Substation design involves more than
installing apparatus, protective devices, and equipment. The
significant monetary investment and required reliable continuous
operation of the facility requires detailed attention to preventing
surges (transients) from entering the substation facility.
These surges can be switching surges,
lightning surges on connected transmission lines, or direct strokes
to the substation facility. The origin and mechanics of these surges,
including lightning, are discussed in detail in Chapter 10 of The
Electric Power Engineering Handbook (CRC Press, 2001).
This article focuses on the design
process for providing effective shielding (that which permits
lightning strokes no greater than those of critical amplitude [less
design margin] to reach phase conductors [IEEE Std. 998-1996])
against direct lightning stroke in substations.
The Design Problem
The engineer who seeks to design a
direct stroke shielding system for a substation or facility must
contend with several elusive factors inherent in lightning phenomena,
namely:
• The unpredictable, probabilistic
nature of lightning
• The lack of data due to the
infrequency of lightning strokes in substations
• The complexity and economics
involved in analyzing a system in detail
There is no known method of providing
100% shielding short of enclosing the equipment in a solid metallic
enclosure. The uncertainty, complexity, and cost of performing a
detailed analysis of a shielding system has historically resulted in
simple rules of thumb being utilized in the design of lower voltage
facilities. Extra high voltage (EHV) facilities, with their critical
and more costly equipment components, usually justify a more
sophisticated study to establish the risk vs. cost benefit.
Because of the above factors, it is
suggested that a four-step approach be utilized in the design of a
protection system:
1. Evaluate the importance and value of
the facility being protected.
2. Investigate the severity and
frequency of thunderstorms in the area of the substation facility and
the exposure of the substation.
3. Select an appropriate design method
consistent with the above evaluation and then lay out an appropriate
system of protection.
4. Evaluate the effectiveness and cost
of the resulting design.
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