On-Load Tap-Changing Power Transformer

I. Basic Definitions

On-Load Tap-Changing Power Transformer (OLTC Transformer): A power transformer that stabilizes output voltage by changing the turns ratio through switching winding taps via an on-load tap changer while under load and without power interruption.

II. Core Working Principle

- Based on the turns ratio formula: U₁/U₂ = N₁/N₂

- Switches high-voltage winding taps to change the effective number of primary turns N₁, thereby stabilizing the secondary voltage U₂

- The on-load tap changer uses a "make-before-break" mechanism with transition resistors/vacuum arc quenching to complete switching within 10–20ms, ensuring uninterrupted power supply and no significant arcing

III. Key Component: On-Load Tap Changer (OLTC)

- Core Function: Safe switching of winding taps while energized

- Main Composition: Vacuum interrupter, transition resistor, rapid operating mechanism, sealed oil chamber

- Lifespan: Mechanical life > 500,000 cycles, electrical life > 200,000 cycles

IV. Main Features

- ✅ Voltage regulation without power interruption: Adjustable during load operation, ensuring uninterrupted power supply

- ✅ Wide voltage regulation range: Typically ±10% (total 20%) for 110kV and above; common configurations include ±8×1.25% or ±4×2.5%

- ✅ Automatic/Manual Control: Can be equipped with intelligent control systems for automatic voltage stabilization

- ✅ Improves power supply quality: Suppresses voltage fluctuations and adapts to scenarios with severe load variations

- ⚠️ Higher cost and larger size: More expensive and complex to maintain than same-capacity off-circuit tap-changing transformers

V. Typical Application Scenarios

- Main transformers in 110–500kV substations (load centers, areas with frequent voltage fluctuations)

- High-reliability power supply scenarios such as urban distribution networks, industrial parks, precision manufacturing, and data centers

- Grid connection for new energy sources like photovoltaic and wind power, to suppress voltage fluctuations

- Critical loads where voltage regulation during power outages is not allowed (hospitals, transportation hubs, etc.)

VI. Differences from Off-Circuit Tap-Changing

- On-Load Tap-Changing: Switching during energized operation, automatic/manual, wide range, high cost

- Off-Circuit Tap-Changing: Requires power interruption, manual operation, narrow range, low cost