Bushing

PRESENTATION ON BUSHINGS

S V S PRASAD Engineer (Design), Switchgear Products Division, Vijai Electricals Limited

Jump to first page

TOPICS TO BE COVERED   

  

What is a bushing Types of bushings Salient design features Manufacturing aspect. Testing of bushings Do’s and don’ts on bushings

BUSHING 



Bushings is one of the most important component that are fitted to the electrical equipments like the transformer, the switchgear etc. It is an insulating structure for carrying the H V conductor through an earthed barrier A bushing has to provide  Electrical

insulation to the conductor for the working voltage and for various over voltages, which occur in service.

 Mechanical

support against various Mechanical forces & Carry the full load current.

TYPES OF BUSHINGS 1.1 Application

i) Generator bushing ii) Transformer bushing iii) Bushings for switch gear iv) Wall bushing or roof bushing 1.2 Application: i) Indoor type ii) Outdoor type

Generator Bushing

Indoor & Out Door Bushing

Wall bushings

Roof bushings

TYPES OF BUSHINGS 2.1 Type of Bushing: i) Pull-through type ii) Stem type 2.2 Type of Bushings: i) Condenser core type ii) Non-condenser core type (Hollow Porcelain type) iii) Gas filled bushing iv) Solid Bushing Solid Ceramic Bushing  Epoxy Bushing 

Solid Ceramic Bushing

Epoxy Bushing

TYPES OF BUSHINGS 3. Type of insulation used in manufacturing Condenser Bushing: i) Synthetic Resin Bonded Paper (SRBP)

ii) Oil Impregnated Paper (OIP) iii) Resin impregnated Paper (RIP)

TYPES OF BUSHINGS 



Non - Condenser Bushing In its simplest form a bushing would be a cylinder of insulating material , porcelain, glass resin etc, with the radial clearance & axial clearance to suit the electric strengths. Condenser Bushing This is also known as capacitance graded bushing as it contains capacitance layers in between the kraft paper insulation at predetermined positions to build up a stress controlling condenser insulator.

CONDENSER BUSHING

DESIGN OF BUSHINGS. Minimum Specification required for Design: 1. Highest System Voltage. 2. Rated Voltage. 3. Basic Insulation Level. 4. Length of CT Accommodation . 5. Creepage Distance Required. 6. Altitude. 7. Application.

SALIENT DESIGN FEATURES  Self

Earthing Test Tap for measurement of Tan Delta and Capacitance at site.

 Environmental

 Bushings

friendly.

for special applications can be designed and supplied to suit client’s specifications.

SALIENT DESIGN FEATURES  Electrostatic

Field Controlled design of Condenser Core by Computer  Suitable for Operation under heavily polluted atmosphere  Reliable, relatively easy erection and practically maintenance free.  Better Dielectric Strength and Partial Discharge free up to Working Voltage  Space for mounting of Ring Type Current Transformer

145 KV OIP CONDENSER BUSHING

MAJOR PARTS OF CONDENSER BUSHING 1. Condenser Core 2. Porcelain. 3. Oil level Gauge 4. Top cap. 5. Test Tap. 6. Mounting Flange. 7. Oil side Stress Shield.

CONDENSER CORE The core of bushing consists of a hollow metallic tube or solid metal rod, over which high grade electrical kraft paper is wound .For condenser cores, conducting layers of metallic foil are introduced at predetermined diameters to make uniform distribution of electrical stress. The winding of condenser core is done in a dust free chamber. The core is then processes. This comprises of drying in a high degree of vacuum ( 0.005mm) , and then impregnating with high quality, filtered and degassed transformers oil.

INSULATORS The function of an insulator is to resist flash over in adverse conditions. This is determined by a) The profile of the dielectric/ Creepage Distance.* b) The mounting arrangement of the insulator i.e. , vertical, horizontal, or inclined. c) The properties of the surface, i.e., hydrophobic, toughness etc. The shortest distance along the surface of the porcelain between the metal parts which normally have the operating voltage between them.

FOR DIFFERENT POLLUTION LEVELS POLLUTION LEVELS

CREEPAGE DISTANCE

I. Light ( areas without industries,

16mm/kV

agricultural or mountain areas )

II. Medium ( areas with industries

20 mm/kV

not producing pollution smoke, Low density of house areas not exposed to the coast)

III. Heavy (Areas with high density

25mm/kV

of industries, high density of Heating plants producing pollution, areas close to the sea.)

IV. Very Heavy (Areas close to

31mm/kV

the coast and exposed to sea spray, desert areas, areas exposed to strong winds carrying salt and sand etc

Example: for 145 kV Bushing with heavy pollution level minimum creepage distance works to be. 145 kVX 25mm/kV = 3625mm

DIFFERENT PROFILES OF PORCELAINS 

Plain.



Anti-fog.



Alternate.

OIL LEVEL GAUGES These are provided in the outer surface of the bushing for monitoring the oil level in the bushing. These are generally of following types. 1. Magnetic Oil level Gauges 2. Prismatic Oil level Gauges. 3. Button Type Oil level Gauges.

TOP CAP This is a metallic housing for the spring pack . it services as an in built oil conservator to cater for oil expansion , and has an oil indicator. In many cases, it also serves the purposes of a corona shield.

MOUNTING FLANGE This is used for mounting the bushing on an earth barrier, such as a transformer tank or a wall. It may have the provisions for following:

a) CT accommodation length. b) Rating plate giving the rating and identification details of bushing.

TEST TAP The test tap is provided for measurement of the power factor and capacitance of the bushing during testing and service. The test tap is connected via a tapping lead to the last condenser foil of the core within the bushing. During normal services, this tapping is electrically connected to the mounting flange through a self-grounding arrangement.

OIL SIDE STRESS SHIELD This is provided at the oil side of the bushing to controlled the electrostatic field inside the transformer after installation

FACILITES REQUIRED FOR MANUFACTURE BUSHING 1.

2. 3. 4.

5. 6.

Dust free / pressurized winding chamber Winding Machines Heating oven for baking. Process Plants for Drying and Vacuum Impregnation at very fine Vacuum (0.0050.001mm of Mercury) Oil Treatment Plants for oil processing Assembly stand and Power pack for Tightening

MANUFACTURE OF BUSHINGS This is done in four stages. 1. Winding .

2. Baking. 3. Assembly. 4. Drying and impregnation

WINDING This is done in an enclosed chamber which is dust free . The winding machine are primarily drives the bushing conductor by three roller mounted 120 Degree. These rollers maintained at temperature of about 120 ºC uniformly by thermic fluid for driving out initially moisture from the insulating paper. Full width paper is passed on these rollers enabling it to wound on bushing conductor under tension and pressure. At predetermine diameters a thin and pure aluminium foil is inserted to form capacitances to control the electrostatic field around the bushing. On attaining the final diameter bushing is wrapped with glue coated paper.

BAKING The bushing is then be unloaded and transferred to a heated the oven on a suitable trolley with in 2 hours and heated at 125°C to 130°C for a period of 24 Hrs. minimum. The temperature of oven is then reduced to 50°C before bushing is taken out and allowed to cool to ambient temperature. The bushing shall then be protected with polythene sheet and stored in a safe place till it is taken for assembly.

ASSEMBLY. The bushing from baking oven should be shifted near assembly stand. The polythene cover should be removed. An insulating lead should be soldered to the last aluminium foil. The bushing assembly is done sequencely as shown. Whole assembly is tightened by a power pack at pre-determined load to achieve leak proof assembly.

DRYING AND IMPREGNATION Assembly of bushings are placed inside a drying and impregnation plant for processing. These are subjected to following after closing the plant. 1.

DRYING PROCCESS: The temperature of the impregnation plant Is maintained at 95 ± 5 º C for a minimum period of 48 Hrs.

2.

VACUUM PROCESS: The temperature of impregnation plant is reduced to 60± 5 º C simultaneously pulling vacuum inside the plant. When a vacuum of 0.005 to 0.001 mm Hg is obtained, the same is maintained for a minimum period of 72 Hrs.

3. IMPREGNATION PROCESS: On Completion of vacuum drying, filtered & degassed oil of the following parameters is admitted inside the bushing at a slow speed at vacuum of 0.005 to 0.001 mm of Hg. Speed of filling is so controlled that the bushing is completely filled in about 12 Hrs time.

i)

TEST Electric strength

VALUES REQUIRED 70 min.

ii) iii)

(BDV in kV) Tan delta at 90 º C Resistivity at 90 °C

0.02 max 10 x 1012min

iv)

(Ohm - cm) Water content {mg/kg or

10 max.

ppm by weight)

4. SOAKING PROCESS: Bushings are then allowed to soak the filled oil naturally for a period of minimum 3 days. In between oil is topped up to the desired level in the bushing whenever needed.

Bushings are then finally closed and final assembly done before taking to test.

OIL IMPREGNATION PLANT

ROUTINE TESTS Routine test: Tests are carried out on each bushing to check requirement which are likely to vary during production. These are follows:

1.

2. 3. 4. 5. 6.

Measurement of dielectric dissipation factor (Tan Delta) and capacitance at ambient temperature. Dry power frequency voltage withstand test. Measurement of partial discharge quantity. Tests of tap insulation. Pressure test on liquid filled and liquid insulated bushings. Tightness test at flange/ Fixing Device.

TESTING FACILITIES 

Tests on Bushings are conducted in electromagnetically shielded Test Laboratory which is provided with. Impulse Generator  Test Transformer for power frequency voltage.  Standard capacitors  Voltage Divider.  PD measuring equipment  Schering bridge 

DOs AND DO NOTs FOR BUSHINGS DOs 1.

Check the packing externally for possible damage before un packing.

2.

Unpack with care to avoid any direct blow on the bushing or porcelain insulator.

3.

Store the bushing in a shed or covered with tarpoline to protect it from moisture and rains. If removed from the crate keep it indoors with lower end protective intact.

4.

Handle the bushing with manila rope slings without any undue force on porcelain insulators.

DOs AND DO NOTs FOR BUSHINGS

DOs 5.

Clean the porcelain insulator thoroughly before taking any measurement or mounting the bushing on the transformer. Check for any foreign body adhered to the bottom porcelain.

6.

Check the oil level by making the bushing vertical

7.

Check for leakage of oil from any of the gasket joints. Each bushing is tested with the oil immersed in oil tank and so some traces of oil can be found which is actually not leakage.

8.

Check tan delta and capacitance ( if possible on mounted bushing without making any connections ).

9.

Maintain the log book of records of periodical checks.

DOs AND DO NOTs FOR BUSHINGS DO NOTs 1.

DO NOT unpack the bushing from crate unless required to be mounted on the transformer.

2.

DO NOT use metal slings on porcelain and avoid un due jerks while handling.

3.

DO NOT store the bushing outdoors with out any protective covering

4.

DO NOT measure IR value and tan delta with out thoroughly cleaning the porcelain and oil end portion.

DOs AND DO NOTs FOR BUSHINGS DO NOTs....... 5.

DO NOT fill oil in the bushing without specific instructions from BHEL .

6.

DO NOT climb the porcelain to tighten the top terminal. Use elevators or seperate ladder for this purpose.

7.

DO NOT dismantle or attempt to repair the bushing in

case the defect or mechanism is not clear.

REFERENCES IS: 2099 IS: 3347 IEC: 60137

SITE TESTS ON BUSHING BEFORE INSTALLATION ON TRANSFORMER After thoroughly cleaning the bushing with non-fluffy cloth, mount the bushing on a stand before carrying the tests at site. 1. Measurement of Tan Delta and Capacitance at 10 kV. (For this purpose a Schering Bridge Kit with 10kV in-built Source is needed along with shielded cable

SITE TESTS ON BUSHING AFTER INSTALLATION ON TRANSFORMER After mounting the bushing on the transformer thoroughly clean the bushing with non-fluffy cloth, 1. Measurement of Tan Delta and Capacitance at 10 kV. (For this purpose the same Schering Bridge Kit should be Used.)