Development of Broadband Resistive–Capacitive Parallel–Connection Voltage Divider for Transient Voltage Monitoring

The on-site measurement of transient voltages is of great significance in analyzing the fault cause of power systems and optimizing the insulation coordination of power equipment. Conventional voltage transformers normally have a narrow bandwidth and are unable to accurately measure various transient voltages in power systems. In this paper, a wideband parallel resistive–capacitive voltage divider is developed, which can be used for online monitoring of transient voltages in a 220 kV power grid. The structures of the high-voltage and low-voltage arms were designed. The internal electric field distribution of the high-voltage arm was analyzed. The influence factors and improvement techniques of the upper frequency limit were studied. The parameters of the elements of the divider were determined. The voltage withstand performances and scale factors under lightning impulses and AC and DC voltages, the temperature stabilities of scale factors and the step response and bandwidth of the developed voltage divider were tested. The results show that the deviations of the scale factors under various voltage waveforms and different temperatures ranging from −20 to 40 °C are within 3%. The withstand voltage meets the relevant requirements specified in IEC60071-1-2011. The step response 10~90% rise time is approximately 29 ns, and the 3 dB bandwidth covers the range of DC to 10 MHz.

Issues and Challenges for HVDC Extruded Cable Systems

The improved features of AC/DC converters, the need to enhance cross-country interconnections, the will to make massive remote renewable energy sources available, and the fear of populations about overhead lines have fostered HVDC cable transmission all over the world, leading in the last two decades to an exponential increase of commissioned HVDC cable projects, particularly of the extruded insulation type. Comprehensive surveys of the issues to be faced by HVDC extruded cable systems appeared in the literature some years ago, but they are not so up-to-date, as HVDC extruded cable technology is developing fast. Therefore, the contribution this paper aims at giving is a systematic, comprehensive and updated summary of the main present and future issues and challenges that HVDC cable systems have to face to further improve their performance and competitiveness, so as to meet the growing quest for clean and available energy worldwide. The topics covered in this review–treated in alphabetical order for the reader’s convenience–are accessories, higher voltage and power, laying environment (submarine and underground cables), modeling, multiterminal HVDC, operation and diagnostics, recyclable insulation, space charge behavior, testing, thermal stability, transient voltages.

Suppression of very fast transients in 245 kV gas insulated substation

Very Fast Transient Over voltages (VFTOs) generate during operation of the disconnector switches or circuit breakers in a gas insulated substation (GIS). These transient voltages stress the insulation system in GIS and further result in mal-operation of the control circuitry as well as the secondary equipment connected to it. Since the VFTOs affect the reliability of GIS, it is necessary to mitigate them. This paper presents the mitigation of the very fast transient over voltages and the associated very fast transient currents (VFTCs) using the nano-crystalline rings in a 245 kV GIS. The mitigation in the peak amplitude of VFTOs and VFTCs at various locations in the GIS for various switching configurations are analyzed in this paper. The nano-crystalline rings of different geometries and permeabilities are considered to understand the damping effect being offered. The nano-crystalline rings are considered at all the possible switches of 245 kV GIS bays. An equivalent circuit model of the nano-crystalline ring is obtained through an iterative algorithm. The very fast transients have been calculated using Electromagnetic Transients Program software. Further, attenuation in the dominant frequency components of VFTOs and VFTCs at different GIS components is also studied for various switching configurations with nano-crystalline rings.

Transient Voltages on Grounding Grids buried in Stratified Soils

Grounding grids (GG) play a fundamental role in the protection of personnel and prevention of damages in equipment during surge transients on substations caused by lightning discharges on power systems. In this context, a precise GG modeling must consider several factors such as the arrangement and the soil compacted in stratified layers. This paper proposes a lumped approach for GG buried in several stratified soils to compute the transient node voltages when subjected to lightning strikes. The vertical and horizontal electrodes are modelled separately by lumped circuit approach. The vertical electrode impedances buried in a stratified soil are computed by the numerical Method of Moments (MoM) in the full-wave electromagnetic software FEKOR , directly in frequency domain, and then, an electric circuit is obtained by the Vector Fitting technique. The horizontal electrodes are modelled based on the electromagnetic radiation theory, where each segment of the electrode can be regarded as a lamental currentcarrying conductor. Lightning currents of fast and slow-front waveforms, are employed in the simulations. Results show that when stratified soils are considered, the differences of the transient voltage peaks, in comparison with the ones calculated for the homogeneous soil is more pronounced as the thickness of soil decreases.

Computation of Transient Voltages on the Interconnected Grounding Grids

Grounding grids play a fundamental role to provide safety during electromagnetic transients and reliable operation of any electrical system under normal conditions. In this context, when lightning strikes a transmission line, surge currents will propagate to the electrical substations where grounding grids must dissipate these impulsive currents into the soil. Grounding grids are composed of horizontal bars welded with vertical forming a large mesh. Additionally, the interconnected grids are largely employed to guarantee safety for personnel and equipment in facilities. Due to several natural conditions, horizontal bars can be damaged and compromise the performance of the grounding grid in power plants. In this paper, transient voltages are computed for whole and damaged grids when lightning strikes a transmission line. Results have shown that there is a difference in the transient voltage peaks in these two conditions. To decrease these voltages, vertical rods are installed in the border of the grounding grids, which has been shown as an effectivesolution to lower these voltage peaks in the damaged grounding grids.