What is a Magnetizing Current?
When power transformers are switched ON without loading secondary winding, a huge amount of current is drawn from the supply which is about 5 to 7 times of rated current. This current is called a magnetizing current.
If the applied voltage to the transformer is sinusoidal then the core flux should also be sinusoidal. If the magnetization curve of the transformer material would have been linear then the magnetizing current should have been sinusoidal but due to economic reasons, modern transformers are operated with high flux density which drives the transformer core in deep saturation. Obviously, the magnetizing current becomes highly non-linear. With such a non-linear magnetization curve a sinusoidal flux may only be obtained with a peaky magnetizing current containing a dominant peaking third harmonic component.
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| Peaky Current in Power Transformer |
The third harmonic component of the magnetizing current can flow if the electric circuit permits. Hence it can easily flow in 1 phase transformers employed in single-phase circuits. However, in 3 Phase transformers, the third harmonic component can flow only in star neutral is connected to source neutral or it can flow in the closed delta.
If the electric circuit does not permit the flow of the third harmonic component of magnetizing current then the magnetizing current remains almost sinusoidal if higher harmonics are ignored. A sinusoidal magnetizing current results in flat-topped flux containing a depressing third harmonic flux component.
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| Flat-Topped Flux in Power Transformer |
The third harmonic flux can get established in the core only if the magnetic circuit permits. Hence it can easily establish in those transformers that have magnetically independent circuits such as in three single-phase transformers in a three-phase bank,3 phase shell type transformers, and 5 limb or 4 limb core type transformers.
What is an Oscillating Neutral Phenomenon?
The presence of third harmonic flux in magnetically independent transformers results in peaky induced emf in both winding that stresses the insulation and results in highly objectionable phenomena known as oscillating neutral.
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| Peaky Induced EMF |
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| Oscillating Neutral |
How to avoid an oscillating neutral phenomenon?
The problem of oscillating neutral and high insulation stress in transformers having independent magnetic circuit crop up because the electric circuit did not give permission for the flow of a third harmonic current. Obviously, the solution lies in providing a path for the third harmonic current in the 3 phase transformers connections. One of the solutions in a star-star connection is to provide a returned path for the third harmonic current by joining the star neutral to the source neutral. However, this option is not exercised because the third harmonic generated voltage of the generator phase would now appear across the primary winding of the transformers. Obviously, this third harmonic voltage would be transferred into the secondary side resulting in the flow of third harmonic currents in the transmission lines causing objectionable telephonic communication interference.
Therefore the only option left is to provide a delta at least on side of the transformer. Thus star-delta,delta-star, and delta-delta connections should be used. However, if a star-star connection has o be used then it should be provided with a tertiary delta. The presence of tertiary delta in star-star connection helps in minimizing shifting neutral problems, prevents current choking, and facilitates single-phase loading.
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| Tertiary Delta In Star-Star Transformer |