Ferroresonance: Tva Experience & Mitigation Methods

Ferroresonance is a phenomenon which can occur in part of a power system where capacitive reactance is in series with the nonlinear inductive reactance of a transformer. When the two values approach each other in magnitude, extremely high voltages can be applied to local equipment and insulation, sometimes resulting in damage and faults.

Over the past 50 years, the Tennessee Valley Authority (TVA) has experienced several ferroresonance events, both in transmission and distribution switchyards.

In transmission switchyards, the ferroresonance has resulted from the series combination of the inductance of bus potential transformers (PT) with grading capacitors across circuit breaker contacts. In most cases the condition has occurred during switching procedures, but in at least one case it followed an automatic bus trip. In the more recent cases, it came as a surprise to some of us that even single-contact circuit breakers can have “grading” capacitors which designers specify to increase the breaker fault interrupting rating.

The mitigation for transmission switchyards has been to: (1) replace breakers with those not having grading capacitors, (2) adding bus capacitance (via coupling capacitor or high-capacitance CCVT), (3) adding secondary resistance loading to the PT secondaries, (4) remove grading capacitors (only if manufacturer approves AND breakers not overstressed), or (5) using PTs with gapped cores. Meanwhile, TVA has developed and communicated operating guidelines on how to minimize (not mitigate) the risk of ferroresonance during switching.

In distribution switchyards, the condition has occurred on 13kV buses fed from delta-connected power transformer windings. A grounding transformer is provided to provide a zero-sequence path for ground faults, but ferroresonance has occurred during switching procedures where the grounding transformer was intentionally disconnected. In most of these cases, three-phase gang-operated switches were used, but in the most recent case, single-phase switches were used.

For three-phase switching, the mitigation for distribution switchyards has been to require the grounding transformer to be connected during the switching procedure. However, for single-phase switching, it has been proven by time-domain transient studies that having the grounding transformers connected is not effective. Note this is regardless of whether a neutral grounding switch (where applied) is open or closed. The most effective way to mitigate ferroresonance on 13kV delta tertiaries seems to be requiring three-phase switching (i.e., circuit breaker or gang-operated disconnect) with the grounding bank connected and its neutral grounded.