When is a transformer considered overloaded? Can transformers operate under overload?

In the operation of transformers, exceeding the current specified on the nameplate is considered overload operation. In general, long-term overload operation is not allowed. Overloading a transformer will cause the temperature to rise. The main factor determining the service life of a transformer is the degree of insulation aging, and temperature plays a decisive role in insulation aging. Research has shown that for every 8℃ increase in temperature during insulation operation, the service life will be reduced by half.

In actual operation, the load of most transformers is not always stable, and the load varies daily and seasonally. The insulation aging during low load periods is smaller than during rated load periods. Therefore, it is allowed to overload the transformer for a certain period of time without affecting its service life. That is to say, the transformer can be overloaded during peak load periods and in winter under the premise of not damaging the winding insulation and not reducing the service life. The allowable overload value should be determined based on the load curve of the transformer, the temperature of the surrounding cooling medium, and how much load the transformer has already carried before overloading.

Transformers have a fixed capacity, and when the electric load exceeds this capacity, it is considered overloaded. For example, a 10kV/400V transformer with a capacity of 1000KVA has a rated current of 58A and 1440A on the primary and secondary sides, respectively. When the secondary current exceeds 1440A, the primary current will inevitably exceed 58A, indicating that it is overloaded.

When a transformer exceeds its rated current and the temperature gradually rises, it is considered overloaded. For example, a 400KVA transformer with a secondary current of 600A is considered overloaded when the equipment is operated at 650-700A. In fact, a 600A transformer is already heavily loaded. Normal transformers work best at 60%-75% of their rated load.

Can transformers be overloaded?

The on-load voltage regulation technology of power transformers refers to the ability to adjust the transformer's turns ratio under load conditions. Transformers that use on-load voltage regulation methods belong to a type of stationary electrical equipment, which converts AC voltages within a certain range into different or multiple voltage values. 

1. Traditional on-load voltage regulation methods 

The traditional on-load voltage regulation device used in transformers is a mechanical tap changer. Taking the double-transition resistor as an example, the conversion switch is switched in order from right to left or left to right after the tap is selected. The drive gear of the mechanical switch is prone to cause operational accidents, which can reduce the reliability of the transformer and pose certain safety risks. Additionally, when the mechanical switch is actuated, it can generate an arc, causing slow burnout of the switch contacts. Therefore, after a certain number of operations, the contacts must be replaced. Another issue to consider is that the generated arc can cause the transformer's oil quality to decrease, thereby reducing the insulation ability of the winding and leading to phase-to-phase or turn-to-turn short circuits. According to some research data, accidents and failures of tap changers account for 10% to 20% of all transformer accidents per year during the period when traditional on-load voltage regulation methods were mainly used, and the failure rate of tap changers for 500 kV transformers was once as high as 25%, indicating that accidents and failures occurred frequently. Because the reaction time of mechanical switches is generally around 5 seconds, it takes a long time, so transformers that use on-load voltage regulation technology in the traditional sense can only be used for voltage regulation in stable states. 

2. New on-load voltage regulation methods 

Because of the above-mentioned shortcomings of traditional mechanical switches, various countries have actively researched and developed new on-load voltage regulation devices, which can be divided into three types according to the type of tap changer: mechanical improvement type, electronic switch type, and auxiliary coil type. 

Mechanical improvement type on-load voltage regulation technology 

This type of transformer is transformed from a traditional transformer by adding electronic circuits to the switch. Its tap changer only needs a small amount of thyristors and a transition resistor. The mechanical switch and electronic switch cooperate with each other to limit the generation of arcs during operation. The thyristor is connected to both sides of the traditional switch. When one branch is disconnected, the thyristor is triggered by the voltage of the contact. The gate current provided by the diode flows through the other branch, and the thyristor is cut off because the current passes through zero, so there is no arc. When the switch branch is closed, the current on the thyristor side is divided, and the original branch current is reduced, which can also limit the arc. The advantage of this type of device is that there is no need for a timing control circuit, and the requirements for the capacity of the thyristor are also low. Even if the thyristor loses control, it will not damage the tap changer of the transformer. The disadvantage is that the operating speed is slow. 

Auxiliary coil type on-load voltage regulation technology 

In fact, as early as the 1980s, researchers proposed to stack adjustable voltages on transformers by controlling the conduction angle of thyristors. Three-phase transformers and boost transformers can be connected to each other, and a reverse-connected thyristor can be used as a connection medium. If the trigger of the thyristor does not delay, it will be triggered when zero is crossed, and the voltage will be phase-loaded onto the load. If the trigger of the thyristor is delayed, the short-circuit switch set in advance can prevent the open circuit of the boost transformer. Later, other scholars have made improvements on this theoretical research basis and added a certain amount of auxiliary voltage to the transformer to ensure that the overlaid voltage is in phase with the original voltage. At almost the same time, Siemens proposed another on-load voltage regulation method based on auxiliary coils to achieve arcless operation. The specific method is to use a coil that can adjust the rated voltage, which is coupled to the transformer and has a voltage regulation range of positive and negative 0.63%. 

Electronic switch type on-load voltage regulation technology 

Due to the development and progress of electronic power technology, the capacity of thyristors has been raised to a new level, making it possible to directly control the electronic power switch of thyristors using a microprocessor without the need for mechanical switches as an auxiliary. The appropriate triggering time is used to minimize the power consumption of the thyristor. Currently, this technology is still in the experimental stage, and some problems such as the easy damage of thyristors and the short circuit caused by thyristor failures still need to be studied and solved. 

Comparison and analysis of the three new on-load voltage regulation technologies of power transformers 

The mechanical improvement type of on-load voltage regulation has the characteristics of high economy, low harmonic content, slow speed, restricted arcs, and no impact on the operation of the transformer when the thyristor loses control. The auxiliary coil type of on-load voltage regulation has the characteristics of high harmonic content, unrestricted arcs. The electronic power switch type of on-load voltage regulation has the characteristics of fast speed, a large impact when the thyristor loses control, low harmonic content, and high price. From this, we can see that the three types of on-load voltage regulation methods have their own advantages and disadvantages. Currently, our country has not conducted a systematic study of the on-load voltage regulation technology of new power transformers. If we can learn from some advanced research results abroad and improve the current on-load voltage regulation technology according to local technology, economy, and other actual conditions, the working performance of power transformers will be greatly improved.