of reducing the TOR. Thus, the solutions proposed so far do not achieve minimum stator winding losses in the entire (that of the MT strategy) TOR. This paper presents the full-range minimum losses (FRML) post-fault control strategy, which minimizes the losses in the whole TOR, for multiphase machines with sinusoidally distributed windings under single open-phase fault. The FRML strategy is evaluated for different types of machines, phase numbers and winding arrangements. Experimental results are provided.

IEEE Transactions on Industrial Electronics, 2016

Multiphase (MP) induction machines (IMs) provide important advantages over three-phase (3P) ones.... more Multiphase (MP) induction machines (IMs) provide important advantages over three-phase (3P) ones. Sensorless speed estimation allows to obtain high-performance control and monitoring without the inconveniences of speed sensors. In 3P IMs, the speed estimation methods based on rotor slot harmonics (RSHs), normally on the principal RSHs (PSHs), are well established. A difficulty of these techniques, in 3P IMs, is that RSHs are usually extremely small. Additionally, as previously assessed concerning 3P IMs, the number of rotor bars should be carefully selected; otherwise, the PSHs might not even arise in the stator current. However, no publications have addressed the magnitude of PSHs in MP IMs, in comparison to 3P ones, or selection criteria of MP IMs for speed-sensorless drives. In this paper, it is shown that in MP IMs larger (easier to detect) PSHs can be obtained, due to the low impedances in their additional stator planes, by appropriately selecting the number of bars and poles so that the PSHs are mapped into such planes. This finding is supported by newly developed stator equivalent circuits, which include the effects of rotor bars. Accordingly, criteria are presented to select MP IMs for speed-sensorless drives. Experimental and finite element results confirm the theory.

IEEE Transactions on Industrial Electronics, May 2015

Low-order odd current harmonics arise in practical multiphase drives due to machine and converter... more Low-order odd current harmonics arise in practical multiphase drives due to machine and converter nonlinear behavior (e.g., deadtime and flux saturation). If the windings are distributed, some harmonics cause torque ripple, whereas others produce losses. The latter is aggravated by the small impedance in the no-torque subspaces. Current harmonics can be compensated without steady-state error by proportional–integral controllers in multiple synchronous frames (SFs); however, a heavy computational load is required. In three-phase systems, the computational burden of this multiple SF (MSF) scheme is often avoided by implementing instead resonant controllers (RCs) tuned at the harmonics that are multiples of six in an SF rotating with the fundamental frequency. A similar structure has been proposed for nonlinearities compensation in asymmetrical six-phase machines. This paper extends this multiple RC (MRC) strategy to symmetrical machines of any phase number. The optimum frequencies for the RCs and for the SF in each plane, so that the number of RCs is minimized, are established. Then, the computational load of the resulting generic MRC scheme is assessed and compared with that of the MSF structure. The conditions in which the former is particularly preferable over the latter are identified. Experimental results are provided.

IEEE Transactions on Power Electronics, 2015

Sinusoidal current regulation of voltage source converters is an aspect of paramount importance t... more Sinusoidal current regulation of voltage source converters is an aspect of paramount importance to achieve a high level of performance in a lot of different applications, such as ac motor drives, active power filters, wind turbines, static synchronous compensators, photovoltaic inverters or active rectifiers.

One of the most extended types of current regulators are resonant controllers, which achieve zero steady-state error at selected frequencies, while providing a good combination of simplicity and high performance. Nevertheless, there are certain aspects with regard to these controllers that have not been approached in the technical literature on the matter, and that should be investigated in order to take advantage of their actual potential.

Most studies devoted to resonant controllers have been carried out in the continuous domain; however, their observations and conclusions cannot be directly applied to digital devices, which work in the discrete-time domain. In nowadays scenarios, most current controllers are implemented in digital platforms, so the influence of the discretization process should not be ignored. Several discrete-time implementations of resonant controllers have been proposed, but a comparison among the performance obtained by a wide variety of discretization techniques applied to resonant controllers has not been presented at this point. One of the contributions of this thesis consists in an in-depth comparison among the effects of discretization strategies when applied to resonant controllers. The discretization process is proved to be of great importance in these regulators, mainly because of their resonant characteristics. The optimum discrete-time implementation alternatives are assessed, in terms of their influence on the resonant peak location and the phase versus frequency response.

The implementations of resonant controllers based on two interconnected integrators are widely employed due to their simplicity regarding frequency adaptation. However, it is proved in this thesis that these schemes require lower resource consumption, but at the expense of important inaccuracies that significantly worsen the performance, except for very low resonant frequencies and sampling periods. Alternative implementations based on two integrators are proposed in this dissertation, which achieve higher performance by means of more accurate resonant peak locations and delay compensation, while maintaining the advantage on low computational burden and good frequency adaptation of the original schemes.

Finally, the analysis and design of resonant controllers is approached. The existing methods, which are mainly based on the phase margin criterion, present some limitations, specially when there are multiple 0 dB crossings in the gain versus frequency response. This situation arises in cases such as selective control and when relatively high resonant frequencies with respect to the switching frequency are required (e.g., in high power converters, where the switching frequency should be low in order to reduce the commutation losses). In this thesis, resonant controllers are analyzed by means of Nyquist diagrams. It is proved that the minimization of the sensitivity peak permits to achieve a greater performance and stability rather than by maximizing the gain or phase margins. A systematic method, supported by closed-form analytical expressions, is proposed to obtain the highest stability and performance, even when there are multiple 0 dB crossings.

Contributions of this dissertation have been published in three JCR-indexed journal papers and presented at two international conferences.

IEEE Transactions on Power Electronics, 2016

2015 17th European Conference on Power Electronics and Applications (EPE'15 ECCE-Europe), 2015

Preventing voltage waveform distortion and integrator windup in multifrequency current control du... more Preventing voltage waveform distortion and integrator windup in multifrequency current control during saturated operation is a difficult combined problem, which so far has only partially been solved. In this paper, a scheme based on distortion-free saturation of the resonant-part outputs is proposed. Its effectiveness is experimentally verified.

2016 IEEE Energy Conversion Congress and Exposition (ECCE), 2016

Multiphase machines (MPMs) have become serious contenders in several applications, such as offsho... more Multiphase machines (MPMs) have become serious contenders in several applications, such as offshore wind energy and electric vehicles. Low-order current harmonics arise in actual drives due to converter and machine nonlinearities, thus producing losses and torque ripple. In comparison to three-phase machines, in MPMs this effect is aggravated because of the existence of low-impedance subspaces. To cancel these harmonics, a multiple resonant controller (RC) (MRC) structure has recently been proposed for MPMs, which combines RCs and synchronous frames (SFs). The MRC scheme allows a significant computational saving in comparison to the multiple SF (MSF) strategy, which includes a proportional-integral controller in an SF per each harmonic. However, such MRC method is only suitable for MPMs with symmetrical winding arrangement (SWA), while asymmetrical winding arrangement (AWA) is also a common choice. In this paper, the MRC strategy is extended to MPMs with AWA. Different neutral configurations, whose effect on the harmonic mapping is more complicated than for SWAs and has hardly been studied so far, are considered. The optimum combinations of frequencies at which the RCs and the SFs should be tuned for AWAs are assessed. Simulation results are provided.

IEEE Transactions on Power Electronics, 2022

Two common defects in induction machines (IMs) are eccentricity and interturn faults, which shoul... more Two common defects in induction machines (IMs) are eccentricity and interturn faults, which should be diagnosed to prevent performance degradation and further damage. A popular fault-detection approach is the current signature analysis (CSA), because of its simplicity and non-intrusiveness. Under closed-loop control, it is combined with analogous voltage-reference (VR) signature analysis (VRSA). However, by using these methods in three-phase IMs it is difficult to discriminate between these faults, which cause similar symptoms. Multiphase machines provide remarkable advantages such as inherent tolerance to open-phase faults. Six-phase IMs are particularly attractive since they allow adopting three-phase converters. Among them, those with symmetrical spatial arrangement of the stator phases offer superior fault tolerance. Nonetheless, the distinction between eccentricity and interturn failures in these IMs has not been addressed so far. This paper studies the discrimination between eccentricity and interturn faults in symmetrical six-phase (S6) IMs by CSA or VRSA. It is shown that, conversely to three-phase IMs and most other multiphase IMs, in S6 ones these two types of failures can be easily distinguished: interturn faults considerably alter the currents or VRs in the so-called x-y plane, whereas eccentricity leads to current/voltage symptoms only in the alpha1-beta1 plane. Experimental results confirm the theory.

IEEE Transactions on Power Electronics, 2020

In three-phase converters it is frequently important to control current components other than the... more In three-phase converters it is frequently important to control current components other than the positive-sequence fundamental. Adequate voltage saturation and antiwindup (VSA) should also be included to prevent malfunction under output-voltage saturation due to, e.g., large voltage disturbances or dc-link-voltage reduction. A multifrequency VSA technique was recently proposed, based on modifying the current references so that the control output voltage is brought back to within the hexagonal boundary. Thus, it avoids the additional distortion that would arise if the control output surpassed (even partially) the hexagon. However, no current limitation was considered. If the current references demanded by the VSA are saturated, windup of the integrator contained in said VSA scheme may occur. Furthermore, current saturation also causes steady-state overmodulation; then, the actual current differs from that fed back to the control (due to the internal-model-control structure included in the VSA), and hence it is not effectively limited. This paper adds current-saturation functionality to the multifrequency VSA by several relevant modifications. Windup of the integrator of the VSA when current saturation occurs is prevented. The actual current is effectively limited to the desired value also when there is steady-state overmodulation. Experimental results with a three-phase permanent-magnet machine are provided.

IEEE Transactions on Power Electronics, 2018

Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating ... more Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating and enhanced fault tolerance. For multiphase machines, multifrequency current control (MCC) is often required, e.g., for harmonic cancellation or injection. In certain situations, converter output voltage (OV) saturation occurs, which can cause windup and additional OV distortion. For MCC, obtaining antiwindup and distortion-free OV saturation, with full dc-bus exploitation, entails extra complexity. Recent publications address this problem in three-phase and dual three-phase (treated as two independent three-phase) systems with one and two isolated neutrals, respectively. However, extending such solutions to drives of any phase number n, winding arrangement (symmetrical or asymmetrical) and neutral configuration is not straightforward. This paper proposes a general MCC scheme, including antiwindup and distortion-free OV saturation with full dc-link utilization, for n-phase machines with different winding arrangements (symmetrical or asymmetrical) and neutral configurations. The computational burden required by the proposed antiwindup and saturation strategy is studied; it is concluded that, although it is significant in comparison with the resources needed by the rest of the MCC, the total computational load is acceptable in most cases. Experimental results with two multiphase machines are provided, including a comparison with other approaches.

IEEE Transactions on Industrial Electronics, 2021

Five-phase induction machines are attractive due to inherent benefits such as lower current ratin... more Five-phase induction machines are attractive due to inherent benefits such as lower current rating than threephase ones. On the other hand, ac motor drives often need to operate in the overmodulation (OVM) region, e.g., to increase the maximum speed or to work with reduced dc-link voltage. Most of the existing OVM strategies for five-phase drives are based on injecting low-order harmonics in the no-torque x-y plane while avoiding low-order α-β harmonics (at least up to a modulation index of 1.2311 p.u.), so as to prevent the torque ripple associated with the α-β components. However, in practice the x-y impedance is very small, and hence this approach can easily lead to overcurrent. This paper proposes an OVM method for five-phase induction motors that adaptively modifies the injected components so that at each moment the addition of low-order x-y harmonics is favored over α-β ones but without surpassing the maximum current rms of the drive. In case the total stator copper loss (SCL) tends to exceed its rating during OVM in a given scenario, the proposed scheme automatically reduces the x-y injection to keep the SCL at its rated value. Experimental results verify the theory.

IEEE Transactions on Power Electronics, 2019

Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating ... more Multiphase drives offer important advantages over three-phase ones; e.g., lower per-phase rating and enhanced fault tolerance. For multiphase machines, multifrequency current control (MCC) is often required, e.g., for harmonic cancellation or injection. In certain situations, converter output voltage (OV) saturation occurs, which can cause windup and additional OV distortion. For MCC, obtaining antiwindup and distortion-free OV saturation, with full dc-bus exploitation, entails extra complexity. Recent publications address this problem in three-phase and dual three-phase (treated as two independent three-phase) systems with one and two isolated neutrals, respectively. However, extending such solutions to drives of any phase number n, winding arrangement (symmetrical or asymmetrical) and neutral configuration is not straightforward. This paper proposes a general MCC scheme, including antiwindup and distortion-free OV saturation with full dc-link utilization, for n-phase machines with different winding arrangements (symmetrical or asymmetrical) and neutral configurations. The computational burden required by the proposed antiwindup and saturation strategy is studied; it is concluded that, although it is significant in comparison with the resources needed by the rest of the MCC, the total computational load is acceptable in most cases. Experimental results with two multiphase machines are provided, including a comparison with other approaches.

IEEE Transactions on Power Electronics, 2017

Multiphase (i.e., with more than three phases) induction machines (MIMs) are particularly attract... more Multiphase (i.e., with more than three phases) induction machines (MIMs) are particularly attractive for safety-critical applications, because they are able to operate under open-phase fault. On the other hand, algorithms for sensorless speed estimation are of great interest, since, e.g., they improve reliability and decrease cost and maintenance. Among these methods, the approach based on rotor slot harmonics (RSHs) is especially popular, due to advantages such as its independence from time-varying parameters (e.g., resistances and inductances). In healthy machines, usually the fundamental or any given harmonic component has ideally equal magnitude in all stator phases. The previous RSH-based techniques rely on this characteristic. Conversely, under open-phase fault there is current imbalance. Moreover, due to the postfault strategies for optimum current reference generation, such imbalance varies with the load. Thus, the existing RSH-based methods are not suitable for open-phase fault, and alternative ones should be developed. This paper addresses the sensorless speed estimation based on RSHs for MIMs under open-phase fault. The main RSHs obtained in faulty situation are studied. Then, accordingly, a speed estimation technique for MIMs under open-phase fault is developed, able to work with good accuracy despite the variable current imbalance. Finite-element and experimental results are provided.

IEEE Transactions on Power Electronics, 2018

Dual three-phase machines (D3PMs) are particularly attractive due to their fault-tolerance capabi... more Dual three-phase machines (D3PMs) are particularly attractive due to their fault-tolerance capability. In general, under open-phase (OP) faults, the current references are adapted following a minimum-loss strategy (MLS) or a maximum-torque strategy (MTS). The MLS offers lower copper loss than the MTS for any given torque, but a smaller torque operation range. The full-range MLS (FRMLS) has recently been proposed, providing minimum copper loss, for each torque value, in the whole torque operation range (the MTS torque operation range); however, only one OP was considered. Furthermore, neither the torque operation range of the MLS nor the copper loss of the MLS or MTS have been assessed with two OPs. Consequently, it is difficult to infer a priori the relevance that the FRLMS may have for two OPs. This letter evaluates the extension of the FRMLS to D3PMs with two OPs. First, the procedure to carry out this extension is described. Then, the convenience of the FRMLS for two OPs is studied for different situations. It is concluded that for two OPs in most cases the FRMLS is substantially better than the MTS and MLS in terms of copper loss and torque operation range. Experimental results verify the theory. Index Terms-Dual three-phase machine (D3PM), fault tolerant control, multiphase drives, variable speed drives. ACRONYMS D3PM Dual three-phase machine. FRMLS Full-range minimum-loss strategy. OP Open phase. MTS Maximum-torque strategy. MLS Minimum-loss strategy I. INTRODUCTION D UAL three-phase machines (D3PMs), a.k.a. asymmetrical six-phase machines, are an attractive alternative to three-Manuscript

IEEE Transactions on Power Electronics, 2016

In three-phase applications, the synchronous- reference frame phase-locked loop (SRF-PLL) is a st... more In three-phase applications, the synchronous- reference frame phase-locked loop (SRF-PLL) is a standard PLL, which benefits from a simple structure and satisfactory perfor- mance under symmetrical and undistorted grid conditions. Under unbalanced and harmonically distorted conditions, however, it suffers from a very poor performance in the detection of grid voltage parameters. To deal with this challenge, incorporating different filters inside its control loop or before its input has been proposed. Recently, using the moving average filter (MAF) as the SRF-PLL prefiltering stage has been suggested in several works. The MAF is a linear-phase filter that can behave like an ideal low- pass filter under certain conditions. The main aim of this letter is to derive the small-signal model of the SRF-PLL with MAF-based prefiltering stage (briefly called the PMAF-PLL), which has not been presented before. This model enables the designer to simply analyze the stability condition and dynamic behavior of the PMAF-PLL. After developing the model, a simple modification to enhance the PMAF-PLL performance under frequency varying environments is presented. Finally, the equivalence of PMAF- PLL and the space-vector Fourier Transform based PLL (SVFT- PLL), which is a well-known PLL in three-phase applications, is proved. This equivalence implies that the small-signal model of the PMAF-PLL and the method presented to enhance its performance are valid for the SVFT-PLL

IEEE Transactions on Power Electronics, 2017

Low-order current harmonics arise in ac drives due to nonlinearities, producing torque ripple and... more Low-order current harmonics arise in ac drives due to nonlinearities, producing torque ripple and extra losses. In multiphase machines, which offer advantages over three-phase ones, the latter is aggravated because some harmonics map in low-impedance no-torque subspaces. A multiple-resonant-controller (MRC) structure, combining resonant controllers and synchronous frames, was proposed for harmonic cancellation. It permits substantial computational saving over the multiple-synchronous-frame (MSF) strategy, which includes proportional-integral control in one synchronous frame per harmonic. However, such MRC method is only suitable for symmetrical winding arrangements (SWAs), while asymmetrical winding arrangements (AWAs) are also widespread. Adapting the MRC for AWAs is not straightforward, since the harmonic mapping differs significantly from SWAs, and the effect of neutral configurations on it is more complicated and has hardly been studied. In this paper, an MRC strategy for multiphase machines with AWA is developed. Different neutral configurations are considered; particularly, it is shown and taken into account that for a single isolated neutral, unlike with SWAs, certain subspaces are coupled and unbalanced. The optimum frequencies of the resonant controllers and synchronous frames are assessed. The computational burden of the MRC and MSF schemes is compared, and the differences with SWAs are established. Experimental results are provided.

—Dual three-phase machines are attractive due to advantages such as inherent fault tolerance. Sev... more —Dual three-phase machines are attractive due to advantages such as inherent fault tolerance. Several strategies for current reference generation have been proposed to improve the postfault performance under open-phase fault. However, for the development and analysis of these strategies, only the stator winding losses were considered, but not the converter ones. In fact, there are no studies so far evaluating the converter losses during postfault operation. Aiming to fill this gap, this letter addresses this topic. Namely, it compares the main postfault control strategies in terms of converter losses for dual three-phase machines with sinusoidally distributed windings under single open-phase fault.

IEEE Transactions on Power Electronics, 2016

Traditionally, the current control of grid-tied converters with LCL filter is based on proportion... more Traditionally, the current control of grid-tied converters with LCL filter is based on proportional-resonant or proportional-integral controllers, which often need an additional active damping method to achieve stability. These solutions do not permit to place the closed-loop poles in convenient locations when dealing with such high-order plants. This constraint results in degraded reference-tracking and disturbance-rejection responses. On the other hand, the existing methods based on direct pole placement or other modern control strategies, do not control with zero steady-state error both positive and negative sequences of the grid current, but only the positive one. This limitation is undesirable under unbalanced grid conditions. This article presents a current controller for grid-tied converters with LCL filters based on direct discrete-time pole placement. The proposed controller makes it possible to control both positive and negative sequences of the grid-side current with zero steady-state error. Contrarily to the classical resonant controllers, the closed-loop poles can be placed in convenient locations, yielding a fast response with negligible overshoot and low controller effort. Moreover, no additional damping methods of the resonance are necessary to achieve stable operation, regardless of the switching frequency and LCL filter used. Simulation and experimental results that validate the proposal are presented.

Fault tolerance is an advantageous characteristic of multiphase machines when compared with three... more Fault tolerance is an advantageous characteristic of multiphase machines when compared with three-phase ones. During open-phase fault, the current references need to be adapted to provide ripple-free torque. As a consequence of this modification, the post-fault phase currents might be larger than the rated current. Such situation leads to overheating, and to preserve the integrity of the system, some limits are set to the post-fault phase currents. Two main strategies have been proposed for the post-fault situation: maximum torque (MT) and minimum losses (ML). The MT strategy allows to obtain the widest torque operation range (TOR) in the post-fault situation but does not minimize the stator winding losses; conversely, the ML strategy provides the minimum stator winding losses for each torque value, at the expense of reducing the TOR. Thus, the solutions proposed so far do not achieve minimum stator winding losses in the entire (that of the MT strategy) TOR. This paper presents the full-range minimum losses (FRML) post-fault control strategy, which minimizes the losses in the whole TOR, for multiphase machines with sinusoidally distributed windings under single open-phase fault. The FRML strategy is evaluated for different types of machines, phase numbers and winding arrangements. Experimental results are provided.

IEEE Transactions on Industrial Electronics, 2016

Multiphase (MP) induction machines (IMs) provide important advantages over three-phase (3P) ones.... more Multiphase (MP) induction machines (IMs) provide important advantages over three-phase (3P) ones. Sensorless speed estimation allows to obtain high-performance control and monitoring without the inconveniences of speed sensors. In 3P IMs, the speed estimation methods based on rotor slot harmonics (RSHs), normally on the principal RSHs (PSHs), are well established. A difficulty of these techniques, in 3P IMs, is that RSHs are usually extremely small. Additionally, as previously assessed concerning 3P IMs, the number of rotor bars should be carefully selected; otherwise, the PSHs might not even arise in the stator current. However, no publications have addressed the magnitude of PSHs in MP IMs, in comparison to 3P ones, or selection criteria of MP IMs for speed-sensorless drives. In this paper, it is shown that in MP IMs larger (easier to detect) PSHs can be obtained, due to the low impedances in their additional stator planes, by appropriately selecting the number of bars and poles so that the PSHs are mapped into such planes. This finding is supported by newly developed stator equivalent circuits, which include the effects of rotor bars. Accordingly, criteria are presented to select MP IMs for speed-sensorless drives. Experimental and finite element results confirm the theory.

IEEE Transactions on Industrial Electronics, May 2015

Low-order odd current harmonics arise in practical multiphase drives due to machine and converter... more Low-order odd current harmonics arise in practical multiphase drives due to machine and converter nonlinear behavior (e.g., deadtime and flux saturation). If the windings are distributed, some harmonics cause torque ripple, whereas others produce losses. The latter is aggravated by the small impedance in the no-torque subspaces. Current harmonics can be compensated without steady-state error by proportional–integral controllers in multiple synchronous frames (SFs); however, a heavy computational load is required. In three-phase systems, the computational burden of this multiple SF (MSF) scheme is often avoided by implementing instead resonant controllers (RCs) tuned at the harmonics that are multiples of six in an SF rotating with the fundamental frequency. A similar structure has been proposed for nonlinearities compensation in asymmetrical six-phase machines. This paper extends this multiple RC (MRC) strategy to symmetrical machines of any phase number. The optimum frequencies for the RCs and for the SF in each plane, so that the number of RCs is minimized, are established. Then, the computational load of the resulting generic MRC scheme is assessed and compared with that of the MSF structure. The conditions in which the former is particularly preferable over the latter are identified. Experimental results are provided.

IEEE Transactions on Power Electronics, 2015

Sinusoidal current regulation of voltage source converters is an aspect of paramount importance t... more Sinusoidal current regulation of voltage source converters is an aspect of paramount importance to achieve a high level of performance in a lot of different applications, such as ac motor drives, active power filters, wind turbines, static synchronous compensators, photovoltaic inverters or active rectifiers.

One of the most extended types of current regulators are resonant controllers, which achieve zero steady-state error at selected frequencies, while providing a good combination of simplicity and high performance. Nevertheless, there are certain aspects with regard to these controllers that have not been approached in the technical literature on the matter, and that should be investigated in order to take advantage of their actual potential.

Most studies devoted to resonant controllers have been carried out in the continuous domain; however, their observations and conclusions cannot be directly applied to digital devices, which work in the discrete-time domain. In nowadays scenarios, most current controllers are implemented in digital platforms, so the influence of the discretization process should not be ignored. Several discrete-time implementations of resonant controllers have been proposed, but a comparison among the performance obtained by a wide variety of discretization techniques applied to resonant controllers has not been presented at this point. One of the contributions of this thesis consists in an in-depth comparison among the effects of discretization strategies when applied to resonant controllers. The discretization process is proved to be of great importance in these regulators, mainly because of their resonant characteristics. The optimum discrete-time implementation alternatives are assessed, in terms of their influence on the resonant peak location and the phase versus frequency response.

The implementations of resonant controllers based on two interconnected integrators are widely employed due to their simplicity regarding frequency adaptation. However, it is proved in this thesis that these schemes require lower resource consumption, but at the expense of important inaccuracies that significantly worsen the performance, except for very low resonant frequencies and sampling periods. Alternative implementations based on two integrators are proposed in this dissertation, which achieve higher performance by means of more accurate resonant peak locations and delay compensation, while maintaining the advantage on low computational burden and good frequency adaptation of the original schemes.

Finally, the analysis and design of resonant controllers is approached. The existing methods, which are mainly based on the phase margin criterion, present some limitations, specially when there are multiple 0 dB crossings in the gain versus frequency response. This situation arises in cases such as selective control and when relatively high resonant frequencies with respect to the switching frequency are required (e.g., in high power converters, where the switching frequency should be low in order to reduce the commutation losses). In this thesis, resonant controllers are analyzed by means of Nyquist diagrams. It is proved that the minimization of the sensitivity peak permits to achieve a greater performance and stability rather than by maximizing the gain or phase margins. A systematic method, supported by closed-form analytical expressions, is proposed to obtain the highest stability and performance, even when there are multiple 0 dB crossings.

Contributions of this dissertation have been published in three JCR-indexed journal papers and presented at two international conferences.

IEEE Transactions on Power Electronics, 2016

2015 17th European Conference on Power Electronics and Applications (EPE'15 ECCE-Europe), 2015

Preventing voltage waveform distortion and integrator windup in multifrequency current control du... more Preventing voltage waveform distortion and integrator windup in multifrequency current control during saturated operation is a difficult combined problem, which so far has only partially been solved. In this paper, a scheme based on distortion-free saturation of the resonant-part outputs is proposed. Its effectiveness is experimentally verified.