Need for protection and causes of faults

Need for a protection system

A power system cannot be designed to be fault-free. When a fault occurs, very high current flows and voltage collapses. Protection is required to:

• Disconnect the faulty part quickly, so that equipment (generators, transformers, cables) is not damaged by heating and mechanical forces.
• Maintain continuity of supply to healthy parts of the system.
• Preserve system stability (keep generators in synchronism).
• Prevent fire, explosion and danger to human life.
• Detect abnormal conditions like overload, under/over voltage and unbalance before they become faults.

Causes of electrical faults

1. Insulation failure: ageing, overheating, moisture, contamination and over-stress break down insulation of cables, transformers and machines.
2. Lightning: direct strokes or induced surges cause flashover of line insulators (the most common cause of overhead line faults).
3. Switching surges: over-voltages during switching of lines and capacitor banks can flash over weak insulation.
4. Mechanical causes: falling trees, birds, animals and kites bridging conductors; conductor breakage by wind or ice; excavation damage to cables.
5. Environmental pollution: salt or industrial dust deposits on insulators cause flashover in wet conditions.
6. Human error: wrong switching operations, closing on earthed equipment, poor maintenance.
7. Equipment failure: defects in breakers, CTs, joints and terminations.
8. Overloading: sustained overload overheats conductors and insulation, leading to breakdown.

Most faults (about 70–80 % on overhead lines) are single line-to-ground faults; three-phase symmetrical faults are rare (about 5 %) but are the most severe.