• Energy Research

A dc arc fault can be a serious threat to the safe operation of a low-voltage dc system. Typically, a fair amount of electromagnetic radiation (EMR) accompanies the arcing process. To investigate the characteristics of EMR of the series dc arc fault, a fourth-order Hilbert fractal antenna is used to detect the EMR signals generated by the dc arc. The factors influencing the characteristics of EMR including electrode material, diameter, current, and pressure are considered. The amplitudes of the EMR pulses at the arc initiation moment are measured, and fast Fourier transform is used to analyze the characteristics of the frequency spectrums. The test results indicate that the amplitudes of the EMR pulses generated by stainless steel electrodes are higher than those generated by brass, copper, and aluminum electrodes. Furthermore, the trends of the amplitudes of the EMR pulses with increasing current are different for electrodes with different diameters. Finally, it is observed that the influence of the pressure is different for different electrode materials. The characteristic frequency band of the EMR pulses of the dc arcs generated under various test conditions is in a certain range (36–41 MHz), which can be used as a detecting characteristic parameter for the series dc arc.

Dry-type air-core reactor used as AC filter reactors is one of the main noise sources in HVDC converter stations. The research on the vibration characteristics of the dry-type air-core reactor is of great significance for the design of noise reduction measures. Based on the assumption of linear vibration system, IEC 60076-6:2007 suggests to use sum frequency, difference frequency and double frequency of the loading current frequency as the decomposition of the total excitation, to measure the acoustic sound level of dry-type air-core reactor. To verify the assumption, this paper aims at the nonlinearity of the dry-type air-core reactor vibration system. The radical vibration of the tested reactor was firstly measured by a laser Doppler vibrometer. The measurement results show that: under single-frequency current, except for the double frequency, the vibration spectrum also contains quadruple frequency and the sixth harmonic frequency components; while under multiple frequency current, other frequency components also exist besides the sum, difference and double components of the current frequency. The ultra harmonic phenomenon is a typical feature of nonlinear systems. Then the vibration equation was also theoretically established. Considering the coupling of magnetic field and vibration, the perturbation method is used to solve the vibration equation. The theoretical analysis results are consistent with the experiments. It is concluded that the nonlinearity of the vibration system of dry-type air-core reactors is not negligible, and one cause of the nonlinearity is the coupling of the magnetic field and the motion of the reactor.

This paper presents a heat transfer matrix and superposed thermal field-based method that can rapidly calculate steady state temperature rises in three-core cable groups laid in ducts. The temperature rise of any cable is a combination of self-reaction and interaction from other cables. A heat transfer matrix is formed by self-reaction coefficient and interaction transfer coefficient representing the thermal characteristics of the cable groups in ducts. The heat transfer matrix can be obtained by enough samples including temperature rise and cable losses based on numerical analysis, trial or commercial software. Rapid calculations can then be performed by simplify iteration of temperature-loss. Extraction of the transfer matrix using CYMCAP ampacity and temperature calculation software and application of the fast calculation method are illustrated and compared, confirming the feasibility and relative simplicity and accuracy of the fast calculation method. This paper presents a heat transfer matrix and superposed thermal field-based method that can rapidly calculate steady state temperature rises in three-core cable groups laid in ducts.

Ladder network parameters identification of transformer winding based on frequency response analysis data is crucial to winding fault diagnosis. In the authors’ previous study, the Gauss–Newton iteration algorithm (GNIA) had been proposed to efficiently identify the network parameters. The core in GNIA is the Jacobian matrix, composed of derivative of frequency response function (FRF) to network components. This study proposes a generic algorithm to calculate this Jacobian matrix. Two key problems concerning the algorithm are solved elaborately. The first problem is the method for derivative calculation toward an arbitrary FRF based on adjoint network method (ANM). The other issue is the mathematical model construction of double ladder network and its adjoint network toward different FRFs to obtain all network branch voltages and currents, which are required in the derivative calculation by ANM. This generic algorithm can efficiently and effectively calculate the Jacobian matrix, which can be applied on ladder network parameters identification and winding fault diagnosis.

Transfer from monopolar ground return mode to monopolar metallic return mode is essential and critical for HVDC transmission systems. During the transfer, the metallic return transfer breaker plays the key role to confirm the success of the transfer. To study the transient characteristics of the systems when metallic return transfer breakers (MRTBs) operate, we construct a detailed model of a MRTB by accounting for dynamic resistance of the switching arc. Also, the transfer failure caused by arrester fault is simulated and analyzed. The simulation results show that decrease unbalanced energy distribution of arresters can cause breakdown of arrester, which is a most common reason of transfer failure. The findings provide reference for improvement of parameters design of MRTB.

Arc faults threaten the safe operation of photovoltaic (PV) systems. An arc fault detection and localization approach using parallel capacitors is proposed. A PV system has been analyzed and tested with five capacitors paralleled with the branches in the system. Series and parallel arc faults at nine locations have been tested in the system. When an arc occurred, current pulses were generated in the capacitors and their amplitudes and polarities were obtained through Hall current sensors. Discrete wavelet transformation was performed on the capacitor currents and the distributions of their amplitudes, frequency spectrums, and polarities are here reported. The results indicate that the distributions are unique under different fault types and locations, which could be used to detect and localize arc faults in PV systems. Moreover, the amplitudes of the capacitor currents can also help to localize a series arc fault within a PV string. Finally, the proposed approach is validated by a double-fault test.

Vibration of AC filter capacitor generates loud noises in HVDC converter stations, which would reach 105dB. Currently, vibration features are not investigated thoroughly. The objective of this work was to measure vibration velocity distribution on AC filter capacitor surfaces and to investigate the vibration features. In the experiments, the capacitor was applied currents consist of three components of 50 Hz, 550 Hz and 650 Hz. The root mean squared values were 30 A, 25 A and 4.6 A, respectively. A portable digital vibrometer was conducted to measure the vibration velocity on capacitor surfaces. In order to obtain velocity data simultaneously, voltage squared was used as a datum signal to calibrate the time offset. It was found that velocities on different measure points on a capacitor surface were not synchronous, and had different phases. On the bottom surface, vibrations performed entirely transverse oscillating. On the broad side surfaces, there were two bending wave generated from the edges connected to bottom and top surfaces. It was concluded that the internal vibration of elements excited firstly on the bottom surface, and then through the edges of bottom and broad side surface, it transmitted on broad side surfaces. A damping structure was designed and applied on the bottom surface to increase the damping coefficient between contact surfaces and to reduce vibration transmitted to broad side surfaces. The sound measurement result shown that the damp-structure capacitor had sound power which was 73.3% of that generated from the conventional capacitor.

In this paper, a method based on time domain response and superposition principle of heat conduction field is studied in this paper. It can realize the quick calculation of transient temperature rise of direct buried cable. The combined action of cable groups is discrete as a combination of multiple cables alone. Self — heating of a single cable can be described by a self — response model. The mutual influence between the cable can be described by mutual response model. And then the integrated temperature rise of the cable core can be obtained through node temperature rise coupling method. So the fast calculation of the transient temperature rise is realized. In the example verification, the model parameters are extracted by using the common software CYMCAP. And then the transient temperature rise process of the typical working condition can be calculated. Finally, compared with the software results, the validity of the method is verified.

Audible noise of can-type filter capacitors in high- voltage direct current converter stations originates from the vibration of capacitor cases, so it is meaningful to investigate the vibration characteristics. However, no experiment under practical excitations or thorough theoretical analysis was presented for the vibration characteristics. This study theoretically and experimentally investigated the vibration characteristics of can-type filter capacitors with stacked elements parallel to bottom. The vibration amplitude was found to be proportional to voltage squared. If the frequency spectrum of excitation voltage included several harmonics (more than three), vibration amplitude on some specific frequencies was related with the phases of these voltage components. Each capacitor surface had a concavo-convex vibration shape and the vibration of the bottom surface and the side surfaces were in opposite phases. The vibration amplitude of the bottom surface was the largest. Moreover, frequency characteristics of capacitor vibration were studied. The frequency response of the bottom surface was bigger than that of narrow and broad side surfaces for lower frequencies, while was smaller for higher frequencies. It was inferred that bottom surface vibration was mainly caused by elastic forces from direct contact and side surfaces vibration mainly came from damping forces from dielectric liquids.