Filling the transformer with oil

With the term “filling” the transformer we mean both filling newly installed equipment for the first time on the final site, and filling as a result of maintenance or internal inspections that have requested the equipment to be emptied.

In both cases, filling a transformer (especially those with a maximum voltage greater than 72.5 KV is an extremely delicate operation that requires the work of qualified experts, suitable operating procedures, suitably trained personnel and state of the art equipment.

Note that approximately 10-15% in terms of weight of the oil used for factory tests (before transporting the transformer to the operating site) remains inside the transformer (adsorbed by the solid insulation and deposited on the inner surfaces) even after it being emptied before transporting it to the final site. Given that often the oil used in factory tests is NOT the one purchased or accepted by the owner of the transformer, if this oil is contaminated there may be cases of cross contamination.This means that the new and compliant oil, used on the final site, might be contaminated by the oil previously used, having remained in the inner parts of the transformer.

Operating mode

Described below are the phases that make up the filling process of electric power equipment with a maximum voltage greater than 72.5 KV.

  1. Control analysis of the filling oil
  2. Preliminary treating the filling oil
    The filling oil is transferred into tanks and subjected to physical treatment (filtration, degassing and dehydration in a vaccuum). The purpose is to obtain an oil with parameters that conform to IEC 60296.
  3. Preparation of the transformer with a vacuum
    The accessories and components which can not resist the vacuum are isolated by closing the shut-off valves or applying blind flanges. Then a sensor is applied to measure the vacuum and a vertical pipe to monitor the oil level inside the tank during filling.
  4. Application of vacuum
    The vacuum is applied until a residual pressure of less than 2 mbar (200 Pa) is obtained and maintained for the duration of between 12 and 36 hours according to the maximum voltage of the machine to be filled. During this draining step the tightness of the seals is checked
  5. Partial filling with a vacuum
    The oil is inserted into the caisson through the bottom gate until completely covering the reels, while maintaining the suction of the vacuum in the upper part. Upon reaching a level of about 20 cm with respect to the lid, the vacuum pipe is shut off and the filling is continued, filling up to the Buchholz relay.In cases where the connection is to the tank, it is completely filled (see next step).
  6. Final filling and checking of levels and vents
  7. Filling accessories
    The separated accessories (variators, radiators, AT passers tanks) that are resistant to the vacuum are filled simultaneously with the main caisson, the ones that are not resistant to the vacuum are filled after the transformer.
  8. Final oil treatment
    The oil is subjected to a final filtration treatment, degassing and dehydration with a vacuum by continuously circulating it in a closed circuit. The oil treatment should ensure that all regulatory requirements are met  (see table 3 IEC 60422 ed. 4 2013), as well as technical and contractual ones and therefore counter any cross contamination.
  9. Final analysis of the oil for quality control

Saturation time before applying energy
Once the transformer has been completely filled it is advised to wait for the cellulosic insulation to become saturated before applying energy to the transformer. Typically, manufacturers specify a saturation time between 12 and 96 hours on the basis of the equipment’s maximum voltage.

Oil treatment: limit of the parameters according to the standards

Table 3 of IEC 60422 ed. 4 2013


Operational Safety

– Automatic interruption of the oil circulation in the event of an alarm even if the system is unsupervised
– Full electrical supply blocks in case of serious alarms (temperature,leakages or power surges)
– Automatic reports to the responsible technician via a GSM call, or if there is no GSM network, the call is made by means of the client’s centralized network.

Process Control

– Oil temperature in the high and low part of the transformer,
– Residual pressure in the degassing chamber,
– Hourly flow rate of oil circulation,
– Oil temperature at the main treatment stages (system entrance, heater, degassing chamber, system output),
– The maximum pressure and clogged filters
– At the same time, water extraction and condensation and the oil discharge voltage (IEC 60156) are monitored daily

The treatment units' small dimensions

mean they are operational even under extreme logistical constraints (e.g . in bunkers, and mines)

Highly qualified personnel

(training and continuous assessment) in possession of specific qualifications (EN 50110 electrical hazard)

Numerous international case histories

that highlight the strict compliance with standard requirements and contractual specificationsi

Diagnosis of functional or critical environmental issues

to eliminate (potential) cross-contamination derived from filling for factory tests

Accredited certification of the individual analytical results

with key risk measurements (ISO IEC 17025); internationally accredited laboratory (Accredia-ILAC) with certificates signed by registered chemists


Sea Marconi is committed to obtaining the following results, measured by the sample taken at the end of the final treatment.

Properties Test Method Value
Water content (mg/kg) IEC 60184 = 10,00*
Breakdown Voltage (kV) IEC 60156 > 70
Total Dissolved Gases (%v) IEC 60567 < 2 %