Aspects of Protection system

Reliability

• Dependability - protection should operate when it should operate
• Security - protection should not operate when it should not operate

Speed
We need fast protection to minimize the damage

Selectivity/Discrimination
Zones of protections are determined by the CTs
Understand the type of the fault and fault location

Cost
Cost/beneficial analysis
fast operation and duplication require additional cost
Back up protection

ANSI reference numbers
21 - Distance relay
50 - Instantaneous over current
51 - time delayed over current
52 - circuit breaker
67 - Directional over current
87 - Differential