FERRORESONANCE IN DISTRIBUTION TRANSFORMERS BASIC INFORMATION AND TUTORIALS

Ferroresonance is the name given to the phenomenon where the exciting reactance of the transformer can become nearly equal to the capacitive reactance of the line to ground, forming a resonant circuit. Such a resonant circuit can distort the normal line impedance to ground so that one line of a 3-phase circuit can rise to a destructive voltage.


Distribution transformers are generally considered as transformers of 500 kVA, and smaller 67,000 V and below, both single-phase and 3-phase. Older installations are primarily pole-/platform-mounted units. Newer installations are frequently pad-mounted units.

Typical applications are for supplying power to farms, residences, public buildings or stores, workshops, and shopping centers. Distribution transformers have been standardized as to high- and low-voltage ratings, taps, type of bushings, size and type of terminals, mounting arrangements, nameplates, accessories, and a number of mechanical features, so that a good degree of interchangeability results for transformers in a certain kVA range of a given voltage rating. They are now normally designed for 65 C rise.


Such a ferroresonance practically never occurs in a normal circuit configuration with the transformers loaded, but it can exist under a combination of the following circumstances which usually occur only during switching of a 3-phase bank or blowing of a fuse in one line:


1. System neutral grounded, ungrounded transformer neutral

2. No load on the transformer

3. Relatively large capacitance line-to-ground such as may exist in cable circuits (underground distribution) or very long overhead lines (although ferroresonance can be and has been corrected by adding still more capacitance which presumably throws the combination out of resonance again)

Although ferroresonance has been studied at some length, it still does not seem possible to reliably predict its occurrence. Experience indicates that it is possible to prevent ferroresonance during switching on a transformer bank if all three transformers are resistance-loaded to 15% or more of their rating, or if special switches are used to assure that the three lines close simultaneously.

SUPERCONDUCTING TRANSFORMERS BASIC INFORMATION

Low-temperature superconducting (LTS) transformers were first proposed in the 1970s, and designed to operate at 6◦K to 14◦K (−268◦C to −260◦C). The invention of high temperature superconducting (HTS) materials increased the prospects for superconducting units designed to operate between 20◦K to 77◦K. A three-phase 630 kVA, 18.7 kVl−−l/420 Vl−−l demonstration transformer based on HTS winding technology is presently under test on the power grid.

Superconducting transformers have about half the weight of conventional oil-filled transformers, and they require less space due to their reduced size, which is important for urban locations. They are nonflammable and employ environmentally benign liquid nitrogen as the cooling medium.

But perhaps the key advantage is their capability for overcapacity operation, due in part to the low temperatures at which HTS windings operate. Heat is the principal enemy of the paper-oil electrical insulation system of conventional power transformers.

HTS transformers operate in the ultra cold range of 20◦K to 77◦K (−253◦C to −196◦C), where insulation materials will not degrade. They can operate up to twice rated power, and they have a low series impedance, improving voltage regulation.

Conventional transformers typically have ηpower = 99.3% to 99.7% for the 30 MVA class. HTS transformers have a higher efficiency, to the extent that the reduced loss in a HTS unit can more than pay for its initial capital cost over its lifetime.

HTS units have a similar construction to the liquid-filled conventional transformer: the magnetic core carries super conducting windings cooled by liquid nitrogen, which is the only safe and low-cost cryogen available in liquid form in the 20◦K to 77◦K temperature range.

The superconducting windings are manufactured either as wires or as flat tapes using BSCCO-2223 material. To date there are not many data available concerning the reliability of HTS units. Most publications concede that a superior, cost-effective HTS transformer technology might take two decades to become available.