Worthy of mention //
Ferroresonance occurs when line capacitance resonates with the magnetizing reactance of a core while it goes in and out of saturation.
Ferroresonance is usually associated with potential transformers, which are instrument transformers that are used to develop voltages used by relays; however, it can also occur with power transformers under special circumstances.
Because these connections are routinely avoided in practice, ferroresonance is not encountered very often and there isn’t much information about it in the literature.
Ferroresonance is worthy of mention, however, because it can utterly destroy a transformer.
The necessary conditions for ferroresonance are established in the system shown in Figure 1.
In the example shown in Figure 1, the ∆-connected tertiary winding of a large three-winding substation transformer supplies a distribution type station-service transformer with a Grd.-Y primary winding. The supply lines to the station-service transformers are through a set of shielded cables. If the cable runs are fairly long, a significant amount of phase-to-ground capacitance may exist.
Each of the inductances shown as L1, L2, and L3, will have instantaneous inductance values that are proportional to the effective permeability of the core at any given instant in time. These inductances form parallel L-C circuits that are in series with one another and in series with the source voltage.
Since L1, L2, and L3 are constantly varying along with the effective permeability of the core, it is almost certain that a series resonant condition will exist at least part of the time during every cycle.
The nonlinear nature of this problem makes mathematical analysis virtually impossible, but the phenomenon has been observed both in the field and experimentally, and the voltages have been measured and recorded.
In the example above, the conditions for ferroresonance can be disrupted by the simple expedient of ∆-connected secondary winding to the station service transformer.
The ∆-connected winding assures that the vector sum of the voltages of all three phases add to zero, stabilizing the neutral point of the Y-connected primary winding and preventing excessive voltage across the windings. The presence of a ∆-connected secondary winding will essentially “snuff out” ferroresonance in this circuit.
Dennis Merchant on ferro-resonance in distribution transformers.
Reference: Power Transformers Principles and Applications – John J. Winders, Jr.
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