The U.S. Department of Energy's Wind Program and the National Renewable Energy Laboratory (NREL) recently published a study conducted in collaboration with the Electric Power Research Institute (EPRI) and the University of Colorado (CU). Researchers examined how the contribution of wind power providing active power controls (APC) could benefit the total power system economics, increase revenue streams, and improve the reliability and security of the nation's power system, all while having negligible impacts on the turbine and its components. Various forms of APC can help stabilize the electric grid as generating sources increase or decrease power output to meet the constantly fluctuating needs of the load demand and avoid events that can cause brownouts and power failures. The NREL study captures a holistic view of how each of these impacts and benefits can be realized.
The study focused on three forms of control: synthetic inertial control, primary frequency control, and automatic generation control. Inertial control is the immediate response to a power disturbance based on a frequency change. This response is currently given by synchronous (spinning) machines that immediately provide energy to the grid by slowing down their rotation, thereby converting rotational energy from the machine to electrical energy in the grid. Primary frequency control follows the inertial response and increases the output of generators to balance generation and load. This response—also called primary control, frequency-responsive reserve, and governor droop—is typically provided by conventional generators with governor controls that adjust output based on the frequency deviation and the generator's governor droop characteristic. Automatic generation control, the slowest of the three, is used during both emergency events and normal conditions. Automatic generation control will return the frequency back to its nominal set point (which, in North America, is 60 hertz) after a large disturbance. It also reduces the area control error continuously to ensure that frequency and interchange energy schedules between regions are kept to set points.
These forms of control all provide active power output adjustments to help support the generation and load balance at different time scales. The NREL study analyzed time frames ranging from milliseconds to minutes to the lifetime of wind turbines and scales ranging from components of turbines to large wind plants to entire synchronous interconnections. Researchers analyzed how wind plants could earn additional revenue from providing APC when power markets are designed a certain way. Simulations were performed to see how system reliability was impacted from high penetrations of wind power without APC, and how providing APC greatly improved those conditions. The study also included a number of simulations and actual field tests using turbines at the National Wind Technology Center at NREL to understand new designs and what, if any, impacts aggressive controls may have on the turbines or their components.
Although many of the control strategies have been proven technically feasible and are used in many regions of the world, currently only a limited number of wind turbines in the United State provide active power controls. This may be due to differences in perspective among various stakeholders—system operators, manufacturers, and wind plant owners. The holistic research approach used in this study seeks to close these gaps and gain recognition of the potential benefits of active power controls from wind.
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