OIL IMMERSED VOLTAGE TRANSFORMERS

APPLICATION
Direct measurement of voltage in High Voltage System is Not possible because of insulation problem of measuring instruments. It is also not possible to use direct voltage for the system protection purpose due to its high value and high insulation problem protective relays. Therefore voltage transformers are used to step-down the high system voltage to low standard value accurately in proportion to their ratio.

BASIC FUNCTIONS OF VOLTAGE TRANSFORMERS 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 an installation.
To sense abnormalities in voltage and give signals to protective relays to
isolate the defective system.

CONSTRUCTION VOLTAGE TRANSFORMER MAINLY CONSISTS OF :

Primary & Secondary winding
Electromagnetic Core
Bottom Tank & Oil Expansion Chamber
Porcelain Bushing

PRIMARY & SECONDARY WINDING : Copper enameled wire is used for winding. Primary is wound with multilayer 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 pipe. Secondary is separately wound and inserted in the primary winding as per the requirement. Winding and tapping of V.T. is done in dust-free atmosphere.
	ELECTROMAGNETIC CORE : C.R.G.O. Silicon Steel is used for building up Electromagnetic core. Shell type construction is used to minimise leakage reactance.
BOTTOM TANK & 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 clearing 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 galvanised 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 confirming to I.S. 563/IEC 815. Bushing with collar at both the ends is clamped using Aluminium Flanges. Nitrile and Neoprene Gaskets are used at both sides of collar to form flexible joint. This joint can sustain vibrations without damaging bushing. Bushing with Cemented Flanges are also used.
INSULATION : High Quality Electrical Grade Kraft Paper crepe paper is used for insulating primary and secondary winding of V.T. The high voltage connection is brought out through paper condensor formed on metallic pipe using fine grading of insulation. semi conducting shield is used to give linear distribution of electric stress along 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
Packing & Transportation
Maintenance

HOW TO SELECT THE V.T. :
It is important to specify correct parameters of V.T. while ordering for optimum
design. Following are main factors for selecting Voltage Transformers.

SERVICE VOLTAGE : System Voltage in which V.T. is to be installed e.g. 11kv, 22kv, 33kv etc.
INSULATION : Whether OUTDOOR OR INDOOR
ATMOSPHERIC CONDITIONS : Such as condition of Pollution, Altitude, Ambient Temperature etc.
INSULATION LEVEL : If insulation level other than associated with service voltage is required, it should be specifically mentioned.
RATED PRIMARY VOLTAGE : Rated primary voltage is generally rated system voltage for unearthed type V.Ts and rated system voltage divided by square root of three for earthed type V.Ts the V.Ts. can be manufactured suitable for more than one system voltage. In such case, different primary voltages required may be indicated.
RATED SECONDARY VOLTAGE : Standard values of secondary voltages are 110v or 110v 3 depending on application of the secondary winding. V.Ts. with different secondary voltages other than those mentioned above can be manufactured and supplied.
VOLTAGE FACTOR : All V.Ts are manufactured suitable for continuous voltage factor 1.2 As per I.S./I.E.C. Specification, short time voltage factor is different for different earthing systems. Therefore appropriate system earthing conditions may be specified.
NUMBER OF SECONDARY WINDINGS, THEIR BURDENS & ACCURACY CLASSES :

Number of secondary windings, their burdens and accuracy classes are selected on the basis of application. Two types of classes are available, one for metering and other for protection. Unlike current transformers, accuracy of one winding is influenced by loading of other winding. Due to this reason burden of each winding should be correctly mentioned. Accuracy of V.T. is guaranteed at the secondary terminals of the V.T. the impedance of cable connecting secondary terminals to load (Relay of Meter) offers series drop in output voltage. The caused additional errors of cable and is proportional to the load on the secondary. Therefore, it is advisable to select separate metering and protection winding. Alternatively, single winding can be used for metering and protection if separate cables are used for connections. (Cable of 4 sq. mm. cross section offer 2% voltage drop at 100 meters distance with 100 VA burden of secondary voltage of 110V/3).

RESIDUAL VOLTAGE TRANSFORMER :

               RESIDUAL VOLTAGE TRANSFORMER is used to detect unbalanced voltage in three phase system and to supply voltage to directional earth-fault relay.
     For directional earth-fault relay, it is necessary that the voltage applied to voltage coil of the relay corresponds in phase to that of the current in current coil. Such voltage will be the Residual voltage of the system and will be the phasor sum of the three line-to-earth voltages.
     Residual voltage can be achieved by connecting secondaries of three single phase V.Ts connected in three different phase, in open-delta fashion. It is however, economical to use three phase V.Ts instead of Three nos. single phase V.Ts 'SE' manufactures three phase P.V.Ts suitable upto 33kv system voltage.