The working principle and basic knowledge of transformers

I. Types and Components of Power Transformers

By phase number: single-phase, three-phase

By cooling method: dry self-cooling, air cooling, forced oil circulation air cooling/water cooling

According to the voltage regulation mode: on-load voltage regulation and no-load voltage regulation

By insulation class: A, E, B, F, H

Main components

Core, winding, bushing, oil tank, oil conservator, radiator. 

2. Wiring group of transformer winding

Wiring method of Y0/Δ11:

High-voltage side (Y connection) : A-X, B-Y, C-Z

Low-voltage side (Δ connection) : a-x-b-y-c-z

Potential vector diagram: Star-shaped on the high-voltage side, triangular on the low-voltage side, with the low-voltage side lagging behind the high-voltage side by 30°.

Iii. The Mystery of Excitation Inrush Current

Definition: Transient current in the winding during full-voltage charging (up to 6 to 8 times the rated current).

Cause: The superposition of residual magnetic flux and voltage magnetic flux leads to core saturation.

Features: It contains DC and high-order harmonic components. The attenuation time is approximately 5 to 10 seconds for large capacities and about 0.2 seconds for small capacities.

Iv. The Necessity of Transformer Impulse Tests

Objective: To verify the insulation strength and inrush current resistance capacity.

Frequency: 5 times for new transformers and 3 times after major overhauls.

V. Three Ironclad Laws for Parallel Operation of Transformers

The transformation ratio is equal.

The short-circuit voltages are equal.

The wiring groups are the same.

Consequences of violation: balanced current, capacity waste, equipment burnout!

Vi. The uniqueness of Autotransformers

Features: There is an electrical and magnetic connection between the primary and secondary sides, with a small size and low cost.

Operation note: The neutral point must be grounded to prevent overvoltage and short-circuit current.

Vii. The Secrets of Pressure Regulation Methods and Taps

On-load voltage regulation: Dynamic regulation during operation (line end/neutral point).

No-load voltage regulation: Regulation during power-off maintenance.

The position of the tap: mostly on the high-voltage side (with low current and easy insulation handling).

Viii. Over-excitation: The Hidden Killer of Transformers

Cause: Voltage ↑ or frequency ↓→ magnetic flux saturation.

Consequences: Iron loss ↑, overheating, insulation aging.

Protection: Limit voltage and install over-excitation protection.

Ix. Safety Guardians of Transformers

Oil pillow: Buffers the volume change of oil and prevents oxidation.

Moisture absorber: Silica gel absorbs moisture (replace when it turns red). Explosion-proof pipe/pressure valve: Pressure relief protection in case of internal failure.

Gas protection: Light gas alarm, heavy gas trip. X. Core Differences of Transformers

Current transformer: The secondary side is open-circuited, which is equivalent to a current source.

Voltage transformer: Short circuit on the secondary side is prohibited, and it is equivalent to a voltage source.

Xi. Overload Rules for Transformers

Normal overload: Equivalent aging of lifespan, hot spot temperature ≤140℃.

Accident overload: Sacrificing service life to ensure power supply, current ≤2 times the rated value.

Twelve. The Necessity of Neutral point Grounding

Autotransformer: Prevents overvoltage on the low-voltage side.

Graded insulation transformer: The neutral point insulation level is low and it must be grounded.

Thirteen. Temporary Switching of gas protection

Signal reconnection scenarios: oil filtration, maintenance, DC grounding, severe oil leakage.

Fourteen. Emergency Time for Cooling System failure

Forced oil circulation transformer: 125MVA and below: 20 minutes

For 125MVA and above: 10 minutes (can be extended to 1 hour when the oil temperature is ≤75℃).

Summary

Transformers are the "heart" of power systems. Mastering their principles and key points of operation and maintenance is a compulsory course for electrical professionals! From excitation inrush current to parallel operation, from voltage regulation techniques to overload protection, every detail is related to system safety.