TRANSMISSION AND DISTRIBUTION PLANNING

UNIT 2TRANSMISSION AND DISTRIBUTION PLANNING

Transmission system:Transfers bulk power from the generating system to distribution system

Sub transmission system: segregates power into dist. systems

Distribution system: Between transmission and consumersVOLTAGE LEVELSGeneration : 11 KVPrimary transmission :220 KV

Secondary transmission :33 KV

Primary distribution :6.6 KVSecondary distribution : 415 VConsumers : 415 / 230 V

TRANSMISSION NETWORKInterconnection permits power exchangePlanning of transmission system involves :Power flow requirementsSystem stabilitySelection of voltage levelsVoltage and reactive power flowConductor selection Losses Insulation levelsType of structure Right of way

TRANSMISSION NETWORKCriteria for planning of transmission system :Availability of generation and demandVoltage levelsSize and configuration of systemsDistanceRight of waysResource constraints

Due to increasing demand, network interconnection are done

Indian loading 90 % ( Healthy loading 50 % )Network reaches alert state of operation at peak loadSmall disturbance causes major network collapseDISTRIBUTION NETWORKSub transmission system:33 220 KVDelivers energy to distribution substationDistribution substation : converts to lower primary distribution system voltage Boosts voltages for better voltage regulation of primary voltagePrimary circuits of feeders : 11-33 KV Supplies bulk load directly and distribution transformersDistribution transformers : 10 2500 KVA Transform the primary voltage to utilisation voltage at 110 440 VSecondary circuits : carry energy from the D.T along streetsService lines : deliver energy from secondary circuits to consumer premisesTYPE OF DISTRIBUTION NETWORKRADIALOnly one supply sourceVulnerable to long interupptionLow reliabilitySuitable for small loads

PRIMARY LOOPProvides power from 2 feedersSupply depends on switch status of sectionalizes and reclosersLoop is normally operated with tie sectionaliser switch openOutage time due to fault is reducedAdditional line increases frequency of faults

TYPE OF DISTRIBUTION NETWORKPRIMARY SELECTIVE Each transformer gets supplied from two sources (feeders)Transfer of feeder in event of fault is automatic , therefore fault duration is minimisedSystem reliability is highUsed for large essential or continous process industrial consumersTYPE OF DISTRIBUTION NETWORK

SECONDARY SELECTIVE A primary feeder has its own transformer, feeding a load eachTie Switch is normally open and interlocked between secondary feeder switchesUsed for industrial plants and large institutions like hospitalsPrimary operational switching is eliminatedDuplicate transformers eliminate interupption timeEach feeder + transformer system must have capacity to supply entire loadTransfer is automatic upon loss of voltage on either feeder with static switching equipment

TYPE OF DISTRIBUTION NETWORK

SPOT NETWORKHas maximum service reliability Transformers are operated in parallelThe LV bus is continuously energized by all D.Ts operating in parallelAutomatic disconnection is obtained by reverse power relays Feeder fault is eliminated by isolating the feeder, while supply is continued through remaining linesUsed in high load density , metropolitan areas, for essential servicesLong duration outages dont exist in this system

TYPE OF DISTRIBUTION NETWORK

GRID NETWORKHas maximum service reliability and operational flexibilityMost economical and effective methodFor high density loads in metropolitan citiesGrid is supplied from multiple feedersNo outage even during feeder maintenanceImproved voltage regulation due to parallel operation of transformersGrid can handle abrupt load changes and disturbancesVoltage fluctuation due to fault is very localizedTYPE OF DISTRIBUTION NETWORK

HV TRANSMISSION (HVAC / HVDC)HV preferred due to :increasing requirement of bulk power transfer over long distancesFeasibility i.e. economical and ecological advantages of generating power near the fuel source instead of load centres

Voltage level of transmission Power handling capacity of a line : depends on :Quantity of power Distance to be transmitted

HV TRANSMISSION (HVAC / HVDC)Observation :The capital cost per MW-km decreases with Higher VoltageP(1*800 KV line) = P(4*400 KV lines) for same distanceFor PL(800KV) = 1/10th of PL(400 KV)V (kV)X (ohm/km)r (ohm/km)PL (in %)4000.3270.0315.128000.2720.01362.710000.2310.00360.8512000.2310.00270.64HVDC LINKSAsynchronous HVDC links is used whereConnecting system widely differsAC mode of connecting systems is impossible

HVDC operation procedure: AC DC AC Uses 2-wires (bipolar transmission) instead of 3-wires for same powerEconomical only for long distances

TYPES OF DC LINKSBased on location of converting stations :

Two terminal D.C. linePower transfer is economical over long distances

Back-to-back D.C. linewith long A.C. feederalong the borderPower transfer between areas is easy

HVDC V/S HVAC TRANSMISSION

HVDC TRANSMISSIONTechnical advantages of HVDC links :Power flow can be controlled independent of system operating conditions.System operating at different frequencies may be interlinkedImproves stability

Common reasons for using HVDC links:Lower line costs 2 wire systemLower losses no reactive power flowAsynchronous connection-two different frequencies Controllability- semiconductor technology yielded better reliability ,controllabilityMajor electrical networks long distance transmission interconnection between systems high power underground distribution system feeders

HVDC TRANSMISSIONIncreased transferability : the same voltage will transfer higher power, with the same infrastructure A single AC circuit can be re-arranged into bipolar DC line- transmission capacity increases by 150 %Right of way retained

Conversion from AC to DC system is easy :Utilize the same infrastructure same towers and conductorsSame conductors can be bundledDC insulators to be used in place of AC insulators

HVDC TRANSMISSIONOther advantages of using HVDC links :DC cables are cheaper compared to ACOne single cable can take up to 500-1000 MWA DC cable does not contribute to short circuit powerCostly and difficult overhead line paths in a city centre can be avoided by cablingIt ensures better conductor utilizationIt provides for three times the capacity, using the same conductorsIt has an even higher capacity with new towers in an existing right of wayIt makes it possible to control reactive power in a city centreIt ensures increased system stabilityIt provides for increased power capacity in parallel AC linesIt provides for controlled power flowIt provides for double circuit performance of a converted single circuit AC lineThere is higher power without increased short circuit powerThere is better control of the line load factorHVDC TRANSMISSIONDisadvantages of using HVDC links :Economical only for long distancesHarmonics are generated due to semiconductor devices used- checked by filters

Costs in HVDC links :Converter stations : the valves made of series connected thyristors modules, no parallel connection neededConverter transformers : to achieve galvanic separation between DC and AC sideSwitchgear : used for clearing faults, must be highly reliable and maintainableSELECTION OF VOLTAGE LEVELSEconomy of supply depends on selection of voltages in the T & D systems

Factor influencing the selection of voltages :Load densityTransmission distance Transmission powerSteady load growth-varying with time and geographically

In medium HV range 11KV is the minimum voltageCities 11-22 KVRural areas 11-33 KV

SELECTION OF VOLTAGE LEVELSEconomical voltage step :3-6 times from medium HV to HVcan be higher/lower depending on load density

voltagedesignationRange of application< 1 KVLVDistribution system for small consumers1-36 KVMHVDistribution systems for large consumers36-150 KVHVDistribution and subtransmission systems for MHV systems, cities, railways< 150 KVEHV Transmission systems for interconnected operationsSELECTION OF VOLTAGE LEVELSProblems in selecting HV : insulation becomes a problemEconomical design of HV overhead lines because line cost is higher in proportion to total costAvailability of existing plant- prevents use of new voltage due to capital already invested

Voltage levels should be max 3 above LV valueEHV level / lower level > 3HV level / lower level > 5Reduction in no. of levels allows increased length of MV systems

Load levelSupply voltageUpto 60 KWLT supply 60 KV - 50 MW11 KV5 30 MVA33 / 66 KV30 50 MVA132 KV>50 MVA220 KVPLANNING CRITERIA : STRATEGYSEB : To provide transmission systems for state gridsPOWERGRID :To lay the transmission system network for :facilitating power transfer by central sector to various constituentsFor strengthening regional power gridsFormation of national power grid Private companies can also develop the transmission systemTransmission network planning :Should be with a long and medium term perspectiveShould have an integrated approach to transfer power from all sources to all beneficiariesOptimal design with reliability, security and economy in mindGood voltage profile should be maintainedNetwork should integrate within the region and inter-regions Configuration should be such that optimal dispatch is possible

THERMAL LOADINGLoading limit is decided by :ambient temperature varies with location and seasonmaximum permissible conductor temperature-specified for standard sizesLoading(MVA) based on max. conductor temp ofAmbient temp65 deg70 deg258 sq. mm ACSR40 deg225 25745 deg189225345sq. mm ACSR40 deg943107745 deg785943DESPATCHABILITY : LOADINGSystem should be planned such that :It is self sufficient as well as suitable for sharing with neighbours

Maximum angular separation between bases : 40 deg for steady state

Should stand outage of 2 circuits of 220 KV system/1 circuit of 400KV / 1 pole of HVDC bipole / 1 EHV transformer without load shedding or rescheduling of generation

Should ensure full evacuation of maximum possible output from generating stations even under transmission line outage

Transmission lines should not pose any constraint on scheduling of generation during outage

Reactive compensation should be possibleDESPATCHABILITY : TRANSMISSION SYSTEM OUTAGE CAPABILITYTransmission system should be capable of withstanding the following outages :

Simultaneous outage of two 220 KV circuitsOutage of one 400 KV SCOutage of a 800/765 KV SCOutage of one bipolar HVDC lineOutage of one generating unitDESPATCHABILITY : SECURITY