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Renewables-Windfarm

Wind Energy Generator Step up (GSU) Transformers

With the advent of renewable energy, the number of wind and solar farms has been on the increase, and economical benefits were originally found in the use of padmount transformers to step-up the voltage as produced by the turbine generator. The rate of failure 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 that we have adopted to improve robustness.

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 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 – this class of failures due to general and localized heating are 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 are a lot higher than expected in a distribution transformer application.
    • Increased risks of hot spots due to gassing.

The occurrence of a factor listed above may not directly result in the failure of the transformer. However, there is considerable interrelationship between the factors, and the occurrence of a factor could lead to the manifestation of another factor, and so on, till eventually one or more factors are substantive enough to result the failure of the transformer.

As a manufacturer of custom transformers, we have applied design considerations to cater for each of the issues faced by WTGSU application. Among these are:

  1. Core and coil designs have been modified in the following areas to mitigate the issues identified above:
    • Three-limbed stacked cores eliminates the issue of hydrogen gases that were an issue with the five-limb wound cores

                
Five-limb wound core                                        Three-limb stacked core

  • Core designs allows for an additional 5% continuous overvoltage over what is prescribed by ANSI standards
  • The use of disc winding for the HV coil offers increased mechanical strength required for WTG 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 to prevent the transfer of voltage spikes from one winding to the other.

 

  1. 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 allow for more effective fluid circulation
    • Gradient and top oil rise are limited to cater for overload conditions during design

These solutions in addition with Virginia-Georgia Transformer’s experience in design and manufacture of transformers make our WTGSU transformers more robust and suitable for the application. This effort can be complemented by the customer/ end user who could:

  1. Adequately size the transformer to meet design conditions of the wind turbine and site conditions.
  2. Implement a robust farm infrastructure which includes grounding transformers to minimize overvoltage in fault conditions.
  3. Close monitoring of transformer performance for early detection of issues which can be resolved before escalating to a dangerous failure. Monitoring should include but not limited to liquid level, temperatures and pressures, as well as periodic DGA of the oil.

CONCLUSION: We have been able to identify failure modes of padmount transformers when used in WTGSU application, and have used our experience in over 10,000 custom designs to develop design considerations to improve robustness of padmount transformers when used in WTGSU transformer application.

There are other economic considerations not covered in this article, so contact us to assist in developing a robust specification. Call us at 540-354-9892 to discuss your concerns.

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