Electrical simulations model large wind farms

Tony Lennon/Industry Manager – Industrial Automation and Machinery/Mathworks/www.mathworks.com

Wind-farm developers must make accurate calculations about a farm’s capability to maintain its electrical output under scenarios that include wind availability, turbine conditions, and transmission-system interaction at the grid point of connection. Similar calculations are useful for a better understanding of the farm’s specified performance compared to its actual power output. Some studies use simulations that assume average wind values and turbine power curves but, these basic calculations do not reveal the impact of the wind turbines’ power electronics on the quality of electricity, the capability of the farm to stay connected to the grid, or the effect of the transmission system on the wind farm and individual turbines. A realistic understanding of a wind farm’s electrical capability requires a simulation using higher fidelity models that capture electrical and mechanical dynamics of individual turbines.

The Simulink model comes from a 3-phase electrical simulation for a 140-turbine wind farm. The chart graphs the active power output of the first 10 turbines. Each turbine is driven by a unique wind profile.

Several situations in the transmission system can impact the wind farm. Regard-less of voltage and frequency fluctuations in the transmission system, a farm is generally required to stay connected to the grid during transient conditions. Low voltage ride through (LVRT) capability is common in today’s modern turbines and often a requirement of the system operator in new wind-farm installations. The grid operator may request that the farm provide supplemental reactive power during a transmission system transient for unspecified periods. While commands to the wind farm are delivered through its SCADA system, each wind turbine must properly respond to the wind driving it. Simulations incorporating turbines that exhibit electromagnetic dynamics help wind-farm designers and operators better understand the farm’s ability to react and compensate for grid faults and transients.

Other effects impact the capacity and operations of a wind farm. As they expand, wind farms may incorporate different turbine types with varying power outputs. Over time, a turbine’s power output degrades and fluctuates as major components age, making a single turbine a source of electrical disturbance from farm to grid. And then the grid can produce subsynchronous resonance that will affect individual turbines. This phenomenon can damage turbine drive trains, mainly double-fed induction generators, which do not use a full power converter.

Simulation is the most practical way to model the performance of a wind farm and test its ability to respond to grid transients, system operator requests, and other phenomenon associated with wind turbines. The challenge is to model the wind farm at a level of detail that balances fidelity of the entire system with speed of simulation. A useful simulation will also contain models of the farm’s supervisory control logic, electromagnetic models of the turbine power converters and generators, grid compensation, grid loads, and wind profiles. Electromechanical effects in the turbine models, such as blade pitch control, can be simulated as variable efficiency factors converting wind speed into mechanical rotation.

Today’s simulation software ranges in capabilities, some focusing strictly on electrical models, while others offer multidomain models that include controls, electrical, mechanical, thermal, and other domains. Almost all simulation software offers some kind of scripting language that lets users develop custom models. Another software feature useful to farm operators is the capability to tune model parameters by directly comparing simulations to operational data. Doing this increases the accuracy of a wind-farm model, letting operators confidently use the results of the simulations over a range of conditions. Software using ordinary differential equation solvers is more appropriate to simulations involving electrical transients because the software offers a tradeoff between model fidelity and speed. To increase simulation speed, the power electronics of converters in the turbine may be simulated using average value models instead of full switching models while maintaining full electromagnetic transient effects. WPE