Showing papers on "Output impedance published in 2014"

Impedance Modeling and Analysis of Grid-Connected Voltage-Source Converters

Abstract: This paper presents small-signal impedance modeling of grid-connected three-phase converters for wind and solar system stability analysis. In the proposed approach, a converter is modeled by a positive-sequence and a negative-sequence impedance directly in the phase domain. It is further demonstrated that the two sequence subsystems are decoupled under most conditions and can be studied independently from each other. The proposed models are verified by experimental measurements and their applications are demonstrated in a system testbed.

Impedance Shaping of the Grid-Connected Inverter with LCL Filter to Improve Its Adaptability to the Weak Grid Condition

Abstract: The current-controlled grid-connected inverter with LCL filter is widely used in the distributed generation system (DGS), due to its fast dynamic response and better power quality features. However, with the increase of power injected into the grid, control performances of the inverter will be significantly influenced by the nonideal grid conditions. Specifically, the possible wide variation of the grid impedance challenges the system stability. Meanwhile, background harmonics of the grid can greatly distort the injected current. Therefore, the control of the inverter should be designed with strong stability-robustness and high harmonic-rejection-ability, both of which correlate closely with the inverter output impedance. However, it is difficult to shape the output impedance into the one with a desirable characteristic simply by adjusting the current loop gain. In this paper, an impedance shaping method is proposed with virtual impedances, and the current control loop can be designed independently. The implementation and parameter design of the virtual impedances are studied under the practical considerations. With this proposed method, the grid-connected inverter can work stably over a wide range of the typical inductive-resistive grid impedance and exhibit strong rejection ability of grid-voltage harmonics. Experimental results from a 6-kW single-phase grid-connected inverter confirm the effectiveness of the proposed method.

Adaptive Control of Grid-Connected Inverters Based on Online Grid Impedance Measurements

Abstract: Stability of a grid-connected inverter depends on the ratio of the grid impedance to the inverter impedance. Since the grid impedance changes during normal power system conditions, this paper proposes a gain-scheduling adaptive control system that uses online grid impedance measurements. For grid impedance measurement, an impulse-response analysis method is programmed in the digital-signal processor (DSP) of the grid-connected inverter. For adaptation, a Routh-Hurwitz stability analysis approach is used to derive, analytically, the stable operation boundaries of the interconnected system. To simplify the analytical derivations, the grid impedance is assumed inductive at low frequencies and curve fitted to the online impedance measurements. Experimental measurements demonstrate the improvement in system stability, when the impedance identification and adaptive control algorithms are programmed together in the DSP of a three-phase inverter, which is connected to a grid with a variable feeder impedance.

Voltage Stability and Control of Offshore Wind Farms With AC Collection and HVDC Transmission

Abstract: This paper investigates the stability and control of offshore wind farms employing medium-voltage ac collection and high-voltage dc (HVDC) transmission to the onshore power grids. Type-IV (full power conversion) turbines and HVDC rectifier based on voltage-source converters are assumed. Output impedance models of the wind turbines and input impedance models of the HVDC rectifier in the positive- and negative-sequence are developed using the harmonic linearization method. An impedance-based stability criterion is then applied to determine the stability of the offshore ac collection bus. Possible instability of the ac bus voltage and resonance between the wind farm and the HVDC rectifier are examined through analysis of the system impedance model. The analytical impedance models are used to identify the root causes of such instability and resonance problems, and to develop possible solutions. Detailed circuit simulation is used to validate the analysis. Individual converter impedance models are also validated by experimental measurements of scaled-down prototypes.

A vibration-based MEMS piezoelectric energy harvester and power conditioning circuit.

Abstract: This paper presents a micro-electro-mechanical system (MEMS) piezoelectric power generator array for vibration energy harvesting. A complete design flow of the vibration-based energy harvester using the finite element method (FEM) is proposed. The modal analysis is selected to calculate the resonant frequency of the harvester, and harmonic analysis is performed to investigate the influence of the geometric parameters on the output voltage. Based on simulation results, a MEMS Pb(Zr,Ti)O3 (PZT) cantilever array with an integrated large Si proof mass is designed and fabricated to improve output voltage and power. Test results show that the fabricated generator, with five cantilever beams (with unit dimensions of about 3 × 2.4 × 0.05 mm3) and an individual integrated Si mass dimension of about 8 × 12.4 × 0.5 mm3, produces a output power of 66.75 μW, or a power density of 5.19 μW∙mm-3∙g-2 with an optimal resistive load of 220 kΩ from 5 m/s2 vibration acceleration at its resonant frequency of 234.5 Hz. In view of high internal impedance characteristic of the PZT generator, an efficient autonomous power conditioning circuit, with the function of impedance matching, energy storage and voltage regulation, is then presented, finding that the efficiency of the energy storage is greatly improved and up to 64.95%. The proposed self-supplied energy generator with power conditioning circuit could provide a very promising complete power supply solution for wireless sensor node loads.

Online Measurement of Battery Impedance Using Motor Controller Excitation

Abstract: This paper presents a fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle. Impedance measurements on lithium-ion batteries between 1 Hz and 2 kHz give information about the electrochemical reactions within a cell, which relates to the state of charge (SOC), internal temperature, and state of health (SOH). We concentrate on the development of a measurement system for impedance that, for the first time, uses an excitation current generated by a motor controller. Using simple electronics to amplify and filter the voltage and current, we demonstrate accurate impedance measurements obtained with both multisine and noise excitation signals, achieving RMS magnitude measurement uncertainties between 1.9% and 5.8%, in comparison to a high-accuracy laboratory impedance analyzer. Achieving this requires calibration of the measurement circuits, including measurement of the inductance of the current sense resistor. A statistical correlation approach is used to extract the impedance information from the measured voltage and current signals in the presence of noise, allowing a wide range of excitation signals to be used. Finally, we also discuss the implementation challenges of an SOC estimation system based on impedance.

A-Source Impedance Network

Abstract: A novel A-source impedance network is proposed in this letter. The A-source impedance network uses an autotransformer for realizing converters for any application that demand a very high dc voltage gain. The network utilizes a minimal turns ratio compared to other magnetically coupled impedance source networks to attain a high voltage gain. In addition, the proposed converter draws a continuous current from the source, and hence it is suitable for many types of renewable energy sources. The derived network expressions and theoretical analysis are finally validated experimentally with an example single-switch 400-W dc–dc converter. For the closed-loop control design and stability assessment, a small signal model and its analysis of the proposed network are also presented in brief.

An Online Battery Impedance Measurement Method Using DC–DC Power Converter Control

Abstract: This paper presents a simple online impedance measurement method for electrochemical batteries, including lithium-ion, lead-acid, and nickel–metal-hydride chemistries By using the proposed online impedance measurement method, there is no need to disconnect the battery from the system or to interrupt system operation, and there is no need to add ac signal injection circuits, costly response measurement, and analysis circuits/devices In practical battery-powered systems, a power converter is usually used to interface the battery with the load for voltage/current regulation purposes In this paper, through the control of the power converter and duty-cycle perturbation, the ac impedance of the battery can be determined The proposed method provides a low-cost and practical solution for the online measurement of the ac impedance of batteries Moreover, the proposed method can be either continuously or periodically performed without interrupting the normal operation of the battery system and the power converter In addition, this paper provides an example where the obtained impedance data are utilized for online state-of-charge estimation of lithium-ion batteries The proposed online impedance measurement method is validated by experiments conducted on a 26-Ah 18650-size lithium-ion battery interfaced to the load via a bidirectional dc–dc boost/buck converter

Active Mitigation of Subsynchronous Interactions Between PWM Voltage-Source Converters and Power Networks

Abstract: Pulse-width-modulated (PWM) voltage-source converters (VSCs) are gaining widespread acceptance in modern power systems. It has been shown recently that full-scale high-power PWM VSCs can induce negative electrical damping at subsynchronous frequencies. However, active reshaping of the VSC incremental output impedance to minimize the negative impacts of a VSC on subsynchronous damping is not reported. To fill out this gap, this paper presents: 1) an extended analysis of the output impedance of a PWM-based two-level VSC; and 2) more importantly, three simple and robust active reshaping techniques to maximize the positive electrical damping in the subsynchronous frequencies without affecting the converter control performance. The first reshaping technique uses the grid voltage and an active-damping controller to generate active impedance that modifies the VSC impedance in the subsynchronous range. The second reshaping technique uses an internal active damping controller to modify the dynamics of the phase-locked loop, which has significant contribution to the negative impedance of the VSC. The third reshaping technique combines the first and second techniques. The proposed active mitigation methods show excellent performance in reshaping the VSC impedance and inducing positive electrical damping to mitigate possible subsynchronous interactions between the VSC and the power network. Further, the proposed compensators show robust control performance at different output power levels of the VSC without significant impact on the converter control performance. A theoretical analysis and comparative time-domain simulation and experimental results are presented to verify the validity and effectiveness of the proposed techniques.

A 60-dB Gain OTA Operating at 0.25-V Power Supply in 130-nm Digital CMOS Process

Abstract: This paper presents a 60-dB gain bulk-driven Miller OTA operating at 0.25-V power supply in the 130-nm digital CMOS process. The amplifier operates in the weak-inversion region with input bulk-driven differential pair sporting positive feedback source degeneration for transconductance enhancement. In addition, the distributed layout configuration is used for all the transistors to mitigate the effect of halo implants for higher output impedance. Combining these two approaches, we experimentally demonstrate a high gain of over 60-dB with just 18-nW power consumption from 0.25-V power supply. The use of enhanced bulk-driven differential pair and distributed layout can help overcome some of the constraints imposed by nanometer CMOS process for high performance analog circuits in weak inversion region.

Impedance Matching and Power Division Using Impedance Inverter for Wireless Power Transfer via Magnetic Resonant Coupling

Abstract: Future applications of wireless power transfer will include powering various devices in a room, charging electric vehicles in a parking area, charging moving robots, and so on. Therefore, practical wireless power transfer must be able to support complicated configurations, for example, combination of multireceiver and repeaters. Many past works have discussed methods for improving efficiency and, more recently, extended the methods to multireceiver systems. However, controllable power division among receivers is also an important feature as receivers nearer to the transmitter tend to absorb more power compared to further ones. In this paper, a new impedance matching and power division method utilizing impedance inverters only at the receiver sides is proposed. The mathematical equations in the proposed method are then generalized for arbitrary number of receivers and arbitrary number of repeaters.

Online Grid Impedance Measurement Using Discrete-Interval Binary Sequence Injection

Abstract: Grid impedance affects the stability and control performance of grid-connected power electronic devices, such as inverters used to integrate wind and solar energy. Adaptive control of such inverters, to guarantee stability under different grid conditions, requires online measurement of the grid impedance performed in real time. Such online measurement can be performed by injecting a current perturbation from the inverter into the grid and by reading the grid voltage responses. To minimize the impact on the inverter operation, the injection must be kept as small as possible while producing enough voltage perturbation that can be reliably measured and processed to extract its various frequency components. This paper proposes the use of a discrete-interval binary sequence (DIBS) for this application to minimize the injection. The DIBS is a computer-optimized binary sequence, where the energy is maximized at specified harmonic frequencies based on the expected grid-impedance characteristics. Experimental results based on a three-phase grid-connected inverter are presented to demonstrate the effectiveness of the proposed methods.

Hybridizing Triboelectrification and Electromagnetic Induction Effects for High-Efficient Mechanical Energy Harvesting

Abstract: The recently introduced triboelectric nanogenerator (TENG) and the traditional electromagnetic induction generator (EMIG) are coherently integrated in one structure for energy harvesting and vibration sensing/isolation. The suspended structure is based on two oppositely oriented magnets that are enclosed by hollow cubes surrounded with coils, which oscillates in response to external disturbance and harvests mechanical energy simultaneously from triboelectrification and electromagnetic induction. It extends the previous definition of hybrid cell to harvest the same type of energy with multiple approaches. Both the sliding-mode TENG and contact-mode TENG can be achieved in the same structure. In order to make the TENG and EMIG work together, transformers are used to match the output impedance between these two power sources with very different characteristics. The maximum output power of 7.7 and 1.9 mW on the same load of 5 kΩ was obtained for the TENG and EMIG, respectively, after impedance matching. Benefiting from the rational design, the output signal from the TENG and the EMIG are in phase. They can be added up directly to get an output voltage of 4.6 V and an output current of 2.2 mA in parallel connection. A power management circuit was connected to the hybrid cell, and a regulated voltage of 3.3 V with constant current was achieved. For the first time, a logic operation was carried out on a half-adder circuit by using the hybrid cell working as both the power source and the input digit signals. We also demonstrated that the hybrid cell can serve as a vibration isolator. Further applications as vibration dampers, triggers, and sensors are all promising.

A Wearable 8-Channel Active-Electrode EEG/ETI Acquisition System for Body Area Networks

Abstract: This paper describes an 8-channel gel-free EEG/electrode-tissue impedance (ETI) acquisition system, consisting of nine active electrodes (AEs) and one back-end (BE) analog signal processor. The AEs amplify the weak EEG signals, while their low output impedance suppresses cable-motion artifacts and 50/60 Hz mains interference. A common-mode feed-forward (CMFF) scheme boosts the CMRR of the AE pairs by 25 dB. The BE post-processes and digitizes the analog outputs of the AEs, it also can configure them via a single-wire pulse width modulation (PWM) protocol. Together, the AEs and BE are capable of recording 8-channel EEG and ETI signals. With EEG recording enabled, ETIs of up to 60 kΩ can be measured, which increases to 550 kΩ when EEG recording is disabled. Each EEG channel has a 1.2 GΩ input impedance (at 20 Hz), 1.75 μVrms (0.5-100 Hz) input-referred noise, 84 dB CMRR and ±250 mV electrode offset rejection capability. The EEG acquisition system was implemented in a standard 0.18 μm CMOS process, and dissipates less than 700 μW from a 1.8 V supply.

High Efficiency Photovoltaic Source Simulator with Fast Response Time for Solar Power Conditioning Systems Evaluation

Abstract: Photovoltaic (PV) source simulators serve as a convenient tool for the dynamic evaluation of solar power conditioning systems and maximum power point tracking algorithms. High efficiency and fast transient response time are essential features of any PV source simulator. This paper proposes a new type of PV source simulator that incorporates the advantages of both analog and digital-based simulators. The proposed system includes a three-phase ac-dc dual boost rectifier cascaded with a three-phase dc-dc interleaved buck converter. The selected power stage topology is highly reliable and efficient. Moreover, the multiphase converter helps improve system transient response though producing low output ripple which makes it adequate for PV source simulators. The simulator circuitry emulates precisely the static and the dynamic characteristics of actual PV generators under different load and environmental conditions. Additionally, the system allows the creation of the partial shading and bypass diodes effect on PV characteristics. The paper investigates the dynamic performance of a commercial solar power inverter using the proposed PV source simulator in steady-state and transient conditions. Closed-loop output impedance of the proposed PV source simulator has been measured and verified at different operating regions. The impedance profile--magnitude and phase--matches the output impedance of actual PV generators. Mathematical modeling and experimental validation of the proposed system is thoroughly presented based on a 2.0 kW hardware prototype. The proposed simulator efficiency, including the active-front-end rectifier and the converter stages, peaks at 96.7%.

Wideband Four-Way Out-of-Phase Slotline Power Dividers

Abstract: Two new wideband four-way out-of-phase slotline power dividers are proposed in this paper. The half-wavelength slotlines are employed to construct the presented compact power dividers. Based on the proposed power-dividing circuit, a four-way power divider is implemented with compact size and simple structure. To obtain high isolation among the four output ports and good output impedance matching, another four-way out-of-phase slotline power divider with improved isolation performance is designed by introducing an air-bridge resistor and two slotlines with isolation resistors. The simulated and measured results of the proposed power dividers demonstrate reasonable performance of impedance matching, insertion loss, amplitude balancing, and isolation among the output ports.

Theory of Image Impedance Matching for Inductively Coupled Power Transfer Systems

Abstract: The optimum design of an inductively coupled power transfer system is accomplished by applying the Roberts's theory on conjugate image impedance. The input and output ports' impedance and the loading capacitances for maximum power transfer are obtained as functions of the self-inductances, the mutual inductance, the copper losses, and the iron loss, which are used to define the figure-of-merit of the system. It is found that the loading capacitances necessarily satisfy the resonance conditions individually in the input and output circuits. The capacitors may be connected in series or in parallel. The equivalent transform formulas between the series and parallel topologies are given for the capacitances and for the port impedances. The port impedance in the series (parallel) topology is proportional (approximately inversely proportional) to the inductive coupling coefficient. The image impedances of the lossless symmetric systems are real or pure imaginary depending on the frequency, and the image impedance patterns are classified as “Hill”, “Valley”, and “Waterfall". The reactance network approach for such lossless systems gives in-depth insight into the system behaviors and facilitates the practical design.

An Impedance Protection Scheme for Feeders of Active Distribution Networks

Abstract: Distribution networks (DNs) gradually become more active and flexible with the massive integration of renewable resources. The conventional protection methods are unsuitable for the promising active DNs. This paper proposes a protection scheme on the basis of measured impedance, which includes an impedance differential method and an inverse-time low-impedance method. The former approach identifies fault instant and provides time reference for the exchanged data. As primary protection, it is able to clear different types of faults as soon as possible. The latter method operates in an automatic coordination way with inverse-time characteristic. It can be used as backup protection or provide protection for single-end lines alone. Both of the methods are independent of operation modes and are able to clear faults with high sensitivity. The efficiency of the protection scheme is validated by PSCAD/EMTDC.

Y-source impedance network

Abstract: This paper introduces a Y-shaped impedance network for realizing converters that demand a high voltage gain while using a small duty ratio. To achieve that, the proposed network uses a tightly coupled transformer with three windings, whose obtained gain is presently not matched by existing networks operated at the same duty ratio. This capability has been demonstrated by mathematical derivation for the proposed network in comparison with other recently reported networks. To further prove the network performance, a single-switch dc-dc converter has been implemented with the network, before testing it experimentally. The results obtained clearly verify the network performance in addition to its higher power density that can generally be achieved by coupled magnetics.

Transmission line resistance compression networks for microwave rectifiers

Abstract: This work presents a development of multi-way transmission-line resistance compression networks (TLRCNs) and their application to rf-to-dc conversion. We derive analytical expressions for the behavior of TLRCNs, and describe two design methodologies applicable to both single- and multi-stage implementations. A 2.45-GHz 4-way TLRCN network is implemented and applied to create a resistance-compressed rectifier system that has narrow-range resistive input characteristics over a 10-dB power range. It is demonstrated to improve the impedance match to mostly-resistive but variable input impedance class-E rectifiers over a 10-dB power range. The resulting TLRCN plus rectifier system has >50% rf-to-dc conversion efficiency over a >10-dB input power range at 2.45 GHz (peak efficiency 70%), and SWR <;1.1 over a 7.7-dB range.

A 60-dB Gain OTA operating at 0.25-V power supply in 130-nm digital CMOS process

Abstract: This paper presents a high gain bulk-driven Miller OTA operating at just 0.25-V power supply in a 130-nm digital CMOS process. The amplifier operates in weak-inversion region with input bulk-driven differential pair with negative resistance source degeneration. In addition, distributed layout configuration is used for all transistors to mitigate the effect of halo implants for higher output impedance. Combining these two approaches, we experimentally demonstrate a high gain of over 60-dB with just 18-nW power consumption from 0.25-V power supply.

Model-Based Nonlinear Embedding for Power-Amplifier Design

Abstract: A fully model-based nonlinear embedding device model including low- and high-frequency dispersion effects is implemented for the Angelov device model and successfully demonstrated for load modulation power-amplifier (PA) applications. Using this nonlinear embedding device model, any desired PA mode of operation at the current source plane can be projected to the external reference planes to synthesize the required multi-harmonic source and load terminations. A 2-D identification of the intrinsic PA operation modes is performed first at the current source reference planes. For intrinsic modes defined without lossy parasitics, most of the required source impedance terminations will exhibit a substantial negative resistance after projection to the external reference planes. These terminations can then be implemented by active harmonic injection at the input. It is verified experimentally for a 15-W GaN HEMT class-AB mode that, using the second harmonic injection synthesized by the embedding device model at the input, yields an improved drain efficiency of up to 5% in agreement with the simulation. A figure-of-merit is also introduced to evaluate the efficacy of the nonlinear embedding PA design methodology in achieving the targeted intrinsic mode operation given the model accuracy.

Control of Inverters Via a Virtual Capacitor to Achieve Capacitive Output Impedance

Abstract: Mainstream inverters have inductive output impedance at low frequencies (such inverters are called L-inverters). In this paper, a control strategy is proposed to make the output impedance of an inverter capacitive at low frequencies (such inverters are called C-inverters). The proposed control strategy involves the feedback of the inductor current through an integrator, which is actually the impedance of a virtual capacitor. The gain of the integrator or the virtual capacitance is first selected to guarantee the stability of the current feedback loop and then optimized to minimize the total harmonic distortion (THD) of the output voltage. Moreover, some guidelines are developed to facilitate the selection of the filter components for C-inverters. Simulation and experimental results are provided to demonstrate the feasibility and excellent performance of C-inverters, with the filter parameters of the test rig selected according to the guidelines developed. It is shown that, with the same hardware, the lowest voltage THD is obtained when the inverter is designed to be a C-inverter. A by product of this study is that, as long as the current ripples are kept within the desired range, the filter inductor should be chosen as small as possible in order to reduce voltage harmonics. This helps reduce the size, weight, and volume of the inductor and improve the power density of the inverter.

Design and analysis of DC gain and transconductance boosted recycling folded cascode OTA

Abstract: A new technique for improving the transconductance and low frequency output impedance of recycling folded cascode (RFC) amplifiers is presented. This enhancement was achieved by using a positive feedback and upgrading the recycling structure. The new structure profits from better transconductance, slew rate, and DC gain in comparison with conventional folded cascode (FC) amplifier. Moreover, the input referred noise is reduced and the phase-margin improved. The enhanced amplifier, simulated in 0.18 μm CMOS technology, exhibits a DC gain enhancement of 16.3 dB as well as 115.5 MHz increase in gain bandwidth compared to conventional FC configuration. The amplifier consumes 360 μW @ 1.2 V which makes it suitable for low-voltage applications.

An impedance-based stability analysis method for paralleled voltage source converters

Abstract: This paper analyses the stability of paralleled voltage source converters in AC distributed power systems. An impedance-based stability analysis method is presented based on the Nyquist criterion for multiloop system. Instead of deriving the impedance ratio as usual, the system stability is assessed based on a series of Nyquist diagrams drawn for the terminal impedance of each converter. Thus, the effect of the right half-plane zeros of terminal impedances in the derivation of impedance ratio for paralleled source-source converters is avoided. The interaction between the terminal impedance of converter and the passive network can also be predicted by the Nyquist diagrams. This method is applied to evaluate the current and voltage controller interactions of converters in both grid-connected and islanded operations. Simulations and experimental results verify the effectiveness of theoretical analysis.

Split-phase control: Achieving complete soft-charging operation of a dickson switched-capacitor converter

Abstract: Switched-capacitor (SC) converters are gaining popularity due to their high power density and suitability for on-chip integration. Soft-charging techniques can be used to eliminate the current transient during the phase switching instances, and improve the power density and efficiency of SC converters. In this paper, we propose a split-phase control scheme that enables the Dickson converter to achieve complete soft-charging operation, which is not possible using the conventional two-phase control. An analytical method is extended to understand and design split-phase controlled Dickson converters. The proposed technique and analysis are verified by both simulation and experimental results. An 8-to-1 step-down Dickson converter is built to demonstrate the reduction in output impedance and improvement in efficiency as a result of the split-phase controlled soft-charging operation.

A LDO Regulator With Weighted Current Feedback Technique for 0.47 nF–10 nF Capacitive Load

Abstract: A Weighted Current Feedback (WCF) technique for output capacitorless low-dropout (OCL-LDO) regulator is presented in this paper. Through feedback of a weighted current, the WCF permits smart management of the output impedance as well as the gain from the inter-gain stage. Based on the Routh-Hurwitz stability criterion, the WCF can avoid the right-half plane (RHP) pole and push the left-half plane (LHP) non-dominant complex pole pair to a higher frequency. Besides, it provides good regulator loop gain and fast transient response. Validated by UMC 65 nm CMOS process, the simulation and measurement results have shown that the WCF LDO regulator can operate at a load capacitance (C L ) range from 470 pF to 10 nF with only 3.8 pF compensation capacitor. At a supply of 0.75 V and a quiescent current of 15.9 μA, the proposed circuit can support a maximum load current (I L ) of 50 mA. When I L switches from 0 to 50 mA in 100 ns, the output can settle within 400 ns for the whole C L range. For a case of single capacitor (C L 470 pF), the settling time is only 250 ns. The comparison results have shown that the WCF LDO regulator offers a comparable or better transient figure-of-merit (FOM) and additional merit to drive wide load capacitance range.

One-Dimensional Optimization for Proportional–Resonant Controller Design Against the Change in Source Impedance and Solar Irradiation in PV Systems

Abstract: The variation in source impedance and solar irradiation effects on photovoltaic (PV) system control performance is investigated. A proportional-resonant (PR) controller in a stationary frame in place of a proportional-integral controller in a synchronous frame was adopted to modulate a single-phase grid-tied inverter for PV systems. Although the PR controllers have gained some momentum lately due to advantages such as instantaneous tracking capability and low-cost computational resources, the tracking performance may decline due to changes in source impedance and solar irradiation where the conventional PR design rule is void. To adapt PR controllers over diverse operating conditions without incurring excessive tracking error, a one-dimensional optimization (ODO) algorithm that dictates the quality of the tracking performance is proposed to search for an appropriate factor χ between the real part of dominant and nondominant eigenvalues for the PV systems. The control gains of the PR controller can be then altered in light of the optimal χ through a simple algebraic conversion. The experimental results confirm the performance of the proposed strategy.

Mitigation of Inverter-Grid Harmonic Resonance by Narrow-Band Damping

Abstract: This paper deals with damping of resonance between grid-connected inverters and the power grid. This paper proposes a narrow-band digital filtering technique to resolve the resonance problem by shaping the inverter output impedance at selected frequencies without affecting the characteristics of the inverter outside the selected frequency range. A frequency-sampling method is used to implement the narrow-band digital filter which allows direct control of the filter's amplitude and phase response in accordance with damping requirements. The experimental results are presented to demonstrate the performance of the proposed method.