Transformer Dry Out Procedure

CHAPTER V 5.0

DRYING OUT OF TRANSFORMERS/REACTOR

5.1

As we aware that the transformer winding is highly hygroscopic and it has tendency to absorb moisture from atmospheric humidity when exposed for longer time during erection or repair, the dry out of transformer to remove moisture before putting into service is essential. The process of dry out of a transformer is one requiring care and good judgment. If the dry out process is improperly performed, great damage may result to the transformer insulation/oil through overheating etc. In no case should a transformer be left unattended during any part of the dry out period. The transformer be carefully watched throughout the dry out period and all observations should be carefully recorded. Also if the weather is rainy or the atmospheric conditions are humid, the transformer windings should not be exposed, otherwise the drying out process will be very long and cumbersome. There are many methods of dry out being adopted by different agencies; some of them are as under: • • • • •

5.2

Hot oil circulation with high vacuum filter machine. Vacuum and nitrogen cycle with additional heating arrangement. Induction heating method. Short circuit method. Hot oil spray under vacuum.

HOT OIL CIRCULATION WITH HIGH VACUUM FILTER MACHINE The most practical method of drying out is by circulating hot oil through high vacuum filter machine. To start with oil is drawn from the bottom and fed into the transformer tank top. This will remove any settled moisture/impurities. After 12 to 24 hours of circulation in this manner, the circulation is reversed i.e. oil is drawn from top of tank and fed at bottom for normal circulation during dry out period. The oil temperature of tank,

filter machine outlet temperature, vacuum in filter machine and IR. value of winding should be recorded regularly and a logbook for the same is to be maintained. It is necessary to cover/lag the transformer tank properly during dry out period to maintain the oil temperature and prevent from loss of heat. The care must be taken during dry out that the temperature should not exceed 60 degree C of main tank oil and 65 degree C of filter machine. After oil circulation at consistent temperature for 4 - 5 days and achieving oil parameters within limits i.e. oil BDV 70 KV & PPM 10, the dry out may be considered as complete. The separate hot oil circulation of cooler bank and OLTC oil compartment should also be done before energisation. 5.3

VACUUM AND NITROGEN CYCLE WITH ADDITIONAL HEATERS This method of dry out is used in wide range of transformers where exposure is more. To extract the moisture faster from winding it is essential to raise the temperature of core and winding around 60 to 65 degree C. Indirect heating at 65 degree C and vacuum of 0.3 torr shall be applied initially and monitoring extraction of moisture. If the extraction rate is less than 50 ml per hour for at least 3 hours this process is stopped. After that dry N2 with dew point of -50 degree C is filled into the transformer tank and maintained for 24 hours. Pressure of N2 shall be between 0.1 5 and 0.2 kg/cm2. After 24 hours dew point of N2 is measured and if it is -30 degree C or so, drying is deemed complete. If not, heating and vacuum shall be repeated. Dew point measurement varies with N2 temperature. Preferably 2 KW concealed heaters are used for heating purpose. After achieving temperature of 65 degree of tank (filled with N2 gas) and maintaining it for 24 hours, evacuation from tank should be started through vacuum pump and continued for more than 24 hours. During vacuum pulling the water is collected. If water extraction is more, the N2 and vacuum cycle may be repeated for two to three more cycles. The N2 in tank is working as media for heat transfer to core and coil. To prevent loss of heat, the tank should be properly covered during entire process of dryout. The dew point measurement of N2 gas in the cylinder before filling (-50 degree C) and of tank after 24 hours of filling may be taken and if the value of tank dew point is nearer to the cylinder dew point (approx. -30 deg C), the dry out is considered as complete. Also the water from the tank outlet is

collected during drying process. At the time of termination of dryout, the water collection from transformer tank may he monitored continuously and this value should be constant for the last three readings taken hourly and should be less than 50 ml. per hour. After completing the dry out, oil filling and other activities may be carried out. The separate hot oil circulation for cooler and OLTC oil compartment should also be done before charging of transformer. 5.4

INDUCTION HEATING METHOD This method of heating is generally adopted to raise the transformer temperature during drying out for faster evaporation of moisture. In this method the transformer tank is wound with cable (preferably welding cable) by separating with wooden straps. The three phase or single phase supply source is connected to the coil and allow current to flow in coiled loop. The tank wall gets heated due to induction effect and heat will be transferred to core and coil through oil. The evaporation will start very fast due to high temperature of winding. The temperature of tank wall can be monitored by using thermometers on tank walls. The extraction of moisture will take place through oil under circulation. The remaining procedure of dry out will be same as stated in method I & 2.

5.5

SHORT CIRCUIT METHOD In this method the LV winding of transformer is kept shorted with copper bar/cable suitable to take full load current. The limited voltage is applied on HV winding to allow the flow of desired current (less than full load current). This will help in raising temperature of core and coil to accelerate the evaporation of moisture. The temperature of oil and winding should be monitored properly with the help of OTI and WTI. In no case WTI temperature should exceed more than 75 degree and OTI 65 degree. However, this method is not recommended for large transformers with directed oil flow having no ONAN rating due to chances of local overheating. HOT OIL SPRAY UNDER VACUUM In this method the hot oil is directly spread over core and coil under partial vacuum. The heat of oil is transferred to core and winding and it accelerates the evaporation of moisture under vacuum. Also the loss of heat is less due to vacuum in tank. The spray of hot oil should be continued by connecting filter machine to draw oil from bottom and fed at top.

The arrangement of spray of oil on winding is done by making a suitable pipe having a bus pipe attached with numbers of branch pipes of suitable size with wholes in it at suitable distance on its full length for dropping of oil on coil. The fitting of this pipe with filter machine can be done by using different opening on tank. The measurement of temperature of core & coil can be done by thermocouple. The continuous spray of hot oil on core and winding under partial vacuum to be continued till extraction of moisture through vacuum pump is complete. After completion of dry out the oil will be filled in tank under vacuum unto top yoke. The pipe fitted for oil spray now can be removed. The separate hot oil circulation of OLTC oil and cooler oil should be done before charging.