The conversion of AC arc furnaces to DC

Why need conversion of AC arc furnaces to DC arc furnaces ?

In the steel smelting industry, electric arc furnaces serve as critical smelting equipment, with alternating current (AC) arc furnaces being the earliest and most widely adopted type. However, as steel production demands increasingly stringent requirements for efficiency, quality, energy consumption, and environmental protection, AC arc furnaces have gradually revealed a series of issues.
AC electric arc furnaces suffer from arc instability. Due to the periodic zero-crossing of alternating current, the arc frequently extinguishes and reignites. This not only disrupts the stability of the smelting process but also makes it difficult to control the temperature and composition uniformity of molten steel, ultimately affecting steel quality. Additionally, AC electric arc furnaces consume significant amounts of electricity. During smelting, a large portion of electrical energy is expended on repeatedly igniting and maintaining the arc, resulting in low energy utilization efficiency. Furthermore, AC electric arc furnaces exert significant impact on power grids. The high-order harmonics and reactive power they generate can disrupt normal grid operations and compromise the stability of other electrical equipment.
In contrast, DC electric arc furnaces offer advantages such as stable arcs, lower power consumption, and minimal grid impact. To enhance competitiveness in steel production and meet increasingly stringent manufacturing and environmental requirements, converting AC electric arc furnaces to DC electric arc furnaces has become a critical choice for numerous steel enterprises.

Technical Proposal for AC Electric Arc Furnace Retrofit

(I) Main Circuit Retrofit

1. Rectifier Installation: Remove certain components from the existing AC power supply system of the AC arc furnace, such as AC contactors and reactors, and install a high-performance rectifier unit. Utilizing thyristor rectification technology, this unit converts three-phase AC power into DC power, providing a stable DC power supply for the arc furnace. The rectifier must exhibit high rectification efficiency and reliability, ensuring stable DC output under varying load conditions.
2. DC Busbar Design: Design and install a DC busbar to transmit the rectified DC power from the rectifier to the furnace’s electrode system. The busbar utilizes high-conductivity copper bars, with cross-sectional dimensions precisely calculated based on the furnace’s rated current. This ensures minimal power loss during transmission while preventing busbar overheating.
3. Filter Device Configuration: To reduce ripple in the DC power supply and enhance power quality, filter devices are installed on the DC bus. Primarily composed of capacitors and reactors, these devices effectively filter high-frequency ripple from the DC power source through optimized selection of capacitance and inductance parameters, delivering a stable DC current to the arc furnace.

II) Electrode System Modification

1. Electrode Structure Replacement: AC arc furnaces typically employ three graphite electrodes, whereas DC arc furnaces generally use one or two electrodes. In accordance with DC arc furnace design requirements, the original three AC electrodes and their associated holding mechanisms were dismantled and replaced with an electrode structure suitable for DC power supply. The new electrode structure utilizes high-strength graphite material, offering superior electrical conductivity and high-temperature resistance to meet the demands of long-term stable operation in DC arc furnaces.
2. Electrode Lifting and Adjustment System Upgrade: DC arc furnaces demand higher precision in electrode lifting speed and positioning to achieve accurate arc length control. Therefore, the existing electrode lifting system must be upgraded. Replace the original hydraulic or pneumatic drive with servo motor drive to enhance response speed and control accuracy during electrode lifting. Simultaneously, high-precision position sensors are installed to monitor electrode position in real time. Position signals are fed back to the control system, enabling closed-loop control of electrode positioning.

(III) Cooling System Retrofit

1. Cooling Method Optimization: AC arc furnaces typically employ water-cooling systems, though cooling efficiency and reliability require improvement. During retrofitting, the cooling system was redesigned to replace existing tubular heat exchangers with high-efficiency water-cooled plate heat exchangers, enhancing cooling efficiency. Concurrently, redundant cooling circuit design was implemented to ensure continuous furnace operation by maintaining functionality in one circuit should the other fail.
2. Cooling Water Quality Control: Equipment such as electrodes and rectifier units in DC arc furnaces demand high-quality cooling water. Impurities or excessive hardness in cooling water can lead to scaling and corrosion, compromising equipment lifespan and cooling performance. Therefore, water treatment devices—including filters and softeners—were integrated into the cooling system to strictly control water quality, ensuring impurity levels and hardness meet equipment specifications.

(IV) Control System Retrofit

1. PLC Control System Upgrade: The existing AC electric arc furnace control system will be dismantled and replaced with a new control system based on a Programmable Logic Controller (PLC). This system utilizes high-performance PLC modules with robust logic processing and data handling capabilities, enabling comprehensive control over the electric arc furnace smelting process. This includes electrode positioning, current/voltage regulation, and cooling system management.
2. Human-Machine Interface Design: Develop an intuitive human-machine interface (HMI) enabling operators to monitor real-time furnace parameters such as current, voltage, electrode position, and molten steel temperature. The HMI allows operators to configure and adjust settings according to production requirements. Additionally, it incorporates fault diagnosis and alarm functions, promptly signaling equipment malfunctions while displaying root causes and troubleshooting recommendations to facilitate rapid resolution.
3. Implementation of Automated Control Functions: Achieve automated control of the electric arc furnace smelting process through a PLC control system and associated sensors/actuators. For instance: – Automatically adjust electrode position and current/voltage based on molten steel temperature and composition readings to enable automatic heating, temperature maintenance, and composition adjustment. – Automatically regulate cooling water flow and temperature according to the cooling system’s operational status to ensure effective equipment cooling.

III. Analysis of Renovation Benefits

(A) Economic Benefits

1. Reduced Electricity Consumption: DC arc furnaces maintain stable arcs, eliminating the repeated arc ignition losses inherent in AC furnaces. This reduces electricity consumption by 10%-15%. Taking a 100-ton rated capacity furnace as an example, electricity consumption per ton of steel was 650 kWh before renovation. post-conversion, this drops to 550-585 kWh per ton. Based on an annual production of 100,000 tons of steel, this translates to annual electricity savings of 650,000-1,000,000 kWh. At an industrial electricity rate of 0.6 yuan/kWh, this yields annual electricity cost savings of 390,000-600,000 yuan.
2. Enhanced Production Efficiency: DC electric arc furnaces feature stable arcs and high thermal efficiency, accelerating molten steel heating and shortening smelting cycles. Post-retrofit, smelting cycles can be reduced by 15%-20%. Taking the original 90-minute cycle per furnace as an example, the cycle time can be shortened to 72-76.5 minutes post-retrofit, increasing daily output by 1-2 furnaces. Based on an annual production capacity of 100,000 tons of steel, this translates to an additional output of 15,000-30,000 tons per year. With a profit margin of 500 yuan per ton of steel, annual profits could increase by 7.5-15 million yuan.
3. Reduced Electrode Consumption: DC arc furnaces exhibit relatively low electrode consumption. The stable arc slows electrode burn-off rates, reducing consumption by 20%-25%. Taking an original electrode consumption of 3kg per ton of steel as an example, post-retrofit consumption drops to 2.25-2.4kg per ton. For an annual production of 100,000 tons of steel, this translates to a reduction of 6-7.5 tons of electrode consumption annually. At an electrode price of 15,000 yuan per ton, this yields annual savings of 90,000-112,500 yuan in electrode costs.

(B) Technical Benefits

1. Enhanced Steel Quality: The stable DC arc in the furnace ensures uniform mixing of molten steel, effectively reducing inclusions and improving purity. Precise temperature control during smelting significantly enhances the uniformity and stability of steel composition, thereby improving mechanical properties and surface quality to meet high-end steel production requirements.
2. Improved Grid Operating Environment: DC arc furnaces generate fewer high-order harmonics and reactive power, minimizing grid impact. Post-retrofit, they reduce grid interference, enhance power factor, improve grid stability, and decrease production disruptions caused by grid issues.

(C) Social Benefits

1. Reduced Energy Consumption and Carbon Emissions: The lower power consumption of DC arc furnaces translates to decreased use of energy sources like coal, thereby reducing greenhouse gas emissions such as carbon dioxide. This aligns with national energy conservation and emission reduction policies, contributing to environmental protection.
2. Enhanced Corporate Competitiveness: The upgrades boost production efficiency and product quality while lowering production costs, strengthening the enterprise’s market competitiveness and supporting long-term development. Additionally, the upgrades demonstrate the company’s application of new technologies and innovation capabilities, elevating its social image.

 Conclusion

Converting AC electric arc furnaces to DC electric arc furnaces yields significant economic, technical, and social benefits. The conversion enables reduced power consumption, enhanced production efficiency, decreased electrode wear, improved steel quality, and optimized grid operation conditions. Concurrently, it lowers energy consumption and carbon emissions while boosting corporate competitiveness. Although challenges exist—including substantial initial investment, high technical complexity, and extensive personnel training requirements—these can be effectively addressed through prudent planning and execution. This includes selecting appropriate financing methods, engaging specialized technical teams, and implementing comprehensive training programs.
Overall, converting AC electric arc furnaces to DC electric arc furnaces is a feasible and necessary project with significant implications for the sustainable development of steel enterprises. It is recommended that steel enterprises with the necessary conditions actively advance this conversion to align with industry trends and enhance their market competitiveness.

Anyang YND own leading technology of DC electric arc furnaces from 50KVA to 30000KVA.   Please contact us to learn more about the AC electric arc furnace upgrade.