In view of the actual random behavior of lightning strikes, an efficient algorithm is proposed in this paper for statistical analysis of the lightning transients on wind turbines. The equivalent circuits are established for blades, moving contact site, tower and grounding arrangement. A set of formulas is provided for evaluating their respective circuit parameters. With the equivalent circuits connected in sequence, a complete circuit model of wind turbines (WTs) is constructed for calculating lightning transients. The lightning transient responses can be obtained at different locations on a wind turbine by using the circuit model to perform the transient calculation. For checking the validity of the circuit model, the experiment measurement is also made on a laboratory-scale WT. An approximate match appears between the measured and calculated results. On the basis of the circuit model, the transient calculation is performed sequentially for each discrete amplitude in a large interval of lightning current. The random distribution of lightning current is taken into account by a cumulative probability function of exponential type. The corresponding probability density function is derived to weight the peak values of the lightning transient responses. The statistical values of the lightning transient responses can be determined by totalizing the weighted peak values in the interval of lightning current. Then, a case study is carried out to contrast the statistical values with the non-statistical ones calculated from the standard lightning current amplitudes, and a significant difference is discovered between them.

Corona discharge characteristics are measured in a corona cage. The difference is found between the q–u curves under double exponential and damped oscillation surges. The behavior of the minor loops is revealed for the q–u curves under positive and negative damped oscillation surges. An extended improvement is made on the traditional approach for modeling of the q–u curves under damped oscillation surges. The extended approach has the capability of describing the complicated trajectory feature of the minor loops. On the basis of the extended approach, an efficient method is proposed for performing lightning surge analysis of overhead lines considering the corona effect. In the proposed method, an overhead line with corona is divided into a certain number of line segments. Each segment is converted into a circuit unit consisting of a non-linear branch and a linear circuit. With these circuit units connected in sequence, a complete equivalent circuit is constructed for the overhead line with corona. The transient responses can be obtained from the solution to the equivalent circuit. Then, the calculated results are compared with the field test results on a test overhead line.

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A moisture sensing technique for real-time monitoring of the moisture content in transformer oil based on an S-taper fiber structure, is proposed and experimentally demonstrated, with the advantages of high sensitivity, excellent repeatability, simple fabrication, compact structure and resistance to ambient temperature variation. By analyzing the physical model of the S-taper fiber, the quantitative relationship between the wavelength change of the transmission dip in the transmission spectrum of the S-taper fiber and the moisture content is established. Then the S-taper fibers with different structural parameters, such as the waist diameter and the axial offset, were fabricated in the lab, and actual measurements in transformer oil samples with different moisture content are carried out. The results show that the transmission dip experiences a red-shifts with decreasing moisture, which could be used to correlate/trace moisture content. It is demonstrated that the S-taper fiber achieves higher detection sensitivity with a decreasing waist diameter or increasing axial offset. For the Staper fiber with a waist diameter of 50 μm and an axial offset of 110 μm, the sensitivity and the lower detection limit reach up to 0.48 nm/ppm and 2.19 ppm, respectively. Therefore, the S-taper fiber sensor could effectively in-situ monitor the moisture content in the transformer oil in real-time, which helps to detect the insulation damp problem in the early stage of the transformer in time and ensure its long-term safe operation.

An effective method is proposed in this paper for calculating the transient magnetic field and induced voltage in the photovoltaic bracket system under lightning stroke. Considering the need for the lightning current responses on various branches of the photovoltaic bracket system, a brief outline is given to the equivalent circuit model of the photovoltaic bracket system. The analytic formulas of the transient magnetic field are derived from the vector potential for the tilted, vertical and horizontal branches in the photovoltaic bracket system. With a time–space discretization scheme put forward for theses formulas, the magnetic field distribution in an assigned spatial domain is determined on the basis of the lightning current responses. The magnetic linkage passing through a conductor loop is evaluated by the surface integral of the magnetic flux density and the induced voltage is obtained from the time derivative of the magnetic linkage. In order to check the validity of the proposed method, an experiment is made on a reduced-scale photovoltaic bracket system. Then, the proposed method is applied to an actual photovoltaic bracket system. The calculations are performed for the magnetic field distributions and induced voltages under positive and negative lightning strokes.