Ferroresonance and Its Causes

Ferroresonance and its causes
Due to the multitude of various sources of capacitances and non-linear inductances in

a real power network and the wide range of operating conditions, configurations under
which ferroresonance can occur are endless. Experience has, however, made it
possible to list the main typical configurations that may lead to ferroresonance.
A six most common examples are explained below:
1. Voltage transformer energized through grading capacitance of one (or more) open
CB(s)
2. Voltage transformers (VT) connected to an isolated neutral system
3. Transformer accidentally energized in only one or two phases
4. Voltage transformers and HV/MV transformers with isolated neutral
5. Power system earthed through a reactor
6. Transformer supplied by a highly capacitive power system with low short-circuit
power
1. Voltage transformer energized through

grading capacitance of one (or more) open
circuit-breaker(s)
In EHV, certain switching operations (padlocking a bus coupler or switched busbar
circuit-breaker, removal of a fault on a busbar section) can drive voltage
transformers (VT) connected between phases and earth into ferroresonance.
These configurations can be illustrated by the circuit in figure 1. Opening of circuit

breaker D initiates the phenomenon by causing capacitance C to discharge through
the VT which is then driven into saturation. The source delivers enough energy
through the circuit breaker grading capacitance C d to maintain the oscillation.
Figure 1 Ferroresonance of a voltage
transformer connected in series with an open circuit breaker
Capacitance C corresponds to all the capacitances to earth of the VT and the

connection supplied by means of the grading capacitances of the open circuit-
breaker(s).
Ferroresonance is of the subharmonic type.
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2. Voltage transformers (VT) connected to an

isolated neutral system
This earthing system can be chosen, can result from the coupling of an isolated
neutral emergency source or from a loss of system earthing. Transient overvoltages or
overcurrents due to switching operations on the power system (load rejection, fault-
clearing etc.) or to an earth fault, can initiate the phenomenon by driving into
saturation the iron core of one or two of the VTs of the parallel ferroresonant circuit in
figure 6.
Ferroresonance is then observed both on the phase-to-earth voltages and on the

neutral point voltage (VN).
Figure 2 Ferroresonance of a VT between phase and earth in an isolated neutral system
The neutral point is displaced and the potential of one or two phases rises with respect
to earth, which may give the impression of a single phase-to-earth fault in the system.
Overvoltage values may exceed normal phase- to-phase voltage under steady
state condition, and cause dielectric destruction of the electrical equipment.
Depending on the relative values of the magnetizing inductance of the VT and
the capacitance C0, ferroresonance is fundamental, subharmonic or quasi-periodic.
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3. Transformer accidentally energized in only

one or two phases
A few examples of configurations at risk are given in figure 3 below. These
configurations can occur when one or two of the source phases are lost while the
transformer is unloaded or lightly loaded, as a result of a fuse blowing on an MV
power system, of conductor rupture or of live works.
For example when commissioning a remote controlled breaking cubicle.

Figure 3 Examples of unbalanced systems at risk from ferroresonance
The capacitances can be in the form of capacitance of underground cable or an
overhead line supplying a transformer whose primary windings are wye connected
with isolated or earthed neutral, or delta connected.
For example the series ferroresonant circuit is made up of the connection in series of
the phase to earth capacitance (between circuit breaker and transformer) of the open
phase and the magnetizing impedance of the transformer. The modes are
fundamental, subharmonic or chaotic.

The phase-to-phase and phase-to-earth capacitances, the primary and secondary
windings connections, the core configuration (three single-phase, free flux or forced
flux), the voltage source system neutral earthing (solidly earthed, earthed,
isolated) and the supply mode (one or two phases energized) are all factors involved
in the establishment of a given state. Isolated primary neutral is more susceptible to
ferroresonance.
To avoid such risks, use of multi-pole breaking switchgear is recommended.
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4. Voltage transformers and HV/MV transformers

with isolated neutral
Ferroresonance may occur when the HV and MV neutrals are isolated, and unloaded
VTs are connected on the MV side between phase and earth (see fig. 4a).
When an earth fault occurs on the HV side upstream from the substation transformer,
the HV neutral rises to a high potential. By capacitive effect between the primary and
secondary, overvoltages appear on the MV side, and may trigger ferroresonance of
the circuit made up of the voltage source E0, the capacitances Ce and C0 and
the magnetizing inductance of a VT (see fig. 4b ).
Figure 4 Ferroresonance of a VT between phase and earth with an isolated neutrals source
transformer
Where:
E0 zero-sequence voltage on the HV side

Ce capacitance between HV and MV winding
C0 zero-sequence capacitance of the MV power system
Once the HV fault has been removed, the voltage of the HV neutral due to a natural
unbalance of the system, may be enough to sustain the
phenomenon. Ferroresonance is fundamental.
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5. Power system earthed through a reactor

The two configurations in figure 5 are susceptible to ferroresonance between an
inductance connected between neutral and earth, and the capacitances to earth of the
network.
As regards LV power systems with isolated neutral, standards recommend (IEC 364)
or stipulate (NF C 15-100) implementation of a Permanent Insulation Monitor (PIM).
Permanent Insulation Monitor Vigilohm, by

Schneider Electric
Some PIMs measure insulation impedance of a power system by injecting direct

current between the system and the earth. Their impedance is mainly inductive (low
impedance for the direct current, and high impedance at power frequency).
They can be a factor contributing to ferroresonance.

Figure 5 Ferroresonance in the case of a system earthed through a reactor
Overvoltages may cause sufficient potential rise of the neutral point to initiate
ferroresonance between the inductance of the PIM and the capacitances to earth of
the network
(see fig. 5a ).
In MV, in order to limit earth-fault currents and help the fault to self-extinghish, a coil of
inductance L (such that 3LC002 = 1 where C0 is the zero-sequence
capacitance of the MV power system and 0 the power pulsation) is connected
between the MV neutral of an HV/MV transformer and earth (Petersen coil).
Excitation and start of resonance of the circuit consisting of series connection
of inductance L and capacitance 3C 0 may occur in the following cases:
1. HV neutral of the HV/MV transformer earthed, and HV fault flowing through the
earth conductor of the substation,
2. Iron core saturation of the HV/MV transformer,
3. Transformer design dissymmetry,
4. Natural dissymmetry of the capacitances (C 1, C2, C3 in figure 5b ) to earth.
This may result in saturation of the iron coil, thus initiating or sustaining
ferroresonance.
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6. Transformer supplied by a highly capacitive

power system with low short-circuit power
Ferroresonance may occur when an unloaded power transformer is suddenly
connected to a low short-circuit power source compared with transformer rated
power through an underground cable or a long overhead line.
This is the case, for example, on return to service in an MV (underground cable) urban
or industrial power networks, but also in very extended rural public MV power
networks (see figure 6 ) or where underground cables are increasingly used (reliability
and aesthetics).
Figure 6 Equivalent diagram of unloaded power transformer supplied by a capacitive system
This parallel ferroresonance (capacitance parallel-connected on the transformers

magnetizing inductance) is normally three- phase, of the fundamental or of the quasi-
periodic type.
In short:
1. Configurations under which ferroresonance can occur are endless.

2. There are many different types of ferroresonance: single-phase, three-phase,
common mode, differential mode.
3. Experience, however, makes it possible to identify some configurations at risk
which command some attention. These are:
a. Voltage transformer connected between phase and earth on an isolated neutral
system,
b. Transformer fed through long and/or capacitive lines,
c. Fuse protection, blowing of which results in non-multi pole breaking,
d. Unloaded or lightly loaded power or voltage instrument transformer.
4. The phenomena most likely to trigger ferroresonance are:
a. Switching operation of capacitor banks and unloaded lines,
b. Insulation faults,
c. Lightning,
d. Switching operation of unloaded transformers.
Preventing or damping ferroresonance

A number of practical measures can be taken to prevent ferroresonance, whose
overvoltages, overcurrents and distortions wave forms result in thermal and
dielectric stresses which may be dangerous for electrical equipment (failure,
reduction in performance and lifetime of insulators).
The various methods used are based on the following principles:
1. Avoid, by proper design and/or switching operations, configurations susceptible

to ferroresonance. This may involve prohibiting certain system configurations,
and/or certain power system switching operations and/or certain switchgear.
2. Ensure that system parameter values are not included (even temporarily) in an
area at risk and if possible provide a safety margin with respect to danger areas.
3. Ensure that the energy supplied by the source is not sufficient to sustain the
phenomenon. This technique normally consists of introducing losses which damp
out ferroresonance when it occurs.
Publication 71 of the IEC states that temporary ferroresonance (and resonance)
overvoltages shall be prevented or limited (by one of the above means). They shall
not normally be considered as the basis for the surge arrester rated voltage, or for the
insulation design unless these remedial measures are not sufficient.
This means that the insulation co-ordination procedure does not normally take into
account the overvoltage levels, and that, consequently, surge arresters (whose
residual voltage is usually higher than the overvoltages due to ferroresonance) do not
theoretically provide protection against it.
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Reference // Ferroresonance by Ph. Ferracci; Schneider Electric

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Ferroresonance and its causes

Due to the multitude of various sources of capacitances and non-linear inductances in

A six most common examples are explained below:

1. Voltage transformer energized through

These configurations can be illustrated by the circuit in figure 1. Opening of circuit

Capacitance C corresponds to all the capacitances to earth of the VT and the

Ferroresonance is of the subharmonic type.

2. Voltage transformers (VT) connected to an

Ferroresonance is then observed both on the phase-to-earth voltages and on the

3. Transformer accidentally energized in only

For example when commissioning a remote controlled breaking cubicle.

fundamental, subharmonic or chaotic.

To avoid such risks, use of multi-pole breaking switchgear is recommended.

4. Voltage transformers and HV/MV transformers

E0 zero-sequence voltage on the HV side

5. Power system earthed through a reactor

or stipulate (NF C 15-100) implementation of a Permanent Insulation Monitor (PIM).

Permanent Insulation Monitor Vigilohm, by

Some PIMs measure insulation impedance of a power system by injecting direct

They can be a factor contributing to ferroresonance.

6. Transformer supplied by a highly capacitive

Figure 6 Equivalent diagram of unloaded power transformer supplied by a capacitive system

This parallel ferroresonance (capacitance parallel-connected on the transformers

1. Configurations under which ferroresonance can occur are endless.

Preventing or damping ferroresonance

The various methods used are based on the following principles:

1. Avoid, by proper design and/or switching operations, configurations susceptible

Reference // Ferroresonance by Ph. Ferracci; Schneider Electric

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