Power System Transient Studies

Power system transient studies are critical for ensuring the stability, reliability, and safety of modern electrical grids. Transients are short-duration electrical phenomena that occur due to sudden disturbances such as switching events, lightning strikes, or faults in the system. These disturbances, which can last from microseconds to seconds, can lead to over-voltages, oscillations, and even equipment insulation failure if not properly mitigated. With the increasing integration of renewable energy sources, the need for effective transient analysis has become more important than ever.

In our latest article, we explore the core aspects of transient studies and the crucial role they play in safeguarding electrical infrastructures.

Key Takeaways:

• Why Are Transient Studies Important?
• Timescales of Power System Dynamics and Transients
• Core Aspects of Power System Transient Studies
• Simulation Tools for Transient Studies
• EPS Expertise in Transient Studies

Why Are Transient Studies Important?

Transient events can significantly impact the performance of electrical systems, leading to voltage spikes, insulation breakdowns, stability issues, and power outages. They are particularly critical in high-voltage environments, where over-voltages can severely damage equipment and stability issues can lead to blackouts. Conducting thorough transient studies helps engineers predict system behaviour during such events, allowing for the design of protective measures that safeguard critical infrastructure.

Timescales of Power System Dynamics and Transients

The diagram below illustrates the timescales of various power system dynamics and transient phenomena in power systems, shown on a logarithmic scale ranging from microseconds (10^-7 seconds) to days (10^5 seconds).

The phenomena are classified based on the speed of occurrence:

 1. Microseconds (10^-7 to 10^-5 seconds):

• Lightning Propagation: Extremely fast, nearly instantaneous propagation of lightning-induced surges across the power system.
• Switching Surges: Transient overvoltages caused by the rapid switching of components like circuit breakers, resulting in high-frequency voltage spikes.

2. Milliseconds (10^-5 to 10^-3 seconds):

• Inverter-Based Controls: Quick response actions by power electronic converters in systems with renewable generation or other grid components, such as voltage-source converters.

3. Seconds to Minutes (10^-3 to 10^1 seconds):

• Stator Transients and Sub-Synchronous Resonance: Electrical and mechanical interactions in synchronous machines or power systems, often related to turbine-generator dynamics.
• Transient Stability: The ability of the power system to maintain synchronism after a disturbance, like a fault or a large load change.

4. Minutes to Hours (10^0 to 10^3 seconds):

• Governor and Load Frequency Control: Frequency regulation to maintain the balance between generation and demand.
• Boiler Dynamics and Voltage Stability: Control of boiler systems and voltage regulation to ensure continuous system stability.

5. Hours to Days (10^3 to 10^5 seconds):

• Power Flow: Steady-state analysis of electrical flows across the grid to ensure efficient and reliable operation.
• Unit Commitment: Long-term scheduling of generating units to meet forecasted demand, ensuring cost-effective and reliable electricity supply.

Core Aspects of Power System Transient Analysis

1. Switching Transients

Switching transients occur when devices like transmission lines, circuit breakers, transformers, or capacitors are switched on or off. These sudden changes in current and voltage create over-voltages and oscillations that can propagate through the network.

• Circuit Breaker Operations
  Opening or closing a circuit breaker can cause transient voltages, leading to oscillations in high-voltage systems. Optimising breaker operations is essential to minimising
  these transients and preventing equipment damage.
  
• Transformer Energisation
  Energising transformers results in high inrush currents, which can lead to insulation damage, mechanical stress and protection malfunction. Accurate modelling of these currents helps engineers develop strategies to mitigate
  their impact.

2. Lightning Induced Over-Voltages

Lightning strikes are some of the most significant causes of transient over-voltages in power systems. A lightning strike can induce severe over-voltages, propagating through the system and potentially causing insulation breakdown.

• Surge Protection
  Lightning-induced transients can travel along power lines, causing significant over-voltages far from the point of impact. These transients can damage transformers, circuit breakers, and other critical equipment.
  
• Protection Mechanisms
  Simulating lightning transients helps optimise the placement of surge protection devices like surge arresters and ensures effective earthing systems. Insulation coordination studies help design systems that withstand these over-voltages.

3. Transients in Gas-Insulated Substations (GIS)

Gas-insulated substations (GIS) are widely used in modern power systems due to their compact design and reliability. However, their unique insulation properties make them susceptible to severe transient phenomena.

• Resonance in GIS
  Due to the high degree of compactness, resonance phenomena can be more severe in GIS than in air-insulated substations. Transient studies ensure the integrity of gas-insulated equipment and minimise the risk of resonance and overvoltage effects.

4. Ferro-Resonance

A nonlinear phenomenon that occurs in power systems containing capacitance and inductance, particularly in circuits involving transformers. This condition can lead to excessive over-voltages, equipment damage, and insulation failure.

• Nonlinear Behaviour
  Ferro-resonance often leads to prolonged over-voltages and excessive heating in transformers. Detailed simulations of ferro-resonant conditions help design systems that avoid such issues, ensuring stable and safe operation.

5. Sub-Synchronous Resonance (SSR)

Sub-synchronous resonance (SSR) is a phenomenon that occurs in power systems when the natural frequency of synchronous machines interacts with the sub-synchronous frequency components generated by other system elements, such as turbine-generator sets and power electronics. This interaction can lead to excessive oscillations and potential damage to equipment.

SSR is particularly concerning in systems with high levels of renewable energy integration and in those employing long transmission lines or series compensation. The primary effects of SSR include:

To mitigate SSR, engineers employ various strategies, including:

By understanding and addressing SSR, power systems can maintain stability and ensure reliable operation under varying load conditions.

6. CT Saturation and HV Capacitor Bank Switching

Current transformer (CT) saturation and high-voltage capacitor bank switching are critical aspects of transient studies, particularly in HV systems. Both phenomena can induce transients that affect system performance.

• CT Saturation
  Saturation of CTs occurs when a transient event drives the CT’s magnetic core into saturation, leading to inaccurate current measurement and potential protection relay misoperation. This requires detailed modelling to ensure accurate system protection.
  
• Capacitor Bank Switching
  The switching of high-voltage capacitor banks generates inrush currents and over-voltages that can impact system stability. Proper control strategies, such as point-on-wave switching, are essential to mitigate these effects.

7. Integration of Renewables

The integration of renewable energy sources, such as wind and solar, into power systems introduces new transient dynamics due to their variable and intermittent nature. This poses additional challenges to maintaining grid stability.

• Power Quality Issues:
  Variability in renewable energy generation can lead to voltage fluctuations, harmonics, and frequency deviations. Transient studies help assess the impact of renewable energy integration on the grid and develop strategies to mitigate power quality issues.

8. HVDC Circuit Transients

High Voltage Direct Current (HVDC) systems have unique transient characteristics due to the nature of their converter-based technology. Transient studies in HVDC systems focus on ensuring smooth operation and interaction with AC systems.

• Commutation Failures and Resonance:
  In HVDC systems, transient studies focus on mitigating commutation failures, controlling resonance effects, and ensuring the smooth interaction between AC and DC components. This helps maintain system stability and prevent disturbances in the grid.

9. Motor Starting Transients

Large motors, when started, induce transients that can lead to voltage dips and high inrush currents, causing disturbances in the system.

• Voltage Dips and Inrush Currents
  Starting large motors causes significant voltage dips, affecting other loads on the grid. Inrush currents from motors also pose a threat to system stability. Motor starting transients can be minimised using soft starters, variable frequency drives (VFDs), or staggered starting techniques.

10. Insulation Coordination

Insulation coordination ensures that the insulation systems of power equipment can withstand normal operating conditions and transient overvoltage events, such as switching and lightning strikes. The goal is to prevent insulation failure and maintain system integrity.

• Design Optimisation
  Insulation coordination studies are used to design the insulation levels of equipment and ensure that surge arresters are correctly placed to mitigate transient over-voltages. This is critical in high-voltage systems, where insulation breakdown can lead to catastrophic failures.

11. Point-on-Wave Switching

Point-on-wave (POW) switching is a technique that involves timing the opening or closing of circuit breakers to specific points in the voltage waveform. This minimises the transient effects caused by switching operations.

• Controlled Switching
  By controlling the exact moment of switching, POW reduces the magnitude of transient over-voltages. It is particularly effective in mitigating transformer energisation transients
  and in capacitor bank switching, where large inrush currents can be avoided.

12. Transient Stability

Transient stability is the ability of a power system to maintain synchronism following large disturbances, such as faults or sudden load changes. This stability is critical for preventing cascading failures and ensuring reliable operation.

Transient stability analysis involves simulating the system’s response immediately after a disturbance, typically within a few seconds to minutes. Key factors influencing transient stability include:

• System Configuration: The layout and interconnections of the power grid.
• Fault Clearing Time: Quick detection and removal of faults are essential to maintain synchronism.
• Governor and AVR Settings: The performance of automatic voltage regulators and governors affects the system’s dynamic response.

Using numerical simulation tools, engineers conduct transient stability studies to assess the system’s behaviour, identify critical parameters, and design control strategies to enhance resilience.

Simulation Tools for Power System Transient Studies

There are several advanced software tools available to conduct power system transient simulations. These include:

• PSCAD/ EMTDC
  This tool specialises in the simulation of electromagnetic transients, offering detailed modelling for transformers, HVDC systems, and renewable energy integration.
  
• EMTP-RV
  Known for its flexibility, EMTP-RV is a powerful tool used to simulate both steady-state and transient phenomena in power systems.
  
• ATP-EMTP
  This free version of EMTP is widely used for transient studies, particularly for insulation coordination, switching transients, and motor starting analysis.

EPS Expertise in Transient Studies

At Engineering Power Solutions (EPS), our power system consultants have extensive expertise in conducting transient analysis. Using industry-leading software such as PSCAD and EMTP-RV, we offer a comprehensive range of services, including:

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