A main lightning discharge is characterized by rapidly rising current that peaks at about 200,000 Amps and averages about 30,000 Amps over its duration. Even though the event is over in milliseconds, there is great potential for harm to personnel and damage to equipment. Personnel working around such a hazard that often strikes wind turbines want to know that they are safe when entering a tower. Electrical grounding is the foundation for an expected level of safety and that begins with a properly designed and installed electrical grounding system.
A designer of such equipment must accept two major goals for the operation of a safe wind-turbine ground system:
• Assure that people in the vicinity of the grounded facility are not exposed to the danger of electric shock
• Provide a means to dissipate electric currents into the earth without exceeding the equipment’s operating limits.
One expects a grounding system to work after commissioning a farm, but what about its longterm reliability? A good grounding system that dissipates lightning current and clears faults quickly helps improve the overall safety and reliability of an electrical system. Reliability must be “built and installed” from the start of the project.
Furthermore, design and material requirements should become more stringent as OEMs build larger structures with greater power outputs. Design considerations for a wind turbine’s grounding system should include:
• Initial soil resistivity tests. These provide a basis for the design.
• A ground-grid footprint and geometry tell how much area there is to work with.
• Line-to-ground fault current specs.
• Specified design resistance of a ground grid. This is often in OEM product specifications and warrantees.
• Possible hazards to individuals working in electrical substations, including stepand- touch potential effects of a Ground Potential Rise (GPR). IEEE Standard 80 defines GPR as a maximum electrical potential that a substation grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage, GPR, is equal to the maximum grid current times the grid resistance. Step potential voltage is the difference in surface potential experienced by a person bridging a distance of 1 m by foot without contacting any grounded object. And touch voltage is a potential difference between the ground potential rise and the surface potential at a point where a person is standing while touching a grounded structure.
• Quantity of buried conductor and ground rods
• Bonding to the foundation rebar and anchor bolts
Site location, the first point, often involves areas of high soil resistivity. In addition, the increased height of more recent wind-turbine designs makes the threat of a lightning strike more likely. Although soil and height make it difficult to design a low impedance grounding system, designers must consider their importance at every wind turbine.
Wind farms consist of interconnected low voltage electrical apparatus and mechanical equipment to form large electro-mechanical systems. Although there are inherent operational dangers, the systems are intended to operate without danger when appropriate routine procedures, and suitable tools and work equipment are correctly used. The purpose of grounding equipment is to maximize the surface-area contact with soil. Lowering the resistance and improving the surge impedance of the grounding hardware helps dissipate a lightning impulse that has a fast-rising edge and a high fundamental frequency, while minimizing the ground potential rise. A typical waveform associated with the lightning impulse reveals high and low-frequencies. The high frequency is associated with the fast rising “front” (typically < 10 μs to peak current) of the lightning impulse, while the lower frequency component resides in the long, high-energy “tail” or follow-on current in the impulse. The grounding system appears to the lightning impulse as a transmission line.
Hence, wind-turbine grounding must satisfy three criteria:
• The system has to effectively dissipate the lightning energy.
• Provide sufficient ground-reference potential to assure the proper operation of the electrical equipment.
• Satisfy step-and-touch potential requirements for the safety of personnel.
Despite the importance of grounding system impedance, the grounding system is typically evaluated with measurements of low-frequency resistance. Many windturbine manu-facturers require a particular value for the resistance, such as 10 Ω for each turbine. This helps protect the generator and other sensitive electrical equipment, and honor a warranty for the whole wind turbine. This value is required even in high soil resistivity (over 5,000 Ωm) and limited space for a wind turbine’s grounding system. These variables work directly against each other.
Although a fixed footprint may make it difficult to meet a specified dc-resistance value, proper design can help maximize the efficiency of the grounding grid. A given area limits the amount of grounding equipment it can support. Therefore, grounding systems are typically treated on a case-by-case basis. Doing so provides an effective and economical grounding solution.
A grounding-system resistance of 10 Ω is sufficient for dissipating light-ning energy, but resistance for the power distribution system should be significantly lower, typically less than 5 Ω. Interconnecting the individual ground systems on each turbine greatly reduces the resistance for an entire grounding network.
Several international standards support the 10-Ω value for an individual wind turbine, but a safe wind turbine work environment still depends on site evaluation (resistivity testing) and proper design of the electrical-grounding system.