How does inter-area oscillation affect grid stability?

Short Answer:

Inter-area oscillation occurs when large groups of generators in one part of the power system swing against generators in another area due to disturbances like sudden load changes or faults. These oscillations are usually low-frequency (0.1 to 1 Hz) and, if not damped quickly, can cause voltage fluctuations, power swings, and even system separation or blackouts.

Such oscillations reduce grid stability because they challenge the balance of power flow between interconnected areas. If control systems fail to respond, they may lead to loss of synchronism, damaging equipment and threatening system reliability.

Detailed Explanation:

Inter-area oscillation and its effect on grid stability

In large interconnected power systems, especially those covering vast geographical areas, different regions operate with groups of generators. These generator groups are supposed to work in synchronism, sharing the load and maintaining system frequency. However, when a disturbance like a fault, sudden load variation, or switching event occurs, the mechanical and electrical power balance gets disturbed. This leads to a phenomenon called inter-area oscillation, where the rotor angles of generator groups in one area begin to swing back and forth against those in another.

These oscillations are typically low-frequency (0.1 to 1 Hz) and can last for several seconds. If not controlled, they grow in magnitude and result in unstable power transfer, threatening the security and reliability of the entire power grid.

How Inter-Area Oscillation Affects Grid Stability:

  1. Power Flow Instability:
    • Oscillations affect the active power exchange between areas.
    • Unstable swings may cause power flow to reverse direction rapidly, stressing transmission lines and increasing the risk of overloads or trips.
  2. Loss of Synchronism:
    • If the oscillation increases beyond limits, generators can lose synchronism, i.e., they rotate out of phase with others.
    • This can lead to the islanding of a region or full system collapse.
  3. Voltage Fluctuations:
    • Oscillating generator output causes voltage instability at different points in the grid, especially at weak buses.
    • Sensitive equipment and industrial processes may be affected.
  4. Increased Line Loading:
    • The repeated shifting of power due to oscillations can overload lines, leading to protective relays tripping and disconnection of lines.
  5. Reduced Power Transfer Capability:
    • To avoid oscillation-related instability, operators may have to reduce the power transfer limits across critical tie-lines, which limits economic operation and reserve sharing.
  6. Poor Damping and Amplification:
    • If the system has poor damping (e.g., due to low system inertia or inadequate control response), the oscillation amplitude may increase.
    • This is especially common when more inverter-based resources (like solar and wind) are used without proper damping systems.

Typical Causes of Inter-Area Oscillations:

  • Long transmission lines connecting distant areas
  • Weak interconnections between regions
  • Sudden switching operations or generator trips
  • High penetration of power electronics-based generation
  • Poorly tuned power system stabilizers (PSS)

Mitigation Techniques:

  1. Power System Stabilizers (PSS):
    • Installed on generators to inject a damping signal into the excitation system and suppress oscillations.
  2. Flexible AC Transmission Systems (FACTS):
    • Devices like STATCOM, SVC, and TCSC can control power flow and add damping to suppress oscillations.
  3. Wide Area Monitoring Systems (WAMS):
    • Use phasor measurement units (PMUs) to detect and analyze oscillations in real-time.
  4. Better Grid Planning:
    • Strengthening inter-area connections and balancing generation sources across regions.
  5. Energy Storage Systems:
    • Batteries or flywheels can inject or absorb power quickly to help damp oscillations.
Conclusion:

Inter-area oscillations significantly affect grid stability by causing power flow disturbances, synchronization loss, and voltage problems between regions. These low-frequency swings, if left uncontrolled, can escalate into major faults or blackouts. To ensure system stability, grid operators must use damping devices like PSS and FACTS, strengthen inter-area links, and monitor real-time oscillations using modern control systems. Effective handling of these oscillations is essential for the reliable and secure operation of interconnected power systems.