In recent years, researchers and scientists have focused on the economical utilization of wind energy on a large scale. Improvement in design of turbines and increasing use of power electronics converters for VAR compensation and/or frequency conversion has given a boost to this industry. In the area of wind and other power generation systems where the input resource power varies considerably, variable-speed generation (VSG) is more attractive than fixed speed systems. In these systems, a maximum power point tracker adjusts a system quantity (such as the rotational speed in the case of wind turbines) to maximize turbine power output. The maximum power point tracking controller and associated power electronics converters set the operating point of the generator to capture the maximum power of fluctuating wind energy.
To maximize their return on investment, developers are aggressively pursuing building larger and larger wind plants. In areas where wind resources support such development, wind plants with total power ratings in excess of 200 MW are becoming the norm. Larger wind plants are designed with a mixture of overhead and underground collector circuits having feeder circuits with individual feeder length exceeding 10 miles in some cases. The plant may also include a collector/interconnect substation, and in some cases a transmission line from the collector substation to the interconnect substation, as well as a separate interconnect substation. The distance from the collector substation to the interconnect substation ranges from several miles to tens of miles, depending on the routing of existing transmission lines and the point of interconnect. The majority of installed wind plants in the US have 34.5 kV collector circuits since in North America most of the medium-voltage infrastructure is based on 35-kV class equipment.
As the penetration and size of wind plants increase, their impact on transmission grids requires a more thorough analysis and understanding. One demanding issue with wind farms is the power quality and stability of the grid. With restructuring of the electric power industry, rules and regulations tend to impact the wind industry through Federal Energy Regulatory Commission (FERC) actions. FERC Orders 661 and 661A address the need for wind plants to support power system voltage by requiring new wind generators to have the capability of fault-ride through and also to control their reactive power within the 0.95 leading to 0.95 lagging range. In addition to the continuing trend to variable speed operation, wind farms can be operated as peak power plants (onshore and offshore). This calls for better control and more enhanced power electronics converter solutions.
For turbine ratings up to around 2 MW, a converter-less structure has resulted in a simple, effective system. High performance turbines have been built with variable speed systems, either using doubly-fed induction generators with a small converter or gearless systems with full-scale converters. Low-voltage technology has been applied successfully at all power levels. At converter power levels in excess of around 500 kVA, a parallel connection of converter modules is typically used to fulfill the technical requirements. However, low voltage wind generators are associated with high connection costs since the effective current that loads the connections between a nacelle (which is a structure present at a top of a wind tower, and which can be 100's of feet in the air) and tower bottom is very high. In a 690 V system a phase current of 1700 A is reached at about 2 MW. This requires a parallel connection of multiple cables per phase and a substantial voltage drop. This disadvantage can be mitigated by placing the electrical conversion system, including the transformer into the nacelle.
However, the structure to support the nacelle weight introduces extremely higher costs. Besides, due to the necessity to connect low-voltage converter modules in parallel, the space needed by the converters in the nacelle increases roughly in proportion to its power. The nacelle dimensions and weight increase considerably and complicate the mechanical stability and the logistics during turbine erection. Thus a need exists for improved power conversion in power generation systems.
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