以后我应该会将课程内容要点写在博客里，一是以求备份，二是做个分享。而已。

What is Protective Relay?

A relay whose function is to detect defective lines or apparatus or other power system conditions of an abnomal or dangerous nature and to initate appropriate control circuit action. --IEEE PES

"detect" and "initate" is what exactly relay does. One point needs to be attention: Protection ≠ Prevention.

Fuse is a kind of "over current protective device".

Goal: Minimaize the impact of outages from two aspect, time and size.

When we design a protective relay system, there are follows need to be considered:

1. Relay Setting

2. Maintainence Practices

3. Exogenous Conditions

4. Maximum sensitivety to faults (decrease number of false negative)

5. Avoid relay aperation under all tolerable conditions (False Positive)

So, what exactly is false negative and false positive? given two examples.

False negative: Current is nomal but temperature isn't due to spillage of oil. Something is wrong but relay system keeps quiet.

False positive: 110% of rated current in Motor.

Relay System

Input-->sensing-->taming-->amplification-->output-->circuit breaker

Relay contains all parts before circuit breaker. Again, relay is NOT breaker.