The “PCBs in oil and transformer” criticality is caused by the implementation of improper management practices of transformers and insulating liquids. This condition is generally classified as “crossed contamination”. The improper practices indicated above have an impact on the various phases of the life cycle of electrical equipment with insulating liquids.
Causes and Steps of the life cycle
| Causes of the criticality “PCB in oil and transformer” | When can occur (steps of life cycle) |
| Incomplete inventory of contaminated equipment, outdated inventory or implemented without correct methodologies |
Inventory |
| Lack of or improper formalization of new oil purchase requirements (see. IEC 60296) |
Requirements and procurement |
| Inadequate quality control for individual lots or individual oil supplies |
Oil acceptance |
| Use of analytical procedures that are not suitable for the verification of the PCBs contamination |
Oil acceptance, factory test, commissioning and pre-energization, operation, ageing, post mortem |
| Topping, impregnation or fills with oil and materials contaminated by PCBs | Construction of transformer, factory test, commissioning and pre-energization, operation, ageing |
| Oil treatments with equipment contaminated by PCBs | factory test, commissioning and pre-energization, operation, ageing |
| Maintenance with the use of tanks and drums contaminated by PCBs | factory test, commissioning and pre-energization, operation, ageing |
| Release of oil from impregnated materials contaminated by PCBs | Factory test, commissioning and pre-energization, operation, ageing, post mortem |
| Recycling of oil and other materials contaminated by PCBs | Post mortem |
Real example of contamination
Foreword – According to the standard IEC 60296, the content of PCBs in a new oil must be “non-detectable”, analysed in accordance with IEC 61619, where the limit of the method is 2 mg/kg of total PCBs (intended as the sum of the congeners present in the sample analysed). The same type of threshold concentration is prescribed prior to the energising (see IEC 60422 ed. 4 2013).
Assuming we have a transformer with a total weight of 88,000 lbs – 40,000kg of which 22,000 lbs – 10,000 kg of oil and 66,000 lbs – 30,000 kg of remaining parts, during the factory test phase or during the installation or even during a maintenance top-up operation, are just enough 1.75 on – 50 gr of PCBs (equivalent to a cup of coffee) to have a contamination of 5 mg/kg of PCBs, that is above the “non- detectable” threshold prescribed by the standards.
