HUARUI 
On Load Tap-Changing Oil Immersed Transformer
An on-load tap-changing oil-immersed transformer is a power device that, based on a conventional oil-immersed transformer, is equipped with an on-load tap-change (OLTC). It allows for automatic or manual adjustment of the transformer's output voltage without power interruption or load interruption.
Its core working principle is: Multiple taps (tap terminals) are set on the high-voltage or medium-voltage winding of the transformer via the on-load tap-change. During load operation, the tap positions are smoothly switched, thereby changing the transformer's turns ratio and achieving precise and stable control of the output voltage. This product is widely used in power grid nodes, industrial and mining enterprises, and high-power consumption scenarios where voltage stability is extremely important, and is a core device for ensuring power quality.
It enables smooth adjustment of the output voltage while the transformer is operating normally and the load is not interrupted, ensuring the continuity and stability of power supply.
Typically equipped with a tap range of ±8×1.25% or ±9×1.25%, offering multiple voltage regulation levels and precise compensation for line voltage drop, ensuring the output voltage remains stable within the acceptable range.
The electric operating mechanism enables remote or automatic rapid voltage regulation, with a response time typically of 5–15 seconds per level, adapting to the needs of rapid fluctuations in grid load.
Can be equipped with an automatic voltage controller (AVC) to automatically determine and execute voltage regulation commands based on the output voltage signal, achieving unattended operation.
Particularly suitable for scenarios with large load changes and drastic voltage fluctuations (such as metallurgy, steel rolling, electrified railways, etc.), effectively improving power quality.
Employing resistive or reactive transition switching technology, ensuring no arcing or short circuits occur during switching, resulting in smooth and reliable switching.
On-load tap changers are typically equipped with an operation counter and an online oil quality monitoring interface, which can record the number of switch operations and provide early warnings of maintenance needs.
| Item | Parameter Range / Description |
| Rated Capacity | 4000 kVA ~ 31500 kVA (Customization available) |
| HV Rated Voltage | 6 kV ~ 110 kV (Voltage levels customizable) |
| LV Rated Voltage | 0.4 kV ~ 35 kV (Customization available) |
| Voltage Regulation Method | On-Load Tap-Changing (OLTC) |
| Tap Range | ±8×1.25% / ±9×1.25% / ±10×1.25% / ±2×2.5% (User optional) |
| Tap Switch Type | Vacuum On-Load Tap Switch / Oil-Immersed On-Load Tap Switch (Resistive / Reactive Transition) |
| Cooling Method | ONAN (Oil-Immersed Self-Cooling) / ONAF (Oil-Immersed Air-Cooling) / OFAF (Forced Oil Circulation Air-Cooling) |
| Connection Group | Dyn11, Yyn0, YNd11, Yy0, etc. (Optional) |
| Insulation Heat Resistance Class | Class A (105℃) |
| Impedance Voltage | 4% ~ 12% (Depending on capacity and voltage level) |
| Losses | Complies with GB 20052, IEC 60076, DOE 2016, ANSI C57.12, etc.; low-loss optimization available |
| Number of Phases | Single-phase / Three-phase |
| Rated Frequency | 50 Hz / 60 Hz |
| Temperature Rise Limit | Top oil temperature rise ≤ 60 K Maximum winding temperature rise ≤ 65 K |
| Ambient Temperature Range | -40℃ ~ +45℃ |
| Voltage Regulator | Optional Automatic Voltage Controller (AVC); supports remote communication (RS485 / IEC 61850) |
Core
Constructed from laminated high-permeability grain-oriented silicon steel sheets, effectively reducing eddy current and hysteresis losses.
High and Low Voltage Windings
Winded with high-quality copper or aluminum wire. The high-voltage winding has multiple taps for connecting to the on-load tap changer.
On-load Tap Changer (Core Component)
Divided into two main parts: the changeover switch and the tap selector, working together to switch taps under load. Common types include vacuum switch type and oil-immersed changeover switch type.
Electric Operating Mechanism
Installed on the top or side wall of the transformer tank, including a motor, reduction gear, position indicator, and control circuitry, enabling electric or automatic voltage regulation.
Automatic Voltage Controller (AVC)
An intelligent control device that monitors the output voltage in real time and compares it with the set value, automatically issuing voltage increase or decrease commands.
Insulating Oil
Mineral oil or environmentally friendly natural ester oil (FR3), providing insulation, heat dissipation, and arc extinguishing functions. The oil in the on-load tap changer switching chamber must be independent or interconnected with the main oil tank.
Protection Accessories
In addition to standard gas relays, pressure relief valves, oil thermometers, and oil level gauges, additional safety devices such as tap changer action counters, oil pressure monitors, and explosion-proof diaphragms are included.
1. Core Product Advantages
① Stable and Reliable Voltage Quality: On-load tap changing effectively compensates for line voltage drops, ensuring that the voltage at the user end remains stable within the standard range, especially suitable for scenarios with long power supply lines and large load fluctuations.
② High Power Supply Continuity: The voltage regulation process does not interrupt the load, avoiding production interruptions and economic losses caused by power outages during voltage regulation, particularly suitable for continuous production industrial and mining enterprises.
③ High Degree of Automation: Combined with an AVC automatic controller, it achieves automatic voltage regulation, significantly reducing manual intervention and adapting to the development trend of smart grids and unmanned substations.
④ Extended Equipment Lifespan: Stable output voltage prevents damage to electrical equipment due to excessively high or low voltage, indirectly protecting downstream load equipment and extending the overall system lifespan.
⑤ Flexible Application Scenarios: It can be used in substation main transformers and industrial power distribution; it supports both three-phase and single-phase operation; it has a wide capacity coverage to meet various engineering needs.
⑥ Strong Fault Tolerance: When the on-load tap changer fails, it can switch to bypass operation mode or be manually locked to a certain tap position to ensure the transformer can continue to supply power (derating operation), avoiding a complete power outage.
2. Service Life
Under standard operating conditions and standardized maintenance, the transformer body's service life can reach 20-30 years, comparable to ordinary oil-immersed transformers. The operating life of the on-load tap changer is typically 200,000-500,000 cycles (depending on the model and switching capacity), requiring regular maintenance or replacement of the tap changer core according to the operation counter prompts.
1. On-Load Tap Changer Special Maintenance
① Operation Count Recording: Regularly record the on-load tap changer operation counter readings. When the cumulative operation count approaches the manufacturer's set maintenance cycle (usually 50,000 to 100,000 times), maintenance should be scheduled.
② Switchgear Chamber Oil Quality Testing: The oil quality in the on-load tap changer's switching chamber deteriorates faster than in the main oil tank (due to arcing during switching). It is recommended to sample and test the oil every 1-2 years or every 5,000 operations, filtering or replacing it as necessary.
③ Electric Mechanism Inspection: Regularly inspect the motor, gearbox, limit switches, and braking devices of the electric operating mechanism to ensure flexible operation and accurate position indication.
④ Controller Calibration: Regularly calibrate the automatic voltage controller (AVC) settings, voltage sampling accuracy, and communication functions to ensure the correct automatic voltage regulation logic.
⑤ Switching Test: During power outage maintenance, a manual/electric full-range tap changer switching test can be performed to check whether the indications of each tap are consistent and whether the switching is smooth and without abnormal noise.
2. Routine Transformer Maintenance (Same as Oil-fired Transformer)
① Daily Inspection: Check oil level, oil temperature, and operating sound; troubleshoot oil leaks and other problems.
② Oil Quality Testing: Routine testing of the main oil tank, with the same frequency as ordinary oil-fired transformers.
③ Component Inspection: Verify protective accessories such as gas relays and pressure relief valves.
④ Cleaning and Heat Dissipation: Regularly clean dust and debris from the radiator surface.
3. Safety Regulations (Same as Oil-fired Transformer, with added safety precautions for voltage regulation operations)
① Safety Precautions for Voltage Regulation Operations: On-load voltage regulation operations should only be performed after confirming that the voltage sampling circuit is normal and the controller parameters are correct. For remote automatic voltage regulation, ensure that no personnel are performing live work on site.
② No-Load Operations Prohibited: It is strictly forbidden to use a no-load tap changer for voltage regulation under load (this can easily cause arcing and burnout). On-load tap changing must rely on a dedicated on-load tap changer.
③ Other safety requirements are the same as for oil-immersed transformers (power outage testing, fire and explosion prevention, environmental safety, etc.).
1. Power Grid Substations: As the main transformer in regional substations, it automatically adjusts to ensure power quality during grid voltage fluctuations.
2. Industrial and Mining Enterprises: In industries with high power consumption and drastic load fluctuations, such as steel, metallurgy, chemicals, papermaking, and textiles, it ensures stable voltage for production equipment.
3. Electrified Railways: As the main transformer in traction substations, it adapts to drastic load changes caused by train operation, ensuring stable contact network voltage.
4. Data Centers and Precision Manufacturing: In applications with extremely high voltage quality requirements, on-load tap changing transformers work in conjunction with UPS systems to ensure power quality.
5. New Energy Grid Connection: As a step-up substation for large wind farms and photovoltaic power plants, it adapts to voltage changes caused by fluctuations in new energy power generation output.
6. Urban Distribution Network: Used as a line voltage regulator or distribution transformer at the end of long-distance distribution lines or in areas with dense loads.
1. Environmental Risks (Same as oil-fired transformers, plus specific risks related to voltage regulation)
In addition to the environmental risks of conventional oil-fired transformers (leakage pollution, high-temperature aging, low-temperature operation, corrosion/salt spray, fire, noise, vibration, etc.), on-load tap changers require special attention for the following:
1. Arcing Risk: On-load tap changers generate arcs during switching. It is necessary to ensure that the insulating oil in the switching chamber is of good quality and has sufficient arc-extinguishing performance. Vacuum-type on-load tap changers do not have this problem and are not affected by oil quality deterioration.
2. Controller Failure Risk: Automatic voltage controller (AVC) failure may lead to incorrect voltage regulation or failure to operate. A dual-controller redundancy or manual emergency operation plan should be configured.
3. Motor Mechanism Failure Risk: Motor or gearbox failure may cause the tap changer to jam in the intermediate position. A manual operating device should be configured as an emergency backup.
4. Communication Interruption Risk: In remote monitoring scenarios, ensure redundant and reliable communication links to prevent loss of voltage regulation commands due to communication interruptions.
2. Installation Restrictions (Same as Oil-fired Transformers)
The installation restrictions of conventional oil-fired transformers (indoor installation restrictions, space limitations, fire safety compliance, altitude compatibility, etc.) are fully applicable. Additional Notes:
Sufficient operating and maintenance space must be reserved for the on-load tap changer control box (usually ≥ 800mm in front).
Control cable channels must be laid separately and isolated from power cables to avoid electromagnetic interference affecting controller signals.
This product supports full-dimensional personalized customization. The main customization dimensions are as follows:
| Customization Dimension | Optional Range / Description |
| Capacity & Voltage | 4000 kVA ~ 100,000 kVA; HV ≤ 110 kV, LV ≤ 35 kV |
| On-Load Tap Changer Brand & Type | Vacuum OLTC: maintenance-free, suitable for frequent voltage regulation Oil-immersed OLTC: economical, requires regular maintenance International brands: MR (Germany); Domestic brands: Huaming, etc. |
| Tap Range | ±8×1.25% / ±9×1.25% / ±10×1.25% / ±2×2.5% (etc., user optional) |
| Connection Group | Dyn11, Yyn0, YNd11, Yy0, ZNyn11, etc. |
| Cooling Method | ONAN / ONAF / OFAF |
| Insulating Oil Type | Mineral oil / Environmentally friendly natural ester oil (FR3) / Synthetic ester oil |
| Winding Material | All copper / All aluminum / HV aluminum + LV copper |
| Voltage Regulation Control Method | Manual / Automatic (AVC) / Remote (SCADA) |
| Communication Protocol | RS485 / IEC 61850 / Modbus / DNP3.0 (Optional) |
| Accessories | Intelligent temperature controller, online oil level monitoring, OLTC operation monitoring, remote data transmission, fault alarm, automatic voltage regulation function (Optional) |
| Standards | GB 1094 / IEC 60076 / ANSI C57.12 / CSA / DOE energy efficiency rating |
| Color & Marking | User-specified color code (RAL, etc.); nameplate and warning labels in Chinese, English, or bilingual |
