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Inrush current en Transformadores


Cuando un Transformador es inicialmente conectado a una fuente de voltaje AC, surge una corriente a través del primario del Transformador llamada INRUSH CURRENT (corriente de arranque). Esta corriente (INRUSH) es análogo a las que presentan las corrientes cuando inician los motores eléctricos cuando es conectada a la fuente de poder y es causada por diferentes fenómenos.

Se refiere al máximo instantánea de entrada actual de corriente en aparato eléctrico cuando se enciende por primera vez.  Por ejemplo, un simple caso de encender las bombillas incandescentes donde se producen altas corrientes  hasta que sus filamentos entrar en calor y aumenta su resistencia (mientras no aumenta la resistencia se produce aumento de intensidad de la corriente eléctrica).  La corriente alterna en los motores eléctricos y transformadores pueden variar su intensidad normal de plena carga al ser conectado, por unos cuantos ciclos de la onda de entrada. Convertidores de potencia también ofrecen irrupción de corrientes altas en relación con sus corrientes de estado estacionario.  Esto es típicamente la corriente de carga de la entrada de capacitancia.  La selección de los dispositivos de protección contra sobretensión como fusibles y disyuntores se hace más complicada cuando existen corrientes altas y deben ser capaces de tolerar estas corrientes.  La protección de sobrecorriente debe reaccionar con rapidez a una sobrecarga o corto circuito, pero no debe interrumpir el circuito cuando se generan los (generalmente inofensivo) los flujos de corriente de irrupción.

Cuando un transformador se energiza por primera vez una corriente transitoria de hasta 10 a 50 veces mayor que la corriente nominal del transformador puede fluir durante varios ciclos.  Esto ocurre cuando el devanado primario está conectado en todo el paso por cero de la tensión primaria.  Para grandes transformadores, corriente de arranque puede durar varios segundos.  Los transformadores toroidales pueden tener hasta 80 veces más grande la irrupción, ya que el magnetismo remanente es casi tan alto como el magnetismo de saturación  “de la histéresis del bobinado”.  Esto es causado porque el transformador siempre tendrá algún residuo de intensidad de flujo y cuando el transformador  ha revitalizado el flujo entrante se sumarán al flujo actual ya que hará que el transformador pase a saturación.  Entonces sólo la resistencia de los bobinados primarios y secundarios de la línea de alta tensión está limitando la corriente.

Se pueden limitar estos efectos instalando resistencias o Relés “transformer switching relay” en los transformadores (esta última es la mejor opción).

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. This is analogous to the inrush current exhibited by an electric motor that is started up by sudden connection to a power source, although transformer inrush is caused by a different phenomenon.We know that the rate of change of instantaneous flux in a transformer core is proportional to the instantaneous voltage drop across the primary winding. Or, as stated before, 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 90o. Since flux (Φ) is proportional to the magnetomotive 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 90o: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, but actually no more rapidly than under normal conditions: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.Alternatively, let us consider what happens if the transformer’s connection to the AC voltage source occurs at the exact moment in time when the instantaneous voltage is at zero. 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:A significant difference exists, however, between continuous-mode 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 half-cycle: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:

This is the mechanism causing inrush current in a transformer’s primary winding when connected to an AC voltage source. As you can see, 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. Because of this, transformer overcurrent protection devices are usually of the “slow-acting” variety, so as to tolerate current surges such as this without opening the circuit.

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  1. Pingback: Inrush current en Transformadores | Rama Estudiantil del IEEE de la UCSA

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