Understanding Electric Furnace Transformers: Essential Guide

Overview of Electric Furnace Transformers

Electric furnaces are important industrial equipment. They are used in metallurgy, chemical engineering, and machinery. In metallurgy, they smelt alloy steels and ferroalloys. In chemical engineering, they make yellow phosphorus, calcium carbide, and synthetic resins. In machinery, they smelt cast steel and cast iron.

The electric furnace transformer gives power to the furnace. People often call it the heart of the furnace. The transformer lowers the grid voltage and gives the power that the furnace needs. It has high power and high efficiency. The stable operation of an electric arc furnace is crucial. It’s crucial to purchase a suitable and reliable transformer for electric furnace. A thorough understanding of the operating principles, load characteristics, and transformer requirements of an electric arc furnace is crucial.

Working Principle of an Electric Arc Furnace

Electric arc furnaces are mainly used to refine high-quality and alloy steels. Modern arc furnaces use direct heating with a non-conductive furnace bottom. Electric arc furnaces operate based on the principle of direct heating of the metal. An arc forms between each electrode and the metal charge, with the molten metal forming the neutral point of the load.

Steelmaking Process

The electric arc furnace steelmaking process is divided into three stages:

1. Melting Period： The electric arc furnace transformer lowers the high grid voltage (6-110 kV) to the voltage needed for the furnace to work. This voltage is used on the three electrodes of the electric arc furnace. It creates an arc between the electrodes and the material inside. This heat, reaching temperatures exceeding 3500°C, melts the charge. The melting time increases with furnace capacity, generally taking 1-2.5 hours. Energy consumption is highest during the melting phase, and the transformer operates under overload.

2. Oxidation Period: This time is used to remove impurities from the molten steel. It also helps keep the temperature and composition of the steel the same throughout. The reduction phase, also called the refining phase, further removes impurities and adjusts the chemical composition of the steel to the desired steel grade.

3. Reduction Phase: Further refines the steel and adjusts the chemical composition to the target standard. During the oxidation and reduction phases, power demand is reduced, and the transformer operates at a constant secondary output current and reduced voltage.

Load Characteristics of Arc Furnace Transformers

During the initial melting phase, the heat absorbed by the cold charge is significant, requiring a high power input. To shorten the melting time, the transformer is typically operated at a 20% overload. During the oxidation phase, power is consumed by heat loss, slag heating, and melting, so the power input is reduced. During the reduction phase, the power input to the furnace is even lower because the metal is overheated and exothermic chemical reactions are occurring.

A typical load diagram for an arc furnace transformer is shown in the figure. During the melting phase, the load current is allowed to be controlled at 1.2 times the rated current.
The permissible duration is 0.55 t (the cycle time t refers to the time required from charging to tapping a heat of steel), but the maximum permissible duration is 2.5 hours.

Specific manifestations:

Large Load Fluctuations: In the melting phase, the arc is unstable. The current and voltage change a lot. The load is very unstable.

Strong Short-Term Overload Capacity: The transformer must handle short surges that are several times higher than the rated current.

Significant Operating Cyclicity: The melting phase, the oxidation phase, and the reduction phase require different voltage and current. This makes a clear cycle.

Frequent Inrush Current: During the start phase and the arc burning phase, the current fluctuates significantly. This puts high stress on insulation and mechanical strength.

Low Power Factor: It is usually less than 0.7. This makes the grid harder to run.

High Harmonic Content: Arc discharge makes many harmonics. These can affect both the grid and the transformer.

Technical requirements for arc furnace transformers

During the melting period and the initial oxidation stage, the arc furnace is often in an unavoidable short-circuit state. The number of short-circuits during the entire melting period can reach dozens or even hundreds of times. Since short-circuits are inevitable, arc furnace transformers must be designed to handle working short-circuit conditions. The technical requirements for arc furnace transformers are mainly as follows:

(1) During the melting period and the initial oxidation stage, frequent working short-circuits will bring great danger to the transformer. In order to stabilize the arc burning and limit the working short-circuit current value, the transformer’s own impedance should be increased or a reactor should be connected in series in the transformer circuit to ensure that the working short-circuit current does not exceed 3 times the rated current. When the capacity is equal to or greater than 10,000 kVA, since the short-circuit impedance is large enough, there is no need to increase the transformer’s own impedance or attach a reactor. In the late oxidation and reduction stages, this danger is basically over. In order to reduce losses and improve the power factor, the tap changer can be used to reduce the transformer’s own impedance or remove the reactor.

(2) The arc furnace transformer is allowed to be overloaded by 20% during the melting period. According to its typical load curve, the root mean square current in its cycle

Thus, the calculation of the transformer’s load loss, temperature rise, and the selection of switches, bushings, current-carrying conductors, etc., should all be considered based on 1.121H

(3) Since the arc in the arc furnace often extinguishes and ignites suddenly, the arc furnace transformer is often in a transient state, resulting in repeated operation overvoltage. It is often the case that the insulation of the windings, switches, bushings, etc., cannot withstand such overvoltage and is shut down. Especially for transformers with a large voltage regulation range, when operating at a higher output voltage, the idle primary winding will generate a non-negligible power frequency voltage rise, resulting in a high potential in certain parts of the windings and switches. Therefore, the insulation of the arc furnace transformer should be carefully checked according to the actual structure, and reliable measures should be taken to improve operational reliability.

(4) Due to the drastic changes in the load current of the arc furnace, the winding vibrates. If the preload is insufficient, the large axial electromagnetic force may cause insulation and spacers to suffer from mechanical oscillation due to the appearance and disappearance of the “air gap”, resulting in insulation damage. Therefore, the design of the arc furnace transformer should reasonably select the geometric dimensions and insulation thickness of the conductor, and the winding compression force should be guaranteed in manufacturing to resist the impact of the electromagnetic force. When the working short-circuit current is set at 3 times the rated current and lasts for 6 seconds, all parts of the arc furnace transformer should be free of damage. In addition, the arc furnace transformer should be able to withstand the effect of a sudden short circuit with a stable value of the short-circuit current not exceeding 10 times the rated current and lasting for 2 seconds without damage.

Practical Applications and Maintenance of Electric Furnace Transformers

When using an electric furnace transformer, please note the following:

1. Regular Inspection: Check the electric furnace transformer often. This makes sure it works well. It also helps find problems early and fix them.

2. Temperature Control: The transformer makes heat when it works. Keep the temperature right to stop overheating and faults.

3. Load Balancing: Keep the furnace load balanced. This lowers stress on the transformer and makes it last longer.