Ferroresonance Team Presentation

Pinaki Chattopadhyay (1358009) Pragyanshree Samantaray (1358010) Pratap Bhanu Mishra (1358011) Pratik Ranjan Behera (1358012)

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The word ferroresonance was firstly used by P. Boucherot in 1920 to describe a complex resonance oscillation in a series RLC circuit with nonlinear inductance.

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Ferroresonance is a name given to a situation where the nonlinear Ferroresonance magnetic properties of iron in transformer iron core interact with capacitance existing in the electrical network to produce a nonlinear tuned circuit with an unexpected resonant frequency. frequency.

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Now-a-days, ferroresonance ferroresonance is a widely studied phenomenon in power systems involving capacitors, saturable inductors and low losses.

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Ferroresonance can be understood as a complex oscillatory energy Ferroresonance exchange between magnetic field energy of nonlinear transformer cores and electric field energy of nearby capacitances.

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The ferroresonance phenomenon appears after transient disturbances (transient overvoltage, lightning overvoltage or temporary fault) or switching operations (transformer energizing or fault clearing).

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Its effects are characterized by : high sustained overvoltages and overcurrents with maintained levels of current and voltage waveform distortion producing extremely dangerous consequences.

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Also, the ferroresonance phenomenon depends on many other factors and conditions such as initial conditions of the system, transformer iron core saturation characteristic, residual fluxes in the transformer core, type of transformer winding connection, capacitance of the circuit, point-of-wave switching operation or total losses.

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Overvoltages and overcurrents

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Sustained levels of distortion

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Loud noise (magnetostriction)

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Misoperation of protective devices

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Overheating

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Electrical equipment damage

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Insulation breakdown

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Flicker

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De-energization of a voltage transformer by the opening of a circuitbreaker.

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As transformer is still fed through grading capacitors across circuitbreaker, this may lead to zero voltage at the transformer terminal.

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The phenomena of Ferroresonance can also occur when an unloaded 3-phase system consisting mainly of inductive and capacitive components is interrupted by single phase means.

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In general it is normally initiated after some type of switching event such as load rejection, fault clearing, transformer energization or deenergization, single-phase switching or loss of system grounding.

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In order to restrain ferroresonance in Coupling Capacitor Voltage Transformers (CCVT), Ferroresonance Suppression Circuits (FSCs) are used.

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It is necessary to understand the phenomenon, predict it, identify it and avoid the situation.

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Connections are made according to the circuit diagram.

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AC voltage was given slowly to the circuit via 1-ф Variac and all the meter readings were noted down.

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The supply voltage to the transformer was increased slowly and V1, V2 and V3 readings were recorded, at certain value of supply voltage there was sudden rise in the readings of V2 and V3 to a high value, this was called ferroresonance condition. Readings of V1, V2 and V3 were noted.

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Then supply voltage was decreased slowly from that point and readings were taken at different values, again at certain value of supply voltage there was a sudden fall of readings of V2 and V3, this was also a ferroresonance condition. Readings of V1, V2 and V3 were noted.

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Graph of the curve between supply voltage (V1) and transformer primary voltage (V3) was plotted.

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Reduce the transformer nonlinear reactance by designing the transformer to operate in the linear part.

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Avoid operating the transformer at no load condition by disconnecting the transformer primary terminals from supply in this case.

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In 3-ф circuits the phenomenon can be avoided by connecting a minimal resistive load on the transformer secondary or by interrupting the applied voltage by a 3-phase interrupting device such as a ganged (3 pole) circuit breaker.

1. What is ferroresonance?

ANS: Ferroresonance or nonlinear resonance is a type of resonance in electric circuits which occurs when a circuit containing a nonlinear inductance is fed from a source that has series capacitance, and the circuit is subjected to a abnormal conditions (like opening of transformer secondary etc). 2. What are different types of ferroresonance?

ANS: The four different ferroresonance types are: 1.

Fundamental mode

2.

Sub-harmonic mode

3.

Quasi-periodic mode

4.

Chaotic mode.

3. How ferroresonance is different from resonance?

ANS: Linear resonance that occurs when inductive and capacitive reactance of a circuit are equal. In linear resonance the current and voltage are linearly related in a manner which is frequency dependent. In the case of ferroresonance it is characterised by a sudden jump of voltage or current from one stable operating state to another one. The relationship  between voltage and current is dependent not only on frequency but also on a number of other factors such as the system voltage magnitude, initial magnetic flux condition of transformer iron core, the total loss in the ferroresonant circuit and the point on wave of initial switching. 4.What is magnetostriction?

ANS: Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization.

5.Which type material is used to make transformer core?

ANS: FERROMAGNETIC material like soft iron is preferred to make transformer core as soft iron can withstand high level of magnetic field at ambient temperature. 6.Name different kinds of magnetic materials.

ANS: The origin of magnetism lies in the orbital and spin motions of electrons and how the electrons interact with one another. The best way to introduce the different types of magnetism is to describe how materials respond to magnetic fields. This may be surprising to some, but all matter is magnetic. It's just that some materials are much more magnetic than others. The main distinction is that in some materials there is no collective interaction of atomic magnetic moments, whereas in other materials there is a very strong interaction between atomic moments. The magnetic behavior of materials can be classified into the following five major groups: •

Diamagnetic

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Paramagnetic

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Ferromagnetic

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Ferri-magnetic

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Anti-ferromagnetic

7.Define permeability. ANS: Permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. The relation between the magnetizing field H and the magnetic field B can also be expressed as the magnetic permeability: or the relative permeability , where is the vacuum  permeability. The permeability of ferromagnetic materials is not constant, but depends on H. In saturable materials the relative permeability increases with H to a maximum, then as it approaches saturation inverts and decreases toward one.

8.What are effects of ferroresonance and how it can be prevented? ANS: Effects of ferroresonance: ›

High sustained Over-voltage (ф-ф, ф to neutral).

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High sustained over-current.

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High sustained level of distortion to the current and voltage waveforms.

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Loud noise of transformer (magnetostriction).

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Over heating of transformers.

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Thermal and Insulation breakdown.

Prevention of ferroresonance: ›

Reduce the transformer nonlinear reactance by designing the transformer to operate in the linear part.

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Avoid operating the transformer at no load condition by disconnecting the transformer primary terminals from supply in this case.

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In 3-ф circuits the phenomenon can be avoided by conne cting a minimal resistive load on the transformer secondary or by interrupting the applied voltage by a 3-phase interrupting device such as a ganged (3 pole) circuit breaker .

9.What makes a transformer “HUM” ?

ANS: Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself when magnetized. When the magnetization is taken away, it goes back to its original condition. This  phenomenon is scientifically referred to as magnetostriction. A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and contracted twice during a full cycle of magnetization. The magnetization of any given point on the sheet varies, so the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to reduce losses and moderate the ensuing heating effect. The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving erratically with respect to its neighbor, so you can see what a moving, writhing construction it is when excited. These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of magnetostriction and hence, the noise level.

10.What do you mean by capacitive and inductive components in an electrical distribution system? ANS: The inductive components in power system can be due to: a) The magnetic core of a wound type voltage transformer, b) Bank type transformer, c) The complex structure of a 3 limb three-phase power transformer (core type transformer), d) The complex structure of a 5 limb three-phase power transformer (shell-type transformer). The circuit capacitive components in power system can be due to: a) The circuit-to-circuit capacitance, b) Parallel lines capacitance, c) Conductor to earth capacitance, d) Circuit breaker grading capacitance, e) Bus bar capacitance, f) Bushing capacitance. 11.What do you mean by capacitive and inductive components in an electrical distribution system? ANS: The inductive components in power system can be due to: a) The magnetic core of a wound type voltage transformer, b) Bank type transformer, c) The complex structure of a 3 limb three-phase power transformer (core type transformer), d) The complex structure of a 5 limb three-phase power transformer (shell-type transformer). The circuit capacitive components in power system can be due to: a) The circuit-to-circuit capacitance, b) Parallel lines capacitance, c) Conductor to earth capacitance, d) Circuit breaker grading capacitance, e) Bus bar capacitance, f) Bushing capacitance.

12.What is inrush current?

ANS: Inrush current input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases. Alternating current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. 13.Describe the causes of low load or no-load conditions at transformer secondary.

ANS: The causes are: •

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Manual switching of unloaded cable fed three phase transformers where only one phase is closed. Manual switching of unloaded cable fed three phase transformers where only one of the phases is open. One or two riser-pole fuses may blow leaving a transformer with one or two phases open.

14.How no-load conditions at transformer secondary can be avoided?

ANS: 

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Reduce the transformer nonlinear reactance by designing the transformer to operate in the linear part. Avoid operating the transformer at no load condition by disconnecting the transformer primary terminals from supply in this case. In 3-ф circuits the phenomenon can be avoided by connecting a minimal resistive load on the transformer secondary or by interrupting the applied voltage by a 3-phase interrupting device such as a ganged (3 pole) circuit breaker.

15.Why switch-gears should be adopted instead of fuses in high power electrical system especially at load side of transformers?

ANS: Switchgear should be adopted instead of fuses for better protection and to maintain stability of the power system. Incase of fuses once facing abnormal condition they permanently break the circuit which may lead to unloaded transformer secondary but if switchgears are adopted then they will make the circuit again after abnormal conditions are gone.

16.What are the common indicators of ferroresonance?

ANS: Common indicators of ferroresonance in power system are: •

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Audible noise: During ferroresonance, there may be an audible noise, often likened to that of a large bucket of bolts being shook, whining, a  buzzer, or an anvil chorus pounding on the transformer enclosure from within. Overheating: Transformer overheating often, although not always, accompanies ferroresonance. This is especially true when the iron core is driven deep into saturation. Since the core is saturated repeatedly, the magnetic flux will find its way into parts of the transformer where the flux is not expected such as the tank wall and other metallic parts. High overvoltages and surge arrester failure: When overvoltages accompany ferroresonance, there could be electrical damage to both the primary and secondary circuits. Surge arresters are common casualties of the event. Low-voltage arresters in end-user facilities are more susceptible than utility arresters, and their failure is sometimes the only indication that ferroresonance has occurred. Flicker: During ferroresonance the voltage magnitude may fluctuate wildly. End users at the secondary circuit may actually see their light  bulbs flicker

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Ferroresonance was observed at input voltage of 144 V (while increasing voltage across transformer primary) and at input voltage of 37 V (while decreasing voltage across transformer primary). The graph was plotted between V (inductive, V3) and V (input, V1).

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Ferroresonance is a widely studied phenomenon but it is still not well understood because of its complex behaviour. Its effects on electrical equipments are still considerable nowadays.

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Power System ferroresonance can lead to very dangerous and damaging over-voltages, but the condition can be avoided by careful system design.