Yy Connection Combination: What are its characteristics?

The Yy connection combination, in addition to possessing the characteristics of the Y-shaped winding, exhibits distinct features based on the different structures of the core.

For core-type transformers:

a. Absence of third harmonic current results in the presence of third harmonic magnetic flux in the magnetic circuit. However, this flux cannot close within the core and can only circulate along the same direction through the three core columns, exiting the core through oil and the tank before returning. Due to the significant magnetic resistance outside the core, the third harmonic magnetic flux is relatively small, and consequently, the third harmonic voltage is usually not more than 5% of the fundamental. However, the third harmonic magnetic flux can cause eddy current losses in metal structural components.

b. Suitable for supplying unbalanced loads using a four-wire system, with minimal drift in the neutral points. Both neutral points can be grounded without conditions.

c. Under rated operating conditions, the maximum phase-to-ground voltage is only 1/√3 of the line voltage, and the voltage at the neutral point is practically zero. As a result, the insulation of the winding is subjected to lower voltage stress. Therefore, the Yy connection combination is commonly used in small-capacity three-phase core-type transformers. However, for large-capacity core-type transformers, it is not advisable due to significant eddy current losses in metal structural components.

For three-phase shell-type transformers or three sets of single-phase transformers:

a.The third harmonic magnetic flux circulates entirely within the core, leading to a significant third harmonic voltage. When the maximum magnetic density of the core aligns with the optimal value for the core structure, the third harmonic voltage can reach 30% to 60% of the fundamental, posing a severe challenge to winding insulation.

b.Under rated operating conditions, the maximum phase-to-ground voltage for each phase is also 1/√3 of the line voltage. However, when the maximum magnetic density of the core aligns with the optimal value for the core structure, the voltage at the neutral point is not actually zero. The neutral point voltage is zero only when the maximum magnetic density of the core does not exceed the inflection point of the magnetization curve. In this scenario, grounding the neutral point is allowed.

c.Grounding the neutral point alters the third harmonic voltage at each phase terminal. The capacitive current flowing through the winding induces an increase in the third harmonic voltage, causing interference with communication. If the neutral points of the generator and transformer on the primary side are not connected, the third harmonic voltage can cause significant interference with communication.

Therefore, the Yy connection combination is not suitable for three-phase shell-type transformers or three sets of single-phase transformers.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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