Preface: With the rapid development of the wind power industry, various operational problems caused by grounding issues have gradually emerged. Due to the lack of corresponding industry and national standards for wind turbine grounding in the early days, grounding standards for wind turbines were quite chaotic, with some projects referencing enterprise standards while others referenced relevant power industry standards, and the selection of grounding materials also presented problems. After years of operation, a large number of wind turbine units in domestic wind farms are currently operating under high grounding resistance conditions. This article only offers some personal views on grounding engineering design.
1. Grounding characteristics of wind turbine generator sets
The grounding of wind turbine generators is mainly divided into two parts: common power supply grounding and lightning protection grounding. Common power supply grounding is primarily determined by the power supply, distribution, and transmission system of the wind turbine generator. Currently, most wind turbine generators use the TN-C system (three-phase four-wire system) for low-voltage distribution. The generator converter output and the prefabricated substation are connected by three live wires and one neutral (N) wire, with the N wire connected to the foundation reinforcement cage, effectively becoming the NPE (Network Protected) wire. Lightning protection grounding includes both the foundation itself and the additional grounding grid outside the foundation. A typical design involves evenly distributing three 60×6mm grounding flat steel strips on the foundation as lightning protection grounding wires connected to the foundation ring. This effectively serves as an auxiliary to the power supply N wire grounding grid and increases the area of the foundation grounding grid. From a lightning protection perspective, this increases the grounding grid area and, given a fixed soil resistivity, reduces the power frequency grounding resistance.
2. Grounding Engineering Design
2.1 Calculation of grounding resistance at wind turbine generator site
Because the geological conditions of the equipment sites are different, the soil resistivity is also different. Therefore, the grounding resistance design of the equipment sites should be different. As shown in Figure 1, the soil resistivity of different equipment sites is different. It can be seen that the soil resistivity of the same equipment site at different depths varies greatly.
Figure 1 shows the typical distribution of soil resistivity at the 9+ turbine location. The surface soil resistivity is approximately 2300 Ω·m; the middle layer reaches 3500 Ω·m; and the resistivity decreases to around 800 Ω·m at the deeper layers. For the grounding resistance design of this location, the grounding formula for non-uniform soil resistivity should be used. When the geological conditions have a two-layer resistivity structure, the grounding resistance is:
Formula (1)
By comparing formula (1) with the uniform resistivity R, the equivalent resistivity can be calculated as follows:
in:
Because the resistivity of the upper and lower soil layers varies significantly in actual engineering calculations, making regression difficult, a coefficient K is introduced into the calculation to represent the equivalent soil resistivity.
Formula (4)
This allows for the conversion of K values with larger rates of change to values between 0 and 1. When K=0,
Yes, when K=1, therefore the regression of can be simplified to the regression of K.
In fact, under two-layer geological conditions, the K value of a horizontal geogrid is mainly influenced by several factors, including the resistivity of the upper soil layer, the resistivity of the lower soil layer, the thickness S of the upper soil layer, the area A of the geogrid, and the density M of the geogrid grid. This can be expressed as:
Formula (5) K=K0+Km
Km is a function of the grid number m, composed of Kmn and KmA. From the above formula, the simplified formula for calculating the grounding resistance of the grounding grid under the condition of double-layer soil resistivity can be derived as follows:
Formula (11)
Of course, in actual engineering design, when encountering situations similar to 9+ machine positions, the actual process of soil replacement is adopted to reduce resistance. The theoretical calculation of the grounding resistance after soil replacement can also be derived according to formula (11).
2.2 Calculation of thermal stability of ground wire in AC grounding grid under common grounding conditions
When the unit's grounding grid is connected to the neutral (N) line, the minimum cross-sectional area of the grounding wire, without considering corrosion, should meet the following requirement:
Formula (12)
In the formula:
Sg—Minimum cross-sectional area of the grounding electrode, mm2;
Ig—The expected maximum short-circuit current flowing through the ground wire, in A;
te—Equivalent duration of the short circuit, in seconds;
c -- Thermal stability coefficient of the grounding material.
Therefore, based on the above formula and the short-circuit current of 800A for a 750kW unit, the minimum cross-sectional area of its shared grounding flat steel should meet 16mm2, i.e., 404mm2 of hot-dip galvanized flat steel. For a 1500kW unit, the minimum cross-sectional area of the shared grounding flat steel should be greater than 505mm2. If the effective corrosion rate of the flat steel is taken into account, the minimum cross-sectional area of the grounding flat steel should be no less than 606mm2.
2.3 Selection of the minimum cross-sectional area of the ground wire in the grounding grid
From the perspective of considering the thermal stability of the grounding grid, the minimum cross-sectional area of the grounding flat steel needs to be taken into account. Similarly, from the perspective of lightning protection, the cross-sectional area of the grounding wire also needs to be calculated. Under normal conditions, there is no large current in AC grounding grids and lightning protection grounding grids. Only under power frequency short circuits to ground and lightning current impulses can large currents appear on the grounding flat steel. When current appears on the grounding wire, the grounding flat steel will inevitably generate heat. Based on the adiabatic process, a simplified formula for the heat balance equation of the grounding wire can be written:
Formula (13)
Since the current shunting ratio of each ground wire in the actual lightning current shunting process may be very small, it is necessary to consider thermal stability and corrosion prevention. In coastal areas and areas with high lightning strike density, the cross-sectional area of the grounding flat steel should be increased by 20% redundancy based on the calculated value.
2.4 Lightning current backflashover of adjacent units during a lightning strike
Some sources suggest that all turbine locations in a wind farm should be connected to form a super-large ground grid. However, since the grounding resistance of adjacent turbine locations cannot be equal, and based on actual operating experience, connecting adjacent turbine locations with different grounding resistances together means that when struck by lightning, the turbine location with higher grounding resistance is less likely to experience ground potential backflash. In more cases, the turbine location with lower grounding resistance among the adjacent locations will be affected.
Figure 2. Distribution of lightning ground current at adjacent locations.
Where Lt is the inductance of the generator tower, Ls is the equivalent inductance of the parallel grounding grid tie line, i is the lightning current, and the waveform of the lightning current is a 10/350s waveform with amplitude I, wavefront t, and steepness a. Therefore, the lightning current at the wavefront is I = at. The lightning current flowing into the ground through the tower is:
Formula (14)
Wherein is the shunt coefficient of the tower, that is, the ratio of the tower current to the lightning current. At this time, the potential on the base of the nacelle is:
The flow diversion coefficient of the tower can be obtained through the equivalent circuit shown in Figure 3.
The current shunting coefficient of the tower is related not only to the tower's inductance Lt and the impulse resistance Rch of the foundation ring grounding, but also to the inductance Ls of the tie line between the grounding grids. A larger Ls results in a larger current flowing through the tower. In a real-world environment, if Unit #2 is struck by lightning, Unit #1, connected to it, is likely to suffer damage due to ground potential backflash caused by the increased current component in the tie line, while Unit #3 will be spared from lightning strikes because the impulse current component flowing through the tie line is smaller. This explains why wind turbine generators require independent grounding grids, rather than large-area combined grounding grids. Superficially, it's understood that large-area combined grounding grids are beneficial for lightning current dissipation, but in reality, under the influence of impulse current and impulse resistance, the flat steel under normal conditions will generate a large inductive reactance component, which in turn generates a very high backflash electromotive force under impulse current.
3. Conclusion
This article only presents a design model through theoretical calculations for various geological conditions and engineering problems that may be encountered in grounding projects. In actual engineering, many practical problems will arise; some can be solved through existing theoretical calculations, while others require years of engineering experience. Because the wind power industry lacks relevant industry standards, there are still some shortcomings in the design, construction, and acceptance of turbine grounding. It is believed that as the industry gradually develops, the corresponding standards will become increasingly完善 (perfected/improved).
