Diego Perez-Estevez

Space-Vector PWM With Common-Mode Voltage Elimination for Multiphase Drives

Switching common-mode voltage (CMV) generated by the pulse width modulation (PWM) of the inverter causes common-mode currents, which lead to motor bearing failures and electromagnetic interference problems in multiphase drives. Such switching CMV can be reduced by taking advantage of the switching states of multilevel multiphase inverters that produce zero CMV. Specific space-vector PWM (SVPWM) techniques with CMV elimination, which only use zero CMV states, have been proposed for three-level five-phase drives, and for open-end winding five-, six-, and seven-phase drives, but such methods cannot be extended to a higher number of levels or phases. This paper presents a general (for any number of levels and phases) SVPMW with CMV elimination. The proposed technique can be applied to most multilevel topologies, has low computational complexity and is suitable for low-cost hardware implementations. The new algorithm is implemented in a low-cost field-programmable gate array and it is successfully tested in the laboratory using a five-level five-phase motor drive.

Selection Criteria of Multiphase Induction Machines for Speed-Sensorless Drives Based on Rotor Slot Harmonics

Multiphase (MP) induction machines (IMs) provide important advantages over three-phase (3P) ones. Sensorless speed estimation makes it possible 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.

Control Strategy for Multiphase Drives With Minimum Losses in the Full Torque Operation Range Under Single Open-Phase Fault

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 postfault phase currents might be larger than the rated current. Such a situation leads to overheating, and to preserve the integrity of the system, some limits are set to the postfault phase currents. Two main strategies have been proposed for the postfault situation: maximum torque (MT) and minimum losses (ML). The MT strategy allows us to obtain the widest torque operation range (TOR) in the postfault 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) postfault 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.

Enhanced Resonant Current Controller for Grid-Connected Converters With LCL Filter

Conventional resonant controllers (RCs) are commonly used in the current control of grid-tied converters with LCL filter due to their advantages, such as zero steady-state error at both fundamental sequences, easy design process, and straightforward implementation. Nevertheless, these traditional solutions do not permit to place the closed-loop poles of the system in convenient locations when dealing with a fourth-order plant model such as the LCL filter plus the computation delay. Therefore, the reference tracking and the disturbance rejection are deficient in terms of transient behavior and depend on the LCL filter. Furthermore, an additional active damping method usually has to be designed in order to ensure stability. This paper presents an enhanced current RC with stable and fast response, negligible overshoot, good disturbance rejection, and low controller effort for grid-tied converters with LCL filter. The developed solution uses a direct discrete-time pole-placement strategy from the classical control theory (using transfer functions), involving two extra filters, to enhance the performance of the RC. In this manner, the complexity of state-space methods from modern control theory is avoided. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.

Positive- and Negative-Sequence Current Controller With Direct Discrete-Time Pole Placement for Grid-Tied Converters With LCL Filter

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 paper 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.

Current harmonic compensation for n-phase machines with asymmetrical winding arrangement

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.

Comparison of Postfault Strategies for Current Reference Generation for Dual Three-Phase Machines in Terms of Converter Losses

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.

Current Harmonic Compensation for

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.

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This paper presents a grid-side current controller for grid-tied inverters with LCL filter, including harmonic current elimination. Contrarily to previously proposed harmonic-current controllers, the presented solution offers a generalized method that gives a consistent and stable performance irrespectively of the number of current harmonics to be canceled and of the resonant frequency of the LCL filter. The response to reference commands is completely damped and fast. The speed is set in accordance with the low-pass characteristic of the LCL filter so as to limit the control effort. Concerning the disturbance rejection, the controller offers an infinite impedance to any disturbances (such as grid voltage harmonics) at a set of arbitrarily specified frequencies. In addition, the performance of the presented controller is evaluated in terms of a fundamental tradeoff that exists between robustness and the number of frequency components rejected. Finally, simulations and experimental results that validate the proposal are presented.

-Phase Machines With Asymmetrical Winding Arrangement and Different Neutral Configurations

Dual three-phase machines are attractive due to advantages such as inherent fault tolerance. During postfault situation, the same magneto-motive force as in healthy operation should be maintained. Furthermore, it is also important to minimize the copper losses. Several strategies have been proposed to improve the postfault performance under open-phase fault. The maximum-torque (MT) strategy obtains the widest torque operation range (TOR) in faulty situation, but without optimizing the stator winding losses. The minimum-losses (ML) strategy minimizes the stator winding losses, at the cost of reducing the TOR. The full-range ML (FRML) strategy ensures minimum stator winding losses, for each torque value, in the whole TOR (that of the MT strategy). For the development and analysis of these strategies, only the stator winding losses were taken into account. Since the losses in a converter represent an important part of the overall system losses, they should also be considered. This paper compares the main postfault control strategies in terms of converter losses with carrier-based pulsewidth modulation (PWM) with sinusoidal references for dual three-phase machines with sinusoidally distributed windings under single open-phase fault. Furthermore, a strategy aiming to optimize both the converter and the stator copper losses is obtained, which results nearly identical to the FRML one. It is concluded that the FRML strategy is optimum in the whole TOR not only in terms of stator winding losses, but also of converter losses. Experimental results are provided.