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Transformer Common Knowledge Q&A

What are the types of power transformers, and what are their main components?

Answer: With the development of power systems, the demand for power transformers has increased, leading to various types. Classified by phase, there are single-phase and three-phase transformers. Based on the position of windings and core, there are core-type and shell-type transformers. According to cooling methods, there are dry-type, air-cooled, forced oil circulation air-cooled, and water-cooled transformers. Regarding neutral point insulation, transformers can be fully insulated or partially insulated. Based on winding materials, there are A, E, B, F, and H insulation classes. Different types of transformers have specific operational requirements. The main components of a power transformer include the core, windings, bushings, oil tank, oil pillow, radiator, and associated equipment.

What is inrush current in transformers, and what causes it?

Answer: Inrush current in transformers refers to transient currents generated in the windings during the initial voltage energization.

When residual magnetic flux in the transformer core aligns with the working voltage's magnetic flux during energization, the total magnetic flux greatly exceeds the core's saturation flux, resulting in a significant inrush current. The peak value of the inrush current can be 6-8 times the transformer's rated current. The magnitude of the inrush current depends on factors such as the system voltage phase angle during energization, the residual magnetic flux in the transformer core, and the impedance of the power source system. Maximum inrush current occurs at the instant the voltage passes through zero (peak flux). Inrush current contains DC and high-order harmonic components and decays over time, influenced by circuit resistance and reactance, typically taking 5-10 seconds for large transformers and around 0.2 seconds for small transformers.

What are the methods of voltage regulation in transformers?

Answer: There are two methods of voltage regulation in transformers: on-load voltage regulation and off-load voltage regulation.

On-load voltage regulation allows adjustment of the tap position during operation to change the transformer turns ratio and achieve voltage regulation. Two common types of on-load voltage regulation are line tap and neutral-point tap. The neutral-point tap has advantages like reduced insulation requirements but requires direct grounding during transformer operation.

Off-load voltage regulation involves adjusting the tap position when the transformer is de-energized or during maintenance to achieve voltage regulation.

What is a fully insulated transformer, and what is a partially insulated transformer?

Answer: A fully insulated transformer, also known as level insulation, has consistent insulation levels across its windings. In contrast, a partially insulated transformer, or graded insulation, has lower insulation levels near the neutral point than the winding ends.

What is the difference in operating principles between voltage transformers and current transformers?

Answer: Voltage transformers are primarily used for measuring voltage, while current transformers are used for measuring current.

(1) The secondary side of a current transformer can be short-circuited but not open-circuited. In contrast, the secondary side of a voltage transformer can be open-circuited but not short-circuited.

(2) Relative to the load on the secondary side, the primary impedance of a voltage transformer is negligible, making it practically a voltage source. In contrast, a current transformer has a high primary impedance, effectively behaving as a current source with infinite internal resistance.

(3) In normal operation, the magnetic flux density of a voltage transformer is close to saturation, causing voltage drop during system faults. On the other hand, a current transformer operates with low magnetic flux density during normal conditions, but in the case of a short circuit, the primary current increases, leading to significantly higher flux density, potentially causing increased output current errors. Therefore, it's advisable to choose current transformers that are less prone to saturation.

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