Mexico’s power system operator, the Comisión Federal de Electricidad (CFE) relies on more than 470 substations with a total of circa 2700 power transformers from 115 kV to 400 kV. Most of these are equipped with oil impregnated paper bushings (OIP) while there is lesser use of resin bonded paper (RBP) bushings as well as resin impregnated paper (RIP) bushings. Operating experience has demonstrated that more than a quarter of all transformer failures (27.7%) are due to failed bushings, about half the time with catastrophic results.
This past INMR article, contributed by LAPEM and the Comisíon Federal de Electricidad (CFE), discussed the utility’s experience mainly with OIP HV bushings taken out of service for the purpose of diagnostics, maintenance or failure analysis.
Due to growing commercial and industrial activity in Mexico, electricity demand is increasing by about 5% per year. The CFE operates a grid covering 9 different Regions. This network is comprised of more than 57,000 km of overhead transmission lines and some 470 substations with nominal voltages of 115 kV, 161 kV, 230 kV and 400 kV and where a total of 2760 power transformers equipped with 6500 HV bushings are in service.
Analysis of power transformer accidents between 2000 and 2014 has demonstrated that bushings are often the ‘weakest link in the chain’ and also the second leading cause of such failures (see Fig.2). Indeed, historical data shows that 27.7% of all power transformer failures in Mexico were due directly to problems with the bushings.
Moreover, statistics reveal that 400 kV bushings account for the highest proportion of such failures, amounting to 50% over the 15 year period studied. It is also important to note that, since the CFE’s main transmission grid is spread over Mexico at 400 kV, recovery time becomes an important issue in maintaining a service level with a high standard of quality and reliability.
Another important issue is that of all power transformer failures due to issues with bushings, 53% had the catastrophic result of the transformers burning. Moreover, analysis shows that failures of power transformers from 2000 to 2014 due to bushing problems carried a high cost because a total of 114 MVA was disconnected from the grid, even without consideration of the downtime needed for servicing, the cost of repair or replacement of the bushings or the cost of cranes, materials and manpower needed to restore service to the affected transformer.
Past studies within the CFE have shown that polymeric-housed bushings are an attractive option for polluted and/or seismically active service areas because of their inherent surface hydrophobicity, light weight, easier handling and superior mechanical stability with more resistance to bending motion. There is also no risk from explosive failures that, in the case of ceramic-housed bushings, could see high velocity shards of porcelain ejected onto adjacent equipment.
However, notwithstanding these benefits, there have been some reported problems related to differences in formulation of the silicone rubber material, different manufacturing processes and different laboratory test methodologies to evaluate these parameters.
Technical Reference for Laboratory Testing
The CFE’s technical specification for power transformers allows the manufacturer to select suitable bushings based on technical requirements and design. At the same time, this technical specification is based on IEC 60137 and lists the main requirements that must be fulfilled for HV bushings installed on power transformers. This standard indicates the bushing reference values for testing performed on C1 at laboratory facilities, i.e.:
• Tan δ of 0.007
• Max Δ tan δ 0.001 for OIP (at Um & 1.05Um/√3)
• Partial discharges of 10 pC @ Um and 5 pC @ 1.05Um/√3
The experiences show us that for Tan δ, most of the manufacturers have lower values.
Tan δ ≤ 0.005 for OIP.
In the diagnostic process the above values are used only as reference. A technique that combines several tests is then applied, such as measuring C1, tan δ (before and after), power frequency (80 to 85%), testing withstand voltage and measuring partial discharges. There are no tests specified for the insulating oil.