Ferroresonance and its mitigation techniques

Ferroresonance is a complex and highly nonlinear electrical phenomenon that occurs in power systems, particularly involving inductive and capacitive components. It arises due to the interaction between the nonlinear magnetizing inductance of power transformers, instrument transformers (e.g., voltage transformers), or reactors, and the system capacitance. This resonance can cause abnormally high voltages, currents, and severe oscillations, which may lead to equipment damage, failure of protective devices, or malfunctions in the power system.

Ferroresonance is different from linear resonance, as it occurs in nonlinear systems (such as transformers), where the inductance varies with the applied voltage. The phenomenon is often triggered by switching operations, fault conditions, or insulation breakdowns in unbalanced power systems.

Conditions for Ferroresonance

Ferroresonance typically occurs under the following conditions:

1. Nonlinear inductance: Power transformers or instrument transformers in the unsaturated and saturated regions.

2. System capacitance: Long power cables, unloaded transformer windings, or capacitor banks.

3. Absence of damping: Limited load on the system, which provides minimal damping, allowing the resonance to grow.

4. System disconnection or switching: Disconnection of one phase of a transformer in an ungrounded or isolated system, creating an imbalance.

Effects of Ferroresonance

Ferroresonance can have several detrimental effects:

- Overvoltage: Voltages can rise to several times the nominal value, stressing insulation and potentially damaging equipment.

- Overcurrent: The resonant condition can lead to extremely high current surges.

- Harmonic Distortion: Nonlinear oscillations can cause excessive harmonic generation, leading to power quality issues.

- Noise and vibrations: Audible noise and mechanical vibrations in transformers and reactors due to the high-frequency oscillations.

- Failure of devices: Protective relays, surge arresters, transformers, and other electrical components can malfunction or suffer catastrophic failure.

Mitigation Techniques for Ferroresonance

To prevent or mitigate ferroresonance, various strategies can be employed, focusing on design, system configuration, and operational controls. Below are the main techniques:

1. Grounding of the Neutral

- Solid grounding of the transformer or distribution network neutral can prevent ferroresonance by keeping the system balanced and providing a path for current during ferroresonance conditions.

- Effective grounding ensures that voltages remain at predictable levels, avoiding abnormal resonant oscillations. Grounding prevents unbalanced conditions that can trigger ferroresonance, especially during single-phase switching events.

2. Use of Damping Resistors

- Damping resistors are inserted into the system to absorb the energy associated with ferroresonance oscillations, reducing the amplitude of resonant voltages and currents. This damping prevents the oscillations from reaching damaging levels.

- Resistors can be installed across open-phase disconnect switches, in series with transformer windings, or in the neutral path.

Application Example:

- In voltage transformer circuits, damping resistors can be used across the secondary winding to suppress ferroresonant overvoltages when the primary voltage is high, particularly during switching operations.

3. Controlled Switching

- Controlled switching devices (also known as pre-insertion resistors or synchronized switching relays) ensure that switching operations (e.g., connecting or disconnecting transformers, lines, or capacitors) occur at the optimal point in the AC cycle, avoiding the conditions that could lead to ferroresonance.

- These devices monitor the system voltage and current and switch at a zero-crossing point or another favorable part of the waveform, reducing the risk of triggering ferroresonant oscillations.

4. Limiting Transformer Energization during Low Load

- Avoid energizing transformers under no-load conditions. Energizing a transformer without sufficient load increases the likelihood of ferroresonance because the inductive reactance of the transformer and the capacitance of the system may interact.

- Ensuring a minimum load on the transformer during energization can mitigate the risk of ferroresonance, providing natural damping to the system.

5. Phase-to-Phase Switching

- In isolated or ungrounded systems, switching operations that leave one phase disconnected (open-phase switching) increase the likelihood of ferroresonance.

- Switching all three phases simultaneously (phase-to-phase switching) reduces the possibility of ferroresonance, as it avoids the unbalanced conditions that often trigger the phenomenon.

6. Installation of Surge Arresters

- Surge arresters can protect the system from the overvoltages caused by ferroresonance by clamping the voltage when it exceeds a certain threshold. While surge arresters don’t prevent ferroresonance, they can help limit the magnitude of overvoltages, thus protecting equipment.

- Care must be taken when selecting surge arresters, as ferroresonance can lead to sustained overvoltages, which could overstress typical surge arresters designed for brief surges.

7. Capacitor Bank Design and Management

- Ferroresonance can occur when capacitor banks interact with nonlinear inductive components. To mitigate this:

- Capacitor banks should be sized and configured to avoid natural resonant frequencies close to the system’s operating frequency.

- Series reactors can be added to capacitor banks to shift the resonance frequency and prevent interaction with transformer magnetizing inductance.

8. Transformer Design and Configuration

- Design transformers with low magnetizing inrush currents and nonlinear characteristics to reduce the risk of ferroresonance.

- Multi-grounded transformer configurations (e.g., grounded wye-wye or wye-delta) are less prone to ferroresonance than ungrounded transformer banks. This design ensures that the system remains balanced during switching events.

9. Avoid Long, Unloaded Cables

- Ferroresonance is often triggered in circuits with long, unloaded cables that create large capacitance. To mitigate this:

- Avoid leaving long cables or transmission lines unloaded or disconnected when transformers are energized.

- Sectionalize cables or install damping devices when the use of long, unloaded cables is unavoidable.

Conclusion

Ferroresonance is a serious and potentially damaging condition in power systems, particularly in systems with significant inductive and capacitive components. Proper system design, including grounding, damping, and controlled switching, plays a key role in mitigating ferroresonance. Awareness of operating conditions such as switching configurations and system load levels is also crucial in preventing ferroresonance from occurring.

By applying these mitigation techniques, utilities and engineers can reduce the likelihood of ferroresonance, protecting their systems from damage and ensuring reliable and stable operation.