Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Active Power Feeder, a Ground-breaking Power Solutionfor Feeding Electric Arc Furnaces

P r i m e t a l s Te c h n o l o g i e s G e r m a n y
S a l e s Te a m - A c t i v e P o w e r F e e d e r
Julian Magin
https://www.primetals.com/portfolio/
automation-digital-plants/pow er

Active Power Feeder (APF) is a modern power supply system specially designed for AC
Electric Arc Furnaces (EAF). APF utilizes an electronic actuator, the Modular Multilevel
Converter (MMC), to supply real power from the supplying medium voltage bus to the electric arc
furnace in a very grid-friendly way. High power quality, i.e., low flicker, low harmonic distortionand excellent power factor are achieved. A compensation system is not required and additional
power consumers on the bus are compensated. A ground-breaking dynamic process control
increases the stability and efficiency of the furnace operation. Furnace current, frequency, and arc
voltage are automatically adjusted according to the operational situation, to minimize the current
fluctuation, avoid overcurrent and to increase the arc stability. The fast, stepless digital control
maintains optimum furnace working points, reduces the power-on time and energy consumption.
The strict limitation of overcurrent situations reduces the stress and wear for the electrical andmechanical furnace equipment and results in reduced maintenance and electrode consumption.
With APF, the conventional EAF becomes a digital green EAF.

|1. Explanation of APF
Main features of APF, a new power supply for EAF are:
- To feed active power from the grid to the EAF.
- To support and stabilize the grid voltage and frequency.
- Digitally control the EAF melting process, i.e., highly dynamic and efficient.
- To deliver high energy efficiency, high productivity, stable operation, and highavailability.
- To make additional compensation systems (SVC: Static Var Compensator, STATCOM:Static Synchronous Compensator etc.) unnecessary.
- To make electro-mechanical equipment, like reactors, tap-changers, furnace switches
unnecessary.
The main advantage of APF is that EAF side and grid side are decoupled from each other,and each side is individually controlled by a fast and stepless digital actuator (Figure 1).
An MMC connected to the medium voltage level is preferred as electronic power supply, for
its lower electrical losses, perfect voltage and current shapes with lowest harmonic distortion, fastand precise actuation, highest availability and installation flexibility.
The converter can be installed outside of the melt shop building at a convenient, economic
location and has an optimal footprint (Figure 2). This is a significant advantage in plants withlimited space and in modernization projects.
The APF converter controls the grid side and the EAF side independently:- Control on grid side
With its Active Front End (AFE) the APF has full control of the power quality up to thePCC (point of common coupling) (Figure 2). The AFE maximizes the power factor toward
the grid and eliminates harmonic distortions and flicker. An additional compensation
system is not required.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
2- Control on EAF side
With its fast dynamic process control, the APF ensures a fast, smooth and safe melting
process. In comparison with conventional EAF where the furnace can only be controlled
slowly by electro-mechanical actuators like tap changer and electrode arm movements,
APF’s fast and stepless digital actuator can react instantly to process disturbances and canstabilize the operation.

Figure 1

APF, a new power supply system for EAF

Figure 2

Installation flexibility with APF|2. Comparison with conventional technology
In comparison with conventional EAF where the furnace is directly attached to the furnacebus, the MMC of the APF is connected in between the furnace and the furnace bus (Figure 3).
Due to the violent melting process of scrap, the conventional electric arc furnace as electricalload is very unstable (Figure 4). The dynamic behavior of the arc inevitably leads to strong
perturbations in the current like loss of arc, short circuit, load fluctuation and unbalance. Finally,
this causes high flicker, high harmonic distortion and poor power factor.
The perturbations are significantly faster than the control speed of the electro-mechanicalpositioning system of the electrodes and actuations with the on-load tap changer of the furnacetransformer. To comply with the grid code requirements, additional compensation systems such as
SVC or STATCOM are mandatory to minimize network perturbations and maintain a high powerfactor.
In comparison APF does not require additional compensation systems and filters. Still thecompensation capability of APF is significantly better than a STATCOM with filters. With APF avery high flicker reduction factor better than 10 can be achieved. The dirty furnace bus becomes aclean bus and an additional ladle furnace can be compensated without additional compensationequipment.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
3Figure 3

Transition to digital Green EAF with APF

Figure 4

Power quality issues caused by EAF operation (example)|3. New dynamic process control
The MMC is the new fast, stepless actuator for high dynamic digital furnace control.
Setpoints are provided by the electrode control, the Melt Expert.
Current and frequency are actuated by the converter. Arc length (arc voltage) is actuated by
the electrode arm hydraulic system.
Each electrical phase is actuated individually to control hot or cold spot.
Steel grade-related power recipes are stored in meltdown profiles and are activated according
to the melt down process condition, i.e., boring phase, melt down phase, flat bath phase.
The main control activities are realized by the fast and stepless current control of theconverter and preferably at a fixed transformer tap. The movements of the hydraulic system andherewith the electrodes are minimized and mainly follow the scrap melt down.

|4. Improvement and saving potential by the regulated furnace
currents
Regulated furnace currents are the basis for increasing the active power input into the furnaceand finally to decrease the power-on time and increase the furnace productivity. The simulation inFigure 5 compares the furnace currents in conventional operation (blue color) with APF operation(red color) for a two-basket melting.
The dynamic process control minimizes process disturbances during boring/melting phasevery efficiently and we observe significantly reduced current fluctuations, increased arc stability
and a strict limitation of over current situations. This makes operation safer and allows to increase
the active power input into the furnace, especially during boring and melting phase. The simulationin Figure 6 shows the positive impact on the power-on time which can be reduced by -10% in thisexample.
Energy saving in the boring and melting phase result from the shortened power-on time, i.e.,from reduced heat losses and from reduced electrical losses. In the example this corresponds toreduced electrical energy consumption per ton of produced steel of -8%. In the case of stable

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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arcing, additional savings in the melting phase, flat bath phase are possible by a decreased
operation frequency with reduced electrical losses, e.g., 40 Hz instead of 60 Hz.
The reduced current fluctuation result in additional savings in the operation cost like reduced
electrode tip consumption (-5% in Figure 6), reduced electrode breakages. The electrical andmechanical stress in the equipment is significantly reduced, resulting in a smoother operation withhigher availability of the furnace and reduced maintenance cost for existing furnace transformer,
on-load-tap-changer, furnace breaker, flexibles, electrode arms.

Figure 5 Regulated furnace current with APF

Figure 6 Increased active power input, reduced power-on time and reduced
electrode consumption with APF|5. Future prospects
APF will be installed in a 50-ton electric arc furnace in Germany in 2025 to demonstrate theenvironmental, technical, and commercial benefits of the new technology. The furnace will be
powered by an APF featuring a 3-phase indirect MMC. APF is well suited to power a wide rangeof AC furnaces, including scrap- and DRI-based furnaces. Capacity wise large furnaces up to 300
tons and higher are on the APF roadmap.