Passive damping is the most adopted method to guarantee the stability of LCL-filter-based grid converters. The method is simple and, if the switching and sampling frequencies are sufficiently high, ...the damping losses are negligible. This letter proposes the tuning of different passive damping methods and an analytical estimation of the damping losses allowing the choice of the minimum resistor value resulting in a stable current control and not compromising the LCL-filter effectiveness. Stability, including variations in the grid inductance, is studied through root locus analysis in the z -plane. The analysis is validated both with simulation and with experiments.
Voltage-oriented PI control of three-phase grid-connected pulsewidth-modulation rectifiers with LCL filters is addressed. LCL filters require resonance damping. Active resonance damping is state of ...the art to face the problem, but it is still under investigation because of the manifold solutions. It is often realized using many sensors and/or complex control algorithms. In contrast, pure PI control requires only one set of current sensors, and its implementation and design are rather simple and well known from the L filter control. PI control has already been shown to be a suitable solution also for LCL filters, but there are limitations. These are investigated in this paper. System stability is analyzed with respect to different ratios of LCL filter resonance and control frequencies. The latter are important parameters for system design and control. Both line and converter current control are analyzed. For a certain range of frequency ratios, the voltage-oriented PI control gives stable performance without additional feedback, but for ratios outside this range, stable operation is impossible. Experimental tests validate the theoretical results. In addition, an experimentally determined LCL filter transfer function is shown in this paper, which shows a lower resonance peak as expected from commonly used filter models.
Background
Cytomegalovirus (CMV) infections are a major cause of disease among immunocompromised patients. Prolonged antiviral therapy is often necessary to prevent or treat CMV disease, and may lead ...to development of antiviral resistance (AVR). Timely identification of viral mutations conferring resistance is essential for effective patient management.
Methods
Amplification by polymerase chain reaction followed by bi‐directional nucleotide sequencing was performed for relevant regions of the CMV UL97 and UL54 genes. Results from 570 samples submitted to a commercial reference laboratory for testing were reviewed and characterized with respect to the frequency of mutations detected, association with viral load (VL), and consistency of results in a subset of patients with multiple samples. Only AVR mutations confirmed by marker transfer experiments were included in the analysis.
Results
AVR mutations were identified in 176 (30.9%) of the 570 samples evaluated. A total of 17 different UL97 mutations and 29 different UL54 mutations were detected. A single mutation per sample was most commonly observed, although 61 samples (10.7%) had >1 mutation, with 40 samples (7.0%) having mutations in both UL97 and UL54 genes. The VL of samples with AVR mutations ranged from 2.03–7.15 log10 copies/mL, and the VL did not differ significantly from samples without AVR mutations. A subset of patients (N = 85) had >1 sample tested, and 48.2% of these patients returned the same result for each sample analyzed, while the remainder had a different results.
Conclusions
Genomic mutations conferring resistance in CMV to antiviral drugs were commonly identified in samples submitted from clinical patients to a reference laboratory for AVR testing. Mutations were identified over the full range of VLs and no correlation was identified between VL and the presence of AVR mutations. In patients with multiple samples submitted for analysis, approximately half of the patients had samples with variable results when the initial result was compared to subsequent results.
Pulsewidth modulation (PWM) voltage source converters are becoming a popular interface to the power grid for many applications. Hence, issues related to the reduction of PWM harmonics injection in ...the power grid are becoming more relevant. The use of high-order filters like LCL filters is a standard solution to provide the proper attenuation of PWM carrier and sideband voltage harmonics. However, those grid filters introduce potentially unstable dynamics that should be properly damped either passively or actively. The second solution suffers from control and system complexity (a high number of sensors and a high-order controller), even if it is more attractive due to the absence of losses in the damping resistors and due to its flexibility. An interesting and straightforward active damping solution consists in plugging in, in cascade to the main controller, a filter that should damp the unstable dynamics. No more sensors are needed, but there are open issues such as preserving the bandwidth, robustness, and limited complexity. This paper provides a systematic approach to the design of filter-based active damping methods. The tuning procedures, performance, robustness, and limitations of the different solutions are discussed with theoretical analysis, selected simulation, and experimental results.
Design and analysis of PI state space control for grid-connected pulsewidth modulation (PWM) converters with LCL filters based on pole placement approach is addressed. State space control offers ...almost full controllability of system dynamic. However, pole placement design is difficult and usually requires much experience. In this paper, a suitable pole placement strategy is proposed, which ensures fulfilling the requirements, which are commonly specified with respect to rise time, overshoot, and proper resonance damping. Controller parameter expressions are derived in terms of system parameters and specified poles and zeros. Hence, straightforward controller tuning for a particular system setting is possible. Performance is analyzed by means of transfer function-based calculations, simulations with MATLAB, and experimental tests. Dynamic performance and robustness against grid impedance variations are addressed as well as harmonic rejection capability and other practical issues.
Voltage-oriented PI control of three-phase grid-connected pulsewidth- modulation rectifiers with @@iLCL@ filters is addressed. @@iLCL@ filters require resonance damping. Active resonance damping is ...state of the art to face abstract truncated by publisher.
The development of discrete sliding mode control for three-phase PWM converters with LCL filter is presented. This robust control method is investigated for the current control loop. Done in a ...discrete state space system, a feedforward feedback structure is utilized. A reference state trajectory and feedforward control in αβ coordinates, discrete integral SMC in dq coordinates are designed and analysed. Simulation and experimental results are shown.
In this paper a rather new nonlinear control method based on differential flatness is applied to the induction machine fed by a voltage source inverter. The control design is done by designing a ...feedforward using the flatness of the system to lead it to the reference trajectory. Due to nonidealities an additional stabilizing linear feedback controller is inserted. A speed and rotor flux control with inner current loops is designed and tuned for the induction machine. Simulation results are presented, analyzed and a comparison with the today's standard control method for ac electrical drives, the field-oriented control, is carried out. Thereby the flatness-based control shows a good performance even though further opportunities for optimizations are left
This publication presents the investigation of active damping of resonance oscillations with virtual resistor for grid-connected PWM rectifiers with LCL-filter for different filter parameters. Using ...the voltage-oriented PI current control with converter current feedback, additional active damping of the filter resonance is necessary for stable operation. In the literature different methods are proposed that differ in number of sensors and complexity of control algorithms. If higher damping of the switching ripple current is required LCL-filters with lower resonance frequencies can be used. Resulting low ratios between resonance frequency and control frequency challenge the control with respect to damping of resonance. Moreover, some active damping methods are not suitable for these filter settings. Here the active damping concept based on virtual resistor is analyzed concerning stability for two significant filter configurations. It turns out that it is applicable for configurations with higher resonance frequency, whereas systems lower resonance frequencies can poorly be damped. Additionally the method exhibits the advantage of simple implementation but the disadvantage of additional current sensors. Theoretical analyses and of the selected method with time-discrete implementation are shown in this paper. Theory is verified by experimental results.
Pulse width modulated voltage source converters (PWM-VSC) could be the universal interface for distributed power producers, loads, storage, power conditioning and transmission systems to the future ...smart grid. The differential mode filter used to connect the PWM-VSC to the electric grid has a crucial role in the proper operation of the converter and in filtering PWM switching harmonics. The paper aims to offer a complete characterization of differential mode filters with attention paid to the model of the inductors. Experimental results validate the theoretical study.
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