Premature failures of padmount transformers in wind farms: a cause and cure analysis
By Balram Ramamurthy
A brief background: Renewable energy has received significant impetus, with the advent of wind and solar power as the prime drivers. The development of wind farms has been on the increase, and economical benefits were originally found in the use of padmount transformers to step-up the voltage produced by the turbine generator.
However, the rate of failures of these transformers in service has led to the investigation of the total cost of ownership in trying to balance the low cost of padmount transformers versus the cost of premature failures. This article looks at identified failure modes and mitigation, with certain newer developed designs that have been adopted addressing issues and greatly increasing the robustness.
Failures and the causes: The reliability, or lack thereof, of padmount transformers used as Wind Turbine Generator Step Up (WTGSU) transformers has been well documented. Broadly, this failure is exhibited in one of three ways:
1. HV winding failures - this is a class of failures primarily caused by
- Over-voltage due to fault conditions and load rejection, as well as transient over-voltages from switching operations that can lead to winding failure if not mitigated.
- Increased mechanical and electrical stress due to the loading cycles of WTGSU.
- Increased risk of partial discharge due to gassing (see 2 below).
2. Gassing - this is a class of failures due to elevated gas generation of
- Hydrogen gas - as a result of five-legged wound core construction that was predominantly used in padmount transformer designs.
- Combustible (hydrocarbon) gases - due to the presence of oil-immersed fuses and load break switch and localized heating (see 3 below).
3. Heating failures - these failures are due to general and localized heating attributable to
- Overloading due to sizing variations of transformers to wind turbines, especially during periods of high wind speeds.
- Harmonics seen by the transformer winding during turbine start-up and operation, resulting in additional eddy and stray losses that are much higher than expected in a distribution transformer application.
- Increased risks of hot spots due to gassing.
Newer custom-engineered designs that have taken the renewable energy characteristic into consideration have very meticulously applied design considerations to cater to each of the issues faced by WTGSU application.
Key design and construction elements:
1. Core and coil designs have been modified to mitigate the issues identified above:
- Three-limbed stacked cores eliminate the potential gassing issue of the five-limb wound cores.
- Core designs allow for an additional five percent continuous overvoltage over what is prescribed by ANSI standards.
- The use of disc winding for the HV coil offers increased mechanical strength required for wind turbine generator application.
- Use of high temperature insulation in the winding hotspot region and other actions are also taken to limit hotspot temperature.
- Use of electrostatic shield to complement the harmonics filtering by the wind turbine inverter and prevent the transfer of voltage spikes from one winding to the other.
2. Improved cooling to mitigate overheating and resultant combustible gas generation:
- Use of liberal K-factor to account for the effect of harmonic losses
- Winding construction with winding sticks and key spacers allows for more effective fluid circulation.
- Gradient and top oil rise are limited to cater for overload conditions during design.
These solutions have been incorporated in custom designs that only a few manufacturers have considered and adopted. This custom design needs to be complemented by the end user with the following:
- Adequately size the transformer to meet design conditions of the wind turbine and site conditions.
- Implement a robust wind farm infrastructure that includes grounding transformers to minimize overvoltage in fault conditions.
- Closely monitor of transformer performance for early detection of issues, which can be resolved before escalating to a dangerous failure. Monitoring should include, but not be limited to, liquid level, temperatures, and pressures as well as periodic DGA of the oil.
Conclusion: Failure modes of padmount transformers when used in WTGSU application are different from standard distribution transformers due to the unique nature of the application that it addresses, and they have to possess special design considerations to improve robustness when used in wind farms.
Balram Ramamurthy heads renewable energy initiatives at Virginia & Georgia Transformer Corp. USA (www.vatransformer.com). They have designed and supplied several such custom-built padmount transformers for wind farms in operation in several parts of the U.S. and Canada. Balram can be reached at Balram_Ramamurthy@vatransformer.com