Transient Analysis and Prevention Measures of Lightning Strikes on Step-up Transformers in Wind Farms

Wind farms are mainly located in open and windy mountainous areas with complex climatic and topographic conditions, and the operation of equipment is inevitably affected by thunderstorms. In this paper, an electromagnetic transient analysis model for lightning strikes on collector lines is established with the use of the ATP-EMTP software. The overvoltage situation of step-up transformers subjected to lightning strikes on towers is analysed, and it is pointed out that existing lightning protection measures cannot effectively prevent the three-phase invasion caused by large current lightning, and a lightning protection strategy of installing a set of lightning arresters in the 35 kV bushing of the main transformer is proposed. A transformer trapezoidal equivalent circuit analysis model is established, and the effectiveness of the lightning protection measures are verified through simulation calculations, providing a technical reference for lightning protection in wind farms in multi-thunderstorm areas.


Introduction
Transformers are subject to various factors, such as strong electric fields, mechanical external forces, and electrochemical corrosion, during long-term operation, which can affect the insulation performance of the windings to a certain extent and lead to transformer failures.This paper takes the main transformer of a mountainous wind farm as the research object, analyses the accident process, and causes of the 220 kV booster station main transformer, and proposes a retrofit scheme for installing lightning arresters [1][2][3][4][5].To analyse the impact of lightning on the booster transformer, this paper establishes a lightning electromagnetic transient analysis model for the collection line based on the electromagnetic transient analysis software ATP-EMTP and verifies the effectiveness of the lightning protection measures [6][7][8][9][10].

Overview of Wind Farm Collection Line
Part of the wind farm collection line is shown in Figure 1 where the low-voltage side of the main transformer of the wind farm's booster station is connected to the switchgear through insulated hard wires.The switchgear is connected to Tower #1 through cables, and the overhead transmission line at Tower #1 is connected to the incoming cable through hard wires.A set of lightning arresters is installed at the junction of the hard wire and cable.Before the accident, there was a strong thunderstorm weather and all the wind turbines were running normally, with the turbines operating at full load.

Model building
To analyse the wave processes inside the transformer, it is necessary to establish a distributed parameter circuit for the winding.For a multi-winding transformer, the equivalent circuit based on the trapezoidal theory is shown in Figure 2, where L1 and L2 represent the unit length inductances of the high (medium) voltage winding and low voltage winding, respectively; C1 and C2 represent the unit length capacitances to the ground of the high (medium) voltage winding and low voltage winding, respectively; C3 represents the mutual capacitance per unit length between the corresponding parts of the high (medium) voltage winding and low voltage winding, and λ C3 represents the mutual capacitance per unit length between adjacent parts of the high (medium) voltage winding and low voltage winding (the mutual capacitance between more distant windings is not considered); K1 and K2 represent the longitudinal capacitances per unit length of the high (medium) voltage winding and low voltage winding, respectively [11][12][13].To further investigate the causes of the fault, we take the topology diagram of a partial wind farm as shown in Figure 1 as an example to establish a trapezoidal equivalent circuit analysis model for the main transformer of the substation and analyse the internal overvoltage of the transformer when a large amplitude lightning current hits the collector line, as shown in Figure 3.The winding is divided into 10 equal parts, and 11 observation points are set from the first section to the last section.The entrance capacitance is simplified for non-critical transformers.The wind turbines are connected to the box transformers by cables, and the cables are connected to the overhead lines via hard wires parallel to the tower body.A 35 kV surge arrester (YH5WZ-51/134) is installed at the junction of the cable and the overhead line [14][15].

Simulation analysis
When a 100 kA lightning current strikes the top of Tower #1, the waveforms of the entrance and internal over-voltages of the main transformer are shown in Figure 4.The lightning overvoltage suffered by the first and last sections of the phase coil under the attack of 100 kA lightning current is particularly severe, manifested by an increase in voltage amplitude and a significant prolongation of the tail time.The threephase same-jump caused by the large amplitude lightning current attack poses the greatest threat to the insulation of the transformer.However, the existing surge arrester is installed far away from the bushing in the switchgear, which weakens its protective effect on the main transformer.Therefore, it is necessary to further improve the lightning protection capability of the equipment.

Analysis of the Effectiveness of Lightning Protection Measures
To prevent the three-phase invasion caused by the large lightning current, a lightning protection measure was proposed to supplement the installation of lightning arresters at the main transformer 35 kV side casing, considering that the main transformer 35 kV side is connected to the switchgear through insulated rigid busbars with heat-shrinkable sleeves, and the main transformer 35 kV lightning arrester is only installed inside the switchgear, with a distance of about 10m from the lightning arrester to the main transformer 35 kV casing.To compare and analyse the changes in the internal voltage waveform of the main transformer before and after the measures were taken, a set of lightning arresters was added at the main transformer 35 kV side casing, and the overvoltage waveform when the 100 kA lightning current counterattacks the top of Tower #1 is shown in Figure 5. From Figures 4 and 5, the lightning overvoltage borne by the head and tail segments of the phase coil under the 100 kA lightning current counterattack is particularly severe, mainly manifested in the increase of the voltage amplitude and the significant extension of the tail time.After a set of lightning arresters was added to the main transformer 35 kV casing, the amplitude of the overvoltage borne by the entrance and unit length winding dropped significantly, indicating that the installation of lightning arresters can effectively prevent damage to the main transformer 35 kV winding caused by the threephase invasion of large lightning currents.

Conclusion
The terrain and landforms of the corridor of the mountain wind farm collector line are complex, and the shielding effect of the ground against lightning is reduced.The high-amplitude lightning current frequently hits the overhead lines, and the intrusion of lightning current causes frequent equipment damage accidents on the line.Taking a specific accident of a wind farm's step-up transformer as an example, this paper establishes a transient electromagnetic analysis model of the collector line lightning strike and analyses the overvoltage of the transformer under lightning strike.Regarding the installation of lightning arresters, it is suggested that the lightning arrester on the main transformer 35 kV side should not be too far away from the casing, otherwise, under the action of the three-phase invasion of lightning, the overvoltage problem exceeding the lightning protection level of the transformer 35 kV side may easily occur.For the main transformers that have already installed lightning arresters on the 35 kV side in the switchgear far away from the casing, a set of lightning arresters should be added at the casing.

Figure 1
Figure 1 Wind power plant collector line topology 3. Establishment of an Electromagnetic Transient Model for the Substation under Lightning Strikes

Figure 2
Figure 2 Multi-winding transformer trapezoidal equivalent circuit schematic

Figure 3
Figure 3 Potential distribution model in transformer windings

( a )Figure 4
Figure 4 The internal voltage waveform of the main transformer winding without an arrester installed

( a )Figure 5
Figure 5 The internal voltage waveform of the main transformer winding after a set of arresters is installed in 35 kV side bushing.