Power System Grounding: Understanding Lightning Strikes

Lightning is an electrical discharge of the accumulation of electrostatic electricity from cloud to cloud, within a cloud, or from cloud to Earth. 

Lightning poses a stark danger, leading electric supply industries to systematically study atmospheric discharges and their impact on electrical power systems. This article emphasizes the lightning that takes place between the cloud and the electric power system. 

 

The Lightning Problem

Due to its extraordinary manifestation and the hazards for both lives and structures, lightning is a phenomenon that impacts society. Research around lightning develops valuable tools and procedures to recognize severe thunderstorms and protect people and equipment.

 

Figure 1. Lightning poses a hazard to both living creatures and structures it comes in contact with.

 

Earlier lightning investigations gathered an understanding of the discharge process. Nowadays, many researchers worldwide continue to successfully resolve the many remaining questions. However, the complexity of the phenomena challenges quick and complete explanation.

A lightning strike is a high current discharge that lasts only several millionths of a second.

A widely accepted theory of lightning holds that clouds acquire charge or are at least polarized. They contain separate negative and positive charges that attract opposite polarity charges within the cloud and between it and neighboring masses such as the Earth and other clouds, creating strong electric fields.

The potential gradient in the air between charge centers within a cloud or between a cloud and Earth is not uniform but is greatest where the charge concentration is highest. The highest charge concentration and most significant voltage gradient in cloud-to-Earth discharges generally occur in the cloud.

Whenever the voltage gradient reaches the dielectric limit for air, the air in the high-stress concentration region ionizes. A breakdown or lightning flash occurs — this current discharge is frequently of high magnitude.

While intracloud and cloud-to-cloud lightning do not create a direct hazard to structures or persons on the ground, the induced voltages in long cables present a risk to control and signal equipment employing electronic or semiconductor devices.

Around 90% of the discharges logged by the National Lightning Detection Network in the USA are cloud-to-cloud. Cloud-to-cloud lightning constitutes dangerous electromagnetic interference (EMI) threats to aircraft, prompting avionics designers to study them.

Cloud-to-ground lightning discharges are of utmost interest because of their hazards to life and property. The high current flowing during the lightning flash can melt conductors, ignite fires, damage equipment, trip power circuits, and produce deadly voltages for living creatures.

Despite its short duration, lightning is the most significant single cause of power outages, as concluded in many operating companies’ reports worldwide. Lightning strikes that produce problems for the power engineer happen on or near a power system. 

 

The Accumulation of Electricity in Clouds

There are several theories to explain the accumulation of electricity in clouds. Two of them are:

1. Wilson’s Ionization Theory
2. Simpson and Scrase’s Breaking-drop Theory

 

Wilson’s Ionization Theory 

C.T.R Wilson (1920) explains his theory by following the progress of water droplets through the rising air currents of a thunderstorm, attributing the droplets’ electrification to contact with air ions. The atmosphere commonly presents many small positive or negative ions with mobilities of about 1 cm/s under the action of a 1 V/cm field. There are also many large ions of much smaller mobility.

According to Wilson, the number of these ions increases in thunderclouds due to the substantial electrical fields. The raindrops falling or rising in the air currents of the thunderstorm shall meet the ions. These drops are polarized with a positive charge on their lower surface and a negative charge on their upper surface, which is the field's normal direction. Later, they attract negative charges to themselves.

The atmosphere contains clusters of ions of both signs at all times and the raindrops capture them selectively, acquiring a negative charge and leaving a preponderance of positive charge in the air. Updrafts carry the positive air and lighter drops to the top of the cloud. The larger raindrops bring a negative charge to the base of the cloud. Thus, according to Wilson, the cloud’s upper region becomes positively charged and the lower area becomes negatively charged. The polarization of most thunderclouds happens this way.

 

Simpson and Scrase’s Breaking-Drop Theory

Simpson and Scrase (1937) made investigations in clouds with instruments sent up in balloons, measuring the potential gradient’s magnitude and its polarity throughout its height and at different portions of the cloud. They also measured the electric field at the Earth’s surface beneath a thundercloud as it passed overhead.

According to this theory, when water droplets break-up they get a positive charge and the surrounding air obtains a negative charge.

Figure 2 shows what Simpson and Scrase believe happens in the cloud. Positive and negative signs indicate the charges within the cloud. The cloud’s progress is right to left, and the solid lines represent streamlines of air, with their separation proportional to the wind velocity. This separation shows the high winds that appear as the storm approaches.

 

Figure 2. A generalized diagram showing air currents and distribution of electricity in a typical heat thunderstorm. Simpson and Scrase, 1937.

 

The air enters the storm from the left and passes under the cloud’s front, where it takes an upward direction. This upward current prevents raindrops from falling through it. Drops falling in this region are broken up, and the charges separate.

The lower region of positive charge is associated with a strong upward current. To the rear of this region, the vertical wind is weaker and the resulting heavy rain is positively charged. Apart from this local area of positive charge, the lower half of the cloud is negative and the top is positive.

The region of separation between the negative charge and the upper positive charge occurs at levels with temperatures between 0°C and -20°C. These temperatures are below the freezing point. For this reason, the deduction is that the generation of the upper charge depends on the presence of ice crystals and not on the existence of water drops.

The air near the top of the cloud tends to become positively charged, while the negatively charged ice crystals move slowly down to melt and recycle or fall as rain. The position of the lower positive charge supports the idea that the breaking-drop process generates it.

Simpson and Robinson confirmed these conclusions in 1941. These intricate and active charge patterns create conditions favorable for a lightning strike.

 

The Mechanism of a Lightning Strike

Schondland et al. gave an excellent description of lightning in a series of papers published from 1934 to 1938. A lightning strike to Earth starts when the charge along the cloud base produces a concentration of opposite charge on the Earth (Figure 3).

 

Figure 3. The cloud leads to the accumulation of opposite charges on the Earth.

 

Whenever the voltage gradient reaches the limit for air, the air in the region of high-stress concentration ionizes or breaks down, producing an ionized channel to Earth. The electric field intensity to cause breakdown at atmospheric pressure is approximately 30 kV/cm. In the cloud, considering the moisture content and lower pressure, the voltage gradient is lower, on the order of 10 kV/cm.

Observations made with the “Boys camera” – developed by Charles V. Boys in 1926 to produce a time-resolved image of the phenomenon – indicate that the breakdown creates a stepped leader strike. 

 

Figure 4. Charles V. Boys with his camera developed specifically to take pictures of lightning.

 

The stepped leader is a discharge that progresses somewhat unexpectedly by short steps from the cloud to the Earth. Figure 5 shows a schematic diagram of a Boys camera.

 

Figure 5. Schematic diagram of a Boys camera. C.V. Boys, 1926.

 

The cloud’s charge flows through the ionized channel, sustaining the high voltage gradient at the channel’s tip, keeping the breakdown process ongoing. The establishment of a lightning strike is a gradual breakdown of the arc path instead of the air’s instantaneous breakdown for the total channel’s length.

Figure 6 shows downward leaders spreading from a cloud to Earth.

 

Figure 6. Stepped leaders propagate toward Earth.

 

A leader step is about 50 m long, completed in approximately 1 µs. The leader takes at least several microseconds to reach the Earth’s surface due to the irregular path and pauses between pushes.

The leader’s direction is toward Earth, but every step’s specific angle of departure is random. Each step approaches Earth at a different angle, giving the overall lightning flash its typical zigzag appearance.

The reason for the step leader recesses seems to be a depletion of the charge centers, reducing the electric gradient at the tip below the critical value for ionization at that position. The leader progresses quickly when receiving a new charge from the cloud.

In 1958, Griscom proposed the prestrike theory as a stepping mechanism. This theory states that a discharge similar to the leader rises from Earth to meet the leader before it reaches the ground. As the stepped leaders approach the Earth, the electric field at the surface grows until it exceeds the critical magnitude to originate upward connecting strikes. Then, upward strikes, usually from high points in the vicinity, intercept the downcoming leaders (Figure 7).

 

Figure 7. A lightning strike to Earth, showing upward strikes.

 

The launch of an upward strike from Earth starts the attachment process. When downward and upward discharges meet, they complete the connection.

A high-current power return strike moves quickly up the leader´s ionized channel after connection to Earth. This strike is more intense and faster than the leader. The result is the neutralization of the charge in the leader’s channel or the channel’s gradual discharge to Earth (Figure 8). The leader and the return strike contribute to transport charge from cloud to ground.

 

Figure 8. Power return strikes from Earth to cloud.

 

The leader originating the first return strike takes what looks like an optically intermittent course. Frequently, there will be several strikes to Earth down the initial channel. What looks like a single flash of lightning is the effect of several high-amplitude, short-duration current impulses or strikes — as many as 30 or 40.

The leaders triggering the return strikes that follow move continuously as a downward dart through the preceding return strike path and are called dart leaders.

 

The Empire State Building Study

What happens when the ground is a tall object, like a building, tree, or electric power line?

Between 1935 – 1941, McEachron and colleagues photographed strikes on top of the Empire State Building in New York City employing the Boys camera. The study was discontinued during the war and resumed in 1948.

The Empire State Building is a steel-frame structure topped by a tower reaching a height of 380m. An elaborate procedure using a set of instrumentation was employed to record as much data as possible. A fundamental discovery made was that in virtually all cases, the first stepped leader advanced upward from the top of the building to the cloud, rather than downward from the cloud as found in flat land. Only in a few cases did they find the original stepped leaders were downward.

There wasn’t a return streamer from the cloud after the upward stepped leader. But succeeding discharges consisted of a continuous downward leader and an upward return streamer.

Another discovery was that a small current, perhaps of a few hundred amperes, continued to flow between current peaks. The researchers concluded that it was a direct-current arc likely to persist for the strike’s entire duration with superimposed current peaks of several magnitudes.

More recent research determined that upward lightning discharges occur only from entities taller than about 100m or bodies of lesser height stationed on mountain tops.

 

A Review of Lightning Research and Characteristics

Lightning surges and strikes can be very destructive to life and power system equipment. They are a frequent cause of power outages and damage to property.

The buildup of electricity in clouds is associated with ionized air, moisture in the atmosphere, and upward winds.

The impact of ice on ice in the cloud’s upper regions may produce a separation of electric charge, similar to raindrops breaking.

Usually, the lower portion of the cloud is mostly negative, and the upper part mainly positive, with a region of mixed charge at levels with temperatures between 0°C and -20°C.

Another mechanism in the accumulation of charges is the water to ice transition in the cloud.

Photographs of lightning strikes taken with the Boys camera led to the following conclusions regarding the mechanism of the lightning strike to relatively flat terrain or low structures:

• Most strikes recorded originated from negative polarity clouds.
• The process opens with a stepped leader flowing from cloud to Earth.
• Each leader approaching Earth instigates upward connecting strikes.
• After connection to Earth, a high-current power return strike flows rapidly up the leader´s ionized channel.
• Successive strikes have a continuous or dart leader proceeding downward from the cloud.
• The strikes consist of many separate discharges.

A study on the Empire State Building discovered a difference in the strike mechanism: most of the original stepped leaders proceed upward from the top of the building to the cloud, rather than downward from the cloud as is the case with flat terrain and lower structures, and no return streamers followed. The following discharges’ stepped leaders were downward from cloud to Earth, and all the return strikes were upward from Earth to cloud.