Control Of Grid-connected Power Converters Research Articles

Control of Low-Inertia Power Systems

Electric power systems are undergoing an unprecedented transition from fossil fuel–based power plants to low-inertia systems that rely predominantly on power electronics and renewable energy resources. This article reviews the resulting control challenges and modeling fallacies, at both the device and system level, and focuses on novel aspects or classical concepts that need to be revised in light of the transition to low-inertia systems. To this end, we survey the literature on modeling of low-inertia systems, review research on the control of grid-connected power converters, and discuss the frequency dynamics of low-inertia systems. Moreover, we discuss system-level services from a control perspective. Overall, we conclude that the system-theoretic mindset is essential to bridge different research communities and understand the complex interactions of power electronics, electric machines, and their controls in large-scale low-inertia power systems.

Small-Signal Stability Improvement of Microgrid With Battery Energy Storage System Based on Real-Time Grid Impedance Measurement

Grid impedance has a significant impact on the small-signal stability and control of grid-connected power converters used for connecting multiple distributed energy resources and in various motor drive applications. To apply advanced control schemes, such as adaptive control, real-time grid impedance is a necessary parameter that ensures stability margins of the system during operation. For real-time grid-impedance measurement purpose, broadband frequency-response measurement techniques based on the pseudorandom binary sequence can be used. In this article, a computationally effective perturbation signal quadratic residue binary (QRB) sequence is proposed for real-time grid impedance measurement. Unlike the conventional perturbation signals, the QRB sequence has low crest factor and less perturbation duration resulting in minimal disturbance during impedance measurement. To extract the grid impedance in real-time, the QRB sequence is added to the reference <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d-axis</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q-axis</i> currents of the battery energy storage system (BESS). Next, perturbed voltage and current signals are extracted in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">abc</i> -frame and converted to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d-q</i> -frame, and grid impedance is assessed. Subsequently, the system stability is analyzed using the generalized Nyquist stability criterion. Later, an adaptive control algorithm is proposed and applied to the energy storage system (ESS) to enhance stability margins of the system based on the measured grid impedance. The effectiveness of the proposed perturbation sequence for impedance measurement and stability improvement algorithm are tested on a standard CIGRE microgrid system in the MATLAB/Simulink simulation environment. Experiments are conducted on a reduced scale 3 kVA laboratory testbed and results are presented to validate the efficacy of the proposed perturbation sequence and stability improvement algorithm.

Synchronous Power Control of Grid-Connected Power Converters under Asymmetrical Grid Fault

Control of grid-connected power converters is continuously developing to meet the grid codes, according to which the generation units should keep connected to the grid and further provide ancillary services, such as voltage and frequency support, negative sequence current injection, inertia emulation, etc. A virtual admittance controller is proposed in this paper for the objective of voltage support under asymmetrical grid faults. By using independent and selective admittances for positive and negative sequence current injection, the unbalanced voltage can be significantly compensated during asymmetrical faults. The controller is based on the generic control framework of the synchronous power controller (SPC), which is able to control a power converter with emulated and improved synchronous generator characteristics. Simulation and experimental results based on two paralleled 100 kW grid-connected power converters demonstrate the controller to be effective in supporting unbalanced voltage sags.

Three-phase grid voltage synchronization using sinusoidal amplitude integrator in synchronous reference frame

Abstract Synchronization with the grid voltage is critical in the control of grid-connected power converters. Under unbalanced grid condition, the conventional synchronous reference frame phase-locked loop (SRF-PLL) does not work well due to the effect of the fundamental negative sequence component. This paper proposes a new synchronization method, the sinusoidal amplitude integrator based phase locked loop (SAI-based PLL), to efficiently eradicate the effect of the fundamental negative sequence component. As a result, it can accurately and quickly estimate the amplitude and phase angle of the fundamental components of grid voltages in unbalanced grid conditions. In addition, when dc offset and harmonics are contained in the grid voltage, a multi-SAI-based PLL method is developed to extract the synchronization signal. The proposed SAI-based PLL incorporates the sinusoidal amplitude integrator into the conventional SRF-PLL. Its operation is analyzed using the complex-vector notation. Simulation and experimental results on the performance of the proposed SAI-based PLL and comparison with other synchronization methods are presented.

Grid Synchronization Systems of Three-Phase Grid-Connected Power Converters: A Complex-Vector-Filter Perspective

Due to the significance of extracting the grid voltage information, the grid synchronization system plays an important role in the control of grid-connected power converters, and various grid voltage synchronization schemes have been proposed. This paper adopts the complex-vector-filter method (CVFM) to analyze the grid synchronization systems. With this method, the pairs of scalar signals, for example, the α- and β-axis components in the stationary α-β frame, are combined into one complex vector. As a consequence, the grid synchronization systems can be described with the complex transfer functions, which is very convenient to evaluate the steady-state performance, for example, the fundamental and harmonic sequence decoupling/cancellation, and dynamic performance of these systems. Moreover, the CVFM also provides a more generalized perspective to understand and develop the grid synchronization systems. Therefore, some of the representative systems are reanalyzed with the CVFM in this paper. A generalized second-order complex-vector filter and a third-order complex-vector filter are proposed with the CVFM to achieve better dynamic performance or higher harmonic attenuation. Moreover, a brief comparison of the complex-vector filters analyzed in this paper is presented. The effectiveness of the CVFM and the proposed two complex-vector filters are verified by the simulation and experimental results.

Fundamental Positive- and Negative-Sequence Estimator for Grid Synchronization Under Highly Disturbed Operating Conditions

The control of grid-connected power converters, such as in photovoltaic generation systems, requires a fast, accurate, and robust synchronization method, particularly during faults and when operating under highly disturbed conditions. This paper presents a method to estimate the frequency, amplitude, and phase of the positive- and negative-sequence components. A filtering stage in a rotating reference frame based on a particular type of notch filter is designed to reject the most relevant disturbances, including voltage sags, frequency variations, phase jumps, voltage imbalances, and harmonics. Furthermore, an accurate estimation of the grid frequency is also carried out. Extensive simulation and experimental results are provided to show the performance of the estimation method in different scenarios considering the aforementioned disturbances. The method is able to precisely provide the estimated values between one and one and a half cycles of the fundamental frequency. Furthermore, the computational cost of implementation of the notch filters employed is considerably small.

A Stationary Reference Frame Grid Synchronization System for Three-Phase Grid-Connected Power Converters Under Adverse Grid Conditions

Grid synchronization algorithms are of great importance in the control of grid-connected power converters, as fast and accurate detection of the grid voltage parameters is crucial in order to implement stable control strategies under generic grid conditions. This paper presents a new grid synchronization method for three-phase three-wire networks, namely dual second-order generalized integrator (SOGI) frequency-locked loop. The method is based on two adaptive filters, implemented by using a SOGI on the stationary αβ reference frame, and it is able to perform an excellent estimation of the instantaneous symmetrical components of the grid voltage under unbalanced and distorted grid conditions. This paper analyzes the performance of the proposed synchronization method including different design issues. Moreover, the behavior of the method for synchronizing with highly unbalanced grid is proven by means of simulation and experimental results, demonstrating its excellent performance.

Frequency tracking of digital resonant filters for control of power converters connected to public distribution systems

Resonant filters implemented in the control of grid-connected power converters should adapt their coefficients in real time using a grid frequency estimation, for example, obtained by a phase-locked loop (PLL). Otherwise, perfect reference tracking and total disturbance rejection of ac signals are not achieved. An optimised implementation of resonant filters, which avoids shortcomings such as inaccurate pole placement and excessive resource consumption, is provided in this study. Normal grid frequency ranges of operation according to the Standard EN 50160 have been taken into account to optimise the trade-off between accuracy and resources. A shunt active power filter prototype (1 kVA rated) with frequency adaptive proportional+resonant current controller has been built to obtain the key figures. Main experimental tests prove that zero error in steady-state tracking of ac current references is achieved, even in the presence of significant frequency deviations (up to ±2 Hz). The influence of PLL estimations quality on the current controller is also experimentally verified (in practice, grid frequency estimations are not constant, so adaptive resonant filters are non-linear). More specifically, the sensitivity to ripple in estimations has been tested and its acceptable level is established. Finally, robustness in the presence of transients such as load changes, frequency steps and grid faults is proved.

Decoupled Double Synchronous Reference Frame PLL for Power Converters Control

This paper deals with a crucial aspect in the control of grid-connected power converters, i.e., the detection of the fundamental-frequency positive-sequence component of the utility voltage under unbalanced and distorted conditions. Specifically, it proposes a positive-sequence detector based on a new decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL), which completely eliminates the detection errors of conventional synchronous reference frame PLL's (SRF-PLL). This is achieved by transforming both positive- and negative-sequence components of the utility voltage into the double SRF, from which a decoupling network is developed in order to cleanly extract and separate the positive- and negative-sequence components. The resultant DDSRF-PLL conducts then to a fast, precise, and robust positive-sequence voltage detection even under unbalanced and distorted grid conditions. The paper presents a detailed description and derivation of the proposed detection method, together with an extensive evaluation using simulation and experimental results from a digital signal processor-based laboratory prototype in order to verify and validate the excellent performance achieved by the DDSRF-PLL