Ferroresonance is usually described as an irregular and chaotic type of resonance. This phenomenon occurs because of the nonlinear characteristic of iron-core (saturable) inductors - ferromagnetic material such as a transformer. Ferroresonance is often times associated with unwanted and destructive overvoltages, but has found some helpful applications in Constant Voltage Transformers, which can mitigate power quality problems like voltage sags.
Basically, ferroresonance involves series connection of saturable inductors and system capacitance due to shunt capacitor banks, series capacitors, internal capacitance of transformers, overhead lines and cables. It does not normally occur in distribution systems but certain conditions may be established such as an open phase during single-pole switching. In connection, transformer bank configurations become a significant factor that affects the occurrence of ferroresonance.
Loss of phase/s or having an open phase is the most common condition that makes ferroresonance highly possible to occur (but not always). Some of these events are listed below:
1. One or two distribution cut-out fuses may blow leaving a transformer with one or two phases open. This is also true with single-phase reclosers.
2. Switching manually an unloaded three-phase transformer or transformer bank (cable-fed) where only one phase is closed. Ferroresonance may occur when the first line is closed for energization, or before the last phase is opened on deenergization.
3. Manual switching of an unloaded, cable-fed, three-phase transformer where one of the lines is open either during energization or deenergization.
Moreover, there are ordinary system conditions that help increase the possibility of the ferroresonance phenomenon including, but not limited to:
· Three-phase systems with single-phase switching devices (e.g. cut-out fuses)
· Ungrounded transformer primary connections
· Higher distribution voltage levels – above 15 kV
· Long underground cable circuits
· Manual switching and cable damage during construction of underground cable systems
· Switching of unloaded or lightly loaded transformers
· Low-loss transformers (e.g. amorphous)
· Unstable and weak systems (low short-circuit MVA)
How Does Ferroresonance Occur?
The figure below shows an effectively grounded three-phase source supplying an unloaded three-phase transformer with Delta Primary. In between the connection are single-pole switches and shielded cables, which have considerable capacitance to ground (C).
|Sample Ferroresonance Circuit|
When the switch for phase A is closed, two of the transformer’s phases are energized via the cable capacitances from B-G and C-G. In AC circuits, recall that capacitance appears as a short circuit at the instant of closing. This causes the transformer windings of legs A-B and A-C to draw the normal inrush current.
The transformer iron during the first cycle of applied voltage could saturate due to closing at or near zero voltage and/or due to residual flux in the transformer core. Consequently, saturation produces large current pulse through the transformer windings and capacitances of phases B and C. Then, the transformer iron drops out of saturation leaving an ample trapped charge (voltage) on the cable capacitance. In the succeeding cycles, the transformer may go into saturation in the opposite direction, thus, changing the polarity of the trapped charge on the capacitance.
If the transformer continues to go into and out of saturation, line-to-line and line-to-ground overvoltages shall occur, which can cause over-excitation of the transformer, and failure of arresters and insulation in the transformer or system.
Furthermore, closing the second phase may result to no better condition than as described above. Nonetheless, if all three phases are closed, ferroresonance will cease.
Effects and Indicators
Ferroresonance can usually lead to the following malfunctions that can be noted and measured:
Peak voltages (i.e. line-to-line and/or line-to-ground) may reach up to five or more times the system nominal voltage. Surge arresters, particularly low-voltage types are commonly damaged, which indicates the occurrence of ferroresonance. Other devices such as electronics may also fail due to high voltage.
b. Excessive Noise in the Transformer
This is mainly caused by the magnetostriction of the steel core being driven into saturation. The noise is described as whining, rumbling or rattling and is louder and different than the normal hum of a transformer.
c. Irregular voltage and current wave shapes
Flicker is an example of this abnormality, which considerably affects electronic devices - immediate failure and/or shortens expected life.
d. Transformer Overheating
In cases when the core is saturated repeatedly, the magnetic flux will find reach the tank wall and other metallic components, which are portions of the transformer where the flux is not expected. Heating can cause the bubbling or charring of the paint on the top of the tank.
IEEE C57.105-1978. Guide for Application of Transformer Connections in Three-Phase Distribution Systems