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High voltage equipment

Prepared by

Dr.-Ing. Thanapong Suwanasri

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Contents1. Power transformer

2. Instrument transformer

3. Disconnecting switch

4. Power circuit breaker

5. Surge arrester

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History 1831 discovery of induction law by Faraday and Henry. 1856 First transformer construction in iron core.

1889-1891 Invention of AC transformer in 3 legs form by M.Dolivo Dobrowolsky

Power Transformer (ANSI Std. C 57.12.00)

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Transformer Outline (Courtesy of Mitsubishi)

1,4) Oil filter valve2) Conservator3) Buchholz relay5) Pressure-relief vent6,7) HV, LV bushing

10) Tank 11) Off-load tap changer17) Oil drain valve

21) Grounding terminal23) Coil29) Radiator32) Oil level gauge

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1) Magnetic Core Core form for small transformer in KVA rating. Shell form for large transformer > 50 MVA.

Material is thin flat laminations of soft iron (0.3mm thick) mixedwith low carbon silicon alloyed to reduce friction loss orhysteresis loss. Eddy current loss is reduced by insulate betweenlaminated or thin sheet with oil - proof coating or vanish.Nowadays we mix phosphate or magnesium during productionprocess.

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Winding arrangement

Core Form Transformer

Shell-Form Transformer

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2) Winding Copper Coil

good mechanical strength and electrical conductivity. Aluminium Coil

Cheap, good thermal dissipation and light weight.

Insulation uses craft paper and pressboard paper with enamelseparate between turn and layer of winding (dry in oven with 120-130C for 3-4days)

Thermal problem!!

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Winding configurations

1) Core form transformer- Cylindrical (Layer - Wound , Helical , Spiral) winding

Cylindrical, Helical, Squared circle cylindrical winding(Courtesy of Mitsubishi)

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Disk (Single and double - wound) winding

2) Shell form transformer uses interleaved rectangularpancake coil for furnace transformer with high mechanicalstress.

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3. Tank Enclosure3.1) Free breathing or open type

air can go in or out depending on temperature andpressure via dehydrating breather to filter moisture

3.2) Conservator or expansion Tank Type consists of expansion

tank on the top with 3 - 10% of tank volume and filled with oiluntil half of conservator tank

air can go in or out of conservatortank via dehydrating breather

( Courtesy of Factory Mutual )

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Dehydrating Beather(Courtesy of Mitsubishi power transformer)

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3.3) Sealed - Tank TypeOn top of oil is filled with inertgas under pressure. Top coveris welded to prevent leakageand pressure-vacuum relief

device to control pressurebetween -8 to +8 psi

( Courtesy of Factory Mutual )

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4. BUSHING INSULATOR

Solid Type Bushing To 25KV Oil Filled to Tally Enclosed Type Bushing 69-92 KV Condenser Type Bushing

Capacitor - type oil - filledbushing for V > 69kV uses oil -

impregnated paper withaluminium foil separated intolayer inside porcelain cylinder

for equal voltage distributionbetween each layers.

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Off load tap ChangerTransformer must be disconnected from system on both side

before changing tap. It can be changed both from inside oroutside tank. Disadvantage is service interruption therefore it is

suitable to use with seasonal load. On load tap changer1) as impedance to prevent short circuit on tap

2) 2 circuits for load current flow and tap changing

2 Types of on load tap changer

High Speed Resistor Type2 compartments, first part is selector switch working while noload current, second part is switch and resistor which causes arcduring operation.

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1

2

3

4

5

R2 R1

Installation inside tank

outside tank Long life because of

short arcing duration

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Reactor type usingdiverter switches

Reactor type usingvacuum switches

1

2

3

4

5

Diverter switch

Selector switch #1 Selector switch #2

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Rating Data

Rated KVA and Phasepower output that transformer can supply within specifictime at rated voltage and frequency and temperature rise< 65 oC (by resistance)IEEE Std. C57.12.00 single phase transformer rated inKVA from 5KVA - 33MVA and 3 Phase from 15KVA- 100MVA

Cooling Classes

Liquid-Immersed, Air-CooledOA, OA/FA, OA/FA/FA means liquid-immersed, self-cooled/ forced air- cooled/ forced air- cooled

frequency

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Voltage Ratings and Taps Nominal System Voltage Maximum System VoltageRated voltage specified under no load condition and based on

turn ratio. ConnectionDepends on transformer type according to ANSI C57.12.70

ImpedanceStandard impedance depends on transformer type based on

average winding temperature rise (by resistance) plus 20C.

Total LossesNo-load loss (T REF 85 oC) plus load loss (T REF 20oC)without losses by ventilation & cooling system, heater, pumpand auxiliary device.

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Insulation Level

It depends on system voltage1) Class1 V < 69kV2) Class2 for 115kV and above

Temperature Rise1) Winding Rise average temperature rise from

ambient should be less than 65o

C by resistancemeasurement and < 80 oC by winding hot spot.2) Temperature rise of liquid insulation < 65 oC

when measures at the top of main tank.