Lightning phenomena (continued)

Stroke formation 

Types of strokes

There are a number of different types of lightning strokes. These include strokes within clouds, strokes between separate clouds, strokes to tall structures, and strokes that terminate on the ground. The positive and negative strokes terminating on the ground are the types of most interest in designing shielding systems and the following discussion will be confined to those types.

Stepped leaders

The actual stroke development occurs in a two-step process.  The first step is ionization of the air surrounding the charge center and the development of stepped leaders, which propagate charge from the cloud into the air. Current magnitudes associated with stepped leaders are small (in the order of 100 A) in comparison with the final stroke current.  The stepped leaders progress in a random direction in discrete steps from 10 to 80 m in length. Their most frequent velocity of propagation is about 0.05% the speed of light, or approximately 150 000 m/s. This produces electric fields near ground with rise times on the order of 100 to 500 microseconds. Electric fields of 250 microseconds from switching surge overvoltages tend to produce the minimum electrical strength of large air gaps compared to 1.2/50 microsecond lightning overvoltages. It is not until the stepped leader is within striking distance of the point to be struck that the leader is positively diverted toward this point. Striking distance is the length of the last step of leader under the influence of attraction toward the point of opposite polarity to be struck.  

Return stroke

The  second  step  in  the  development  of  a  lightning  stroke  is  the  return  stroke.  The return stroke is the extremely bright streamer that propagates upward from the earth to the cloud following the same path as the main channel of the downward stepped leader. This return stroke is the actual flow of stroke current that has a median value of about 24 000 A and is actually the flow of charge from earth (flat ground) to cloud to neutralize the charge center. The velocity of the return stroke propagation is lower than the speed of light and varies with atmospheric conditions; an approximate value can be 10% of the speed of light. The  amount  of  charge  (usually  negative)  descending  to  the  earth  from  the  cloud  is  equal  to  the  charge (usually positive) that flows upward from the earth. Since the propagation velocity of the return stroke is so much greater than the propagation velocity of the stepped leader, the return stroke exhibits a much larger current flow (rate of charge movement). The various stages of a stroke development are shown in Figure 2.

Approximately 55% of all lightning flashes consist of multiple strokes that traverse the same path formed by the initial stroke. The leaders of subsequent strokes have a propagation velocity much greater than that of  the  initial  stroke  (approximately  3%  the  speed  of  light)  and  are  referenced  as  "dart  leaders".

Figure: 02

Lightning phenomena

Charge formation in clouds

Numerous theories have been advanced regarding the formation of charge centers, charge separation within a cloud, and the ultimate development of lightning strokes.

1. ·        One theory attributes charge separation to the existence of both positive and negative ions in the air and the existence of a normal electric field directed toward the earth.  Large  drops  of  water  in  the  electric  field  are  polarized,  the  upper  sides  acquiring  a negative charge and the lower sides a positive charge. As the polarized drops of water fall due to gravity, the undersides (positive sides) attract negative ions, while no such action occurs at the upper surfaces. As a result of this action, the drops accumulate negative charge.  Thus, the original charges, which were distributed at random and produced an essentially neutral space charge, become separated. The large drops of  water  carry  the  negative  charges  to  the  lower  portion  of  the  cloud,  causing  the  lower  portion  to  be negatively charged and the upper portion to be positively charged.
2. ·        Another theory is that the interaction of ascending wind currents in the leading head of a cloud breaks up the water droplets causing the resulting droplets to be positively charged and the air to be negatively charged. The positively charged water droplets are unable to fall through the ascending wind currents at the head of the cloud, which causes this portion of the  cloud  to  be  positively  charged  while  the  remaining  larger  portion becomes negatively  charged. 
3. ·        Yet another theory suggests that there are regions of subzero temperature within a cloud and the subsequent formation of ice crystals is an essential factor in the explanation of the charge centers within clouds.

The  important  fact  to  the  designing  engineer  is  that  a  charge  separation  does  occur  in thunderstorm clouds. 

Experiments using balloons equipped with electric gradient  measuring  equipment  have  been  performed  to  investigate  typical  charge  distribution  in thunderclouds,  and  these  experiments  have  shown  that,  in  general,  the  main  body  of  a  thundercloud  is negatively charged and the upper part positively charged. A concentration of positive charge also frequently exists in the base of the cloud. Such charge distribution in a cloud causes an accumulation of charge of the opposite polarity on the earth’s surface and on objects (e.g., trees, buildings, electric power lines, structures, etc.) beneath the cloud.

A typical charged cloud and the resulting electric fields are shown in Figure 1 (Note that the plot in Figure 1 is of the electric gradient as the cloud moves over the ground, not the amount of charge below the cloud.)

Figure: 01

The electrical charge concentrations within a cloud are constrained to the size of the cloud. The cloud size, in relation to the earth, is small. Therefore, the electrical gradient that exists in the cloud is much greater than at the earth. Because of this, an electrical discharge tends to be initiated at the cloud rather than at the ground.

Substation lightning shield designing standards

There two main standards in practice for lighting shield designing. And each contains different calculation methods.

1. IEEE Guide for Direct Lightning Stroke Shielding of Substations (IEEE 998-2012).

2. Protection against lightning (IEC 62305-1,2,3,4).











One of the popular method included in IEEE 998 is the electrogeometric model (EGM) by rolling sphere method.