Transformer Applications in a Modern Wind Farm
Category: TransformersToday's modern utility wind farm consists of a collection of wind turbines distributed in an array that provides the greatest exposure to the local wind flows. Often the array is composed of a series of radial lines of turbines connected in parallel to feed a common “collector” bus. The number of transformers on each radial and the number of radials varies with the site terrain, available area, and individual switching and protection schemes. See Figure 1 for a typical wind farm array layout.
Depending upon the design, wind farms may use transformers for six unique functions.
Wind Turbine Step-Up Transformers
Each turbine is equipped with a transformer to step-up the turbine generator output voltage to the collector system voltage. This transformer also serves as a source for the turbine’s auxiliary power requirements when the turbine is off line or generating insufficient power. Proximity to the turbine is critical to limit the length of costly and inefficient high current, low voltage, cables. The typical distribution transformer is often ill-suited to the requirements of a wind farm. We have found that the mismatch between requirements and typical transformer design contributes to the present high rate of step-up transformer failures at wind farms.
Grounding Transformers
Under normal conditions, the common collector bus will be connected to the substation main transformer. Depending on the system configuration, grounding transformers may be required to provide a system ground as protection circuitry operates. Also, grounding transformers themselves provide a protective function by preventing a faulted phase from staying at ground potential and limiting overvoltage conditions on the un-faulted phases. Grounding transformer sizing is based on the ampere capacity of the radial collector circuit and the time required for de-energization of the turbine and collector circuits when a fault occurs.
Substation Main Transformers (Sometimes Known as Collector Transformer)
Voltage from the collector system is stepped-up to the sub-transmission or transmission level voltages by the substation main transformer. This intermediate step helps limit the transformation ratio required by the individual wind turbine step up transformers. Some installations might use more than one substation main transformer to limit its size or take advantage of site logistics, depending upon station design philosophy.
Transmission Auto-Transformers
These transformers provide the flexibility to interconnect to multiple transmission lines with dissimilar voltages.
Station Service and Auxiliary Power Transformers
Because wind farms are usually located far from developed urban or residential lands, they must be self-sufficient and not require power from external sources. This is particularly important for off-shore installations. Thus, power for such local functions as lighting, heating, switching, tripping, relaying, metering, and communications is provided by the station service transformer connected to the main transmission lines. It is possible to provide some of these functions from a grounding transformer, but this approach is not popular with designers and would not provide continuing power in the case of a fault.
Voltage Conditioning Equipment
Dynamic-VAR compensation equipment can be used to limit damage to the turbine equipment due to under-voltage conditions and to provide system stability. This equipment requires integral transformers of sizes as high as 8 MVA. The transformers are integral to this equipment, so they are not normally ordered separately or specified by the system designers, as the case is for the other type of transformers in the wind farm environment. |
