Nadir Baloch T.A 175

Switchgear & Maintenance

SWITCHGEAR

 switchgear is the combination of electrical disconnect

switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.

Switchgear

Types:LV SwitchgearMV SwitchgearGenerally electrical switchgear rated upto 1 KV is termed as low voltage switchgear The most common use of LV switchgear is in LV distribution board.

THE ROLE OF SWITCHGEAR IS :

ELECTRICAL PROTECTIONSAFE ISOLATION FROM LIVE PARTSLOCAL OR REMOTE SWITCHING

The basic functions of Switchgear

Periodic MaintenancePreventive Maintenancea) 2 yearly Maintenanceb) 4 yearly MaintenanceImportance Of PMThrough such PM, troubles can be detected in the Early stages and corrective actions can be taken before extensive damage occurs.

The most common use of LV switchgear is in LV distribution board. This system has the following partsIncomerThe incomer feeds incoming electrical power to the incomer bus. The switchgear used in the incomer should have a main switching device. The switchgear devices attached with incomer should be capable of withstanding abnormal current for a short specific duration in order to allow downstream devices to operate.Generally Air Circuit Breakers are preferably used as interrupting device.Sub – Incomer These sub – incomers draw power from main incomer bus and feed this power to feeder busFeedersDifferent feeders are connected to the feeder bus to feeds different loads like, motor loads, lighting loads, industrial machinery loads, air conditioner loads, transformer cooling system loads etc.

Main Transformer Incomer

Main Components Of Switchgears

BusbarCircuit Breakers (ACB)TransformerCT (For Measuring Current)Relays ( IM30 Over Current+Earth Fault )Control Circuit ( For Control Supply 110v)

BusbarBusbar is a strip or bar of copper, brass or aluminium that conducts electricity within a switchboard, distribution board, substation,The cross-sectional size of the busbar determines the maximum amount

of current Used to interconnect the loads and sources of electrical power.

Busbar SpecificationManufacturer: Nova Magrini GallilioType of Busbar : Copper ( Insulated with Silver coated joints)Busbar Rating : 1500 AmpMethod Of Cleaning : Clean with Lint Free ClothBusbar Dimensions : 2*100*8 mm

Busbar Maintenance Full Bus Bar Inspection check for overheating signs, visual

inspection of Cu plate and nut bolts. cleaning of incoming/outgoing spouts.

Remove dust with vacuum cleaner. Cleaning and lubricating mechanical links with contact grease. Check operating mechanism of shutter. Contact Resistance of Electrical joints. Check IR to earth and between phases on busbar and outgoing

circuits @ 2500V

Air Circuit Breaker (ACB)A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow

This type of circuit breakers, is those kind of circuit breaker which operates in air at atmospheric pressure.

 ACBs are still preferable choice up to voltage 15 KV

It is also good choice to avoid the risk of oil fire, in case of oil circuit breaker.

Operation Of ACBThe first objective is usually achieved by forcing the arc into contact with as large an area as possible of insulating material. Every air circuit breaker is fitted with a chamber surrounding the contact. This chamber is called ‘arc chute’. The arc is driven into it. If inside of the arc chute is suitably shaped, and if the arc can be made conform to the shape, the arc chute wall will help to achieve cooling. 

Air Circuit Breakers : Charging methodCircuit breaker closing springs may be compressed manually or by means of a small electric motor.The closing coil is charged by manual charging handle. For closing, first charge the spring by using the charging handle

Maintenance of Air Circuit Breaker LV Circuit Breakers operating at 600 volt and below, should be inspectedMaintained every 1 to 4 years depending on their services and operatingConditions.

Conditions that make frequency maintenance and inspection are necessary:

High humidity and High tempDusty or Dirty AtmosphereCorrosive Atmosphere

Inspection & Cleaning Insulating parts including bushing should be cleaned

of dust Clean & inspect Arc Chutes for Cracks, Broken Parts

. Replace Damage Parts Inspect Breaker Operating Mechanism for loose

hardware & missing or broken etc Examine for wears Check Rack in/ out mechanism for normel operation Lubricate if necessary Check tightness of all terminations

Contacts maintenance Arcing contacts to be verified for burn offTo be adjusted or exchanged that arc contacts simultaneoslu when closingPermissible tolerance 1mmAdjust lead of arcing contacts from main contacts such that when arc contacts Meet then distance between

Insulation Resistance Test (Megger)Load and line conductors should disconnected from the breaker to make Test. IR between phases and phases w.r.t ground with breaker closed.

the result is compared against manufacturer/ pervious data For breaker being tested.

Cycling the breakerOn & Off the breaker several times to check the correct operationOf breaker.

Verify For Loop CheckInsert circuit breaker at test position and on & off the breakerfrom remote location

Current TransformerA current transformer (CT) is used for measurement of alternating electric currents

Current transformer are used in electrical power system for stepping down currents of the system for metering and protection purpose.A CT functions with the same basic working principle of electrical power

transformerCT steps down rated system current to 1 Amp or 5 Amp.Because the relays and meters are generally designed for 1 Amp, 5 Amp and

110V

Relay

Relays is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. these contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped

Protective Relay Testing

Why do we use protective relays?Their main duty is to isolate a faulty section within few cycles

Periodic Maintenance TestingAfter a relay is commissioned it’s important to carry out regular

maintenance tests.Some of the advantages of such testing can be stated as, it will pin point

a defective relay before it fails to act during a faultMethods of TestingSteady State: steady state testing is for checking the relay pick up.

Injected current, voltage. when the current is raised and thenfluctuated around pick up.Dynamic state Time for relay operation is measured.

Freja 300

The FREJA 300 relay testing system is a computer-aided relay testing system by injecting current and voltage.

Testing PrinciplesThere are two main principles as primary injection and secondary injection.• Primary injection- High current is injected to primary side of the CT. Test carried out covers CT, conductors, relay and sometimes circuit breaker as well. The relay unit has to be isolated from the power system. Usually this principle is used at commissioning and also if the secondary of the CT is not accessible.• Secondary injection - Relay is disconnected from the CT and the stepped down current isdirectly injected to relay.

General Precautions to be takenCheck the Electrical Isolation FirstDuring maintenance Earth MUST be connectedBefore maintenance of breaker spring must be discharge.Don’t use compressed air on breakerDon’t use file or emery paper for contacts cleaning

Inspection Inside Switchgear RoomSwitchgear environment (switch room access and surrounds,

including fence and external walls if outdoors.Signs of water Signs of unauthorized accessCondition of firefighting equipment and warning noticesGeneral Housekeeping