**1. Introduction**

As a new form of clear power generation, photovoltaic (PV) power generation has attracted more and more attention around the world. Currently, first and second-generation PV technologies are already included for building integration photovoltaic (BIPV) and building attached/applied photovoltaic (BAPV) application in the form of roof, window, wall and shading elements. In addition, third-generation PVs are under exploration [1–3]. With BIPV, it can not only generate power for the building consumers or even power companies, but also save cost and space when constructing the distributed PV stations on buildings. The application of BIPV modules is the development trend of green buildings. It also represents the future of urban and building energy development [4–6].

Due to the high installation location, BIPV modules suffer from lightning hazard greatly. Lightning is a natural phenomenon of a strong discharge, releasing tremendous energy. When lightning strikes BIPV, it will cause deformation and melting of metal framework and damage to PV modules [7,8]. In total, 5% to 10% of solar installations are damaged by direct lightning or lightning electromagnetic pulse every year [9].

**Citation:** Bian, X.; Zhang, Y.; Zhou,Q.; Cao, T.; Wei, B. Numerical and Experimental Study of Lightning Stroke to BIPV Modules. *Energies* **2021**, *14*, 748. https://doi.org/ 10.3390/en14030748

Academic Editor: Adam Dy´sko

Received: 27 December 2020 Accepted: 27 January 2021 Published: 1 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

To ensure PV systems safe and reliable, lightning protection design attracts more and more attention. At present, there is much research on direct lightning and lightninginduced overvoltage in PV power plants [10–13]. Y. G. Jia [14] and F. Y. Guo [15] pointed out the design of the green building and the key point of lightning protection. N. H. Zaini [16] simulated the lightning stroke of different waveforms and amplitudes on different parts of the photovoltaic system, and found that the transient current would appear at the nearest point to the lightning striking, while the transient voltage would appear on the AC side of the inverter. Jae-Young Cho [17] analyzed the damage of lightning overvoltage to PV array infrastructure, and considered the influence caused by different lightning striking locations and distances on PV array. C. Dechthummarong [18] studied the lightning withstand of insulation materials of field-aged PV modules, applied a pulse of 1.2/50 μs between impulse voltage generator (IVG) and PV modules, and realized the mathematical model of partial discharge in PV insulation gap. The experimental results showed that insulation defects caused by aging produce partial discharge between PV modules and aluminum frame. K. Tamura [19] conducted a lightning stroke test on a centralized PV system with an area of 75 m2, a height of about 10 m and an installed capacity of 14 kW. The test showed that the lightning current attached the fabricated aluminum chassis and flowed into the ground, which verified the lightning energy withstand capability of centralized PV modules. K. M. Coetzer [20] conducted a lightning current test in a high voltage lab, and found that poor wiring between PV modules leads to the arising of the excessive induced current. Then, a better wiring method was proposed to reduce the amplitude of the induced current.

As an important part of the integrated roof, the BIPV modules face the threat of direct lightning strike. The configuration of BIPV is quite different from the traditional PV modules which are installed on the fixed PV brackets on the roof. However, the PV modules of BIPV are integrated with the metallic roof without any PV bracket. It changes the relative position between PV modules and surrounding metallic structures compared to the traditional PV modules. Therefore, the study of traditional PV modules is not available for BIPV directly. The study of lightning stroke to BIPV modules is very little addressed nowadays.

In order to protect BIPV modules against lightning damage, a study about the lightning stroke to BIPV modules is conducted with numerical and experimental methods in this paper. Since lightning effect to BIPV modules can be divided into two aspects, i.e., lightning attachment and lightning energy withstand, the study is also divided into two parts, the attachment characteristics in Section 2 and the energy withstand capability in Section 3 of this paper. The study conclusions of this paper will be guidance for the lightning protection of BIPV modules used in rapidly growing distributed PV stations. It would be expected that the combination of PV and building is one of the most important areas for future PV applications. It improves the lightning protection requirements of green buildings and PV systems.

#### **2. Study of Lightning Attachment Characteristics to BIPV Modules**

## *2.1. Numerical Simulation and Analysis*

#### 2.1.1. Modeling of Lightning Stroke to BIPV Modules

During a thunderstorm, the lightning downward leaders develop step by step from the cloud to the ground. As the lightning downward leaders approach to the ground, upward leaders will be generated when the electric field strength of the object on the ground reaches a certain level. Once the downward leader connects with the upward leader from a certain object such as BIPV modules on a building, the object will be struck by the lightning, as Figure 1 shows.

**Figure 1.** Schematic diagram of lightning stroke to building integration photovoltaic (BIPV) modules.

The connection is dependent on the distance between the downward leader and the upward leader reaching the breakdown threshold of the electric field. This threshold is defined as the lightning striking distance which can be interpreted as a function of lightning current, as Equations (1) and (2) shows.

$$r\_{\varepsilon} = K I\_P^b \tag{1}$$

where *rc* is the lightning striking distance, m; *IP* is the amplitude of lightning current, kA; *K* and *b* are coefficients to account for different striking distances to a mast, a shield wire, or the ground plane [21].

According to buildings or transmission lines at different altitudes, scholars have carried out a lot of research on the three important parameters of the striking distance [22–27]. The lightning distance *rg* should be corrected by Equation (2). The specific values of the lightning striking distance parameters from different literature are listed in Table 1.

$$r\_{\mathcal{K}} = \mathbb{K}\_{\mathcal{K}} r\_c \tag{2}$$

*Kg* is the correction factor to correct the striking distance *rc* of the earth; *rg* is the corrected lightning distance.


**Table 1.** Striking distance parameters from different literatures.

According to the striking distance parameters given in IEEE Std 998-2012 [27], where *K* = 8, *b* = 0.65, *Kg* = 1. Because 50% of current amplitude *IP* = 30 kA [28], the lightning striking distance *rg* is calculated to be 73 m.

The downward leader is simplified as a rod electrode when studying the lightning striking position. The electrode voltage is taken as 50% breakdown voltage of negative lightning discharge of the rod–rod gap in air. According to Equation (3), the electrode voltage is set as 43.9 MV.

$$L\_{50\%} = 110 + 6d \tag{3}$$

where *U*50% is 50% breakdown voltage of lightning impulse, kV. *d* is the distance between poles, cm.

In the FEA software, an electrostatic field model of PV modules is established under lightning downward leader. As shown in Figure 2, the downward leader head is equivalent to a rod electrode, and the relative spatial distance between the rod electrode and the PV module is *rg*. The PV module consists of an inner solar cells and an outer metal frame as Figure 1 shows. The material of metal frame is set to aluminum. The material of solar cells is set to silicon. Moreover, the metal frame is set to zero potential as grounding. The lightning interception position of different PV modules can be determined by evaluating the distribution of static electric field strength on the PV modules. The position with the maximum field strength is prone to be struck with maximum possibility.

**Figure 2.** Simulation model of photovoltaic (PV) module under electrostatic field.

2.1.2. Electrostatic Field Theory

The electric field, due to a given charged lightning stepped leader, can be calculated using Equations (4)–(7) of electrostatics.

$$
\nabla \times \mathbf{E} = \mathbf{0} \tag{4}
$$

$$
\nabla \bullet \mathcal{D} = \rho \tag{5}
$$

$$E = -\nabla \varphi \tag{6}$$

$$
\nabla^2 \varphi = -\frac{\mathcal{C}}{\varepsilon} \tag{7}
$$

where *E* is the electric field tensor; *D* is the electric displacement vector; *ρ* is the charge density; *ϕ* is the electric potential; and *ε* is the permittivity of the free space.

It should be noted that the lightning distance *rg* determines the relative spatial position between the lightning downward leader and BIPV modules in the computational domain, and also determines the locations of the zero-electric potential boundary conditions that are associated with the metal frame.
