The core of selecting an overload transformer lies in balancing “short-term overload capacity” and “long-term operational reliability”. It requires comprehensive consideration around five core dimensions: load characteristics, overload parameters, technical performance, scenario adaptability, and economy. This is to avoid equipment overheating and damage, power supply interruptions, or investment wastes due to improper selection. The specific key factors are as follows:

1. Clarify load characteristics: the core basis for model selection

The load characteristics directly determine the core parameters of an overloaded transformer, and the following three points need to be analyzed:

Rated load and peak load: First, determine the long-term rated load (Pn) of the transformer, which is the average load during daily operation (usually designed to be more economical with a load rate of 70% to 80%); then specify the peak load (Pmax) – including equipment start-up impact load (such as motor and compressor start-up current), time-of-day peak load (such as electricity peak usage and process switching load), ensuring that the peak load does not exceed the transformer’s short-term overload limit (the overload factor marked by the manufacturer × rated capacity).

Overload duration and frequency: The overload capacity of an overload transformer is strictly limited by time (e.g., 1.1 times overload for 2 hours, 1.2 times overload for 1 hour, 1.3 times overload for 30 minutes). It is necessary to record the single duration of peak load (e.g., the evening peak load lasts for 3 hours every day, equipment startup lasts for 10 minutes) and the monthly/annual overload frequency (e.g., 10 overloads per month, overloads only in summer), to avoid long-term frequent overloads that accelerate insulation aging.

Load type: Distinguish between impact loads (such as machine tool and crane startup, with short duration and sudden current increase) and continuous peak loads (such as air conditioning peak, with longer duration and stable load); inductive loads (such as motors) have high starting current, requiring focus on the transformer’s impact resistance; capacitive loads (such as capacitor compensation devices) may generate harmonics, requiring consideration of the transformer’s harmonic resistance.

II. Confirm overload parameters: match the limit capability of the equipment

The overload performance of an overload transformer is determined by the manufacturer’s design. When selecting a transformer, it is necessary to specify the following parameters to avoid exceeding the limits:

Overload multiple and duration: These are core parameters that need to be selected based on the peak load and duration – for example, if the peak load is 1.2 times the rated capacity and lasts for 1.5 hours, a product labeled “1.2 times overload for 2 hours” or above should be selected; note: the higher the overload multiple, the shorter the allowable duration, and there needs to be sufficient “cooling recovery period” after overload (such as operating at no load or low load for 1-2 hours after overload to avoid continuous overload).

Insulation and heat resistance level: The temperature of transformer windings will rise sharply during overload. The insulation level determines the short-term high-temperature resistance capability – commonly used insulation levels are Class F (*allowable temperature 155℃) and Class H (180℃). For overload scenarios, it is recommended to choose Class F or above to ensure that the insulation does not carbonize or age during overload.

Cooling method: The cooling capacity directly affects the overload duration. Transformers with natural air cooling (ONAN) have a weaker overload capacity, while those with forced air cooling (ONAF) or forced oil-circulating cooling (OFAF) exhibit higher cooling efficiency and can support longer overload periods (for example, in ONAF mode, the duration of 1.2 times overload can be extended to 2 to 3 hours). For outdoor installation, the impact of ambient temperature on cooling should be considered. In high-temperature areas, it is recommended to choose products with enhanced cooling capabilities.

III. Evaluating technical performance: ensuring long-term stable operation

In addition to overload capacity, the basic technical performance of the transformer must meet the needs of the specific scenario, with a focus on:

Winding material and structure: Copper windings are preferred (with good conductivity, low loss, and high temperature resistance) and are more suitable for overload scenarios compared to aluminum windings. The winding structure needs to be compact, utilizing vacuum impregnation technology to enhance insulation strength and heat dissipation, thereby avoiding local overheating during overload.

Core performance: The core utilizes low-loss silicon steel sheets (such as grade 30Q130) to reduce no-load losses and no-load current, thereby minimizing energy consumption during non-overload periods. The core structure must be sturdy to prevent vibration and noise caused by electromagnetic forces during overload conditions.

Protection device configuration: Overloaded transformers require comprehensive protection functions – temperature protection (winding thermometer, oil temperature thermometer, overtemperature alarm or tripping in case of overload), overcurrent protection (overcurrent relay to avoid long-term overload), and short circuit protection (fuse or circuit breaker to prevent short circuit faults); intelligent transformers can be remotely monitored for temperature and load, facilitating timely adjustment of operating conditions.

Voltage regulation capability: In certain scenarios (such as voltage fluctuations in the power grid or voltage drops due to load variations), transformers must possess voltage regulation functions. It is advisable to choose either on-load voltage regulation or off-load voltage regulation models (on-load voltage regulation allows for voltage adjustments during operation, making it more suitable for scenarios with significant load fluctuations), to ensure stable output voltage.

IV. Applicable Scenarios and Installation Conditions: Avoidance of Usage Restrictions

The installation environments and power supply requirements vary across different application scenarios, necessitating targeted considerations:

Installation environment: For outdoor installation, products with a protection level of IP54 or higher should be selected to withstand wind, rain, and dust. In high-temperature environments (such as metallurgical workshops or outdoor summer conditions), models with high temperature resistance and enhanced heat dissipation should be chosen. In humid environments (such as basements or ports), proper moisture-proof measures should be taken to prevent insulation from being affected by moisture. In areas with an altitude of over 1000 meters, plateau-type transformers should be selected. As the altitude increases, heat dissipation efficiency and insulation strength decrease, necessitating derating (for example, if the altitude is 2000 meters, the rated capacity should be derated by 10%).

Power supply system requirements: Confirm the grid voltage level (such as 10kV, 35kV) and frequency (50Hz/60Hz), ensuring that the transformer’s rated voltage matches the grid; for three-phase loads, a three-phase transformer should be selected. In scenarios with unbalanced loads (such as a high number of single-phase devices), the transformer’s ability to withstand unbalanced loads should be considered to avoid neutral line overload.

Space and maintenance conditions: For large industrial settings, it is necessary to consider the footprint and weight of transformers to ensure that the installation site meets the required load-bearing capacity. Maintenance space (such as space on both sides of the oil tank and overhead space) should be reserved to facilitate regular checks on oil temperature and cleaning of the radiator. In outdoor or densely populated areas, attention should be paid to noise levels (typically required to be ≤65dB) to avoid affecting the surrounding environment.

V. Balancing economy and scalability: balancing investment and long-term costs

Initial investment and equipment utilization: The initial investment of an overload transformer is slightly higher than that of an ordinary transformer, but “over-sizing” should be avoided. If the peak load duration is long (such as exceeding 4 hours per day) or the overload frequency is high*, it is recommended to directly choose an ordinary transformer with a larger rated capacity rather than relying on overload capacity. If the peak load is short and the frequency is low, choosing an overload transformer can reduce initial investment and improve equipment utilization (avoiding the situation where an ordinary transformer is “over-sized”).

Operational loss and energy consumption cost: The load loss (copper loss) of a transformer increases with the load. When overloaded, the loss rises sharply (the loss is proportional to the square of the load). It is necessary to calculate the energy consumption cost of long-term overload – for example, frequent 1.2 times overload may increase the annual energy consumption cost by 10% to 20%. The pros and cons of saving initial investment and increasing energy consumption in the later stage need to be weighed.

Capacity expansion and compatibility: Considering future load growth (such as factory expansion and user increase), select an overload transformer with a rated capacity that allows for a certain margin (usually 10% to 20%) to avoid replacing the equipment again in the short term. At the same time, ensure that the transformer is compatible with the existing power distribution system (such as circuit breakers, fuses, and cables), so that the transformer will not trip due to insufficient rated capacity of supporting equipment during overload.

VI. Verify manufacturer qualifications and after-sales service: ensure product quality

Manufacturer Qualifications: Select a formal manufacturer with a power transformer production license and ISO9001 quality system certification. The products must pass type tests (such as short circuit tests and overload tests) to ensure that the overload performance meets standards. Prioritize manufacturers with experience in special design for overload transformers to avoid modifications to general-purpose transformers (modified products have poor overload reliability).

After-sales and warranty: Confirm the warranty period (usually 1-3 years), request the manufacturer to provide guidance on overload operation (such as overload duration and cooling requirements); clarify the after-sales response time, especially for industrial and emergency scenarios, where it is necessary to choose manufacturers with dense after-sales service networks and the ability to provide quick on-site maintenance, to avoid long downtime caused by failures.

Summary of core selection logic

The key to selecting an overload transformer is to “match the overload characteristics of the load” – neither choosing a product with insufficient overload capacity (leading to frequent tripping and equipment damage), nor blindly pursuing a high overload multiple (resulting in investment waste and increased energy consumption). It is necessary to first clarify the “overload multiple, duration, and frequency” through load statistics, and then comprehensively screen based on technical performance, scene conditions, and economy, to achieve the goal of “reliable short-term overload and economical long-term operation”.