Title of Invention

DRY OUT OF TRANSFORMERS AND REACTORS AT SITE BY GAS METHOD

Abstract The invention relates to a method for drying out transformer and reactors at site, comprising the steps of: (a)applying full vacuum to the transformer tank for sufficient tune: (b)monitoring condensate collection while vacuum pulling: (c)pushing dry nitrogen inside the transformer tank; (d)heating the transformer tank using heaters provided around the transformer tank; and (e)repeating the steps (a) to (d) until the desired parameters are achieved This invention relates also to a devrce for drying out transformers and reactors at site comprising : - a heating chamber (2), - a plurality of heaters (4) mounted inside said heating chamber (2) surrounding the transformer tank (3), - vacuum pump (5) for applying vacuum to the transformer tank (3) with a cold trap (6) for monitoring the condensate collection: and - arrangement for pushing dry nitrogen gas inside said transformer tank..
Full Text FIELD OF THE INVENTION
The present invention relates to a method of drying out transformers and reactors at site.
In the construction of transformer, the insulation system plays an important role and therefore needs maximum attention at all stages of handling the transformer.
During the manufacturing stage or during refurbishing, overhauling or repair of the transformer, the core and windings get exposed to the elements resulting in deposit or absorption of dust, dirt and moisture. In order to obtain optimum insulating properly, it is necessary to purge out all these deposits or absorbed materials. This requires careful drying out of the transformer.
While undertaking repair work on transformer at site, all the necessary measures are to be taken for effective dry out of the transformer. The method of moisture evacuation from the transformer which was exposed to the atmospheric condition during its exposure for repair, is of utmost importance for ensuring satisfactory performance of the transformer after repair.
Drying out of the transformer is required if the tank is removed for more than 48 hours. It is also required if the transformer is shut down for a long period, like several months.
In case of prolonged storage in nitrogen gas and after formation of condensation during operation also, drying out of the transformer may be required. BACKGROUND OF THE INVENTION
The method generally employed at site for drying out of transformer is by using an oven for drying the core and winding coils. Sometimes these can be dried by short circuiting. However, both these methods require a considerable amount of care and in some cases the transformer winding may catch fire.
In short circuit method current is circulated for heating the coil. As the coil insulation is oily, it can catch fire easily so careful operation is needed. In some of the sites fire has taken place and damaged the complete core and coil during this process.
A third method for drying out a transformer is by using a filter machine. However, this method of drying out transformer is not very effective. As the oil is heated in filter machine and circulated in transformer, it gets oxidized and therefore charcteristic of oil gets deteriorated, specially resistivity reduces and tan delta increases and therefore insulation resistance and tan delta value of transformer become poor.
A fourth method used for drying out a transformer is by sprinkling hot oil from top in a closed chamber. Such a method however, requires a lot of care.
In sprinkling method the oil is used by spray method for drying out. The oil carries dirt, sludge, carbon etc and oxidized due to contact in air and thereby characteristics of oil deteriorated in this process. This oil therefore is considered is waste and not used for the transformer. SUMMARY OF THE INVENTION
The main object of the present invention therefore, is to provide a novel method of drying out transformers and reactors which can be safely adopted at site and to achieve parameters comparable with those achievable at works without the problems and disadvantages of known methods.
These objects are achievable by the drying out process of the present invention by vacuum, nitrogen gas filling and heating.
Thus the present invention provides a method for drying out transformers and reactors at site, comprising the steps of:
(a) applying full vacuum of 760mm mercury or less than 1 torr to the transformer tank for sufficient time;
(b) monitoring condensate collection while vacuum pulling;
(cj pushing dry nitrogen inside the transformer tank wherein the dew
point of said nitrogen is between - 50°C and - 70°C and the pressure
of nitrogen inside the tank is 0.15kg/ cm2; (d) heating the transformer tank using heaters provided around the
transformer tank wherein the temperature of core and windings of the
transformer is maintained between 65°C and 70°;
(e) repeating the steps (a) to (d) until the desired parameters of condensate collection of ≤50 grams per hour and dew point of - 30°C of the condensate are achieved when the tank/winding insulation temperature is 30°C and the nitrogen gas pressure in the transformer tank is 0.15kg/ cm2;
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Fig . 1 shows the arrangement for carrying out the method of the present
invention.
DETAILED DESCRIPTION
Transformers, after completion of repair work at site inside an enclosed shed are suitable blanked with gaskets and blanking plates. A temporary steel structure alongwith the four sides of transformer tank 3 is erected for mounting heaters 4 inside a heating chamber 2. The heater 4 can be mounted surrounding the transformer tank 3 at 1 to 1.5 feet away from transformer tank 3 and at suitable height depending upon the size of transformer.
Finned industrial air heaters can be used in adequate numbers depending upon the size of the transformers. The heaters 4 are clamped this structure. An asbestos cloth/tarpaulin/thermocol cover is provided for complete coverage of the transformer and heaters to prevent heat less to the atm osphere.
After carrying out the preparations as described above, full vacuum is applied to the transformer tank 3 with the help of
a vacuum pump 5 for sufficient time. The vacuum applied can be
760mm of mercury or less than 1 torr and it can be applied for 48
hours. The condensate collection through cold trap 6 should be
monitored while vacuum pulling.
The heating chamber 2 is provided with an arrangement ( not
shown) through which nitrogen gas is pushed inside the
transformer tank for achieving positive pressure. The dew point
o o
of nitrogen pushed is between -50 C and -70 C and the posi-
2 tive pressure achieved is 0.15 Kg/cm .
The transfoermer tank 3 is then heated for minimum 48 hours
through heaters to raise the inside temperature of transformer.
Temperature is to be measured by thermocouples 8.The temperature
o of core/winding inside the transformer is maintained at 65 C
o to 70 C during the heating cycle.
The temperature measured can be shown on a temperature indicator
7.
The heaters 4 are to be switched off, full vaccum is to be pulled again and condensate collection to be monitored till desired parameters are achieved.
The cycle of pushing dry nitrogen gas under vacuum,heating, vacuming and collecting condensate is to be repeated until rate of condensate collection according to requirement has been
achieved. The transformer is to be filled again with dry nitrogen gas , dew point of dry nitrogen gas inside the transformer tank is to be measured and temperature and gas pressure are to be recorded.
Adequate heating of transformer tank, vacuum pulling and condensate monitoring cycle to be continued till the required parameters are achieved.
The dry out cycle to be terminated after achieving the desired parameters.
The desired parameters for termination of dry out cycle are:
Condensate : ,≤ 50 gram per hour.
o Dew point : -30 C at tank/winding insulation
o temperature of 30 C and nitrogen
2 gas pressure of 0.15 Kg/cm .
A study has been carried out on dry out of several transformers at site by employing this method. The above dry out technique has resulted in satisfactory performance of the transformer after re-commissioning.
Transformer details and dry-out time
SL.NO TRANSFORMER DETAILS DRY-OUT
TIME (DAYS)
1 250 MVA-3 phase 420 kVGT 25
2 200 MVA-1 phase 420 20
3 315 MVA- 3 phase 420 kV BT 30
Dry out of transformers, which had long exposure during repair/inspection at site, was carried out at various sites, by vacuum pulling, nitrogen gas filling and heating. This method has resulted in not only better IR values of transformer winding but also reduced evolution of normal gases in oil during its operation.
This novel technique is effective and safe for dry out of transformer, which improves the properties of insulating material and extends the life of the transformer.
While the invention has been described with reference to a specific method and device, it should be appreciated that modifications and improvements in both method and device may occur to those skilled in the art, without deviating from the scope and spirit of the invention.











We claim:
1. A method for drying out transformers and reactors at site, comprising
the step of:
(a) applying full vacuum of 760mm mercury or less than 1 torr to the transformer tank for sufficient time;
(b) monitoring condensate collection while vacuum pulling;
(c) pushing dry nitrogen inside the transformer tank wherein the dew point of said nitrogen is between - 50°C and - 70°C and the pressure of nitrogen inside the tank is 0.15kg/ cm2;
(d) heating the transformer tank using heaters provided around the transformer tank wherein the temperature of core and windings of the transformer is maintained between 65°C and 70°;
(e) repeating the steps (a) to (d) until the desired parameters of condensate collection of <_50 grams per hour and dew point of the condensate are achieved when tank insulation temperature is nitrogen gas pressure in transformer cm2> 2. A device for drying out transformer and reactors at site by the method
as claimed claim 1 comprising:
a heating chamber (2),
a plurality of heaters (4) mounted inside said heating chamber (2)
surrounding the transformer tank (3),
a vacuum pump (5) for applying vacuum to the transformer tank
(3) with a cold trap (6) for monitoring the condensate collection;
and
arrangement for pushing dry nitrogen gas inside said transformer
tank.
3. The device as claimed in claim 2, wherein said plurality of heaters (4) are an adequate number of finned industrial air heaters.
4. The device as claimed in claim 2 or 3, wherein said heaters are mounted at ft to 1.5 ft away from the transformer tank (3) at a suitable height.
5. The device as claimed in claim 3, wherein a thermocouple is provided
for measuring the temperature of the transformer core winding.
6. The device as claimed in claim 5, wherein a temperature indicator (7) is provided for showing the measured core/winding temperature.
7. A method for drying out transformers and reactors substantially as herein described and illustrated in the substantially as herein described and illustrated in the accompanying drawing.

Documents:

894-del-2003-abstract.pdf

894-del-2003-claims.pdf

894-del-2003-complete specification (granded).pdf

894-del-2003-correspondence-others.pdf

894-del-2003-correspondence-po.pdf

894-del-2003-description (complete).pdf

894-del-2003-drawings.pdf

894-del-2003-form-1.pdf

894-del-2003-form-19.pdf

894-del-2003-form-2.pdf

894-del-2003-form-3.pdf

894-del-2003-gpa.pdf


Patent Number 217596
Indian Patent Application Number 894/DEL/2003
PG Journal Number 38/2008
Publication Date 19-Sep-2008
Grant Date 27-Mar-2008
Date of Filing 14-Jul-2003
Name of Patentee BHARAT HEAVY ELECTRICALS LIMITED
Applicant Address BHEL HOUSE, SIRI FORT, NEW DELHI-110049, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SHRIVASTAVA CHANDRA MOHAN AGM/TXX, BHEL, BHOPAL.
PCT International Classification Number HO2H 5/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA