Showing papers on "Electrical impedance published in 2018"

Unified Impedance Model of Grid-Connected Voltage-Source Converters

Abstract: This paper proposes a unified impedance model of grid-connected voltage-source converters for analyzing dynamic influences of the phase-locked loop (PLL) and current control. The mathematical relations between the impedance models in the different domains are first explicitly revealed by means of complex transfer functions and complex space vectors. A stationary ( αβ -) frame impedance model is then proposed, which not only predicts the stability impact of the PLL, but also reveals its frequency coupling effect. Furthermore, the impedance shaping effects of the PLL on the current control in the rotating ( dq -) frame and the stationary ( αβ -) frame are structurally comapred. The frequency-domain case studies on a three-phase grid-connected converter are next presented, and subsequently validated in time-domain simulations and experimental tests. The close correlations between the measured results and theoretical analysis confirm the effectiveness of the stationary-frame impedance model.

Constant Current/Voltage Charging Operation for Series–Series and Series–Parallel Compensated Wireless Power Transfer Systems Employing Primary-Side Controller

Abstract: This paper proposes a new control technique, which only employs the primary-side controller and load identification approach to adjust charging voltage/current for series–series (SS) and series–parallel (SP) compensated wireless power transfer (WPT) systems. The advantages are that dual-side wireless communication for real-time charging current/voltage adjustment is avoided as well as it is suitable for different charging modes, e.g., constant voltage (CV) and constant current (CC) charging defined by the battery charging profile. The load identification approach, which utilizes reflected impedance theory and quadrature transformation algorithm for calculating the active power, is proposed to estimate the equivalent load resistance of battery. Then, the CV/CC charging for both SS and SP compensation are achieved by the PI-controlled phase-shift H-bridge inverter. The simulation and experimental results validate the feasibility of proposed control method. During the CC charging, 3.01 and 3.03 A for SS and SP compensation with the error of 1.2% and 1.4% are achieved. During the CV charging, 25.8 and 25.7 V for SS and SP compensation with the error of 1.1% and 1.3% are realized. The proposed method improves the performance of both SS- and SP-compensated WPT systems to be more suitable for the applications that require compact and lightweight receiver.

Design of a High-Efficiency Wireless Power Transfer System With Intermediate Coils for the On-Board Chargers of Electric Vehicles

Abstract: In this paper, a high efficiency inductive wireless power transfer system for the on-board chargers of electric vehicles is proposed. In order to improve the power transfer efficiency, the proposed system adopts two additional intermediate coils with resonant capacitors, which increases the effective magnetizing impedance between the transmitter and receiver coils with no ferrites. The resonant tank of the proposed system is designed to operate the converter as a current source and as a voltage source at two different frequencies to implement the constant current (CC) mode charge and constant voltage (CV) charge, respectively. Since the proposed converter operates at a fixed frequency in each mode of charge operation, full soft switching of all the switching devices is possible and the zero phase angle condition can be achieved in both the CC and CV mode operations. A theoretical analysis based on a Thevenin model to come up with a suitable design for the battery charger and its closed-loop controller is presented and its superior performance is demonstrated by experimental results. A 6.6 kW prototype is implemented with a 200 mm air gap to demonstrate the validity of the proposed method. Experimental results show that the dc to dc conversion efficiency of the proposed system is 97.08% at 3.7 kW of output power in the CV mode charge.

Analysis of Resonance Between a VSC-HVDC Converter and the AC Grid

Abstract: A 1270 Hz resonance occurred between ±350 kV/ 1000 MW Luxi back-to-back voltage source converter based high-voltage dc transmission (VSC-HVDC) converter and the 525 kV ac grid after disconnection of several ac transmission lines. To understand the resonance and find a solution, the impedance-based stability analysis model considering different equipment is first established. Then, the resonance is analyzed and repeated in the simulation based on the established model. The system stability can be judged by the ratio of grid impedance to the equivalent impedance of all parallel-connected equipment with the converter. To evaluate the occurrence and risk of resonance, the frequency range where the impedance has a negative-real-part has been searched and studied. In order to narrow the negative-real-part region to avoid potential resonance, solutions such as control strategy improvement and passive or active impedance adapter may be applicable and are discussed. For a complex system containing various equipment, the equipment can be divided into several subsectors to avoid modeling all possible combinations of equipment, which can be exhausting. And analysis has shown sufficient but not necessary condition to stabilize the system is to avoid the negative-real-part region in each sector.

An Improved Design of Current Controller for LCL -Type Grid-Connected Converter to Reduce Negative Effect of PLL in Weak Grid

Abstract: For three-phase LCL -type grid-connected converter, when it is attached to weak grid, current control interacts with phase-locked loop (PLL) via point of common coupling voltage. Consequently, PLL dynamic might deteriorate grid current control and even result in system instability. However, the conventional design method of current controller neglects the impact of PLL, and therefore, it is hard for the current controller to mitigate the negative effect of PLL. In this paper, an improved design of current controller, i.e., proportional–integral controller and capacitor-current-feedback active damping, is proposed to reduce the negative effect of PLL on current control. First, a small-signal impedance model is developed to analyze the impact of PLL on current control. Then, the effect of current controller parameters on the converter output impedance is analyzed, and a design guideline to improve the current controller parameters is presented. With the improved parameters, in precondition of satisfying system stability margin under both stiff and weak grid, the negative effect of PLL on current control can be effectively mitigated without employing additional control strategies. Furthermore, the current control has a strong robustness against wide-range variation of grid impedance. Finally, the experiment demonstrates the effectiveness of the proposed design method.

Frequency-Fixed Beam-Scanning Leaky-Wave Antenna Using Electronically Controllable Corrugated Microstrip Line

Abstract: An electronically controllable microstrip leaky-wave antenna (LWA) to steer the radiations at a fixed frequency is presented. The proposed LWA is composed of a corrugated microstrip line loaded by the varactor diodes with triangular-modulated surface impedance. Due to the periodical modulation of the surface impedance, the guided waves can be converted into the leaky-wave radiations efficiently with frequency-scanning property. Furthermore, the surface impedance of the LWA can be reconfigured by changing the capacitance of the varactor diode through dc bias voltage, which will make the radiation beam steer in a large angle range accordingly at a fixed frequency. Both numerical simulations and experimental results show that the radiation beams can be controlled for continuously steering at each frequency from 5.5 to 5.8 GHz by changing the dc bias voltage from 0 to 20 V, in which the scanning angle can reach as high as 45°.

Stability Improvement for Three-Phase Grid-Connected Converters Through Impedance Reshaping in Quadrature-Axis

Abstract: Three-phase AC−DC and DC−AC power converters have been extensively employed as grid-interfaces in various applications, e.g., distributed generation and energy storage systems. In these applications, power converters should always synchronize with the mains grid so that active and/or reactive power can properly be regulated while maintaining desired waveforms of grid currents. Grid synchronization necessitates accurate information of grid voltages, which is normally obtained through phase-locked-loops (PLLs). However, the employment of PLLs may bring in stability concerns. Previous research revealed that the inclusion of PLLs shapes the impedance of power converters into a negative resistance in the quadrature-axis ( q -axis), and this should be responsible for instability. To resolve the instability issue caused by PLLs, this paper proposes an impedance controller for reshaping the q -axis impedance into a positive resistance in the low-frequency band. Without any extra burden on system hardware, the proposed controller can easily be implemented by directly relating the q -axis voltage to the q -axis current reference. As a result, the presented three-phase power conversion system can operate stably even under a severely weak grid condition, which are verified by simulation and experimental results.

Low-Frequency Stability Analysis of Single-Phase System With dq -Frame Impedance Approach—Part I: Impedance Modeling and Verification

Abstract: The utilization of a large number of power electronic converters in the high-speed railway system results in the low-frequency instability for the single-phase vehicle-grid system. In order to probe into the problem, the impedance-based approach is adopted. This paper focuses on the dq -frame impedance modeling and verification of the single-phase converters in the electric multiple units. The closed-loop input impedance modeling methods for both single converter and interleaved converters considering the second-order generalized integrator-based phase-locked loop (PLL) are put forward. Then, the dq -frame impedance measurement method for single-phase systems based on the Hilbert transform is proposed, and the modeling approach is verified by the frequency-domain simulations on the switching models. The good agreement of the model and simulation illustrates that the modeling method is suitable and accurate enough until half of the switching frequency. It is elucidated that the single-phase rectifier's impedance has the off-diagonal characteristic and the negative-impedance characteristic on the dd channel at low frequencies. Moreover, both the current and voltage controls have influences on the negative impedance shaping, while PLL does not.

A New Fault Location Technique in Smart Distribution Networks Using Synchronized/Nonsynchronized Measurements

Abstract: This paper proposes a new impedance-based technique to locate all fault types in distribution networks with/without distributed generators. A new procedure to form an impedance matrix using only series impedances of the distribution lines is introduced. The impedance matrix along with the prefault and during-fault voltage phasors at few buses is used to estimate the injection fault current via the least-squares technique. Linear least-squares estimator is utilized if microphasor measurement units ${({\mu} \rm{PMUs)}}$ are installed along the network. However, a nonlinear least-squares problem solved by the trust-region-reflective algorithm is used when only the voltage magnitudes are provided by smart meters. The operation of the standard protective devices in the distribution networks is used to reduce the computational burden of the proposed method. Also, a generalized measurement placement algorithm is studied using the discovered features of the impedance matrix. In addition, the Sobol's sensitivity analysis is conducted to quantify the importance of different input factors on the fault location accuracy. The effectiveness of the proposed method is validated on a real 134-bus, 13.8 kV distribution network under several fault scenarios and noisy measurements.

Mutual coupling reduction between elements of UWB MIMO antenna using small size uniplanar EBG exhibiting multiple stop bands

Abstract: In this paper, mutual coupling reduction between elements of UWB MIMO antenna using small size uniplanar EBG is presented. Proposed UWB antenna geometry consists of two circular shaped monopole radiator swith a slot in ground plane for proper impedance matching. A UC-EBG (Uniplanar Electromagnetic Band Gap) cell of size 6.8 mm × 6.8 mm is inserted between the antenna elements in 4 × 1 array configuration to improve the isolation. Bandgap of the UCEBG is determined using dispersion diagram and suspended stripline method. The proposed antenna is fabricated on 1.6 mm thick, low cost FR4 substrate, possessing an overall size of 27.2 mm × 46 mm. The antenna has been fabricated and experimentally verified. The antenna shows simulated and measured −10 dB impedance bandwidths of 14.6 GHz (3–17.6 GHz) and14.8 GHz (3.6–17.9 GHz) respectively. UCEBG structure exhibits multiple stop bands and suppresses E-plane coupling over these bands. Isolation better than −18 dB is achieved over the complete impedance bandwidth. Radiation efficiency and peak gain of the antenna varies from 78% to 96.7% and 1.4 dB to 4 dB respectively. MIMO parameters, i.e. error correlation coefficient (ECC) better than0.018 and Total active reflection coefficient (TARC) better than −26 dB is achieved over the impedance bandwidth.

Analysis of Output Current Characteristics for Higher Order Primary Compensation in Inductive Power Transfer Systems

Abstract: Constant output current is desirable for some wireless power transmission applications such as charging batteries and driving light-emitting diodes. Converters having an inherently small output current fluctuation versus variations of load and coupling are attractive solutions since such converters can be controlled to achieve constant current with minimum control efforts. In this paper, the impedance conditions and the output current characteristics for different compensation types are presented. It is shown that the types of compensation producing load- and coupling-independent output current at the designed operating point as well as having a unimodal dependence of the output current on coupling would achieve better output current performance against variations of load and coupling. The sensitivities of the output current to parameter variation of typical compensation types are quantitatively analyzed and compared. Analytical results indicate that the primary parallel–series and secondary series compensation, which produces output current that exhibits a unimodal relationship with coupling and is nearly immune to coupling variation at the designed operating point, has the smallest output current fluctuation versus variation of the coupling coefficient. Finally, a comparison of experimental results of different types of compensation is presented for verification.

Graphene Based Terahertz Phase Modulators

Abstract: Electrical control of amplitude and phase of terahertz radiation (THz) is the key technological challenge for high resolution and noninvasive THz imaging. The lack of active materials and devices hinders the realization of these imaging systems. Here, we demonstrate an efficient terahertz phase and amplitude modulation using electrically tunable graphene devices. Our device structure consists of electrolyte-gated graphene placed at quarter wavelength distance from a reflecting metallic surface. In this geometry, graphene operates as a tunable impedance surface which yields electrically controlled reflection phase. Terahertz time domain reflection spectroscopy reveals the voltage controlled phase modulation of π and the reflection modulation of 50 dB. To show the promises of our approach, we demonstrate a multipixel phase modulator array which operates as a gradient impedance surface.

Sliding Mode Controller in a Multiloop Framework for a Grid-Connected VSI With LCL Filter

Abstract: This paper proposes a multiloop framework for current control of grid-connected voltage source inverters (VSIs) with LCL output filter. The discrete-time model is of third-order with a nonminimum phase zero when controlling the grid side current. This poses some difficulties on the design of most types of controllers. Motivated by this problem, the inner loop is implemented by a discrete-time sliding mode control law to ensure the tracking of the converter side current. This can be achieved regardless the grid impedance and voltage, making the converter to behave like a current source inverter with a capacitive+inductive ( CL ) filter. Thus, the problem of current control falls from a third-order system to a second-order system. Several types of controllers can be designed to implement the outer loop based on the equivalent CL circuit. In this work, a resonant controller with a virtual resistor was applied. Simulations and experimental results are presented to validate the proposal.

Impedance Characterization and Modeling of Lithium-Ion Batteries Considering the Internal Temperature Gradient

Abstract: Battery impedance is essential to the management of lithium-ion batteries for electric vehicles (EVs), and impedance characterization can help to monitor and predict the battery states. Many studies have been undertaken to investigate impedance characterization and the factors that influence impedance. However, few studies regarding the influence of the internal temperature gradient, which is caused by heat generation during operation, have been presented. We have comprehensively studied the influence of the internal temperature gradient on impedance characterization and the modeling of battery impedance, and have proposed a discretization model to capture battery impedance characterization considering the temperature gradient. Several experiments, including experiments with artificial temperature gradients, are designed and implemented to study the influence of the internal temperature gradient on battery impedance. Based on the experimental results, the parameters of the non-linear impedance model are obtained, and the relationship between the parameters and temperature is further established. The experimental results show that the temperature gradient will influence battery impedance and the temperature distribution can be considered to be approximately linear. The verification results indicate that the proposed discretization model has a good performance and can be used to describe the actual characterization of the battery with an internal temperature gradient.

Inherent Self-Interference Cancellation for In-Band Full-Duplex Single-Antenna Systems

Abstract: We propose a new analog self-interference cancellation (SIC) technique for in-band full-duplex transmission in single-antenna systems. We use an RF circulator to separate transmitted (Tx) and received (Rx) signals. Instead of estimating the SI signals and subtracting them from the Rx signals, we use the inherent secondary SI signals at the circulator, reflected by the antenna, to cancel the primary SI signals leaked from the Tx port to the Rx port. We modified the frequency response of the secondary SI signals using a reconfigurable impedance mismatched terminal (IMT) circuit, which consists of two varactor diodes at the antenna port. We can also adjust the frequency band and the bandwidth by controlling the varactor diodes bias voltages. The IMT adjustability makes it robust to antenna input impedance variations and fabrication errors. We analyze and fabricate a prototype of the proposed technique at 2.45 GHz. We achieved more than 40-dB cancellation over 65 MHz of bandwidth. Our technique is independent of the RF circulator and antenna type and it can be applied to any frequency band. It is also very relevant to small mobile devices because it provides a simple and low-power and low-cost adjustable analog SIC technique.

High-Precision Electrical Impedance Tomography Data Acquisition System for Brain Imaging

Abstract: In electrical impedance tomography (EIT), it is difficult to obtain the intracranial impedance due to the highly resistive skull enclosing the brain. Therefore, a high-precision data acquisition system is required for brain EIT. In this paper, we used a high-precision digital synthesis method and a digital demodulation technique with high noise immunity to eliminate random errors. Moreover, we focused on two problems encountered during EIT data acquisition: 1) the shunt effect on the excitation current due to the distributed capacitance between electrodes and ground and 2) high common-mode voltages in the boundary measurements. We designed a new electrode interface to reduce the influence of the distributed capacitance and a programmable current source to accurately compensate for the excited current. We also proposed a new voltmeter circuit with improved CMRR. Overall, this EIT data acquisition system can produce a programmable current with SNR greater than 89 dB. It can also measure the voltage difference precisely with CMRR higher than 75 dB with a 1- $\text{k}\Omega $ impedance imbalance. The results on a calibration model show that this system has a high SNR of 83 dB and a low reciprocity error of 0.125%. In addition, EIT imaging results were acquired using a brain physical phantom. The system can detect small disturbances of 0.35% in volume (1.99% of the cross-sectional area) and 17% in resistivity. Experiments on healthy volunteers also suggest that small intracranial impedance variations due to temporary occlusion and reperfusion of the unilateral carotid artery may be monitored by the system.

Low-Frequency Stability Analysis of Single-Phase System With $dq$ -Frame Impedance Approach—Part II: Stability and Frequency Analysis

Abstract: Nowadays, the power electronics-based systems sometimes present the low-frequency instability owing to the interaction between the power converters and the grid, such as the low-frequency oscillation (LFO) happening in the single-phase vehicle-grid (electric multiple units (EMUs)—traction power grid) systems of high-speed railway. To address this problem, the impedance-based stability analysis method is adopted. Given the grid impedance and the $dq$ -frame converter impedance in the EMUs, several popular impedance-based stability analysis methods are compared, and their features and applicability are summarized. The generalized Nyquist stability criterion is then applied to more cases to validate the proposed impedance model for the single-phase converters, and excels in good estimation of the stability boundary and oscillation frequency. The analysis results are validated by the simulation and dSPACE tests. Through the comprehensive analysis in this paper, the mechanism of LFO in high-speed railway system is further elucidated, and it is suggested that the negative impedance of the converters will contribute to the LFO.

Analysis and Design of Dual-Band Rectifier Using Novel Matching Network

Abstract: In this brief, a compact dual-band impedance matching network is introduced and applied to the design of rectifying circuits. The matching network can work at two arbitrary frequencies with arbitrary complex impedance simultaneously. Theoretical analysis is carried out and the closed-form design formulas are derived. For validation, a dual-band rectifier working at 0.915 and 2.45 GHz is implemented. The measured maximum RF-to-dc efficiencies are 77.2% and 73.5% at 0.915 and 2.45 GHz, respectively.

A High-Transmittance Frequency-Selective Rasorber Based on Dipole Arrays

Abstract: This paper presents a frequency-selective rasorber whose transmission window locates at the higher frequency of absorption band. The accomplished rasorber is composed of dipole-like and slot arrays, and has realized the transmissive/absorptive performance. In every unit cell, each pair of dipole-like elements connected by vias is printed on the two sides of the substrate, and the coupling between long and short dipoles is suppressed by this structure. A guiding circuit is studied based on the analysis of the current path, and the insertion loss of transmission window is significantly reduced by the surface current at the pass-band that is hindered to pass through lossy elements. The presented rasorber acts as an absorber at the low frequencies, while providing a high transmittance window at 5.6 GHz. This design is elaborately optimized to achieve low reflection and angle-insensitive performance. Finally, the presented structure is validated by numerical simulations and experimental measurements. This rasorber could be used for secrecy communications among stealth facilities while providing stable broad-band absorptive properties.

Online Grid-Impedance Measurement Using Ternary-Sequence Injection

Abstract: Grid impedance affects the stability and control performance of grid-connected power-electronics 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. Wideband frequency-response-measurement techniques based on the pseudo-random binary sequence (PRBS) or impulse injection have been often applied for grid-impedance measurements. However, while using the PRBS or impulse, it is assumed that the system under study is linear. Because such an assumption does not typically hold for grid-connected systems, the measured impedances are usually prone to distortions caused by nonlinearities. This paper proposes the use of periodic ternary sequences for online grid-impedance measurements. Using appropriately designed periodic ternary signals, the linear component of the grid impedance can be identified, eliminating errors from even-order nonlinear distortions. In addition, the ternary sequences can be designed for a much wider range of sequence length compared to the conventional PRBS, enabling more efficient optimization of computing power and frequency resolution. Experimental measurements are shown from a grid-connected photovoltaic inverter.

Fully Soft-Switched Dual-Active-Bridge Series-Resonant Converter With Switched-Impedance-Based Power Control

Abstract: Both conduction loss and switching loss can contribute significantly to the overall power loss of an isolated bidirectional dual-active-bridge series-resonant dc–dc converter (DABSRC) operating at high frequency. To achieve soft switching and minimum-tank-current operation under wide-range variations in output voltage and current, a switched-impedance-based DABSRC is proposed. Minimum-tank-current operation aims to reduce conduction loss arising from circulating current at the low voltage, high-current side of DABSRC. Full-range soft switching is achieved in all switches, thus, switching loss is significantly reduced. With this new topology, power control is achieved by controlling a switch-controlled capacitor in the series resonant tank while ensuring minimum-tank-current operation and soft switching in all switches. The proposed topology and modulation scheme are validated by means of a 1-kW experimental prototype of DABSRC operating at 100 kHz designed to interface a 250-V dc bus to a supercapacitor with a rated output voltage of 48 V. The effectiveness of the proposed topology for charging/discharging a supercapacitor at a maximum rated power of 1 kW is verified by simulations and experimental results with a maximum efficiency of 97.5%.

Design and Analysis of Piezoelectric Metamaterial Beams With Synthetic Impedance Shunt Circuits

Abstract: We present an electromechanical modeling framework and a detailed numerical investigation for the design and analysis of piezoelectric metamaterial beams whose unit cells with segmented electrode pairs are shunted to synthetic impedance circuits. This framework aims to extend the well-studied locally resonant piezoelectric metamaterials and resulting finite metastructures with specified boundary conditions to novel concepts beyond bandgaps associated with simple inductive shunts. Overcoming the bandgap limitations of the locally resonant design requires more advanced considerations in the electrical domain. To this end, we bridge piezoelectric metamaterials and synthetic impedance shunts, and present a general design and analysis framework along with numerical case studies. A general procedure is implemented based on the root locus method for choosing the shunt circuit impedance, with an emphasis on vibration attenuation and practical design considerations. Case studies are presented for systems with locally resonant bandgaps with or without negative capacitance, as well as systems with multiple distinct bandgaps, and the necessary shunt admittance is derived for each case. Simulations are performed for a typical finite meta material beam with synthetic impedance shunts, accounting for the finite sampling rate and circuit dynamics. Time-domain simulations using these synthetic impedance circuits are compared to the ideal frequency-domain results with very good agreement.

A High-Bandwidth Integrated Current Measurement for Detecting Switching Current of Fast GaN Devices

Abstract: Gallium nitride (GaN) devices are suitable for high-frequency power converters due to their excellent switching performance. To maximize the performance of GaN devices, it is necessary to study the switching characteristics, which requires measuring the switching current. However, GaN devices have a fast switching speed and are sensitive to parasitic parameters, so the current measurement should have a high bandwidth and should not introduce excessive parasitic inductance into the power converters. Traditional current measurements are difficult to meet these requirements, especially for fast GaN devices. This paper presents a high-bandwidth integrated current measurement for detecting the switching current of fast GaN devices. By effectively utilizing the parasitic inductance in the circuit, a single-turn coil is embedded in the printed circuit board. This coil could pick up a sufficiently strong voltage signal, which is then processed to reconstruct the switching current. Moreover, corrections are carried out to further improve the accuracy. The current measurement has a small insertion impedance and a high bandwidth with a small influence on the parasitic inductance of the converter. The accuracy of the current measurement is experimentally verified by a 40 V GaN-based double pulse test circuit with a load current up to 25 A.

Sequences Domain Impedance Modeling of Three-Phase Grid-Connected Converter Using Harmonic Transfer Matrices

Abstract: The sequence-domain impedance using harmonic linearization technique can well describe the harmonic behavior of the interconnected system consisting of the grid and converter. However, fast phase-locked loop (PLL) design, unbalanced current controller structure will cause the unneglectable off-diagonal components in impedance matrix of converter. Due to the frequency coupling and sequence coupling effect in the sequence domain, the traditional single input single output transfer function lacks the capability to accurately express this frequency shift relationship. Therefore, in this paper, the harmonic transfer matrix (HTM) is used to model the frequency shift existing in the sequence-domain impedance model. By unifying positive sequence and negative sequence components, the complexity of impedance model derivation can be reduced. The linear time-periodic system is transformed to a linear time-invariant system in harmonic domain. And then, Nyquist criteria is applied to analyze the stability of the interconnected system. Finally, simulation results verify the proposed HTM-based sequence-domain impedance modeling method.

Observer-Based Pulsed Signal Injection for Grid Impedance Estimation in Three-Phase Systems

Abstract: This paper addresses the estimation of the grid impedance and the control of grid-tied converters by combining pulsed signal injection (PSI) and observer-based techniques. A Luenberger-based observer is used for controlling the grid-side current of an LCL filter by only measuring the converter-side currents and the grid-side voltage. This configuration mitigates the effects of parameter variation at the LCL filter. Under grid impedance changes, the observer control signal will be used for triggering the signal injection. A PSI approach is employed for estimating online the grid impedance using a recursive least square algorithm. Compared with existing grid-impedance estimation techniques, the proposed method can: 1) identify a generic RL grid impedance, even under unbalanced conditions; 2) reduce the distortion induced by the excitation signal by relying on the observer to triggering the injection when a grid impedance change is detected; and 3) identify grid impedance values much lower than the converter filter impedance, which is the usual situation when the converter rated power is well below the grid rating. Finally, it is worth pointing out that the proposed estimation technique is well-suited to be incorporated into an adaptive current controller scheme.

Sequence Impedance Modeling and Converter-Grid Resonance Analysis Considering DC Bus Dynamics and Mirrored Harmonics

Abstract: Sequence impedance models have been developed for two-level voltage source converters by assuming constant dc bus voltage and ignoring coupling over frequency effects. There have been efforts to overcome these limitations but the resulting models are very complex and the formulated system models require generalized Nyquist criterion for stability analysis. This paper presents a new modeling and system analysis method that solves these problems. The method starts by modeling the converter current response at different frequencies to a perturbation in the ac terminal voltage while keeping the dc bus voltage constant, and to a perturbation in the dc terminal voltage while keeping the ac terminal voltage constant. The individual transfer functions are then connected to build the overall converter models that include both the effects of dc bus dynamics and coupling over frequency. For converter-grid system stability analysis, the effects of coupling over frequency are shown to be equivalent to an additional impedance in parallel with the sequence impedance. The developed models are validated individually as well as in system resonance analysis.