Sub-Synchronous Control Interaction (SSCI) Studies for Renewable Power Plants

1. Introduction

With the rapid integration of renewable energy sources such as wind and solar into modern power grids, dynamic interactions between power electronic converters and the network have become increasingly significant. One such phenomenon is Sub-Synchronous Control Interaction (SSCI) — a control-driven instability that occurs between Inverter-Based Resources (IBR) and series-compensated transmission systems.

SSCI poses a serious risk to system stability, potentially leading to oscillatory over currents, converter tripping, or even equipment damage. Therefore, detailed simulation studies and mitigation techniques are critical during the design and grid compliance stages of renewable power projects.

SSCI arises due to control-loop dynamics of the converter interfacing the grid. When a renewable plant is connected through a series-compensated line, the converter control interacts with the network’s sub-synchronous frequencies (typically 10–50 Hz below the fundamental 60Hz). This interaction can lead to negative damping at these frequencies, resulting in unstable sub-synchronous oscillations.

3. System Modeling for SSCI Studies

To accurately capture SSCI behavior, detailed electromagnetic transient (EMT) simulations are required using tools such as PSCAD/EMTDC as preferred by most of the grid operators around the globe.

Key Modeling Components:

-Renewable plant model: Detailed representation of converter controls (inner current loop, outer voltage/reactive power control, PLL, etc.)

– Grid network: Transmission line parameters with realistic series compensation.

– Transformer and filters: Proper representation of impedance characteristics.

–  Control system interactions: Inclusion of DC link dynamics and PLL behavior.

The study generally investigates multiple operating conditions, such as varying compensation levels, plant output power, and grid strength (short-circuit ratio).

4. Study Objectives

SSCI studies are conducted to achieve the following objectives:
– Identify the risk of sub-synchronous oscillations in the renewable power plant network.
– Determine critical compensation levels that could trigger SSCI.
– Evaluate the effectiveness of damping controls or SSCI mitigation schemes.
– Ensure compliance with grid codes and utility interconnection requirements (e.g., ERCOT, NERC, SEC, AEMO or CEA guidelines).

5. Typical Analysis Procedure

1. Model Development: Create an EMT-level model of the renewable plant and the grid interface.
   2. Frequency Scan Analysis: Identify resonance points and impedance characteristics at sub-synchronous frequencies.
   3. Time-Domain Simulation: Introduce disturbances or changes in compensation levels and observe converter current response.
   4. Impedance Scan Analysis: Assess system stability margins and identify modes with negative damping.
   5. Mitigation Design: Implement and test damping solutions such as converter control tuning, SSCI damping controllers, or reduction of series compensation percentage.

6. Case Study Example

In a 300MW solar farm connected via a 40% series-compensated 220 kV line, SSCI was observed around 30–40 Hz. The converter’s reactive power control interacted with the network’s resonant frequency, leading to oscillatory torque and voltage fluctuations. By optimizing the PLL bandwidth and incorporating an active damping loop, the sub-synchronous oscillation was successfully suppressed, restoring system stability.

Scope of Work

• Comprehensive SSCI analysisfor a 300 MW Solar Plant
• EMT-level modeling of inverter controls, PLL, filters, and DC-link dynamics
• Sub-synchronous resonance screening and stability assessment
• Frequency scan and impedance scan evaluation under various grid strengths (SCR conditions)
• Time-domain simulations to detect negative damping and oscillatory behavior
• Assessment of series compensation impact and critical compensation thresholds
• Design and validation of mitigation strategies including PLL tuning and active damping
• Compliance review aligned with Saudi Grid Coderequirements

Key Outcomes

This study will ensure that the plant operates without sub-synchronous instability risks, maintains stable converter performance, and meets all grid-integration criteria. It will also provide clear, actionable recommendations for improving damping, enhancing controller stability, and supporting long-term reliable operation of large-scale solar facilities.

7. Mitigation Techniques

– Control-based damping: Modifying converter control parameters (e.g., proportional gains, bandwidths).
– Supplementary damping controllers: Adding feedback loops targeting sub-synchronous frequency bands.
– System-based measures: Adjusting series compensation, using TCSC or SSSC devices.
– Hybrid solutions: Coordinating converter damping with FACTS device controls for system-level stability.

8. Conclusion

Sub-Synchronous Control Interaction (SSCI) represents a modern grid stability challenge in renewable-dominated networks. As grid codes evolve, SSCI analysis has become a mandatory study for renewable project grid integration. Through advanced EMT simulations, resonance screening, and robust control tuning, SSCI risks can be effectively identified and mitigated, ensuring reliable and resilient renewable power systems.