Power system protection is an integral part of every power system. All power equipment including power generators, step-up transformers, step-down transformers, transmission lines, power capacitors, electric motors, other loads, etc needs protection. The necessity for protection is incurred by all kinds of contingencies such as equipment failure due to insulation deterioration, lightning strike, short-circuit by nature force or creature-made happenings, inappropriate operation of power system, and other inadvertent incidences. Some power equipment is very expensive such as MW generators which could cost millions of dollars. Furthermore, an outage due to the failure of the power system causes severe damage to the economy and inconvenience to people’s daily life. A properly designed protection can ensure power supply is cut to minimum users yet continue to supply power to other end users in the case that a fault occurs in the system. It is a sophisticated art that needs systematic study in order to master. All these calls for a new module for undergraduate students to learn in the field of power system protection.

1. Basics of Protection

2. Agenda
• Why protection is needed
• Principles and elements of the protection system
• Basic protection schemes

3. Disturbances: Light or Severe
• The power system must maintain acceptable
operation 24 hours a day
– Voltage and frequency must stay within certain limits
• Small disturbances
– The control system can handle these
– Example: variation in transformer or generator load
• Severe disturbances require a protection system
– They can jeopardize the entire power system
– They cannot be overcome by a control system

4. Power System Protection
Operation during severe disturbances:
– System element protection
– System protection
– Automatic reclosing
– Automatic transfer to alternate power supplies
– Automatic synchronization

5. Protection System
A series of devices whose main purpose is to
protect persons and primary electric power
equipment from the effects of faults
The “Sentinels”

6. Blackouts
• Loss of service in a large
area or population
region
• Hazard to human life
• May result in enormous
economic losses
• Overreaction of the
protection system
• Bad design of the
protection system
Characteristics Main Causes

7. Short Circuits Produce High Currents
Fault
Substation
a
b
c
I
I
Wire
Three-Phase Line
Thousands of Amps

8. Protection System Elements
• Protective relays
• Circuit breakers
• Current and voltage transducers
• Communications channels
• DC supply system
• Control cables

9. Three-Phase Diagram of the Protection Team
CTs
VTs
Relay
CB
Control
Protected
Equipment

10. Circuit Breakers

11. Current Transformers
Very High Voltage CT

12. Voltage Transformers
High Voltage

13. Protective Relays

14. Examples of Relay Panels
Old Electromechanical
Microprocessor-
Based Relay

15. THANK YOU