With population growth, worldwide industrialization, and the emergence of new technology, expanding energy infrastructure and investing in sustainable power sources is becoming more critical than ever. Many utilities are looking to windpower as a viable alternative to traditional fossil fuel based power plants, as it is a safe and constant source of power that has minimal environmental impact. Since 1996, nearly 280 GWs of wind power have been added to our electrical grid, making wind energy a significant contributor to our power supply. Although the deployment of wind energy has many benefits to our economic and environmental well-being, the proliferation of wind farms can have an adverse impact on grid performance and stability.
Wind energy is intermittent and non-dispatchable by nature, making it difficult to align energy supply with demand. Often times, there are periods when wind speeds are high but energy load is low, or wind speeds are low and energy demand is high. This dynamic can result in grid instability and power quality issues such as voltage unbalance and fluctuations, power losses on the transmission line, poor power factor, and ultimately the wind farm tripping offline. As a result, grid operators and regulators have implemented a set of rules, which vary by electrical network, to minimize the impact of wind farms on the grid, often referred to as grid codes. At the point of common coupling (PCC) the wind farm is required to comply with a set of specifications that address reactive power control, active power control, power quality, tolerance to system faults, response to frequency changes, and requirements to “ride through” voltage fluctuations.
Wind farm developers and owners have implemented a variety of strategies to meet these requirements and increase plant performance. One common method to increase grid code compliance is the deployment of full power converter wind turbines. By using these advanced wind turbine generators (WTG) with variable speed synchronous generators, the wind turbine generator rotates at synchronous speed and therefore has the same behavior as a conventional generator. Through the use of power electronics, the WTG interacts with the grid letting the plant stay online despite voltage fluctuations.
There are some limitations to this approach. In order for the WTG to fully ride through low voltage dips, the components need to be designed to mechanically withstand the electromagnetic forces that are produced during grid events. Risks of mechanical failure per wind turbine increase substantially the more the wind turbine needs to re-synchronize itself back into the grid. The components for these advanced wind turbines are designed to endure high mechanical stresses, resulting in a larger, heavier and more expensive nacelle. Increasing weight and size of the nacelle can negatively impact the total cost of energy for the wind farm as additional resources are needed to install the turbine and provide structural support. This cost adder is magnified by the number of wind turbines used in any given wind farm. The ability for these advanced wind turbines to satisfy stringent grid codes is further limited the longer the distance there is between the turbine and wind farm collector system. Wind farms that are offshore or in remote locations often require extra transmission equipment to maintain reactive power support, regardless of the individual turbine technology.
Another approach to complying with grid codes requirements and ensuring maximum wind farm energy output is installing a static synchronous compensator (STATCOM) either at the wind farm collector grid or at the point of common coupling (PCC). By applying a STATCOM system at the PCC, the wind farm owner or operator is able to automatically control the amount of reactive power, addressing any voltage stability concerns. When the system voltage is low, the STATCOM injects reactive power on to the network helping to ‘boost’ or raise the connection bus voltage. Alternatively, when voltages levels are high, the STATCOM is able to absorb excess reactive power helping to suppress or lower system voltage. The STATCOM is able to provide this level of control quickly and respond automatically to events, as it detects and compensates for voltage fluctuations with unparalleled precision.
Another STATCOM advantage is overcoming long distances not just from the wind farm to the load centers, but within the wind farm itself. Wind farm collector grid layouts can become prohibitive in terms of power factor capability, serviceability, and maintenance, as many wind farms span tens to hundreds of miles in distance. In such instances, the inherent reactive power capability of wind turbines may be limited by the reactive losses of the generator step-up transformers (GSU), collector grid impedance, and plant power transformer. In the instance of using advanced wind turbines, additional compensation is needed simply to overcome the design of the collector grid. A STATCOM will be able to quickly respond to grid events (with a response time of one to two cycles), providing dynamic voltage control, regardless of the wind farm layout. Even on weak grids, the STATCOM device has the capability to control reactive power, limiting grid impedance, and ultimately enhancing the power output of the wind farm.
Article By Michelle Meyer, Senior Product Manager within Power Conversion at ABB Inc.
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