Sub-Synchronous Oscillations (SSO)

Sub-Synchronous Oscillations (SSO) (also called Sub-Synchronous Resonance etc.), why they matter today, what causes them, how they are classified, and what mitigation strategies exist.

What are Sub-Synchronous Oscillations (SSO)

• Definition: SSO refers to oscillations in an electric power system at frequencies belowthe fundamental grid frequency (i.e. below 50 Hz or 60 Hz depending on region). In such oscillations, energy is exchanged between electrical and mechanical parts of the system, or between control / power electronic devices and the network, at these lower frequencies.
• Historically, these were observed in systems with large synchronous generators plus series-compensated transmission lines, where mechanical components (like turbine generator shafts) interact with electrical network resonance modes.

Why SSO is Relevant Now

Several reasons why SSO is getting more attention in recent years:

1. High Penetration of Inverter-Based Resources (IBRs): Solar, wind, battery storage systems are tied to grid via power electronics. These devices have fast control loops that can interact (resonance, control interaction) with network dynamics at sub-synchronous frequencies.
2. Lower System Inertia & System Strength: With fewer large rotating machines, the inertia in the grid declines. Weak grid conditions mean less damping, making it easier for small disturbances or poorly damped resonances to grow.
3. More Complex Network Topologies and Control Systems: Use of series compensation, HVDC links, long transmission lines, lots of power electronics with varying control schemes increases potential interaction paths.
4. Equipment Damage Risk: If undamped or growing, SSOs can lead to mechanical damage (shaft fatigue, torsional overloads), protection mis-operation, or even disconnection events.

Classification / Types of SSO

The literature divides/sub-classifies SSO into different types depending on what exactly is interacting, how, and the source of oscillation. Key categories:

Mechanisms / Causes

Some of the more specific causes or mechanisms:

• Series compensationin transmission lines: The use of capacitors in series with lines to improve voltage support or reduce losses can create resonance circuits. Combined with generators (mechanical inertia), this can trigger SSR.
• Negative resistance effects: Some network components or machines can appear to provide negative damping at sub-synchronous frequencies, making oscillations grow rather than decay.
• Control loops in inverters / converters: Fast control, such as PLLs (phase-locked loops), current regulators, or converter control strategies can excite or amplify certain modes. When multiple inverters act together, or an inverter interacts with network impedance, oscillations can result.
• Weak grid / low damping: Where grid strength is low (long lines, low short circuit capacity, few synchronous machines), damping is less effective. Disturbances are less suppressed.

Detection & Monitoring

To identify SSOs, the following are used:

• Time-domain observations of voltage/current oscillations (often using PMUs / high speed measurement). Looking for sustained or growing oscillations below grid frequency.
• Frequency domain analysis / spectral analysis. Identify resonant peaks, see which modes are present, their damping (or lack thereof).
• System / network modelling: electromagnetic transient (EMT) models, or phasor models for dynamic studies, to simulate potential SSO under various operating conditions and disturbances.

Mitigation & Control Strategies

What can be done to reduce or suppress SSOs:

• Design & Planning Measures

• Ensuring that series compensation is sized or controlled to avoid strong resonance with mechanical modes.
• Grid codes/interconnection requirements for IBRs to study and limit/control their potential to excite SSO.
• Building system strength: maintaining sufficient synchronous generation (or equivalent inertia), adding synchronous condensers, etc.
  • Control Strategies / Controllers
• Additional damping controllers (could be in the inverter / converter control or in systems like PSS – Power System Stabilizers) to inject damping at problematic frequencies.
• Active disturbance rejection control (ADRC) based suppression in converter controllers. For example, in renewable integration via MMC-HVDC, controllers have been designed to suppress SSO.
  • Operational Actions
• Re-configuring network topology when required. E.g., temporarily disabling or reducing series compensation during certain conditions.
• Monitoring and, if needed, constraining or adjusting the operation of inverter-based resources during weak grid conditions.

Recent / Real-World Examples

• In Great Britain, episodes of sub-synchronous oscillations were recorded (e.g. North Scotland in 2021) at around ~8 Hz. These events were mild but required investigation.
• In Kauaʻi Island power system (Hawaii), an 18-20 Hz oscillation followed a plant trip; this event is reported to be among the first transmission system-wide subsynchronous oscillations clearly driven by IBRs.

Challenges & Open Research Areas

• Predicting all possible interaction paths in complex grids with many IBRs; often simulation models may not capture every nuance.
• Ensuring that measurement tools (PMUs, monitoring) have sufficient bandwidth/time resolution for detecting SSOs.
• Designing controller/firmware in IBRs that are robust under many conditions, so they do not inadvertently excite unstable modes.
• Harmonization of grid codes, standards to ensure SSO study requirements are included everywhere.
• Mitigation strategies that are cost-effective and scalable, particularly for renewable-rich regions.