Ferro-Ressonance

DOE-HDBK-1092-2004

iii. Transformer size that permits saturation of the iron core at the operating voltage

iv. Transformer unloaded or very lightly loaded.

Prevention or control of ferro-resonance may be accomplished by any of the following

measures:

1. Using a wye-wye transformer connection with both neutrals grounded and tied to the system

neutral.

2. Using only phase-to-neutral (not phase-to-phase) transformer connections for single-phase

transformers.

3. Limiting length of underground cable between transformers and single-pole or poorly

synchronized three-pole switching devices.

4. If single-pole or poorly synchronized switching devices must be used, ensuring that

transformer and underground cable are loaded in excess of 2% resistive load of the

transformer capacity.

5. If transformer primary is ungrounded-wye, temporarily grounding the neutrals of the

transformers being switched.

6. Installing close-coupled, high-speed, three-pole switching devices to minimize the duration

of the single-phase condition during opening and closing of the circuit.

7-24