ferroresonance team presentation

8/13/2019 Ferroresonance Team Presentation

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The word ferroresonance was firstly used by P. Boucherot in 1920 todescribe a complex resonance oscillation in a series RLC circuit withnonlinear inductance.

Ferroresonance is a name given to a situation where the nonlinear

magnetic properties of iron in transformer iron core interact withcapacitance existing in the electrical network to produce a nonlineartuned circuit with an unexpected resonant frequency.

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

Ferroresonance can be understood as a complex oscillatory energyexchange between magnetic field energy of nonlinear transformercores 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) orswitching operations (transformer energizing or fault clearing).

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

Also, the ferroresonance phenomenon depends on many other factors

and conditions such as initial conditions of the system, transformeriron core saturation characteristic, residual fluxes in the transformercore, type of transformer winding connection, capacitance of thecircuit, point-of-wave switching operation or total losses.


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Overvoltages and overcurrents

Sustained levels of distortion

Loud noise (magnetostriction)

Misoperation of protective devices

Overheating

Electrical equipment damage

Insulation breakdown

Flicker


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De-energization of a voltage transformer by the opening of a circuit-breaker.

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

The phenomena of Ferroresonance can also occur when anunloaded 3-phase system consisting mainly of inductive and capacitivecomponents is interrupted by single phase means.

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


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Connections are made according to the circuit diagram.AC voltage was given slowly to the circuit via 1- Variac and all themeter readings were noted down.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 V3were noted.Then supply voltage was decreased slowly from that point andreadings were taken at different values, again at certain value of

supply voltage there was a sudden fall of readings of V2 and V3, thiswas also a ferroresonance condition. Readings of V1, V2 and V3 werenoted.Graph of the curve between supply voltage (V1) and transformerprimary voltage (V3) was plotted.


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

Avoid operating the transformer at no load condition bydisconnecting the transformer primary terminals from supply in thiscase.

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


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1. What is ferroresonance? ANS: Ferroresonance or nonlinear resonance is a type of resonance inelectric circuits which occurs when a circuit containing a nonlinearinductance is fed from a source that has series capacitance, and the circuitis 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 mode2. Sub-harmonic mode3. Quasi-periodic mode4. Chaotic mode.


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3. How ferroresonance is different from resonance?ANS: Linear resonance that occurs when inductive and capacitivereactance of a circuit are equal. In linear resonance the current and voltage

are linearly related in a manner which is frequency dependent. In the caseof ferroresonance it is characterised by a sudden jump of voltage orcurrent from one stable operating state to another one. The relationship

between voltage and current is dependent not only on frequency but alsoon a number of other factors such as the system voltage magnitude, initial

magnetic flux condition of transformer iron core, the total loss in theferroresonant 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 ofmagnetization.


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5.Which type material is used to make transformer core?ANS: FERROMAGNETIC material like soft iron is preferred to maketransformer core as soft iron can withstand high level of magnetic field atambient temperature.

6.Name different kinds of magnetic materials.ANS: The origin of magnetism lies in the orbital and spin motions of electronsand how the electrons interact with one another. The best way to introduce thedifferent types of magnetism is to describe how materials respond to magneticfields. This may be surprising to some, but all matter is magnetic. It's just thatsome materials are much more magnetic than others. The main distinction is thatin some materials there is no collective interaction of atomic magnetic moments,whereas in other materials there is a very strong interaction between atomicmoments.

The magnetic behavior of materials can be classified into the following five majorgroups: Diamagnetic Paramagnetic Ferromagnetic Ferri-magnetic Anti-ferromagnetic


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7.Define permeability.ANS: Permeability is the measure of the ability of a material to support theformation of a magnetic field within itself. The relation between the magnetizingfield 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, butdepends on H. In saturable materials the relative permeability increases with H toa 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 ( -, to neutral). High sustained over-current. High sustained level of distortion to the current and voltage waveforms. Loud noise of transformer (magnetostriction). Over heating of transformers. Thermal and Insulation breakdown.

Prevention of ferroresonance: 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- 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 .
http://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Circuit_breaker


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9.What makes a transformer HUM ?ANS: Transformer noise is caused by a phenomenon which causes a pieceof magnetic sheet steel to extend itself when magnetized. When themagnetization is taken away, it goes back to its original condition. This

phenomenon is scientifically referred to as magnetostriction. Atransformer is magnetically excited by an alternating voltage and currentso that it becomes extended and contracted twice during a full cycle ofmagnetization.The magnetization of any given point on the sheet varies, so the extensionand contraction is not uniform. A transformer core is made from manysheets of special steel to reduce losses and moderate the ensuing heatingeffect. The extensions and contractions are taking place erratically all overa 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 normallyvisible to the naked eye. However, they are sufficient to cause a vibration,and consequently noise. Applying voltage to a transformer produces amagnetic flux, or magnetic lines of force in the core. The degree of fluxdetermines the amount of magnetostriction and hence, the noise level.


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

11.What do you mean by capacitive and inductive components in anelectrical distribution system?ANS: The inductive components in power system can be due to: a) Themagnetic core of a wound type voltage transformer, b) Bank type transformer,

c) The complex structure of a 3 limb three-phase power transformer (core typetransformer), d) The complex structure of a 5 limb three-phase powertransformer (shell-type transformer).The circuit capacitive components in power system can be due to: a) Thecircuit-to-circuit capacitance, b) Parallel lines capacitance, c) Conductor toearth capacitance, d) Circuit breaker grading capacitance, e) Bus bar

capacitance, f) Bushing capacitance.


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12.What is inrush current?ANS: Inrush current input surge current or switch-on surge refers to themaximum, instantaneous input current drawn by an electrical device whenfirst turned on. For example, incandescent light bulbs have high inrushcurrents until their filaments warm up and their resistance increases.Alternating current electric motors and transformers may draw severaltimes their normal full-load current when first energized, for a few cyclesof the input waveform.

13.Describe the causes of low load or no-load conditions attransformer secondary. ANS: The causes are:

Manual switching of unloaded cable fed three phase transformerswhere only one phase is closed.

Manual switching of unloaded cable fed three phase transformerswhere only one of the phases is open.

One or two riser-pole fuses may blow leaving a transformer with oneor two phases open.


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16.What are the common indicators of ferroresonance? ANS: Common indicators of ferroresonance in power system are:

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 fromwithin.

Overheating: Transformer overheating often, although not always,accompanies ferroresonance. This is especially true when the iron coreis driven deep into saturation. Since the core is saturated repeatedly,the magnetic flux will find its way into parts of the transformer wherethe flux is not expected such as the tank wall and other metallic parts.

High overvoltages and surge arrester failure: When overvoltagesaccompany ferroresonance, there could be electrical damage to boththe primary and secondary circuits. Surge arresters are commoncasualties of the event. Low-voltage arresters in end-user facilities aremore susceptible than utility arresters, and their failure is sometimesthe only indication that ferroresonance has occurred.

Flicker: During ferroresonance the voltage magnitude may fluctuatewildly. 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 voltageof 37 V (while decreasing voltage across transformer primary). Thegraph was plotted between V (inductive, V3) and V (input, V1).

Ferroresonance is a widely studied phenomenon but it is still not wellunderstood because of its complex behaviour. Its effects on electricalequipments are still considerable nowadays.

Power System ferroresonance can lead to very dangerous anddamaging over-voltages, but the condition can be avoided by carefulsystem design.

ferroresonance team presentation

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