Desiccation cracking of soils: A review of investigation approaches, underlying mechanisms, and influencing factors

Adopting non-contact capacitive coupling for voltage monitoring is promising as it avoids electrical connection with high-voltage transmission lines. However, coupled voltage transformation matrix to correlate voltage of overhead transmission lines and induction bars has not been achieved mathematically due to the lack of equivalent electric circuit model for analyzing the physical phenomenon. Moreover, exact spatial positions of overhead transmission lines are typically unknown and dynamic in practice. In this paper, a technique based on non-contact capacitive-coupling and assisted by magnetic-field sensing for monitoring voltage of overhead transmission lines was designed and implemented. The technique in this paper is demonstrated on a single-circuit transmission line as an example, while it is also applicable for multi-circuit transmission lines. The capacitive coupling between overhead transmission lines and induction bars were modeled as lumped capacitors, and then, the equivalent electric circuit model was established. The coupled voltage transformation matrix to correlate voltage of overhead transmission lines and induced voltage of induction bars mathematically was formed accordingly. This paper was also carried out to analyze the effect of ground wires, sensitivity of induction bars, the ability of high-frequency transient measurement, and the intrinsic capacitance of a measurement instrument. The exact spatial positions of overhead transmission lines were acquired by integrating magnetic-field sensing with the stochastic optimization algorithm. The methodology was verified by simulation on the 10-kV single-circuit three-phase overhead transmission lines taking non-ideality of signal measurement in account, and wavelet de-noising algorithm was supplemented to filter the interferences. A scaled testbed to experiment the technique was built to monitor 220 V overhead transmission lines in the lab, and also the typical waveform of a high-frequency switching surge (up to 1 kHz), which was generated by a programmable ac source. The reconstructed results match well with the actual values. This technique can largely improve transient-fault identification over traditional potential transformers by the virtue of the increased upper measurement limit and bandwidth through capacitive coupling. Moreover, it can be implemented with low-cost copper induction bars and compact magnetoresistive sensors, enabling large-scale application to realize sectional monitoring in the wide area.

Non-Contact Capacitive-Coupling-Based and Magnetic-Field-Sensing-Assisted Technique for Monitoring Voltage of Overhead Power Transmission Lines

The pollution performance of insulators installed in transmission and distribution lines plays a key role in maintaining the reliability, safety and cost-effectiveness of power systems. Among the different insulator monitoring techniques, leakage current stands out as one of the most meaningful pollution performance indicators as it gives a measure of how close the insulators string is to flashover. This paper presents a novel methodology for the prediction of the leakage current on insulator strings considering the environmental and weather information of the insulators location. It is based on the combination of a new developed Cumulative Pollution Index (CPI), which estimates the soluble pollution deposit on the insulator string, and a machine learning technique such as Random Forests algorithm. The method is valid for ceramic insulators, i.e., toughened glass, as well as RTV silicone-coated insulators with hydrophobic transfer properties. The research is supported by an extensive field monitoring program where three different insulator strings composed by non-coated, half-coated (bottom part) and full-silicone-coated glass insulators were monitored in a period covering twenty-two consecutive months. Finally, the performance of the proposed prediction model is evaluated using real data.

A Cumulative Pollution Index for the Estimation of the Leakage Current on Insulator Strings

In this study, the authors propose a novel non-linear mathematical model of a hydro-energy generation system in the load rejection transient considering the elastic water hammer-impact of the penstock and the non-linear characteristics of the generator. More specifically, six dynamic transfer coefficients of the hydro-turbine are introduced to the generation system. Furthermore, the dynamic behaviours of the system are investigated using bifurcation diagrams and time waveforms from the point of engineering. Finally, these methods and analytical results will supply some principles for designing and running a hydropower station.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rpg.2015.0579

Dynamic analysis and modelling of a Francis hydro-energy generation system in the load rejection transient

Numerical analysis of lightning attachment to wind turbine blade

Soil electrical resistivity is an important parameter in grounding system design. In this paper, laboratory measurements of soil electrical resistivity are carried out using a soil box. Two measurement methods are used and compared, namely, a two-electrode method and a four-electrode method. The measurements indicate that the results reported by the two-electrode method are affected by the contact resistance between the electrodes and the soil, which increases as the soil water content increases. Moreover, the soil electrical resistivity was observed to decrease as the test signal frequency increases. Using the four-electrode method and a 50-Hz ac signal, the effects of soil water content, soil porosity, pore fluid composition, and temperature on the soil electrical resistivity are then investigated. The results show that increasing the soil saturation level results in a power function decrease in the soil electrical resistivity. For a given gravimetric water content, as the soil porosity decreases, the resistivity decreases. In addition, owing to the mobility of the ions, different electrolytes in the pore fluid contribute differently to the soil electrical resistivity. Finally, the dependence of soil electrical resistivity on soil temperature is found to be divided into three stages: above 0 °C, around 0 °C, and below 0 °C. An abrupt change in the soil electrical resistivity is observed at around 0 °C.

Laboratory Investigations on Factors Affecting Soil Electrical Resistivity and the Measurement

Far-field measurements of lightning-produced electromagnetic fields are of high importance for lightning location systems. Data from recorded lightning strokes can be, for example, used to assess the risk of lightning-induced damage in some geographical areas. Artificially triggered lightning, usually generated using rocket-wire technology, plays an important role in the study of lightning physics. Multiple-station observations of far (typically larger than 20 km) electric field waveforms produced by rocket-triggered lightning are quite rare. In 2014, electric field waveforms were measured for return strokes through the rocket-and-wire technique at Conghua. They were measured by electric field sensors at nine sites belonging to the lightning locating system in Foshan, Guangzhou, China. A total of 38 events were recorded and overall statistics of those waveforms are presented. The results were compared to previous experimental data related to natural and artificially triggered lightning to validate their similarity.

Multiple-Station Measurements of a Return-Stroke Electric Field From Rocket-Triggered Lightning at Distances of 68–126 km

This paper presents the induced voltage measured at the terminal of a 10 kV distribution line when triggered lightning taking place at a distance of 40 m. It is shown that the induced voltage waveforms exhibit three components including a slow increase preceding the initial negative peak, following a rapid polarity reversal and the attenuated oscillations. The waveform parameters are defined in this paper. The observed negative peak value ranges from −47.1 kV to −7.5 kV. The positive peak value ranges from 5.3 kV to 20.9 kV, and the peak to peak value ranges from 12.8 kV to 63.2 kV. The test results show that the negative peak value of the induced voltage at the terminal of distribution generated by nearby negative triggered lightning generally exceeds the positive peak value, and in some cases the negative peak value even reaches twice the positive peak value. The negative 10%-90% rise time ranges from 2.3 μs to 88.5 μs, and the average oscillation period is about 18 μs. Both negative and positive peaks of induced voltages show good linear relationship with the peaks of lightning return stroke current.

Observation of Induced Voltage at the Terminal of 10 kV Distribution Line by Nearby Triggered Lightning

General grounding method of overhead ground wires adopted in Chinese 500-kV transmission lines is usually optical fiber composite ground wire (OPGW) grounded at each tower while common ground wire (CGW) insulated with one point grounded. Energy loss of OPGW and high induced voltage of CGW is a problem to be concerned as to its values and control methods. Both field measurement and ATP-EMTP simulation of induced current and induced voltage of overhead ground wires in a 500-kV single-circuit transmission line are carried out in this paper. Quantitative data of induced current and energy loss show the urgent necessity to reduce the energy loss and change the grounding method of overhead ground wires. A no-loss method for ground wires is proposed by comparing advantages and disadvantages of three grounding methods of overhead ground wires, where both OPGW and CGW are insulated with one point grounded. Induced voltage is calculated in different transposition pitch and segment length of ground wires with grounding position at the end of the line or at the midpoint of transposition pitch and segment. It shows that induced voltage of ground wires grounded at the midpoint of transposition pitch and segment is half of which grounded at the end. Moreover, induced voltage of overhead ground wires can be reduced to 500 V or other limited values by choosing a reasonable transposition pitch and grounding position.

Induced Voltage of Overhead Ground Wires in 50 0-kV Single-Circuit Transmission Lines

This paper presents the analysis and observation results of the induced voltage waveforms caused by nearby natural lightning on overhead power lines in the Guangzhou field experiment site during the summer of 2008. Peak values of the induced voltage for 36 natural lightning return strokes ranged from 0.21 to 3.87 kV. And the double-peak oscillation phenomenon can be found in the waveforms. The interval time of adjacent return strokes ranged from 17.1 to 430.1 ms. The average 10%-10% peak value duration time of induced voltage waveform of the first return strokes is 244.6 μs, and that of the subsequent return strokes is 83.2 μs. The induced voltage waveforms on the live and neutral conductors are almost the same, while the induced voltage amplitude on the live conductor is slightly larger than that on the neutral conductor. The lightning location system data indicate that the induced voltage on the live and neutral conductor may be of either positive or negative polarity, depending on the lightning ground strike points. However, the amplitude of the induced voltage calculated by the Rusck model is smaller than those of the observation results, with the former being 60% of the latter.

Jianguo Wang

Papers

Laboratory Investigations on Factors Affecting Soil Electrical Resistivity and the Measurement

Multiple-Station Measurements of a Return-Stroke Electric Field From Rocket-Triggered Lightning at Distances of 68–126 km

Observation of Induced Voltage at the Terminal of 10 kV Distribution Line by Nearby Triggered Lightning

Induced Voltage of Overhead Ground Wires in 50 0-kV Single-Circuit Transmission Lines

Observation of Natural Lightning-Induced Voltage on Overhead Power Lines