IEC 60186: Voltage Transformers — Ferroresonance Is the Unique Fatal Risk

Voltage Transformers: The “Ears” of Protection Systems — Just as Critical as CTs, Often Overlooked

IEC 60186 specifies voltage transformer requirements. If CTs are the “eyes” of protection, VTs are the “ears” — distance protection, directional overcurrent, and synchro-check devices all depend on VT voltage signals.

Ferroresonance: a VT-unique risk. In unearthed-neutral systems, VT inductance and line-to-earth capacitance can form a series resonant circuit. After a single-phase fault clears, the transient can excite ferroresonance — voltages reaching 2–3× rated, lasting seconds, capable of thermally overloading the VT or blowing its fuse. Mitigation: a damping resistor (typically 25–100 Ω, 100–500 W) connected across the open-delta secondary winding.

Voltage Factor: The nameplate parameter that determines survival. 1.2 U_n continuous (effectively earthed systems), 1.5 U_n / 30 s (unearthed), or 1.9 U_n / 8 h (healthy-phase voltage rise during a line-to-earth fault on an unearthed system). Selecting the wrong voltage factor means the VT suffers thermal runaway from overexcitation during a system earth fault.

TN Lab — VT selection goes beyond ratio. Ferroresonance protection and voltage factor are the VT-unique parameters that matter most.