In this paper, a Frequency Adaptive Selective Harmonic Control (FA-SHC) scheme is proposed. The FA-SHC method is developed from a hybrid SHC scheme based on the internal model principle, which can be designed for gridconnected inverters to optimally mitigate feed-in current harmonics. The hybrid SHC scheme consists of multiple parallel recursive (nk±m)-order (k = 0, 1, 2, . . ., and m ≤ n/2) harmonic control modules with independent control gains, which can be optimally weighted in accordance with the harmonic distribution. The hybrid SHC thus offers an optimal trade-off among cost, complexity and also performance in terms of high accuracy, fast response, easy implementation, and compatible design. The analysis and synthesis of the hybrid SHC are addressed. More important, in order to deal with the harmonics in the presence of grid frequency variations, the hybrid SHC is transformed into the FA-SHC, being the proposed fractional order controller, when it is implemented with a fixed sampling rate. The FA-SHC is implemented by substituting the fractional order elements with the Lagrange polynomial based interpolation filters. The proposed FA-SHC scheme provides fast on-line computation and frequency adaptability to compensate harmonics in gridconnected applications, where the grid frequency is usually varying within a certain range (e.g., 50±0.5 Hz). Experimental tests have demonstrated the effectiveness of the proposed FA-SHC scheme in terms of accurate frequency adaptability and also fast transient response.
In order to minimize the effect of the grid harmonic voltages, harmonic compensation is usually adopted for a gridtied inverter. However, a large variation of the grid inductance challenges the system stability in case a high-order passive filter is used to connect an inverter to the grid. Although in theory, an adaptive controller can solve this problem, but in such case the grid inductance may need to be detected on-line, which will complicate the control system. This paper investigates the relationship between the maximum gain of the controller that still keeps the system stable and the Q-factor for a grid-tied inverter with an RL series or an RC parallel damped high-order power filter. Then, a robust passive damping method for LLCL-filter based grid-tied inverters is proposed, which effectively can suppress the possible resonances even if the grid inductance varies in a wide range. Simulation and experimental results are in good agreement with the theoretical analysis.
Electrolytic capacitor with a DC-side inductor is a typical DC-link filtering configuration in grid-connected diode rectified Adjustable Speed Drives (ASDs). The criteria to size the DC-link filter are mainly from the aspects of stability and power quality. Nevertheless, the reliability of the DC-link filter is also an essential performance factor to be considered, which depends on both the component inherent capability and the operational conditions (e.g., electro-thermal stresses) in the field operation. Nowadays, unbalanced voltage has the most frequent occurrence in many distribution networks. It brings more electrical-thermal stress to the component, affecting the reliability of the capacitors. In order to study the reliability performance of the LC filter in an ASD system quantitatively, this paper proposes a mission profile based reliability evaluation method for capacitors. Different from the conventional lifetime estimation, a nonlinear accumulated damage model is proposed for the long-term estimation, considering the nonlinear process of ESR growth and capacitance reduction during the degradation. Based on the proposed lifetime estimation procedure, four case studies are investigated: 1) Lifetime benchmarking of capacitors in LC filtering and slim capacitor filtering configurations; 2) Scalability analysis for the lifetime of capacitors in terms of system power rating and grid-unbalanced levels; 3) Lifetime estimation of capacitors in DC-link filter with long-term mission profile, and 4) The impact of the capacitor sizing on the lifetime of DC-link capacitor under grid-balanced and grid-unbalanced conditions. The results serve as a guideline for proper selection of DC-link configurations and parameters to fulfill a specification in adjustable speed drives.
This paper proposes a mission profile-based reliability prediction method for Modular Multilevel Converters (MMCs). It includes key modeling steps, such as long-term mission profile, analytical power loss models, system-level and component-level thermal modeling, lifetime modeling, Monte-Carlo analysis, and redundancy analysis. Thermal couplings and uneven thermal stresses among sub-modules are considered. A case study of a 15-kVA down-scale MMC has been used to demonstrate the proposed method and validate the theoretical analysis. The outcomes serve as a first step for developing realistic reliability analysis and model-based design methods for full-scale MMCs in practical applications.
Along with the increasing penetration of distributed generation with voltage-source converters (VSCs), there are extensive concerns over the potential virtual rotor angle stability, which is characterized by oscillations of power and frequency during the dynamic process of synchronization in the grid. Several control strategies have been developed for VSCs to emulate rotating inertia as well as damping of oscillations. This paper classifies these strategies and provides a small-signal modeling framework including all kinds of VSCs in different applications for virtual rotor angle stability. A unified perspective based on the famous Phillips-Heffron model is established for various VSCs. Thus, the concepts of equivalent inertia and the synchronizing and damping coefficients in different VSCs are highlighted, based on the similarities with the synchronous generator (SG) system in both physical mechanisms and mathematical models. It revealed the potentiality of various VSCs to achieve equivalence with the SG. This study helps promote the unity of VSCs and traditional SGs in both theories and methods for analyzing the dynamic behavior and enhancing the stability. Finally, future research needs and new perspectives are addressed.
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