Protecting wind turbines through effective grounding

Authors: Sameer Kulkarni and Dr Ahmed El-Rashed

The share of wind power in electricity generation is expected to increase, and with that comes a requirement for this carbon-free source to be more reliable. The wind turbine, which is the most important component of a wind power system, is exposed to harsh environmental conditions and electrical transients, such as lightning strikes. Naturally, understanding the lightning protection scheme of a wind turbine and checking its integrity is vital to protecting it during lightning strikes so that continued, reliable operation is achieved.

Recent international studies have shown that in one European country, 80 % of insurance claims on wind turbines resulted from lightning-related damage. Similarly, a major US utility reported that over 85 % of its wind turbine downtime was due to lightning-related damage.

This article provides a general overview of the lightning protection system of a wind turbine, best practice for lightning protection on wind turbines, and verification of effectiveness. It discusses the need and advantages of various tests performed to verify the continued integrity of lightning protection systems, and shares reference values for testing parameters along with expected results, while reviewing some practical and safety considerations.

Wind power

Renewable energy — and wind power in particular — is growing at a rapid pace. In 2020, new wind power installations provided 93 GW globally. The year-on-year growth is 53 %, with both the United States and China leading the world in new installations of wind power generation. Wind power answers the pressing needs and circumstances of today. It is a relatively inexpensive and green energy source that addresses constrained infrastructure budgets as well as climate change policies. Most market analysts indicate that wind power will continue to grow at a fast rate because all the driving factors for its adoption persist.

This is great news for the electrical power industry, as there will be growth and opportunity for many years to come. However, this growth will require improved maintenance programs to protect investments and maximise the profits from wind power.

Lightning strikes

The biggest maintenance problem for wind power is lightning strikes (Figure 1a and Figure 1b). According to Vestas CEO Henrik Andersen, intense lightning strikes were the biggest driving force behind the record warranty claims that amounted to €175 million (US $212 million) in the second quarter of 2020 alone. Wind turbine manufacturers and installers, such as Vestas, recognise the immense danger of lightning strikes and take great care in the design of turbines. Nevertheless, operators and owners of wind turbines must implement a robust and effective maintenance program for their assets.

Figure 1a: Lightning damage to a wind turbine

Figure 1b: Lightning damage to a wind turbine 

Lightning protection systems

A growing number of studies speculate that rotating wind turbines may be more susceptible to lightning strikes than stationary structures. Wind turbines are at a high risk of being struck by lightning due to their height and the locations used for wind farms, and lightning faults cause more loss in wind turbine availability than other faults. Wind turbines are equipped with lightning protection to minimise damage from direct lightning strikes and to shield sensitive equipment integral to wind turbine operation. Lightning strikes not only produce large current flows but also impress unwanted electromagnetic fields across components housed in the nacelle and base of the tower. The lightning protection system (LPS) performs the function of directing the current from strikes to ground.

Lightning protection zones

To facilitate the coordination of protection functions, it is prudent to divide the wind turbine into lightning protection zones (LPZ). The lightning protection zone concept is a structuring measure for creating a defined, electromagnetically compatible environment in an object while being cognisant of the object’s stress withstand capability. IEC 62305, Standard for Lightning Protection, defines the LPZ for structures and can be applied to a wind turbine. The zones are classified as external or internal based on their exposure to direct lightning.

External zones „

• LPZ 0A is the zone where the threat is due to the direct lightning flash and the full lightning electromagnetic field. The internal systems may be subjected to full lightning surge currents.
• LPZ 0B is the zone protected against direct lightning flashes but where the threat is due to the full lightning electromagnetic field. The internal systems may be subjected to partial lightning surge currents.

The rolling sphere method is used to determine LPZ 0A — the parts of a wind turbine that could be subjected to direct lightning strikes, and LPZ 0B — the parts of a wind turbine that are protected from direct lightning strikes by external air-termination systems or airtermination systems integrated in parts of a wind turbine (for example in the rotor blade), as seen in Figure 2 and Figure 3.

Figure 2: Simplified wind turbine, external LPZ

Figure 3: Air termination systems installed for wind turbine nacelle

Internal zones

• LPZ 1 is the zone where the surge current is limited by current sharing and isolating interfaces and/or by surge protection devices (SPD) at the boundary. Spatial shielding may attenuate the lightning electromagnetic field. „
• LPZ 2 to LPZ n are the zones where the surge current may be further limited by current sharing and isolating interfaces and/or by additional SPDs at the boundary. Additional spatial shielding may be used to further attenuate the lightning electromagnetic field. The LPS essentially works by providing a low resistance path-to-ground. The path goes from the blade’s tip to the base of the turbine. This path is shown in Figures 4 and 5.

 Figure 4: Current path for lightning discharges

Figure 5: Foundation earth electrode at wind turbine base

In the event of a lightning strike, current will flow to ground through the LPS, not the sensitive equipment in the wind turbine. As lightning current is dissipated through the grounding system, it is important that it should not cause thermal or mechanical damage or arcing that may lead to fires or injuries to personnel. To ensure that the protection will work effectively when needed, the resistance of the path-to-ground should be measured at regular intervals to check that it meets the limits specified by the turbine manufacturer (typically limited to 15 to 30 mΩ, depending on turbine size). For these tests, use of a low resistance ohmmeter is recommended.

Methods for verifying lightning protection systems

Measurement of low resistance is affected by factors such as measurement type, test current magnitude, length of test leads, and placement of leads/probes.

Four-wire method

The four-wire method (Figure 6) is most appropriate because it uses separate current probes to inject direct current (DC) and separate potential probes to measure the voltage drop across the test specimen.

In some practical cases, a Kelvin measurement, where current and potential probes are 180 ° apart, is also employed to measure low resistance values. The use of any other methods such as a two-wire method may not be suitable, as the measurement will include the contact resistance values of the probes, which makes the results less certain.

Figure 6: Four-wire method

Testing wind turbine lightning protection

The most important test on an LPS is to test the conductor from the blade tip to the down conductor inside the hub that ultimately connects to the ground grid, as was shown in Figure 5 and is depicted in Figure 7 and Figure 8.

Figure 7: Lightning conductor resistance measurement at blade tip

Figure 8: Lightning conductor resistance measurement at wind turbine hub

This conductor is placed under significant strain as the blade flexes with the wind during normal operation. Under strain, the conductor may fracture. Unfortunately, it is not enough to simply check continuity because, if the fractured conductor is touching at the break point during a continuity test, the result of the test will be misleading. Because of this, a test current magnitude of 1 A or more is recommended for this test.

The length of a typical turbine blade can be seen in Figure 9. The size of the turbines poses a problem because low resistance ohmmeter test leads are typically very short. Due to the size of the wind turbines, extralong leads are required, often up to 100 m. This is a huge increase in length over standard test leads for low resistance ohmmeters. The long leads must be designed with a low enough resistance to ensure that a measurement is still possible. To achieve this, it is important to understand the test instrument design.

Figure 9: Wind turbine blade before installation

Some instruments have a compensation factor to allow for power loss in standard test leads. When using long test leads, this compensation will no longer be sufficient and the test range of the instrument will be reduced. When the resistance of the test leads is increased, the value of R in the following equation will also increase.

P = I2R

Where:

• R is (resistance of load) + (resistance of test leads)
• P is output power of the test instrument
• I is output current of the test instrument

Since the maximum power output (P) of the test equipment cannot change, the rise in test lead resistance will cause the maximum current (I) to be reduced. Table 1 shows how lead length impacts the ability of an instrument to measure low resistances. It is clear that accurate and repeatable measurements will depend on a combination of test current, lead length, and resolution.

As seen in Figure 10, the performance of the low resistance tester at 1 A (2.5 W) is the most suitable for the lead lengths that are typically employed for testing wind turbine LPSs. For wind turbine applications, it is important to use an appropriate range and test current because it is essential for the length of test leads to accommodate the length of the wind turbine blades.

Results

In one such example, the LPS on a wind turbine with 32 m (105 ft) blades was tested using a low resistance ohmmeter. The instrument was used in its ‘long test lead’ mode, which applies a 1 A test current and can measure accurately down to 0.01 mΩ when using 100 m long (330 ft) test leads. Testing consisted of measuring the system’s resistance from the tip of each blade to the hub, and from the hub to the base. The lightning system in this case terminated with interconnected ground rods at the base of the turbine tower.

Table 1: Resistance range for varying test current magnitudes for a popular low resistance tester

Each measurement was taken three times to evaluate repeatability. The variance meter on the instrument automatically recorded three measurements in a row and calculated their variance. The raw results from this test can be seen in Table 2; total results are shown in Table 3.

Table 2: Raw measurements, variance, and averages

Table 3: Total resistance values and results

The low variance provides confidence in the measurement. In the field, test engineers must take every care to remain safe and follow best practice. This will provide the best possible measurements.

The manufacturer of this wind turbine prescribes a pass level for the lightning system of 20 mΩ or less. This test proves that the lightning system has been installed correctly and is in good working order. Therefore, this turbine has good lightning protection in line with the manufacturer’s specification.

Conclusion

Lightning is a hugely damaging threat to wind turbines and, as wind power installations continue to spread across the world, the requirement to protect these assets is becoming ever more important.

Manufacturers of wind turbines take great care in designing lightning protection systems, but owners and operators of turbines must ensure that these systems have been installed correctly. Additionally, the owners and operators must regularly check the lightning protection system as part of the maintenance program.

Testing and verifying the lightning protection system is based primarily on low resistance measurements. There are some challenges to measuring resistances at milliohm level when dealing with large structures like a wind turbine, so a balance between test energy, accuracy, resolution, and test lead length must be established. However, the right tools for the task make it a simple job.

It is highly recommended to make lightning protection system maintenance a key regular task for owners and operators of wind turbines. This will minimise the risk of lightning damage and ensure that these valuable assets are properly protected.

About the authors

Sameer Kulkarni, PE, is an Applications Engineer at Megger. He previously worked for Entergy at River Bend Nuclear Generating Station as a systems engineer responsible for power distribution, large power transformers, and NERC. Sameer obtained his BS in Mumbai, India, and graduated with an MS in electrical engineering from Arizona State University. He obtained his Professional Engineer licence in June 2019 and is an IEEE Member.

Dr Ahmed El-Rasheed is a Business Development Director at Megger and has over 14 years’ experience in electrical engineering. He is a member of several international standards organisations and has published papers on ground testing, insulation testing, and multi-sensor integration using AI.

BIBLIOGRAPHY

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