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.



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.



The selection of a cooling system based on liquids permits a greater overload capability. Liquid-filled units are cooled in a variety of ways. Some of them protect the coolant from oxidation by sealing the transformer and inserting inert gas in the air space.

(1) Oil-Immersed Self-Cooled The insulating mineral oil circulates by natural convection within the tank, which has either smooth sides, corrugated sides, integral tubular sides, or detachable radiators.

(2) Oil-immersed self-cooled and forced-air cooled The same as type 1, but the addition of fans increases the rate of heat transfer from the cooling surfaces, thereby increasing the permissible transformer output.

(3) Oil-Immersed Self-Cooled and Forced-Oil–Forced-Air Cooled The rating of an oil-immersed transformer may be further increased by the addition of some combinations of fans and oil pumps.

(4) Oil-Immersed Forced-Oil-Cooled with Forced-Air Cooler Heat transfer from oil to air is accomplished in external oil-to-air heat exchangers with oil pumps and fans.

(5) Oil-Immersed Water-Cooled Cooling water runs through pipes that are in contact with the cooling oil of the transformer. The oil flows around the outside of these pipe coils by natural convection, thereby effecting the desired heat transfer to the cooling water.

(6) Oil-Immersed Forced-Oil-Cooled with Forced-Water Cooler External oil-to-water heat exchangers are used in this type of unit to transfer heat from oil to cooling water.

Depending upon the geometric duct dimensions and the pressure applied by the oil pumps, the oil velocities for laminar flow range from 0.005 m/s to 0.05 m/s. A great disadvantage of mineral oil is its flammability.

For this reason nonflammable synthetic oils were developed, such as those with the brand names Askarel, Inerteen, Pyranol (USA), Permitol (England), Aroclor (France), and Clophen (Germany). Unfortunately, most of these have proven to be undesirable from an environmental and health point of view, and are not used in new transformer designs.



The nominal power efficiency ηpower of a transformer is the ratio of rated real power output to rated real power input: ηpower = Pout/Pin = 1− (Ploss/Pin). Total losses Ploss are the sum of the no-load and load losses. No-load losses consist of eddy-current and hysteresis losses within the core (|˜ic|2 Rc, the loss caused by the core-loss component ic of the exciting current iφ;), ohmic loss |˜iφ|2 Rp, and dielectric loss: that is, all losses that occur at full voltage with the secondary circuit open.

Load losses are |˜ip(t)|2 Rp+|˜is(t)|2 Rs caused by the primary [ip(t)] and secondary [is(t)] load currents. Eddy-current losses also occur, induced by stray fluxes within the solid transformer structure, and similar losses are generated in the windings, varying with the load current.

No-load losses are measured at rated frequency and rated secondary voltage (if the secondary side is the low-voltage side) and are considered to be independent of load. Load losses are measured at rated frequency and rated secondary current, but with the secondary short-circuited and with reduced voltage applied to the primary, the high-voltage side. Load losses can be assumed to vary as the square of the load current.

Most units are not fully loaded all the time, and therefore one defines the energy efficiency of a transformer, where lightly loaded periods are also taken into account during a load cycle. For low-power-efficiency transformers (ηpower < 96%) the loss can be measured from the relatively large difference between the input power Pin and the output power Pout.

However, for high power efficiency units (ηpower > 96%), the errors in measuring Pin and Pout and the small difference between the two make an efficiency determination meaningless. If two current transformers (CTs, maximum errors εCT1 = εCT2 = 5 mA, CT ratio = 20) and two potential transformers (PTs, εPT1 = εPT2 = 0.24 V, PT ratio = 30) as well as two ammeters (εA1 = εA2 = 5 Ma) and voltmeters (εV1 = εV2 = 0.3 V) with full-scale errors of 0.1% are used, then the maximum error in the measured losses for a 25 kVA, ηpower = 98.44%, 240 V/7200 V single-phase transformer at cos φ1 = 1 is #Ploss = (240 V ± εPT1 ± εV1)(5.20835 A ± εCT1 ± εA1) × 20 − 30 (240 V ± εPT2 ± εV2 (3.472 A ± εCT2 ± εA2) = (240.54 V) × (104.367 A) − (7183.8 V) × (3.462 A) = 234.1 W, so that #Ploss/Ploss = ± (234.1/390)100% ≈ 60%.

This means the conventional method of measuring the losses and therefore the power efficiency of high-efficiency units does not produce accurate results, and other methods must be used.



Effects of Overcurrent.
A transformer may be subjected to overcurrents ranging from just in excess of nameplate rating to as much as 10 or 20 times rating. Currents up to about twice rating normally result from overload conditions on the system, while higher currents are a consequence of system faults.

When such overcurrents are of extended duration, they may produce either mechanical or thermal damage in a transformer, or possibly both. At current levels near the maximum design capability (worst-case through fault), mechanical effects from electromagnetically generated forces are of primary concern.

The pulsating forces tend to loosen the coils, conductors may be deformed or displaced, and insulation may be damaged. Lower levels of current principally produce thermal heating, with consequences as described later on loading practices. For all current levels, the extent of the damage is increased with time duration.

Protective Devices. 
Whatever the cause, magnitude, or duration of the overcurrent, it is desirable that some component of the system recognize the abnormal condition and initiate action to protect the transformer. Fuses and protective relays are two forms of protective devices in common use.

A fuse consists of a fusible conducting link which will be destroyed after it is subjected to an overcurrent for some period of time, thus opening the circuit. Typically, fuses are employed to protect distribution transformers and small power transformers up to 5000 to 10,000 kVA.

Traditional relays are electromagnetic devices which operate on a reduced current derived from a current transformer in the main transformer line to close or open control contacts, which can initiate the operation of a circuit breaker in the transformer line circuit. Relays are used to protect all medium and large power transformers.

All protective devices, such as fuses and relays, have a defined operating characteristic in the current-time domain. This characteristic should be properly coordinated with the current-carrying capability of the transformer to avoid damage from prolonged overloads or through faults.

Transformer capability is defined in general terms in a guide document, ANSI/IEEE C57.109, Transformer Through Fault Current Duration Guide. The format of the transformer capability curves is shown in Fig. 10-35.

The solid curve, A, defines the thermal capability for all ratings, while the dashed curves, B (appropriate to the specific transformer impedance), define mechanical capability. For proper coordination on any power transformer, the protective-device characteristic should fall below both the mechanical and thermal portions of the transformer capability curve.

(See ANSI/ IEEE C57.10-38 for details of application.)
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