Master Small Signal Oscillations for a Resilient Clean Energy Future 

In the evolving landscape of power engineering, small signal oscillations in mixed source power grids present a unique set of challenges and opportunities. As power engineers, understanding these oscillations is crucial for maintaining grid stability and efficiency. This blog post delves into the intricate world of small signal oscillations, exploring their causes, impacts, and the innovative solutions that are shaping the future of mixed source power grids.

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Understanding Small Signal Oscillations

What Are Small Signal Oscillations?

Before diving into the complexities, let’s define what we mean by small signal oscillations in the context of power grids. Small signal oscillations are minor fluctuations in electrical power systems that can occur due to various disturbances. These oscillations, often of low amplitude, can significantly affect the performance and stability of power grids, especially those incorporating mixed energy sources like solar, wind, and traditional fossil fuels.

Why They Matter in Mixed Source Power Grids

In mixed source power grids, the integration of renewable energy sources adds layers of complexity to grid management. Unlike traditional power plants, renewable energy sources are often intermittent and less predictable. This variability can lead to more frequent and less predictable small signal oscillations, challenging grid stability.

Causes of Small Signal Oscillations

Integration of Renewable Energy Sources

The incorporation of renewable energy sources, with their variable output, is a primary factor in the increase of small signal oscillations. Wind turbines and solar panels, for instance, can create fluctuations in voltage and frequency due to changes in wind speed or solar irradiance.

System Disturbances

External disturbances such as sudden load changes, equipment failures, or line outages also contribute to small signal oscillations. These events can cause abrupt changes in power flow, leading to oscillations that can propagate throughout the grid.

Impact on Grid Stability

Reliability Concerns

Small signal oscillations, if not properly managed, can escalate, leading to significant stability issues. These can range from reduced power quality to large-scale outages, posing a threat to the reliability of power delivery.

Interference with Grid Operations

Ongoing oscillations can interfere with the normal operation of grid control systems. They can mask other important signals or trigger false alarms, complicating grid management and maintenance.

Real-world Case Study: Australian Grid Oscillations

Australia has been facing issues with grid stability and frequency oscillations due to the increase in non-synchronous power sources, such as wind and solar power 1234567891011121314.

Non-synchronous power sources do not provide rotational inertia, which is essential for maintaining grid stability and preventing blackouts24. The Australian Energy Market Commission is consulting on a rule change for a spot market in inertia provision to address this issue2.

The limitations of the physical infrastructure that carries electricity from the point of generation is also jeopardising the renewable energy industry in Australia10. The prevalence of rooftop solar and other distributed resources makes the transition to a system without synchronous generators even more complex19. To mitigate these issues, Australia is exploring new ways to maintain system strength, such as grid-forming inverters and synchronous condensers1219.

Synchronous Condensers and Grid Oscillations

Synchronous condensers (also known as syncon’s) can play a vital role in stabilising the power grid, especially in areas with weak networks or high penetration of non-synchronous generation. Syncons provide short-circuit power, inertia, and reactive power for dynamic loads22. They can modulate the reactive power output and terminal bus voltage, which helps to dampen inter-area oscillations in wind farms2324.

The impact of synchronous condensers on sub/super-synchronous oscillations in wind farms is dependent on their capacity and leakage reactance22. The use of synchronous condensers in conjunction with a power oscillation damper (POD) can dampen inter-area oscillations in a network with high renewable generation26.

Synchronous condensers can also be used in a hybrid configuration with battery energy storage systems (BESS) to stabilise the grid through increased short-circuit current, increased frequency support and system inertia, decreasing ROCOF, and reactive power control27.

The relationship between Synchronous Condenser Shaft Damage and Grid Oscillations

Sub-synchronous Resonance (SSR) occurs when an electrical network exchanges energy with a source at a frequency lower than the system’s synchronous frequency. This energy exchange induces torsional stress on generator shafts28, posing a risk of damage or fracture.

There have been cases where grid oscillations have been implicated in damage, including damage to shafts. In a study on sub-synchronous oscillation events in grid-connected wind farms, it was reported that shafts were severely damaged, although the root cause of the incident was not clearly identified29.

Additionally, research has focused on the impact of synchronous condensers on sub/super-synchronous oscillations in wind farms, emphasising their role in stabilising the power grid and mitigating oscillations30. Synchronous condensers can provide essential support to address grid stability issues, but the specific cases of damage to syncon shafts due to grid oscillations may vary and require further investigation.

Grid Oscillation Monitoring and Control to prevent Synchronous Condenser Shaft Damage

Monitoring and control systems can be used to adjust the operation of synchronous condensers to prevent damage due to grid oscillations. These systems can detect grid oscillations and adjust the synchronous condenser’s operation accordingly.

Addressing this risk demands new ways of monitoring and managing the grid. VECTO includes unique phasor measurement capabilities that delivers the experience of an Oscillation’ Phasor Measurement Unit (oPMU). The system captures GPS time synchronised data through a fleet of wave synchronised edge-computers installed throughout the grid, continuously recording and streaming oscillation phasor data using the established IEEE C37-118 protocol. Oscillation phasors captured at different locations is similar to synchrophasor data, allowing for interaction with all existing Phasor Measurement Units currently on the market.

The VECTO System concurrently identifies up to three dominant oscillation phasors within the range of 0.1Hz to 43Hz.

  1. Low Frequency Range — The algorithm’s low-frequency range, spanning from 0.1Hz to 1Hz, is designed to identify inter-area oscillations. These oscillations typically arise from the interaction of significant energy sources that are geographically distant. This type of oscillation is commonly observed within the frequency range of 0.2Hz to 0.8Hz.
  2. Medium Frequency Range — The algorithm’s mid-frequency range, ranging from 1Hz to 10Hz, is tailored to identify local-area oscillations. These oscillations typically emerge from the interaction among energy sources situated in closer proximity. A common cause is the phenomenon known as “hunting” among a group of generators.
  3. High Frequency Range — The algorithm’s high-frequency range, spanning from 10Hz to 43Hz, is designed to pinpoint oscillations arising from control loop instability among different Inverter-Based Resources (IBR) energy sources.

The Dominant Oscillation Phasor Monitoring (oPMU) feature of the VECTO is a licensed software module that can be loaded onto an existing VECTO Measurement Platform.

These systems can help prevent damage to synchronous condensers by detecting and responding to grid oscillations in real-time.

Mitigation Strategies

Advanced Control Systems

Modern control systems, equipped with real-time monitoring and adaptive response capabilities, are crucial in managing small signal oscillations. These systems can quickly detect fluctuations and adjust grid operations to mitigate their effects. The VECTO 3 is one such modern monitoring and control device.

Grid-Scale Energy Storage

Energy storage systems, such as battery storage, can act as a buffer, absorbing and releasing energy to smooth out fluctuations caused by renewable sources. This not only helps in damping small signal oscillations but also enhances overall grid resilience.

Dynamic Line Rating

Implementing dynamic line rating systems can optimize the capacity of power lines based on real-time conditions. This approach helps in managing the flow of electricity more effectively, reducing the risk of oscillations.

Looking Ahead: Future Innovations

AI and Machine Learning in Grid Management

Advancements in artificial intelligence (AI) and machine learning offer promising avenues for predicting and managing small signal oscillations. By analysing vast amounts of electrical grid data, these technologies can anticipate potential issues and optimise grid operations.

VECTO Grid OS data is built on Cassandra, a vector search ready data-store. This means that all the power data recorded on the electrical grid and streamed to VECTO Grid OS is available in a ML and AI ready format for researchers.

The VECTO System has the ability to trigger on events (defined by rules) that are then available as raw electrical data with long pre- and post-roll. This provides a rich data-set for researchers.

An additional feature available within the VECTO System is the XrossTrigger™ feature; this allows you to remotely trigger any configured group of geo-distant VECTO 3 monitoring devices, on any defined event(s). This feature records raw electrical data, with pre- and post-roll, on all the devices in the defined group, whenever the configured event occurs. This event data is then available via remote access.

Smart Grid Technologies

The evolution of smart grid technologies, including advanced sensors and edge-computing monitoring devices (such as the VECTO 3), provides enhanced visibility and control over grid dynamics. These technologies enable more efficient handling of the complexities introduced by mixed source power grids.

Conclusion: Embracing the Challenge

Small signal oscillations in mixed source power grids represent a critical aspect of modern power engineering. As our energy landscape evolves, the need to understand, mitigate, and manage these oscillations becomes increasingly important. Through a combination of advanced technologies and innovative strategies, we can ensure the stability and reliability of our power grids, even in the face of growing complexity.

In conclusion, small signal oscillations pose a significant challenge but also present an opportunity for innovation and advancement in power engineering. By embracing these challenges and continuing to develop and implement effective solutions, power engineers play a pivotal role in shaping a stable, efficient, and sustainable energy future.

The VECTO System oPMU module (available on the VECTO 3) gives power engineers the all the familiar capabilities available with PMU’s but provides high resolution detection and control signals for small signal oscillations.

Further Reading …

Small signal oscillations in mixed source power grids reveals their potential impact on grid stability. Here are the key findings from recent research:

  1. Grid Synchronization Stability with Three-Phase Converters: Synchronization instability in voltage-source converters (VSCs), often used in grids for efficiency and renewable energy utilization, can be caused by the negative incremental resistance behavior of grid-tied inverters. Altering the inverter’s phase-locked loops (PLLs) design can stabilize the system (Wen et al., 2014).
  2. Stability of Wind Power Systems with Full-Load Converter Interfaced Wind Turbines: The stability of power systems with high penetration of wind turbines, especially those using full-load converters, is crucial. Modal analysis highlights the impact of these turbines on small-signal stability, suggesting the need for detailed models and control strategies (Knueppel et al., 2012).
  3. Low Frequency Oscillation Analysis in Power Grids: Low-frequency oscillations, despite the use of power system stabilizers, can be induced by disturbances and can threaten grid stability. An energy function approach can provide insights into these oscillations and help in locating disturbance sources (Lei, 2012).
  4. Grid-Connected Inverter System Stability: Small-signal stability issues arise when connecting inverters to weak grids. A model shows how grid impedance and PLL interactions can lead to oscillations, with proposed control parameters to suppress these oscillations (Xie et al., 2020).
  5. Impact of Inverter-based Resources on Stability: The shift to distributed energy resources, including more inverter-based systems, affects small-signal stability. Different inverter controls (grid following, droop-controlled grid forming, and virtual oscillator control) have varying impacts on stability, with grid-forming controls potentially improving it (Lin et al., 2021).

In summary, small signal oscillations in mixed source power grids can indeed pose risks to grid stability. However, these risks can be mitigated through careful design of system components and control strategies.

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