Tech Insights

Controlling Frequency Dips and Forced Oscillations in Wind Turbines

April 11, 2024 by Shannon Cuthrell

Two new methods could minimize frequency dips and forced oscillations in power systems with a high concentration of wind turbines. 

Wind energy deployment is rapidly expanding worldwide, but industry-standard variable-speed turbines can cause instability in power systems subjected to sudden shifts in supply and demand, as is typical in renewable energy generation. 

Forced oscillations and power system frequency control issues are significant limitations in the ongoing transition to intermittent resources like wind turbines and solar panels. Disruptions can knock out power for millions of customers, damage equipment, and reduce power transfer capacity. 

 

The first offshore turbine at the Dogger Bank Wind Farm started delivering power to the U.K. coast in 2023.

The first offshore turbine at the Dogger Bank Wind Farm started delivering power to the U.K. coast in 2023. Image used courtesy of Dogger Bank Wind Farm

 

A research team from the U.K.’s University of Birmingham has developed two easy-to-integrate control methods to counter these side effects. After confirming the technologies’ efficacy in two published studies, the university has filed two patent applications and seeks commercial partners for further collaboration. 

The novel control strategies come amid fast-growing wind energy penetration in the U.K., meeting 29% of the nation’s demand. According to WindEurope statistics, its offshore wind fleet generated 46 TWh in 2023, almost equivalent to Greece’s electricity demand. Nearly 1.4 GW of wind power was installed last year alone, including 553 MW onshore (primarily in Scotland) and 833 MW offshore, bringing its cumulative capacity near 30 GW. 

Several projects are underway as the nation targets 50 GW of offshore capacity by 2030. Last October, Scotland’s largest fixed-bottom installation, the 1.1 GW Seagreen project, started operating more than 110 Vestas 10 MW turbines. The 3.6 GW Dogger Bank project in the North Sea installed the first of 227 GE Haliade turbines about 80 miles from the U.K. coast. The unit now serves power to customers via a high-voltage direct current connection to the national grid. 

 

Minimizing Frequency Drops Through a Speed Recovery Strategy

Amid an imbalance of supply and demand, grids experience frequency dips in two phases. First, it triggers a frequency nadir, which measures stability after a disturbance in the power system. After the second drop, the frequency recovers to a settled state but still causes losses in efficiency and revenue. 

The University of Birmingham researchers cited a historical example highlighting the need to raise the frequency nadir and eliminate the second dip for grids with a high share of wind connections. In August 2019, a massive power cut impacted over a million customers across England and Wales. According to a 2020 analysis by the University of Cambridge, the event demonstrated traditional under-frequency load shedding disconnected all users on the disconnected feeders, including frequency response units and embedded generation. 

 

The control structure of a traditional fast frequency support method.

The control structure of a traditional fast frequency support method. Image used courtesy of the study’s authors (Figure 2) 

 

The University of Birmingham researchers found a way to hasten the frequency recovery to the settling state within 20 seconds and practically eliminate the subsequent frequency drop, keeping power capture losses under 1%. The rotor speed automatically adjusts with the frequency in the secondary control rather than the primary. 

The novel method employs an adaptive gain function to improve frequency nadir based on real-time rotor speed and wind power penetration. The adaptive gain control keeps variable-speed wind turbine systems stable by maintaining the rotor speed above the minimum limit. 

 

The University of Birmingham’s fast frequency control concept.

The University of Birmingham’s fast frequency control concept. Image used courtesy of the study’s authors (Figure 3)

 

The method was tested in six simulated scenarios under medium and high wind speeds and different wind penetration conditions. 

The frequency control strategy can be applied to existing control systems in variable-speed wind turbines. The researchers explained that such turbines operate at a maximum power point tracking mode to support large volumes of energy. They don’t regulate their active power to lend support during frequency fluctuations. 

 

Suppressing and Isolating Forced Oscillations

The University of Birmingham researchers also developed a solution tackling forced oscillations, in which external disturbances trigger oscillations almost identical to a power system’s natural oscillations. Such disruptions could include anything from upstream wakes to wind shear and turbulence, which could reduce output and damage equipment. Worse, equipment-related oscillations can cause ripple effects thousands of miles away if spread throughout the power grid. Thus, isolating and suppressing forced oscillations from turbines or the grid is critical. 

 

The novel control method on a two-machine system at a wind farm.

The novel control method on a two-machine system at a wind farm. Image used courtesy of the study’s authors (Figure 1) 

 

The researchers noted the technique applies to wind farms with grid-following or grid-forming principles. The control method absorbs or releases active and reactive power quickly to prevent oscillations from spreading. 

Simulations at different wind speeds confirmed that the technology could absorb power opposite from the oscillating power. It was also simulated for various wind farm sites. Overall, the method causes negligible wind capture losses and dampens natural oscillations. 

The technology is easy to add to existing systems with no additional energy storage or power electronics devices.