Active Power Control of Wind Power Plants for Grid Integration
- 91 Downloads
Increasing penetrations of intermittent renewable energy sources, such as wind, on the utility grid have led to concerns over the reliability of the grid. One approach for improving grid reliability with increasing wind penetrations is to actively control the real power output of wind turbines and wind power plants. Providing a full range of responses requires derating wind power plants so that there is headroom to both increase and decrease power to provide grid balancing services and stabilizing responses. Results thus far indicate that wind turbines may be able to provide primary frequency control and frequency regulation services more rapidly than conventional power plants.
KeywordsActive power control Automatic generation control Frequency regulation Grid balancing Grid integration Primary frequency control Wind energy
Wind penetration levels across the world have increased dramatically, with installed capacity growing at a mean annual rate of 17% over the last decade (Broehl and Asmus 2018). Some nations in Western Europe, particularly Denmark, Ireland, Portugal, Spain, the United Kingdom, and Germany, have seen wind provide more than 18% of their annual electrical energy needs (Wiser and Bolinger 2018). To maintain grid frequency at its nominal value, the electrical generation must equal the electrical load on the grid. This balancing has historically been left up to conventional utilities with synchronous generators, which can vary their active power output by simply varying their fuel input. Grid frequency control is performed across a number of regimes and time scales, with both manual and automatic control commands. Further details can be found in Diaz-Gonzalez et al. (2014) and Ela et al. (2011).
Wind turbines and wind power plants are recognized as having the potential to meet demanding grid stabilizing requirements set by transmission system operators (Ela et al. 2011; Aho et al. 2013a, b; Diaz-Gonzalez et al. 2014; Ela et al. 2014; Fleming et al. 2016). Recent grid code requirements have spurred the development of wind turbine active power control (APC) systems (Diaz-Gonzalez et al. 2014), in some cases mandating wind turbines to participate in grid frequency regulation and provide stabilizing responses to changes in grid frequency. The ability of wind turbines to provide APC services also allows them to follow forecast-based power production schedules.
For a wind turbine to fully participate in grid frequency control, it must be derated (to Pderated) with respect to the maximum power (Pmax) that can be generated given the available wind, allowing for both increases and decreases in power, if necessary. Wind turbines can derate their power output by pitching their blades to shed aerodynamic power or reducing their generator torque in order to operate at higher-than-optimal rotor speeds. Wind turbines can then respond at different time scales to provide more or less power through pitch control (which can provide a power response within seconds) and generator torque control (which can provide a power response within milliseconds) (Aho et al. 2016).
Wind Turbine Inertial and Primary Frequency Control
Inertial and primary frequency control is generally considered to be the first 5–30 s after a frequency event occurs. In this regime, the governors of capable generators actuate, allowing for a temporary increase or decrease in the utilities’ power outputs. The primary frequency control (PFC) response provided by conventional synchronous generators can be characterized by a droop curve, which relates fluctuations in grid frequency to a change in power from the utility. For example, a 3% droop curve means that a 3% change in grid frequency yields a 100% change in commanded power.
Although modern wind turbines do not inherently provide inertial or primary frequency control responses because their power electronics impart a buffer between their generators and the grid, such responses can be produced through careful design of the wind turbine control systems. While the physical properties of a conventional synchronous generator yield a static droop characteristic, a wind turbine can be controlled to provide a primary frequency response via either a static or time-varying droop curve. A time-varying droop curve can be designed to be more aggressive when the magnitude of the rate of change of frequency of the grid is larger.
Stability issues arising from the altered control algorithms must be analyzed (Buckspan et al. 2013; Wilches-Bernal et al. 2016). The trade-offs between aggressive primary frequency control and resulting structural loads also need to be evaluated carefully. Initial research shows that potential grid support can be achieved while not causing increases in average structural loading (Fleming et al. 2016). Further research is needed to more carefully assess how changes in structural loading affect fatigue damage and operations and maintenance costs.
Wind Turbine Automatic Generation Control
Secondary frequency control, also known as automatic generation control (AGC), occurs on a slower time scale than PFC. AGC commands can be generated from highly damped proportional integral (PI) controllers or logic controllers to regulate grid frequency and are used to control the power output of participating power plants. In many geographical regions, frequency regulation services are compensated through a competitive market, where power plants that provide faster and more accurate AGC command tracking are preferred.
Active Power Control of Wind Power Plants
Summary and Future Directions
Ultimately, active power control of wind turbines and wind power plants should be combined with both demand-side management and storage to provide a more comprehensive solution that enables balancing electrical generation and electrical load with large penetrations of wind energy on the grid. Demand-side management (Callaway and Hiskens 2011; Palensky and Dietrich 2011) aims to alter the demand in order to mitigate peak electrical loads and hence to maintain sufficient control authority among generating units. As more effective and economical energy storage solutions (Pickard and Abbott 2012; Castillo and Gayme 2014) at the power plant scale are developed, wind (and solar) energy can then be stored when wind (and solar) energy availability is not well matched with electrical demand. Advances in wind forecasting (Pinson 2013; Okumus and Dinler 2016) will also improve wind power forecasts to facilitate more accurate scheduling of larger amounts of wind power on the grid. Finally, considering active power control in conjunction with reactive power and voltage control of wind power plants is also important for the stability and synchronization of the electrical power grid (Jain et al. 2015; Karthikeya and Schutt 2014).
A comprehensive report on active power control that covers topics ranging from control design to power system engineering to economics can be found in Ela et al. (2014) and the references therein.
- Aho J, Buckspan A, Pao L, Fleming P (2013a) An active power control system for wind turbines capable of primary and secondary frequency control for supporting grid reliability. In: Proceedings of the AIAA aerospace sciences meeting, Grapevine, Jan 2013Google Scholar
- Aho J, Buckspan A, Dunne F, Pao LY (2013b) Controlling wind energy for utility grid reliability. ASME Dyn Syst Control Mag 1(3):4–12Google Scholar
- Aho J, Fleming P, Pao LY (2016) Active power control of wind turbines for ancillary services: a comparison of pitch and torque control methodologies. In: Proceedings of the American control conference, Boston, July 2016, pp 1407–1412Google Scholar
- Broehl J, Asmus P (2018) World wind energy market update 2018. Navigant Research, Sep 2018Google Scholar
- Buckspan A, Pao L, Aho J, Fleming P (2013) Stability analysis of a wind turbine active power control system. In: Proceedings of the American control conference, Washington, DC, June 2013, pp 1420–1425Google Scholar
- Ela E, Milligan M, Kirby B (2011) Operating reserves and variable generation. Technical report, National Renewable Energy Laboratory, NREL/TP-5500-51928Google Scholar
- Ela E, Gevorgian V, Fleming P, Zhang YC, Singh M, Muljadi E, Scholbrock A, Aho J, Buckspan A, Pao L, Singhvi V, Tuohy A, Pourbeik P, Brooks D, Bhatt N (2014) Active power controls from wind power: bridging the gaps. Technical report, National Renewable Energy Laboratory, NREL/TP-5D00-60574, Jan 2014Google Scholar
- Pickard WF, Abbott D (eds) (2012) The intermittency challenge: massive energy storage in a sustainable future. Proc IEEE 100(2):317–321. Special issueGoogle Scholar
- Wiser R, Bolinger M (2018) 2017 Wind technologies market report. Lawrence Berkeley National Laboratory Report, Aug 2018Google Scholar