RAJESH ARORAASSISTANT MANAGER Delhi Transco LimitedSUB STATION BASICS &

OVERVIEW OF EQUIPMENTS

SUB STATION• An electrical sub-station is an assemblage of electrical components

including busbars, switchgear, power transformers, auxiliaries etc. These components are connected in a definite sequence such that a ckt. can be switched off during normal operation by manual command and also automatically during abnormal conditions such as short-ckt.

• Basically an electrical substation consists of No. of incoming ckt. and outgoing ckt. connected to a common Bus-bar systems. A sub-station receives electrical power from generating station via incoming transmission lines and delivers elect. power via the outgoing transmission lines.

• "Sub-station are integral parts of a power system and form important links between the generating station, transmission systems, distribution systems and the load points."

GENERAL VIEW OF SUB STATION

Substation VIEW

MAIN TASKS ASSOCIATED WITH MAJOR SUB-STATIONS

• Protection of transmission system.• Controlling the Exchange of Energy.• Ensure steady State & Transient stability.• Load shedding and prevention of loss of synchronism. Maintaining the

system frequency within targeted limits.• Voltage Control; reducing the reactive power flow by compensation of

reactive power, tap-changing.• Securing the supply by proving adequate line capacity.• Data transmission via power line carrier for the purpose of network

monitoring; control and protection.• Fault analysis and pin-pointing the cause and subsequent improvement in

that area of field.• Determining the energy transfer through transmission lines.• Reliable supply by feeding the network at various points.• Establishment of economic load distribution and several associated

functions.

TYPES OF SUBSTATION • .• Classification based on Outdoor or Indoor : Outdor substation is under open

skv. Indoor substation is inside a building.• Classification based on voltage levels, e.g. : A.C. Substation : EHV, HV, MV,

LV; HVDC

• Classification based on configuration, e.g. :• Conventional air insulated outdoor substation or• SF6 Gas Insulated Substation (GIS)• Composite substations having combination of the above two

• Classification based on application• Step Up Substation : Associated with generating station as the generating voltage is low.• Primary Grid Substation : Created at suitable load centre along Primary transmission lines.• Secondary Substation : Along Secondary Transmission Line.• Distribution Substation : Created where the transmission line voltage is Step Down to supply

voltage.• Bulk supply and industrial substation : Similar to distribution sub-station but created

separately for each consumer.• Mining Substation : Needs special design consideration because of extra precaution for safety

needed in the operation of electric supply.• Mobile Substation : Temporary requirement.

SUBSTATION PARTS AND EQUIPMENTS

• Outdoor Switchyard• Incoming Lines• Outgoing Lines• Busbar• Transformers• Bus post insulator & string insulators• Substation Equipment such as Circuit-beakers, Isolators, Earthing Switches, Surge

Arresters, CTs, VTs, Neutral Grounding equipment.• Station Earthing system comprising ground mat, risers, auxiliary mat, earthing

strips, earthing spikes & earth electrodes.• Overhead earthwire shielding against lightening strokes.• Galvanised steel structures for towers, gantries, equipment supports.• PLCC equipment including line trap, tuning unit, coupling capacitor, etc.• Power cables• Control cables for protection and control• Roads, Railway track, cable trenches• Station illumination system

MAIN PARTS OF SUB STATION

• Main Office Building• Administrative building• Conference room etc.

• 11 KV Switchgear, LV• 11 KV Indoor Switchgear

• Switchgear and Control Panel Building• Low voltage a.c. Switchgear• Control Panels, Protection Panels

• Battery Room and D.C. Distribution System• D.C. Battery system and charging equipment• D.C. distribution system

• Mechanical, Electrical and Other Auxiliaries• Fire fighting system• D.G. Set• Oil purification system•

Transformer Substation bus (rigid or strain bus) Switches (Vertical, side, center, double, etc. break) Switchgear Bus Support Insulators Suspension insulators Arresters Breakers Voltage regulators Current transformers Potential Transformers Coupling Capacitors Line Traps Other

MAIN EQUIPMENTS OF SUB STATION

SINGLE LINE DIAGRAM

Transformers

Very large three-phase network transformer.

Transformers

A B C

a b c

NVAN VBN VCN

VcaVbcVab

A three-phase wye-delta transformer with three-legged iron core.

MAIN PARTS OF TRANSFORMER

• MAIN TANK (magnetic core, windings )• CONSERVATOR TANK• COOLING SYSTEM (ANON/AFON/AFOF)• OLTC• PRV• BUCCHOLZ RELAY• BUSHINGS• CONTROL CABINET

BUSBAR SYSTEMS (BUSBAR LAYOUTS)

• Single busbar system.• Single sectionalised busbar system.• Duplicate busbar system with one breaker per

circuit.• Duplicate busbar system with two breakers per

circuit.• Main and transfer busbar.• Ring busbar.• Breaker and a half busbar system.• Mesh busbar scheme.

Simplest and cheapest bus bar schemeSimplest and cheapest bus bar scheme

Maintenance and extensions of busbars Maintenance and extensions of busbars are not possible without shutdown of the are not possible without shutdown of the substation.substation.

Operation & maintenance of bus bar is Operation & maintenance of bus bar is easyeasy

• Similar to the single bus Similar to the single bus scheme except the scheme except the sectionalising breaker or sectionalising breaker or isolator.isolator.

• By keeping the sectionaliser By keeping the sectionaliser open one section can be in open one section can be in service and the other can be service and the other can be taken for maintenance or taken for maintenance or extension.extension.

• If a bus section breaker is If a bus section breaker is provided busbar protection provided busbar protection can detect fault on any can detect fault on any section and trip the breakers section and trip the breakers connected to that section connected to that section and isolate it.and isolate it.

• Individual CB can be taken Individual CB can be taken out for maintenance on-load out for maintenance on-load at a time.at a time.

• The transfer bus coupler The transfer bus coupler acts as the breaker for the acts as the breaker for the circuit under by pass.circuit under by pass.

• Individual circuits have a Individual circuits have a bypass isolator to connect bypass isolator to connect to the transfer bus and this to the transfer bus and this isolator will be closed during isolator will be closed during bypass operation of that bypass operation of that particular circuit.particular circuit.

• Most commonly used bus Most commonly used bus scheme.scheme.

• Normally load will be Normally load will be distributed on both the buses distributed on both the buses and the bus coupler will kept and the bus coupler will kept closed.closed.

• For maintenance & extension of For maintenance & extension of any one of the buses the entire any one of the buses the entire load will be transferred to the load will be transferred to the other bus.other bus.

• On load transfer of a circuit On load transfer of a circuit from one bus to the other bus is from one bus to the other bus is possible through bus isolators possible through bus isolators provided the bus coupler is provided the bus coupler is closed and thereby two buses closed and thereby two buses are at the same potential.are at the same potential.

• On load bypassing of any On load bypassing of any circuit for breaker maintenance circuit for breaker maintenance is not possibleis not possible.

• In this bus scheme, in In this bus scheme, in addition to the main buses addition to the main buses there will be a separate there will be a separate transfer bus also. transfer bus also.

• Since separate transfer bus Since separate transfer bus is available there will be no is available there will be no need of transferring the load need of transferring the load from one bus to the other from one bus to the other bus unlike in a double main bus unlike in a double main cum transfer bus cum transfer bus arrangement.arrangement.

• Other features are similar to Other features are similar to the one described in double the one described in double bus cumbus cum

• In this layout, two circuit have In this layout, two circuit have three breakers, the middle three breakers, the middle breaker ties the two circuits and breaker ties the two circuits and hence is called the tie breaker. hence is called the tie breaker.

• The combination of three The combination of three breakers is called a diameter.breakers is called a diameter.

• Parallel feeders from the same Parallel feeders from the same source should not be source should not be connected to the same connected to the same diameter.diameter.

• One great advantage of breaker One great advantage of breaker and half scheme is that even and half scheme is that even both the buses are out of both the buses are out of service , power can be service , power can be transferred from feeder to transferred from feeder to another.another.

• This arrangement is This arrangement is very expensive one very expensive one and hence followed and hence followed in very important in very important circuit only. circuit only.

• In this arrangement In this arrangement breaker breaker maintenance for maintenance for each circuit is each circuit is possible.possible.

WHAT IS A CIRCUIT BREAKER

• Circuit Breakers are the switching and current interrupting devices. Basically a circuit breaker comprises a set of fixed and movable contacts. The contacts can be separated by means of an operating mechanism. The separation of current carrying contacts produces an arc. The arc is extinguished by a suitable medium such as dielectric oil, air vacuum, SF6 gas. The CBs are necessary at every switching point in the substation.

CIRCUIT BREAKER

• IN SIMPLE WORDS, A C.B is an equipment which can open/close a circuit under all conditions i.e. no load, full load, fault conditions.

CIRCUIT BREAKER(220 KV)

CIRCUIT BREAKER FOR 400 KV

HOW DOES CB WORKS

• When a fault occurs, the resulting over current in the C.T primary winding increases the secondary e.m.f. This energises the relay coil, hence the tripping coil of C.B operates.

• It is interesting to note that relay performs the function of detecting a fault whereas the C.B does the actual circuit interruption.

CURRENT TRANSFORMERS

• A CURRENT TRANSFORMER IS USED TO STEP DOWN THE CURRENT FLOWING IN THE CIRCUIT FOR THE FOLLOWING PURPOSES :

• 1.PROTECTION (O/C,DISTANCE,DIFF.)

• 2 METERING & INDICATIONS

Live Tank CTs

Dead tank CTs

Hairpin design and Eye Bolt design

CAPACITIVE VOLTAGE TRANSFORMER

• A CVT IS MORE ECONOMICAL AN ELECTROMAGNETIC VOLTAGE TRANSFORMER WHEN THE VOLTAGE LEVEL IS ABOVE 66KV.

• IT IS USED FOR :

• 1 PROTECTION

• 2 METERING AND INDICATION

• 3 PLCC

C V T

ISOLATORS & EARTH SWITCH

• ISOLATORS ARE USED TO PROVIDE ISOLATION FROM THE LIVE PARTS FOR THE PURPOSE OF MAINTENANCE.

• EARTHING SWITCH IS USED TO DISCHARGE THE VOLTAGE ON THE CIRCUIT TO EARTH FOR SAFETY

ISOLATORS

SURGE ARRESTERS ( L A )

• SURGE ARRESTOR IS USED TO DIVERT THE TRASIENT OVER VOLTAGE SURGES TO EARTH &

• THUS PROTECT THE SUB STATION EQUIPMENTS FROM LIGTHNING AND SWITCHING OVER VOLTAGE SURGES.

SURGE ARRESTERS

WAVE TRAP

• WAVE TRAP IS CONNECTED IN SERIES OF CIRCUIT WHICH ALLOWS ONLY NORMAL FREQUENCY SIGNAL TO PASS AND CHECK ( STOP ) THE PLCC HIGH FREQUENCY SIGNALS

WAVE TRAPS

DC SYSTEM (BATTERY BANK )

Lead-Acid Battery• Batteries use a chemical reaction to do work on charge and produce

a voltage between their output terminal

PROTECTION AND CONTROL PANELS

• CONTROL PANELS HAVE 1. INDICATION METERS / ANNUNCIATION WINDOWS

2. ENERGY METERS

3. CONTROL SWITCHES

RELAY PANELS HAVE1. DIFFERENT RELAYS FOR PROTECTIONS

OF SWITCHGEARS INSTALLED IN THE SWITCH YARD

VIEW OF CONTROL PANEL

CAPACITOR BANK

IS USED TO CONTROL POWER FACTOR AND VOTAGE PROFILE

EARTHING SYSTEMPurpose of earthing……

Safety of the living beings around the vicinty of the substation

Proper functioning of the protection system under fault condition

To limit the touch and step potential within tolerable limits

Human SafetyHuman SafetyCurrent Range Effects on Humans

1 mA Threshold of perception 1-6 mA Let go currents 9-25 mA Pain full, hard to let go 25-60 mA Muscular contractions 60-100 mA Ventricular fibrillation

Maximum Body Current: Ik = for t = .03s to 3s

0.116

√t

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