Introduction: Addressing SSCI in Modern Power Systems
As renewable energy resources expand, inverter-connected installations like Utility-Scale Wind Power Sites and Commercial Solar Energy Infrastructure bring a risk of sub-synchronous control interactions (SSCI). These low-frequency oscillations, arising from interactions between inverters and compensating devices, challenge grid stability and asset lifespan. This article delves into SSCI impact on frequency stability, compliance, and the risk of operational curtailment due to power quality.
The Importance of SSCI in Frequency Stability and Compliance
SSCI occurs when inverter-based resources, such as wind turbines and solar systems, interact at frequencies below nominal, risking frequency stability. A significant case, involving Xcel Energy’s Type-3 wind farms (using Doubly-Fed Induction Generators (DFIGs)) with series compensation, highlighted SSCI‘s destabilising potential. This configuration is prone to SSCI when inadequately protected against low-frequency oscillations, primarily in the 8-13 Hz range. When left unchecked, these oscillations jeopardise compliance with grid codes, particularly in weak grid environments, where control dynamics of inverters lead to unstable low-frequency conditions.
The nature of these low-frequency oscillations presents a substantial threat to both grid stability and the integrity of the involved equipment. The equipment, including turbine generators and associated converters, might experience increased mechanical stresses and thermal loading, leading to a reduced service life or immediate failures. Thus, understanding the dynamics of SSCI is crucial as networks evolve with increased integration of inverter-based resources (IBR).
This vulnerability often leads to grid operators enforcing curtailment, as non-compliance risks operational reliability and safety.
The Role of System Studies in Identifying SSCI Risks
Accurately identifying SSCI risks requires robust system studies using harmonic impedance scanning, dynamic frequency scanning, and time-domain simulations. These tools allow engineers to map sub-synchronous resonances and assess inverter damping characteristics critical for stabilising the grid.
Effective identification of SSCI risks in inverter-connected systems relies on sophisticated methodologies within system studies, including harmonic impedance scanning, dynamic frequency scanning, and time-domain simulation. Each technique plays a vital role in mapping the electrical resonance conditions that could lead to SSCI.
- Harmonic impedance scanning is a passive technique used to understand network impedance as a frequency function. By performing scans with software tools like PSCAD, it helps recognize sub-synchronous frequencies where resonance might occur. This method, however, generally ignores control influences.
- Dynamic frequency scanning, on the other hand, introduces low-magnitude harmonic currents into inverter models to measure the damping characteristics of power electronic converters specifically at sub-synchronous frequencies. This method often involves harmonic voltage waveform modulation and provides a dynamic response plot, offering insight into the system’s negative resistance properties. It complements impedance scans by evaluating system behavior under disturbances.
- Finally, time-domain simulation builds on these analyses by recreating specific network configurations and evaluating their performance under predicted worst-case scenarios. Software such as PSCAD and EMTP is instrumental in these simulations, ultimately determining whether SSCI risks necessitate mitigation strategies. The outcome informs necessary actions like controller tuning, topology alterations, or implementing protection solutions, crucial for securing modern electrical grid reliability.
Such detailed studies are integral in determining whether mitigation is necessary, such as controller tuning (a method used in the West Murray Zone in Autralia) or introducing protection solutions. System studies, that include using high-resolution real-world electrical grid data, are paramount; they ensure the reliability and stability of electrical grids by preemptively identifying SSCI risks and proposing corrective actions, reinforcing the resilience of today’s electrical infrastructure.
Effective SSCI Monitoring and Risk of Curtailment Due to Power Quality Issues
Power quality monitoring is essential to avoid SSCI-induced curtailment. Technologies such as Phasor Measurement Units (PMUs) and SSCI-specific relays help monitor and address SSCI in real time. For instance, PMU data has been used to detect oscillations caused by renewable energy plants, with algorithms pinpointing the exact location of oscillatory sources. In particular, the Oscillatory Phasor Measurement Unit (oPMU) algorithm, developed by CT LAB has been particularily effective in detecting SSOs in the West Murray Zone in Australia.
Additionally, the energy dissipation method allows operators to track where oscillation energy accumulates, thereby addressing SSCI before it forces curtailment. Implementing these monitoring methods reduces operational downtime by enhancing grid compliance.
Preventative Strategies and Protection Solutions
Preventing SSCI involves specific strategies:
- Tuning inverter controllers – controller tuning adjusts inverters’ dynamic responses, effectively dampening oscillations.
- Modifying network topology, such as bypassing series compensators.
These protective measures, serve as direct preventative tactics. These approaches effectively prevent equipment damage by intervening when SSCI conditions exceed pre-defined thresholds. However, challenges exist, such as accurately predicting SSCI conditions and setting precise thresholds without compromising grid performance.
Using the combined, real-world, high-resolution sample rate Synchrophasor and Power Quality data from the VECTO 3, power engineers operating in the West Murray Zone in Australia were able to resolve SSO‘s in the higher frequency range through tuning inverter controllers. This demonstrates the need for real-time grid oscillation monitoring and detection that captures the data during events. Engineers can then use this data to fine tune the parameters of the IED‘s contributing to the sub-synchronous oscillations. Recording this detailed and comprehensive data is essential for engineers configuring thresholds and parameters in protection devices such as SSCI relays.
Through these measures, engineers can prevent SSCI-related incidents and reinforce grid stability despite the increasing presence of renewable resources.
Mitigation SSCI Relay Technology
Sub-Synchronous Control Interaction (SSCI) relays have become essential in addressing challenges associated with inverter-connected renewable systems. These relays are specially designed to detect low-frequency oscillations that conventional relays might miss, thus preventing equipment damage and enhancing grid stability.
Real-world applications of SSCI relay technology have shown significant efficacy in various scenarios. For instance, in case studies involving Type-3 wind farms, the relays demonstrated flexibility in adapting to different network conditions. They function by measuring voltage and current signals within defined frequency ranges and can adjust based on user-defined settings. This operational adaptability ensures the protection relays can be signalled to suit specific site requirements, providing tailored protective measures.
One illustrative example includes installations in a large-scale wind farm where relays successfully identified and mitigated SSCI conditions, substantially reducing thermal stress on turbine components and prolonging their operational life. Additionally, using relays in photovoltaic systems has underscored their utility in maintaining stable power output despite fluctuating environmental conditions.
The adoption of these SSCI relay technology often faces challenges, such as setting optimal configurations and integrating with existing protection systems. Nonetheless, comprehensive event data, such as that provided by the VECTO 3 ensures seamless implementation.
Through these applications, SSCI relay technology exemplifies a robust front in the ongoing quest to secure renewable energy systems against destabilising phenomena, paving the way for more resilient future energy grids.
Conclusion
As renewable energy integration accelerates, SSCI poses a complex challenge for grid stability and compliance. Comprehensive studies, proactive SSCI monitoring, and targeted protection strategies are essential to mitigate SSCI risks, preventing curtailment and ensuring compliance. Ongoing research is critical to refining these solutions, safeguarding modern power systems as they evolve with renewable resources.
References
What is Dynamic Frequency Scanning?
In the context of sub-synchronous control interactions (SSCI), dynamic frequency scanning is particularly important. As renewable energy sources like wind and solar power are integrated into the grid, they often use inverters that can interact with the grid in complex ways. These interactions can lead to low-frequency oscillations, which can destabilise the power system.
By employing dynamic frequency scanning, operators can identify these oscillations in real-time, allowing for timely interventions to maintain grid stability. This technique is essential for ensuring that the integration of renewable energy does not compromise the reliability of the power supply.
The VECTO 3 has an optional software module that provides real-time dynamic frequency scanning, detection, while VECTO Grid OS provides consolidated event matching for sub-synchronous oscillations (SSOs).
Overall, “dynamic frequency scanning” is a critical tool in modern power system management, especially as the energy landscape evolves with increased reliance on renewable sources. It helps in monitoring and maintaining the stability of the grid, thereby addressing the challenges posed by SSCI and ensuring a reliable energy supply.
SSCI Relays
SSCI relays detect sub-synchronous oscillations (SSOs) through advanced signal processing techniques that isolate and analyse sub-harmonic frequencies in real-time. Here’s a technical breakdown of how they operate:
- Frequency Filtering and Bandpass Processing: SSCI relays incorporate band-reject filters to remove the fundamental frequency (50 or 60 Hz), allowing the relay to focus on the sub-synchronous range. A band-pass filter isolates specific sub-harmonic frequencies where oscillations are likely to occur, enabling the detection of disturbances that fall outside standard protection relays’ frequency range.
- Discrete Fourier Transform (DFT) and Sliding DFT (SDFT): Many SSCI relays use Discrete Fourier Transform algorithms, particularly Sliding DFT, to continuously analyse the signal for oscillations at specific frequencies. By computing the magnitude and frequency of the SSO, the IED (such as the VECTO 3 can assess whether oscillation amplitudes exceed set thresholds. The Sliding DFT’s computational efficiency and noise immunity make it suitable for real-time, accurate monitoring in challenging grid environments.
- Pickup and Trip Logic: Once an SSO frequency is detected, the monitor sends a SCADA message to the protection relay to decide whether the oscillation warrants a trip signal. The relay can issue a trip command if oscillations exceed thresholds, effectively isolating impacted equipment to prevent further damage.
- Optional Supervisory Elements: Advanced SSCI relays, such as the VECTO 3, include security features like Rate of Change of Frequency (RoCoF) supervision and undervoltage blocking to prevent unnecessary tripping from transient conditions or non-SSCI-related disturbances. These features add an additional layer of discrimination, reducing the risk of false positives.
These technologies represent current industry standards for managing SSCI, ensuring that mitigation is precise, fast, and adaptable to the unique demands of inverter-heavy power grids. This approach is crucial for maintaining compliance with grid codes and minimising curtailment due to power quality issues.
