Pablo Fernandez-Comesana

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.

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.

Grid-synchronization methods for power converters

Grid synchronization is an important part in the control of grid-connected power electronic converters. The fundamental phase-angle at the point of common coupling should be tracked on-line in order to control energy transfers. Digital implementation allows to implement high performance algorithms, which are robust in the presence of power quality phenomena. However, different kinds of distortion cause a reduction of the effective bandwidth, and hence, affects to the transient response of the equipment. This paper reviews some of the highest performance algorithms for grid synchronization: phase locked loops (PLL), schemes based on synchronous reference frames (SRF) and digital filtering and finally, stochastic filtering based methods. The pros and cons of each one are assessed and some interesting techniques to enhance the dynamics are provided. The assessment in the presence of frequency deviations is analyzed in detail. The most significant techniques to provide a better frequency adaptation are enumerated and analyzed in the last section of this paper.

An Optimized Implementation of Phase Locked Loops for Grid Applications

This paper presents an optimized digital implementation of phase locked loops (PLLs) for grid applications suitable for implementation in low-cost industrial devices. A robust PLL is crucial in most of power converter applications, particularly in distorted environments. That is, the phase estimation should not be affected by power quality phenomena, given by Standard EN 50160, such as harmonics, imbalance, line notching, and voltage sags. The PLL dynamics is optimized as follows. A notch filter inside the loop is implemented to enhance the steady-state filtering. The bandwidth is maximized to get a fast postfault retracking (transient response). As justified in this paper, this approach is very suitable for both single- and three-phase PLLs. A low-resource-consuming implementation of the digitally controlled oscillator is provided: A digital model based on an RC electronic oscillator implements the needed trigonometric functions. This reduces the needed digital resources without reducing the performance. The proposed PLLs have been implemented and tested in a fixed-point DSP TI TMS320LF2407. These PLLs have been tested using different distorted inputs. Experimental results show that fast and rippleless phase estimations are achieved by the proposed implementations.

On the discrete-time implementation of resonant controllers for active power filters

Current control is crucial in active power filters. The current controller should provide perfect tracking of reference and total rejection of disturbance (grid voltage). The use of resonant controllers has been often proposed as one of the highest performance alternatives for alternating current and voltage control. In this work it is studied the effect of the discrete-time implementation of resonant controllers in their performance, specially when tracking high order components. It is proved that some methods cause a displacement of the resonant peaks, so infinite gain is not located at the desired frequencies. On the other hand, other options, such as impulse invariant, maintain the resonance at the design location, allowing for a superior performance. It is also studied the effect of zeros distribution on the behavior. A single-phase active power filter laboratory prototype has been built. The theoretical analysis is validated by experimental results, demonstrating the importance of the discrete-time implementation and the superiority of the impulse invariant method.

Graphical Diagram for Subspace and Sequence Identification of Time Harmonics in Symmetrical Multiphase Machines

The use of multiphase motor drives is an increasingly important strategy nowadays. These multiphase machines are usually modeled by a reference frame transformation to avoid the cross-coupling of variables. This transformation decomposes the original n-dimensional vector space into orthogonal subspaces. Mapping the voltage and current harmonics into the subspaces in distributed machines is important because it allows to identify which components are related to the torque and which ones just increase the machine losses. The sequence identification of each harmonic is also important in closed-loop current harmonic compensation to set the controllers. In addition, the harmonic mapping is interesting in multimotor systems to know how harmonics from one machine can affect the other machines in the system. In this paper, a simple graphical method for time harmonic subspace and sequence identification is proposed. This method is valid for symmetrical machines of any phase number n, it provides full subspace and sequence identification and it can be used in multimotor systems. Experimental results using a five- and a six-phase motor in single-drive configuration and a series-connected two-motor six-phase drive validate the proposed method.

A fast, accurate and robust algorithm to detect fundamental and harmonic sequences

This paper proposes a modified weighted least square estimation (WLSE) technique used in combination with a phase locked loop for fast and accurate reference generation, a key part in the control of ac power converters. Simulation and real-time implementation (dSpace DS1103) tests emulating realistic (distorted) environments have been performed. These results prove the excellent behavior and robustness of the WLSE employed as a specific sequence detector. A comparison in terms of transient response with other high performance methods is also contributed. The computational burden of the proposed WLSE is assessed.

Torque ripple minimization in surface-mounted PM drives by means of PI + multi-resonant controller in synchronous reference frame

Non-sinusoidal back electromagnetic forces cause torque ripple in permanent magnet synchronous motors. These harmonics should be reduced by means of an appropriate current control to improve performance. Good results have been obtained with linear regulators such as repetitive controllers in synchronous reference frame (SRF) and resonant controllers in stationary frame. Repetitive controllers present drawbacks such as difficult frequency adaptation, due to its requirement for variable sampling frequency. On the other hand, previous approaches that employ resonant controllers in stationary frame pose several problems. These implementations are quite resource-consuming, since they need the online computation of trigonometric functions, and when the compensation is not limited to just low frequencies, instability occurs due to the system delay. In this paper, it is proposed to use resonant controllers in SRF for torque ripple minimization in surface-mounted permanent magnet machines, so that it is possible to reduce their number by four. Additionally, cosine terms calculation is optimized by Taylor series, and a frequency adaptive phase lead is included in each resonant controller to compensate the plant delay. In this manner, an effective torque ripple minimization is achieved with optimum resource-consumption and good stability margins, even if high and variable frequency components are compensated.

Evolutive algorithm to optimize the power flow in a network using series compensators

The actual lines have many operation problems like oscillations, a important reactive power flow in the lines… other kind of problems are the planification of the network (power of the generators, reactive power flow in the lines…) or the bad distribution of the power flow in the lines of the network, for example one line overloaded and the other lines below their nominal power, this case happens, for example, in a network when a line falls. Many techniques have been used to solve this second type of problems, actually techniques based in the evolution and natural selection (like genetic algorithms or evolutionary algorithms) are being used combined with electronic equipment. The FACTS are a good electronic equipment to be used in these problems, because the converter can stay in bypass until a line falls, in this moment the converter can be used to solve the problem. The line impedance can be changed using SSSC converters, because they can inject a voltage in quadrature with the line current, so the effect of the converter is a capacitive or a inductive reactance in series with the line impedance. This paper presents an evolutive algorithm that solves the problem of placing SSSC converters in a network to optimize the power flow across all the lines of those network, minimizing the total installed power. The number of converters to be placed and their power will be chosen by the algorithm too.

Harmonic subspace and sequence mapping in a series-connected six-phase two-motor drive

The torque and flux of sinusoidally distributed multiphase machines can be controlled by using only two stator d-q components. Thus, in these machines there exist additional degrees of freedom, which in multimotor drive systems are used to independently control a group of series-connected multiphase machines. Series connected machines share stator currents, therefore it is interesting to study how harmonics produced by non-linearities, faults or imbalance of the feeding system are distributed in the two-motor system, in terms of subspaces and sequences. This paper analyzes low-order harmonic distribution and its effects in a series-connected six-phase two-motor drive through experimental results collected from a laboratory setup.