Jacob Aho

A tutorial of wind turbine control for supporting grid frequency through active power control

As wind energy becomes a larger portion of the worlds energy portfolio and wind turbines become larger and more expensive, wind turbine control systems play an ever more prominent role in the design and deployment of wind turbines. The goals of traditional wind turbine control systems are maximizing energy production while protecting the wind turbine components. As more wind generation is installed there is an increasing interest in wind turbines actively controlling their power output in order to meet power setpoints and to participate in frequency regulation for the utility grid. This capability will be beneficial for grid operators, as it seems possible that wind turbines can be more effective at providing some of these services than traditional power plants. Furthermore, establishing an ancillary market for such regulation can be beneficial for wind plant owner/operators and manufacturers that provide such services. In this tutorial paper we provide an overview of basic wind turbine control systems and highlight recent industry trends and research in wind turbine control systems for grid integration and frequency stability.

An Active Power Control System for Wind Turbines Capable of Primary and Secondary Frequency Control for Supporting Grid Reliability

Wind energy is producing a larger share of power on many utility grids as more wind turbines are installed, providing motivation for wind turbines to provide ancillary services that are necessary for power grid reliability. The ancillary services considered in this paper consist of actively controlling the power output of the wind turbines to track power set-point commands, participate in frequency regulation, and provide frequency response. This paper focuses on the development of a wind turbine control system that is capable of varying the turbine’s active power output upon receiving de-rating power set-point commands, manual power commands, and automatic frequency regulation commands to meet the system operators’ needs in below-rated and above-rated wind speeds. The turbine is de-rated by operating at a higher than optimal tip speed ratio, storing additional inertia in the rotor, which can be used to assist in frequency regulation by providing a primary response to fluctuations in the grid frequency.

Combining droop curve concepts with control systems for wind turbine active power control

Wind energy is becoming a larger portion of the global energy portfolio, and wind penetration has increased dramatically in certain regions of the world. This increasing wind penetration has driven the need for wind turbines to provide active power control (APC) services to the local utility grid, as wind turbines do not intrinsically provide frequency regulation services that are common with traditional generators. Large scale wind turbines are typically decoupled from the utility grid via power electronics, which allows the turbines to synthesize APC commands via control of the generator torque and blade pitch commands. Consequently, the APC services provided by a wind turbine can be more flexible than those provided by conventional generators. This paper focuses on the development and implementation of both static and dynamic droop curves to measure grid frequency and output delta power reference signals to a novel power set point tracking control system. The combined droop curve and power tracking controller is simulated and comparisons are made between simulations using various droop curve parameters and stochastic wind conditions. The tradeoffs involved with aggressive response to frequency events are analyzed. At the turbine level, simulations are performed to analyze induced structural loads. At the grid level, simulations test a wind plants response to a dip in grid frequency.

Optimal trajectory tracking control for wind turbines during operating region transitions

Control systems for wind turbines have become an active area of research over the past decade as the wind industry has grown and more turbines are installed. Properly sited turbines experience many transitions between below rated operation, where the goal is to extract maximum energy from the wind, and rated speed operation, where the goal is to regulate the power capture to the rated power of the turbine. Many of the largest structural loads are induced during the transition between these operating regions. This paper focuses on using preview wind speed measurements to schedule, optimize, and track a desired trajectory of the wind turbine states and inputs during region transitions between below-rated and above-rated operation. The goal of this control system is to reduce the structural loading on the turbine components through smoother region transitions. The wind speed preview measurements are used to generate an initial desired trajectory of the turbine. This trajectory is optimized by finding a regulation trajectory that lies on the turbine trajectory manifold which is close to the desired trajectory in a weighted L2 sense. The regulation trajectory is then used as a reference for a time-varying linear quadratic optimal controller.

Stability analysis of a wind turbine active power control system

Wind penetration levels have increased dramatically in recent years. This has motivated the need for wind turbines and wind plants to provide grid frequency regulation services. Wind turbines do not inherently provide frequency regulation services in the same manner as conventional synchronous generators because modern wind turbines are decoupled from the grid via their power electronics. Consequently, active power control (APC) must be performed to allow wind turbines to participate in grid frequency regulation. In this paper, we consider a wind turbine APC system, which uses a generator torque controller to operate in a derated manner, allowing the turbine to maintain overhead power. Derated operation is achieved by operating the turbine at a higher-than-optimal tip-speed ratio. We show the stability of the APC torque controller and the asymptotic stability of the derated operating points for a constant input. Further, the control system is shown to be input-to-state stable for time-varying inputs.

Active power control of wind turbines for ancillary services: A comparison of pitch and torque control methodologies

As wind energy generation becomes more prevalent in some regions, there is increased demand for wind power plants to provide ancillary services, which are essential for grid reliability. This paper compares two different wind turbine control methodologies to provide active power control (APC) ancillary services, which include derating or curtailing power generation, providing automatic generation control (AGC), and providing primary frequency control (PFC). The torque APC controller provides all power control through the power electronics whereas the pitch APC controller uses the blade pitch actuators as the primary means of power control. These controllers are simulated under various wind conditions with different derating set points and AGC participation levels. The metrics used to compare their performance are the damage equivalent loads (DELs) induced on the structural components and AGC performance metrics, which are used to determine the payments for AGC services by system operators in the United States. The simulation results show that derating the turbine reduces structural loads for both control methods, with the APC pitch control providing larger reductions in DELs, lower AGC performance scores, and higher root-mean-square pitch rates. Providing AGC increases the structural loads when compared to only derating the turbine, but even the AGC DELs are generally lower than those of the baseline control system. The torque APC control methodology also allows for more sustained PFC responses under certain derating conditions.

Analysis of gain-scheduling implementation for the NREL 5-MW turbine blade pitch controller

Gain scheduling for the widely-used NREL 5-MW proportional-integral blade pitch controller can be implemented either by the original documented method (Integrate First): integrate the generator speed error before multiplying by the gain-scheduling correction factor, or the Multiply First method: multiply the generator speed error by the gain-scheduling correction factor, then integrate. The results of the Multiply First implementation are straightforward: the effective proportional and integral gains are simply the unscheduled gains multiplied by the value of the gain-scheduling function evaluated at the pitch operating point for the average wind speed. However, the original (Integrate First) implementation effectively reduces the proportional and integral gains significantly further than the Multiply First implementation, with the difference becoming more severe as wind speed increases. Integrating First results in effective gain reductions of 34% at 13 m/s and 41% at 18 m/s compared to Multiplying First. Further, this effective gain reduction disappears if the baseline pitch controller is augmented with a feedforward control signal whose average value is the pitch operating point. These effects are explained through analysis of a simplified version of the gain-scheduling feedback loop and verified through simulation. Simulation results also show that when effective gains are corrected to match each other, Integrating First improves performance compared to Multiplying First. This gain correction is not feasible for real-world implementation, but a simple change to the gain-scheduling function f(u) instead is feasible and is shown to have almost the same effect. Lifetime performance metrics for Integrating First with the new f(u) show a 4% reduction in tower base moment, a 2% reduction in blade root moment, a 4% reduction in RMS pitch rate, and a 10% reduction in RMS power error compared to Multiplying First.