How to choose the magnetic flux density in the core of transformer-class products?

When selecting the magnetic flux density in the core, several considerations need to be taken into account.

1. Material of electrical steel sheets: If the magnetic flux density is chosen too high, the no-load current and no-load losses will increase rapidly. This is because electrical steel sheets exhibit magnetic saturation, and the relationship between magnetic flux and the current generating the flux is not linear. With an increase in voltage, the magnetic flux density also increases. When the magnetic flux density reaches a certain level, further increases in voltage result in minimal increases in saturated magnetic flux density. Therefore, for oil-immersed transformers, the magnetic flux density of cold-rolled electrical steel sheets should not exceed 1.75T, and the upper limit should be chosen below the saturation point.

2. Operating characteristics: Considerations for selecting magnetic flux density should account for normal and fault operation characteristics:

a. Normal operating characteristics: Power transformers need to be able to operate continuously at rated capacity when the voltage exceeds 5% of the corresponding tap voltage, or up to 10% when in no-load conditions. For test transformers and voltage regulators, the output voltage waveform must be sinusoidal. In three-phase transformers with Yym winding for high voltage as the neutral point in a grounded system, a reduction in magnetic flux density is necessary due to the displacement of the neutral point causing an increase in phase voltage when the low-side centerline current is at 25% of the low-side rated current.

b. Fault operating characteristics: Power transformers have short-circuit protection during operation, and short-circuit currents are allowed only for a very short duration. Although the magnetic flux density may increase by 1.39 times and 1.73 times during single-phase grounding, the selection of magnetic flux density can still be based on normal operating conditions. For Yyn winding three-phase double-winding transformers and single-phase three-winding voltage transformers, the normal operating magnetic flux density generally needs to be reduced to 50% of the saturation value. For three-phase three-winding five-column voltage transformers, due to different excitation characteristics in each phase to prevent parallel resonance overvoltage, the magnetic flux density also needs to be moderately reduced.

3. Winding connection scheme: The winding connection scheme affects the waveform of the magnetic flux. In the Yd winding, as the third harmonic current can form a loop in the d-shaped winding, the magnetic flux is sinusoidal, resulting in a sinusoidal induced voltage. In the Yy winding, as the third harmonic current has no loop, the excitation current is sinusoidal. However, due to the saturation characteristics of electrical steel sheets, the magnetic flux contains a third harmonic component. This leads to a flat-topped waveform for the magnetic flux and a peaked waveform for the secondary induced voltage. However, Yy winding is only used for three-column cores, where the third harmonic magnetic flux passes through air, resulting in a large magnetic reluctance. Therefore, the third harmonic component in the magnetic flux is not significant, and the magnetic flux remains close to a sinusoidal waveform. Thus, the magnetic flux density does not need to be excessively high. For single-phase voltage transformers with a V-shaped winding used in neutral-point ungrounded systems up to 35kV, there is generally no loop for the third harmonic excitation current. Therefore, the magnetic flux contains third harmonic magnetic flux. To maintain a sinusoidal waveform for the secondary induced voltage, the magnetic flux density should not be chosen too high.

4. Core temperature rise: The higher the magnetic flux density during normal operation, the higher the core temperature rise. If the core temperature rise cannot be reduced even after adding oil passages or gas passages, it is necessary to reduce the magnetic flux density during normal operation to meet the requirements of the thermal characteristics.

5. No-load noise: No-load noise is related to the magnetic flux density. A reduction of 0.1T in magnetic flux density can result in a reduction of approximately 2 to 3 dB(A) in no-load noise.

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.

Know more about power transformer and reactor : www.lstransformer.com