Surge analysis for lightning strike on overhead lines of wind farm
Introduction
Wind farm installations have increased substantially over past decades to satisfy the growing demand of electricity [1], [2], [3]. Besides enhancing the power supply reliability, the use of wind energy also reduces the fossil fuel hazard and minimizes the operating cost. The wind energy can be either integrated into AC or DC grids. It is estimated that the installation capacity of wind turbines (WTs) would exceed one thousand gigawatts by the year 2020. Wind turbines are tall structures more than tens of meters height and located in regions which might span across mountain ridges or in other areas of elevated ground to obtain good wind condition. The wind turbines are located on open space and exposed to atmospheric thunderstorms at a high risk [4], [5], [6], [7]. Many incidents have been reported worldwide on this issue. As the wind turbine blades are often made of glass fiber reinforced polymer composite materials, some blades are thermally damaged and broken down under the strong effect of heavy electric arc current; the lightning sometimes hits the ground and devastate the underground cable in collector line as well as the control system [8], [9]. The increase in wind farm gives rise to arising damage from lightning storm.
Lightning is a rather stochastic natural phenomenon. The occurrence in wind farm plant is influenced by many factors, like the geographical location, the altitude, and the topographical and orographical effects. The mechanism of lightning strikes rotating wind turbine blade has been extensively investigated; the evolution of upward leader is captured using high-speed CCD camera; the lightning protection space is divided in multi-zones; a blade lightning protection zone for rating the lightning risk is defined [10]. The configuration and number of receptors on the interception effect are analyzed. Moreover, an improved electro-geometric model has been proposed to elucidate the evolution of lightning streamer [11]. Wind turbine blades can produce electric discharges, and ions would be accumulated near the tip of blade receptor. Ions are likely to migrate along the rotating direction and electric field generated by thundercloud. The rotation has different effects on the lightning - attracting ability of wind turbine for short and long gaps.
In the case of lightning strike on wind turbine, the heavy current would pass through the tower, the grounding system and finally dissipate into soil. The entire electromagnetic transient of wind turbine has been studied by means of EMTP, and the transients for many locations inside the wind turbine are computed [12, 13]. The frequency of lightning current generally ranges from 10 kHz to 3 MHz [14]. Mathematically, the receptor and down conductor inside wind turbine blade are divided into a number of segments [15]. Each segment is represented by a RLC π-circuit; the moving contacts and shaft bearing are treated as a set of resistance and capacitance in parallel; the wind turbine tower is a tubular circular truncated cone for analysis. In addition to single blade strike, multi-blade lightning strike is also considered [16]. Several reduced-scale prototypes have been set up in laboratory. Standard lightning impulses are applied to the tip of blade receptor to validate the simulation results [17]. The coupling effect between wind turbine tower and signal cable are discussed and the induced voltage on metal sheath and cable conductor are obtained [18]. The transient jointly depends on the lightning current magnitude and wave shape, the impedance of wind turbine structure and the grounding system. The magnitude of transient overvoltage decreases from the blade tip to the wind turbine body. A small grounding resistance is essential. The off-shore wind generation system, which has a monopile foundation soaked in seawater, is completely different from that of on-shore wind farm.
In practice, most of onshore wind farms are built on remote mountainous terrain to obtain good wind condition. Compared to offshore wind farms, the optimization of large-scale onshore wind farms is more dependent on the surroundings, such as the topography of construction area and the presence of neighbouring residential areas. The wind turbines are electrically connected to power grid through overhead lines. The right-of-way of transmission lines often takes a large area, and the lines as well as transmission towers are also vulnerable to lightning strikes. The structure and topology of wind farm are different from traditional thermal plants. There are various designs of collection systems for wind farm substation, including radial connection, radial-loop connection, and AC star/cluster connection. Meanwhile, a series-parallel connection of turbines with HVDC has been proposed, e.g., DC radial connection, DC series/daisy chain connection, and DC series-parallel connection [19]. In the case of shielding failure or back flashover, the lightning surge would propagate along the overhead line and intrudes into transformers. Subsequently, the surge transfers between primary and secondary windings and into the generator. Part of lightning current would propagate to the substation. The basic lightning impulse insulation level (BIL) of the wind turbine generator is relatively low. The lightning surge also passes through the converter. As a kind of electronic device, it is sensitive and prone to damage under severe lightning condition. Furthermore, the arrangement of wind farm substation is also different from traditional substation. As a consequence, the transient response is changed. The characteristics of lightning surge are completely different from that of blade receptor, little attention has been paid to this aspect so far, and that is the main objective of our work.
Section snippets
Wind farm description
The schematic diagram for the lightning surge into wind farm is illustrated in Fig. 1. When a strong lightning strikes the shield wire or transmission tower, the current would dissipate into ground and cause a potential rise across the insulator string, resulting in the back flashover. On the other hand, the lightning sometimes directly hits the phase conductor, leading to the so-called shielding failure. Then, the surge would travel through the overhead line, step up transformer, power cables
Mathematical model
The wind farm consists of transmission tower, overhead lines, power transformer, wind turbine converter, and double-fed wind induction generator (DFIG). The mathematical models are described as follows.
Simulation model
To elucidate the transient response of wind farm, the 220 kV Huishangang wind farm substation located in Hunan of southern China is considered, and the system configuration is illustrated in Fig. 13. This step-up substation is built to collect all energy generated by the turbines and received through cables. The substation is composed by medium voltage system and high voltage system. Each system consists of the general busbar, circuit breakers, disconnectors, current transformer together with
Lightning surge propagation in wind farm substation
In addition to wind turbine, the lightning surge would also propagate into substation along the line and threaten the safety of power equipment. The transient response under lightning strike is plotted in Fig. 24, and the voltage rises at crucial nodes of wind farm are presented in Table 5.
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Since the lightning strike point is close to Unit 3, the traveling wave first reaches TR3H and WT3, and then Unit 2 and Unit 4. Due to the close distance, the wave reflection effect is relatively small, and
Conclusions
The conclusions are drawn as follows.
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The characteristics of surge due to lightning direct strike on overhead line are completely different from that of blade receptor. The influences of lightning current waveform, lightning strike mode and location are discussed. The surge transfer between primary and secondary windings of step-up transformer plays a key role in the system transients; the shorter the front of lightning current, the larger the amplitude of the overvoltage is.
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As the lightning
CRediT authorship contribution statement
Qiuqin Sun: Methodology, Resources, Funding acquisition, Writing - review & editing. Lei Yang: Investigation, Software, Validation, Writing - original draft. Qian Li: Writing - review & editing. Xiaorong Zhang: Writing - review & editing. Feng Wang: Conceptualization, Formal analysis. She Chen: Data curation. Lipeng Zhong: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China under Grant 52077068 and 51677061.
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