Understanding Complex Oscillations in High-RE Power Systems: Beyond What Meets the Eye

As power systems evolve towards higher renewable energy (RE) penetration, engineers are increasingly encountering oscillatory phenomena that don’t fit traditional mental models. One particularly interesting case is when we observe low-frequency oscillations in the 6-7 Hz range – but is that really what we’re seeing? This article explores how these visible oscillations might be telling us a deeper story about our power system’s behaviour.

The Fundamentals: Frequency, Modulation, and Aliasing

Before diving into power system specifics, let’s refresh our understanding of some fundamental concepts that will help us interpret what we’re seeing:

Beat Frequencies and Modulation

When two signals of different frequencies interact, they create what’s known as a “beat frequency” – a periodic variation in amplitude at the difference of their frequencies. If you have signals at

   

​ you’ll observe a beat at

   

Mathematically, this is described by the trigonometric identity:

   

Application to Power Systems

In our power systems context, when we observe a 6-7 Hz oscillation, it might actually represent:

• A beat frequency between the fundamental (50/60 Hz) and a higher frequency component
• A modulation of the fundamental by a control system or interaction
• An aliased observation of a higher frequency phenomenon

Why This Matters in High-RE Systems

Modern power systems with high RE penetration are particularly susceptible to complex oscillatory behaviours for several reasons:

1. Reduced System Inertia
   • Traditional synchronous generators provide natural damping
   • Inverter-based resources (IBR‘s) don’t inherently provide this damping
   • System response to perturbations becomes faster and potentially more oscillatory
2. Multiple Control Timeframes
   • Grid-following inverters have PLL controls operating at high frequencies
   • Power controls operate at medium frequencies
   • Voltage controls typically operate at lower frequencies
   • These create multiple potential interaction points
3. Control Interactions
   • Between different inverters
   • Between inverters and synchronous machines
   • Between inverters and FACTS devices
   • Between plant-level and device-level controls

Investigating Observed Oscillations

When you encounter a 6-7 Hz oscillation, consider these investigation paths:

1. Frequency Analysis
   • Use FFT analysis to identify all frequency components
   • Look for sidebands around the fundamental frequency
   • Check for harmonics and sub-harmonics
2. Control System Audit
   • Map out all control loops and their natural frequencies
   • Identify potential resonance points
   • Check for control parameter interactions
3. System Conditions
   • Document system strength (SCR) at key points
   • Note the loading conditions of nearby synchronous machines
   • Record RE penetration levels when oscillations occur

Technical Deep Dive: Finding the Real Oscillation

Let’s work through an example. If we observe a 7 Hz oscillation, possible sources include:

1. Direct Oscillation
   • Actually 7 Hz (rare in power systems)
   • Could be mechanical or control system based
2. Beat Frequency
   • Could be 57 Hz interacting with 50 Hz
   • Could be 53 Hz interacting with 60 Hz
   • Multiple combinations are possible
3. Control System Interaction
   • PLL bandwidth typically 20-50 Hz
   • Power control bandwidth typically 5-10 Hz
   • Voltage control typically 1-5 Hz

The observed 7 Hz might be:

   

Where

   

is 50 or 60 Hz, and

   

is the actual system oscillation.

Mitigation Strategies

Once you’ve identified the true nature of the oscillation, consider these mitigation approaches:

1. Control System Tuning
   • Adjust PLL parameters
   • Modify power control gains
   • Review plant controller settings
2. System Level Solutions
   • Grid-forming inverter deployment
   • Synchronous condenser installation
   • Strategic placement of damping devices
3. Operational Measures
   • Maintain minimum synchronous generation
   • Limit RE penetration under weak grid conditions
   • Implement adaptive control settings

Conclusion

When investigating power system oscillations, especially in high-RE networks, don’t take the observed frequency at face value. The visible 6-7 Hz oscillation might be a symptom of higher-frequency interactions that require different mitigation approaches than what you’d use for a true low-frequency oscillation.

Key Takeaways:

1. Always consider the possibility of frequency modulation and beating
2. Use comprehensive frequency analysis tools
3. Map control system interactions
4. Consider system strength and operational conditions
5. Document patterns and correlations