Lightning is one of the most captivating and dangerous natural phenomena accompanied by a single or multiple strokes within the same flash. The cumulonimbus clouds are responsible for producing lightning flashes though, still, certain other environmental factors, such as instability in the atmosphere often categorized as convective available potential energy (CAPE) indicating the presence of strong updrafts, humidity, temperature, and pressure also play a vital role for the development of thunderstorm conditions.
Classification of lightning strikes
The most hazardous cloud-to-ground lightning results in interruptions in a power system by causing direct as well as indirect lightning strikes. In the former case, the lightning directly hits the component of the power system, such as towers; however, in the latter case, the stroke occurs in the vicinity of the power system, such as the ground, as depicted in Fig. 1. The magnitude of the voltage produced due to a direct strike is relatively high, compared to an indirect strike. These strikes can cause malfunctioning of the transformer and substation-associated equipment, flashovers of the insulators, fire, and explosions in structures and power equipment, damage to metallic structures because of excessive current flows, electrical shock hazards, rise in the potential of the ground, electromagnetic interference, and human safety hazards.
Impact of lightning strikes on transmission and distribution electrical power systems
To mitigate the perilous impacts of lightning for power systems, the design engineers should appropriately select the insulation of overhead transmission as well as distribution lines. The probability of direct strikes is higher for transmission systems; therefore, surge protective devices such as surge arrestors and shield wires are used along with grounding systems. For distribution systems, indirect strikes are more frequent due to the limited height and lower insulation strength of overhead distribution lines. When an indirect strike occurs, significant overvoltages are induced across the distribution overhead line due to the coupling between electromagnetic fields produced by the lightning strike and the conductor of the overhead line. The magnitude of the induced voltage is a critical aspect from an engineering perspective, as accurately determining it will lead to proper insulation coordination and increased reliability of the power system.
Factors of dependence of induced over voltages
Here are the factors of dependence of the induced over voltages produced as a result of an indirect strike:
- Configuration of the ground, which relies upon soil characteristics comprising ground resistivity and ground permittivity. Various soil types such as moraine, sand, gravel, silt, clay, and peat soil are characterized by various values of resistivity as well as permittivity. Furthermore, the moisture level of the soil as well as layering (also known as stratification, which can be vertical or horizontal) of the soil, also affects the magnitude of the induced overvoltage. The regional topography, including the geometry of the terrain, mountain geography, mixed-media environment (land and sea interface), and various types of lakes characterized by different conductivities, play a vital role in determining the hazard potential of these lightning-induced voltages.
- The waveshape and the magnitude of the lightning current, which are random parameters. The measured lightning currents can be accurately modeled using the waveform of the Heidler function due to its close resemblance. However, the stochastic nature of the lightning current can be modeled using the log-normal probability distribution.
- The distance of the strike point from the overhead line.
Techniques for the evaluation of induced overvoltages by indirect strikes
Various methods, such as analytical and numeric techniques, are employed to accurately assess induced overvoltages. Each method has its own merits and demerits, such as the simultaneous incorporation of the aforementioned factors of dependence and computational burden requirements.
Analytical techniques
Amongst analytical techniques, the first formula was presented by Rusck by considering an ideal ground i.e., ignoring the soil resistivity and assuming the step function of the current as given by $\eqref{eq:a}$
$$ \begin{equation} V = \frac{Z I_m h}{d} \times g(v) \label{eq:a} \end{equation} $$
where $Z = \frac{1}{4\pi \sqrt{\frac{\mu_0}{\epsilon_0}}} = 30 \Omega$, and $g(v) = 1 + \frac{v}{c \sqrt{2} \sqrt{1 – 0.5 \left( \frac{v}{c} \right)^2}}$
where $V$ represents the induced over voltage, $I_m$ represents the peak lightning current, $h$ represents the height of the distribution overhead line, $d$ represents the stroke location, $v$ represents the speed of the lightning current wavefront, and $c$ represents the speed of light.
Later, many researchers incorporated the influence of soil resistivity, such as Darveniza and Paulino, and the formulae presented by them are specified in $\eqref{eq:b}$ and $\eqref{eq:c}$, respectively. The influence of the lossy ground has been incorporated in $\eqref{eq:b}$ and $\eqref{eq:c}$, but for very high resistivity grounds, these formulae result in underestimated values of the peak voltages induced by indirect lightning strikes. Recently, it has been established that the ground permittivity is quite influential for the soils exhibiting high resistivity; correspondingly, there is ongoing research in this domain, and extensions are being proposed in existing analytical formulae.
$$ \begin{equation} V = \frac{Z I_m h^*}{d} \times g(v) \label{eq:b} \end{equation}$$
where $h^* = h+ 0.15 \sqrt{\rho}$ and $\rho$ represents the ground resistivity.
$$ \begin{equation} V = k(\rho, h) \left\{ \sqrt{3} \left(\frac{v}{c} \right)^{1/3} I_m \sqrt{\frac{\rho}{d}} + \frac{Z I_m h}{d} \times g(v) \right\} \label{eq:c} \end{equation}$$
where $k(\rho,h)=1$ for $\rho = 0$ and $h=0$, and $k(\rho,h) =0.85$ otherwise.
Numerical techniques
On the other hand, numerical techniques provide full-wave solutions to Maxwell’s equations for the computation of electromagnetic fields produced by the lightning strike, as shown in Fig. 1. These methods include the hybrid electromagnetic model, method of moments, finite-element method, and finite-difference time-domain method. The complex ground configurations, as discussed above, can be modeled using these methods, but higher computational time is required along with memory requirements. But currently, graphic processing units are also being employed to reduce the computational effort involved in these techniques.
Machine Learning (ML) techniques
Nowadays, machine learning techniques are also in practice for localizing and detecting lightning strikes. Moreover, methods like regression and artificial neural networks have also been used by a few researchers for accurate estimation of the magnitude of the induced over voltages. ML techniques overcome the downsides of conventional techniques: approximations used in analytical methods and enhanced computational and memory requirements in numeric techniques. Thus, ML techniques are paving the way to avoid the hazards posed by lightning by accurately assessing the magnitude of the induced overvoltage and considering the lightning parameters’ random and stochastic nature.
Hazard Prevention
Lightning rods, down conductors, grounding systems, surge arrestors, and the use of single or multiple shield wires with or without periodic grounding and equipotential bonding are some of the conventional lightning protection schemes.
But due to a boom in ML techniques, now lightning nowcasting is being done, and remedial actions are taken before the occurrence of lightning, which also eliminates the cost incurred in installing the lightning detection sensors.
Adoption of standards and codes
While installing the lightning protection systems, compliance with specific standards and codes is ensured to meet the safety requirements in engineering practices and mitigate the adverse effects of lightning.
- NFPA 780 standard is meant for lightning protection system installation
- IEC 62305 series deals with the protection of electronic equipment and hazards imposed on the structures from the perspective of physical damage. These also include the tolerable limits of the touch and the step potentials for the safety of the personnel.
- IEC 62561 deals with the requirements of the components used in lightning protection systems.
Written By: Noor Ul Ain