How to Prevent Ferroresonance in Voltage Transformers?

In the daily operation of medium and high-voltage grids, the destruction of Voltage Transformers (VTs)—also known as Potential Transformers (PTs)—due to ferroresonance remains a prevalent technical challenge. Understanding its underlying mechanisms and implementing effective preventive measures is critical for ensuring the operational safety of switchgear and other distribution equipment.

What is Voltage Transformer Ferroresonance?

In ungrounded neutral systems, when the primary side of a voltage transformer is connected to the grid, its magnetizing reactance forms a parallel circuit with the system’s coupling capacitance to ground, as illustrated in the system model below:

As shown, the coupling capacitance on the left consists primarily of the dielectric between the phase conductors and the ground. On the right, the magnetizing reactance of the potential transformer varies according to the magnetic flux through its core. This combination creates a non-linear LC circuit where simple voltage transients—such as switching overvoltages or ground faults—can trigger resonance frequencies.

Formation Mechanism and Hazards of Voltage Transformer Resonance

Once ringing initiates, the voltage across the magnetizing reactance and the coupling capacitance components can escalate rapidly. If the VT is lightly loaded, this ringing becomes difficult to dampen. In such cases, the VT’s burden plays a vital role in limiting the amplitude of oscillatory currents, as the load resistance acts as a shunt, diverting a portion of the current to the ground (a phenomenon corroborated by load-limit charts in the IEEE Red Book).

During oscillation, the current can drive the magnetizing force into a state where the VT core reaches saturation. When saturated, the VT’s reactance to ground decreases, leading to a surge in the excitation current through the primary winding. As the sine wave approaches its zero-crossing, the VT may come out of saturation, but in low-loss systems, the charge stored in the system’s coupling capacitance remains high.

As the polarity of the sine wave reverses, the process repeats itself. During saturation intervals, the excitation inrush current through the VT primary can far exceed rated values. Since this current often fails to reach the instantaneous trip threshold of primary fuses, they rarely blow in time to prevent damage. Consequently, these sustained current surges typically lead to insulation failure, short circuits, or the complete burnout of the transformer.

Common Mitigation Strategies for Voltage Transformer Ferroresonance

To suppress the amplitude of resonance, industry standards typically involve introducing artificial damping into the VT circuit, either on the primary or secondary side:

1. Open-Delta Connection (Broken Delta): Connecting the secondary windings in an open-delta configuration with a damping resistor. A common rule of thumb is to select a resistor with a power rating equal to 50% of the rated VA of a single VT.
2. Secondary Parallel Resistors: Connecting resistors in parallel across the secondary leads of each phase. Potential transformer suppliers generally recommend precise resistance values tailored to specific models.
3. Installation of Primary Neutral Dischargers (Non-linear Resistors): These are connected between the VT primary neutral point and ground. Typically utilizing Silicon Carbide (SiC) non-linear resistors, they provide low resistance during saturation to limit voltage and absorb resonance energy. They provide effective damping and inrush current limitation under various conditions, including normal operation (high impedance), single-phase grounding (low impedance), or arcing grounds.

Conclusion

While the onset of ferroresonance can be stochastic and elusive, the potential threat it poses to electrical infrastructure cannot be ignored. In the stages of medium voltage switchgear design and equipment selection, accurately forecasting the system’s operating environment and configuring appropriate damping measures—such as high-performance damping resistors or primary neutral dischargers—is essential to effectively suppress ringing, protecting VTs from excitation inrush damage and extending their service life.

As a specialized instrument transformer manufacturer, we recommend performing a rigorous scientific evaluation of VT excitation characteristics and thermal stability capacity during the early stages of product selection, incorporating simulations of potential site conditions to match the most suitable suppression accessories. Through proactive technical support and precision product matching, a more robust and reliable power distribution protection system can be established.