POWER SYSTEM PROTECTION

DEVICESP R A K A S H K U M A R

THIS SEMINAR WILL COVER:

OBJECTIVE OF PROTECTION

COMPONENTS OF POWER SYSTEM NEED TO BE PROTECTEDPROTECTION DEVICESRALAYS AND ITS TYPECIRCUIT BREAEKRS AND ITS TYPEOTHER BREAKERS

OBJECTIVE OF POWER SYSTEM PROTECTION

• To isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.

Selection of protective relays requires compromises:• Maximum and Reliable protection at minimum

equipment cost• High Sensitivity to faults and insensitivity to

maximum load currents• High-speed fault clearance with correct selectivity• Ability to operate correctly under all predictable

power system conditions.

ART & SCIENCE OF PROTECTION

COMPONENTS OF POWER SYSTEMNEEDS TO BE PROTECTED.• GENERATORS• TRANSFORMERS• BUS-BARS• TRANSMISSION LINES

PROTECTIVE SYSTEM ELEMENTS• FUSES• PROTECTIVE RELAYS• CIRCUIT BREAKERS• INSTRUMENT TRANSFORMERS (C.T. & P.T.)• DC SUPPLY SYSTEM

FUSES•  A fuse  is a type of low resistance resistor that acts as a sacrificial device to

provide overcurrent protection.• A fuse interrupts an excessive current so that further damage by overheating

or fire is prevented.

RELAYSA protective relay is a compact device designed to sense the abnormal condition and trip a circuit breaker when a fault is detected

TYPES OF RELAYS:• ELECTROMECHANICAL RELAYS• STATIC RELAYS• NUMERICAL RELAYS• THERMAL RELAYS

ELECTROMECHANICAL RELAYS

STATIC RELAYS• Static relays have no moving parts . Measuring elements of static relays

have been successfully and economically built up from diodes, zener diodes, p-n-p and n-p-n transistors, field effect transistors or their combinations

• Higher sensitive than purely electromechanical relays. • provide fast operation, long life and low maintenance.

THERMAL RELAYSThe basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts.

NUMERICAL RELAYS

The relay has multiple microprocessor design. The microprocessors perform the software function such as signal processing , protection algorithm , and handling the human interface.

CIRCUIT BREAKER• A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by Overcurrent/overload or short circuit. Its basic function is to interrupt current flow after Protective relays detect faults condition.

• The commonly-available preferred values for the

    rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 

    32 A, 40 A, 50 A, 63 A, 80 A, 100 A and 125 A.

• The circuit breaker is labeled with the 

    rated current in amperes, but without 

    the unit symbol "A". Instead, the ampere

    figure is preceded by a letter "B", "C" or "D",

     which indicates the instantaneous tripping current

 

TYPES OF CIRCUIT BREAKER According to their arc quenching media

the circuit breaker can be divided as-• Oil circuit breaker.• Air circuit breaker.• SF6 circuit breaker.• Vacuum circuit breaker.

According to the voltage level of installation types of circuit breaker are referred as-• High voltage circuit breaker.• Medium voltage circuit breaker.• Low voltage circuit breaker.

LOW VOLTAGE CIRCUIT BREAKER.• Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and industrial application, and include:

• MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. 

MEDIUM VOLTAGE CIRCUIT BREAKER• Medium-voltage circuit breakers rated between 1 and 72 kV may be

assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation.

HIGH VOLTAGE CIRCUIT BREAKER• Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher.

OTHER BREAKERS

• Recloser—A type of circuit breaker that closes automatically after a delay. These are used on overhead electric power distribution systems, to prevent short duration faults from causing sustained outages.

• Polyswitch —Also known as resettable fuse , made of crystalline polymers. Used in computer power supply , aerospace and nuclear application and loudspeakers.

EARTH LEAKAGE CIRCUIT BREAKEROne terminal of the relay coil is connected to the metal body of the equipment to be protected against earth leakage and other terminal is connected to the earth directly.If any insulation failure occurs or live phase wire touches the metal body, of the equipment, there must be a voltage difference appears across the terminal of the coil connected to the equipment body and earth. This voltage difference produces a current to flow the relay coil.

 Residual Current Circuit BreakerOne CT core is energized from both phase wire and neutral wire. The polarity of the phase winding and neutral winding on the core is so chosen that, in normal condition mmf of one winding opposes that of another. In normal operating conditions the current goes through the phase wire will be returned via neutral wire if there's no leakage in between.

• Current transformers are used to step primary system currents to values usable by relays, meters, SCADA, transducers, etc.

• CT ratios are expressed as primary to secondary; 2000:5, 1200:5, 600:5, 300:5

• A 2000:5 CT has a “CTR” of 400

Current Transformers

VP

VS

Relay

• Voltage (potential) transformers are used to isolate and step down and accurately reproduce the scaled voltage for the protective device or relay

• VT ratios are typically expressed as primary to secondary; 14400:120, 7200:120

• A 4160:120 VT has a “VTR” of 34.66

Voltage Transformers

DC SUPPLY SYSTEMAll the circuit breakers of electrical power system are DC (Direct Current) operated. Because DC power can be stored in battery and if situation comes when total failure of incoming power occurs, still the circuit breakers can be operated for restoring the situation by the power of storage battery.

CONCLUSION Power system is protected to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.

The selection of the components or devices should be done carefully to mitigate the faults.

REFERENCEShttp://www.electrical4u.com/protection-system-in-power-system/

https://en.wikipedia.org/wiki/Power-system_protection

www.electrical-engineering-portal.com

Protection-and-Switchgear-by-U.A.Bakshi-and-M.V.Bakshi