Characteristics Of Impedances Research Articles

Improved scheme of grid-connected inverters based on virtual PCC and impedance remodeling with enhanced steady-state and dynamic performance

The issue of low-frequency oscillation (LFO) becomes more prominent when considering the phase-locked loop (PLL) impact of grid-connected inverter (GCI) under weak grid. Impedance analysis shows that the frequency interaction point outside the capacitive negative damping region can effectively avoid the oscillation. Therefore, a modified PLL scheme is designed to shape the characteristics of the GCI output impedances, which effectively tracks the grid phase through the virtual point of common coupling and adjusts the phase- frequency characteristic of the GCI equivalent output impedance towards an increasing phase angle. On this basis, the impedance remodeling strategy is used to improve the system, which further broadens the feasible range of stable operation and ensures the phase margin of the GCI. The comparison results indicate that proposed scheme demonstrates enhanced capability in suppressing LFO and exhibits a wider range of adaptability to grid impedance. Experimental validation has verified the accuracy of the theoretical analysis, leading to the conclusion that the GCI with proposed scheme possesses superior dynamic performance.

Analytical method of DC input impedance for stability analysis of IPM motor drive system

AbstractThis paper proposes an analytical calculation method of frequency characteristics of DC input impedances of an IPM motor drive system for design and stability analysis by the impedance method. The proposed method is based on an averaged frequency model linearized at an operating point. The developed small‐signal model considers control principles and mechanical perturbations. It is an analog and digital mixed model which is controlled by a digital signal processor system. Analysis examples of frequency characteristics for the impedance method by the proposed method are investigated and validated analytically and experimentally.

Permanent fault identification and location scheme for MTDC grid based on equivalent impedance phase characteristics and distributed parameter line

To ensure the reliability of the MTDC grid's recovery system during the reclosing stage, identifying permanent faults is crucial. Therefore, a permanent fault identification and location scheme is proposed based on equivalent impedance phase characteristics and distributed parameter lines. The equivalent impedances with distributed parameter lines in permanent and temporary faults are investigated. The proposed scheme for identifying a permanent fault is based on the phase characteristics of the equivalent impedances. The proposed scheme for permanent fault location is achieved by solving the fault location equation considering distributed parameter lines to determine the fault distance. Simulation results indicate that the proposed scheme is suitable for long-distance DC transmission lines and is less affected by the sampling rate, fault locations, fault resistance, and noise. The proposed scheme improves the reliability and sensitivity of permanent fault detection.

A Convolution Power-Based Protection Scheme for Hybrid Multiterminal HVDC Transmission Systems

Compared with conventional HVDC systems, the hybrid multiterminal HVDC (Hybrid-MTDC) system has a more sophisticated network topology composed of line commutated converter (LCC) and modular multilevel converter (MMC) stations, which leads to dc fault discrimination being difficult and ambiguous. Furthermore, for long transmission lines, high fault resistances, and different converter impedance characteristics, the reliable discrimination of external and internal faults located at the remote end and T areas is particularly demanding. Therefore, considering the frequency characteristics of dc boundaries and equivalent converter impedances and dc line models, this article constructs a time-domain convolution power. The performance of the convolution power in various fault scenarios is mathematically elaborated. Due to its good decomposition ability for short-time nonstationary signals, wavelet decomposition is utilized to extract the specific frequency bands (FBs) of traveling waves; thus, the convolution power in different FBs can be calculated. On this basis, a convolution power-based protection scheme is proposed for identifying external and internal faults, locating a dc bus fault, and selecting faulty poles. PSCAD/EMTDC simulation results show that various dc faults can be correctly identified within 3 ms, and the proposed scheme has high reliability and sensitivity, which is beneficial for the protection of Hybrid-MTDC systems.

Fault Analysis and Design of a Protection System for a Mesh Power System with a Co-Axial HTS Power Cable

The uses of high-temperature superconducting (HTS) cables pose a challenge of power system protection since the impedance of the HTS cable is varied during fault conditions. The protection systems should be designed properly to ensure the reliability and stability of the whole system. This paper presents a fault analysis of the co-axial HTS cable in the mesh system and proposes a coordinated protection system. In the proposed protection system, the main protection is the differential current relay whereas the backup protections are the overcurrent and directional overcurrent relays. The normal and abnormal relay operations are considered to analyze the transient fault current in the HTS cable and evaluate the performance of the proposed coordinated protection system. Characteristics of cable impedances and temperatures under various fault conditions are presented. The proposed protection scheme is validated by the simulation in the PSCAD/EMTDC program. Simulation results show that the coordinated protection scheme could successfully protect the HTS cables in both normal and abnormal relay operations.

Development of negative-group-delay circuit for high-frequency ultrasonic transducer applications

Compared with low-frequency (<15 MHz) ultrasonic transducers, high-frequency ultrasonic transducers provide higher spatial resolutions while reducing the echo-signal amplitudes and bandwidths owing to the size reduction of the piezoelectric materials. Therefore, performance improvement of high-frequency ultrasonic transducers can enhance the performance of the whole ultrasound instrument. To improve the echo-signal quality, such as the amplitudes, signal distortions, and bandwidths, negative-group-delay circuits for high-frequency ultrasonic transducers have been applied. The components of the negative-group-delay circuit can be constructed using the measured magnitude and phase characteristics of the electrical impedances of the ultrasonic transducers. Therefore, the relationship between the negative group-delay circuit and ultrasonic transducers must be analyzed to identify the adequate components of the negative-group-delay circuit. To confirm the proposed idea, the performances of negative-group-delay circuits were measured and compared. The differences in the measured phase angles of the electrical impedances between the ultrasonic transducer and the combined resistor limiter, negative-group-delay circuit, and preamplifier (including a coaxial cable) were 0.66°, 1.31°, and 9.45° at the resonant, center, and anti-resonant frequencies, respectively (<10°). The calculated and measured maximum negative group delays with the resistor limiter and preamplifier were –21.64 ns at 31.65 MHz and –17.41 ns at 27.38 MHz, respectively.To verify our concept experimentally, we performed a typical pulse-echo test using the designed negative-group-delay circuit integrated with a preamplifier and using the preamplifier alone. The peak-to-peak amplitude, pulse width, –6-dB bandwidth, and total harmonic distortions of the echo signal for the negative-group-delay circuit with the preamplifier (0.232 Vp-p, 0.36 μs, 81.12%, and –60.72 dB, respectively) were improved compared with those for the preamplifier alone (0.154 Vp-p, 0.74 μs, 51.98%, and –28.61 dB, respectively). Therefore, the negative-group-delay circuit is useful for improving the echo-signal quality of high-frequency ultrasonic transducers. The proposed approach is simple and effective for improving the echo-signal quality of ultrasonic transducers without increasing the performance of the power amplifiers and preamplifiers.

A Simple Method to Determine Synchronous Machine Quantities Using Frequency Characteristics of Operational Impedances

SUMMARYThis paper presents a simple method for determining synchronous machine quantities: d‐ and q‐axes time constants and reactances. This method determines them only by drawing additional lines in the frequency characteristics of operational impedances. A new systematic drawing strategy for determining transient/subtransient open‐circuit time constants and the d‐axis transient reactance is proposed. The frequency characteristics of operational impedances are obtained by the standstill dc test using a small dc power supply. Since the rotational test becomes unnecessary, the proposed method is suitable for tests in a factory. The validity of the proposed method was demonstrated with a numerical calculation example on a large‐capacity machine (800 MVA, 25 kV, two poles, 60 Hz) and an implementation test on a small‐capacity machine (10 kVA, 200 V, 31.9 A, four poles, 50 Hz).

演算子インピーダンスの周波数特性を利用した同期機諸定数の簡易算出法

This paper presents a simple method for determining synchronous machine quantities: d- and q-axes time constants and reactances. This method determines them only by drawing additional lines in the frequency characteristics of operational impedances. A new systematic drawing strategy for determining transient/subtransient open-circuit time constants and the d-axis transient reactance is proposed. The frequency characteristics of operational impedances are obtained by the standstill DC test using a small DC power supply. Since the rotational test becomes unnecessary, the proposed method is suitable for tests in a factory. The validity of the proposed method was demonstrated with a numerical calculation example on a large-capacity machine (800MVA, 25kV, 2 poles, 60Hz) and an implementation test on a small-capacity machine (10kVA, 200V, 31.9A, 4 poles, 50Hz).

토중방전을 수반하는 접지봉의 임펄스응답에 대한 기초적 연구

This paper deals with the fundamental characteristics of transient and conventional ground impedances associated with soil discharges when the lightning impulse voltage was applied to a small-sized ground rod. The potential rise responses of the test ground rod to impulse currents were observed. The current-voltage (I-V)curves, transient and conventional ground impedance curves were examined.. To analyze the transient behaviors of ground rod attendant upon soil discharges, the impulse responses of ground rod were compared with those of non-inductive resistor. As a consequence, the potential rise and impedance of ground rod were reduced and the currents dispersed into the earth were increased due to soil discharges near the ground rod. Soil discharges are classified into two regimes; ionization and breakdown phenomena. The potential rise and impulse impedance of ground rod are significantly decreased by breakdown processes in soil.

An approach to measurements of electrical characteristics of lithium-ion battery with open-circuit voltage function

Comprehension of open-circuitvoltage (OCV) function and battery impedance can be used to evaluate the residual energy and dynamic voltage response of lithium batteries. However, noise contamination during data sampling may affect the accuracy of the derived OCV function and battery impedance extraction, resulting in a poor evaluation performance. By aiming at formulating a more representative circuit-based model, this study proposes a hybrid averaging method along with an impedance network observation approach that is embedded within a time-frequency localisation method, anticipating eliminating the influences of sampling noise such that the characterisation of the OCV function and battery impedances can be better ensured. In such an integrated approach, the trimmed relevance vector regression is also added, which is followed by two cascaded R-C networks in order to fit the characteristics of battery impedances. Through the method proposed in this paper, it is found that the average error can be decreased to be less than 20 mV. Both battery residual energy and I-V performance are more effectively comprehended.

An analysis on the Frequency-dependent grounding impedance based on the ground current dissipation of counterpoises in the two-layered soils

Abstract Because of the increased usage of power conversion equipment and their susceptibility to lightning strikes, high frequency fault currents flowing through grounding systems are increased. As grounding systems change from individual groundings to common or integrated groundings, the improved performance of grounding systems against high frequency fault currents is urgently required. When regarding the common or integrated grounding systems, most of them are required to satisfy the ground resistance defined by standards or regulations. The grounding system performance against high frequency fault currents is defined by the grounding impedance. A number of studies have reported that grounding system performance is reduced due to the increase of the grounding impedance with increasing of the frequency of fault currents. In this paper, in order to investigate the characteristics of the frequency-dependent grounding impedances of counterpoises installed in the two-layered soils, the ground current dissipation rates in each 10 m section of the counterpoises were measured and analyzed as functions of the length of grounding electrodes and the current injection point. As a result, most of the ground currents above the frequency of 100 kHz are dissipated into the earth near the current injection point and the high frequency grounding impedance of long counterpoises is converged into that of short counterpoises. The high frequency grounding impedance measured at the low soil-resistivity end of the counterpoise buried in the two-layered soils is much lower than that measured at the high soil-resistivity end.

A Study on the Harmonic Contributions of Residential Loads

This paper presents a measurement technique to determine the harmonic sources and impedances of residential houses at the utility metering point. The results are then applied to quantify harmonic and current contributions of the residential premises. Four residential houses are investigated by using the proposed method. The characteristics of the load-side harmonic impedances and sources are studied, and their harmonic contributions are determined. The results show that voltage distortion is affected mainly by background harmonic sources that exist within the supply system. The current harmonics are affected by the residential loads and the supply system.

THE INFLUENCE OF A NON-RIGID BAFFLE ON INTERMODAL RADIATION IMPEDANCES OF A FLUID-LOADED BILAMINAR PLATE

Theoretical formulae for intermodal radiation impedances (including self- and interaction radiation impedances) are derived for a bilaminar rectangular plate which is mounted on a non-rigid baffle and immersed in a fluid. Green function and Fourier transform methods are used to solve the governing vibration equation. Numerical calculations of the intermodal radiation impedances of a square plate are presented for different baffle impedances, and the effects of baffle impedance on the acoustic radiation impedance are discussed. The results show that intermodal radiation impedances are strongly dependent on the baffle impedance. For different baffles, the frequency characteristics of the intermodal radiation impedances vary. For certain baffle impedances they are highly frequency dependent at low frequencies but asymptotically approach a constant at high frequency. It is concluded that it is possible, by means of a judicious choice of baffle material, to control sound radiation in the near field.

Variational model for the radiation impedance of class IV transducers II

In a previous paper [H. C. Robinson and E. A. McLaughlin, 3318(A) (1994)], the variational method of determining modal radiation impedances was applied to a pair of class IV transducers. The self‐ and mutual radiation impedances for the quadrupole, octopole, and monopole modes were presented. This paper extends the analysis to include the effects of other asymmetric modes of the class IV. The in‐vacuo mode description of the transducer’s surface velocity will be improved by using three‐dimensional mode shapes. The trial functions describing the surface pressure are modified as well. Changes in the impedance characteristics for the symmetric modes due to this improved transducer description will be shown. The characteristics of the nonsymmetric modal impedances are compared to those of the symmetric modes as a function of dimensionless major axis ka and dimensionless separation distance kd. Suitable methods for combining these modal impedances into a total radiation impedance for the transducer will be presented. The effect of relative orientation of the transducers on the radiation impedance is investigated. The results will be compared to equivalent circuit models and experimental data. [This work was sponsored by the Office of Naval Research.]

Resonance analysis of figure-8 coil type permeance meters

An equivalent circuit for a figure-eight coil type permeance meter is derived to analyze its LC resonance characteristics. The equivalent circuit for the pickup coils is derived using the same techniques used for thin film inductor analysis. The driving sheets and twisted lead wires are modeled according to the theory of distributed constant networks. Permeance meters are made, the frequency characteristics of the coils' impedances are measured, and the validity of the equivalent circuit used is demonstrated. The LC resonance caused by the stray capacitance between the pickup coils and the driving sheets is discussed. >

Analysis and application of NRD-guide excitors

In this paper, the NRD-guide excitors using a length of coaxial line which consists of a rectangle external conductor and a thin round inner conductor are presented, and the characteristics of the input impedances of LSE 11 o mode and LSM 11 o mode at the insertion points are analysed and calculated by using the method of the rector Green's function respectively. The computation results show that the mode excitors are able to improve the operating modes matching between NRD-guide and the quasi-TEM transmission line. The structures are suitable for fabrication LSE 11 o mode oscillator and LSM 11 o mode oscillator to compose millimeter wave NRD-guide integrated front ends. An example of an LSE 11 o mode oscillator in Ka-band is given. Its oscillation frequency and output power could be easily adjusted by changing the parameters of the excitor.

単杭の鉛直・水平振動特性の一簡易評価法

For single piles under vertical and horizontal ,vibrations, dynamic Winkler stiffnesses of subgrade reaction on pile circumference and at pile tip are proposed as simple expressions. A simplified estimation for vibrational characteristics of impedances and receptances at pile head presented by solving equations of motion of the pile as rod and Timoshenko's beam and by introducing the proposed subgrade reactions. It is verified that this method is sufficiently accurate in the practical situation from comparison with the rigorous and other solutions and this application to a building suported on piles in a case history is readily useful.

Corrigenda for vol 71, Pages 259-270

Pages 259–270: M. Mishima, K. Kawakami, K. Higashiya, T. Fukunaga, T. Ooka, and K. Kuno. “Frequency characteristics of airway and tissue impedances in respiratory diseases.” Page 262: A duplicate of Fig.5 was printed in place of Fig.4. We regret the error. Figure 4 should appear as follows:

Frequency characteristics of airway and tissue impedances in respiratory diseases

We measured the frequency characteristics (at 10-40 Hz) of airway (Za) and tissue (Zt) impedances in cases of chronic obstructive pulmonary disease [asthmatic bronchitis (AB), chronic pulmonary emphysema (CPE)] and interstitial pneumonitis (IP) by use of an improved random noise oscillation and body box method. The results were then compared with those obtained for normal subjects. The real part of Za was markedly elevated in patients with AB but only slightly elevated in those with CPE. To interpret these data we used an electromechanical analogue including serial inhomogeneity with shunt impedance. From this model we concluded that AB causes both the central and peripheral airway resistances to increase, while CPE brings about a rise mainly in peripheral resistance. In IP patients, only the imaginary part of Zt decreased, which might reflect the decrease in both lung and chest wall compliance. In CPE patients, but not in AB patients, the real part of Zt fell. These data were consistent with the assumption that the decrease in mass per unit volume of lung tissue and hyperinflation of the chest wall in CPE patients might lower the tissue resistances.

Nonlinearities in the charge-division encoding of position-sensitive detectors

The equations governing the charge-division encoding of a position-sensitive detector, represented as a distributed resistance-capacitance line with realistic end loading, are developed and discussed. Sources of the differential nonlinearity in the position encoding are identified and related to characteristics of the dynamic load impedances, and four practical means for reducing or eliminating such nonlinearities are proposed. The various methods for reducing nonlinearities are evaluated both by numerical solution of the circuit equations and by testing with an operating position-sensitive neutron detector and encoding system. All measurements and calculations show close agreement with one another and with the theory.