A transformer can incur in core saturation conditions in either of the following cases:
- When operating above rated power
- When operating above rated voltage
The first situation can arise during peak demand periods, and the second case can occur during light load conditions, especially if utility capacitor banks are not disconnected accordingly and the feeder voltage rises above nominal values.
A transformer operating on the saturation region will show a nonlinear magnetizing current, which contains a variety of odd harmonics, with the third dominant. This effect will be more dominating with increment in the load. In an ideal lossless core, no hysteresis losses are produced. The magnetic flux and the current needed to produce them are related through the magnetizing current of the material used in the core construction.
When the hysteresis effect is considered, this non sinusoidal magnetizing current is not symmetrical with respect to its maximum value. The distortion is typically due to triplen harmonics (odd multiples of three), but mainly due to the third harmonic. This spectral component can be confined within the transformer using delta transformer connections. This will help maintain a supply voltage with a reasonable sinusoidal waveform.
In three-legged transformers, the magneto motive forces (mmf) of triplen harmonics are all in phase and act on every leg in the same direction. Therefore, the trajectory of the magnetic flux for the triplen harmonics extends outside the boundaries of the core. The high reluctance of this trajectory reduces the flux of triplen harmonics to a very small value. The components of fifth and seventh harmonics can also be considerable (5 to 10%) to produce considerable distortion and ought not be ignored.
In electric power distribution networks, harmonics due to transformer magnetizing current reach their maximum value when the system is lightly loaded and voltage level is high. When a transformer is de-energized, it is possible that it retains residual magnetic flux in the core. On re-energization, this flux come together with the magnetizing flux produced by the inrush current, and the two combined can yield peak values three times or higher the nominal flux at rated load. The resulting effect may cause the transformer core to reach extreme saturation levels involving excessive turn amps within the core. Consequently, magnetizing currents as large as 5 to 10 % of nominal current (compared with 1 to 2% of nominal magnetizing current during steady-state operating conditions) can develop. The duration of the magnetizing current is mainly a function of the primary winding resistance. For large transformers with large winding resistance, this current can remain for many seconds.
The harmonic content of steady-state currents in three-phase systems does not involve even harmonics, which appear under waveform asymmetry when the positive and negative half cycles are not of the same amplitude. However, under energization, a distribution transformer develops all kinds of low-order harmonics involving even harmonics which are often used for restraining the operation of differential protection.
The presence of the even harmonics and their decaying nature are typical under transformer saturation. Here we can reproduce the harmonics created during transformer saturation by injecting harmonic currents similar to the typical harmonic spectrum into an AC source and a short feeder representation..
Transformer saturation can also take place following a voltage dip because a sudden change in voltage leads to a DC component in the magnetizing flux.
This phenomenon is seen in a auto manufacturing machine Shop in Maharashtra. Conventional way of 14% detuned reactor capacitor bank could not solve the problem of triplen and even harmonics.
The best alternative is the combination of active and passive combination to remove triplen and take care of varying voltage during the course of the day.