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C U R R E N T A N D VO LTA G E I N S T R U M E N T T R A N S F O R M E R S
16
I N S T R U C T I O N S F O R I N S TA L L AT I O N , U S E A N D M A I N T E N A N C E
Appendix 3
Damping ferroresonance for voltage transformer type range TJ
Technical background
Ferroresonance is a phenomenon usually characterized by
over-voltages and very irregular wave shapes and is associ-
ated with the excitation of one or more saturable inductors
through capacitance in parallel with nonlinear inductor.
The saturable inductor is usually present in the form of an in-
strument transformer, power transformer or reactor which
utilizes an iron core.
Ferroresonance of single-pole insulated transformers in un-
earthed network is one of the most common ferroresonance
cases. Depending on the supply voltage, capacitance and in-
ductance the oscillation can be either periodic (over- or
sub-harmonic or with fundamental frequency) or aperiodic.
Using damping resistor or VT Guard in the residual voltage
secondary, shown in figure below, can considerably reduce
the risk for ferroresonance.
There is an additional factor that can in some cases reduce
or totally eliminate the risk for ferroresonance and it is
over-voltage factor. According to IEC standard is the rated
over-voltage factor 1.9xUn/ 8h. Higher rated over-voltage
factor shifts the operating point towards lower flux values
of voltage transformer. It results in smaller sensitivity of
transformer. Some kind of transients usually initiates fer-
roresonance.
Recommendation
Rated voltage factor: We recommend using the voltage
transformers with the over-voltage factor in the range (2.5-
3) xUn/8h. We cannot guarantee the value of the over-volt-
age factor if the requirements for the secondary winding are
too high.
Voltage of residual
Value of Rdamp
winding
100:3 V
22 Ω
27 Ω
110:3 V
Tab. 5. Recommended values of damping resistor
Power Dissipation
450 W
450 W
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