In modern power distribution systems, dry-type transformers are widely used in commercial buildings, industrial plants, data centers, and new energy projects due to their safety, environmental friendliness, and low maintenance costs. However, in actual operation, many users allow dry-type transformers to operate under low-load conditions for extended periods, meaning the actual load is far below the rated capacity. So what exactly are the consequences of low-load operation of dry-type transformers? Xinhong Electrical combines product technical parameters and industry experience to provide you with an in-depth analysis.
Low-load operation generally refers to a dry-type transformer operating long‑term at less than 30% of its rated capacity, or even at no‑load or near‑no‑load conditions. For example, if a SCB type dry-type transformer rated at 1000kVA operates long‑term with an actual load below 300kVA, it can be considered low‑load operation. Although this may seem "safer" because the load is light, such operation actually leads to a series of technical, economic, and safety consequences.
The total losses of a dry-type transformer consist of no-load losses (core losses) and load losses (copper losses). No-load losses depend mainly on the core material and manufacturing process, and are essentially independent of the load level. Load losses, on the other hand, increase with the square of the load current.
Under low-load conditions, load losses are very small, but no-load losses remain almost unchanged. For example, the SCB13-800/10 dry-type transformer produced by Xinhong Electrical has a no‑load loss of approximately 1000W. If it operates long‑term at 10% of rated current, no‑load loss can account for more than 85% of total losses, causing the transformer’s operating efficiency to drop to 80% or even lower.
Low efficiency means that each kilowatt‑hour of electricity is used less effectively, which is directly reflected in the electricity bill – the user pays nearly the same no‑load electricity cost while obtaining very little useful output.
When a dry-type transformer operates under low load, the proportion of excitation current increases. The excitation current is inductive reactive power, causing the system power factor to drop. For substations equipped with reactive power compensation devices, improper compensation control under light‑load conditions may lead to over‑compensation or oscillating switching. If the long‑term power factor falls below the grid standard (e.g., 0.9), the power supply company will impose additional charges according to the "Power Factor Adjustment Electricity Fee Method", resulting in hidden losses.
From a physical life perspective, low load leads to lower winding temperature rise, reducing thermal stress on the epoxy resin insulation material and copper conductors, which indeed helps slow down insulation aging. However, when a dry-type transformer operates under extremely low load or even no‑load for long periods, there may be risks of partial discharge or moisture ingress – especially in high‑humidity environments where the transformer's own temperature is far below its design upper limit, internal moisture is not easily expelled, which may instead reduce insulation reliability. Xinhong Electrical uses vacuum casting technology in the manufacture of all its dry‑type transformers, ensuring partial discharge levels below 5pC. Nevertheless, users still need to pay attention to the operating environment and cannot neglect routine maintenance simply because of low load.
Standard dry-type transformers do not have on‑load tap changers. Under low load, the internal impedance voltage drop of the transformer is very small, so the secondary output voltage approaches the no‑load voltage, typically 2%~5% higher than the rated voltage. For precision electronic equipment or lighting loads, long‑term overvoltage can shorten equipment life or even cause failures. If capacitors are also present in the system, there may be a risk of ferroresonance.
Many users mistakenly believe that "the lower the load, the more electricity is saved", but the opposite is true. Take a 1000kVA dry-type transformer as an example:
Annual no‑load loss: 2000kWh (at 240);
Load loss decreases with the square of the load. At 10% load, load loss is only 1% of rated copper loss;
Total loss is almost equal to no‑load loss, yet the active power delivered is only about 80kW (assuming power factor 0.8).
Loss per unit of active power is far higher than in the economic load zone (typically 40%~65% of rated capacity).
Therefore, long‑term low‑load operation is an uneconomical "big horse pulling a small cart" condition. The proper approach is to select a transformer capacity that matches the actual load profile, or use multiple transformers in parallel with dynamic switching.
As a professional power equipment manufacturer, Xinhong Electrical is committed to providing high‑efficiency, energy‑saving dry‑type transformer products and scientific operation guidance. For existing or potential low‑load scenarios, we offer the following recommendations:
During the project design phase, consider load growth over the next 5‑10 years and avoid blindly oversizing capacity. Xinhong Electrical offers dry‑type transformers with multiple efficiency levels, including SCB10, SCB12, SCB13, and SCB14. The SCB14 series, for example, has no‑load losses more than 30% lower than national standards, reducing absolute losses even under low‑load conditions.
For locations with large load fluctuations (such as schools, shopping malls, seasonal factories), consider using two smaller dry‑type transformers in parallel. During low‑load periods, operate only one; during high‑load periods, operate both. Xinhong Electrical can design automatic switching control systems for you to track the economic load range.
Install multifunction energy meters and temperature control systems on the low‑voltage side of the dry‑type transformer to monitor losses, load factor, and winding temperature in real time. When the load factor remains below 20% for a set period, the system can issue an alarm or recommend switching operation modes.
Even under low load, follow the "Dry‑type Transformer Operation and Maintenance Regulations" by measuring insulation resistance and absorption ratio every six months, and checking whether the core grounding is proper. All Xinhong Electrical products come with a detailed maintenance manual and remote technical support.
Low‑load operation of a dry‑type transformer is not a harmless condition. Its main consequences include: reduced operating efficiency, degraded power factor, increased loss per unit of output power, as well as potential overvoltage at the secondary side and moisture ingress risks. Although low load can slow down thermal aging of insulation, considering overall economics and system reliability, long‑term extremely low‑load operation is not recommended.
Choosing a Xinhong Electrical dry‑type transformer means not only obtaining a high‑quality product with low losses, low partial discharge, and high reliability, but also having a professional team to support you throughout the entire lifecycle – from selection to operation and maintenance. For more information on dry‑type transformer load optimization, energy efficiency retrofits, or capacity evaluation, please visit our Google-indexed official website or contact your local sales engineer.
