Sub-Synchronous Oscillations (SSO) Mitigations and Preventions:-

Sub-synchronous oscillations (SSO) can occur in power systems, particularly in networks with large synchronous generators and series-compensated transmission lines, or where power electronic converters like HVDC, STATCOMs, or wind turbines interact with the grid. SSO mitigation and prevention aim to ensure grid stability, reliability, and safe operation.

SSO Types

1. Sub-Synchronous Resonance (SSR): Interaction between the mechanical modes of a generator and the electrical network, typically in series-compensated systems.
2. Sub-Synchronous Torsional Interaction (SSTI): Interaction between torsional modes of the generator shaft and the power system.
3. Sub-Synchronous Control Interaction (SSCI): Interaction caused by control systems of power electronic devices like wind turbines or STATCOMs.

SSO Mitigation Methods

Mitigation strategies aim to reduce or eliminate SSO by adjusting system components or implementing control techniques.

1. Damping Controllers

• Power System Stabilizers (PSS):Installed on generators to provide damping for oscillatory modes, including sub-synchronous frequencies.
• Supplementary Damping Controllers (SDC):Used with power electronic devices like STATCOMs, HVDC, and FACTS to dampen sub-synchronous oscillations.

2. Series Compensation Tuning

• Reduced Compensation Levels:Adjust the level of series compensation to avoid resonance at sub-synchronous frequencies.
• Thyristor-Controlled Series Capacitors (TCSC):Dynamically adjust the compensation level, thereby avoiding resonance.

3. Filtering Techniques

• Sub-Synchronous Filters:Install filters tuned to block sub-synchronous frequencies from propagating through the network.

4. Shaft Protection Systems

• Shaft Torsional Monitoring and Relays:Protect the generator shaft from torsional stresses caused by SSO by detecting oscillations and tripping the generator if necessary.

5. HVDC System Modifications

• HVDC Modulation:Modulate the active power in HVDC links to provide damping for sub-synchronous frequencies.
• Hybrid HVDC Systems:Combine VSC and LCC technologies to manage interactions effectively.

6. Wind Turbine Control Modifications

• Sub-Synchronous Control Damping:Implement control algorithms in wind turbine converters to actively dampen SSCI.
• Frequency Scanning Analysis:Analyze the grid and turbine interaction to design control systems that prevent SSO.

7. FACTS Devices

• Static VAR Compensators (SVC):Provide dynamic reactive power support and stabilize oscillations.
• STATCOM with SSO Dampers:Actively inject damping signals to counteract sub-synchronous frequencies.

8. Grid Modifications

• System Strengthening:Add transmission lines or shunt reactors to increase grid strength and reduce the likelihood of resonance.
• Line Outage Management:Monitor and manage line outages to prevent network configurations that may trigger SSO.

SSO Prevention Strategies

Preventive measures are designed to avoid conditions that may cause SSO before they occur.

1. Network Design

• Avoid high levels of series compensation unless necessary.
• Use simulation tools during the planning stage to identify potential resonance issues.

2. Equipment Selection

• Install damping resistors in series capacitors to avoid resonance.
• Choose generators with sufficient damping in their mechanical system.

3. Control System Tuning

• Design control systems for power electronics to avoid sub-synchronous frequency interaction.
• Perform detailed electromagnetic transient (EMT) studies to optimize settings.

4. Monitoring and Diagnostics

• Real-time monitoring of oscillations using Phasor Measurement Units (PMUs) or Wide Area Monitoring Systems (WAMS).
• Deploy online tools to detect and alert operators of SSO risks.

5. Operational Strategies

• Limit power transfer over series-compensated lines during high-risk conditions.
• Adjust generator setpoints or HVDC power flow to avoid SSO-prone conditions.

6. Training and Awareness

• Train grid operators to recognize SSO scenarios and take corrective actions.
• Develop operating procedures to handle contingencies involving SSO.

Case Example: Mitigation of SSO

1. Wind Farm SSCI Mitigation: A wind farm connected to a series-compensated line experienced SSCI. Mitigation involved: Adding damping controllers in the wind turbine's converter. Adjusting the series compensation level to shift the resonance frequency.
2. HVDC SSTI Mitigation: HVDC link near a thermal plant caused torsional interactions. Mitigation involved: Modulating HVDC power flow to provide damping. Installing shaft monitoring and protection systems.