SKT-Hydro Electric Power Plants and Grid Connection

8/9/2019 SKT-Hydro Electric Power Plants and Grid Connection

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S.K. Tyagi, Former Chief EngineerConsultant,

Alternate Hydro Energy Centre,Indian Institute of Technology Roorkee

Roorkee 247 667 (Uttarakhand)


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Almost instant availability of service.

Very high reliability.

Quick response to rapidly changing load.

No standby losses.

No change in efficiency with age.

Less man power for O&M.

Running cost very little. No fuel charges.

No problem of fuel transport or refuge disposal.


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Dam or weir.

Inlet water ways.

Power House & Plant.

Tail race or outlet water ways.

Natural lakes at high altitude.

Substantial catchment area.

Favorable geographical, geologicaland meteorological conditions.

Ground survey and aerial survey.

Many factors like terrain, condition ofrocks, slopes of ground, narrownessvalley.


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(a) Water availability:-

(i) Essential data's required Daily, weekly, monthly flow over a year forplant capacity and output for storage schemes one or more dryperiods of years.

(ii) Minimum water flow conditions.

(b) Water Storage:-

- Required to absorb wide variation in rainfall and equalize the flow ofwater through-out year.

- Careful study of topography, geology, ecology and social conditions.

- Location nearest to power plant.

(c) Head of Water:-

- Features which effect head are topography, water flow, level of tailrace turbine discharge and flood.

- P & Q H relation

- All possible efforts are made to secure maximum possible head.

(d) Distance from Load Centre

(e) Access to site:- Roads & other facilities for transport.

BASED ON SOURCE OF WATER

- Run off river scheme

- Storage scheme

Based on head available

- Low Head

- Medium Head

- High Head

Head affect choice of Turbine

- Axial Flow reaction (Kaplan, Propeller) turbine forlow head (1 to 50 m).

- Mixed Flow Reaction (Francis) Turbine for mediumhead (upto 300 m).

- Impulse Turbine for high head (above 300 m).


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1. Dams- Primary function to afford head, create storage.- Principal type concrete, masonry earth or a

combination.- The concrete and masonry dams may be core type &

buttress type. Minimum upkeep & attendance.- Earth dam with or without core (concrete or puddle day).

2. Head regulator- Regulate supply of water to channel, tunnel or

penstocks.

3. Tunnel or channel- Conveys water to the forebay with little loss of head.- Semi-circular or trapezoidal as these give more

hydraulic mean depth than rectangular.- Power channels should be lined to prevent loss of water

due to seepage and erosion of channel banks.- Trash rack at forebay to prevent entry of boulders & logs

which may damage turbine.

4. Forebay- Meets hour to hour requirement of water demand of

station.- Larger the forebay easier the operation.

Screens/Trash RacksPrevent entry to logs, fish, boulders or any otherforeign material.

Penstocks- Follow the best and shortest route.- Proper anchoring through out the route.

Surge Towers- Facilitates stability in operation of Power Station.- It protects channel or tunnel in case of sudden rise of

water pressure due to tripping of machine.- Height comparable to the operating head.

Power House Building- Covering for protection of equipment installed.- Size according to the requirement of plant to be

handled.- Height is mostly depend upon the length of shaft.- It should have adequate space, apart from the main

machines, a repair shop, a store, office, control room,test room, LT room, communication room and also aretiring room and a lunch room for workers.


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Crane & Crane Girders- Important part of the power house.

- Span the width of the power house.

- Should be able to travel for the entire length.

- Elevation of crane rail above working floordepends upon the length of the shaft.

- Desirable capacity depend on the weight ofheaviest assembled part to be lowered in pit-mostly rotor.

HYDRO TURBINE(i)Impulse (a) Pelton : High head

(b) Turgo : Medium head

( c) Cross Flow : Low head

(ii)Reaction (a) Mixed Flow- Francis : Medium head

(b) Axial Flow Kaplan, Semi Kaplan,

Propeller, Tubular, Bulb : Lowhead

Choice of Turbine- Output, working head, nature of load and speed.

- For low head adjustable blade type Kaplan Turbine ispreferred.

- For medium head reaction type, having fixed blade, FrancisTurbine is preferred.

- For high head impulse that is Pelton Turbine is used.


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Governors

- To adjust automatically the guide vanes and runner blades tomeet the load conditions and speed regulation.

- Should have a sensitive and positive control without anydanger of either over running or hunting.

- Principle performance characteristics are:-

- sensitiveness

- stability

- freedom from hunting

- speed regulation

- regulating time

- Type

- Hydro Mechanical Governor

- Electronic Governor

- Load Diverter for machines below 100 kW

Speed and Efficiency of Turbine- Principle factors affecting speed are head and output

required.- For a given output the speed falls if the head is less.- For a given head speed falls with increase in output.- Large output and low heads, operate at low speed.- With constant speed and constant head efficiency of reaction

turbine falls rapidly as the load is reduced.- Desirable to operate reaction turbine near full load as far as

possible.- Critical speed at which turbine operates most efficiently. If

this is increased without decreasing diameter of runner orwheel, the efficiency is reduced.

- Performance of various units operating under differentconditions can be compared by a reference to specificspeed.

- Specific speed denotes the speed of geometrically similarmodel of turbine designed to give an output of 1 B.H.P.under head of 1 ft and 1 cusec water flow.

- Ranges of different type:-Impulse .. 3-6Reaction (a) .. 10-110 (Mixed Flow Francis)

(b) .. 110-225 (Axial Flow, Kaplan,Propeller)


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ELECTRICAL PLANT- Generator- Exciter

- Switchgear

- Transformer

- D.C. Battery and battery charger

GENERATOR- A field or assembly of magnets arranged to produce

magnetic flux.

- An armature or assembly of electric conductorsarranged across the path of magnetic flux.

- The field and armature are mounted on mechanicalparts so that a relative motion is produced betweenthe magnetic flux and electric conductors.

- As the field poles are arranged alternatively positiveand negative, the polarity of induced emf isalternating.

Hydro GeneratorsThere are two types of generators namely synchronous

and Induction. The induction generator is also calledAsynchronous generator based on the super synchronousspeed at which it operates.

For decentralized power station or grid connectedpower station with stand-alone operation synchronous

generator is obviously required. Only for grid connectedstation Induction generator can be employed.

The most important requirement of a hydro generator isits suitability to withstand runaway speed of the turbine.The usual runway speed of different types of turbine ismentioned below:-

.


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S. No. Types of Turbines Range of Runway SpeedOver rated speed (Nr/N)

1. Pelton 1.6-1.82. Turgo Impulse 1.6-1.83. Cross Flow 1.6-1.84. Francis 1.8-2.25. Full Kaplan 2.8.-3.26. Semi-Kaplan 2.5-2.87. Propeller 2.2-2.6

Whereas an Induction generator has to be designed to withstand runawayspeed for an indefinite time quite frequently a synchronous generator has towithstand runaway speed continuously for a period of 15 minutes. This requires avery robust mechanical design for both types of hydro generators.

Generators are selected keeping the capital cost of the plant and the cost of

Generation as paramount factors

Synchronous GeneratorsAs the name suggests the Synchronous generator runs at Synchronous

speed corresponding to the system frequency and number of poles of themachine. An Excitation system to give DC Excitation to the field is requiredfor this type.

Synchronous Generators can operate independently or in parallel with

similar other sets or with grid. Special synchronizing equipment are howeverrequired for synchronizing the generator. They are costlier than inductiongenerators.

The Synchronous Generator has ability to supply reactive KVAR to theGrid and can therefore regulate the power factor. This is another importantadvantage of having a Synchronous Generators.


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Induction Generators

The Induction Generator operates only at super synchronous speed-a speedwell above synchronous speed corresponding to the supply frequency. Thegenerator voltage, frequency and speed are all dictated by the electrical systemwith which is connected. Hence AVR for the generator and speed governor for theturbine are both redundant. The synchronizing equipment is not required tosynchronize the induction generator with the grid. The generator can be simplyswitched ON with the grid at the proper speed. The Induction generator draws itsexcitation current from the grid eliminating separate excitation equipment.However, this imposes additional burden on the grid to supply excitation currentthereby causing stability problems unless the grid is at least 10 to 15 timesstronger than the induction generator connected with it.

Another important point to be considered is compatibility of voltage dips ofthe grid with the pull out torque of the generator which should be substantiallyhigh of the order of well over 200 % - 225 %.

The generation voltage preferred is 415 v for decentralized power station upto 1000 KW and 3.3/6.6/11 KV for above 1000 KW for grid-connected powerstations. The criteria are the overall economy of the entire electrical systemconsisting of the generator, switchgear, transformer, protection system etc.

STATOR

- Built up of welded steel plates made in three or four segments.- Circular steel frame for supporting core and winding.- The core is laminated to prevent losses.- The coils are in the form of bars or diamond form coils laid in slots.

ROTOR

Types

1. The distributed pole type in which the windings are embedded in slotsin the face of cylindrical wheels so that pole pieces are integral with thewheel.- used for high speed generator.- greater strength and compactness.

2. The salient pole type in which winding surrounded, individual polepieces mounted on the periphery of the wheel.- Majority of water wheels have salient pole type fields.- Field poles are laminated to prevent hysteretic and eddy current.- A water wheel is subjected to run away speed of 150 to 200 percentof normal speed. Generator rotor is designed to withstand this speed

safely.


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BEARINGS- Thrust bearing.- Lower guide bearing.- Upper guide bearing.- The thrust bearing is designed to carry the weight of

all rotating parts i.e. runner rotor, exciter armaturesetc. and also unbalanced thrust of water.

- Generally the bearings are oil lubricated. Oil is filledin the housing of the bearing and proper oil level ismaintained for the safe running of bearings.

- In some machine oil is supplied under pressure withthe help of pumps.

- Cooling is done through water piping system.

BRAKES To bring the machine to rest after shutting downs of

the machine as without brakes slight leakage

through the gates might cause machine to rotateindefinitely. It also constitutes an additional safe-guard against

run away speed. The braking units are utilized for jacking purposes

also.

VENTILATION

- Disposal of electrical losses which appearas heat presents serious problem.

- Generator is provided with fan bladesmounted on rotor.

- The systems directing the cooling air areaxial circumferential but all take air at

one or both ends.

PERFORMANCE OF GENERATORS

- Efficiency of generator is the ratio of itsuseful output to its total input.

- Depends upon size, speed, load, powerfactors.


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EXCITATION SYSTEM- An external direct current source is required to maintain

necessary magnetic flux in the field winding of the alternator.

- The amount of excitation depend upon the speed, load, andpower factor.

- Excitation requirements are more with low speed lagging powerfactor and heavy load.

(a) Conventional

- Main exciter

- Pilot exciter permits the smooth regulation of main exciterterminal voltage from zero to rated voltage.

- With the vertical water wheel generators these arevertically mounted type.

- The armature of main and pilot exciter with theircommutator are mounted on extension shaft coupleddirectly to the rotor shaft.

- The magnetic system consisting of yoke, main poles, inter-poles are supported on thrust bearing frame.

- These are mostly open circuit self ventilated.

(b) Static Excitation

(c) Brushless ExcitationAC exciter with rotating diodes

(d) Self Excited, Self Regulated Excitation System (SESR)

AUXILIARY POWER REQUIREMENT The total power requirement is normally 1 to 5 percent.

For Intake gates, compressors, cranes, dewatering anddrainage pumps, workshop, test lab., air conditioners,lighting etc.

This is obtained from station transformers and unitauxiliary transformers.

CABLING AND G.T. CONNECTIONS

The cables are laid in trenches or on cable racks. The generator and step up transformers are connected

by a specially designed bus duct which carries veryheavy current.

CONTROL ROOM The control gear, protective relay and metering &

recording panels are installed in separate room.

The arrangement is either linear or semi-circular.

Some panels may desk type.


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VOLTAGE REGULATION

To overcome following difficulties, automatic voltageregulators are used:-

- If large electrical load is switched off, the rise of

speed of the turbine causes dangerous rise involtage due to the action of alternator and exciter.

- This refers to an alternator operating at high powerfactor where regulation of standard machine (i.e.percentage rise above the rated voltage when fullload is thrown off as about 25% to 30% at 0.8 p.f.and 12 to 15% at unity p.f. assuming the speed andexcitation to remain constant.

ELECTRICAL PROTECTIVE SYSTEM

- To take care abnormal electrical condition resultingfrom undue speed variation faulty operation of gates,electrical fault, mechanical fault occurring duringoperation.

- Automatic over speed, under speed, over voltage, overfrequency relays.

- Generator differential relay.- Over all differential relay not only shut off the machine

but release CO2 also into the generator barrel.

- A circuit breaker is provided to open the exciter circuitsautomatically upon the operation of alternator relays.

- When load is thrown off the speed rise is quicker andthe voltage rise is so rapid that over voltage relay willoperate. This relay may be set to operate at 35 to 40percent above normal voltage.


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TRANSFORMER

- Step up transformer of adequate capacity for stepping upgeneration voltage to transmission voltage.

- The transformer may be self cooled, water cooled or forced oilcooled in order to dissipate losses of transformer, which mayappear in the form of heat.

- The transformer oil should have following qualities:-

- High resistivity & dielectric strength

- Low viscosity

- High flash and burning point

- High thermal conductivity & specific heat

- Chemical neutrality towards metals and insulating material

- Chemical stability at high temperature

- Moisture even in very small quantity reduces dielectric strength

- Drying and purifying of oil is done by centrifuging through streamline filter machine.

SWITCHGEAR

Main functions:-

- To localize the effects of faults by operation or protectiveequipment and so automatically disconnect the faulty plantfrom system.

- To break efficiently short circuits without giving rise todangerous conditions.

- To facilitate re-distribution of loads, isolation andmaintenance on the system.

ISOLATORS

- A short of knife switch for isolating circuit breakers,generators, transformers, arresters and other equipment.

- It acts as section switch in conjunction with double bus barand single circuit breaker.

- It is not designed to work under load.

- It consists of single pole knife with an eye to receive a switchhook mounted on porcelain insulator and suitable metal base.


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CIRCUIT BREAKER

The selection involves following factors:-- normal voltage

- normal current

- abnormal current

- interrupting duty

- operation arrangement of terminals

- space available

- accessibility for repairs

- altitude of installation

- temperature

A breaker is rated on the basis of:-- r.m.s. voltage of circuit

- the normal temperature rise over ambient temperature

- normal frequency of current at normal voltage.

- short time rating based on the maximum r.m.s. current thebreaker will carry safely for 1, 2, 3 or 5 secs. withoutexcessive heating.

Type of Circuit Breaker:-(1) Air Blast Circuit Breaker

- Air under pressure is used to assist in the makingand breaking of the circuit and also afford desireddegree of insulation.

- Auxiliary contacts are provided to open after andclose before the main contacts so as to relieve themain contacts of the burning action.

(2) Oil Circuit Breaker- These are arranged to break under oil.- In small breaker all the 3 poles are arranged in one

tank.- In medium an large breaker each pole has a

separate tank.- Auxiliary contacts as described as above are

provided.- When heavy currents are interrupted by the breaker,

the pressure generated due to heat of the arc isenormous and consequently the tank must be madevery sturdy.

- A vent is provided to escape the gases.

(3) SF6


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BUS BAR ARRANGEMENT

- Single bus bar- Sectionalized single bus bar

- Single ring bus bar

- Duplicate bus bar system

1. Single Bus Bar

- This is seldom used as there is no standby.

- Work on the bus bar cannot be undertaken without interruptingsupply.

2. Sectionalized Single Bus Bar-Reasonable measure of security in case of break-down.

3. Single Ring Bus Bar-This extended form of single bus.-Make possible to isolate any section for repair


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4. Duplicate Bus Bar System

The advantage of spare bus are:-

(i) Cleaning, repairs, modifications and extensionsmay be undertaken without interruption to thesupply.

(ii) Feeders which have undergone repair may betried out separately by a test set before puttinginto the service.

(iii) Feeders may be isolated from the system for

maintenance.(iv) A supply from an inter-connected system may

be received and distributed without running inparallel.

(v) The bus bars may be used to best advantage,as non adjacent sections which are lightlyloaded may be paralleled through spare busand run as one section.

(vi) To take full advantage of this system, provisionshould be made for synchronizing andparalleling the two sets of bus bars.


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Switchyard Equipment

Generally following equipment are to be installed in switchyard as perrequirement and provisions in the single line diagram.Transformers.Circuit Breakers.Current Transformers.Potential Transformers.Lightning Arrestors.PLCC Equipment.

1.Wave trap

2.Coupling capacitor or CVTIsolators.Isolators with earth blades.Bus bar system, insulators and hardwares.Main and Auxiliary structuresFence all aroundEarthing network.


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GRID CONNECTION :

Level of voltage:

415 V

11 KV

33 KV

66 KV

132 KV

1506.1

kVAL+

100

3

6.1

PL+

The voltage is selected through following two empirical formulae.

1. Economical Voltage V=5.5

Where L =Length of line in Km.kVA =Power per phase required to be transmitted.

2. Economical Voltage V = 5.5

Where L =Length of line in Km.P =Power in kW per phase

The above formulae gives an idea of voltage, but final selection is made after detailed studies.


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TRANSMISSION LINESTransmission lines are essential for three purposes.

To transmit power from a hydro generating site to a grid substation. These may be very longand justified because of the subsidy aspect connect with the project.For bulk supply of power to load centers from large HEP and steam stations. These are likelyto be relatively short.For interconnection purposes, that is, for transfer of energy from one system to another incase of emergency or in response to diversity in system peaksFrequent attempts have been made to set up definitions of transmission lines, distributioncircuit and substations. None has proved entirely satisfactory or universally applicable, but forthe purposes of accounting the Federal Power Commission(USA) and various statecommissions(USA) have set up definitions that in essence are read as follows:A transmission system includes all land, conversion structures and equipment at a primarysource of supply; lines, switching and conversion stations between a generating or receiving

point and the entrance to a distribution center or wholesale point, all means and equipment

whose primary purpose is to augment, integrate or tie together sources of power supply.The power flow analysis of the transmission network should be got done through computersbefore finalization of the transmission line parameters, particularly terminating substations,conductor & its configuration.

PRINCIPLES OF LAYOUT

(i)The design of the individual units should be suchthat the risks of failure are reduced to minimum.

(ii)Barriers and partitions should, if possible,separate each unit so that a faulty unit will notinterfere with its neighbors. Complete sub-division

avoids as far as practicable any serious troublespreading and damaging the adjoining units orsections.

(iii)Layout should be such that any section may beisolated without unduly affecting the service.

(iv)To provide easy and safe access formaintenance and general routine inspection.

(v)Provisions should be made for handling oil anddealing with fires.

Documents

SKT-Hydro Electric Power Plants and Grid Connection