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# Transformer riddle no.5 – Ynyn Connection with Additional Delta Connection

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• #158
Shahbazi

As you know, because the B/H curve of the magnetic material forming the transformer Core is not linear.
As you know, because the B/H curve of the magnetic material forming the transformer Core is not linear. If a sinusoidal voltage is being applied for a sinusoidal flux (and hence a sinusoidal secondary voltage), the magnetizing current is not sinusoidal. Thus the magnetizing current of a transformer having an applied sinusoidal voltage will comprise a fundamental component and various harmonics. The magnitude and composition of these harmonics will depend on the magnetizing characteristic of the core material and the value of the peak flux density. If the harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion. Earthed Star connection and delta connection are suitable path for zero sequence currents.
In power transformers with "Ynyn" connection , there are suitable low- impedance path for third harmonic and other zero sequence currents (due to unbalance loads), as we can say: the tertiary delta connection is not necessary when two sides of star-star connection of transformer are earthed directly.

Is it correct? Whether we always don't need any delta tertiary winding in transformers with Ynyn connection?

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• #1322
AMIR KHASHAYAR

In absence of tertiary winding , the current arisen from third harmonic flux and unbalanced loads will pass through neutral point of wye winding and unknown routes up to neutral point of source. This current will interfere with other systems particularly communication systems . Having a tertiary winding leads to circulating of third harmonic flux inside the tertiary winding and unbalanced load currents converts to negative phase sequence current . On the other hand , if the said current passes the neutral point of wye winding , this point will be warm unavoidably and if it’s grounded through impedance , it will fluctuate over earth potential .

#1615
R K Mohapatra

Considering a Star/Star transforemr with Neutral on both sides , it can presumed that the 3rd Harmonic current goes back to the Source through the Neutal . but the provision of a Delta winding reduces the current due to it’s % impedence and grounding a corner point if Delta provides a local path for the Harmonic current through the Neutal of the winding . If it desired the tertiary winding can be kept floating ( ungrounded ) and the Harminic current will flow in the loop stabilizing the voltage .

#1618
Hamid

Quoted by AMIR KHASHAIAR : ” This current will interfere with other systems particularly communication systems .” I agree with AMIR. Indeed we can use delta connection as a three harmonic current trap. Telephone circuits are particularly susceptible to the influence of ground return currents. In rural areas when power or telephone circuits use a ground return, the large inductive loop can have a large influence on the noise produced in telephone circuits. Particular care should be used to minimize nonlinear loads in this case. A voice band telephone channel is normally designed to pass frequencies between 300 and 3000 Hz. Although harmonics in this range of frequencies are very small compared with the fundamental, they still have an effect on the telephone reception. The effect that a

#3224
Nawaz

Core type Yyn transformers with 3 limbs do not necessarily need a tertiary delta winding because the transformer tank itself acts as an effective delta winding. This is known as the tank-delta effect.

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