Theory
A current transformer operates on the principle of balance of Ampere turns in primary and secondary winding. Voltage Es developed across secondary of the current transformer can be represented as ES=ZS x is where Zs is impedance of burden, connection leads and CT secondary. The magnetic flux required to develop voltage Es needs excitation current i.e. this excitation ampere turns are supplied through total primary ampere turns, causing error in the magnetic material. Errors in the current transformer are also non-linear over the current range of 120% to 5%. There are four main factors which determine the capability of current transformer i.e.
 Insulation Level (Service Voltage)
Rated primary current
Short time withstand current
Burden and Accuracy
Current Transformer Must
Withstand operational voltage and over voltage in the network
Withstand rated primary current in continuous operation without exceeding maximum allowed temperature rise
Be capable to sustain thermal and mechanical stresses developed due to system fault current
Feed current to external circuit with specified accuracy at specified primary currents
Design
We manufacture current transformers in both types i.e. Live Tank Design and Dead Tank Design. Current transformer can be designed for single ratio or multi ratio. The ratio selection can be achieved by providing two or four sections of primary for series/parallel reconnection. However, the current ratio shall be in proportion of 1:2:4 the advantage of this type of ratio selection ratio. Ratio selection is also possible by giving taps on the secondary winding. Advantage of this type is that ratio can be achieved in any multiple of the lowest ratio. It is also possible to change the ratio on CT without taking shut down on the man system. However, it is obvious that output rating shall change as per the ratio. For multi ratio CT, 2 or 3 ratios can be provided by primary series-Parallel reconnection and additional ratios by secondary taps. High output (Burden/Knee point Voltage / Accuracy Limiting Voltage) with minimum excitation current and better accuracy class can be provided on the CT if primary current is higher. Higher output can also be obtained with multi-turn primary design. Limitations on no. of primary turns are, however, imposed by short time current rating and peak dynamic current. It is advisable to provide minimum primary turns to limit the dynamic forces and mechanical stress experienced by the CT under fault conditions.
Construction
Core & Secondary Winding: High permeability, CRGO silicon steel is used as core material. Toroidal core from continuous strips are annealed in controlled atmosphere to achieve best quality secondary cores. Secondary winding is done on automatic winding machine and is distributed equally on the periphery of the core to minimize leakage reactance. Best quality enameled copper wire with adequate inter-layer insulation is provided to avoid secondary winding short circuiting in most adverse condition.
Primary Winding
Primary winding is of braided electrolytic copper conductor with double cotton covering. Varnished fiberglass sleeve is provided as an additional insulation on this conductor. In case of live tank construction, the primary is wound on insulated secondary. The primary is positioned property to sustain dynamic forces developed during short circuit condition. In case of Dead Tank construction, the primary is encapsulated in circular rigid fibre glass ring and galvanized pipe to form Eye Bolt construction.
Insulation
High quality crepe insulating paper is used to build up main insulation of the CT in live tank design, insulation is built up on secondary core and secondary leads are brought out through a metallic galvanized pipe. In dead Tank CT primary winding is encapsulated in fibre glass ring and Aluminium pipe. Main insulation is built up on primary winding (paper condenser is formed on the pipe) with 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 CT while CT 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 predetermined pressure.
Porcelain Insulator
Brown Glazed porcelain bushing with different shed profiles to suit different pollution conditions are use. These Bushings are hollow cylindrical type conforming to IS 5621/IEC & 815. Bushings with Collar at both the ends are clamped using Aluminium Flange, Nitrile and Neoprene Gaskets are used at both sides of collar to form flexible joint. This joint can sustain vibrations without damaging bushing. Bushings with cemented flanges are also used.
Tank & Base
Top tank (which also acts as oil Expansion Chamber) of the CT having current less than 800 Amps is made of MS sheet. Stainless steel Tanks is used for CTS with primary current of 800 Amps. And above Base is fabricated from MS plates and channels all MS Tanks and Bases 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. All MS parts can be supplied Hot Dip Galvanized on request.
Testing
Our testing Lab is equipped with modern test facilities to carry out all routine tests including temperature rise test. Partial Discharge test plays vital role in quality control of insulation. All our Cts are tested for Partial Discharge on most modern bridge type Partial Discharge Test Set. All the Cts pass through a standardized quality assurance plan to ensure requisite top quality at every stage and in the final product.
Packing & Transportation
All Cts are packed with strong jungle wood to take care of most adverse conditions of transportation all over the country. Special Seaworthy packing is done for the Cts for Export.
Maintenance
The Cts do not require maintenance apart from occasional cleaning of Bushings and checking of Nitrogen Pressure. |
