December 17, 2013
Traditionally, one of the primary driving characteristics of wind turbine generator (wtg) padmount transformer selection has been low initial capital investment. This has led to the popularity of choosing an often less expensive utility style distribution-type transformer over one with special characteristics. Over the last few years, the wind industry has grown to realize that this standard "off-the-shelf" wtg padmount transformer is not built to withstand the rigorous conditions of the industry. These have been hard lessons learned— resulting in many failures and outages. Thankfully, this trend to use cheaper "off-the-shelf" wtg padmount transformers is expected to reverse going forward with many Owner-Operators opting to select transformers built with added options to perform more reliably under the wind-specific operating conditions. Additionally, this is leading to discussion around creating a wind-specific joint IEC (International Electrotechnical Commission)/IEEE standard to help address some of these concerns.
Some of the electrical characteristics of wind turbines and the variability of their generation can have detrimental effects on the wtg padmount transformer and need to be considered when writing a transformer specification.
They are as follows:
- Typical wind plant collection system voltage and KVA ratings are at the extreme end of the distribution class IEEE Standards.
- The wtg padmount transformer is subjected to thermal cycling that is more severe than in a typical distribution transformer as the output of the turbine is constantly changing with the wind.
- The majority of wind turbines are controlled through solid-state power electronics to meet power factor and voltage 'ride-through' requirements at the terminals and the point the wind plant interconnects to the greater electric grid.
- These power electronics introduce higher order frequency components to the output waveform that can cause various issues with the transformer. The interaction of these high frequency components and the wind plant collection system equipment are not well understood under all operating characteristics.
- Power quality issues can be introduced both by the aforementioned solid-state components as well as lightning events and interactions with other wind turbine generators as they switch on- and off-line. Given the fact that wind plants are a collection of many generators, this introduces a higher likelihood of these events.
- The transformer is subjected to voltages as high as 10 percent over their nominal rating, which is beyond the IEEE Standard for distribution class transformers.
The conditions outlined above place the wtg padmount transformer under considerable stress.
One of the more cost-effective additions to a transformer specification is to increase the BIL (Basic Insulation Level) to 200kV from the IEEE Standard for 34.5kV class equipment of 150kV. In fact, many manufactures are now providing a 200kV BIL as their standard for wind plant transformer applications and may even take exception to a specification calling for the lower 150kV BIL rating. Until only recently, IEEE standard 600A bushings were only available up to 150kV BIL but at least one manufacturer is now offering a 200kV BIL due to the demand of the wind industry. The added insulation around the core and windings will give the transformer a greater margin of operation to better handle the stresses of the wind turbine generator and the nearby electrical system. Additionally, the interactions of the solid-state electronics and the grid conditions are not well understood and the added BIL provides some margin to account for this uncertainty.
An additional approach that works well to mitigate some of these power quality concerns is to install electrostatic shielding between the high and low voltage windings. This shielding will introduce an additional path to ground that effectively blocks the higher frequency components and electrical transients from passing through the low voltage winding to the high voltage winding and then further into the electrical system.
Any decision to increase capital investment must be weighed against the benefits and operational up–time, and choosing wtg padmount transformer options is no different. The approach of some owners is to select the "off-the-shelf" transformer and minimize outage time through additional effective isolation and procuring spare transformers. This is limited as an effective approach because of the potential of serial failures caused by the system level electrical issues outlined above. Increasing the GSU transformer BIL rating and adding electrostatic shielding between the high and low voltage winding and following recommended maintenance practices, are good improvements to transformer specifications. This can have a long-term operational benefit at any wind plant and help minimize outage time for minimal additional investment.
About the Author
A graduate of the University of Minnesota with a Bachelors in Electrical Engineering, Nathan Riggs has worked on various Solar and Wind projects throughout his time at Mortenson. Currently Nathan works as a Senior Electrical Engineer supporting the High Voltage Transmission Group within Mortenson's Renewable Energy groups supporting Wind and Utility construction projects through the United States and Canada.