Electrical Power System

Major Electrical Equipments in Power Stations1Major Components of a Power StationAlternators- Hydro turbo generators, Steam turbine generators.Exciter and Excitation SystemPower TransformersBus BarsVoltage RegulatorsSwitches Air Break Switches, IsolatorsCircuit BreakersProtective RelaysReactors Power Line Carrier Current EquipmentsMetering Equipment-CTs and PTs, Energy Meters, Frequency metersControl Room EquipmentsLighting Arrestor

2Alternators:The generator employed in power plants are three phase synchronous generators, called as alternatorsMain types are Hydro-generators and Turbo GeneratorsUsed to generate electricityInput to Hydro-generator is velocity or pressure of water and output is electrical powerInput to turbo generators is pressurized steam and output is electric powerTurbine used for hydro generators are Kaplan turbine, Francis turbineTurbine used for turbo generators are impulse and reaction turbines3Alternators:Hydro Generators:Salient pole type alternator is used.Field winding is wound on rotors and stator is used as armature.The frequency of supply is depends on f=NP/120.As the speed of hydro turbine is lower, to get required frequency, number of poles is high.To provide large number of poles, the diameter of pole is large as compared to axial length.For impulse turbine the alternator is horizontally mounted (for speed 100-1000 rpm) and for Francis and Kaplan turbine it is vertically mounted (for speed 50-500 rpm)Machines are usually air cooled and sizes varies from 0.5MW to 1000MW.

4Alternators:Steam Generators:Cylindrical rotor (non-salient)type synchronous generator is usedField winding is wound on rotors and stator is used as armature.The frequency of supply is depends on f=NP/120.As the speed of steam turbine is large (3000 rpm or 1500 rpm), to get required frequency, number of poles is 2 or 4.To reduce peripheral speed, the diameter of rotor is small and axial length is increased.Nearly sinusoidal flux distribution in air gapMachines are usually air cooled, hydrogen cooled or oil cooled and sizes varies from 10MW to over 1500MW

5Alternators:Specifications:

Rated Capacity in MVARated Speed in RPMRated frequency in HzRated Stator voltage (kVrms)Rated Stator current.(Amp)Excitation (Rotor) voltage (Vdc) and current (Adc)Maximum Sudden shortcircuit currentWinding connection.Class of insulation.6Alternators:Specifications:

Cooling systemMaximum allowable temperatureBore diameter, frame diameter, core length, shaft diameterRotor weight, stator weightName of the manufacturer.Year of manufacturing.

7Excitation Systems:It required to provide the necessary field current to the rotor winding of synchronous generator

Should available for all time.

Loss of excitation is serious than drop of a generator from line. Unexcited alternator will draw large reactive power from line.

Main requirements are Reliability, easy to control, ease to maintenance, stable for higher transients

8Excitation Systems:The amount of excitation depends onLoad current, Load power factor, speed of machines. Larger the load current lower will be speed and lagging power factor more the excitation required.

Exciter is normally a small ac or dc generator provided on same shaft of machine (centralise or separate one)

Types of Excitation Systems are: --DC excitation system --AC excitation system --Static excitation system9Excitation Systems:DC Excitation System:

Having two exciterPilot exciter is a self excited or permanent magnet dc generator providing the field current to the main exciter. The main exciter is a separately excited dc generator providing the field current to the alternator.Directly coupled exciter are used for small turbo alternatorsDirectly coupled gear driven exciter of large and medium alternatorsLarge time constant (response time is about 3sec) and commutation difficulty are the drawbacks

10Excitation Systems:AC Excitation System:

Consists ac exciter and rotating thyristor systemMain ac exciter-to provide ac input to rectifier --small ac alternator with fixed field and rotating armatureRotating rectifier- provide field current to alternatorThe field of ac exciter may be fed from a rectifier unitAs commutator is not required, it eliminates the problem of commutationShort time constant (response time is less than 0.1 sec)

11Excitation Systems:Static Excitation System:

Supply for excitation is taken from alternator itselfOutput of alternator is connected to three phase star delta step down transformerStar connected primary is connected to alternator output and delta connected secondary to three phase controlled rectifierThe output of this controlled rectifier unit is fed to field winding of alternatorResponse time is 20msec and excellent dynamic performance.Eliminate the exciter winding loss possibility and reduced operating costs

12Excitation Systems:Brushless Excitation System:

Consist of an alternator , rectifier , main exciter and permanent magnet generator pilot exciter. Both the main and pilot exciters are driven directly from the main shaft. The pilot exciter has rotating permanent magnets attached to the shaft and 3 phase stationary armature which feds the main exciters field through 3ph rectifiers. The main exciter has stationery field and rotating armature directly connected through rectifiers to the field of main alternator. As commutator is not required, it eliminates the problem of commutation

13Power Transformers:Used for step up voltage at generating station and step down voltage at main distribution substationUpto 10MVA-Naturally cooled, oil immersed (ON type).Above 10MVA-Blast coolFor higher ratings-Forced oil, water cooing or air blast coolingTo regulate the voltage the on load tap changer is used on hv winding of the transformerIt is generally operated at full load so it is designed to have maximum efficiency at full load.Flux density of 1.5 to 1.77% impedance ranging from 6-18%% regulation is 6-10%

14Power Transformers

Power Transformers

Power Transformers:Specifications of power transformers:kVA RatingRated voltage HV and LVNumber of phasesRated frequencyConnection type (star or delta for both windings)TappingType of coreCooling systemPermissible temperature rise above ambient temperature of oil and windingsVoltage regulation% impedanceEfficiency in % at full load, load, load at unity and 0.8 p.f. laggingCovered under IS 2026-1962

17Bus BarsBus-bar or Bus is a main conductor carrying current and having constant voltage to which all the incoming and outgoing lines are connected.

In some arrangements two buses are provided. One bus is usually called the Main bus and the other auxillarybus.

The busbars used in a substation are usually bare rectangular bars but they can be of other shape also as round solid tubes or square tube. Busbars are made of Copper or Aluminium.

The different arrangements of busbars in a substation areSingle bus-bar arrangementSingle busbar with sectionalisationDouble busbar arrangement

18Bus BarsSpecifications of busbarsRated Current ( e.g. 200, 400, 2000 ) AmpsRated Voltage ( e.g. 6.6, 11, 132, 220 ) kVRated frequencyRated Short time currentSize of busbar

19Voltage Regulators Every alternator in a power plant is provided with an automatic voltage regulator.

It performs following functions.i) Control of system voltage within prescribed limitsii) Proper division of reactive power between the alternators operating in parallel.iii) Prevention of over-voltages due to sudden loss of load on the system.

At present electromagnetic voltage regulators are in use.

For greater response and greater sensitivity electronic regulators are used.

20Power Switches:A Switch is used in electric circuit for making and braking the electric circuit by simple motion of knobSwitches may classified as air break switches and oil switchesAir Break Switches:Horn gap type contact for quenching the arcMay opened singly or in gangMounted such that it opens in downward directionOil SwitchesThe contacts open in oilHave both the blade and the contact mounted in a oil filled tankUsed in very high voltage heavy current circuits

21IsolatorsIsolators:Not equipped with arc quenching devices so can not be used on loadIt is a no load switch.If opened on load arc flashover may take place between contact and earth and causes accident to operatorIsolators are provided on both sides of circuit breaker.

22IsolatorsSpecifications of Isolators:Rated VoltageNormal Current RatingShort time current ratingRated Insulation LevelType of mechanism ( Manual, Spring Operated)Construction (Pantograph, Horizontal break)Operating time for opening.

23Circuit BreakersA circuit breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under normal or abnormal conditions.

Can be operated manually or remote controlled under normal conditions and automatically in abnormal condition.

Must carry normal load currents without overheating and damage and open short circuit current without serious damage with minimum burning contact

Types- Air circuit breaker, oil circuit breaker, SF6 circuit breaker, vacuum circuit breaker

24Circuit BreakersSpecifications of Circuit BreakerType of construction : No. of Poles : Nominal System voltage : Highest System voltage : Rated Frequency : Rated Continuous current : Rated short circuit breaking current : Rated short circuit making current : Rated short time current : Rated duration of short time current : 1 SecRated operating duty : O 0.3 Sec- CO-3 Min CO 25ReactorsThe reactor is a coil having large inductive reactance as compared to its resistance. Mainly used in a circuit or system to limit the short circuit currents to a safe value.two types i) Open type and ii) Oil immersed type.In open type construction circular coils or bars of stranded copper are embedded in a number of concrete slabs. The whole is based on concrete base and mounted on porcelain insulators.The oil immersed type reactors has insulation and cooling arrangement like transformers. Can be located in series with generators, in series with feeders or in bus-bars.

26Instrument Transformers:Potential or voltage transformer (PT or VT) is used to reduce the higher voltage level to measurable voltage (generally to 110V)

Current transformer (CT) is to reduce the higher current level to measurable current (generally to 1A or 5A)

Having two types 1) Measuring instrument transformer 2) Protection instrument transformer

27Current Transformers:Specification of CT:Rated primary currentRated secondary currentRated Burden (VA)Accuracy classFor Protection ( 5P, 10P,...) For Metering ( 05, 1.0 ,..)Rated frequencyInsulation systemsNo of SecondariesCore 1- MeteringCore 2- Protection

Type of CT ( Bar Primary / Wound Primary)

28Current Transformers:

29Potential TransformersSpecification of PTRated primary voltageRated secondary voltageRated Burden (VA)Accuracy classFor Protection ( 5P, 10P,...) For Metering ( 05, 1.0 ,..)Rated frequencyInsulation systemsNo of SecondariesCore 1- MeteringCore 2- Protection30Potential Transformers

31Protective Equipments: RelaysThe protective relay is an device which senses the abnormal or fault conditions and gives command to circuit breaker to open.It ensures the safely of the circuit equipment from any damage from the fault.Fundamental items of relay*Sensing Element: Responds to any change in actuating quantity*Comparing Element: Compares the actuating quantity with pre selected setting.*Control element: Opens the main circuit. Over current Relay, Over voltage, under frequency, distance relay, differential relay

Sensing ElementComparing ElementControlElementTo trip or signal circuit32Lightning ArrestersLightning arrestor and Surge Absorber provides protection against lightning and over voltages

It diverts the high voltage surges to Earth

33Lightning Arresters

34Lightning ArresterSpecifications of Lightning Arrester

Rated Voltage (kV rms)Nominal discharge current (kA Peak)Maximum wavefront and wavetail time.Max. switching surge voltage.Max lightning impulse voltage.

35Lightning Arrester

36Lightning Arrester

37Lightning Arrester

38Power line carrier communication: PLCCIt is installed in an power system for communication, relaying, tele-metering or for supervisory control.The power itself is used for communicationLC type of filter is used to convert carrier signal to audible vice or commandsSeries connected L(Wavetrap) acts as the low pass filter. It allows power frequency (50Hz) signals and blocks Communication (high) frequency signals. Shunt connected C (Coupling capacitor) acts as high pass filter allows Communication (high) frequency signals and blocks power frerquency (50Hz) signals Reliable, fast operating, need not separate line to transfer signalSubstation ASubstation BControl Room39Line trap or Wave trap

40Line trap or Wave trap

41Control Panel and Room:The various controls are performed such as voltage adjustment, load control, emergency control.It consists voltage regulators, relays, ammeters, voltmeters, energy meters, temperature gauges, water level meters etc.Well equipped with suitable display and alarms which shows the current status of the station.It should be neat, clean, away from noise, well ventilated and illuminated, good fire handling facility etc. All the meters should be clearly visible to operator and it should be calibrated.

42Other Equipments:Battery room: used to supply all relays and CB for operationLightning arrestor and Surge Absorber: to provide protection against lightning and over voltagesEarthing and earthing mat: to provide protection against earth fault and prevent from shocks.Instruments: used to measure electrical and mechanical quantities.43Overhead line Insulators44Pin Type insulators

Suspension Type insulators

Silicon Rubber insulators

Silicon Rubber insulators

Post Insulators

Bushings