Sub-synchronous oscillations (SSOs)

Globally (and particularly in the Netherlands), there has been an increase in sub-synchronous oscillations (SSOs) in the power grid. Sub-synchronous oscillations (SSOs) are electrical or electromechanical oscillations that occur at frequencies lower than the fundamental grid frequency (i.e., lower than 50 Hz in Europe or 60 Hz in North America). They represent a dynamic interaction between grid-connected equipment (generators, wind turbines, and power electronics) and the broader power grid, resulting in resonance modes at sub-synchronous frequencies.

Unlike traditional electromechanical oscillations, SSOs are typically localized interactions between specific equipment and parts of the grid. Modern renewable energy sources (wind, solar, batteries) use power electronics inverters whose fast control loops can interact with the dynamics of the power grid, leading to sub-synchronous control interactions.

Why are sub-synchronous oscillations (SSOs) a problem?

If they are not properly cushioned, they can cause the following problems:

1. Equipment stress and damage
2. Grid instability and power outages
Poorly damped oscillations can intensify, causing the system to become unstable or necessitating system disconnection.
3. Power curtailment or shutdown of facilities
Grid operators may be forced to reduce output or shut down facilities to prevent damage.

3,5 Hz project

In the Netherlands, a consortium is conducting a research project on 3.5 Hz oscillations in the power grid. This consortium consists of Tennet, Liander, Enexis, Duurzaam Energie Perspectief, and Stedin.

They found a correlation between the presence of the 3.5 Hz oscillations and when the sun is shining in the Netherlands. They found the following correlations: the 3.5 Hz oscillations occur most frequently:

At times of day when the sun is at its strongest.
In areas of the Netherlands with the highest density of solar panels.
For PV inverters connected to the low-voltage grid (there was no connection to large solar parks on the medium-voltage grid).

Further research will be conducted in 2026 to take additional measurements and verify the hypotheses. The research is being conducted in collaboration with manufacturers of (micro)inverters and with the ElaadNL Test Lab and Power Lab (HAN/Elaad). The results will be presented at the upcoming “International Conference on Harmonics and Quality of Power,” to be held in early September 2026 in Dresden.

Sub-synchronous oscillations (SSOs) are oscillations below the grid frequency caused by interactions between grid components and mechanical or electronic systems. They are becoming increasingly common in modern grids with a high proportion of renewable energy and power electronics, particularly in cases of series compensation or weak grid conditions. SSOs can damage equipment and threaten grid stability if not properly addressed.

For more information, please visit the pages below.

1. 3.5 Hz oscillations in the electricity grid
3. Testing the effects of grid distortion