OIL TRANSFORMER HV/LV

Transcription

1 Rev.02 - Pag. 1/23 Installation, use and maintenance Manual OIL TRANSFORMER

2 Rev.02 - Pag. 2/23 TABLE OF CONTENTS 1 INTRODUCTION Reference standard Electromagnetic compatibility Marking CE GENERAL OUTLINE OF THE TRANSFORMER AND THE ACCESSORIES RECEPTION, HANDLING, STORAGE Reception Handling Storage INSTALLATION Standard installation conditions Ventilation Insulating distances Earthing connections LV and HV connections Regulation of transformation ratio Parallel connection of transformers TRANSFORMER PROTECTIONS Over temperature protection Overloading and short circuit protection Protection against over voltages COMMISSIONING Mechanical checks before the commissioning Electrical checks before the commissioning Insulating fluid level Arcing horns Air dryer Buchholz Relay Thermometer Integrated safety device Operations for commissioning... 20

3 Rev.02 - Pag. 3/23 8 MAINTENANCE Input voltage Conserving the oil level Outside cleaning of radiators / corrugated walls Repainting Oil treatment General reconditioning Extraordinary maintenance... 23

4 Rev.02 - Pag. 4/23 1 INTRODUCTION 1.1 Reference standard The oil transformer referred to the attached test report, has been designed and manufactured by TESAR in order to meet to Italian CEI and international IEC Standards requirements in force at the moment of its manufacturing (unless different agreement). CEI Standards (Italian) - CEI EN Power transformer part 1 : General - CEI EN Power transformer part 2 : Temperature Rise - CEI EN Power transformer part 3 : Insulation levels, dielectric tests and external clearances in air - CEI EN Power transformer part 4 : Guide to the lightning impulse and switching impulse testing - Power transformers and reactors - CEI EN Power transformer part 4 : Ability to withstand short circuit - CEI EN Power transformer- part 10 determination of sound levels - CEI Marcatura dei terminali dei trasformatori di potenza - CEI Guida di carico per trasformatori immersi in olio IEC Standards (International) - IEC Power transformer part 1 : General - IEC Power transformer part 2 : Temperature Rise - IEC Power transformer part 3 : Insulation levels, dielectric tests and external clearances in air - IEC Power transformer part 4 : Guide to the lightning impulse and switching impulse testing - Power transformers and reactors - IEC Power transformer part 4 : Ability to withstand short circuit - IEC Power transformer- part 10 determination of sound levels - IEC Terminal and tapping markings for power transformers - IEC Loading guide for oil-immersed power transformers 1.2 Electromagnetic compatibility The intensity of low frequency magnetic field emitted by the windings is of a limited value and, however, it is equal or lower than those of the field emitted by the connections and the low voltage bars. Its value quickly decreases by increasing of the distance from the transformer. 1.3 Marking CE TESAR does not affix the marking CE on its transformers as foreseen at paragraph of the Guide to the application of the Directive 89/336/EEC which excludes from the Directive application field: High voltage inductor High voltage transformer 2 GENERAL This publication aims to guide users on the properties of power and distribution transformers insulated with mineral, silicone or vegetable oil. Two types of transformers are considered: - Hermetically sealed total filled (Fig. 1) - Breathing with conservator (Fig. 2)

5 Rev.02 - Pag. 5/23 3 OUTLINE OF THE TRANSFORMER AND THE ACCESSORIES Fig. 1 Hermetically sealed total filled transformer Fig. 2 Breathing with conservator transformer

6 Rev.02 - Pag. 6/23 Standard components and accessories: a. Rating plate (Fig. 3 and Fig. 4) b. HV porcelain bushing (Fig. 4) c. LV porcelain bushing (Fig. 4) d. Off-load tap-changer (Fig. 4) e. N 2 earthing terminals (Fig. 4) f. Tow attachment (Fig. 4) g. Lifting eyes (Fig. 4) h. Truck with rollers for lengthways or sideway travel (Fig. 4) i. Thermometer pocket (Fig. 4) j. Drain valve(fig. 4) k. Arcing horns on HV porcelain bushing, for transformer without HV cable box (Fig. 4) l. Air dehydrating breathers, for breathing transformer (Fig. 4) m. Oil level indicator, for breathing transformer (Fig. 4) Optional components and accessories: I. Plug-in HV terminals (Fig. 5) II. Thermometer, to install on the thermometer pocket (Fig. 5) III. RIS or equivalent for hermetically sealed transformer (Fig. 5) IV. Buchholz Relay for breathing transformer (Fig. 5) V. Safety valve (Fig. 5) VI. PT100 thermo-resistance (to install in the thermometer pocket) (Fig. 5) VII. HV and/or LV cable boxes (Fig. 5) Fig. 3 Rating plate

7 Rev.02 - Pag. 7/23 k i b g f c d a j e h m l Fig. 4 Standard components and accessories

8 Rev.02 - Pag. 8/23 I Plug-in for HV terminals II Thermometer III RIS or equivalent (for hermetically sealed transformer) IV - Buchholz Relay V - Safety valve VI - PT100 thermo-resistance (for breathing transformer) VII - HV and/or LV cable boxes Fig. 5 Optional components and accessories

9 Rev.02 - Pag. 9/23 4 RECEPTION, HANDLING, STORAGE 4.1 Reception The transformer is generally supplied totally assembled and ready to be connected to the medium and low voltage line. According to the specification, the transformer is shipped with a polythene protection, or packed into wooden cage, for protection against dust and little shocks, or finally packed into a wooden case for over-sea shipments. On receipt of the transformer, both at the client's plants or at site, it is necessary to carry out the following checks: Check there are no signs of damage on the packing or on the transformer occurred during the transport. The characteristics of the transformer detailed on the rating plate must correspond to those of the shipping documents and with those of the test report, which is attached to the transformer. Check each transformer is complete with the accessories foreseen in the contract (wheels, bushing, etc.). IMPORTANT: In case any anomalies are found, please contact immediately the manufacturer. If within 5 days there will be no notification of anomalies or defects, it can be considered that the transformer has been delivered in perfect conditions. The manufacturer, therefore, cannot be considered responsible neither for what could happen to the transformer during service nor for the eventual consequences.

10 Rev.02 - Pag. 10/ Handling During the transport or the handling, it is recommended to use only the special lifting eyes and tow attachments (see g and f letter in the Fig. 4). IMPORTANT: The transformer cannot be moved pushing the cooling fins or radiators or tank. Ropes must be arranged so as not to interfere with the radiators or cooling fins. Lifting can be made using the appropriate hooks remembering to use ropes long enough so that H is never less than L (Fig. 6). Always use all the existing hooks and not only one of them. Fig. 6 Lifting transformer

11 Rev.02 - Pag. 11/23 Example of handling Fig. 7 Handling by means of bridge crane and forklift truck Fig. 8 Manual handling Fig. 9 Wrong handling

12 Rev.02 - Pag. 12/ Storage Transformer for indoor installation The transformer must be stored in a sheltered, clean and dry ambient maintaining the packing up to the installation Transformer for outdoor installation The transformer may be stored outdoor until the installation time. IMPORTANT: The storage temperature must not be lower than -25 C. 5 INSTALLATION 5.1 Standard installation conditions The maximum height of installation must not exceed 1000 m above sea level. When the transformer is in service, the ambient temperature of the room must be within the following limits: Minimum temperature: -25 C Yearly average temperature: +20 C Monthly average temperature: +30 C Maximum temperature: + 40 C In case the height of the place of installation and/or the ambient temperature values are higher than the ones specified above, it is necessary to specify it at the ordering stage since a particular dimensioning of the transformer depends upon these values. 5.2 Ventilation The electric current passing through the windings, and the effect of the core magnetising current produce electrical losses resulting in localised heating of the windings and core. The transformer is designed in such a way that natural cooling maintains the transformer temperature under the maximum values foreseen by the standards. In order to avoid temperature accumulation in the room where the transformer is installed, it is necessary to provide suitable ventilation. Each TESAR transformer is equipped with a thermometer pocket where a thermometer with contacts may be installed (optional accessory). The suggested values for setting the contacts are following (oil temperature): Description Alarm C Trip C Oil 90 C 100 C

13 Rev.02 - Pag. 13/23 Dimensioning of air louvers Natural cooling In the room where the transformer is installed, it is necessary to install air louvers sufficient to dissipate the heat generated by transformer during service, in order to guarantee the standard service conditions and to prevent exceed the over temperature limits of the transformer. Therefore the room must be provided with a louver on the lower part S, in order to guarantee an appropriate fresh air flow; and a louver S on the upper part of the opposite wall, in order to extract the hot air (see Fig. 10). To calculate the surface of the louvers for air incoming and outgoing, for a nominal annual ambient temperature of 20 C, the following formula may be used: S P S' S H where: P = Sum of the transformer no-load losses and the load losses at 75 C [kw] S = Surface of the incoming louver [m 2 ] S = Surface of the outgoing louver [m 2 ] H = Height between the two louvers [m] Fig. 10 Natural cooling In order not to obstruct cooling is necessary that the transformer is installed at a distance not less than 30 cm from the walls and not less than 60 cm from other transformers. The room shall be a suggested flow of approx. 3,5 m 3 /min for each kw of total losses at 75 C Forced cooling Forced cooling is necessary in the following cases: Frequent overloading Low-dimensioned room Room scarcely ventilated Medium daily temperature higher than 30 C. The forced cooling can be realized by means of installation of air extractor located in the upper part of the room controlled by a suitable thermostat or directly by a transformer thermal protection relay, with a suggested flow of approx. 3,5 m 3 /min for each kw of total losses at 75 C. ATTENTION: an insufficient air circulation, besides reducing the nominal life of the transformer, causes over heating which, in the worst cases, can determine the intervention of the thermal protection relay.

14 Rev.02 - Pag. 14/ Insulating distances The transformer supplied for service without HV/LV cable-box (IP00) must be installed in the room respecting the insulating distances specified in the national standard (in Italy, CEI 11-1). The transformer, also with plug-in bushing, must be protected against direct contacts. 5.4 Earthing connections Tesar is not responsible for the transformer installation. The installation must be carried out according to the standards in force, to the applicable laws and to the present instructions. The following points must be taken into consideration when the installation is carried out: connect the earthing conductors to the relative earth points on the metallic parts of the transformer and cable boxes. connect the low voltage neutral to earth when required or when required by the protection system to earth. 5.5 LV and HV connections Execution without cable boxes (IP00) The cables and the bus bars which are connected to the transformer must be duly fixed to avoid any mechanical stress on the LV and HV transformer terminals. Both upper and bottom cable connections can be performed, being sure, however, to respect the configuration showed in the drawings. In case the connections arrive from the bottom please ensure that there is sufficient depth for the bending of the cables. Execution with cable boxes The cables and/or the bus bars which are connected to the transformer must enter in the cable boxes only through suitable flanges specified at the ordering stage. In any case the cables and/or the bus bars must be duly fixed outside the cable boxes to avoid any mechanical stress on the LV and HV terminals of the transformer and also on the cable boxes. After the installation please check the correct degree of protection is maintained near the passageway of the cables and/or the bus bars. HV cable box: cables coming only from the bottom LV cable box: cables coming from the bottom or from the top Fig. 11 Cables coming from the bottom (left) and from the top (right)

15 Rev.02 - Pag. 15/ Regulation of transformation ratio The variation of the primary voltage is obtained by changing the position of the voltage off-load tapchanger located on the cover of the tank. Ensure always that the position of the tap-changer is properly inserted and not in an intermediate position. In case the primary voltage of the system does not correspond exactly to the voltage of the central tap, it is necessary to modify the position of the tap duly positioning it on one of the other taps in order that the secondary obtained is the no-load voltage shown on the rating plate. Note: At the time of shipping, the position of the voltage off-load tap-changer are located on the central tap. In case of transformers with double ratio, please be sure that the position corresponding to the voltage of the system feeding the transformer is properly connected. Note: At the time of shipping, the position of the voltage off-load tap-changer are connected on the position of the higher primary voltage. Important A) In case of transformers with double ratio, please be sure the tap corresponding to voltage of the system feeding the transformer it-self, is duly connected. B) In case the position of the tap-changer has to be changed after the transformer has been put into service the first time, it is recommended to put the transformer out of service and to connect to ground the LV and HV circuits before approaching the transformer. 5.7 Parallel connection of transformers If the transformer must be connected in parallel with other transformers, please verify the total compatibility of the voltage ratio and of the conditions stated by the IEC standards, and particularly: Identical voltage ratio Identical frequency of functioning Identical vector group Identical short circuit voltage (Tolerance ± 10%) The rated power (kva) of the biggest transformer should not be larger than twice the smallest transformer The LV connections must be the same length from the LV terminals (of two different units) to the switch junction.

16 Rev.02 - Pag. 16/23 6 TRANSFORMER PROTECTIONS 6.1 Over temperature protection The protection can be realized with: - thermometer (optional accessories) - RIS or equivalent equipment (optional accessories for hermetically sealed transformer) The thermometer is generally installed in the thermometric pocket on the cover of the tank. The thermometer may be equipped with two settable contacts, one for alarm, the other for trip. The suggested values are written at paragraph 5.2. The integrated safety device for hermetically sealed transformer (RIS or equivalent) provides: - two settable contacts for temperature, one for alarm and the other for trip. The suggested values are written at paragraph one settable trip contact for pressure (The suggested value is 0.3 bar) - one trip contact for oil level 6.2 Overloading and short circuit protection According to the parameters of the standards shown at point 1.1, the transformer is designed and manufactured in such a way to withstand limited abnormal situation of over-voltages, overloading and short circuit. Therefore, the transformer must be protected against thermal and dynamic effects caused by continuous overloading and secondary short circuits by means of an automatic switch or suitable fuses, able to disconnect the transformer in case of current flow higher than the one fixed by the protection. The protections setting and/or the choice of fuses HV and LV side, must be carried out taking into consideration the primary and secondary rated currents stated in the transformer rating plate, taking also into consideration that when we feed the transformer, a very high magnetizing current is established on the primary, by means of variable starting from a minimum of 10 times the rated current (in the worst conditions of insertion depending from the instant when the feeding circuit is closed, from the electric characteristics of the feeding network, from the reactance and resistance values of the circuit network-transformer, the insertion current can reach also 20 times the rated current), even if the automatic switch located on the secondary is open and therefore without loading. Therefore, it is necessary to duly set the relay of maximum current HV side, in current and time value, by introducing a little delay (approx. some tens of ms), in such a way that the protection relay will not operate. Furthermore, we suggest limiting the number of connections and disconnection of the transformer on the network. 6.3 Protection against over voltages In order to protect the transformers against over-voltages at industrial frequencies or the ones due to atmospheric origin, it is recommended that voltage surge arrester with variable resistance are used. The characteristics of the surge arresters depend on the transformer insulation level and from the characteristics of the distribution system line. It is recommended to install the surge arresters when the transformer is connected, directly or through a short cable, to the main networks.

17 Rev.02 - Pag. 17/23 7 COMMISSIONING 7.1 Mechanical checks before the commissioning Check the earth connections Check of the insulating distance of the parts under voltage towards ground as shown at paragraph 5.3 Check the tightening of HV and LV terminals applying the following values of the tightening torque. Bushing On the central conductor LV/HV in copper or brass Bolts M8 M12 M20 M30x2 M42x3 M48x3 M55x3 M75x3 Torque N*m On the fixing stud screw HV insulators Bolts M10 M12 M16 Torque N*m On the pinch bolts connecting flags Bolts M10 M12 M16 Torque N*m Mechanical parts Bolts M8 M10 M12 M14 M16 M18 M20 M22 Torque N*m With torque wrench set in kgm, divide the values by 10

18 Rev.02 - Pag. 18/ Electrical checks before the commissioning Verify that the position of the tap-changer is inserted correctly. In case of double primary voltages, please check the feed to the transformer correspond to the correct position. Check the correct functioning of the switches located as protection of the transformer on HV and LV side. Check the correct setting and functioning of the overloading and short-circuit protection relay. Check the correct setting and functioning of the over-temperature protection relay and of the thermometer. Check the functioning of the fans. Check the general conditions of the transformer and proceed with the measurement of the insulating resistance by means of a Megger at 2500 V. This measurement must be carried out with HV and LV terminals disconnected from the equipment, it means before connecting the cables and/or the bus bars. The values of the measured resistances must be approx. the following: HV terminals / LV terminals grounded 20 MΩ LV terminals / HV terminals grounded 10 MΩ HV terminals - LV terminals / ground terminal 10 MΩ In case the measured values are lower, please contact our after sale service. ATTENTION: In case a transformer is put into service after a long storage period, or after a long de-energised period, it will be necessary: 1) clean the HV/LV bushings from eventual dust and condensation by means of dry compressed air jets and dry cloth. 2) if the transformer has been held in stock far more than six months in normal climates or far more than a month in damp climates, check the dielectric strength of the insulating oil. Using the procedure established by the IEC standard to check the dielectric strength of the insulating oil, if the test is negative proceed to dry the dielectric oil of the transformer, otherwise contact Tesar. It is recommended, finally, to carry out always a visual check of the transformer in order to verify any unlikely presence on the surface the cooling ducts 7.3 Insulating fluid level Check that the reading on the level indicator approximately correspond to ambient temperature (transformer with conservator or RIS for hermetically sealed). If the insulating fluid needs to be topped up, only fluid with the same characteristic and not containing PCBs must be used and with the required dielectric strength. The oil transformer must be topped up according to the manufacters instruction.

19 Rev.02 - Pag. 19/ Arcing horns If the transformer is equipped with arcing horns, check that the position of the spark gap rods mounted on HV porcelain bushings corresponds to the insulation class, as shown in the following chart. Insulation Class [kv] A [mm] B [mm] Fig. 12 Arcing horns 7.5 Air dryer If the transformer is equipped with air dryer (transformer with conservator), check that the salts of silicagel are orange. If they are neutral colour, they have already picked up moisture and do not fulfil their function as dehydrating air. It is necessary to remove the salts and regenerate the dryer by placing them in an oven at 150 C until they are come back to orange colour. 7.6 Buchholz Relay If the transformer is equipped with Buchholz relay (transformer with conservator) to avoid unnecessary activation, proceed carefully with the leaking operation of the relay, acting on the taps (see instruction relay). If during commissioning or in the first hours of the energization the Buchholz relay should signal the formation of gas, in small quantities, first to believe that there is a fault in the transformer and repeat the leaking operation. 7.7 Thermometer If the transformer is equipped with the thermometer check the threshold of the alarm and trip are set at the operating conditions. The setting temperature are provided to the manufacturers in according to transformer type and the insulation class. 7.8 Integrated safety device If the transformer is equipped with the integrated safety device (RIS or equivalent) check the setting of the alarm and trip of the thermometer and the values of pressure shown are matched to operating conditions. For more additional information, consult the operating manual supplied with the integrated security device, you must be careful not to change the orientation fixing RIS or equivalent (the exact position should be required before delivery).

20 Rev.02 - Pag. 20/ Operations for commissioning Closing of HV switch When the switch is closed the transformer gives out a sharp noise, which decreases in few ms until it stabilises. Check of secondary voltages Before closing the low voltage switch or make additional checks for parallel operation with other transformers, it is necessary: Check by means of a voltmeter the proper value between each two phases and of the three wye-connected voltage. Check the proper phases displacement and the correct vector group. In case the values correspond to those stated in the rating plate, it is possible to proceed and complete the commissioning or to carry out the checks for the parallel service. Run in parallel service with other transformer In case the transformer has to run in parallel with another, it is also necessary: Check the compatibility of the rating plates of both transformers Verify the concordance of the phases by measuring the voltage between the phase "one" of the transformer already in service and the phase "one" of the transformer to be connected in parallel by means of a voltmeter. The value of this result must be zero. Continue in the same way for phase 2 and phase 3 Closing of LV switch Complete the commissioning of the transformer closing the LV switch. IMPORTANT We remind you that qualified technical personnel must carry out the operation, commissioning and energization. Furthermore, during the voltage measurements and the phase displacement checks on the upper terminals of the LV switches, you are recommended to use suitable instruments and suitable insulating gauntlets.

21 Rev.02 - Pag. 21/23 8 MAINTENANCE The transformers are static machines, and therefore in general do not require special care. However, a periodic examination is necessary, particularly during the first months of operation. At longer intervals it is essential to carry out more through inspections and controls. The periodicity of the examinations is subordinated to the environmental conditions and to operating conditions. In fact, if the machine is installed in dry, clean environments and in regular and uniform operating conditions, the necessity for surveillance is relative, and the inspection may be made at much longer time intervals; while in damp, dusty environments and in the presence of frequent variations in ambient temperature or in load, surveillance must be more assiduous and inspections more frequent. The accessories and safety and warning devices must also be submitted to periodic inspections. To better clarify the concept of maintenance, we are providing a table which, in synthetic and summary form, identifies the type of inspection and the periodicity with which it is advisable to make the inspection, in order to obtain good operation and a long life for the transformer. Controls and tests Interventions Periodicity of Inspection Voltage Current Ambient temperature Oil temperature Oil leakage inspections Oil level Oil stiffness Oil acidity Cleaning from dust, dirt, unlikely presence in the tank / bushings / others Tightening of the bolts of the primary and secondary electrical connections Tightening of the mechanical parts bolts and transformer clamping to the ground Earth resistance Inspection, filters salts Inspection, Thermometer and/or Bucholtz relay Inspection, safety valve Before energization X X X X X X X X X X X X X Every day X X Every week X X X Every month X X Every quater X Every 6 months X X X Every year and/or after exceptional events X X X X Every 2 years Every 5 years If the dielectric strength is low Oil treatment General reconditioning X X X Positions

22 Rev.02 - Pag. 22/23 Controls of positions 1 6 can be carried out by the user at sight, utilizing the normal instrumentation available at the plant. Control of the efficiency of the earthed mass installation is normally made by an official public authority (in Italy the USL). For the other controls, it is best to get in contact with laboratories or with specialized companies. The importance of frequent readings and their recording is obvious. It is enough to think that in this way, it is possible to establish up to what point the rated characteristics of the machine and the efficiency of its refrigerant organs are utilizable. For the good operation of the machine, it is advisable to respect a temperature limit which must never be exceeded. This limit for oil transformers with Class A insulation is 105 C. If this limit for the oil temperature is exceeded, this can be due to one or more of the following conditions: 1 the network frequency does not correspond to the one on the registration plate; 2 a load condition greater than the rated one, exists; 3 the machine is over-supplied (the switch is not in the correct position with respect to the value of the input voltage); 4 ventilation in the cabin is not sufficient; 5 the radiators of the transformers have lost part of their efficiency due to an accumulation of extraneous bodies deposited on them (practically they have reduced their dissipating power). Nevertheless, transformers well-built in accordance with existing international regulations can, in determined conditions, put up with periods of over-loading, with greater than rated operating power. This type of operation is codified in the Load guide edited by the Regulatory Authorities (e.g. IEC standard). 8.1 Input voltage The input voltage values indicated on the registration plate must be strictly respected. Three conditions may exist: 1a. the input voltage is at a value greater than the one required by the position of the switch; the machine is over-supplied; consequently, it is noisier and the temperature of the transformer tends to increase. 2a. the input voltage is at a value less than the one required by the position of the switch; the machine is under-supplied; and the output voltage of the transformer (side BT) will have a value lower than the rated one (practically, it is equivalent to lowering the power of the machine). 3a. the input voltage is at a value almost equal to the one required by the position of the switch: the machine is in the ideal conditions for operation. 8.2 Conserving the oil level If the oil level becomes low (this can be seen from the special indicator located on the conservator), it is necessary to bring it back up to normal by using exclusively oil with chemical characteristics equal to that used for the first filling. The main reason for a possible lowering of the oil level can be found in the leakage of liquid through the closing seals; therefore, it must be verified if, in some part of the transformer, there are leaks; try to eliminate them by tightening the closing screws and possibly substituting the seals if they have deteriorated.

23 Rev.02 - Pag. 23/ Outside cleaning of radiators / corrugated walls One probable cause of overheating in the transformer may be the reduction of the dissipating power of the radiators / corrugated walls due to extraneous bodies deposited on them. It is essential, therefore, to remove these extraneous bodies (dust, sand, etc.) which are the cause of a reduction in efficiency. 8.4 Repainting If the transformers are intended for work in unfavourable environmental conditions (very humid climates), after a certain period of time traces of rust may show up on the steel parts of the machine, which if neglected could cause serious water-proofing problems, especially for the radiators. It is advisable to proceed, therefore, to a careful cleaning of possible traces of rust by washing and repainting of the same shade as the original one. 8.5 Oil treatment If from the oil sample taken it results that the oil has lost its original limpidity in relevant quantities, filtrage may be appropriate. It is suggested, however, to proceed to filtrage only after having repeated the stiffness test several times, in order to ensure that the possible low dielectric stiffness does not depend on errors of measurement. If the filtrage is done with equipment capable of submitting the oil also to a hot-oil vacuum operation, the operation will be much more efficient and will restore the best operating conditions to the transformer. The sample will be taken from the gate-valve located close to the tank bottom. 8.6 General reconditioning After long periods of operation with full load and therefore at a high temperature, the oil is subject to oxidation. The primary consequence of this phenomenon is the formation of gummy deposits and sediments on the windings, in addition to the formation of water droplets. For this reason we recommend carrying out a reconditioning at least every 5 years or even a shorter period, extracting the machine from its casing. Where sediment and deposits are found, it is necessary to proceed to a complete washing of the extractable parts with mineral oil. The oil must be submitted to analysis and, should an appreciable physical or chemical deterioration result, it is necessary to proceed to a complete replacement. In the opposite case, careful filtrage may be sufficient. The reconditioning operation must be carried out in a specialized workshop, because only with adequate equipment and trained personnel it is possible to check the position and the conditions of the windings, the good conservation of the switch contacts, the state of the insulations and the replacement of the seals. In any case, replacement of the oil must be preceded by the total drying-out of the active parts of the machine (core, windings). 8.7 Extraordinary maintenance In the case in which the transformer operates in discontinuous service, before of the energization, especially after a long stop, it will be necessary to perform all the checks before commissioning listed in Chapter 7. In the event that the transformer is subjected to exceptional events such as short circuits, overvoltage, flooding, or other events, before of the energization we recommend to call our customer service for check control. For more accurate and complete checks, please contact our Assistance Service Phone (+39) Fax (+39) info@tesar.eu