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The paper introduces the notion of reactive power during circuit transients. The definition the authors propose is based on the concept of local Hilbert space. It reduces to well known quantities in steady state, and it is applicable to systems with arbitrary number of phases. They illustrate this "dynamic" reactive power on a simple example of a linear RL circuit for which closed-form expressions can be derived.

In this paper we present an adaptive controller for the series active filter used to guarantee a balanced voltage on the load side while compensating selected current load harmonics. These objectives are accomplished despite unbalance and harmonic distortion in both the source voltage and current load. To achieve current harmonics compensation, a bank of (passive) band-pass filters is connected in shunt to the load, where each one is tuned at the harmonic concerned. The series active filter then injects voltage harmonic components at the same frequency as the current harmonics and, as a result, it imposes a high impedance on the line path, thus forcing the load current harmonics to flow into the bank of (passive) band-pass filters. The controller also computes the required voltage to be injected by the series active filter to compensate for unbalance and most of the distortion in the source voltage.

The paper addresses torque ripple reduction in current-fed switched reluctance motors (SRMs). Ripple-free torque production in SRMs requires an accurate model that is often too complex for practical implementation. The algorithm proposed here combines the use of a simplified model with adaptation. Explicitly, it includes dynamic estimation of low harmonics of the combined unknown load torque and the ripple in the produced torque (due to model simplification), and adds appropriate terms to the commanded current to cancel these harmonics. The control is based only on the position signal and the speed related information is obtained by an observer. Simulations suggest that our method is indeed effective for constant speed reference commands. Finally, experimental results are included to demonstrate that the algorithm performs well in reducing the torque ripple.

This paper presents an analog implementation of a fast controller for a unity-power-factor (UPF) AC/DC converter. Unlike low bandwidth controllers associated with many state-of-the-art UPF converters this fast controller can exercise control action at a rate comparable to the switching frequency rather than the line frequency. In order to accomplish this while maintaining unity power factor, the fast controller employs a ripple-feedback cancellation scheme. In addition to responding faster to disturbances, the controller facilitates the replacement of bulky bus capacitors with smaller, less expensive alternatives. >

We present analytical results and numerical simulations that quantify effects of low switching frequency in AC drives. These results build on our related results for DC converters. Specifically, we reexamine the models of pulse-width modulated (PWM) inverters that are used in high efficiency AC drives. The standard modeling procedure assumes that the switching frequency of the PWM block is high enough, so that resulting supply waveform can be replaced by its first harmonic in control-oriented models. There are practically important cases when characteristics of available semiconductor switches (or of the heat removal system) limit the switching frequency, thus mandating a re-evaluation of standard models. We use tools from averaging theory to model the PWM policy known as space vector modulation, which is commonly used in high performance drives, and derive results that are relevant for a number of AC drives.

Effects of (low) switching frequency and of delays (due to digital control implementation) on DC servomotor drives with a PWM power supply are examined. A modeling procedure and analytical formulae to quantify deviations from standard (idealized) fixed-gain models are presented, and illustrated by simulations and hardware experiments. Necessary and sufficient conditions for the existence of a periodic steady state in the operation of a DC servomotor drive are also described.

The paper describes practical experience and analytical advances in a nonlinear controller design methodology for series resonant DC/DC converters. The control goal is to maintain the output voltage in the presence of large load perturbations by varying the switching frequency. The proposed methodology utilizes large scale, nonlinear switched and generalized averaged models, and the resulting closed-loop system is exponentially convergent under typical operating conditions. The designer has a direct handle over the convergence rate, and the nonlinear controller requires only the usual output voltage measurements, while the load variations are estimated. In this paper, the previously reported results are extended in two directions: (1) a dissipativity-based nonlinear controller is implemented in affordable analog circuitry, and evaluated experimentally; and (2) the control structure is generalized to allow for both resistive and current loads.

Randomized modulation of switching in power converters holds promise in reducing input filtering requirements and in reducing acoustic noise in motor drive applications. This paper is devoted to issues in analysis and synthesis of randomized modulation schemes based on finite Markov chains. The main advantage of this novel type of randomized modulation is the availability of an explicit control of time-domain performance, in addition to the possibility of shaping the power spectra of signals of interest. Numerical (Monte Carlo) and experimental verifications for our power spectral formulas are presented. We also formulate representative narrow- and wide-band synthesis problems in randomized modulation, and solve them numerically. Our results suggest that randomized modulation is very effective in satisfying narrow-band constraints, but has limited effectiveness in meeting wide-band signal power constraints.

In this paper we present a statistical characterizations of eigenstructure changes of a linearized model describing power system load dynamics. We consider examples of induction machines with different ratings, and allow for wide variations of electrical and mechanical parameters. We describe a stochastic norm approach as means to efficiently describe the model variations without having to perform repeated eigenvalue calculations.

In this paper, the authors present an adaptive controller for the boost-based power factor precompensator which guarantees fast regulation of the output voltage towards a desired constant value with a power factor close to unity. This twofold control objective is fulfilled even in the presence of harmonics on the voltage source and uncertainties on the system parameters and load. The key for the solution to this problem is to express the model in terms of the input current instead of the inductor current. The resulting controller is reduced, through transformations, to the cascade interconnection of two controllers, namely the inner and the outer control loop. It is shown that while the latter turns out to be a simple lead-lag plus integration, the former is composed mainly of second order filters tuned at the frequencies of the considered harmonics and with transfer functions that follow a well defined pattern. Simulations are provided to assess the performance of the proposed controller.

The paper describes an analytical model for unbalanced operation of the unified power flow controller (UPFC). This nonlinear, time-invariant model is expressed in terms of dynamic symmetric components, and it is validated on a benchmark power system example taken from the literature. The model is evaluated via simulations in unbalanced operation and during unbalanced (one phase to ground) faults. The model achieves an accuracy comparable to detailed time-domain models, in addition to a reduction in simulation time.

We address the issue of improving transmission security in a deregulated power market. We propose an optimization procedure that assures that transmission security is maintained, and generates minimal corrections in existing contractual transactions that are necessary to ensure security. Our procedure is based on a DC load flow, and as such it is intended as a screening tool, and a source of candidate control actions that will be later refined by a more accurate model. Our approach may also enable faster calculations of important operational quantities like available transfer capability.

In this paper, the authors present a controller that directly regulates the active and reactive power in a synchronous three-phase boost-type rectifier ensuring good regulation of the output voltage and guaranteeing a close to the unity power factor operation. The controller follows the ideas of the well known direct torque control (DTC) for induction motors. In their case, the active and reactive powers replace the torque and flux amplitude used as the controlled outputs in DTC, thus motivating the name DPQ-control. The authors show that a simple modification to the original algorithm makes the "selection" of the control inputs more accurate. To better visualize this technique, they utilize the concept of output regulated subspaces (ORS). The controller proposed has been tested experimentally and some results are presented here.

The paper reviews the use of dynamic phasors in modeling and analysis of FACTS, and illustrates the capabilities of the methodology on thyristor-controlled series capacitors (TCSCs) and on unified power flow controllers (UPFCs), the large signal phasor models of these subsystems are expressed in continuous time, making them directly compatible with the conventional models used for the other system components.

In this paper we present an adaptive controller for the series active filter to guarantee a sinusoidal balanced voltage on the load side despite of the presence of perturbations in the source voltage such as unbalance, harmonic distortion, sags and swells, as well as disturbances in the current load such as step changes, unbalance and distortion by higher order harmonics. To accomplish this objective, the series active filter is forced to inject the appropriate voltage to the line through a series connected transformer. This task is performed by the proposed controller in three stages (loops). First, an inner control loop forces the injected voltage to track a periodic reference. Second, an outer control loop regulates the voltage level in the capacitor (the storage element) making possible the construction of such a periodic reference. Third, the power flow between the line and the DC capacitor is handled by an outer (higher level) loop that allows the DC capacitor to deliver or to absorb the necessary power to improve transients on load voltage when changes appear in the active power delivered by the source or absorbed by the load.

Several new methods for faulted transmission line parameters estimation in phasor and time domain are proposed, that eventually improve the overall performance of numerical distance relays. The concept of dynamic phasors is introduced to accommodate the time-variant nature of the current and voltage signals during transients and faults. Based on dynamic phasor transmission line models, direct and indirect estimation methods are derived. For the proposed indirect estimation method, stability of prediction error dynamics is assured by using the Lyapunov direct method. Presented estimation techniques are compared with a conventional stationary phasor solution as well as with a recursive least-square estimator derived in the discrete time domain. In the evaluation, more realistic assumptions are considered with regards to distortion of the input voltage and current signals along with the variable fault resistance because of arcing faults. Simulation results and actual field measurements are included for performance evaluation of the proposed estimators.

The paper describes a novel approach to dynamical modeling of asymmetries in electric machines and polyphase systems (e.g., the ones caused by unbalanced supply waveforms). The proposed technique is a polyphase generalization of the dynamic phasor approach from power electronics and electric drives. The technique is applicable to nonlinear models, and offers distinct advantages in modeling, simulation, and control with respect to standard time-domain models. In a steady state, the dynamic phasors reduce to standard phasors from ac circuit theory. We performed experiments and simulations involving a three-phase induction motor and a three-phase synchronous permanent magnet motor, and we demonstrate that models based on dynamic phasors provide very accurate descriptions of observed transients. In a steady state, our approach yields improved equivalent circuits that contain coupling between the positive and negative sequence subcircuits.

Control methods for the pulsation of torque reduction for the surface-mounted permanent magnet motors are discussed. The pulsation torque is a consequence of the non-sinusoidal flux distribution and due to interaction of the rotor's permanent magnets with the changing stator reluctance. The proposed control method is an estimator based. To ensure parameter convergence the Lyapunov direct method is used in the estimator design for the flux Fourier coefficients. A novel nonlinear torque controller based on the flux/torque estimate is introduced to reduce the influence of the flux harmonics. The influence of the cogging torque is considerably reduced at lower motor speed using the internal model principle and adaptive feedforward compensation technique. An overall control scheme and experimental results are also presented.

The paper investigates the use of dissipativity-based control for voltage sourced inverters (VSI) applied as reactive power and unbalance compensators, such as STATCOMs. The approach relies on a frequency domain modeling of system dynamics using both positive sequence and negative sequence dynamic components. The proposed approach can deal with unbalanced supply voltages and perform regulation of AC unbalanced currents, even for those VSI inverters where, due to switching frequency limitations, stationary frame regulators do not have enough bandwidth to cope with reference tracking. The passivity-based procedure is investigated in terms of adaptive terms, anti-wind-up limitations and decoupling between the DC link and AC current regulations, together with stability and convergence proofs. Finally, the paper explores a sequence of simplifications and adjustments that reduce the adaptive controller to PI structures with feedforward for positive and negative sequence components, thus establishing a downward compatibility with standard industrial practice.

Addresses torque ripple reduction in current-fed switched reluctance motors (SRMs). Ripple-free torque production in SRMs requires an accurate model which is often too complex for practical implementation. The proposed algorithm combines the use of a simplified model (to reduce computational burden), with adaptation. Explicitly, it includes dynamic estimation of low harmonics of the combined unknown load torque and the ripple in the produced torque (due to simplification), and adds corresponding terms to the commanded current, to cancel these harmonics. Several simulations are presented, suggesting that our method is indeed effective for both constant and variable reference commands, even when a very crude model is used in control design.

In this paper, the authors present a passivity-based control (PBC) law as nested-loop PBC to which they have added a sliding mode control (SMC) scheme for the design of the switching control sequence of a three phase boost-type rectifier. The controller presented here is indeed a modification of the original PBC scheme, which was necessary in order to eliminate bias components that appear in the input currents, which may cause possible saturation in the magnetic elements. Also, a justification for the PI external loop extensively used in implementations is given here. Using an experimental test bench, the authors have implemented the controller proposed and the results are shown.

This paper presents modeling and analysis of the operation of an induction motor loaded by a position-dependent load in order to derive an accurate time-invariant model. This model is developed in terms of variable frequency dynamic phasors. Comparisons with a more conventional model are carried out in order to illustrate the capabilities of the new model.

It is the purpose of this paper to explore the problem of regulating the output voltage of a series resonant DC-to-DC converter (SRC). Using harmonic estimation, the discontinuous system can be modeled as a static nonlinearity in cascade with a first order system. Thus reducing the complexity of the problem. The authors propose two different output feedback control laws. The first control is adding to a passivity based controller an adaptive law to estimate the unknown resistive load. They show that this controller is downward compatible with a controller recently proposed after some simple considerations. In the second controller, the authors propose to cancel the static nonlinearity part of the system and include a proportional controller plus an adaptive scheme. Using an experimental setup, both converters are implemented and the results are presented as a comparative study.

In this paper we address the issue of monitoring power quality with limited sensing and computational resources. The proposed staggered sampling methodology utilizes close-to-periodic nature of waveforms of interest. The sparse samples obtained in our scheme are reordered (shuffled) before FFT is used for spectral calculations. We provide a complete characterization of the shuffling process in time domain and of the dual inverse shuffling in the frequency domain. The methodology is applied to a number of cases of interest in power quality, demonstrating that harmonics of the fundamental frequency can be recovered without error, and that effective descriptions are possible for flicker, sag, interharmonics, and noisy measurements.

In this paper we are interested in the design of switching control strategies which, as pursued in classical direct torque control (DTC), are aimed to directly regulate two outputs: torque and flux amplitude. A criterion in terms of the error and/or the prediction in one-step-ahead on these outputs is proposed to design the switching sequence. As a result, a control vector, i.e., the switch position, is directly selected without the requirement of an auxiliary space vector or other modulation technique. We consider two types of criterion: quadratic and absolute value. Finally, experimental results following these two approaches are presented and compared with respect to the classical DTC.