DSLP Calculator (Direct Stroke Lightning Protection)

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DSLP Calculator – Corrected DFLB Version

Direct Stroke Lightning Protection (DSLP) Calculation Method

Direct Stroke Lightning Protection (DSLP) is used in substations, switchyards, transformers, and other high-voltage installations to protect equipment against direct lightning strokes. The method determines the protection zone created by lightning masts (LMs) based on mast height and the height of the equipment being protected.

The calculations shown in the uploaded images are based on the procedure given in High Voltage Engineering by Prof. D. V. Razevig, Chapter 31, translated by Dr. M. P. Chourasia.

Purpose of DSLP Calculations

The DSLP method is used to determine:

  • Protective radius of a single lightning mast
  • Allowable spacing between two lightning masts
  • Midpoint protection between two lightning masts
  • Safe protection zone for electrical equipment

These calculations ensure that all live parts remain within the lightning protection zone.

Parameters Used in DSLP Calculations

1. Height of Lightning Mast

\[
h = \text{Height of Lightning Mast}
\]

Unit: meters (m)

2. Height of Equipment to be Protected

\[
h_x = \text{Height of the live part to be protected}
\]

This may be:

  • Transformer top
  • Busbar
  • Gantry conductor
  • Switchyard equipment

Unit: meters (m)

Protective Radius of a Single Lightning Mast

The protective radius is denoted by:

\[
r_x
\]

It represents the horizontal protection distance available at the protected height (h_x).

The formula depends on:

  • Mast height (h)
  • Protected height (h_x)
  • Coefficient (P)

Coefficient for Lightning Mast Height

The coefficient (P) depends on the lightning mast height.

For mast height greater than 30 m

\[
P = \frac{5.5}{\sqrt{h}}
\]

For mast height less than or equal to 30 m

\[
P = 1
\]

Case 1: When ( h_x \le \frac{2}{3}h )

If the protected height is less than or equal to two-thirds of the mast height, the protective radius is calculated as:

For ( h > 30,m )

\[
r_x = 1.5h\left(1-\frac{h_x}{0.8h}\right)P
\]

For ( h \le 30,m )

Since (P = 1),

\[
r_x = 1.5h\left(1-\frac{h_x}{0.8h}\right)
\]

Interpretation of the Formula

The protective radius decreases as:

  • the protected height increases, or
  • the mast height decreases.

Therefore:

  • taller lightning masts provide larger protection zones,
  • higher equipment requires more careful placement of lightning masts.

Allowable Distance Between Two Lightning Masts

When two lightning masts are used together, the spacing between them must be limited to maintain continuous protection.

The allowable spacing is:

\[
S = 7 \times h_a \times P
\]

Where:

\[
h_a = h – h_x
\]

Meaning of Allowable Spacing

  • If spacing is too large, the midpoint between the masts may fall outside the protected zone.
  • Increasing mast height allows greater spacing.
  • Higher protected equipment reduces allowable spacing.

Midpoint Protection Between Two Lightning Masts

The midpoint between two lightning masts has a reduced effective height called:

\[
h_o
\]

It is calculated as:

\[
h_o = h – \left(\frac{S}{7P}\right)
\]

This equivalent midpoint height is used to calculate the midpoint protective distance.

Midpoint Protective Distance

The midpoint protective distance is denoted by:

\[
b_x
\]

Case 1: When ( h_x \le \frac{2}{3}h_o )

For ( h_o > 30,m )

\[
b_x = 1.5h_o\left(1-\frac{h_x}{0.8h_o}\right)P
\]

For ( h_o \le 30,m )

\[
b_x = 1.5h_o\left(1-\frac{h_x}{0.8h_o}\right)
\]

Case 2: When ( \[h_x > \frac{2}{3}h_o\] )

For higher protected objects:

\[
b_x = 0.75h_o\left(1-\frac{h_x}{h_o}\right)
\]

Importance of Midpoint Calculations

Midpoint calculations are extremely important because:

  • the protection zone narrows between two masts,
  • improper mast spacing can leave equipment exposed,
  • transformer bushings and busbars are often located near midpoint regions.

Practical Design Considerations

1. Mast Height Selection

Higher mast heights:

  • increase protection radius,
  • increase allowable spacing,
  • improve overall protection coverage.

However:

  • taller masts increase structural cost,
  • mechanical stability becomes critical.

2. Equipment Height

Higher equipment:

  • reduces protection radius,
  • reduces midpoint protection,
  • requires taller masts or reduced spacing.

3. Mast Spacing

The actual spacing between lightning masts should always satisfy:

\[
S_{actual} \le S_{allowable}
\]

Otherwise:

  • midpoint protection becomes inadequate,
  • equipment may remain exposed to direct lightning strokes.

Applications of DSLP Calculations

DSLP calculations are widely used in:

Conclusion

The Direct Stroke Lightning Protection method provides a systematic approach for designing lightning mast protection zones in high-voltage substations.

The calculations involve:

  • protective radius of single masts,
  • allowable mast spacing,
  • midpoint equivalent height,
  • midpoint protection distance.

Accurate implementation of these equations ensures reliable protection of expensive electrical equipment against direct lightning strokes.

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