Unit - 1 M1

Power System-III (Switch Gear and Protection)

Mani Sankar Matta, Asst. Professor, EEE Dept., AITAM Tekkali

13EE3017

HAPPY 2016


Unit – 1 CIRCUIT BREAKERS

• Module 1 – INTRODUCTION

• Module 2 – ARC

• Module 3 – CIRCUIT BREAKERS


Module – 1 INTRODUCTION


Power System Protection

• Electrical Power Systems - Protection

(Generators, T/Fs, Transmission & Distribution lines)

• Protection from ?

Short circuits – heavy currents

Abnormal conditions – over speeds, overvoltage,

under frequency, loss of excitation, over heating.....


What Components (Equipment) Do We Protect?

• Generators

• Transformers, Reactors

• Lines

• Buses

• Capacitors


Necessity of system protection

The protection is needed to remove as speedily as possible any element of the power system in which a fault has developed • To avoid abnormal operating regions - safety of

the equipment. • safety of the human personnel

• safety to healthy equipment

Mani Sankar Matta, Asst. Professor, EEE

Dept., AITAM Tekkali



Causes of faults

• Failure of Insulation – Short circuits

• Pollution(soot, cement dust, salt) – Insulator string – Flashover

• Failure of conducting path (open- circuit)

• Broken conductor falling on ground – Short circuit

• Tree Contact

• Abnormal loading

• Over voltages due to lightning

• Ice and Snow loading

• Animals (Birds, snakes, squirrels....)

• Earth Quakes, creepers, poor quality of system components



• T/F, Motor, Gen, Cable : Insulation failure, mechanical damage, accidental contact with earth, abnormal loading..

• Poor quality of system components



Fault Statistics

• Overhead line : 50%

• UG cables : 9%

• Transformers : 10%

• Generators : 7%

• Switchgears :12%

• CTs, PTs, Relays,

Control Equipment : 12%


Frequency of occurrence of different faults on OH lines

• Line to Ground (L-G) : 85%

• Line to Line (L-L) : 8%

• Double Line to Ground (2L-G) : 5%

• Three phase (3-Φ) :2%


Switch Gear and Protection

• apparatus used for switching, controlling and protecting the electrical circuits

• switching and interrupting currents - normal and abnormal conditions

• switches, fuses, relays and circuit breakers


• Function : prompt removal from service any element of a power system when it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage with effective operation of the rest of the system.

• Protection system and switch gear – doesn’t anticipate or prevent occurrence of a fault

• Buchholz relay – early warning of incipient faults


Relay

• detects the fault and initiates the operation of Circuit Breaker (CB)

• continuously monitors - electrical quantities (voltage, current, frequency and phase angle) which are different under normal and fault conditions

• operates by virtue of the current and/or voltage supplied to them by current transformers (CT) and voltage transformers (PT)


• If the value of actuating quantity (current, voltage, etc) is above “pick up” value the relay operates

• Operation of a particular relay signals the CB to open the appropriate circuit to isolate the faulty apparatus




Fig: Basic connection of protective relay - single line diagram


Fig : Basic connections of a protective relay

Circuit Breaker

• Isolates faulty part of the power system in case of abnormal conditions

• Relay detects abnormal conditions and sends a “tripping signal” to CB

• Fixed contact and moving contact


• Normal conditions : contacts remain closed

• If a trip signal is received the moving contact is separated from the fixed contact to interrupt the circuit

• Contacts can be opened manually or by remote control whenever desired



• As the contacts separate an arc is struck between the contacts

• Arc delays current interruption and also generates enormous heat

• Main problem in CB : arc should be extinguished in shortest possible time



• CB location - each generator, transformer, bus, transmission line, etc., can be completely disconnected from the rest of the system


Essential qualities of protection system

• Reliability: assurance that the protection will perform correctly.

• Selectivity: maximum continuity of service with minimum system disconnection.

• Speed of operation: minimum fault duration and consequent equipment damage and system instability.

• Simplicity: minimum protective equipment and associated circuitry to achieve the protection objectives.

• Economics: maximum protection at minimal total cost


Course outline

• Unit – 1 Circuit Breakers

• Unit – 2 Electromagnetic and Introduction of Static Relays

• Unit – 3 Generator & Transformer Protection

• Unit – 4 Feeder and Bus - Bar Protection

• Unit – 5 Protection against over voltage and grounding


Reference Books • Badri Ram, D. N. Vishwakarma, “Power System Protection

and Switchgear”, 2nd Edition, TMH Publications

• V K Mehta, Rohit Mehta, “Principles of Power system”, 4th Edition, S. Chand

• C. L. Wadhwa, “Electrical Power Systems”,3rd Edition New Age international (P) Limited

• Sunil S Rao, “Switchgear and Protection” Khanna Publishers


THANK YOU


Unit - 1 M1

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