P.F. Seixas

Comparison of Three Single-Phase PLL Algorithms for UPS Applications

In this paper, the performance assessment of three software single-phase phase-locked loop (PLL) algorithms is carried out by means of dynamic analysis and experimental results. Several line disturbances such as phase-angle jump, voltage sag, frequency step, and harmonics are generated by a DSP together with a D/A converter and applied to each PLL. The actual minus the estimated phase-angle values are displayed, providing a refined method for performance evaluation and comparison. Guidelines for parameters adjustments are also presented. In addition, practical implementation issues such as computational delay effects, ride-through, and computational load are addressed. The developed models proved to accurately represent the PLLs under real test conditions.

A three-phase line-interactive UPS system implementation with series-parallel active power-line conditioning capabilities

This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with series-parallel active power-line conditioning capabilities, using a synchronous reference frame (SRF) based controller, which allows an effective power factor correction, load harmonic current suppression and output voltage regulation. The three-phase UPS system is composed of two active power filter topologies. The first one is a series active power filter, which works as a sinusoidal current source in phase with the input voltage. The other is a parallel active power filter, which works as a sinusoidal voltage source in phase with the input voltage, providing to the load a regulated and sinusoidal voltage with low total harmonic distortion (THD). Operation of a three-phase phase-locked loop (PLL) structure, used in the proposed line-interactive UPS implementation, is presented and experimentally verified under distorted utility conditions. The control algorithm using SRF method and the active power flow through the UPS system are described and analytically studied. Design procedures, digital simulations and experimental results for a prototype are presented to verify the good performance of the proposed three-phase line-interactive UPS system.

A comparative analysis of control algorithms for three-phase line-interactive UPS systems with series-parallel active power-line conditioning using SRF method

This paper presents alternative control algorithms using synchronous reference frame (SRF) based controllers, which allow an effective power factor correction, load harmonic current suppression and output voltage regulation for three-phase line-interactive UPS systems with active series-parallel power-line conditioning capability. The proposed UPS systems are composed of two active power filter topologies. The first one works as a series active power filter and the other works as a parallel active power filter. An analysis of two control algorithms using SRF method, including compensation of the instantaneous real input power is carried out. The control algorithms are described, and next analytically studied to verify their characteristics. Digital simulations are presented to confirm their good performance.

PLL-based repetitive control applied to the single-phase power factor correction using boost converter

The low frequency ripple in single-phase power factor correction (PFC) boost converters dramatically limits the bandwidth in standard control strategies, deteriorating the overall system performance. This paper presents the study and the implementation of a Phase-Locked Loop (PLL)-based repetitive controller in order to improve the dynamics and the input current distortion in a PFC boost converter. The repetitive control applied to the PFC presents a straightforward implementation with satisfactory results when compared to other techniques, as described in the paper. Experimental results validate this claim.

Small-signal stability enhancement of communicationless parallel connected inverters

In this paper, small-signal stability issues of parallel-connected voltage source inverters without control interconnections are addressed. The dynamic analysis performed showed that the system stability margin is reduced when the composition of the inverter connection impedance is neither purely inductive nor purely resistive, which are two common approximations found in the literature. Based on a multivariable approach, this paper proposes the application of decoupling to the control of parallel inverters in order to increase the stability range concerning the connection impedance composition R/X. It is shown that decoupling can significantly improve the system damping even without precise knowledge of actual R/X. Experimental results are presented and corroborate the presented ideas.

Performance analysis of three-phase line-interactive UPS system with active power-line conditioning

This paper describes the active power flow behavior of a three-phase line-interactive uninterruptible power supply system with series-parallel active power-line conditioning capabilities. The control algorithm uses synchronous reference frame (SRF)-based controller, which allows an effective harmonic and reactive power compensation, generated from any configuration of non-linear load. The control of the active power flow in the dc-bus of the UPS is obtained through an additional voltage dc-bus controller. Under normal line conditions, the UPS system works with universal filtering capabilities, such as, compensating the input currents and the output voltages. Two pulse width modulation (PWM) converters are used to perform the series and parallel active power-line conditioning. The active power flow control of the UPS system is discussed taking into account battery charging and undesired conditions, such as, input voltage unbalances, total harmonic distortion of the load currents and displacement factor (cos/spl phi//sub 1/). The control algorithm using SRF method are described and analytically studied. The performance of the UPS system is evaluated in three-phase, three-wire system. Experimental results are presented to confirm the theoretical studies.

Incremental passivity based parallel operation of uninterruptible power supplies without communication — Towards a digital implementation

The digital implementation of an approach to the parallel operation of uninterruptible power supplies (UPS) is investigated. The method is developed in time domain and it relies on the natural tendency to synchronization of interconnected incrementally dissipative nonlinear oscillators. Simulations of two interconnected UPS systems to a given resistive load are carried out, taking into consideration specific problems associated with the practical implementation in a Digital Signal Processor (DSP) based system.

Power system stabilizer for comunicationless parallel connected inverters

In this paper, small-signal stability issues of low-voltage parallel-connected voltage source inverters without control interconnections are addressed by means of dynamic analysis and experimentations. The studies performed showed that the parallel system damping is severely reduced when the traditional droop method is employed and the cabling is long. This paper proposes the application of the PSS (Power System Stabilizer) action to the control of parallel inverters in order to increase the stability range concerning the connection impedance composition R/X. It is shown that the PSS action can significantly improve the system damping. Experimental results for two 1kVA inverters in parallel sharing a common load are presented and corroborate the presented ideas.

A line-interactive UPS system implementation with series-parallel active power-line conditioning for three-phase, four-wire systems

Este artigo apresenta um sistema de energia ininterrupta (SEI) line-interactive trifasico com capacidade de condicionamento ativo de potencia serie e paralelo. Um controlador baseado no sistema de eixo de referencia sincrona (SRF) e usado na compensacao de potencia reativa e harmonica geradas por quaisquer configuracoes de cargas nao lineares. Sob condicoes normais da rede eletrica o SEI trabalha na compensacao das correntes de entrada e das tensoes de saida. Dois conversores controlados em tensao e modulados por largura de pulso (PWM), chamados de filtros ativos serie e paralelo, sao usados para realizar o condicionamento ativo de potencia serie e paralelo. O filtro ativo serie trabalha como uma fonte de corrente senoidal em fase com a tensao de entrada, drenando da rede correntes senoidais, balanceadas com baixas taxas de distorcao harmonica (TDH). O filtro ativo paralelo trabalha como uma fonte de tensao senoidal em fase com a tensao de entrada, fornecendo para a carga tensoes reguladas, senoidais e com baixas taxas de distorcao harmonica. O desempenho do SEI e avaliado para sistemas trifasicos com quatro fios. Resultados experimentais sao apresentados para confirmar os estudos teoricos.

Very low input current distortion for single-phase power factor correction based on PLL-repetitive control strategy

The low frequency ripple in single-phase power factor correction (PFC) boost converters dramatically limits the bandwidth in standard control strategies, deteriorating the overall system performance. This paper presents the study and the implementation of a Phase-Locked Loop (PLL)-based repetitive controller in order to improve the dynamics and the input current distortion in a PFC boost converter. The repetitive control applied to the PFC presents a straightforward implementation with satisfactory results when compared to other techniques, as described in the paper. Experimental results validate this claim.