Electrical Transients

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TRANSFORMER MAGNETIZING INRUSH CURRENT AND FERRORESONANCE Electrical Transient by: Esmeralda Tapiz , REE

Transcript of Electrical Transients

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TRANSFORMER MAGNETIZING INRUSH CURRENT AND FERRORESONANCE

Electrical Transient

by: Esmeralda Tapiz , REE

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Topic:

1. TRANSFORMER MAGNETIZING INRUSH CURRENT

2. FERRORESONANCE

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I. MAGNETIZING INRUSH CURRENT IN TRANSFORMER

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Introduction• 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. The selection of overcurrent protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated

• Transformer magnetizing inrush current is an example of nonlinear properties of circuit elements that can be a potential source of abnormalities.

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When a transformer is first energized, a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles. Toroidal transformers, using less copper for the same power handling, can have up to 60 times inrush to running current. Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage (which for a pure inductance would be the current maximum in the AC cycle).

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In the absence of any magnetic remanance from a preceding half cycle, the effective magnetizing force is doubled compared to the steady state condition. Unless the windings and core are sized to normally never exceed 50% of saturation, (and in an efficient transformer they never are, such a construction would be overly heavy and inefficient) then during such a start up the core will be saturated.

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• Once the core saturates however, the winding inductance appears greatly reduced, and only the resistance of the primary side windings and the impedance of the power line are limiting the current.

• As saturation occurs for part half cycles only, harmonic rich waveforms can be generated, and can cause problems to other equipment.

• For large transformers with low winding resistance and high inductance, these inrush currents can last for several seconds until the transient has died away (decay time proportional to ~XL/R)and the regular AC equilibrium is established.

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• To avoid magnetic inrush, the inductive load needs to be synchronously connected near a supply voltage peak.

• Under normal excitation a transformer draws a magnetizing current of between 0.5% to 2% of its rated current.

• Because of saturation effects in the iron, this is not sinusoidal.

• The amount of distortion depends on the flux density to which the core is worked.

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Fig.1 Energy released by a transformer core when the magnetizing current is chopped

The core is taken around a hysteresis loop like that shown in Figure 1, each cycle the rate of change of flux at every instant producing just sufficient emf to counterbalance the instantaneous voltage supply.

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Fig. 2. Flux φ, voltage E, and magnetization current I for transformer.

Steady state conditions are illustrated in Figure 2 where it will be seen that the current oscillates between as the flux changes

sinusoidally between.

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The hysteresis loop is redrawn in Figure 3 and a magnetization curve has been added. It is evident that as the voltage is increased and more and more flux is demanded from the core, the peak current increases sharply, because the core becomes saturated.

Fig. 3 Hysteresis loop and magnetization curve for predicting transient inrush current

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• To do this we have to go back to the last time the transformer was energized and more particularly to the time when it was switched off. Figure 3 shows that the instant the current passes through zero; there is considerable remanant flux in the core, which is removed only by a reversal of the current.

• Thus we may expect that after a transformer has been disconnected there is a significant flux left in its core.

• It will usually be less thanbecause a transient current will flow in the winding after current ceases in the disconnecting device, as a consequence of the transformer discharging its own capacitance.

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• Let us suppose that this value of flux is . • Let us further suppose that when next the transformer is

energized, the polarity of the voltage is such that it calls for the flux to increase positively.

• If in addition, the applied voltage wave is just passing through zero and is going positive, the flux will have to increase through an increment equal to before the voltage peak is reached and a further before the voltage returns to zero again.

• Since the flux started from an initial value of it will have to reach , before reversing.

• It is very clear from figure 3 that because of magnetic saturation, an enormous current would be drawn from the supply. During the next half cycle the flux will return to , when the current, though negative, will be less than the normal peak. We have to considered here the very worst condition for energizing the transformer, but statistically or a condition close to it has equal probability will all others of occurring.

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Figure 4 Transient inrush current to 1000kVA, 13.8 kV transformer; 1 div=80 A

When this happens it is not unusual for the peak inrush current to be several times the rated load current of the transformer. Figure 4 is an example of this. The apparatus involved was a 1000 kVA, 13.8 kV, three-phase transformer, which has a normal peak magnetizing current of less than 2 A and a rated load current of 42 A. It will be seen from the oscillogram the inrush current is over 150 A.

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• The current does not persist at such a gross level or with such a measure of asymmetry, because what we have here is a series RL circuit.

• It was shown that the asymmetrical current in an RL circuit contains a d.c. component that declines exponentially with a time constant of L/R, making the total current more and more symmetrical as it does so.

• This is true in principle for the transformer inrush current, but because L is nonlinear and R reflects the core losses as well as the winding resistance, the time constant cannot be so readily defined.

• In practice it may take several seconds for the asymmetrical inrush current to approach its steady state final value.

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• The magnetizing inrush current phenomenon produces current input to the primary winding, which has no equivalent on the secondary side.

• The normal bias for the inrush current is not effective and increase of operation setting to a value would make the protection of little value.

• As the magnetizing inrush current phenomenon is transient, stability can be achieved by providing a small time delay.

• A kick fuse can be connected as shunt to an instantaneous relay. (this fuse is so rated that it carries inrush current without blowing)

• However, for internal fault the fuse blows and permits the relay to operate.

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• In case of severe inrush current, the set time delay might be insufficient to give stability.

• Also to minimize the damage to important transformers it may be essential to clear the fault without delay.

• The differential current is passed though a filter which extracts the second harmonic current.

• This component is then applied to produce a restraining quantity sufficient to overcome the operating tendency due to whole of the inrush current which flows in the operating circuit.

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II. FERRORESONANCE

Ferroresonance or nonlinear resonance is a complex electrical phenomenon. It can cause overvoltages and overcurrents in an electrical power system and can pose a risk to transmission and distribution equipment and to operational personnel

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It occurs when a circuit containing a nonlinear inductance is fed from a source that has series capacitance. One example of nonlinear inductance is the magnetic core of a wound type voltage transformer, but it may also arise due to the complex structure of a 3 or 5 limb three phase power transformers. The circuit series capacitance can be due to a number of elements, such as the circuit-to-circuit capacitance of parallel lines, conductor to earth capacitance, circuit breaker grading capacitance, busbar capacitance, or bushing capacitance, etc.

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• Ferroresonance should not be confused with linear resonance that occurs when inductive and capacitive reactances 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 characterized 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.

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i. Conditions

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

• In the electrical distribution field this typically occurs on a medium voltage electrical distribution network of transformers (inductive component) and power cables (capacitive component).

• If such a network has little or no resistive load connected load and one phase of the applied voltage is then interrupted, ferroresonance can occur.

• If the remaining phases are not quickly interrupted and the phenomenon continues, overvoltage can lead to the breakdown of insulation in the connected components resulting in their failure.

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ii. Ferroresonance• In the phenomenon of series resonance a

very high voltage can appear across the elements of a series LC circuit when it is excited at or near its natural frequency.

• Because the voltage across the inductors leads the current in phase by 90° and the capacitor voltage lags the current by the same amount, the phasor diagram appears as in Fig. 5 (b).

• It is seen that both and can far exceed V. Voltage conditions of this kind can be sustained and are therefore more properly called dynamic over-voltages rather than transients.

• However they are sometimes brought about by transient fault conditions.

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Figure 5. Simple Series Resonance

From fig. 5(a) it is evident that the voltages and add to give the applied voltage V.

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In the following we will first develop a basic understanding and then consider some practical examples. There will be significant distortions in the current and in the voltage across the L and C but we will ignore them initially and concern ourselves with the fundamental components only. We will simply neglect resistance in the first instance.

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The circuit of Figure 5.(a) remains relevant but with the understanding that the inductance is saturable. The voltage across the inductance will depend upon the frequency ω and the current through a function we will simply call ), thus we may write:

is plotted as a function of current in Fig. 6.

This voltage will lead the current by 90°. The voltage across the capacitor is given by

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Fig.6 Voltage and Current relationships in a ferroresonant circuit

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The minus sign indicating that it is antiphase with and lags the current by 90°. The total voltage will therefore be

Or

• Showing that has a fixed constituent V and one that is proportional to I.

• It is plotted in figure 6 as the inclined straight line.

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• Since both characteristics represent, the operating point must be where the two lines cross at P.

• The capacitor voltage in this instance is PC and the inductor voltage PB, which modestly exceeds V, whereas the current is given by OB.

• It is interesting to note that were the voltage V applied to the capacitor alone, it would take a much larger current Ic but if applied to the inductor alone, the current would be the smaller current IL.

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Figure 7 Showing the effect of increasing C in a

ferroresonant circuit

The slope of the inclined line is given by

Fig. 7 shows the consequences of changing C, all other parameters remaining constant

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• Indicating that if either ω or C is reduced the slope will increase and the intersecting point P will progress up the curve.

• Simultaneously, the voltage and will increase sharply.

• The figure also shows that in general the capacitor line makes multiple intersections with the ωf(L) line when the complete characteristics is considered.

• There such intersections for one line, designated (a), (b) and (c) illustrate this point. It will be observed that the values of and corresponding to point (a) are negative and far exceed those corresponding to point (b) also for point (a) whereas for point (b).

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• This shows that the current I leads the voltage for condition (a) but lags behind V for condition (b).

• Both (a) and (b) are stable operating points since any slight variation of I when operating at either, will cause voltage changes tending to restore the current to its initial value.

• The outcome depends entirely on the phase of the voltage at the time of energization.

• A momentary variation in I would cause changes in such as to reinforce the deviation and destabilize rather than stabilize.

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Fig.8. Operating conditions with a series-connected capacitor and linear inductor

The straight line AB is the characteristic of the inductor. The characteristic of the capacitor is given JK or J’K’ according to the value of the capacitor. The operating point will be P or P’. We observe that for a given capacitor there is only one operating point: it is at P if and is at P’ if .

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• We also note that if C is reduced JK becomes steeper and J’K’ becomes less steep, so that in the limit when the lines becomes parallel to AB the voltages becomes infinite.

• Resistance is always present in any practical reactor whether linear or nonlinear. This has a limiting effect on the reactor voltage in both instances. Introducing resistance changes the basic equation:

Or

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• This equation can also be solved graphically by plotting the right-hand side and observing where it intersects with the characteristic of the saturable inductance.

• The first term on the right of the equation is an eclipse.

• It is plotted in Fig. 9 for three different values of R first by itself around the horizontal is and then inclined in combination with .

• There are again three points of intersection: points (a) and (b) are stable, (c) is not . The voltage limiting effect of the resistance is quite marked as the value of R is increased and may provide a for of protection in practical circuits.

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Figure 9. Illustrating the effect of resistance in the LC circuit

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As mentioned earlier, our analysis has thus far ignored the waveform distortion inherent in a nonlinear circuit of this kind. However, this can be taken into account. This requires that the V/I characteristic of the inductive element be expressible as analytical expressions. Feldman and capablanca take this one step further by recognizing the hysteresis loop of the core. They approximate this characteristic by two pairs of parallel lines i.e. the hysteresis loop becomes a parallelogram. They show by their model and by test that the transition between modes as the exciting voltage is varied.

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How do ferroresonance situation arise in practice?

Figure 10. Circumstance in which ferroresonance can occur

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Figure 10(a) shows a switch used to energize and dc energize the primary of a transformer. The two are interconnected by a length of cable, a common practice in distributing systems, The switch is shown with only one pole closed. This condition will prevail momentarily on closing at the first pole completes its circuits, or on opening before the last pole disconnects. Some circuits of this kind use a fuse in series with the switch to interrupt fault currents.

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It might appear at first that with only one pole of the switch closed the transformer is not energized. In a way this is true. There nevertheless remains a path for current through two of the phase windings and the cable capacitance, as indicated by the bolder lines in Figure 10(b).

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It is this current that can produce resonance and impress excessive voltages across the transformer and the cables on the unenergized phases. It can, for instance cause lightning arresters connected at the B and C bushings of the transformer to operate. If the condition is sustained, repeated operation can destroy the arresters.It was thought that wye-wye transformers with neutrals grounded were immune to ferroresonaces, but this is not true if the transformers have four or five legged cores.

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• The mechanisms for producing the overvoltage are based on ferroresonance phenomena and are not extremely sensitive to types of generators, types of transformer connections, location of capacitors or other equipment parameters.

• The overvoltages depends on the establishment of a harmonic resonant circuit utilizing transformer saturation characterisitics, the existence of power factor correcting capacitors on the feeder and a source of energy sufficient to maintain the transformer saturation.

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III. FERRORESONANCE IN DISTRIBUTING TRANSFORMERS

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Ferroresonance phenomenon practically never occurs in a normal circuit configuration with the transformers loaded, but it can exist under a combination of the following circumstances which is usually occur only during switching of a three-phase bank or blowing of a fuse in one line:

1. System neutral grounded, ungrounded transformer neutral2. No load on the transformer3. Relatively large capacitance to ground such as may exist in cable circuits (underground distribution) or very long overhead lines.

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Although Ferroresonance has been studied at some length, it still does not seem possible to reliably predict its occurrence. Experience indicates that it is possible to prevent ferroresonance during switching on a transformer bank if all three transformers are resistance-loaded to 15% or more of their rating, or if special switches are used to assure that the three lines close simultaneously.

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Reference:IEC 61000–4–30, Electromagnetic Compatibility (EMC) – Testing and measurement techniques – Power quality measurement methods, Published by The International Electrotechnical Commission, 2003. Electrical Transients in Power System, Second EditionBy: Allan GreenwoodStandard Handbook for Electrical EngineersBy: Donald G Fink and H. Wayne BeatyTransformers

By Bhel, Sanka Sen.,Bhel Wikipedia:

Jacobson, D.A.N., Examples of Ferroresonance in a High Voltage Power System, accessed 2011-09-25 Boucherot, P.,"Éxistence de Deux Régimes en Ferrorésonance", Rev.Gen. de L’Élec., vol. 8, no. 24, December 11, 1920, pp. 827-828Dugan, R. C., Examples of Ferroresonance in Distribution Systems, accessed 2011-09-06Ferracci, Ph., Cahier technique n° 190: Ferroresonance, Groupe Schneider, accessed 2011-09-06