Overlapping zones and qualities of relaying

Why protective zones are overlapped

A zone boundary is fixed by the CT location. If the zones did not overlap, for example if the bus relay took its current from a CT on the bus side of a breaker and the line relay from a CT on the line side, the breaker and the short connection between the two CTs would belong to no zone. A fault there would be a blind spot and could be cleared only by slow back-up protection.

Therefore the CT of each zone is placed on the far side of the common breaker, so the zones overlap across the breaker:

       Zone 1 (bus)
   ◄──────────────────►
 ──●──CT2──[ CB ]──CT1──●── line
         ◄──────────────────►
             Zone 2 (line)
 CT1 feeds bus relay, CT2 feeds line relay

• A fault inside the overlap trips the breakers of both zones; this is acceptable because the overlap is very small.
• Overlap ensures that every point of the system, including the breakers, is protected.
• With dead-tank breakers and bushing CTs, overlap is easily obtained.

Essential qualities of protective relaying

1. Selectivity: trips only the breakers needed to isolate the faulty element.
2. Speed: clears the fault quickly (a few cycles) to reduce damage and maintain system stability.
3. Sensitivity: responds to the minimum fault current, e.g. high-resistance earth faults.
4. Reliability: dependable (always trips when required) and secure (never trips when not required).
5. Stability: remains inoperative for through-faults, power swings and transformer inrush.
6. Simplicity: least number of relays and contacts; easy maintenance.
7. Economy: cost of protection proportional to the importance of the equipment.

These qualities often conflict, e.g. higher speed and sensitivity may reduce stability; good relaying balances them for each application.