Transformer Voltage Regulation: On-Load Voltage Regulation and Off-Load Voltage Regulation
In the vast and intricate "symphony hall" of the power system, transformer voltage regulation technology acts as a skilled "tuner," while on-load voltage regulation and off-load voltage regulation serve as different "tuning tools" in its hands, working together to maintain the harmonious stability of power supply quality.
I. No-Load Voltage Regulation: The "Old-Fashioned Tuning Box" of the Power World
No-load voltage regulation is based on the principle of electromagnetic induction, adjusting voltage by changing the turns ratio of the transformer windings-much like changing the strings of an instrument to alter its pitch. In a 10kV distribution transformer, the high-voltage side typically has 3–5 tap changers, each corresponding to a ±5% adjustment range of the rated voltage. When switching tap changers, the output voltage can vary between 9.5kV and 10.5kV.
However, using this "tuning box" requires the transformer to be "shut down." During operation, staff must first completely de-energize the transformer, wear proper insulated gear, and follow strict safety procedures to complete voltage testing, grounding, and other safety measures before manually operating the tap switch. The entire process not only has a long operation cycle but also poses certain safety risks. Therefore, no-load voltage regulation is suitable for scenarios with stable loads and minimal voltage fluctuations, such as rural power grids. During the transition between peak and off-peak seasons, staff can perform "seasonal tuning" to ensure the basic power supply requirements of the grid during different time periods.
Ⅱ. On-load voltage regulation: the "smart mixing console" of the power system
On-load voltage regulation breaks the limitations of voltage regulation under power outage, enabling smooth voltage adjustment while the transformer is operating under load, making it the "smart mixing console" of the power system. Its core equipment, the on-load tap changer, acts as an "intelligent hub." Based on structure, it can be classified into composite and combination types; based on transition methods, it can be classified into resistive and reactive types. Among these, resistive on-load tap changers are the most widely used due to their stable performance and controllable costs.
Taking the CM-type combined on-load tap changer as an example, during tap switching, the transition resistor acts as a "current buffer zone," playing a critical role. When the tap changer operates, it first connects the transition resistor to the circuit, limiting the circulating current generated during the switching instant, enabling the current to smoothly transfer from one tap to another. The entire switching process is completed within 10–20 ms, and voltage fluctuations are strictly controlled within ±1.5% of the rated voltage, ensuring uninterrupted power supply. In terms of equipment performance, on-load tap changers have a mechanical lifespan exceeding 1 million cycles and an electrical lifespan of 100,000–200,000 cycles, making them suitable for various complex power environments.
Ⅲ. Comparison of the two: Characteristics and applications of different "tuning tools"
No-load voltage regulation is similar to manual tuning, with a voltage regulation range typically of ±5% or ±2×2.5%, fewer voltage regulation levels (usually 3–5 levels), and relatively lower regulation accuracy; On-load voltage regulation, however, is automatic fine-tuning, with a voltage regulation range of up to ±8×1.25% or more, and a voltage regulation step count of 17–33 steps, enabling more precise and flexible voltage regulation.
In actual power grid applications, the two types of voltage regulation are clearly divided and work in conjunction with each other. On-load voltage regulation transformers in 220kV and above transmission grids act as the "chief voltage regulator," responsible for controlling the overall voltage and balancing reactive power; while off-load voltage regulation transformers at the end of distribution grids function as "local voltage adjusters," making static adjustments to local voltage based on seasonal load changes. With the advancement of power technology, the application of new vacuum on-load tap changers and IoT monitoring technology has made the "smart control panel" more precise and reliable, bringing higher-quality "sound quality" to the power system.