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This paper compares four current control structures for selective harmonic compensation in active power filters. All controllers under scrutiny perform the harmonic compensation by using arrays of resonant controllers, one for the fundamental and one for each harmonic of interest, in order to achieve zero phase shift and unity gain in the closed-loop transfer function for selected harmonics. The complete current controller is the superposition of all individual harmonic controllers and may be implemented in various reference frames. The analysis is focused on the comparison of harmonic and total closed-loop transfer functions for each controller. Analytical similarities and differences between schemes in terms of frequency response characteristics are emphasized. It is concluded that three of them have identical harmonic behavior despite the fact that their implementation is significantly different. It emerges that the fourth one has superior behavior and robustness and can stably work at higher frequencies than the others. Theoretical findings and analysis are supported by comparative experimental results on a 7-kVA laboratory setup. The highest harmonic frequency that can be stably compensated with each control method has been determined, indicating significant differences in the control performance. Index Terms--Active power filters (APFs), current control, harmonic compensation, selective harmonic controller.

A new current control scheme for selective harmonic compensation is proposed for shunt active power filters. The method employs an array of resonant current controllers, one for the fundamental, and one for each harmonic, implemented in fundamental reference frame in order to reduce the overall computational effort. The proposed controller design is based on the pole-zero cancellation technique, taking into account the load transfer function at each harmonic frequency. Two design methods are provided, which give controller transfer functions with superior frequency response. The complete current controller is realized as the superposition of all individual harmonic controllers. The frequency response of the entire closed loop control is optimal with respect to filtering objectives, i.e., the system provides good overall stability and excellent selectivity for interesting harmonics. This conclusion is supported by experimental results on a 7.6-kVA laboratory filter, indicating a reduction in current THD factor from 34% to 2%, while the highest harmonic compensated is the 37th harmonic current. Index Terms--Active power filters (APFs), current control, harmonic compensation, selective controller.

This paper describes the control and parallel operation of two active power filters (APFs). Possible parallel operation situations of two APFs are investigated, and then the proposed topology is analyzed. The filters are coupled in a combined topology in which one filter is connected in a feedback loop and the other is in a feedforward loop for harmonic compensation. Thus, both active power filters bring their own characteristic advantages, i.e., the feedback filter improves the steady-state performance of the harmonic mitigation and the feedforward filter improves the dynamic response. Another characteristic of the proposed topology is the possibility of joint operation of both filters either as frequency-sharing or load-sharing, with or without redundancy. The frequency-sharing operation is possible due to the control algorithm, which is based on selective harmonic compensation using equivalent harmonic integrators. Implementation details and a discussion on the efficiency improvement for various switching frequencies are provided. The evaluation of the proposed topology concludes that this approach is very practical for achieving both low and high order harmonic compensation and stable grid operation. This is supported by extensive measurement results on a 15-kVA laboratory setup, indicating a reduction in total harmonic current distortion from the existing 30% to less than 2% for a typical adjustable speed drive application. Index Terms--Active filters, harmonic analysis, interconnected power systems, power system harmonic, pulse width modulated inverters.

Two flux observers for wide speed range direct torque control (DTC) of sensorless induction-motor drives are presented and compared. The first one is a full-order sliding-mode observer with proportional plus integral (PI) compensation, without rotor speed adaptation. The second one is based on a zero phase-delay-improved integrator of the voltage model, which uses only a PI flux amplitude control with stator-flux reference magnitude in the correction loop. In both cases, an estimated dc offset is built up and memorized by the PI integral component and this totally compensates for all dc offsets and drifts originated in the acquisition channels. Two feasible solutions for on-line stator-resistance identification are proposed. Simulation and experimental results prove the accuracy, robustness, and high-dynamic performance of both observers when employed in sensorless DTC drives. The effectiveness of state estimation is confirmed by a steady state and transient sensorless operation at very low speed with rated load torque and step-speed reversal. Index Terms--Flux observers, induction-motor drives, integrators, sensorless control, variable-speed drives, variable-structure systems.

Permanent magnet-assisted reluctance synchronous machine (PM--RSM) starter alternator systems are credited with good performance for wide speed range in hybrid electric vehicles. This paper proposes a motion-sensorless motor/generator control of PM--RSM from zero speed up to maximum speed, using direct torque and flux control with space vector modulation. A quasioptimal stator flux reference with a flux versus torque functional is proposed. A stator flux observer in wide speed range uses combined voltage-current models for low speeds, and only the voltage model for medium to high speeds, both in proportional-integral closed loop. A novel rotor speed and position observer with a fusion strategy employs signal injection and only one D-module vector filter in stator reference--for low speed, combined with a speed observer from the stator flux vector estimation--for medium-high speed. The proposed system is introduced piece by piece and then implemented on a dSpace 1103 control board with a 350-A metal-oxide-semiconductor field-effect transistor-pulse-width modulation converter connected to a 42-Vdc, 55-Ah battery, and a 140-Nm peak torque PM-RSM. Extensive experimental results from very low speed to high speed, regarding observers and drive responses, including artificial loading (motoring and generating), seem very encouraging for future starter-alternator systems. Index Terms--Direct torque and flux control (DTFC), hybrid electric vehicles (HEV), integrated starter-alternators (ISA), permanent magnet-assisted reluctance synchronous motor (PM-RSM), sensorless control.

State observers are key components of modern ac drives. The paper presents a comparative analysis of two state observers for induction-motor (IM) drives: the speed-adaptive observer and the inherently sensorless observer. The adaptive observer employs the time-variable full-order motor model with the rotor speed as the adaptive quantity. Thus, the speed estimation accuracy significantly impacts on the flux observer. It is shown that the popular model reference adaptive system (MRAS) speed estimator displays reduced bandwidth, and does not deliver adequate performance for the flux estimation. The inherently sensorless observer employs a full-order dual reference-frame model in order to eliminate the speed adaptation. In this way, it becomes decoupled from the speed estimator and its performance is superior to that of its adaptive counterpart. Theoretical aspects and comparative simulation results are discussed for both observers. Comparative experimental results for both observers are presented. Very low-speed-operation (3 r/min) capability of the drive with the sensorless observer is demonstrated. Index Terms--Adaptive observers, induction-motor (IM) drives, sensorless control, sliding-mode observers (SMOs), variable-speed drives.

A sensorless induction machine drive is presented, in which the principles of variable-structure control and direct torque control (DTC) are combined to ensure high-performance operation in the steady state and under transient conditions. The drive employs a new torque and flux controller, the 'linear and variable-structure control,' which realizes accurate and robust control in a wide speed range. Conventional DTC transient merits are preserved, while the steady-state behavior is significantly improved. The full-order state observer is a sliding-mode one, which does not require the rotor speed adaptation and provides accurate state estimation in the entire speed range. The proposed scheme is a complete variable-structure solution that allows persistent sensorless operation of the drive at very low speeds, including zero and 3 r/min, with full load. Simulations and extensive experimental results confirm the robustness, accuracy, quickness, and low-chattering operation of the drive. Index Terms--Induction motor (IM) sensorless drives, sliding-mode observers (SMOs), variable-speed drives, variable-structure systems.

A sensorless hybrid direct torque control (DTC) drive for high-volume low-cost industrial, domestic, and automotive applications has been developed using the inexpensive TMS329F243 digital signal processor controller. Under transient operating conditions, the drive is controlled using the classic bang-bang DTC. In the steady state, using linear torque and flux controllers, the control system generates a reference voltage vector for the inverter feeding the induction motor. The vector is then realized using the well-known space-vector pulsewidth modulation (SVPWM) technique. As a result, the irregular switching pattern typical for the classic DTC strategy is replaced with a quasi-optimal one, typical for the SVPWM. Description of the drive, including a novel motor state observer, is followed by selected experimental results confirming its excellent operating characteristics. Index Terms--Direct torque control (DTC), observers, sensorless ac drives, space-vector pulsewidth modulation (SVPWM).

A family of variable-structure controllers for induction machine drives is presented, in which the principles of direct torque control (DTC), variable-structure control (VSC), and space-vector pulsewidth modulation are combined to ensure high-performance operation, both in the steady state and under transient conditions. Three new VSC schemes are designed following the DTC voltage-control-based philosophy. These provide robust, fast, and accurate torque and flux control, without the penalty of high chattering. Comparative results demonstrate that proposed techniques preserve the DTC transient merits, while the steady-state behavior is significantly improved. Experimental results prove the strong robustness, accuracy, quickness, and low-ripple sensorless operation of the drive that uses the new schemes. Index Terms--Direct torque control (DTC), induction motor drives, variable-speed drives, variable-structure systems.

Direct torque control (DTC) is known to produce fast response and robust control in ac adjustable-speed drives. However, in the steady-state operation, notable torque, flux, and current pulsations occur. A new, direct torque and flux control strategy based on variable-structure control and space-vector pulsewidth modulation is proposed for induction motor sensorless drives. The DTC transient merits and robustness are preserved and the steady-state behavior is improved by reducing the torque and flux pulsations. A sliding-mode observer using a dual reference frame motor model is introduced and tested. Simulations and comparative experimental results with the proposed control scheme, versus classic DTC, are presented. Very-low-speed sensorless operation (3 r/min) is demonstrated. Index Terms--Induction motor drives, sliding-mode observers, variable-speed drives, variable-structure systems.

A sensorless induction motor drive is presented, in which the principles of sliding-mode control, direct torque control (DTC), and space-vector modulation are combined to ensure high-performance operation, both in the steady state and under transient conditions. Merits of the classic DTC transient behavior are preserved, while the steady-state operation is significantly improved. The torque and flux controllers, and motor state observer are of the sliding-mode type. The inverter is directly controlled on the basis of torque and flux errors, using space-vector pulsewidth modulation. Computer simulations and experimental results presented demonstrate the robustness, accuracy, quickness, and low-chattering, wide-speed-range operation of the drive. Index Terms--Induction motor drives, sliding-mode observers (SMOs), variable-speed drives, variable-structure systems.

Direct torque control (DTC) is known to produce quick and robust response in ac drives. However, during steady state, notable torque, flux, and current pulsations occur. They are reflected in speed estimation, speed response, and also in increased acoustical noise. This paper introduces a new direct torque and flux control based on space-vector modulation (DTC-SVM) for induction motor sensorless drives. It is able to reduce the acoustical noise, the torque, flux, current, and speed pulsations during steady state. The DTC transient merits are preserved, while better quality steady-state performance is produced in sensorless implementation for a wide speed range. The flux and torque estimator is presented and an improved voltage--current model speed observer is introduced. The proposed control topologies, simulations, implementation data, and test results with DTC and DTC-SVM are given and discussed. It is concluded that the proposed control topology produces better results for steady-state operation than the classical DTC. Index Terms--Direct torque control, estimators, sensorless.