This paper proposes a step-by-step procedure for designing the LCL filter of a front-end three-phase active rectifier. The primary goal is to reduce the switching frequency ripple at a reasonable cost, while at the same time achieving a high-performance front-end rectifier (as characterized by a rapid dynamic response and good stability margin). An example LCL filter design is reported and a filter has been built and tested using the values obtained from this design. The experimental results demonstrate the performance of the design procedure both for the LCL filter and for the rectifier controller. The system is stable and the grid current harmonic content is low both in the lowand high-frequency ranges. Moreover, the good agreement that was obtained between simulation and experimental results validates the proposed approach. Hence, the design procedure and the simulation model provide a powerful tool to design an LCL-filter-based active rectifier while avoiding trial-and-error procedures that can result in having to build several filter prototypes.

Design and control of an LCL-filter-based three-phase active rectifier

This paper presents a new multiresonant frequency-adaptive synchronization method for grid-connected power converters that allows estimating not only the positive- and negative-sequence components of the power signal at the fundamental frequency but also other sequence components at other harmonic frequencies. The proposed system is called MSOGI-FLL since it is based on both a harmonic decoupling network consisting of multiple second-order generalized integrators (MSOGIs) and a frequency-locked loop (FLL), which makes the system frequency adaptive. In this paper, the MSOGI-FLL is analyzed for single- and three-phase applications, deducing some key expressions regarding its stability and tuning. Moreover, the performance of the MSOGI-FLL is evaluated by both simulations and experiments to show its capability for detecting different harmonic components in a highly polluted grid scenario.

/pdf/multiresonant-frequency-locked-loop-for-grid-synchronization-1b1w90po2u.pdf

Multiresonant Frequency-Locked Loop for Grid Synchronization of Power Converters Under Distorted Grid Conditions

Grid synchronization algorithms are of great importance in the control of grid-connected power converters, as fast and accurate detection of the grid voltage parameters is crucial in order to implement stable control strategies under generic grid conditions. This paper presents a new grid synchronization method for three-phase three-wire networks, namely dual second-order generalized integrator (SOGI) frequency-locked loop. The method is based on two adaptive filters, implemented by using a SOGI on the stationary αβ reference frame, and it is able to perform an excellent estimation of the instantaneous symmetrical components of the grid voltage under unbalanced and distorted grid conditions. This paper analyzes the performance of the proposed synchronization method including different design issues. Moreover, the behavior of the method for synchronizing with highly unbalanced grid is proven by means of simulation and experimental results, demonstrating its excellent performance.

A Stationary Reference Frame Grid Synchronization System for Three-Phase Grid-Connected Power Converters Under Adverse Grid Conditions

The phase-locked loops (PLLs) are probably the most widely used synchronization technique in grid-connected applications. The main challenge that is associated with the PLLs is how to precisely and fast estimate the phase and frequency, when the grid voltage is unbalanced and/or distorted. To overcome this challenge, incorporating moving average filter(s) (MAF) into the PLL structure has been proposed in some recent literature. An MAF is a linear-phase finite-impulse-response filter, which can act as an ideal low-pass filter, if certain conditions hold. The main aim of this paper is to present the control design guidelines for a typical MAF-based PLL. The paper starts with the general description of MAFs. The main challenge associated with using the MAFs is then explained, and its possible solutions are discussed. The paper then proceeds with a brief overview of the different MAF-based PLLs. In each case, the PLL block diagram description is shown, the advantages and limitations are briefly discussed, and the tuning approach (if available) is evaluated. The paper then presents two systematic methods to design the control parameters of a typical MAF-based PLL: one for the case of using a proportional-integral (PI) type loop filter (LF) in the PLL, and the other for the case of using a proportional-integral-derivative (PID) type LF. Finally, the paper compares the performance of a well-tuned MAF-based PLL when using the PI-type LF with the results of using the PID-type LF, which provides useful insights into their capabilities and limitations.

/pdf/moving-average-filter-based-phase-locked-loops-performance-2o4bmfz6rb.pdf

Moving Average Filter Based Phase-Locked Loops: Performance Analysis and Design Guidelines

Single-phase phase-locked loops (PLLs) are popular for the synchronization and control of single-phase grid-connected converters. They are also widely used for monitoring and diagnostic purposes in the power and energy areas. In recent years, a large number of single-phase PLLs with different structures and properties have been proposed in the literature. The main aim of this paper is to provide a review of these PLLs. To this end, the single-phase PLLs are first classified into two major categories: 1) power-based PLLs and 2) quadrature signal generation-based PLLs. The members of each category are then described and their pros and cons are discussed. This paper provides a deep insight into characteristics of different single-phase PLLs, and therefore, can be considered as a reference for researchers and engineers.

Single-Phase PLLs: A Review of Recent Advances

In this paper, a phase-locked loop algorithm appropriated for digital-signal-processor-based control implementations, where the operation of a static-power converter needs to be synchronized with an ac network, is presented. The proposed algorithm includes a multiplier, a filter, a feedback closed loop, and a numerically controlled oscillator stage. As a result, a discrete sine (and cosine) signal is generated in synchronism with the fundamental component of an external-reference (ER) signal. Moreover, the sampling period of the algorithm is adjusted at each sampling instant such that an integer number of sampling periods per period of the ER signal is ensured. This is the main feature, and it is achieved by using a discrete rectangular window filter and a discrete controller. The proposed algorithm code is simple, stable, and presents high noise rejection. A comprehensive theoretical justification and various rigorous experimental tests are included.

A Robust Phase-Locked Loop Algorithm to Synchronize Static-Power Converters With Polluted AC Systems

Active front-end rectifiers based on pulse-width modulated voltage source rectifiers (PWM-VSR) are becoming the preferred topology for implementing high performance AC/DC voltage power supplies. This is due to the fact that the input AC current is fully controllable, both in amplitude and distortion, and in phase: close to unity power factor can be obtained. However, in high performance applications, a second order filter is usually introduced on the AC side to reduce the high frequency current injection into the distribution system. To avoid the use of damping resistors, active damping is used. This requires measurement of the filter capacitor voltages, in addition to sensing of the supply currents, the AC supply voltage synchronization, DC voltage for output control and DC current for protection purposes. As a result, a large number of sensors is required and the overall reliability is thus reduced. This paper proposes a complete scheme based on virtual sensors that synthesize the required AC currents and voltages without the need for measuring them. It uses the information provided by DC current and DC voltage sensors in combination with a linear reduced order state observer and a linear parameter identification algorithm. In addition, the scheme handles the nonlinearities inherent in the model of the PWM-VSR. As a result, at least four sensors can be eliminated, while the control strategy maintains an excellent performance. The paper includes a complete formulation of the virtual sensor based algorithm and its application to control the reactive power and the DC voltage of the PWM voltage source rectifier. Simulated and experimental results on a 2 kVA digital signal processor-controlled prototype confirm the validity of the theoretical considerations.

Decoupled control of PWM active-front rectifiers using only DC bus sensing

Recent developments in current source topologies involve the staking of standard topologies to handle large currents as in medium voltage AC-drives. These solutions deal with (a) balancing currents among the topologies, (b) operating the group at unitary displacement power factor, and (c) accomplishing AC input current quality requirements. This paper presents a simple control strategy to achieve the objectives (a) and (b) in a stacked current source topology. In addition, the objective (c) is achieved by integrating the DC link current controllers with a modified master PLL algorithm, thus unity displacement power factor is reached before, during and after transient requirements. On the other hand, the input current quality is improved by using an optimized multilevel modulation technique. Complete mathematical formulations as well as experimental results validate the proposed approach.

Modular current source AC-Drive: A simple control scheme

Three-phase PWM converters are widely used as they can operate at low harmonic distortion and high input displacement power factor. However, these features are achieved by means of modern control techniques which require many electrical variables in order to be properly implemented. This constraint makes these techniques less attractive and simpler control techniques are preferred. On the other hand, virtual sensors have become an alternative to provide the electrical variables without sensing them. They use the already available information to estimate the missing measurements; however, their design uses several parameters that are usually manually tuned. This paper presents a general virtual sensor and defines performance tests in order to evaluate and design optimum operating parameters of the virtual sensor applied to three-phase static power converters. Simulated and experimental results in a 2 kVA prototype are presented to illustrate the proposed approach.

http://ieee-ies.org/iecon01/abstracts/IE01A-988662463.pdf

Design and performance evaluation of a generalized virtual sensor for three-phase PWM AC/DC converters

High performance control strategies applied to pulse-width modulated current source rectifiers (PWM-CSR) require sensing of the supply currents, the input capacitor voltages, and synchronization with the AC supply voltage in addition to the DC voltage and current sensors used for protection purposes. For instance, control strategies developed to provide the necessary input line current damping-thus avoiding the need for damping resistors-and decoupled control of the active and reactive instantaneous powers, require the sensing of the supply currents and input capacitor voltages. As a result, a large number of sensors is needed and the over all reliability is therefore reduced. This paper proposes a technique based on virtual sensors to provide the required AC current and voltage values without actually sensing the electrical variables. The technique takes into account the nonlinear model of the PWM-CSR by using the information from the DC current and DC voltage sensors in combination with a linear state observer and a linear parameter identification algorithm. As a result, at least four sensors can be eliminated, while the features of the control strategy are preserved. The paper includes a complete formulation of the virtual sensor based algorithm and its application to the control of active and reactive instantaneous powers in a PWM-CSR. Results are presented to confirm the validity of the theoretical considerations.

E.A. Araya

Papers

A Robust Phase-Locked Loop Algorithm to Synchronize Static-Power Converters With Polluted AC Systems

Decoupled control of PWM active-front rectifiers using only DC bus sensing

Modular current source AC-Drive: A simple control scheme

Design and performance evaluation of a generalized virtual sensor for three-phase PWM AC/DC converters

Virtual-sensor-based control of PWM current source rectifiers