POWER SYSTEM PROTECTION

Power system

Generators

Transformers

Transmission lines

» Underground cables

» Overhead lines

Distribution systems

Sri Lanka Transmission system

PROBLEMS

• Abnormal situations

» Over loading» Over and under voltages» Under frequency» Over speeding

• Faults» Three phase faults» Phase-phase faults» Earth faults» Open circuits

FAULTS

• Excess current

• Liberates heat

• Damage at the point of fault

• Overheats the upstream system

• Creates voltage drops

Hence faults and abnormal operations have to be detected and cleared as quickly as possible.

SITUATIONS HAVE CHANGED…..

• Expansion of power systems

• Need for an automatic device to detect faulty/abnormal situations

• First automatic protection

FUSE• Very effective

• Still being used in distribution systems

• Complexity of power systems

• Development of protective gear based on automatic devices

- RELAYS

PROTECTIVE RELAYS

Protective relays monitor the state and operating conditions of all

elements of a power system and responds to the occurrence of faults

and disturbance

GENERAL REQUIREMENTS OF PROTECTIVE

RELAYING

• Selectivity

• Speed of Operation

• Sensitivity

• Dependability and Reliability

• Primary and Backup Protection

• Unit and Non Unit Protection