Why do voltage transformers exhibit ferroresonance?

Under normal circumstances, the core of a voltage transformer is unsaturated, and the coil inductive reactance is greater than the line capacitive reactance, so ferroresonance will not occur. However, when the fault disappears, or when an unloaded busbar is connected, the line is grounded, or lightning strikes, the three-phase potential changes, and the inrush current of the capacitor flows through the voltage transformer coil. The current on the voltage transformer increases, causing the coil to saturate and the inductance value to decrease rapidly. When Xl = 1/Xc, ferroresonance occurs.

voltage transformers

The diagram above shows a typical substation busbar wiring diagram, where L is the magnetizing inductance of the instrument transformer, L0 is the busbar-to-ground inductance, and C0 is the busbar-to-ground capacitance.

Therefore, when one busbar is shut down in a double busbar connection, if the busbar circuit breaker has resonance conditions, the primary isolating switch of the busbar transformer should be opened first, and then the circuit breaker should be opened to prevent the ferromagnetic resonance between the bus tie switch's disconnect capacitance and the busbar transformer.

Reasons for Ferromagnetic Resonance in Voltage Transformers

In ungrounded neutral systems, the ferroresonant phenomenon in voltage transformers (PTs) is mainly related to the nonlinear inductance characteristics of the voltage transformer and the LC circuit formed by the system capacitance. The following is a detailed explanation:

Nonlinear Inductance Characteristics of Voltage Transformers

Under normal operation, the magnetization curve of the voltage transformer core is in the linear segment, and the inductance value remains relatively stable. However, when certain disturbances occur in the system, such as a single-phase ground fault, the voltage of the faulty phase decreases, while the voltage of the non-faulty phases increases, leading to core saturation. After core saturation, the inductance value of the voltage transformer decreases with increasing current, exhibiting nonlinear inductance characteristics.

Capacitors and Nonlinear Inductance Forming a Resonant Circuit

In ungrounded neutral systems, there is distributed capacitance between the conductor and ground, and stray capacitance between the phases of the busbar and the line. These capacitances, together with the inductance of the voltage transformer, form an LC resonant circuit. When the JDZW-35 voltage transformer core saturates, the change in its inductance value causes a change in the natural frequency of the circuit. When the frequency of the power supply is equal to or close to the natural frequency of the LC circuit, ferromagnetic resonance may be excited.