Transformers #4276856

Low Tension Current Transformers are used for measurement andor protective applications in 660V Class. In Measuring Current Transformers (Refer TABLE- 1) main concern is the accuracy of the secondary current over a wide operating range. Typically the errors are guaranteed to be within specified limits. The nature of errors are current (Ratio) error and phase displacement. Metering current transformers are specified by (1) a burden and (2) accuracy class.

We manufacture lower range of Cast Resin Transformers (CRT) both single as well as three phase, ranging from 16 KVA to 1000 KVA capacity. Our Cast Resin Transformers are designed effectively to reduce line cost and line losses and improve the quality and reliability of the electrical supply system.The transformers can be made suitable for direct pole mounting, indoor application, hazardous area application etc. These are capable of withstanding mechanical and thermal stresses caused by short circuiting in accordance with Bureau of Indian Standards Specification 11171.

Core :Cores are made out of high grade CRGO Steel. These are cut precisely using special cutting tools to obtain burr free edges and ensure proper alignment, low core losses and low noise generation. Annealed core laminations are assembled on core frames and then clamped with suitable mechanical structures. The core edges are given a protective coating.

Coil Winding :The high and low voltage coils are made out of fibre glass covered copper conductors. They are wound on variable speed machines. The windings are then assembled into precise moulds for the casting process.

Casting :The mould assembly with the completed winding is first preheated and then placed into the casting vessel. The casting mix comprising resin, hardner and additives such as fillers, colour paste, accelerators and plasticisers are closed by precisely controlled pumps into a thin film degassing mixer where it is thoroughly homogenised and degassed. When the vaccum level and temperature in the casting vessel reaches the predetermined value, the resin mix is introduced into the moulds containing the active parts.

Curing :Once the resin filling is over, the mould assembly is transferred to the Polymerising oven where controlled curing cycle is applied, so that the resin system cures without any internal cracks. After curing, the coils are demoulded, finished.

Core Coil Assembly :The cast coils are inserted on the core limbs. Resin Cast spacer blocks, end blocks and separators are used as required. The end frames are assembled at this stage.

Tanking :Precisely welded tanks smoothly finished and strongly built to withstand mechanical shocks and vibrations are used as protective enclosure. The core and coil assembly is lowered into the tank and then locked in position. The type of ventilation and protection offered will be different for different applications.

Testing :Apart from section wide testing of different assembled parts, the fully assembled transformers are tested for voltage ratio, polarity, exciting current and no load loss, impedence and load loss, seperate source high voltage test. Partial discharge measurement is done on all transformers. Special tests can be conducted as specified.

Resistant to short circuits (high mechanical strength).Non inflamable.Resistant to temperature fluctuations.Resistant to Moisture.High overload capacity.Space saving permitting compact installation as Unitised substations.Practically no maintenence.No leakage of oil or pilferage.Long and trouble free service.

INTRANS Transformers are made by using modern Computer Aided Design and reliable process technology. Quality Control procedures are devised and strictly adhered to ensure quality in every stages of manufacture i.e., from the very stages of raw material purchases to final delivery. The designs are chosen so that the total cost including material cost, maintenance cost and the energy cost due to no load and load losses are minimum.

The designing and manufacturing operations are guided and controlled by experts in the field. All the above factors ensure that transformer coming out of Intrans plant is optimum designed, reliable and rugged.

Our Scope of Production include Power and Distribution Transformers of capacity upto 2000KVA and 33KV. Any special transformer within the above power capacity can be designed and manufactured to meet customers requirement.

SALIENT TECHNICAL FEATURES

Core :The core is manufactured from good quality Cold Rolled Grain Oriented Silicon Steel Sheets and is of mitred construction to take advantage of grain orientation of the material. The clamping method is designed to reduce the noise level to the minimum.

Windings :Both the Low and High voltage windings are made from high conductivity, electrolytic, copper or aluminium wires. Generally low voltage coils of Distribution transformers are made from rectangular paper covered strips and is of multi layer, spiral cosntruction. The high voltage coils of these ranges of transformer are manufactured using paper covered round-wire and is of disc cross over type.

Electrical, Thermal and Mechanical effects during operations are being considered while designing the coils. The interlayer and turn to turn insulation of the winding is designed to withstand Power Frequency and Lightning Impluse Test Voltages. The axial and radial brazing of the winding take care of the short circuit forces. Adequate cooling ducts are provided to limit temperature rise and to control hot spot temperature to the allowable level.

Main Insulation :The main insulation is designed to meet all the electrical mechanical and thermal requirement, the material is sourced from reputed manufacturers only.

Tank :The tank is made from lamination free quality steel. The tanks are tested for any leakages before surface treatment or final assembly. The inside of the tank is cleaned and coated with hot oil resistant paint or varnish. The outside is properly coated with stove enamelled good quality paints after 7 tank phosphating process.

Tapping :Generally an off circuit Tap Changer is provided to adjust the HV winding to meet the local voltage condition. Normally +/- 2.5% and +/- 5% tapping are provided on HV side. Any special requirement from customers can also be considered. On Load Tap changers (OLTC) are also provided as per customers requirement.

Terminals :The terminals of the low and high voltage windings are taken out through standard bushings to facilitate connection to the transformers. Either out door type or cable box type terminations can be provided as per the requirement from the customers.

Insulating Oil :In case of oil filled transformers the filling is done with good quality transformer oil meeting the Indian Standard specification IS-335.

Fittings :The standard fittings include

ConservatorDrain / Filter ValvesLifting LugsEarthing TerminalsRating and Diagram PlatesTerminal MarkingBreather (Clear view type)Oil Filling Hole with capThermometer Pocket &Oil Level Indicator

Any additional accessories such as Buchholz relay, Oil temperature indicator, Winding temperature indicator etc. can be provided on request.

In AC circuits, for measuring large voltages a specially constructed accurate ratio Potential Transformers are used. Their function is to reduce the supply Voltage to a value small enough to be easily measured with standard low range AC instruments of moderate size and capacity. The advantages of using Potential Transformers for range extension of AC meters are many fold.

The instrument is isolated from line voltages hence it can be grounded.It is possible to achieve standardisation of instruments and meters at secondary ratings of 110V.Several instruments can be connected from a single transformer.The power connected in measuring circuits is low.

Potential Transformers of dual purpose Metering & Protection are also commonly used. An additional secondary winding is provided which is connected in open delta so that in the event of an earthfault on any of the phases, appropriate voltage is developed across the open delta terminals which is then used to actuate a tripping mechanism.Resin Cast Potential Transformers are dry type transformers the active part of which is completely embedded in quartz loaded Epoxy Resin by using vacuum moulding technique. After moulding the resin is hardened by polymerisation. The main insulation between Primary and Secondary & Earth is obtained by this epoxy resin. These transformers are of compact design with high mechanical and electrical properties. These are non-hygroscopic and hence give very good insulation properties even in given humid conditions.

Details required on placement of orders:

Ratio :This is the Ratio of the primary voltage to the rated secondary voltage. The Primary Voltage can be either 11KV i.e. Connected between the lines or 11KV/ when connected between the lines and earth. The primary voltage also varies depending on the connection i.e 110V or 110V/ root 3 voltage.

Burden :This is the load connected across the secondary winding expressed in VA depending on the number of meters connected and distance of the meters from the Potential transformer.

Class of Accuracy :The choice of accuracy class depends on the duty to which the Potential Transformer is put to use. The measuring PT maintains the Ratio and phase angle error over the operating range of 80-120% of Primary Voltage. The protection PTs are used in protection system for operating differential protection relays and maintain accuracy for 5% of the rated voltage given by the voltage factor.

Resin Cast current Transformers are dry type transformers the active part of which is completely embedded in quartz loaded epoxy resin by using Vacuum moulding technique. After moulding the resin is hardened by polymerisation. The main insulation between primary and secondary and earth is obtained by this epoxy resin and hence CTs have a lot of technical advantages.

ADVANTAGESUse of epoxy resin for insulation

Compact design for high mechanical and dielectric properties. The considerable reduction in dimensions reduces space and switchgear panel width.No fire spread and hence no permanent damage to the equipment; Even after a surface flashover reconditioning is easy by a thorough cleaning of surface.Epoxy is non hygroscopic and hence gives very good insulation properties even in severe humid conditions.Non ageing. The moulding has a non wearable glossy surface and has a very high degree of toughness. Therefore the CT requires no maintanence except for the clearing of dust contamination on the surface.As there is no oil for insulation the CT can be mounted in any direction to suit the switchgear design.The current terminals are provided with sufficient area thereby avoiding any overheating of joints.Based on the dynamic current rating additional clamping facility is provided for the terminals.For higher current ratings window tupe CTs type WR can be used which will give additional saving in space required for terminal connections.For window type CTs equipotentialising facilities are provided with cast nuts and thereby permanent and sure connections can be made.For window type CTs large window openings are provided so that the busbars can be run through without any special bending and with one thickness gap between individual bars as done in the switchgear.Sufficiently long creepage surfaces are provided to ensure better surface tracking properties.

The basic designs are available in three major types to cover the entire range of 11KV Resin Cast CTs for primary currents upto 2500A.

The wound primary cast resin PR type for low ratios.The bar primary cast resin BR type for higher ratios.The window types cast resin WR type without primary for still higher ratios.

The PR type is a wound primary type CT designed to cover primary currents from 25 amps to 300 amps. The bar type BR CTs are designed with a bar as primary. These CTs can cover a range of primary currents from 300 amps to 1000 amps. All higher ratings of primary Currents are covered in WR type of Window CTs.

While ordering please specify the Short Time Rating, Insulation Level, VA burden, type of CT and Model.When specifying the burden please consider the total connected items and see that the actual burden is as close to the rating as possible. However one of the standard rating as per IS2705 may be adopted.When specifying the accuracy class please ensure that the requirements of certain relays or any other specific requirements are taken care of.While selecting the primary current of the CT ensure that the calculated maximum load current in the system falls within the range from 50% to 100% of the selected ratings in general cases.Avoid combining too many ratios in the same CT.Clearly indicate any dimensional limitations in switchgear panel, if any.

These transformers have been successfully type tested for a Short Time Current rating upto 44KA for 1sec at CENTRAL POWER RESEARCH INSTITUTE , Banglore.

INTRANS 11KV CT/PT UNITS (HT metering sets) consists of two Current Transformers and one Three phase Voltage Transformer each with an accuracy class 0.5. These are oil immersed, in a mild steel tank and are suitable for out door platform mounting or pole mounting use.

Current Transformers are Epoxy Resin moulded. The windings and core are completely embedded in quartz loaded Epoxy Resin by using vacuum moulding technique and are then hardened by polymerisation. The Current Transformers have high mechanical and dielectric properties and require no maintenance.

The Voltage Transformer is oil immersed type. The magnetic circuit of the three leg core type made up from Cold rolled Grain Oriented Grade 41 silicon steel laminations. These cores are liberally designed as regards flux density thus avoiding abrupt changes of error with slight excess voltage. The core clamps are of angle iron construction.The low voltage winding consists of a single coil for each leg. The high voltage winding is built up from a number of separate coils, the line coils having reinforced end turn insulation. All coils are thoroughly dried and impregnated with varnish. Both primary and secondary windings are star connected.

Three high voltage HRC PT fuses of 2 amps cap and two bakelite HRC fuse units are fitted. The former are located in the tank on a specially moulded epoxy fuse carrier which can be easily reached by removing the top HT fuse cover. The latter are situated in secondary terminal box.

The secondary terminals are brought out of the tank through a moulded epoxy terminal board with brass terminals with M6 threads. The bakelite fuse holder with HRC fuses are mounted on this terminal board.

The epoxy molded current transformers have, secondary windings rated at 5 amps with accuracy class 0.5 as per IS 2705. The three phase voltage transformer is designed for 110V between secondary terminals as per IS 3156. The transformers are designed to have their specific output and accuracy at 50 cycles per second.

INTRANS Metering sets for outdoor use have the following special features.

The transformers are housed in a sheet steel welded tank. These tanks are pretreated with 7 tank phosphating process and stove enamelled. The transformers are housed in a sheet steel welded tank. These tanks are pretreated with 7 tank phosphating process and stove enamelled.A gradual slope on the tank top cover ensures that no rain water accumulates on the cover.The secondary terminals are located in a box welded on one side of the tank with provision for sealing.Lifting lug, drain plug, oil filling plug, oil level indicator and earthing terminals are provided as standard fittings.Epoxy resin moulded Current Trainsformers enable withstanding of dynamic forces during short circuit conditions.

Complete conformation to all the relevant Indian Standard Specifications.