Coherence and time-frequency analysis of impulse voltage and current measurements

The characteristics of SF6 gas in withstanding impulse voltage within UHV GIS and GIL systems determine their insulation design. This paper established an impulse breakdown voltage test platform to investigate the impulse breakdown properties of SF6 gas under varying gas pressures, voltage polarities, and degrees of electric field non-uniformity. Test results indicate: Under slightly non-uniform and extremely non-uniform fields, the polarity effect on SF6 gas impulse breakdown field strength is pronounced. In slightly non-uniform fields, positive-polarity breakdown field strength exceeds negative-polarity values. In extremely non-uniform fields, negative-polarity breakdown field strength surpasses positive-polarity values. As the electric field non-uniformity coefficient increases, impulse breakdown field strength generally exhibits a trend of rapid initial decline followed by gradual decrease. In extremely non-uniform fields with positive polarity voltage, the SF6 switching impulse breakdown field strength exhibits a hump effect with increasing gas pressure, reaching its minimum value at 0.6 MPa.

Phase sequence exchange (PSE) is an emerging emergency control method for power systems. However, PSE will generate impulse current and voltage. In this research, to suppress the impulse of PSE, a phase control method for zero‐current opening and equal‐voltage closing is adopted, and verified via both physics and experiments. First, an equivalent circuit is established, and the differential equations are solved to validate that this phase control method is the optimal method for the suppression of the impulse current and voltage in PSE. Then, the action sequence scheme and calculation method of the phase control method are advanced. Finally, based on the real‐time digital simulation system (RTDS), a PSE prototype was used to carry out impact experiments. The results verify that the performance of the optimal method is the best compared to those of other control methods, the requirement for the sampling frequency is low, and there is almost no additional cost. In the two experiments, the first peak of the impulse current was found to be reduced by 47.49% and 33.28% on average, respectively, and the first peak of the impulse voltage was found to be reduced by 14.89% and 44.10% on average, respectively.

This paper presents verification of high voltage measuring systems by time convolutions between standard testing waveforms and unit step responses of the measuring systems. The waveforms under investigations are full lightning impulse voltage waveforms, tail-chopped lightning impulse voltage waveforms, front-chopped lightning impulse voltage waveforms and steep-front voltage waveforms according to IEC standards 60060-2 and 61211. This paper considers four different types of the measuring systems. The results show that this technique is applicable, and should be a useful method for impulse voltage measurements in a standard high voltage

Whereas high resolution digital recorders are used in recording the impulse voltage tests in prominent accreditation laboratories worldwide, a usage of an 8 bit recorder is also allowed by IEC publication. An 8 bit recorder is, due to its nature, known to record a waveform with 0.4% vertical resolution and it has not been appreciated for determining a peak value of lightning impulse in the reference measuring system where the scale factor is often evaluated with 0.1% or less uncertainty. This paper demonstrates simulation in which an 8 bit record with an internal noise is fed into the lightning impulse parameter determination software and the computed parameters were compared with theoretical values. The results show that, if suitable software is used, errors in parameters deduced from an 8 bit record is as small as those derived from a 12 bit record, suggesting an inexpensive 8 bit recorder can replace a 12 bit in impulse voltage measurements.

Doppler signals from a disturbed arterial blood flow are very likely to be nonstationary for they usually have a time-varying frequency structure caused by a complicated flow pattern. Therefore conventional Fourier analysis based methods are unsuitable for this application. This paper introduces the application of joint time-frequency analysis (TFA) to estimate the frequency dynamics of arterial Doppler blood flow signals and to detect the presence of small stenoses of the artery. The short time Fourier transform (STFT), Wigner distribution (WD), and Choi-Williams distribution (CWD) were investigated. The results of synthetic signal analysis show that the STFT, WD and CWD follow the transient changes of frequency well, and the STFT and CWD also behave well for multicomponent signals. Doppler spectra were obtained by means of the FFT, STFT, WD, and CWD for different flow rates and positions along a simulated blood vessel for stenoses ranging from 2% to 25% cross sectional area reduction (CSAR) using a blood analogue. The results indicate that all of these methods are to some extent capable of detecting stenoses as small as 2% CSAR by identifying the existence of negative frequency components in the spectra, and the CWD appears at this stage to be superior to the STFT and WD.

Fiber grating sensors have been widely used in industry and has been an important research topic in power systems due to their small size, satisfactory electric shielding ability, and high compatibility with optical fibers. Voltage is a crucial parameter that must be monitored in the power system. Existing PZT-FBG voltage sensors are small, inexpensive, and present acceptable linearity in the range of 50 Hz-20 kHz. However, voltages in power systems cover signals with a frequency from DC to MHz and the existing frequency range is insufficient. Finite element method is adopted to improve frequency characteristics of the PZT-FBG sensor and choose the optimal basic structure by initially calculating resonant frequencies. Structural parameters are then selected via dielectric spectrum analysis. The test platform is established to verify amplitude response, frequency response, three-wave response, temperature characteristics, and impulse voltage response of the frequency-optimized PZT-FBG sensor. Test results showed that the sensor presents satisfactory linearity and the temperature-independent demodulation method improves its stability with temperature changes. Impulse voltage tests indicated that the sensor can respond to impulse waveforms with a wave front time of 250 and 10 μs. Hence, the sensor can respond to a voltage signal of 100 kHz with acceptable consistency. The optimization achieved in this study can provide a reference for further improvement of this sensor and other similar sensors with PZTs.

Independent grounding electrodes are often set in the grounding system for lightning protection. When lightning current enters the ground through the independent grounding electrode, it will cause electromagnetic interference to the nearby secondary cables. When lightning flows into the ground, it is usually accompanied by the generation of soil discharge channels. And the randomness of channel spatial structure will affect the transient electromagnetic field distribution, thereby affecting the electromagnetic interference process. In order to explore the influence of soil discharge on the capacitive coupling process of electromagnetic interference of secondary cables in the ground, an experimental observation platform is built to carry out the simulation experiment of cable capacitive coupling under impulse current, the image data of soil discharge channel and the voltage and current waveform of grounding electrode and shield of cable are collected. The results show that the capacitive coupling current waveform of the shield of cable is consistent with the first derivative law of impulse voltage waveform of grounding electrode; The spatial structure of soil discharge channel and the water and salt content of surrounding soil have a significant effect on the capacitive coupling current.

Narrow-band interference (NBI) is a critical issue for synthetic aperture radar (SAR), in which the imaging quality can be degraded severely. To suppress NBI effectively, a novel interference suppression algorithm using robust principal component analysis (RPCA) based signal separation in time-frequency domain is proposed. The RPCA algorithm is introduced for signal separation in the time-frequency domain for the first time. The fundamental assumption of RPCA is that a matrix can be modeled as a combination of a low-rank matrix and a sparse counterpart. In terms of the SAR echo, the short time Fourier transformation (STFT) matrix of mixed signals (i.e., useful SAR signals and NBIs) well fits the assumption of RPCA. Based on this property, radar echoes are first transformed into the time-frequency domain by STFT to form an STFT matrix. Then, the RPCA algorithm is used to decompose the STFT matrix into a low-rank matrix (i.e., NBIs) and a sparse matrix (i.e., useful signals). Finally, the NBIs can be reconstructed and subtracted from the echoes to realize the interference suppression. The experimental results of simulated and measured data demonstrate that the proposed algorithm not only can suppress interference effectively, but also preserve the useful information as much as possible.

Measurement error which is caused by the position of reference ground point and strong electromagnetic interference is introduced in detail. Based on the results, reliable and effective methods for anti-interference, such as surge absorption circuit and floating measurement, were applied to measurement unit. As the results, in the impulse current system the measuring accuracy can be assured and the performance of electromagnetic compatibility can be enhanced.

Health care applications of autoidentification technologies, such as radio frequency identification (RFID), have been proposed to improve patient safety and also the tracking and tracing of medical equipment. However, electromagnetic interference (EMI) by RFID on medical devices has never been reported. To assess and classify incidents of EMI by RFID on critical care equipment. Without a patient being connected, EMI by 2 RFID systems (active 125 kHz and passive 868 MHz) was assessed under controlled conditions during May 2006, in the proximity of 41 medical devices (in 17 categories, 22 different manufacturers) at the Academic Medical Centre, University of Amsterdam, Amsterdam, The Netherlands. Assessment took place according to an international test protocol. Incidents of EMI were classified according to a critical care adverse events scale as hazardous, significant, or light. In 123 EMI tests (3 per medical device), RFID induced 34 EMI incidents: 22 were classified as hazardous, 2 as significant, and 10 as light. The passive 868-MHz RFID signal induced a higher number of incidents (26 incidents in 41 EMI tests; 63%) compared with the active 125-kHz RFID signal (8 incidents in 41 EMI tests; 20%); difference 44% (95% confidence interval, 27%-53%; P < .001). The passive 868-MHz RFID signal induced EMI in 26 medical devices, including 8 that were also affected by the active 125-kHz RFID signal (26 in 41 devices; 63%). The median distance between the RFID reader and the medical device in all EMI incidents was 30 cm (range, 0.1-600 cm). In a controlled nonclinical setting, RFID induced potentially hazardous incidents in medical devices. Implementation of RFID in the critical care environment should require on-site EMI tests and updates of international standards.

Recently IEC TC 81 decided to set up PWI 62561-8 in order to extend the IEC 62561 family (Lightning Protection Components) by a new part 8 (Requirements for components for isolated LPS). The paper works out the basic electric characteristics of components of isolated LPS which require standardization. It describes and discusses test procedures that cover the dielectric strength and the capability to withstand lightning current of components of isolated LPS in order to link the possibilities of (new) components for isolated LPS with the requirements of the IEC 62305 series. With the definition of a new wave shape 1/200μs which takes the negative first lightning stroke into account , some new approaches and requirements are considered in the test procedures. The paper briefly revisits the basics of the calculation of separation distances according to the current standards - impulse-voltage-time characteristic and the constant-area-criterion. Based on these facts a simplified high voltage impulse test is derived which works with the standardized voltage impulse 1.2/50 μs. This is a high voltage impulse test to cover the needs for testing the dielectric strength of the non-metallic or composite component design of an insulating fastener and the dielectric strength of the insulating layer of cables used as insulated down conductors. The same test also covers the creepage discharge behavior of the potential grading at insulated down conductors. A high current impulse test is discussed in order to classify insulated down conductors due to their lightning current withstand capability. Finally these tests procedures had been applied to insulating fasteners and cables used as insulated down conductors. The preliminary results of these tests are presented and discussed by means of their applicability for standardized testing. The aim of this contribution is to help to provide an acceptable, simple and physically correct basis for suitable electrical test procedures that support the development of the new 62561-8 standard.

Spinal somatosensory evoked potential (SSEP) has been employed to monitor the integrity of the spinal cord during surgery. To detect both temporal and spectral changes in SSEP waveforms, an investigation of the application of time-frequency analysis (TFA) techniques was conducted. SSEP signals from 30 scoliosis patients were analysed using different techniques; short time Fourier transform (STFT), Wigner-Ville distribution (WVD), Choi-Williams distribution (CWD), cone-shaped distribution (CSD) and adaptive spectrogram (ADS). The time-frequency distributions (TFD) computed using these methods were assessed and compared with each other. WVD, ADS, CSD and CWD showed better resolution than STFT. Comparing normalised peak widths, CSD showed the sharpest peak width (0.13+/-0.1) in the frequency dimension, and a mean peak width of 0.70+/-0.12 in the time dimension. Both WVD and CWD produced cross-term interference, distorting the TFA distribution, but this was not seen with CSD and ADS. CSD appeared to give a lower mean peak power bias (10.3%+/-6.2%) than ADS (41.8%+/-19.6%). Application of the CSD algorithm showed both good resolution and accurate spectrograms, and is therefore recommended as the most appropriate TFA technique for the analysis of SSEP signals.

A superconducting Toroidal Field Model Coil (TFMC) was developed by the European Home Team within the framework of the R&D program of the International Thermonuclear Experimental Reactor (ITER) and is presently tested in the TOSKA facility of the Forschungszentrum Karlsruhe. To withstand the Lorentz forces the 98 turns of the ITER TFMC are divided into 10 pancakes, embedded in five stainless steel (SS) radial plates and enclosed in a grounded SS case. To minimize the voltage stress of the conductor insulation the potentials of all five inner pancake joints are connected to their own radial plates. This insulation concept offers the possibility to test three kinds of insulation types: the conductor insulation between the conductor and the surrounding radial plates, the radial plate insulation between two adjacent radial plates, and the ground insulation between all radial plates connected with the conductor and the grounded case. The rated terminal to terminal voltage is 9.8 kV for a symmetrical grounding. Based on the rated voltage, the insulation design and the manufacturers' specifications, a DC and AC test program is executed after each set of defined insulation stresses on every type of insulation at room and cryogenic temperatures. In addition, partial discharge measurements are performed as a nondestructive diagnostic tool for the observation of the status of electrical insulation. It is well known from conventional high-voltage engineering that large electrical systems must be designed to withstand internal overvoltages caused by fast rise times of the applied voltage. Therefore calculations were performed, both in time and frequency domains, to describe the transient electrical behavior of the ITER TFMC. To prove the insulation strength and to verify the calculations, high voltage impulse tests with fast rise times (9.8 kV peak) and low voltage high frequency tests are performed.

As an important feature for micro-motion targets, micro-Doppler is widely used in parameters estimation and target classification. Aiming at the extraction of micro-Doppler, a method based on STFT (Short Time Fourier Transform, STFT) and RANSAC (Random Sample Consensus, RANSAC) is proposed. Firstly, the precession model of coneshaped target is established, and the parameterized expression of micro-Doppler is derived. Secondly, STFT is conducted on the echo to analyze the characteristic in time-frequency domain, and time-frequency maxima is used to acquire the time-frequency points set. Then, according to the parameterized expression, the prescribed fitting function is designed, and RANSAC is adopted to distinguish the valid micro-Doppler frequency points and the invalid points, and thus the micro-Doppler is estimated. Finally, the effectiveness of the proposed method is verified by experiments.

Chirp pulse compression technique can improve signal-to noise ratio of ultrasonic flaw signals. But its disadvantage is the presence of sidelobes which might mask nearby small flaws and make it difficult to separate multiple flaws. In this paper, time-frequency analysis (t-f analysis) is applied to ultrasonic chirp pulse compression signals for range resolution improvement. Short Time Fourier Transform (STFT) is adopted for t-f analysis, followed by the application of Minimization and Polarity Thresholding Algorithm to suppress the sidelobe. In order to achieve the low sidelobe level without suppressing the mainlobe, the STFT is calculated by varying the window length with frequency and both pseudo-twofold and pseudo fourfold frequency cos components obtained from the STFT are utilized. The method is applied to both simulated and measured ultrasonic flaw signals of chirp. The results show a good suppression of sidelobes and a further compression of pulse width with preserving both the polarity and the amplitude of signals.