Transformer Magnetizing Inrush Current PRESENTED BY PROF VG
Transformer Magnetizing Inrush Current PRESENTED BY PROF. VG PATEL
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT PRESENTATION FLOW v. What is Inrush Current? v. How it is produced? v. How it effect the electrical network? v. On which parameters / factors it depends? v. How to minimize it? v. Conclusion. Friday, December 17, 2021 VG PATEL 2
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT What is Inrush Current? Inrush current or input surge current 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. When a transformer is first energized a transient current much larger than the rated trans-former current can flow for several cycles. This is caused because the transformer will always have some residual flux density and when the transformer is reenergized the incoming flux will add to the already existing flux which will cause the transformer to move into saturation. Friday, December 17, 2021 VG PATEL 3
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT The exciting current of transformer is composed of: 1. An inrush current that occurs the instant a transformer is energized but decays in magnitude and time. The inrush current is a complicated function that occurs due to the residual magnetic flux in the iron core being "out of synch" with the magnetic flux that would flow the instant on the voltage sine wave when the transformer is energized. Fusing for a transformer is sized and selected so that a fuse will not operate or melt during the inrush. 2. A small current due to the AC resistance in the primary winding (which does not concern here). 3. A magnetizing current that sets up the original magnetizing flux within the transformer core. The idealized behavior (ignoring resistances) of the inrush currents is governed by Faraday's Law. Friday, December 17, 2021 VG PATEL 4
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Inrush current: - The phenomenon of magnetizing inrush is a transient condition, which occurs primarily when a transformer is energized. It is not a fault condition, and therefore does not necessitate the operation of protection, which, on the contrary, must remain stable during the inrush transient, a requirement which is a major factor in the design of protective systems for transformers. When a transformer is initially connected to a source of AC voltage, there may be a substantial surge of current through the primary winding called inrush current. It is an instantaneous current that ramps up quickly and dies off as the transformer reaches steady state. A transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles. This instantaneous current produces large transient currents that can damage circuits. Friday, December 17, 2021 VG PATEL 5
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT This "original" no load magnetizing flux circulates around the core (reversing its direction 50 times a second in keeping with the magnetizing current) and circulates through both primary and secondary windings. It is this magnetizing flux which induces a voltage in the secondary winding. It is also this "no load" magnetizing flux which also produces two types of core losses: hysteresis loss and eddy current loss. Inrush Inhibit during Transformer Energization: The transformer energization resembles the condition of an internal fault. If no inhibiting mechanism is provided, the differential element will trip. The magnetizing inrush current has significant 2 nd harmo-nic content. The level of 2 nd harmonic current can be used to differentiate between inrush and a fault condition. Friday, December 17, 2021 VG PATEL 6
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Transformer inrush high currents occur to energize the transformer core. The steady-state magnetizing current for a transformer is very low, but the momentary current when it is first energized can be quite high. The concerns are typically the same as with motor starting, except for one important difference - besides being a high current magnitude, the transformer energizing current is full of harmonics. Both even and odd harmonic components occur when a transformer is energized, and they can excite system resonances, resulting in dynamic over voltages. Inrush Inhibit during Transformer Energization: The transformer energization resembles the condition of an internal fault. If no inhibiting mechanism is provided, the differential element will trip. The magnetizing inrush current has significant 2 nd harmonic content. The level of 2 nd harmonic current can be used to differentiate between inrush and a fault condition. Friday, December 17, 2021 VG PATEL 7
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT How it is produced? When a transformer is taken off-line, a certain amount of residual flux remains in the core due to the properties of the magnetic core material. The residual flux can be as high as 50 to 90 % of the maximum operating flux, depending upon the type of core steel. When voltage is reapplied to the transformer, the flux introduced by this source voltage builds upon that already existing in the core. In order to maintain this level of flux in the core, which can be the well into the saturation range of the core steel, the transformer can draw current well in excess of transformer’s rated full-load current. Depending on the transformer design, the magnitude of this inrush current can be anywhere from 3. 5 to 40 times the rated full-load current. The wave form of the inrush current is similar to a sine wave, but largely skewed to the positive or negative direction. This inrush current experiences decay, partially due to loss that provides a damping effect. However, the current can remain well above rated current for many cycles. Friday, December 17, 2021 VG PATEL 8
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT The driving force of the inrush currents is the voltage applied to the primary of the transformer. This voltage forces the flux to build up to a maximum theoretical value of double the steady state flux plus remanance. Therefore, the transformer is greatly saturated and draws a large amount of current. Since the current is of short duration there are not adverse effects to the transformers. The idealized behavior (ignoring resistances) of the inrush currents is governed by Faraday's Law. The Inrush current in power transformers occurs when the polarity and magnitude of the residual flux do not agree with the polarity and magnitude of the instantaneous value of steady-state flux in the transformer core. The loss characteristic of transformer steel has improved considerably so that much higher flux densities are now utilized and the prevalence of heavy current in-rushes with modern transformers is due to this. Friday, December 17, 2021 VG PATEL 9
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Friday, December 17, 2021 VG PATEL 10
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Friday, December 17, 2021 VG PATEL 11
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Friday, December 17, 2021 VG PATEL 12
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT The rate of change of instantaneous flux in a transformer core is proportional to the instantaneous voltage drop across the primary winding. The voltage waveform is the derivative of the flux waveform, and the flux waveform is the integral of the voltage waveform. In a continuously-operating transformer, these two waveforms are phase-shifted by 90 o. Since flux (Φ) is proportional to the magneto motive force (mmf) in the core, and the mmf is proportional to winding current, the current waveform will be in-phase with the flux waveform, and both will be lagging the voltage waveform by 90 o. Both core flux and coil current start from zero and build up to the same peak values experienced during continuous operation. Thus, there is no "surge" or "inrush" or current in this scenario. Friday, December 17, 2021 VG PATEL 13
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT During continuous operation (when the transformer has been powered for quite some time), this is the point in time where both flux and winding current are at their negative peaks, experiencing zero rate-of-change (dΦ/dt = 0 and di/dt = 0). As the voltage builds to its positive peak, the flux and current waveforms build to their maximum positive rates-of-change, and on upward to their positive peaks as the voltage descends to a level of zero. This is the mechanism causing inrush current in a transformer's primary winding when connected to an AC voltage source. The magnitude of the inrush current strongly depends on the exact time that electrical connection to the source is made. If the transformer happens to have some residual magnetism in its core at the moment of connection to the source, the inrush could be even more severe. Friday, December 17, 2021 VG PATEL 14
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT A significant difference exists, however, between continuousmode operation and the sudden starting condition assumed in this scenario: during continuous operation, the flux and current levels were at their negative peaks when voltage was at its zero point; in a transformer that has been sitting idle, however, both magnetic flux and winding current should start at zero. When the magnetic flux increases in response to a rising voltage, it will increase from zero upwards, not from a previously negative (magnetized) condition as we would normally have in a transformer that's been powered for awhile. Thus, in a transformer that's just "starting, " the flux will reach approximately twice its normal peak magnitude as it "integrates" the area under the voltage waveform's first halfcycle. Friday, December 17, 2021 VG PATEL 15
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Switching in at zero voltage • With no residual magnetism At the instant of switching in, there being no residual magnetism in the core, the flux must start from zero, and to maintain the first half cycle of the voltage wave it must reach a value corresponding approximately to twice the normal maximum flux density. The total current may be considered to consist of the normal no-load current and a drooping characteristic transient current superimposed upon it. Due to the initial high saturation in the core, the current waves may be extremely peaked and contain prominent third harmonics. Friday, December 17, 2021 VG PATEL 16
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT All electromagnetic apparatus with iron cores have varying degrees of magnetic remanance, which retains a substantial portion of the magnetic induction to which the core is driven until polarity is reversed at the power source. This characteristic of iron means that when magnetizing power is removed, as with opening a switch, that the core remains magnetized. When power is reapplied to the core, the magnetic state to which the core will next be driven depends on the phase angle of the source relative to the retained magnetic state of the core. If the phase angle is identical to the state when the circuit was switched off, then magnetizing current will remain well behaved, and of very small amplitude. However, if the phase angle is sufficiently out phase and at a voltage zero, a very large inrush current will result. This seminar examines the inrush condition, looking at causal factors, amplitude with size and type of transformer as well as the effect of source impedance. Analytical examination of harmonics is considered as well as the impact on differential protection. Speakers include manufacturers and users in an open dialogue with plenty of opportunity for audience interaction. Friday, December 17, 2021 VG PATEL 17
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT • With maximum residual magnetism Residual magnetism having a polarity opposite to that to which the flux would normally attain under equivalent normal voltage conditions. Instead of the flux wave starting at zero it will start at a value corresponding to the polarity and magnitude of the residual magnetism in the core, and in the first cycle the flux will reach a maximum higher than outlined in (a) by the amount of residual magnetism. The theoretical limit is a flux which corresponds to a value approaching three times the normal maximum flux density, and at this value the initial current in-rush will be still greater. Where the residual magnetism possesses the same polarity as that which the changing flux would normally attain, results in a diminution of the initial maximum values of the flux, and consequently of the current in-rush. Friday, December 17, 2021 VG PATEL 18
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Friday, December 17, 2021 VG PATEL 19
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Friday, December 17, 2021 VG PATEL 20
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT The maximum inrush current occurs if the transformer is energized when the voltage wave is near zero. At this time, the current and flux should be a maximum in the highly inductive circuit, and a half wave of voltage requires a flux change of twice the maximum flux. If there is a residual flux left in the bank from the previous energization, it may either add or subtract from the total flux, thereby increasing or decreasing the inrush current. Power transformers are operated normally near the knee of the saturation curve, so the additional flux requirement of zerovoltage-wave energization plus any residual flux will definitely saturate the iron and increase the magnetizing current components. Inrush current is most severe when connection to the AC source is made at the moment instantaneous source voltage is zero. Friday, December 17, 2021 VG PATEL 21
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT 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). 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. This can also be expressed as the remnant magnetism in normal operation is nearly as high as the saturation magnetism at the "knee" of the hysteresis loop. 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. Friday, December 17, 2021 VG PATEL 22
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH 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. Under normal conditions the magnetic flux in the core, being 90° out of phase with the voltage, reaches its peak value when the voltage passes through zero. Due to this phase displacement it is necessary for the flux to vary from a maximum in one direction to a maximum in the opposite direction in order to produce one half cycle of the required back e. m. f. in the primary winding, so that a total flux is embraced during the half cycle corresponding to twice the maximum flux density. Friday, December 17, 2021 VG PATEL 23
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Switching in at maximum voltage With no residual magnetism: In this case at the instant of switching in, the flux should be zero, due to its 90° phase displacement from the voltage, and as we have assumed there is no residual magnetism in the core, the desired conditions are obtained which produce the normal steady time distribution of the flux. That is, at the instant of switching in the flux starts from zero, rises to the normal maximum in one direction, falls to zero, rises to the normal maximum in the opposite direction and again reaches zero, the wave being symmetrically disposed about the zero axis. The no-load current, therefore, pursues its normal course and does not exceed the magnitude of the normal no-load current. Friday, December 17, 2021 VG PATEL 24
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT With residual magnetism in the core equal to half the normal flux density and of the same polarity as the normal flux density: Effect is equivalent to maximum residual magnetism having a polarity opposite to that to which the flux would normally attain under equivalent normal voltage conditions. In this case the residual magnetism introduces the transient components, so that the initial flux waves are unsymmetrically disposed about the zero axis, high initial maximum flux values are attained, and in the case where the residual magnetism has the same value as corresponds to the normal maximum flux density the current inrush will have a value corresponding approximately to twice the normal maximum flux density. Friday, December 17, 2021 25 VG PATEL
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT This is with maximum residual magnetism having the same polarity as that to which the flux would normally attain under equivalent normal voltage conditions. This is the converse of the foregoing case, and the initial flux waves will again be unsymmetrically disposed about the zero axis. For the same value of residual magnetism the total maximum flux would be the same as above, but both flux and current waves would initially be disposed on the opposite side of the zero axis. Friday, December 17, 2021 VG PATEL 26
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT In an ideal transformer, the magnetizing current would rise to approximately twice its normal peak value as well, generating the necessary mmf to create this higher -than-normal flux. However, most transformers aren't designed with enough of a margin between normal flux peaks and the saturation limits to avoid saturating in a condition like this, and so the core will almost certainly saturate during this first half-cycle of voltage. During saturation, disproportionate amounts of mmf are needed to generate magnetic flux. This means that winding current, which creates the mmf to cause flux in the core, will disproportionately rise to a value easily exceeding twice its normal peak: Friday, December 17, 2021 VG PATEL 27
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Friday, December 17, 2021 VG PATEL 28
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Let us suppose that the primary winding of a transformer is suddenly connected to an AC voltage source at the exact moment in time when the instantaneous voltage is at its positive peak value. In order for the transformer to create an opposing voltage drop to balance against this applied source voltage, a magnetic flux of rapidly increasing value must be generated. The result is that winding current increases rapidly. Friday, December 17, 2021 VG PATEL 29
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT How it affects the electrical network? Effect of high magnetizing inrush current: • High magnitude and long time duration of inrush current reduces transformer life. • More losses. • Breaker contact temperature may rise to very high value, which affects the breaker performance and life. • Can cause interaction problems with other loads in a power system. These inrush currents can cause sags that trip loads. Coupled with the tendency of other constant power devices to increase current to make up for the reduced voltage, the inrush current may cause protection devices to trip. Friday, December 17, 2021 VG PATEL 30
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT • Deterioration of the insulation and mechanical support structure of windings. • Heavy damage can occur to transformer if thermal and magnetic stress capability limit of transformer is crossed. • Transformer core may get saturated. • Operation failure of transformer differential protection. • Besides being a high current magnitude, the transformer energizing current is full of harmonics. Both even and odd harmonic components occur when a transformer is energized, and they can excite system resonances, resulting in dynamic over voltages. Friday, December 17, 2021 VG PATEL 31
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT On which parameters / factors it depends? When a transformer is first energized, a transient magnetizing or exciting inrush current may flow. This inrush current, which appears as an internal fault to the differentially connected relays, may reach instantaneous peaks of 8 to 30 times those for full load. For calculating inrush current magnitude and duration, various formulae are available. According to a 1951 AIEE report, time constants for inrush vary from 10 cycles for small units to as much as few min for large units. After transformer is manufactured and erected in field, the magnetizing inrush current and duration during energizing depends upon following factors: Friday, December 17, 2021 VG PATEL 32
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT • Location of transformer (nearer to power generating source – more magnetizing inrush current, away the power generating source – comparatively less magnetizing inrush current due to involvement of some line impedance i. e. resistance in the power system from the source to the transformer bank. The resistance from the source to the bank determines the damping of the current wave. Banks near a generator will have a longer inrush because the L is high compared to resistance which is very low). • Size of the transformer bank (large transformer units tend to have a long inrush as they represent a large L relative to the system resistance. At remote substations, the inrush will not be nearly so severe, since the resistance in the connecting line will quickly damp the current. ) and size of the power system • Electrical network capacitance to which transformer secondary is connected. Friday, December 17, 2021 VG PATEL 33
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT • Whether transformer charged from HV side OR LV side (HV side - comparatively less magnetizing inrush current, LV side - comparatively more magnetizing inrush current. This phenomenon is because LV winding is nearer to core where gap fluxes are more. (Unit-type generator and transformer combinations have no initial inrush problem because the unit is brought up to full voltage gradually). • Type of iron used in the transformer core and its saturation density. • Prior history, or residual flux level, of the bank. • How the bank is energized. Friday, December 17, 2021 VG PATEL 34
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT How to minimize (mitigate) it? When the unloaded transformer is energized, the voltage rises in the system because of system capacitance. Depending upon the system capacitance, even after removing the source, voltage exists on the transformer terminals till charged capacitor is fully discharged. Some engineers are having false belief that to minimize the transformer magnetizing inrush current, transformer should be energize with secondary side loaded. As the magnetizing current largely depend upon the following aspects of the core, energizing the transformer with load cannot help in reducing the magnetizing current. At the most energizing transformer with load can reduce the oscillations. Friday, December 17, 2021 VG PATEL 35
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Magnetizing current is a function of core and is depends upon following factors: • Flux density (transformers designed for some normal, economical saturation densities, the crest of t will produce super saturation in the magnetic circuit. The result will be a very large crest value in the magnetizing current. ) • Grade of core material • Type of lamination cutting (step lap, mitred, semimitred) Friday, December 17, 2021 VG PATEL 36
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT To minimize the magnetizing current, care to be taken while design stage: • Flux density, Grade of core material and type of lamination cutting. Designing transformer by choosing low flux density, the transformers can be bulky and more expensive. Along with the lower flux density, the burr level of laminations, cutting angle of mitred, assembly, clamping of core is also very important. • To increase no. of turns on primary side (but this will increase losses). • To minimize the inrush problems, transformer manufacturers resort to gaps in the core. Gapping is an expensive production methodology and is difficult to control and test. In addition, gapped transformers become acoustically noisy. Friday, December 17, 2021 VG PATEL 37
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT To minimize the magnetizing current after transformer is manufactured, erected at site and going to be energized, following are the factors worth considering: The main parameters controlling the magnitude of the currents (once the transformer is built) are the remanance, the switching angle and the resistance of the primary circuit. The following techniques are used to control one of the parameters: Friday, December 17, 2021 VG PATEL 38
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT v The magnitude of the inrush current depends on the instant on the AC wave at which the transformer is switched on and also at which instant it was switched off (remanance magnetism). Controlled switching of circuit breaker (also known as synchronous closing of CB). CB contacts will be bridged only at zero current (of wave form). Special programmed relays are available with event logger which will allow CB contacts to close at zero current instant. In foreign countries this system is in use since quite a long time. In India, this system is introduced since about 5 years for 400 KV system. A set of CB and associated relay is to be installed, conventional CB will not work. v Pre-insertion of series resistor: A transformer with an iron core, or a capacitor will have retentivity or require charging energy. When the reactive device is energized, the only limit Friday, December 17, 2021 VG PATEL 39
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT to initial charging current may be its series resistive component. This creates very low initial impedance for ½ to many cycles on AC. When being energized from a low impedance source, these momentary low impedance loads can create high current surges and oscillations, which can result in over-voltages, In some systems the step impedance is in series with the main switching unit and the step contactor is used to bypass the limiting impedance. This is less desirable since it requires both switching units to have full momentary current ratings, a more expensive arrangement. In special applications, a two step interruption as well as closing may be desirable. Again the timing safeties must be used to protect the momentary rated current limits of the step unit. The disadvantage is the addition of series components that reduce the overall reliability and also not effective solution under all operating conditions. Friday, December 17, 2021 VG PATEL 40
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT v Select minimum tap no. (maximum voltage tap) when charging from HV side. Select maximum tap no. (minimum voltage tap) when charging from LV side. v Charge the transformer keeping voltage constant and frequency little bit on higher side. OR keeping frequency constant, apply reduced voltage than the rated. v After calculating system capacitance, a reactor of appropriate reactance is designed and connect to LV terminals of transformer (high voltage reactor will be more costly). Along with CB, reactor breaker also will get closed. System capacitance and reactance of newly inserted reactor will get nullified and after magnetizing inrush current is decayed, reactor will be switched off automatically through circuitry provided. Friday, December 17, 2021 VG PATEL 41
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT v. A neutral resistor based scheme. Even if duration and magnitude of inrush current is known, there is no remedial firm solution available. At the most we can provide resistor in the neutral circuit of transformer. We can calculate the value of R through which neutral should be earthed. v. To minimize residual magnetism on the core, load on transformer should be first reduced to zero before switching off the transformer breaker. v. The most practical solution for such problem is to provide few hundred milliseconds’ intentional time delay in tripping circuit. Friday, December 17, 2021 VG PATEL 42
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT THE BEST OPTION This is the best option if own captive alternator is available. Transformer is energised by running the alternator at rated speed and then gradually raising the excitation to generated voltage. (If AVR/Excitation is on AUTO mode and field breaker is switched on, it will build up about 90% rated voltage. From AUTO mode, the control should be put on MANUAL mode and gradually voltage should be raised up to rated voltage. This will limit the flux level which is proportional to v/f). * Friday, December 17, 2021 VG PATEL 43
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT CONTEOLLED SWITCHING ABB has developed a device (known as CONTEOLLED SWITCHING) by which CB contacts can be made to bridge at any instant of waveform (cycle) as desired by us, eg 300, 450, 600, etc. One can set any angle on the device at which CB contacts should bridge. It will be more clear from following slides. Friday, December 17, 2021 VG PATEL 44
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TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Sympathetic inrush: When a bank is paralleled with a second energized bank, the energized bank can experience a sympathetic inrush. The offset inrush current of the bank being energized will find a parallel path in the energized bank. The dc component may saturate the transformer iron creating an apparent inrush. The magnitude of this inrush depends on the value of the transformer impedance relative to that of the rest of the system, which forms an additional parallel circuit. Again, the sympathetic inrush will always be less than the initial inrush Sympathetic inrush: The total current at breaker C is the sum of the initial inrush of bank A and the sympathetic inrush of bank B. Since this waveform looks like an offset fault current, it could cause misoperation if a common set of harmonic restraint differential relays were used for both banks. Unit-type generator and transformer combinations have no initial inrush problem because the unit is brought up to full voltage gradually. Recovery and sympathetic inrush may be a problem, but as indicated above, these conditions are less severe than initial inrush. Friday, December 17, 2021 VG PATEL 50
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT Sympathetic inrush: A magnetizing inrush can occur in an energized transformer when a nearby transformer is energized. A common case is paralleling a second transformer bank with a bank already in operation. The dc component of the inrush current can also saturate the energized transformers, resulting in an apparent inrush current. This transient current, when added to the inrush current of the bank being energized, provides an offset symmetrical total current that is very low in harmonics. This would be the current flowing in the supply circuit to both transformer banks. Recovery Magnetizing Inrush: During a fault or momentary dip in voltage, an inrush may occur when the voltage returns to normal. This is called the recovery inrush. The worst case is a solid three-phase external fault near the transformer bank. During the fault the voltage is reduced nearly to zero on the bank; then when the fault is cleared, the voltage suddenly returns to essentially normal value. This may produce a magnetizing inrush, but its maximum will not be as high as the initial inrush since the transformer is partially energized. Friday, December 17, 2021 VG PATEL 51
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT When a transformer is switched off, the flux density follows the hysteresis loop of the core material and remains at a level termed the 'residual' value of the flux density. In modern transformer steels this value can be as much as 80% of Bmax. Thus if the switch off and switch on of the transformer have each occurred at the least favourable time we have a worst case flux density of (Bres + 2 Bmax) established in the core. The core material itself can only absorb a flux density of Bsat before saturation of the steel occurs. Any flux density in excess of this value flows in the air space (between the core and the inside of the primary winding) since this flux is flowing in a nonmagnetic medium, very large values of magnetizing current are necessary causing the excessive values of magnetizing current which are the subject of this short note. Friday, December 17, 2021 VG PATEL 52
TRANSFORMER ENCYCLOPAEDIA MAGNETIZING INRUSH CURRENT CONCLUSION Inrush current (peak, 2 nd harmonic and duration) of today’s power transformers differ from those of older designs due to the use of higher grain oriented core steels, the step-lap core joint type and higher rated design core induction values. VG PATEL Friday, December 17, 2021 53
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