Substation Grounding System – Why is it Required?

A. Purpose of Substation Grounding System

The substation grounding system is a crucial component of the overall electrical infrastructure. Proper grounding serves two key purposes:

  1. It enables the safe dissipation of electric current into the earth without surpassing the operational limits of the equipment.
  2. It ensures a secure environment, protecting personnel near grounded facilities from electric shock hazards during fault conditions.

B. Grounding System

The grounding system encompasses all interconnected grounding elements within the substation, including the ground grid, overhead ground wires, neutral conductors, underground cables, foundations, and deep wells. The ground grid consists of interconnected horizontal bare conductors (mat) and ground rods. Its design aims to control voltage levels within safe limits while maintaining cost efficiency.

This information primarily focuses on personnel safety. However, data related to grounding system resistance, grid current, and ground potential rise can also help assess whether equipment operational limits are exceeded.

Safe grounding relies on the interaction of two grounding systems:

  1. Intentional ground – Comprising grounding systems buried at a certain depth below the earth’s surface.
  2. Accidental ground – Temporarily formed when a person is exposed to a potential gradient near a grounded facility.

C. Common Misconception

A common misconception is that any grounded object is always safe to touch. However, low substation ground resistance alone does not guarantee safety. There is no direct correlation between the overall grounding system resistance and the maximum shock current a person may experience. A substation with low ground resistance may still present hazards, while another with high ground resistance could be safe if designed properly.

Multiple factors influence voltage levels within and around a substation. Since these voltages are site-specific, a single grounding design cannot be universally applied. Critical factors such as grid current, fault duration, soil resistivity, surface material, and the size and configuration of the grounding grid significantly impact voltage conditions. If factors like electrode placement, soil characteristics, and grounding geometry result in excessive potential gradients at the earth’s surface, the system may be unsafe, even if it can handle fault currents within the limits set by protective relays.

See also  Identification of Grounded Conductors - Philippine Electrical Code 2017 Edition

During a typical ground fault, if not properly designed, the maximum potential gradients on the earth’s surface may reach hazardous levels, posing a risk to personnel. Additionally, dangerous voltages can develop between grounded structures, equipment frames, and the surrounding earth, further increasing the risk of electric shock.

D. Conditions Leading to Electric Shock Accidents

Electric shock incidents occur when several unfavorable conditions align, including:

  • High fault current to ground relative to the grounding system’s area and its resistance to remote earth.
  • Soil resistivity and ground current distribution that create high potential gradients at points on the earth’s surface.
  • Human presence at a critical location where the body bridges two points of high potential difference.
  • Lack of sufficient contact resistance or other series resistance to limit body current to a safe level.
  • Extended duration of fault and body contact, allowing current to flow through the body long enough to cause harm at a given intensity.

Despite these risks, electric shock accidents are relatively infrequent due to the low probability of all these factors coinciding.

E. Ensuring Personnel Safety

To protect personnel in and around a substation, the grounding system is designed to limit potential differences to safe levels. IEEE Std. 80, IEEE Guide for Safety in AC Substation Grounding, provides guidelines and design equations for developing a safe grounding system.

The guide’s approach is based on permissible body current, ensuring that accidental grounding does not lead to ventricular fibrillation (heart failure). By controlling the voltages that generate body current, the design methodology keeps exposure within safe limits.

References:

  • Electric Power Engineering Handbook, Second Edition, Leonard L. Grigsby
  • IEEE Std. 80, IEEE Guide for Safety in AC Substation Grounding
See also  Three Elements of Good Engineering Design

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.