HV Equipment File 1 of 5

High voltage equipment Prepared by Dr.-Ing. Thanapong Suwanasri

Contents 1. Power transformer 2. Instrument transformer 3. Disconnecting switch 4. Power circuit breaker 5. Surge arrester

Power Transformer (ANSI Std. C 57.12.00)

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

Transformer Outline (Courtesy of Mitsubishi)

1,4) 2) 3) 5) 6,7) 10) 11) 17) 21) 23) 29) 32)

Oil filter valve Conservator Buchholz relay Pressure-relief vent HV, LV bushing Tank Off-load tap changer Oil drain valve Grounding terminal Coil Radiator Oil level gauge

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) mixed with low carbon silicon alloyed to reduce friction loss or hysteresis loss. Eddy current loss is reduced by insulate between laminated or thin sheet with oil - proof coating or vanish. Nowadays we mix phosphate or magnesium during production process.

Winding arrangement • Core Form Transformer

• Shell-Form Transformer

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 enamel separate between turn and layer of winding (dry in oven with 120130C for 3-4days)

– Thermal problem!!

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

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

– Disk (Single and double - wound) winding

2) Shell form transformer uses interleaved rectangular pancake coil for furnace transformer with high mechanical stress.

3. Tank Enclosure 3.1) Free – breathing or open type air can go in or out depending on temperature and pressure 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 oil until half of conservator tank – air can go in or out of conservator tank via dehydrating breather

( Courtesy of Factory Mutual )

Dehydrating Beather (Courtesy of Mitsubishi power transformer)

3.3) Sealed - Tank Type On top of oil is filled with inert gas under pressure. Top cover is welded to prevent leakage and pressure-vacuum relief device to control pressure between -8 to +8 psi

( Courtesy of Factory Mutual )

4. BUSHING INSULATOR • Solid Type Bushing To 25KV • Oil Filled to Tally Enclosed Type Bushing 69-92 KV • Condenser Type Bushing Capacitor - type oil - filled bushing for V > 69kV uses oil impregnated paper with aluminium foil separated into layer inside porcelain cylinder for equal voltage distribution between each layers.

5. Tap Changing Equipment • No-Load Tap Changing • On-Load

– Off load tap Changer Transformer must be disconnected from system on both side before changing tap. It can be changed both from inside or outside tank. Disadvantage is service interruption therefore it is suitable to use with seasonal load. – On load tap changer 1) 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 Type 2 compartments, first part is selector switch working while no load current, second part is switch and resistor which causes arc during operation.

5 4 3 2 1

R2

R1

• Installation – inside tank – outside tank • Long life because of short arcing duration

– Reactor type using diverter switches – Reactor type using vacuum switches Selector switch #1

Selector switch #2 1 2 3 4 5

Diverter switch

Rating Data • Rated KVA and Phase power output that transformer can supply within specific time at rated voltage and frequency and temperature rise < 65oC (by resistance) IEEE Std. C57.12.00 single phase transformer rated in KVA from 5KVA - 33MVA and 3 Phase from 15KVA - 100MVA

• Cooling Classes – Liquid-Immersed, Air-Cooled OA, OA/FA, OA/FA/FA means liquid-immersed, selfcooled/ forced air- cooled/ forced air- cooled

• frequency

• Voltage Ratings and Taps – Nominal System Voltage – Maximum System Voltage Rated voltage specified under no load condition and based on turn ratio. • Connection Depends on transformer type according to ANSI C57.12.70 • Impedance Standard impedance depends on transformer type based on average winding temperature rise (by resistance) plus 20C. • Total Losses No-load loss (TREF 85oC) plus load loss (TREF 20oC) without losses by ventilation & cooling system, heater, pump and auxiliary device.

•

Insulation Level It depends on system voltage 1) Class1 V < 69kV 2) Class2 for 115kV and above

•

Temperature Rise 1) Winding Rise average temperature rise from ambient should be less than 65oC by resistance measurement and < 80oC by winding hot spot. 2) Temperature rise of liquid insulation < 65oC when measures at the top of main tank.