The recently introduced proportional-resonant (PR) controllers and filters, and their suitability for current/voltage control of grid-connected converters, are described. Using the PR controllers, the converter reference tracking performance can be enhanced and previously known shortcomings associated with conventional PI controllers can be alleviated. These shortcomings include steady-state errors in single-phase systems and the need for synchronous d-q transformation in three-phase systems. Based on similar control theory, PR filters can also be used for generating the harmonic command reference precisely in an active power filter, especially for single-phase systems, where d-q transformation theory is not directly applicable. Another advantage associated with the PR controllers and filters is the possibility of implementing selective harmonic compensation without requiring excessive computational resources. Given these advantages and the belief that PR control will find wide-ranging applications in grid-interfaced converters, PR control theory is revised in detail with a number of practical cases that have been implemented previously, described clearly to give a comprehensive reference on PR control and filtering.

Proportional-resonant controllers and filters for grid-connected voltage-source converters

Voltage source inverters connected to the grid in applications such as active rectifiers, active filters, uninterruptible power supplies, and distributed generation systems need an optimal ac current control. To obtain zero steady-state error at the fundamental frequency (i.e., unity power factor), the use of a standard integrator in a rotating frame is as effective as the use of a resonant controller in a stationary frame. However, the grid voltage harmonics influence the current controller and generate current harmonics unless several integrators in multiple rotating frames or resonant compensators in a stationary frame are adopted. In this letter, a hybrid system consisting of a proportional integral (PI) controller plus a generic hth harmonic resonant controller implemented in a frame rotating at the n th harmonic frequency is discussed in detail. The hth harmonic controller is able to decrease both the (h - n)th and (h + n)th harmonics, while the PI controller is able to decrease other harmonics if the synchronization phase signal adopted for the frame transformation is unfiltered. It is demonstrated that the use of a PI and sixth harmonic resonant compensator is effective for both positive and negative sequence fifth and seventh harmonics; hence, four harmonics are compensated with the proportional integral-resonant (PI-RES) controller implemented in a synchronous frame. Simulation and experimental tests validate the proposed analysis

Multiple harmonics control for three-phase grid converter systems with the use of PI-RES current controller in a rotating frame

Review of multilevel voltage source inverter topologies and control schemes

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, ie, the system provides good overall stability and excellent selectivity for interesting harmonics This conclusion is supported by experimental results on a 76-kVA laboratory filter, indicating a reduction in current THD factor from 34% to 2%, while the highest harmonic compensated is the 37th harmonic current

High Performance Current Controller for Selective Harmonic Compensation in Active Power Filters

Resonant controllers have gained significant importance in recent years in multiple applications. Because of their high selectivity, their performance is very dependent on the accuracy of the resonant frequency. An exhaustive study about different discrete-time implementations is contributed in this paper. Some methods, such as the popular ones based on two integrators, cause that the resonant peaks differ from expected. Such inaccuracies result in significant loss of performance, especially for tracking high-frequency signals, since infinite gain at the expected frequency is not achieved, and therefore, zero steady-state error is not assured. Other discretization techniques are demonstrated to be more reliable. The effect on zeros is also analyzed, establishing the influence of each method on the stability. Finally, the study is extended to the discretization of the schemes with delay compensation, which is also proved to be of great importance in relation with their performance. A single-phase active power filter laboratory prototype has been implemented and tested. Experimental results provide a real-time comparison among discretization strategies, which validate the theoretical analysis. The optimum discrete-time implementation alternatives are assessed and summarized.

Effects of Discretization Methods on the Performance of Resonant Controllers

In this paper, a current control scheme, based on proportional-integral regulators using sinusoidal signal integrators (SSIs), is proposed for shunt type power conditioners. The aim is to simplify the implementation of SSI-based current harmonic compensation for industrial implementations where strict limitations on the harmonic distortion of the mains' currents are required. To compensate current harmonics, the SSIs are implemented to operate both on positive and negative sequence signals. One regulator, for the fundamental current component, is implemented in the stationary reference frame. The other regulators, for the current harmonics, are all implemented in a synchronous reference frame rotating at the fundamental frequency. This allows the simultaneous compensation of two current harmonics with just one regulator, yielding a significant reduction of the computational effort compared with other current control methods employing sinusoidal signal integrators implemented in stationary reference frame. A simple and robust voltage filter is also proposed by the authors to obtain a smooth and accurate position estimation of the voltage vector at the point of common coupling (PCC) under distorted mains' voltages. The whole control algorithm has been implemented on a 16-b, fixed-point digital signal processor (DSP) platform controlling a 20-kVA power conditioner prototype. The experimental results presented in this paper for inductive and capacitive loads show the validity of the proposed solutions.

Improved current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

This paper presents a new general adaptation law for sensorless vector control of induction motors using the model reference adaptive system (MRAS) approach with Luenberger observer. The method has been experimentally tested and compared with the classical adaptation mechanism at both low and high speed.

General adaptation law for MRAS high performance sensorless induction motor drives

This paper presents an improved mathematical model of Permanent Magnet Synchronous Machine (PMSM) that takes into account the Cogging Torque (CT) oscillations that appear due to the mu ...

Improved Mathematical Model of PMSM Taking Into Account Cogging Torque Oscillations

Low power factor and increased current harmonic content are some of the negative effects of static power converters usage. To correct these disadvantages, active power filters with complex current control schemes can be used. This paper proposes an new method for tuning the resonant controllers also known as proportional sinusoidal-signal-integrator (P-SSI) controllers, based on Naslin polynomials theory.

Design of current controllers for active power filters using naslin polynomial technique

This paper presents a comparison between the sensorless vector control schemes of the induction motor using the Luenberger observer, the Kalman filter and a neural network observer. The first two methods have been implemented on a digital signal processor (DSP). Different possibilities for reducing the complexity of their implementation are discussed. This is of particular relevance for industrial applications based on DSP microcontrollers. The performance for the third method is appreciated by simulation tests.

Valeriu Bostan

Papers

Improved current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

General adaptation law for MRAS high performance sensorless induction motor drives

Improved Mathematical Model of PMSM Taking Into Account Cogging Torque Oscillations

Design of current controllers for active power filters using naslin polynomial technique

Luenberger, Kalman and neural network observers for sensorless induction motor control