The term ferroresonance refers to a resonance that involves capacitance and iron-core inductance. The most common condition in which it causes disturbances in the power system is when the magnetizing impedance of a transformer is placed in series with a system capacitor due to an open-phase conductor.

Under controlled conditions, ferroresonance can be exploited for useful purpose such as in a constant voltage transformer. In practice, ferroresonance most commonly occurs when unloaded transformers become isolated on underground cables of a certain range of lengths.

The capacitance of overhead distribution lines is generally insufficient to yield the appropriate conditions. The minimum length of cable required to cause ferroresonance varies with system voltage level.

The capacitance of cables is nearly the same for all distribution voltage levels, varying from 40 to 100 nF per 1000 ft, depending on conductor size. However, the magnetizing reactance of a 35-kVclass distribution transformer is several times higher (curve is steeper) than a comparably-sized 15-kV-class transformer.

Therefore, damaging ferroresonance has been more common at the higher voltages. For delta connected transformers, ferroresonance can occur for less than 100 ft of cable.

For this reason, many utilities avoid this connection on cable-fed transformers. The grounded wyewye transformer has become the most commonly used connection in underground systems in North America.

It is more resistant, but not immune, to ferroresonance because most units use a three legged or five legged core design that couples the phases magnetically. It may require a minimum of several hundred feet of cable to provide enough capacitance to create a ferroresonant condition for this connection.

The most common events leading to ferroresonance are
• Manual switching of an unloaded, cable-fed, 3-phase transformer where only one phase is closed (Fig. 23-28a). Ferroresonance may be noted when the first phase is closed upon energization or before the last phase is opened on de-energization.

• Manual switching of an unloaded, cable-fed, 3-phase transformer where one of the phases is open (Fig. 23-28b). Again, this may happen during energization or de-energization.

• One or two riser-pole fuses may blow leaving a transformer with one or two phases open. Single phase reclosers may also cause this condition. Today, many modern commercial loads will have controls that transfer the load to backup systems when they sense this condition. Unfortunately, this leaves the transformer without any load to damp out the resonance.

• Phase of a cable connected to a wye-connected transformer.

FIGURE 23-28 Common system conditions where ferroresonance may occur: (a) one phase closed, (b) one phase open.

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