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.
Causes
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:
a.
Overvoltages
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.
Reference:
IEEE C57.105-1978. Guide for
Application of Transformer Connections in Three-Phase Distribution Systems
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