Ferroresonance — TSBC Incident Case Study

Parameter	Normal Condition ✓	Ferroresonance ⚡
Primary Voltage	14,400 V L-N (8,314 V/phase)	Phase A floating → chaotic oscillation
All 3 Fuses	Closed — symmetrical	Phase A open — L-C path formed
Transformer Load	~250 kVA (50%) — damped	~15 kVA (3%) — near zero damping
Secondary L-N	347 V (rated)	~1,561 V (4.5× rated) ▲▲
Secondary L-L	600 V (rated)	~2,704 V (4.5× rated) ▲▲
Waveform	Clean 60 Hz sinusoid	Distorted · Rich harmonics · Chaotic
HV Cable Capacitance	Balanced, acts as shunt	In series with Lm → resonant tank!
Transformer	Linear, quiet operation	Core saturating · Audible vibration / hum
Surge Arresters	Normal standby	Exploded (not rated for sustained OV)
Revenue Meter	Normal	Destroyed
Transformer Fire	N/A	None recorded ✓
Worker Injury	N/A	None recorded ✓

⚠ What Happened — BC Municipal Facility, 2021

A 500 kVA, 14.4/25 kV pad-mount transformer fed via underground HV cables was being de-energized during maintenance. The building was largely shut down — the transformer was carrying only ~3% of rated load.

Root cause: The Phase A fuse cutout was opened individually (single-phase switching) while phases B and C remained energized.
Mechanism: Phase A's HV cable capacitance (~26.5 nF) appeared in series with the transformer's magnetizing inductance Lm — forming a resonant L-C circuit.
Amplification: Near-zero load meant virtually no resistive damping — the oscillation sustained at 4.5× rated voltage.
Outcome: Surge arresters exploded, revenue meter destroyed, building equipment damaged. Transformer and workers were unharmed.

⚡ The L-C Resonance Mechanism

HV underground cable (25 kV class) has significant shunt capacitance — typically 20–30 nF/phase per km at 25 kV.
Phase A fuse opens → cable for Phase A loses its source connection.
That cable's C now connects in series with the transformer's magnetizing inductance Lm via inter-phase coupling (C_ab, C_ac).
Resonant frequency f_r = 1/(2π√(Lm·C)) drifts near 60 Hz as the core alternately saturates (Lm↓) and recovers (Lm↑).
Each saturation event "pumps" energy back, sustaining 4.5 p.u. oscillations on all phases.

⚡ Why 3% Load = Maximum Danger

Load resistance R is the only damping element in the L-C circuit:

Heavy load → small R → large damping → oscillations collapse quickly.
Light load → large R → minimal damping → oscillations build and sustain.
Per Jacobson (2003): as little as 5% resistive load was enough to eliminate the ferroresonant state in a comparable underground-fed 13.8/24 kV system.
In this incident, load was ~3% — well below that threshold, with essentially zero damping.
Five-legged core transformers (common in padmounts) are especially susceptible due to inter-phase magnetic coupling.

How to Prevent Ferroresonance

1. Three-phase switching only: Use gang-operated, 3-phase cutouts or switches — never open one fuse at a time on underground HV cable runs.
2. Verify load before switching: Confirm the transformer carries >5–20% of rated load, or temporarily connect a resistive load bank before single-phase operations.
3. Follow utility switching procedures: Never energize / de-energize phases individually on cable-fed padmounts.
4. Design review: Long HV cable runs increase C — flag underground runs >1 km for ferroresonance risk assessment on lightly loaded transformers.
5. Arrester selection: Install arresters rated for ferroresonant duty, or add ferroresonance-suppression (ferro-damping) resistors.

TSBC Findings & Takeaways

Single-phase switching of the HV fuse cutout on a near-unloaded transformer directly caused the ferroresonance event.
The underground HV cable run provided sufficient capacitance to form and sustain the resonant L-C path.
The ~3% load at time of switching provided no meaningful circuit damping.
Surge arresters exploded — they were not designed for sustained ferroresonant overvoltage duty.
Revenue meter was destroyed by the overvoltage event.
No transformer fire was recorded. No worker injuries were recorded.

Source: TSBC Incident Summary II-1126869-2021 (#20405) · BC, 2021

The Resonance Math — Scaled to This System

Resonant frequency: f_r = 1 / (2π√(Lm · C))
HV cable C ≈ 26.5 nF/phase (2.3 km, #2 AL, 25 kV class per Jacobson 2003).
500 kVA transformer excitation current ~1–2% → effective Lm ≈ several hundred H (non-linear, collapses in saturation).
At saturation onset, Lm drops sharply → f_r briefly sweeps through 60 Hz → resonance ignites.
Peak overvoltage = 4.5 p.u. → L-N: 347 × 4.5 = 1,561 V · L-L: 600 × 4.5 = 2,704 V.
Suppression: R_load < V²_rated / (0.05 × kVA_rated) → 5% load ≈ 25 kW resistive for this 500 kVA unit.

⚡ Ferroresonance — Transformer Overvoltage Case Study

What is Ferroresonance?