L. Matakas

Phase-Locked Loop Based on Selective Harmonics Elimination for Utility Applications

Phase-locked loops (PLL) are widely used in power electronics equipment connected to the mains. The use of a square wave voltage-controlled oscillator instead of a sinusoidal one eliminates one multiplier, resulting in a simple PLL algorithm, suitable for low-cost processors. In spite of its simplicity, distorted grid voltages cause steady-state phase error. This paper proposes the use of a modified square waveform obtained by the selective harmonics elimination (SHE) method to solve the phase error problem. Simulation and experimental results for the steady state and the transient tests are presented to validate the proposed single-phase and three-phase SHE-PLL methods. The tests using a field-programmable gate array show that the dynamic response of the proposed method is similar to that of classical PLL, with a simpler implementation.

Multiloop controller and reference generator for a Dynamic Voltage Restorer implementation

This paper deals with the design and analysis of a Dynamic Voltage Restorer output voltage control. Such control is based on a multiloop strategy, with an inner current PID regulator and an outer P+Resonant voltage controller. The inner regulator is applied on the output inductor current. It will be also demonstrated how the load current behavior may influence in the DVR output voltage, which justifies the need for the resonant controller. Additionally, it will be discussed the application of a modified algorithm for the identification of the DVR voltage references, which is based on a previously presented positive sequence detector. Since the studied three-phase DVR is assumed to be based on three identical H-bridge converters, all the analysis and design procedures were realized by means of single-phase equivalent circuits. The discussions and conclusions are supported by theoretical calculations, nonlinear simulations and some experimental results.

Transformer Operation at Deep Saturation: Model and Parameter Determination

This paper proposes a model that adequately describes the operation of the transformer at deep saturation, suitable for power-electronics applications, and a method for determining its parameters. Simulation and experimental results are presented to confirm the validity of the model and the method.

Positive sequence tracking Phase Locked Loops: A unified graphical explanation

This paper presents a graphical analysis which explains several three-phase Phase-Locked Loop (PLL) algorithms based on the detection of the positive-sequence component of the power system. It is shown that, by employing vector algebra (dot, inner or scalar product) and space vectors concepts, several PLLs methods presented in literature may be represented by a unique theory.

Gain limits for current loop controllers of single and three-phase PWM converters

This paper analyzes Proportional (P) and Proportional-Integral (PI) techniques applied to current loop controllers of single and three phase Pulse Width Modulated (PWM) Power Converters. Gain limits are derived for both strategies based on intuitive per unit values of the grid and power converter parameters. Tracking response and disturbance rejection performances of each controller in continuous time domain are also evaluated. Simulation and experimental results for continuous time, as well as experimental results for discrete time are presented.

PLL performance under frequency fluctuation-compliance with standards for distributed generation connected to the grid

Grid frequency fluctuation from the nominal value is expected by the standards related to distributed generation (DG) and smart grid (SG) systems. The Phase Locked Loop (PLL) estimates a signal synchronized with the fundamental frequency of the grid voltage that will be used by the controllers of the power converter of a DG system. A discrete model for the PLL, including the phase detector harmonic generation, is proposed. Poor low pass filtering of the PLL phase detector output signal can lead to PLL output distortion and slow time response. This paper analyses and compares the operation of moving average (MAV) filters, and two strategies of adaptive MAV filters, proposed in the literature, under frequency variation. The PLL controller and its tuning presented in these papers are also discussed. An improved PLL based on adaptive MAV filter and a better tuned controller is presented. Simulation and experimental results are provided.

A method of transformer parameters determination for power electronics applications

This paper proposes a model that adequately describes the operation of the transformer at deep saturation, suitable for power electronics applications, and a method for determining its parameters. The magnetizing branch is represented by the parallel association of a fixed resistance and a non linear inductor that can be implemented by using a current source or switched linear inductors. The parameters with linear behavior are obtained by the traditional short circuit and open circuit tests. The non linear behavior of the magnetizing inductance is obtained by processing the voltage and current waveforms measured in the open circuit test. Deep saturation condition is achieved by acquiring these waveforms during the transformer energization at nominal voltage, instead of using steady state data, which would require an oversized power supply and cause excessive thermal stress at the windings. Pre-demagnetization procedures are developed to cope with the remanent flux. Simulation and experimental results are presented to confirm the validity of the model and the method.

MiniDVR-dynamic voltage restorer with active harmonic filter (tests of prototype)

The electric engineering development has provided a more intense use of todays equipment with greater efficiency. But, often, such equipment cause distortions in the networks voltage and current waveforms and also it is sensitive to fluctuations of the network itself. Active filters constitute a feasible technological option for the mitigation of distortions and fluctuations, so as to improve the quality of energy and the development of mitigating technologies for harmonic distortions, through series and shunt voltage sources created by power electronics devices (DVR-dynamic voltage restorer). This project has been developed a miniDVR prototype to: firstly, recompose the voltage transients present and operate as a voltage active filter, and secondly, act also compensating part of the reactive load, in this way, improving the voltage control. This device is in its final test phase at the EPUSP laboratory with the transient recomposition function (sag/swell and voltage harmonics).

Transformer operation at deep saturation: Model and parameters determination

This paper proposes a model that adequately describes the operation of the transformer at deep saturation, suitable for power electronics applications, and a method for determining its parameters. Simulation and experimental results are presented to confirm the validity of the model and of the method.

Reference generator for voltage controlled power conditioners

Voltage reference generation is an important issue on electronic power conditioners or voltage compensators connected to the electric grid. Several equipments, such as Dynamic Voltage Restorers (DVR), Uninterruptable Power Supplies (UPS) and Unified Power Quality Conditioners (UPQC) need a proper voltage reference to be able to compensate electric network disturbances. This work presents a new reference generators algorithm, based on vector algebra and digital filtering techniques. It is particularly suited for the development of voltage compensators with energy storage, which would be able to mitigate steady state disturbances, such as waveform distortions and unbalances, and also transient disturbances, like voltage sags and swells. Simulation and experimental results are presented for the validation of the proposed algorithm.