We took first an 11 KV CT(current transformer) for repair and the method and principle adopted and tested has been applied for other oil filled electrical equpments like potential transformers and distribution transformers also. A CT is an instrument transformer used to measure the value of high current in a circuit in terms of propotionate reduced measurable value.The basic principle of operation of CT is the fact that ,in magnetically coupled circuits the ampere turns(product of ampere and turns)in primary and secondary circuits are equal.Let Np , Ip and Ns , Is be the number of turns and value of current in primary and secondary respectively then
NpxIp=NsxIs
       In a CT assembly there are two CTs ,One with small core cross section for controlling and the other with large cross sectional area .The large cross sectional area in metering is provided to aoid saturatin of flux and achieve accuracy in metering.
In an oil filled CT the value of current in primary side may range from 100 ampere to 1200 ampere . As such for a 400/1 ct (a ct which reduces 400A current to 1 amp) for a current of 1200 in pmy the value of secondry turns will be 400x4=1XNs ie the number of turns in secondary will be 1600 of 1 ampere capacity(the number of turns of a ct is designed on its maximum current carrying capacity). In the same way for 200A load side current with a ct of 200/1 A (pmy turns 1 and secondry current 1A) the number of turns in the secondry will be 200; For a 100/1 amp ct the number of turns in secondry side of ct will be 100;This shows that the number of turns in the secondry is proportional to the current in the primary. this shows that the current in secondary side of ct is proportional to the current in primary side.As such secondary side tapping is set on higher turns for higher current in the primary.
The main cause of failure of CT in high voltage system such as 11,33,132KV and above is gradual decrease of insulation resistance causing flash over bettween live high voltage primary winding winding and eart point(ground point).The insulation in CT decreases due to absorption of moisture and dust in filled transformer oil and the insulating materials.In 11 and 33KV ct's it has been found that rain water mostly enter through valve and in some cases through sealing gaskets( reducicing the insulation Resistance and increasing the capcitance,both undesirable).
       In 11 kv CT the core and coil supporting stands made of bakelite sheets are susceptable to punture due to lowering down of insulation resistance by absorption of moisture or leakage of water. These sheets are available in local market and are replaced easily.The windings are insulated with tape on puntured and damaged points. The main problem after repair is to achieve the required insulation resistance. The maimum insulation resistance found in newly purchased ct is found of the order of 20 mega ohm.which decreases with time and cause failure.
but in our workshop by using transformer oil as medium of heating and drying for 11KV CT we achieved an insulation resitance of the order of 10 giga ohm in 6 hours an unexpected value giving guarantee against CT failure.Drying of CT uptill now has been applied to about 900 ct of varius specifications and found working successfully more reliably than new purchased CT.To the great wonder the failure rate of these repaired ct's are negligible as compared to newly purchase ones.
       The above technique has been applied to 33KV,132KV CT and all are in services for more than 10 years. The insulation resistance of 132KV CT was found 100 Giga Ohm against required 15Giga Ohm after heating for 24 hours.
One thing which is most important is the fact that in investigations of cause of failure rain water influx through gas release valves has been found the main cause of failure of CT. The pulvarised rubber seals in the valves generally rot and decay with time and give path to rain water to enter into T oil filled CT Box.This water causes the decrease of insulation resistance of winding and Bakelite Plate supporting stands resulting in their rupture and failures. To rectify the problem we simply cover the valve with a tea glass that prevents water even if the valve seal gets unserviceable.By 8 yrs working experience of electrical power equipment maintenance I know that 80%of VCB(vaccum circuit breakers)become inoperative on account of failure of CT.The repair process is so simple and inexpensive that it can be carried out any where in a small work shop.
       In an Oil Drying System T oil is used as medium of convection for transfer of moisture from assembled and repaired job to atmosphere .This method of drying is being used since long in different repair work shops by our technicians for CT and distribution transformers. in this method the repaired job is kept in steel tank and a 2 kw 220 volt heater coil wound on half inch thick rectangular wooden block(preferablly Backalite log)placed in an both end open cylinderical insulating Polycarbonate cylinder.this heater is placed in beneath th job. The drum is filled with T Oil. upto the top level of of the job . A 220 volt AC power supply is given to the heater.As the oil gets heated the moisture of the job evaporates to the atmosphere gradually. When the bubbles vanish the job is ready and the required insulation is attained. We measure the IR value of the job by a Meggar. As the oil has already taken the place of porosity of insulating materials and other voids in core and coil assembly there is no possiblity of entering moister in the job and IR does not drop while it is taken out of heating tank. The most important precaution is that the heater should not be taken out of T oil tank without disconnecting the electric supply otherwise fire will catch. When the required IR is achieved the job is placed and fitted in its CT Box with accessories and fressh T oil of proper dielectric strength is filled in it. Measure the parameters most importantly the IR and if foun ok put theit in service confidentally. This method of drying of coil and core of T oil filled electrical equipments is so effective and useful that it has been used for repair of even distribution,power transformers ,CT,PT and etc. and metering units etc. At CT repair workshop Patna this method is being used for repair of 200kva and 315 kva distribution transformers with excellent results.

Advantages of repair is as follows.

1        It had been found that due to a failure of a single CT in a VCB the whole control system becomes inoperative.The defective CT could not be repaired.But by adopting this method today the the defective CT is replaced within 24 hours. Today repaired CT's of different makes are available in all the Mrt stores of bsphcl.
2        The cost of repair is very low compared to purchase of new sets through lengthy purchase processes and delayed and untimely supply.
3        CTs installed in P/S/S are of different makes,models and capacities hence their forecast of failure and procurement in advance is difficult.
4        If we procure CT in Advance and store them in our stores they become unusable due to insulation decremnt with time.
5        Due to changing technology old models may not be available in supplying companies or market. Hence repair is the only solution to keep compatiblity with other equipments.
6        More over the insulation value achieved in 11 KV and 33 KV CT through oil heating method is 500 times more than that achieved through normal heat chamber vaccum oil filling methods.