Fernando Ortiz Martinz

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.

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.

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.

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.

Design criteria for current loop controllers — Continuous time analysis

This work analyzes the proportional (P) and the proportional-integral (PI) control strategies of current loop controllers applied to pulse width modulated (PWM) power converters. Maximum and minimum gain limits are established for both cases based on the converter per-unit (pu) values. The drawbacks of each controller, for the operation in continuous time domain, are also evaluated.

Robust control design for the voltage tracking loop of a DVR

The Dynamic Voltage Restorer (DVR) is a solution for protection of sensitive loads from voltage sags and swells. Its basic function is to inject voltages in the grid line in order to mitigate voltage sags and/or swells. Typically the controller structure for a DVR is composed by an inner current loop and an outer voltage loop. Usually a proportional or proportional integral controller is used for the current loop and a resonant controller is used for the voltage loop. This paper presents the design of a robust controller for the voltage tracking loop of a DVR that guaranties the robust stability against load parameters variation, and assures the tracking of a sinusoidal voltage waveform and the rejection of the non linear load current influence with a pre specified error. The voltage controller design is based on H8 parameter specification approach. All the performance and robustness requirements are specified and analyzed based on the frequency response plot of closed loop transfer function. The proposed controller performance is validated by using PSIM software simulation and by experiments carried out on a DVR.

Optimized tuning method of stationary frame Proportional Resonant current controllers

This paper presents an optimized tuning method of Proportional Resonant (PR) current controllers implemented in the stationary frame. The approach consists of defining three operation regions based on behavior of the closed loop poles, and of determining simplified tracking and disturbance transfer functions for each of these regions. Charts of proportional and resonant normalized gains versus settling time and overshoot are then built, allowing the designer to easily visualize the influence of these gains on the transient response. It is shown that the disturbance response performance is more significant than the tracking one, and that minimum settling time and overshoot for this case are obtained if the resonant gain is set equal to the controller resonant frequency, providing a simple and efficient rule-of-thumb for the designer. Simulation and experimental results are presented to validate the method.

Disturbance calculation based on space vector dot product: Applications to compensators

The ability to individually extract the many disturbances of the AC line currents or voltages is desirable for reference signal calculation algorithms employed in power conditioners, allowing improved utilization of the power converter. This paper proposes a novel algorithm based on the dot product of three dimensional space vectors in the abc coordinate system, for real time calculation of the instantaneous positive, negative and zero sequence components for each individual harmonic. It is presented the functionalities for compensating fundamental reactive power, selected current harmonics and current unbalances. This method is computationally simple and requires no coordinate transformation. The proposed algorithms are validated by simulation and experiments.