All electrical plant require a connection to the general mass of earth in order to:
• minimize shock hazards to personnel and animals,
• establish a low-impedance path for earth-fault currents and hence satisfactory operation of protection,
• improve protection from lightning and retain voltages within reasonable limits, and
• prevent large potential differences being established which are potentially hazardous to both personnel and equipment.
In the UK this subject is referred to as 'earthing' while in the US it is called 'grounding'. The terms maybe considered to be synonymous.
Because earthing has obvious safety implications there is considerable guidance available in both national and international standards for conventional electrical plant. For earthing of AC substations, the US standard (ANSI/IEEE, 1986) is widely applied and Charlton has written an informative guide (Copper Development Association, 1997) which is mainly focused on UK practice.
Wind farms, however, have rather unusual requirements for earthing. They are often very extensive stretching over several kilometres, subject to frequent lightning strikes because of the height of modern wind turbines, and are often on high-resistivity ground being located on the tops of hills. Thus, normal earthing practice tends not to be easily applicable and special consideration is required. The IEEE Recommended Practice (1991), which is no longer current, recommended 'that the entire wind farm installation have a continuous metallic ground system connecting all equipment. This should include, but not be limited to, the substation, transfor mers, towers, wind-turbine generators and electronic equipment.' This practice is generally followed with bare conductor being laid in the power-collection cable trenches to provide both bonding of all parts of the wind farm as well as a long horizontal electrode to reduce the impedance of the earthing system.
A wind farm earthing system is required to operate effectively for both power frequency 50/60 Hz currents and lightning surges which have rise-times typically of less than 10 ,ws. Although it is conventional to use the same physical earthing network for power frequency currents and lightning surges the response of the earthing system to the high-frequency components of the lighting current is completely different to that at 50 Hz.
The performance of a wind-farm earthing system may be understood qualitatively by considering Figure 10.4.
At each wind turbine a local earth is provided by placing a ring of conductor around the foundation at a depth of about 1 m (sometimes known as a counterpoise earth) and by driving vertical rods into the ground. It is common to bond the steel reinforcing of the wind-turbine foundation into this local earthing network. The purpose of this local earth is to provide equipotential bonding against the effects of both lightning and power frequency fault currents and to provide one element of the overall wind-farm earthing system. Regulations differ for the value of this local earth but in the UK it is necessary to achieve a resistance to earth of less than 10 Q (British Standard, 1999) before the connection of any power or SCADA cables. These local turbines' earths are shown as Rturbine in Figure 10.4. As the turbine-earthing network consists of a ring of only, say, 15 m in diameter and local driven rods it may be considered as purely resistive.
However, the long horizontal electrodes linking one turbine to the next have a more complex behaviour (similar to a transmission line) and are represented in Figure 10.4 as a n equivalent circuit. Thus the resistance to ground is indicated by Rshunt while the series impedance is the combination of Rseries and Lseries. Rseries comes about simply from the resistance of the earth wire while L series is the self-inductance of the earth circuit. On the long earthing networks found on large wind farms this series impedance cannot be ignored. It may be seen immediately that for the high-frequency components of a lightning strike on a wind turbine the series inductance acts effectively to reduce the earthing network to only the local turbine earth. Even with 50 Hz fault current the series impedance leads to significantly higher earthing impedances than would be expected with geographically small earthing systems where series impedance may be neglected. In this discussion shunt capacitance has been ignored although this may become significant at very high frequencies.
This behaviour of wind-farm earthing systems has been confirmed by site measurements and Table 10.1 shows the measured values for the earth impedance at the main substations of two UK wind farms.
It may be seen that even at 50 Hz the impedance to earth consists of almost equal resistive and inductive parts (i.e., the X= R ratio of the circuit to earth is almost unity). This has important implications both for the design and testing of windfarm earthing systems. It is not possible to use conventional calculation techniques which were designed for small, fully resistive, earth networks and it is necessary to take into account the effect of the long conductors. Sophisticated computer programs to do this are available and these may be used to consider both 50 Hz and lightning surge behaviour. Similarly, simple testing methods such as those developed by Tagg (1964) are only intended to measure resistance to earth and will give optimistic results on large wind farms. The only effective measurement method to determine the 50 Hz earth impedance of a large wind farm is the so-called current injection test. In this test a current (typically 10-20 A) is injected into the earth electrodes at the wind farm and the potential rise measured with respect to a 'true' earth. However, the return path of the injected current must be remote from the wind farm (usually some 5-10 km away) to ensure that it does not effect measurements at the wind farm. Hence it is conventional to use the de-energized connection circuit of the wind farm to the main power network as the path of the test current. The rise in potential of the wind-farm earthing systems is measured against a remote earth which classically is transferred to the site over a metallic telephone circuit. Ideally the route of the telephone circuit should be orthogonal to the power circuit to avoid induced effects. It may be seen that tests of this type are extremely
Capacity of wind farm
Number of turbines
Route length of
Earth impedance at
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