Oscar Lopez

Effects of Discretization Methods on the Performance of Resonant Controllers

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.

Eliminating Ground Current in a Transformerless Photovoltaic Application

For low-power grid-connected applications, a single-phase converter can be used. In photovoltaic (PV) applications, it is possible to remove the transformer in the inverter to reduce losses, costs, and size. Galvanic connection of the grid and the dc sources in transformerless systems can introduce additional ground currents due to the ground parasitic capacitance. These currents increase conducted and radiated electromagnetic emissions, harmonics injected in the utility grid, and losses. Amplitude and spectrum of the ground current depend on the converter topology, the switching strategy, and the resonant circuit formed by the ground capacitance, the converter, the ac filter, and the grid. In this paper, the ground current in a 1.5-kW PV installation is measured under different conditions and used to build a simulation model. The installation includes a string of 16 PV panel, a full-bridge inverter, and an LCL filter. This model allows the study of the influence of the harmonics injected by the inverter on the ground current.

Multilevel Multiphase Space Vector PWM Algorithm

In the last few years, interest in multiphase converter technology has increased due to the benefits of using more than three phases in drive applications. Besides, multilevel converter technology permits the achievement of high power ratings with voltage limited devices. Multilevel multiphase technology combines the benefits of both technologies, but new modulation techniques must be developed in order to take advantage of multilevel multiphase converters. In this paper, a novel space vector pulsewidth modulation (SVPWM) algorithm for multilevel multiphase voltage source converters is presented. This algorithm is the result of the two main contributions of this paper: the demonstration that a multilevel multiphase modulator can be realized from a two-level multiphase modulator, and the development of a new two-level multiphase SVPWM algorithm. The multiphase SVPWM algorithm presented in this paper can be applied to most multilevel topologies; it has low computational complexity and it is suitable for hardware implementations. Finally, the algorithm was implemented in a low-cost field-programmable gate array and it was tested in a laboratory with a real prototype using a five-level five-phase inverter.

Analysis and Design of Resonant Current Controllers for Voltage-Source Converters by Means of Nyquist Diagrams and Sensitivity Function

The following two types of resonant controllers are mainly employed to obtain high performance in voltage-source converters: 1) proportional + resonant (PR) and 2) vector proportional + integral (VPI). The analysis and design of PR controllers is usually performed by Bode diagrams and phase-margin criterion. However, this approach presents some limitations when resonant frequencies are higher than the crossover frequency defined by the proportional gain. This condition occurs in selective harmonic control and applications with high reference frequency with respect to the switching frequency, e.g., high-power converters with a low switching frequency. In such cases, additional 0-dB crossings (phase margins) appear; therefore, the usual methods for simple systems are no longer valid. In addition, VPI controllers always present multiple 0-dB crossings in their frequency response. In this paper, the proximity to the instability of PR and VPI controllers is evaluated and optimized through Nyquist diagrams. A systematic method is proposed to obtain the highest stability and avoidance of closed-loop anomalous peaks: it is achieved by the minimization of the inverse of the Nyquist trajectory distance to the critical point, i.e., the sensitivity function. Finally, several experimental tests, including an active power filter that operates at a low switching frequency and compensates harmonics up to the Nyquist frequency, validate the theoretical approach.

High-Performance Digital Resonant Controllers Implemented With Two Integrators

Resonant controllers are one of the highest performance alternatives for ac current/voltage control. The implementations based on two integrators are widely employed to achieve frequency adaptation without substantial computational burden. However, the discretization of these schemes causes a significant error both in the resonant frequency and in the phase lead provided by the delay compensation. Therefore, perfect tracking is not assured, and stability may be compromised. This paper proposes solutions for both problems without adding a significant resource consumption by correction of the roots placement. A simple expression to calculate the target leading angle, in delay compensation schemes, is also proposed to improve stability margins by means of a better accuracy than previous approaches. Experimental results obtained with a laboratory prototype corroborate the theoretical analysis and the improvement achieved by the proposed discrete-time implementations.

Comparison of the FPGA Implementation of Two Multilevel Space Vector PWM Algorithms

Multilevel converters can meet the increasing demand of power ratings and power quality associated with reduced harmonic distortion and lower electromagnetic interference. When the number of levels increases, it is necessary to control more and more switches in parallel. Field programmable gate arrays (FPGAs), with their concurrent processing capability, are suitable for the implementation of multilevel modulation algorithms. Among them, space vector pulsewidth modulation algorithms offer great flexibility to optimize switching waveforms and are well suited for digital implementation. In this paper, two algorithms, 2-D and 3-D, are analyzed and implemented in an FPGA. In order to carry out the implementation, both algorithms have been described in very high speed integrated circuit hardware description language, partly hand coded, and partly automatically generated using the system generator tool. Both implementations are compared in terms of implementation complexity and logic resources required. Finally, test results with a neutral-point-clamped inverter are presented.

Tuning of Phase-Locked Loops for Power Converters Under Distorted Utility Conditions

This paper presents a novel approach in the tuning of phase-locked loops (PLLs) for power electronic converters. PLLs are implemented inside a higher level controller to estimate the grid-voltage phase angle and then control the energy transfer between the power converter and the AC mains. The tuning of the PLL is not a trivial task, particularly when considering power-quality phenomena. In a general way, PLLs with a low bandwidth (low-gain PLLs) are required when handling distorted voltages. It is analytically demonstrated in this paper that low-gain PLLs have more tradeoffs than high-gain PLLs (e.g., PLLs for communications); it is not possible to optimize the settling time for a phase jump without making slower the PLL response to frequency variations. Existing tuning methods do not take into account low-gain features, which may result in nonoptimum designs. The proposed PLL tuning methodology is based on inspection of frequency-domain diagrams and, contrary to the other existing tuning methods, takes into account ldquolow-gainrdquo dynamics. It assures an optimized performance in the presence of any kind of disturbances in the grid. From a practical point of view, the proposed tuning procedure is very intuitive for controller designs. Some significant design examples and experimental results, obtained from a discrete implementation (dSpace platform), are provided in order to validate the theoretical approaches.

Assessment and Optimization of the Transient Response of Proportional-Resonant Current Controllers for Distributed Power Generation Systems

The increasing number of distributed power generation systems (DPGSs) is changing the traditional organization of the electrical network. An important part of these DPGSs is based on renewable energy sources. In order to guarantee an efficient integration of renewable-based generation units, grid codes must be fulfilled. Their most demanding requirements, such as low-voltage ride-through and grid support, need a really fast transient response of the power electronics devices. In this manner, the current controller speed is a key point. This paper proposes a methodology to assess and optimize the transient response of proportional-resonant current controllers. The proposed methodology is based on the study of the error signal transfer function roots by means of pole-zero plots. Optimal gains are set to achieve fast and nonoscillating transient responses, i.e., to optimize the settling time. It is proved that optimal gain selection results from a tradeoff between transients caused by reference changes and transients caused by changes at the point of common coupling. Experimental results obtained by means of a three-phase voltage source converter prototype validate the approach. Short transient times are achieved even when tests emulate very demanding realistic conditions: a +90° phase-angle jump in the current reference and a “type C” voltage sag at the point of common coupling.

Multilevel Multiphase Space Vector PWM Algorithm With Switching State Redundancy

Three-phase four-leg voltage-source converters are used in inverter, rectifier and active filter applications to control the neutral current caused by unbalanced or nonlinear loads. From the modulation point of view, a four-leg converter can be considered as a four-phase system. Hence, the modulation task can be carried out with a generic multiphase modulation algorithm. In this paper, a recent multilevel multiphase space vector PWM algorithm with switching state redundancy is particularized for multilevel three-phase four-leg converters. The obtained algorithm is compared with an existing three-dimension modulation technique showing important similarities. Finally, the new algorithm is implemented in a low-cost field-programmable gate array and it is tested with a five-level cascaded full-bridge inverter.

A Generic Open-Loop Algorithm for Three-Phase Grid Voltage/Current Synchronization With Particular Reference to Phase, Frequency, and Amplitude Estimation

This paper presents a new open-loop architecture for three-phase grid synchronization based on moving average and predictive filters, where accurate measurements of phase, frequency, and amplitude are carried out in real time. Previous works establish that the fundamental positive sequence vector of a set of utility voltage/current vectors can be decoupled using Parks transformation and low-pass filters. However, the filtering process introduces delays that impair the system performance. More specifically, when the input signal frequency is shifted above the nominal, a nonzero average steady-state phase error appears in the measurements. To overcome such limitations, a suitable combination of predictive and moving average finite impulse response (FIR) filters is used by the authors to achieve a robust synchronization system for all input frequencies. Moving average filters are linear phase FIR filters that have a constant time delay at low frequencies, a characteristic that is exploited to good effect to design a predictive filter that compensates such time delays, enabling zero steady-state phase errors for shifted input frequencies. In summary, the main attributes of the new system are its good frequency adaptation, good filtering/transient response tradeoff, and the fact that its dynamics is independent of the input vector amplitude. Comprehensive experimental results validate the theoretical approach and the high performance of the proposed synchronization algorithm.