Potential Transformer
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Direct Measurement of voltage in high voltage system is not possible because of difficulty in electrical isolation of measuring instruments. It is also not possible to use voltage for the system protection due to its high value and difficulty in electrical isolation of protective relays. Therefore, Voltage Transformers are used to step-down the high system voltage to standard value accurately in proportion to their ratio.
Basic Functions of voltage Transformer are
 To reduce the line Voltage to a value, which is suitable for standard measuring instruments, relays, etc
To isolate the measuring instruments, meters, relays etc. From high voltage side of an installation
To sense abnormalities in voltage and give voltage signals to protective relays to isolate the defective system
Theory
General principles of power transformer design also apply to the voltage transformers but there are certain considerations of performance which are of particular importance. In the electromagnetic voltage transformers, accuracy depends on leakage reactance and winding resistance. These determine how the phase error and voltage error vary as the burden on secondary increases. The permeability and power dissipation of core material affects the exciting current and thus the error at zero burden. To comply with the requirement of IS:3156/IEC: 60044-2, Flux density employed in Voltage transformer is much less than the value generally used in power transformer. The turns ratio and voltage drop due to leakage |
Design
Single Phase Electromagnetic Voltage Transformer (EMV) is manufactured in two types
Single Pole (To connect between line & Earth)
Both Pole (To connect between Line-to-Line)
Hase Voltage Transformers are of star/star connected or star/Open delta connected (residual Voltage Transformer) type. Single – Phase voltage transformers are manufactured for 11kV to 132kV system voltage, where as 3 phase voltage transformer are manufactured for 11 kV to 33 kV system voltage
Construction
Voltage Transformer Mainly Consists of
Primary & Secondary Winding
Electromagnetic Core
Bottom Tank & Top Oil Expansion Chamber
Porcelain Bushing
Stainless Steel Bellow
Primary & Secondary Winding
Enameled copper wire is used for winding. Primary is wound with multi layer and graded insulation. The diameter and length of each layer is selected such that surge voltage is distributed equally in all layers of the Winding. Multilayer Kraft Paper Insulation is provided between winding layers. Stress equalizing shield is provided on last layer of the winding. H.T. Connection is brought out through metallic galvanized pipe. Secondary is separately wound and inserted in the primary winding as per the requirement. Winding and taping of VT is done in dust free atmosphere.
Electromagnetic Core
CRGO Silicon Steel Lamination is used for building up Electromagnetic Core. Shell type construction is used to minimize leakage reactance.
Bottom Tank & Top Oil Expansion chamber
Bottom Tank and oil expansion chamber are made of M.S. Sheet. All tanks and chambers are painted with oven baked paint, after cleaning by seven tank process. All surfaces which come in contact with oil are painted with oil insoluble paint. M.S. Parts can be hot dip galvanized on request.
Porcelain Bushing
Brown Glazed porcelain Bushing with different shed profiles to suit different pollution conditions is used. These Bushings are Hollow cylindrical Type Conforming to I.S. 5621/IEC 815. Bushing with collar at both the ends is clamped using Aluminium Flange. Nitrile and Neoprene Gaskets are used at both sides of collar to from flexible joint. This joint can sustain vibrations without damaging bushing. Bushings with cemented Flanges are also used.
Insulation
High quality Electrical Grade Kraft. Paper rand Crepe Paper is used for insulating primary and secondary winding of V.T. The high voltage connections is brought through paper condenser formed on metallic galvanized pipe using fine grading of insulation semi-conducting shield is used to give linear distribution of electric stress ailing the length of the bushing. The paper insulation is dried in oven under very high vacuum and strictly controlled conditions. Filtered and de-aerated EHV grade oil is filled in V.T., while V.T. is under vacuum. To seal it, the space left for expansion on the top is filled with dry and pure nitrogen through non-returnable valve at pre-determined pressure.
Testing
Truvolt Engg. Testing Lab is equipped with modem test facilities to carry out all routine test, including temperature rise test. Partial discharge Measurement plays a vital role in quality control of insulation. All our Vts are tested for Partial Discharge on most modern bridge type partial Discharge Test Set. All the Vts pass through a standardized quality assurance plan ensure requisite top quality at every stage and in the final product.
Packing & Transportation
All Vts are packed with strong jungle wood to take care of most adverse conditions of transportation all over the country. Special seaworthy packing is done of the Vts for export.
Maintenance
Vts do not require maintenance apart from occasional cleaning of Bushings and checking of Nitrogen Pressure. |
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