Protection of Substation

PROTECTION OF SUBSTATION

Introduction  A Substation covers various equipment such as Circuit Breakers, Isolators, Earth Switches, Lightning Arresters, Grounding System and etc. Protection scheme is designed to limit the effects effect s of  disturbances in the power system which is allowed to persist, may damage plant and interrupt the supply of electric el ectric energy. The greatest threat to the security of a power supply station is short circuit which imposes sudden and violent change in he power system. Rapid isolation isol ation of the fault by nearest switchgear will minimize the damage. It covers various types of protection prot ection used in switchyard such as relays, circuit breakers and etc.

 ELECTRICAL SUBSTATION 

An electrical substation substation is a subsidiary station of electricity generation, generation, transmission and distribution system where voltage is transformed from high to low or low to high using transformers. Substations generally contains transformers and have switching,  protection and control equipment. equipment. In I n large substations, circuit breakers are used to interrupt any shortshort -circuits -circuits or overloaded currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or  fuses for protection of branch circuits.

 D ESIGN  The selection of the location a substation must have following factors: Sufficient land area is required for installation of equipment 1. with necessary clearances for electrical safety and access to maintain large apparatus such as transformers. transformers. The site must have room for expansion due to load growth or  2.  planned transmission additions. Environmental effects effects must be considered, such as drainage, noise and road traffic effects. Grounding and ground potential rise must be calculated to 3.  protect passers by during a short  ±circuit ± circuit in transmission system. The substation site must be reasonably central central to the 4. distribution area to be served.

 D FOR PROTECTION   NEE  

A protective scheme isolates the faulty section of the system from the healthy sections. An electric power system consists of generators, transformers, transmission transmission lines and distribution lines etc. Short circuits and other abnormal conditions often occur on the power system. system. The heavy current associated with short circuits is likely to cause damage to the equipment if suitable  protection at each section of the power system is not provided.

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A protective scheme scheme includes major circuit breakers and protective relays to isolate the faulty section of a healthy system from healthy sections. A circuit  breaker can disconnect the faulty element of the section when it is called upon to do so by the protective relay. The function of a protective relay is to detect and locate a fault and issue a command command to the circuit breaker to disconnect the faulty element.

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Thus if a fault occurs in an element of a power system, an automatic protective device is needed to isolate the faulty element as quickly as possible to keep the system in healthy operation.

 D CAUSES OF FAULTS   NATURE AN  Faults

are mainly caused by:

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Insulation failure. Conducting path failure. Overvoltages due to lightening or switching surges. Puncturing or breaking of insulators. Accumulation of foreign particles on the surface of string and pin insulators. Failure of conducting path due to broken conductors. Faults on overhead lines due to lightening strokes, ice and snow loading, storms and etc. Tripping of circuit breakers due to errors erro rs in switching operation, testing or  maintenance work, defects in protective devices. poo r quality of system components or because of faulty Faults due to poor system design. Failure of solid insulation due to aging, heat, moisture, overvoltage, accidental contact with with earth or earth screens, flashflash -over over voltages and etc.,

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COMPONENTS OF PROTECTION  The main components used in the protection scheme in a substation to isolate the faulty section of the system from the healthy section are       

FUSES EARTHING INSULATION ISOLATORS RELAYS CIRCUIT BREAKERS LIGHTNING ARRESTERS

FUSE  A fuse is a protective device used for protecting cables and electrical equipment equipment against overloads and short circuits. A fuse was patented by Thomas Edison in 1890. It is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit. In electrical and electronics engineering, a fuse (short for  fusible link) is a type of overover -current -current protection device. Its typical typical component is a metal wire or strip (element) that melts when too much current flows, which interrupts i nterrupts (disconnects) (disconnects) the circuit in which it is connected. connected. Circuit or device failure is often a reason for excessive current. A fuse blows (interrupts excessive current) so that further damage is prevented. A fuse typically is not intended to protect from the initial cause of overover -current. -current.

TYPES OF FUSES  In general fuses are classified into two types: HIGH VOLTAGE FUSE LOW VOLTAGE FUSE B. The low voltage fuses are subdivided into two types: Semi-Enclosed Semi-Enclosed Rewireable Fuse  HighHigh-Rupturing -Rupturing Capacity Cartridge Fuse  The high voltage fuses are categorised into three types: Cartidge Type  Liquid type  Metal Type  A.

 EARTHING  In electrical engineering, engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return  path for electric current, or a direct physical connection to the Earth. The term ground and grounding are used in electrical engineering engineering to represent repr esent electrical equipment equipment that is securely bonded to the ground for safety reasons. In electricity supply systems, an earthing (grounding) system defines the electrical  potential of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the safety and electromagnetic compatibility of the power supply. A functional earth connection serves a purpose other than providing protection against electrical shock. In contrast to a protective p rotective earth connection, connection, a functional functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as surge suppression and electromagneticelectromagnetic electromagnetic netic--compatibility -compatibility filters, some types of antennas and various measurement instruments. Generally the protective earth is also used as a functional earth, though this requires care in some situations.

 INSULATOR An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, equipment, also called insulators or  insulation, intended to support or separate electrical conductors without  passing current through themselves. Insulators used for highhigh-voltage -voltage power transmission are made from glass,  porcelain, or composite polymer polymer materials. materials. Porcelain insulators insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed dirt. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain has a dielectric strength of about 4±  4 ±10 10 kV/mm. Glass has a higher dielectric strength, strength, but b ut it attracts condensation and the thick  irregular shapes needed for insulators are difficult to cast without without internal strains. Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to ceramic materials.

TYPES OF INSULATORS   PIN

TYPE INSULATORS

 SUSPENSION  STRAIN

TYPE INSULATORS

INSULATORS

 SHACKLE

INSULATORS

 ISOLATOR The isolators are used to connect and disconnect the high voltage  power systems under no load load conditions. conditions. CircuitCircuitIsolator  Isolator Isolator disconnec disco disconnect nnectt provides provides three three--pole, - pole, pole, group group--operated, -operated, operated, visiblevisibleair  air--gap -gap isolation in distribution substations. The CircuitCircuit Isolator  Isolator Isolator can be used used to interrupt interru interrupt pt lowlow-level -level charging currents associated with substation bus work and circuitcircuit -breaker - breaker breaker bushings, as well as other lowlow-voltage -voltage currents commonly present in substations. CircuitCircuit-Isolator  -Isolator Isolator is available in ratings ratings from from 69 kV through through 138 138 kV, in 1200 A, 1600 A, and 2000 A versions. SideSide Si de-break, de- break, break, verticalvertical  break, centercenter -break, - break, break, and double double--break - break break styles can be furnished, to suit almost any substation layout.

 RELAYS  A Relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or  many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher   power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier. A relay is a automatic device which detects an abnormal condition in an abnormal condition in an electrical circuit and causes a circuit breaker to isolate the faulty element of the system. In some cases it may give an alarm or visible indication to alert operator.

CLASSIFICATION OF RELAYS BASE   D ON CONSTRUCTION 

Relays are broadly classified into following categories depending on the technology of their construction and operation:  Electromagnetic Relay  Static Relay Microprocessor Relay  CLASSIFICATION CLASSIFICA TION OF RELAYS BASE   D ON THEIR FUNCTION 

Protective relays relays can be classified into following categories, depending on the duty they are required to perform.     

OverCurrent Relays Undervoltage Relays Impedance Relays Under Frequency Relays Directional Relays

CIRCUIT BREAKER A circuit breaker is an automatically automatically--operated -operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. An early form of circuit breaker was described by Edison in an 1879 patent application, although his commercial power distribution system used fuses. Its purpose was to protect lighting circuit wiring from accidental accidental short short-circuits and overloads. A protective relay detects abnormal conditions and sends a tripping signal to the circuit breaker. After receiving the trip command from the relay, the circuit breaker isolates the faulty part of the  power system. A circuit breaker has two contactscontacts- a fixed contact contact and and a moving moving contact. Under normal conditions these contacts remain in closed  position. When the circuit breaker is required to isolate the faulty  part, the moving contact moves to interrupt the circuit.

TYPES OF CIRCUIT BREAKERS 

The circuit breakers are classified as: 

OIL CIRCUIT BREAKER 

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AIR -BLAST AIR-BLAST CIRCUIT BREAKER 

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SULPHUR HEXA FLUROIDE CIRCUIT BREAKER 

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VACUUM CIRCUIT BREAKER 

 LIGHTNING ARRESTER A lightning arrester is a protective device which conducts the high voltage surges on the power system to the ground. Lightning Arresters are protective devices for limiting surge voltages due to lightning strikes or equipment faults or other events, to prevent damage to equipment and disruption of service. Also called surge arresters. A lightning arrester is a protective device which conducts the high voltage surges on the power system to the ground. It consists of a spark gap in series with a linear resistor. One end of the diverter  is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that the normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break down the air insulation from an arc. The  property of the nonnon-linear resistance is that its resistance decreases as the voltage or current increases or vicevice -versa.

CONCLUSION  A protective scheme includes majorly circuit breakers and  protective relays to isolate the faulty section of a healthy system from healthy sections. A circuit breaker can disconnect the faulty element of the section when it is called upon to do so by the  protective relay. The function of a protective relay is to detect and locate a fault and issue a command to the circuit breaker to disconnect the faulty element. Protective scheme isolates the faulty section of the system from the healthy sections. Thus a healthy and protective substation holds circuit breakers,  protective relays, relays, lightning arresters, isolators and insulation for  the protection of every component of its family.

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