Short Answer:

A power swing in power systems is a condition where there are sudden, large, and temporary oscillations in power flow due to a disturbance like a fault or sudden change in load or generation. These swings mainly affect the rotor angles of generators, which may go out of synchronism if not controlled properly.

Power swings are not faults, but they behave similarly in the system, causing fluctuations in voltage, current, and power flow. If the system is stable, these swings gradually reduce; if not, they can grow and lead to system separation or blackouts.

Detailed Explanation:

Power swing in power systems

In a large interconnected power system, stability depends on the continuous balance and synchronism between multiple generators. When the system is disturbed, for example, by a transmission line fault, generator trip, or sudden load change, it can cause generators to swing against each other in terms of their rotor angles. This dynamic condition is called a power swing.

During a power swing, the real and reactive power flows, current, and voltage in the network experience rapid oscillations. The system’s ability to damp these oscillations determines whether the swing will subside or become unstable. If uncontrolled, it may lead to loss of synchronism, generator tripping, or even wide-area blackouts.

Causes of power swing

1. Sudden large disturbances:
   Power swings are typically caused by major events such as:
   • Three-phase faults on transmission lines
   • Sudden disconnection of large loads
   • Generator tripping or synchronization issues
   • Switching operations or breaker misoperations
2. Imbalance in mechanical and electrical power:
   When a generator’s mechanical input (from the turbine) does not match the electrical output after a disturbance, its rotor begins to accelerate or decelerate, causing angular swings.
3. Weak interconnections:
   In a poorly meshed grid or long-distance transmission lines, swings are more likely due to less damping and delayed recovery from disturbances.

Behavior and detection

Power swings cause cyclical variations in electrical quantities, such as:

• Fluctuating current magnitudes
• Rapid voltage changes
• Oscillating real and reactive power flow

These variations resemble those caused by faults, making them difficult to distinguish without proper protection settings. Therefore, distance relays and protective devices are specially configured to detect and block tripping during stable power swings and only act during actual faults.

Some protection relays use a “power swing blocking (PSB)” feature, which allows them to ignore swings and avoid false tripping during stable conditions.

Impact on system stability

• Stable power swing:
  If the system has good damping, the rotor angles gradually settle back to normal, and the system remains stable.
• Unstable power swing:
  If damping is low or the disturbance is too severe, the swing grows, leading to loss of synchronism and possible islanding or blackouts.

To manage swings, system planners use:

• Stability studies and simulations
• Power system stabilizers (PSS)
• FACTS devices for better voltage and power flow control
• Controlled switching and fast fault clearing

Conclusion:

A power swing is a dynamic condition in power systems where generator rotor angles oscillate due to disturbances like faults or sudden load changes. These swings cause fluctuations in power flow, and if not damped, they can lead to system instability. Recognizing and managing power swings using proper protection, controls, and system design is essential for maintaining grid stability and reliability.