Presentation 11.1

© OMICRON 2013 – Instrument Transformer Measurement Forum

Ferroresonance Oscillation – Phenomenon and Mitigation

Erik P. Sperling, PFIFFNER Instrument Transformers, Switzerland

Introduction

This article deals with the phenomenon of ferroresonance oscillations, which may occur under specific conditions in medium voltage and high voltage networks.

For various reasons, permanent optimization of instrument transformers and power transformers has occurred within the last decades. Magnetic flux density has been permanently increased and the market requires equipment that meets higher demands. Because substations are becoming smaller and more modern, the stray capacitance of the system has decreased. Permanently increasing power transmission in existing substations causes the rated system voltage VR to rise to the highest system voltage VM. In addition, the power losses within the transformer itself have been reduced and the burden has decreased to the value found in “low-power” applications (see IEC61859). [2,4]

All the reasons mentioned above have a direct influence on the probability of ferroresonance oscillation occurring within networks.

Theoretical aspects

Ferroresonance oscillation is defined as those complex non-linear oscillations which occur when a magnetic iron core goes into periodical saturation. In principal, this can occur if an inductive component with a ferromagnetic iron core is connected in a resonant circuit together with a capacitance and a power source.

Fig. 1 Simplified LC series resonant oscillation circuit,

without resistive component

Vq(t) Voltage source Vc(t) Voltage drop over capacitance VL(t) Voltage drop over inductance i(t) Circulating current CK Capacitance LH(B(t)) Non-linear main inductance

The non-linear component in figure 1 represents the inductive part. The value of the inductance

depends on the flux density of the magnetic iron core (see formula 1).

( ) ( ( )) Formula 1

The relation between the root-mean square values for the currents and voltages within the oscillation circuit, see figure 1, is illustrated in the diagram in figure 2. The shape of the blue curve VL represents the characteristics of the voltage and current values for the non-linear inductance (magnetization curve). The shape of curve VC, dashed green line, corresponds to the V/I-characteristics of the capacitance CK.

Fig. 2 Current / voltage diagram of the resonance

circuit, as shown in figure 1

The voltage imposed on the inductance consists of the power source voltage minus the voltage drop over the capacitance VL = Vq – VC (green line). The intersection points between the blue curve and the green line are shown as the operating points A and B. Both points represent stable working conditions. For higher capacitance values, the green line rotates in the direction of the x-axis. The intersection point C between the blue curve and the green line defines the unstable operating point. In the case of a very small reduction of the current, the resulting voltage over the inductance VL will decrease to a higher extent (it follows the green line) than is required by the blue curve. This results from the V/I-characteristic of the magnetization curve. From this, it follows that the operating point C jumps to the operating point B and the inductance value decreases to a fraction of its original value. The result is a very fast reversal of the polarity of the electrical charge in the capacitor. The characteristic of the current is capacitive at operating point A and is inductive at operating points B and C. [1]

Presentation 11.2

© OMICRON 2013 – Instrument Transformer Measurement Forum

Classification of ferroresonance oscillations

Ferroresonance oscillations are observed in single-phase networks or for one phase of three-phase systems. Three-phase ferroresonance oscillations are always measured simultaneously in all three phases. Single-phase oscillations as well as three-phase oscillations have different characteristics and appear as a result of different triggering events. For a better understanding, the following discussions are given for both phenomena.

Single phase ferroresonance

During single-phase ferroresonance, only one single voltage transformer is affected. In the case of several voltage transformers, all are affected independently of each other. Typical requirements for the occurrence of such types of oscillation are:

earthed neutral system

capacitive coupling to a switched-off network part

non-grounded network part

Such types of oscillation are caused by the following trigger events:

switching-off of a network part

high load rejection

cut-off of a secondary short-circuit

In the case of an oscillation, the sub-harmonics of the system frequency appearing are typically:

50Hz/60Hz (first order): rare, raised amplitude, very fast dielectric and thermal damage possible

162/3Hz/20Hz (third order): very often,

amplitude in the nominal range, thermal damage possible

10Hz/12.5Hz (fifth order): occurs from time to time, low energy, damage improbable

It is possible to trigger single phase ferroresonance oscillation in test laboratories. For capacitive voltage transformers, the ferro-resonance check is a routine test according to IEC 61869-5. For inductive voltage transformers, ferroresonance tests are mainly used as special tests for experimental purposes or as verification tests for the self-damping characteristic within a specified network condition range.

Three phase ferroresonance

In case such oscillation occurs, all three phases with their voltage transformers are always affected simultaneously. The requirements for the occurrence of such types of oscillations are:

isolated neutral systems

a three-phase group of single-phase inductive voltage transformers

Such types of oscillation are caused by the following trigger events:

switching-on of a network part

disappearance of a single phase-to-earth fault

In the case of such types of oscillation, the following typical behaviour pattern can only be determined during direct measurements and fault analyses: [2]

25Hz/30Hz (second order): always occurs

additional superimposed beat vibration in a range of 0.7Hz up to 7Hz

amplitude is essentially higher than the nominal range

very high energy oscillations

fast dielectric and thermal damage to the transformer

As opposed to single-phase ferroresonance oscillations, simulations or measurements in test laboratories are only possible if very extensive investigations can be made, which is not possible at the present time. No mathematical theory exists which covers all the complex interdependencies and parameters needed to perform simulation calculations and to produce significant results.

Behaviour of ferroresonance oscillations

The behaviour of ferroresonance oscillations can be subdivided into four main fundamental categories. [2]

stationary power-line frequency oscillation

stationary subharmonic oscillation

stationary chaotic oscillation

non-stationary (transient) oscillation

Depending on the substation topography and single-phase or three-phase ferroresonance, different types of curves may be observed.

The examples illustrated in Figure 3 show the three main characteristics of possible ferroresonance behaviour. [3]

In case 1, an additional third subharmonic curve is superimposed on the fundamental oscillation. The resulting voltage amplitude is much higher than under nominal service conditions. As a result, a dielectric breakdown within the insulation system will occur. To save the transformer, a very fast switch-off operation is recommended.

Presentation 11.3

© OMICRON 2013 – Instrument Transformer Measurement Forum

case 1

case 2

case 3

Fig. 3 Example of single-phase ferroresonance behaviour with the characteristic curves

In case 2, a ferroresonance oscillation is illustrated with a resulting voltage amplitude which is lower or equal to that found under nominal service conditions. A thermal overstress will occur, resulting from thermal overheating within the primary winding and the magnetic iron core, which is caused by the additional saturation current. As a consequence, the high-voltage insulation will be aged and a breakdown will occur.

In case 3, an existing ferroresonance oscillation disappears after a certain duration of time. This effect is designated as a self-damping phenomenon. This is always the case when the circuit resonance conditions are no longer fulfilled. Case 3 illustrates optimal behaviour when ferroresonance oscillation occurs.

Results measured in the laboratory

As mentioned in the chapter above, single-phase ferroresonance oscillation can be generated in a test laboratory. In the following, some differing measurement results, observed during investigations at the test laboratory, will be illustrated and described.

Figure 4 shows the results of measurements made on an inductive voltage transformer. No ferroresonance oscillation was detectable during the switching operations. With this combination, the voltage transformer is in ferroresonance-free condition.

Fig. 4 No ferroresonance, channel 1: primary voltage,

channel 2: secondary voltage

The measurement results in figure 5 show an initial ferroresonance oscillation with a self-damping effect after a time of 1.4 seconds. The resonance conditions for a permanent oscillation are no longer fulfilled.

Fig. 5 Self-damping oscillation, channel 1: primary

voltage, channel 2: secondary voltage

A continuous ferroresonance oscillation with an inductive voltage transformer is illustrated in figure 6. The ferroresonance oscillation was not self-damped or damped using an additional damping device. This would cause aging or destroy the transformer. In the case of such an event in a real substation, the transformer would be thermally destroyed over a longer period of time.

Presentation 11.4

© OMICRON 2013 – Instrument Transformer Measurement Forum

Fig. 6 Continuous ferroresonance oscillation, channel

1: primary voltage, channel 2: secondary voltage

Figure 7 shows a measurement using an additional external passive attenuation unit. It was connected to a special additional damping winding which provided the optimal damping voltage. It is noticeable that a high damping current (green curve on channel 4) flows via the attenuation unit. This high current occurs only during the very short damping time. Under standard operating conditions, the leakage-current via the attenuation unit is negligible.

Fig. 7 Passive-damped ferroresonance oscillation,

channel 1: primary voltage, channel 3: secondary voltage, channel 4: current via attenuation unit. (channel 2: not applicable)

For capacitive voltage transformers (CVT), the ferroresonance check is part of routine tests. For example, during this test, the CVT will be connected at 80% of rated voltage and will be short-circuited via a secondary winding for at least 200ms. Simultaneously with the removal of the secondary short-circuit, single-phase ferro-resonance oscillation occurs and must disappear within a specified time (IEC61869-5). Typical behaviour with optimal damping is illustrated in figure 8.

Fig. 8 Ferroresonance check on a capacitive voltage

transformer. Primary voltage: green curve, secondary voltage: blue curve

Countermeasures

In summary, under consideration of the above-mentioned statements, the following conclusions should be respected by the instrument transformer manufacturer as well as by the substation designer and operator.

The mechanical and electrical design is under the control of the instrument transformer manufacturer. Following possibilities are available:

keeping magnetic flux density as low as possible

use of suitable magnetic iron core materials

use of an air gap

providing additional losses within the transformer

When choosing additional external attenuation devices, the manufacturer, as well as the operator, has the possibility of modifying the ferroresonance behaviour of the system. Commonly used additional external damping solutions are:

resistance or/and saturation coils

resonance circuit damping devices

additional burden

electronic damping devices

open-delta winding including damping unit (valid for three-phase)

A damping system which is designed for three-phase ferroresonance oscillation cannot always be used as a single-phase ferroresonance damping system. This also applies to the opposite constellation. The substation designer and operator can have an influence on the behaviour of the network system during the design phase as well as during operation. This could be:

consequent grounding of switched-off network parts

skilful substation design

design of switchgear / grading capacitor

adding capacitors into the network

star-point handling

starting sequence for parts of the network

Presentation 11.5

© OMICRON 2013 – Instrument Transformer Measurement Forum

Because of the very complex and difficult phenomenon of ferroresonance oscillation, close cooperation between the manufacturer and the operator is essential in order to attain optimal impact on the safety of the network system as well as on the safety of the inductive instrument transformers.

Literature

[1] IEH Leibnitz Universität Hannover: Oberstufenlaboratorium Energieversorgung / Hochspannungstechnik Versuch Nr. 4.:Kippschwingungen; Skript; WS2010/2011

[2] Bräunlich et al.: Ferroresonanzschwingungen in Hoch- und Höchstspannungsnetzen, Teil 1-4: Bulletin SEV/AES 2006 - 2009

[3] Erik Sperling; Modern inductive instrument transformers for new challenges; ITMF Symposium Brand; 2011

[4] Marcel Krasner; Ferro-Resonanz im Hoch und Höchstspannungsnetz; TAE Symposium Stuttgart; 2012

About the Author

Erik Sperling studied electrical energy systems and high-voltage technology at the University of Karlsruhe (Germany) and has now been working for a long time in the field of high-voltage technology. Today he is the deputy head of R&D at the PFIFFNER Group and is active in the special

fields of voltage measuring systems both in theory and in practice. In particular, he is working on non-conventional voltage measurement theories and the frequency-dependent transmission behaviour of high-voltage instrument transformers.

Since 2004, he has been a member of the MT20 maintenance team of IEC / TC33 (Power capacitors and their applications) and is working on the new standard IEC 60358-x. Also, he is a member of various working groups in WG37 and WG47 within the framework of IEC / TC38 (Instrument transformers) which are managing the IEC 61869-x family of standards.