What is Inrush current?
Inrush current is the instantaneous high input current drawn by an electrical equipment when it is turned-on. This arises due to the high initial currents required to charge the capacitors and inductor or transformers in the equipment.
This current is also known as the switch–on surge, or the input surge current. The amplitude of this current can be as high as a short circuit current. This current in transformer may be caused by energizing an unloaded transformer, occurrence of an external fault, voltage recovery after clearing an external fault and out-of phase synchronizing of connected generator.
These currents have many unfavorable effects, including operation failure of transformer differential protection, deterioration of the insulation and mechanical support structure of windings and reduced power quality of the system.
Such current in a transformer only lasts for a short period of time (less than a second), but the magnitude of the inrush current ultimately depends on the size and type of transformer. In general, the larger the transformer, the higher the inrush.
The general equation that gives the amplitude of inrush current as a function of time can be expressed as :
where Vm – maximum applied voltage;
Zt – total impedance under inrush, including system;
φ – energization angle; t – time;
t0 – point at which core saturates;
τ – time constant of transformer winding under inrush conditions;
α – function of t0; Kw – accounts for 3 phase winding connection;
Ks – accounts for short-circuit power of network.
Peak value of inrush current
In order to design a protective system for the transformer ,Peak value of inrush current is an important factor and a simplified equation can be used to calculate the peak value of the first cycle of the inrush current. This equation is as follows:
where Vm – maximum applied voltage; L – air core inductance of the transformer; R – total dc resistance of the transformer; BN – normal rated flux density of the transformer core; BR – remanent flux density of the transformer core; BS – saturation flux density of the core material.
Thus from the above two equations it is clear that the value of inrush current is dependent on the parameters of the transformer and operating conditions.
Mitigation
Pre-insertion of series resistors and synchronous closing of circuit breakers are examples of the available mitigation techniques. The residual flux plays a significant role in the development of the magnetizing inrush current and it has been observed that residual flux can be reduced by putting phase-to-ground capacitors at transformer terminals. It is also observed that pre-insertion resistor can reduce inrush current and the combination of all methods can give best results.