Transformers

Types of Transformers mainly used in various EHT Sub-Stations :

According to phases:

A] 3ø Transformers: all EHT Transformers are 3Ø Power transformers. 3ø 220KV/132KV, 132KV/33KV Power Transformers are at transmission level and  33KV/11K and  3ø, 11KV/440V at distribution level.

B]1ø [11KV/Rt3]/[415V to 440V/rt3] mainly used at low voltage areas of only 1ø Power required.

According to winding:

A] Two winding Transformers: This type of transformers are used up to 132/KV33KV Levels of 10/16MVA50/80MVA Maximum.

B] Auto Transformers: This type of transformers are used for voltage levels 220/132KV and above 100MVA to  Maximum 1000MVA and used for instrument transformers like variable voltage transformers [Dimmers] at Low voltage levels of 3Ø 440V and 230V

C] High Ratio transformers: 132KV/11KV Transformers are used at rare cases where voltage required 11KV, but available 132KV source available.

         220KV/33KVTransformers are used where voltage required for 33KV but available 220KV source only.

        11KV/220KV Transformers are used at generation stations for stepping up the voltage.

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Main Accessories of the 132KV/33KV power Transformers:

a] High voltage bushings

b] Radiators

c] On load Tap Changer

d] Silica gel breather

e] main tank conservator

f] OLTC Tank conservator

g] Main tank bucholze relay

h] OLTC bucholze relay

i] oil surge relay

j] FCC panel

k] Diverter switch marshalling box

l] Lifting lugs

m] jacking pads:

n] rollers/skids

o] owing lugs

p]Lashing lugs

r] rail polls

a] High voltage bushings;

The oil impregnated paper condenser type of bushings is utilized up to 145KV, 800A to 245KV, 800A range of voltage and current for 132V/33KV to 220K/132KV Transformers. These bushings filled with high quality transformer oil with sufficient level for housing and isolating the HV terminal connector from body and insulator to maintain high insulation level and low capacitance between high voltage terminals to ground/body of the transformer. The oil filled in the bushing adds dielectric strength of entire bushing. The tan delta point is provided for testing of the tan delta value of the bushing yearly.

     The Video shows how the value of capacitance vary as per atmospheric conditions and other factor
    
    The oil equalizer pipes are connected to three bushings of the transformers at HV side as the bushings are mounted on transformer vertically with slanted from the level of the transformer tank.

Parts of the bushing

1] Terminal cap: the terminal cap is used to terminate the HV conductor.

2] Cable bolt: it is used for maintain the tightness of the HV lead in the bushing not to drop.

3] Pin:

4] Dummy bolt: the dummy bolt provided to press and hold rubber washer for avoiding leak/expose the bushing oil to atmosphere. 

5] Gasket: The gasket is provided in between transformer main tank and bushing bottom to avoid leakage of the bushing oil due to gravity and pressure of the oil in the conservator tank.

 

6] lower spark gap horn and upper spark gap horn: These are provided both  sides of the transformer HV bushing to avoid flash over of the bushing during high voltage conditions. The arrangement acts as horn gap lightening arrestors to avoid damages due to high voltage impulses and lightening impulse voltages.

8] Tan∂ Cap [Measuring tap]:  The measuring tap provided for measuring the tan∂ value where angle ‘∂’ indicates the discharging or loss angle.  A Small amount of the leakage capacitive current in the bushing from HV terminal to body/ground of the transformer through oil and insulator of the bushing exists. If the value of the leakage current increases due to aging and atmosphere moisture enters in the bushing, the capacitance of the bushing increases and damages the bushing in the high voltage conditions further leads to damage of the entire transformer due to fault current and transfer of high heat to the transformer winding. Hence to prevent sudden explosion of the bussing, the tan∂ value is to be measured at least yearly and the value should be >0.1 and capacitive leakage current<1000µA. The Tan cap provided for the high voltage bushing at the voltage>132KV only.

 9] Air vent screw: Air vent screw is provided to release the air in the form of bubbles stored in the bushing to avoid reduction in Insulating Resistance value and dielectric strength of oil in the bushing and mall operation of the protection system due to air in the bushings spreads to main tank of the transformer and further bucholze relay.

Preliminary operations:

I] Open the bushing case carefully and check for transit damage if any. if damage is found, should  inform  immediately to the company personal with brushing sl. No. and make of the bushing.

2] lift the bushings by suitable nylon on manila rope slings and keep it on a soft surface in a position 15⁰ to the horizontal.

3] Thoroughly clean the surface of the bushing.

4] Unscrew the terminal cap

            I] remove the locating pin

Ii] pull out  dummy cable bolt

Iii]the cable bolt to be used on the unit is brasses to the end of the flexible draw through type lead referred.

iv] Remove the blanking plate and un coil the draw through type lead on the transformer.

v] Tie the flexible steel wire to head of the bolt fixed tempararely on the top of the cable bolt.

vi]To facilitate the threading of the main lead through the bushing tube and same time preventing the lead from falling back in to the tank

Vii]Check whether the gasket on the pocket is in order, if not clean the seat of the gasket and replace it by new one.

Mounting/erection

·         Suitably raise the bushing and remove the protective cover at oil end of the bushing cover.

·         Take the bushing over the bushing pocket and lower it over the same. During the lowering operation, thread the flexible steel wire through conductor tube of the bushing. Continue this operation till the flange of the bushing rests on the flange of the bushing pocket. Bolt the two flanges and remove the slings. The final position oil end of the bushing fits in press board cylinder support inside the power transformer. in case during the mounting operation, both the flanges do not match, force should not be used to get the same. Instead the angle of the inclination should be changed such that bushing very smoothly fits on the pocket provided for it in the transformer.

·         Old the cable bolts in position, remove the flexible wire and insert the location pin to hold the cable bolt permanently in position.

·         Fix the terminal cap in position by screwing it on the terminal bolt.

·         For de-aeration of the central tube after oil filling, unscrew the terminal cap and press the terminal bolt downwards.

·         Fix upper and lower spark horns and adjust gap settings as per general arrangement of the transformer.

Caution

Do not fill the oil in the condenser type bushing at site to avert failure of bushing. Do not mount the bushing on the transformer, without ensuring the sufficient oil level in the bushing. Fixation of the bushing should be done such that air release plug is at the top most point.