# New Multilevel Converter System for Electric Arc Furnace Power Supply

Duro BASIC*, Cyrille BAVIERE*, Philippe CLAVIER*, Pierre Louis GARMIER**, Nicolas LAPASSAT*, Christof SIHLER**, Franck TERRIEN**

*GE VERNOVA Power Conversion SAS, 15 Rue Jean Bart, 91300 Massy, France

**GE VERNOVA Power Conversion SAS, 24 Avenue du Marechal Juin, 40437 Belfort, France

E-Mail: duro.basicConfig, cyrille.baviere@ge.com, philippe.clavier@ge.com, pierre-louis.garmier@ge.com, nicolas.lapassat@ge.com, christof.sihler@ge.com, franck.therien@ge.com

URL: https://www.gevernova.com/power-conversion/

## Keywords

«Multilevel converters», «Grid integration», «Flicker», Static Synchronous Compensator (STATCOM)», «Power quality», «Electric Arc Furnaces».

## Abstract

Historically, utility requirements have been satisfied with the use of compensators (SVCs or STATCOMs) connected in parallel to electric arc furnaces (EAFs), to correct the power factor and reduce harmonics, flicker, and unbalance. To further increase power quality and furnace productivity, a series-connected converter system for EAFs has been developed.

## Introduction

European steel industry is currently facing challenges driven by decarbonization objectives, increasing energy prices and stricter power quality requirements. They are mainly driven by an increasing number of inverter-based power generation resources connected to utility grids. Nonlinear loads such as EAFs are at the origin of degradation of power quality with major consequences such as harmonic pollution, voltage fluctuations and generally voltage unbalance. The installation of an active compensator device, such as a Static Var Compensator (SVC) or STATCOM, is typically required to satisfy the grid standard requirements. In industrial applications and especially in weak grids, voltage unbalance and voltage flicker are particularly difficult to control. Flicker is mainly due to reactive, and also active power variations of the load. It requires dynamic active and reactive power

compensation, e.g. by means of a VSC based STATCOM combined with passive filters 0. A STATCOM is more complex than a SVC and uses IGBT (Insulated Gate Bipolar Transistor) technology. In the past decade, STATCOM have become "the new normal" solutions for furnace compensation with harmonic reduction of 2x-3x for most harmonic orders and flicker reduction of up to 5x or 6x [2]. To-date, STATCOMs have been able to meet the utility requirements for harmonic, flicker, unbalance, and power factor, governed by IEC 519 power quality standards globally. Decoupling the EAF frequencies from the upstream system through a converter enables the operation of large electric steelmaking plants in weak power grids with a high share of renewable power generation, and to meet all future power quality requirements. It also provides the flexibility to run the furnace at different electrical frequencies from the utility side, which has potential productivity benefits depending on the furnace application.

## State-of-the-art STATCOM Technology for EAF Power Supply

EAF power supply systems generally require a power factor corrector and balancer that provides dynamic reactive power and inverse sequence current compensations. The compensator is connected in parallel to the EAF load and combined with passive filters, as shown in Fig. 1. Depending on the power rating of the required STATCOM, e.g., above 75 to
<figure><img src="image_1.png" /><figcaption>Fig. 1: Single line diagram of an EAF power supply with passive filters and a VSC based STATCOM providing dynamic active and reactive power compensation.</figcaption></figure>

100 MVA, a modular multilevel converter (MMC) system provides highest operational flexibility and availability, without having to install bulky equipment like transformers as part of the STATCOM installation. Fig. 2 shows the single line diagram and an example installation of a MMC based delta STATCOM. The STATCOM must compensate for the filter

<figure><img src="image_2.png" /><figcaption>Fig. 2: 100 MVA MMC Delta STATCOM for direct connection to 36 kV distribution grid</figcaption></figure>

at no-load operation, therefore the ratio of STATCOM compensation power divided by the passive filter power can vary from 50%/50% to 100% / 0% (no passive filter compensation). Considering the large amount of equipment (MMC & passive filters) and high technical effort required with actively compensating the power profile of an electric arc (high density plasma) that can cause extremely dynamic and chaotic disturbances, a series connected high performance converter system with ability to decouple the EAF from the power system may be the best choice for the next generation of EAF power supplies. Such Direct Feed (DF) power supplies can provide additional productivity benefits through the ability to control the furnace electrode currents at frequencies other than 50 or 60 Hz.

## New Multilevel Converter Topology Developed for DF EAF Supply

A DF EAF power supply requires a power conversion system with high nominal power, robustness, and availability. MV MMC topologies are best suited to provide an EAF direct feed power supply because of their modular structure, excellent power quality, and fault-tolerant operation capability. The MMC converter is based on high power submodules providing increased flexibility for the installation, operational efficiency, and higher availability through the possibility to include n+1 or n+2 redundancy at individual power cell
level. To achieve high robustness and ampacity of the individual power submodules, the DF design is based on proven press pack IEGT technology that has been used in MV drives for almost 20 years [3]. Fig. 4 shows an example of a press-pack stack with 4 IEGTs. The single IEGTs have a 4.5 kV nominal voltage, with several current ratings available, mainly 750A, 1500A, 2000A and 3000A (trench devices). This technology is the base of creating full bridge (FB) and half-bridge (HB) submodules, as shown in Fig. 3. The power stacks are cooled by deionized water. Each IEGT benefits from double side cooling thanks to the press pack technology. Another benefit of press-pack stacked IEGTs is that the main failure mode is a maintained short circuit, which makes the redundancy implementation easier, and avoids the necessity for arc containment casing. This results in a compact and cost-effective design of the high-power density stacks (FB and HB power submodules).

The classical Double Star back-to-back MMC ac/dc/ac converter topology, such as used in HVDC applications, requires to install a comparably high number of actively switched and controlled power submodules and cell capacitors, which typically occupy more than 50% of the weight and size of MMC [4]. Given that the main power flow of EAF power supplies is unidirectional only, a new hybrid multilevel converter system design has been introduced for the EAF supply.

<figure><img src="image_3.png" /><figcaption>Fig. 3: The FB and HB power submodules</figcaption></figure>

It has a Hybrid Modular Multilevel Rectifier Type 1 (HMMR1) front-end, which is shown in Fig. 5. The HMMR1 is a patented topology [5][6] derived from an embedded converter topology [7], which enables reduced volume, weight, cost and increased efficiency compared to the conventional Double-Star (DS) power conversion systems [8]. The HMMR1 is basically a combination of a classical three phase 6-pulse rectifier enhanced by the ac and dc current shaping circuitry, which is composed of chain links of HB submodules (arms) like in classical double star MMC converters. However, the HMMR1, operated at unity Power Factor (PF), requires only half of the modules needed in the DS MMC converters (arm voltage rating reduced to $V_{dc}$/2 instead of full $V_{dc}$). Depending on the signs of the grid voltages, two HMMR1 converter arms belonging to the same phase are connected alternatively either to the upper or lower half dc link voltages as illustrated in Fig. 6. The diode rectifier operates basically in the 3-Level mode while the HMMR1 arm converters are covering the difference between the grid ac and 3-Level rectifier dc voltage. For positive grid ac voltages, the required arm voltages are (1):

<figure><img src="image_4.png" /><figcaption>Fig. 4: Power stack with 4 IEGTs used in MV drives and MV MMC submodules (HB and FB)</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 5: Hybrid multilevel back-to-back ac/dc/ac converter composed of the HMMR1 rectifier (ac/dc converter) stage at the grid side and MMC inverter stage at the EAF side</figcaption></figure>

$$v_{arm1,3,5} = \frac{V_{dc}}{2} - v_{1,2,3} \qquad v_{arm2,4,6} = v_{1,2,3} \qquad (1)$$

For negative grid ac voltages, the required arm voltages are (2):

$$v_{arm1,3,5} = -v_{1,2,3} \qquad v_{arm2,4,6} = \frac{V_{dc}}{2} + v_{1,2,3} \qquad (2)$$

In unity power factor operation, the dc link current is distributed among the diodes and HMMR1 converter arms connected to the dc link rails in a trapezoidal manner (Fig. 7), in synchronism with the respective ac phase

voltages. The arms connected to the dc link midpoint are supplying the complementary currents needed to get sinusoidal ac currents (Fig. 8). Thus the HMMR1 converter arm currents are composed in the following way (for angles $\theta$ between zero and $\pi$): for $\pi/3 \ge \theta \ge 0$ the arm currents are:

$$i_{arm1} = \frac{I_{dc}}{\pi/3} \theta \qquad i_{arm2} = i_{arm1} - i_{s1} \qquad (3)$$

between $2\pi/3 > \theta \ge \pi/3$:

$$i_{arm1} = I_{dc} \qquad i_{arm2} = i_{arm1} - i_{s1} \qquad (4)$$

<figure><img src="image_6.png" /><figcaption>Fig. 6: 3-Level switching states of HMMR1 converter [8]</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 7: Trapezoidal distribution of the dc link current among the HMMR1 converter arms</figcaption></figure>

and finally between $\pi > \theta \ge 2\pi/3$:

$$i_{\text{arm 1}} = I_{\text{dc}} \left( 1 - \frac{1}{\pi / 3} \left( \theta - 2\pi / 3 \right) \right) \quad (5)$$

$$i_{\text{arm 2}} = i_{\text{arm 1}} - i_{s1}$$

The HMMR1 can be operated with controllable non-unity power factors to deliver an amount of reactive power to regulate input power factor, and/or to support the grid during the low-voltage transient and weak grid conditions. In non-unity power factor operation [8], the ac voltage and currents are not in phase. In the HMMR1 the ac currents crossings are dictating the diode commutations and the arm current distribution must be altered to be able to operate the rectifier with the arms which are capable of producing only unipolar arm voltages (Fig. 8). In addition, the required voltage rating of the HMMR1 arms is increased in a function of the desired power factor (6).

$$V_{\text{arm peak}} \approx \frac{V_{\text{dc}}}{2} (1 + |\sin \varphi|) \quad (6)$$

For example, for minimum PF=0.95 the HMMR1 voltage rating has to be increased by about 30% compared to that required for the unity PF operation. The HMMR1 P-Q diagram is shown in Fig. 9. The minimum power factor is defined by the arm voltage rating and it is theoretically limited to a minimum value of PF=0.8.

<figure><img src="image_8.png" /><figcaption>Fig. 9: P-Q diagram of HMMR1</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8: Typical waveforms of the arm voltages and currents of the HMMR1 rectifier (ac/dc converter) in operation with different power factors (1, 0.95 capacitive and 0.95 inductive)</figcaption></figure>
# Converter Design for DF EAF Pilot Plant with 130 MW Power Rating

In a direct feed EAF power supply the grid side converter will be run in non-unity power factor operation ($PF_{min}=0.95$) to deliver some amount of reactive power and support the grid during the low-voltage transients and weak grid conditions. In this setup still one third of the chain links of HB submodules can be replaced by simple diode stacks as shown in Fig. 10. Only 12 HB towers must be installed on the grid side of the converter, together with 12 diode stacks (RD), each with 12 diodes in series (n+1 redundant design). The EAF side of the converter consists of 18 HB towers. The DC link capacitor banks (DCC) provide additional energy storage and de-coupling between the EAF load and the grid.

## EAF power quality and converter control challenges

The mitigation of arc furnace perturbations provides an extreme load scenario for MV equipment. The main control task challenge is to compensate arc furnace impact on the grid during electric arc heating, including the reduction of flicker effects. The (non-regulated) currents from the arc furnace during a heat cycle are very unpredictable, see Fig. 11 (erratic, asymmetric, fast variation...). Peak currents during electrode short-circuit conditions can reach values above 100 kA.

<figure><img src="image_10.png" /><figcaption>Fig. 11: Three-phase EAF currents measured during a heat cycle (nom. current amplitudes, typically 60-80 kA)</figcaption></figure>

To control the furnace currents and actively compensate the power fluctuations caused by the arc voltage fluctuations, a converter control system with very fast response behaviour is required. It is achieved by a distributed control architecture, which reduces the amount of data exchanged with the global control system and optimizes the calculation capacity of the local and global control. The main benefits of this principle is that sampling periods under 40 µs are achievable in high power converter systems with rating higher than 100 MVA [2]. Such fast real-time converter control is not only required in the STATCOM applications (for the shunt compensation of highly transient and imbalanced EAF loads) but also in the DF EAF systems to achieve effective control of the arc furnace currents to a pre-set value. Thus, DF EAF supply systems offer unprecedented performances in the EAF current regulation which are not possible in a classical EAF power supply systems, where the externally imposed supply voltage is constant and defined by the tap setting of the EAF transformer, see Fig. 1.

<figure><img src="image_11.png" /><figcaption>Fig. 10: Main modules of EAF power supply – in total 12+18 HB towers installed for a 130 MW supply, including redundant submodules</figcaption></figure>
The expected benefits of the EAF current controlled supply is more stable EAF operation, higher efficiency and reduced electrode consumption. Validation tests of the converter and real time control system operation, together with the extremely dynamic EAF load, is essential during the detailed development phase and control system tuning. Thus, to validate the new converter architecture, real-time control and all software components, the full converter control hardware has been installed in a Real-Time Simulation (RTS) test platform, see Fig. 12. The overall ac/dc/ac DF converter, grid connection and EAF are represented in the real-time simulation environment by detailed models, the furnace behaviour with an arc voltage generation based on the Cassie–Mayr electric arc model (with the arc voltage perturbations equivalent to these measured in customer EAFs).

<figure><img src="image_12.png" /><figcaption>Fig. 12: Photo of DF converter control hardware installed in RTS platform for validation testing</figcaption></figure>

RTS tests are key to validate the sizing of critical components under normal and extreme operation scenarios, test all control algorithms incl. the interaction of the DF converter system and Level 1 EAF controllers and pre-tune the converter control with real parameters before the actual commissioning of the full system. Fig. 13 shows an exemplary measurement result from RTS testing of the converter system with HMMR1 front-end.

<figure><img src="image_13.png" /><figcaption>Fig. 13: DF converter arm currents and voltages measured in RTS platform during validation tests with control hardware in the loop (HIL)</figcaption></figure>

## Positive effects of DF EAF power supply for EAF process and utility grids

As demonstrated in detailed numerical and RTS simulations with the actual converter control hardware and software in the loop, the DF system effectively limits the EAF current peaks, reduces the number of instances when arcs are interrupted or short-circuited in the furnace, and improves EAF current stability and efficiency of the power conversion process.

The worldwide first DF power supply with a rating above 100 MW was installed and commissioned on an electric arc furnace pilot plant in Germany in the second half of 2024. Fig. 14 shows a photo of an upper arm of the HMMR part of the DF converter. In the background six additional HB towers are visible, which belong to two arms of the inverter part. The electrical commissioning of the full converter system has been successfully completed in Q4 2024. Since then, the DF converter system is tested with the arc furnace, providing active power up to the nominal value to the steel melting process, with effective values of the electrode currents up to 70 kA (measured on the secondary side of the EAF transformer). One key advantage of a DF power supply is the ability to control the electrode currents, which is not possible with prior art (existing) electric arc furnaces that are directly connected to the medium voltage grid. As highlighted in the previous section, asymmetric electrode currents with fast changing amplitudes, such as the one shown in Fig. 11, provide extreme load scenario for the conventional EAF power supplies and MV equipment. With a DF power supply such loads can be significantly reduced, as demonstrated in Fig. 15. With and without electrode current control the electric arc will have
<figure><img src="image_14.png" /><figcaption>Fig. 14: Upper arm of HMMR converter for a rated voltage of 33 kV (nom. active power 130 MW)</figcaption></figure>

voltage perturbations. The level of instability depends on the furnace inventory (e.g. scrap or iron bore), operating conditions (boring, melting, refining) and EAF design parameters. The simulated results shown in Fig. 14 have been validated during the EAF DF operation and control optimization phase that is currently still ongoing (in Jan. 2025), for scenarios with moder-

ate arc voltage perturbations (upper 3 curves) and high arc voltage perturbations (lower 3 curves). In all scenarios the fast feedback control of the DF converter enables to control and symmetrize the electrode currents, which enables better control of the steel melting process and reduces EAF current and power fluctuations and their effects on the electrodes and all MV equipment.

<figure><img src="image_15.png" /><figcaption>Fig. 15: Operation of an electric arc furnace without (left) and with DF power supply (right)</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 16: Active power per phase and total active power on the EAF and grid side</figcaption></figure>

The DF EAF converter also effectively decouples the EAF load from the grid, enabling to run the DF supply with a power factor of 1 or down to 0.95 capacitive, in order to compensate the reactive power of the upstream transformers or other inductive loads in the steel plant.

The DF EAF converter supply also enables to achieve flicker mitigation factors which are far above those achievable with the conventional EAF power supplies which use dynamic reactive power shunt compensation systems, such as shown in Figs. 1 and 2. The main advantage of the DF supply is illustrated in Fig. 16. Only four curves are visible in the upper graph of Fig. 16. The reason is that the three active power curves on the grid side are exactly same, meaning that the power is perfectly balanced among the three phases thanks to the DF converter control. Because of the multilevel converter design composing DC link voltages and currents with negligible harmonic content, no relevant reactive power or reactive power transients can propagate from the furnace to the grid side. As visible in the lower graph, showing the total active power on the furnace and grid side, all fast variations of active power are decoupled from the MV grid, only moderate (acceptable) power variations appear on the grid side of the converter. This decoupling is achieved by means of the converter control and storing energy in the MV DC buffer capacitors included within each submodule (visible in HB tower photo on the right side of Fig. 10) and in the additional energy storage capacitors in the converter DC link (DCC). The possibility to add such storage capacitors is important, particularly when the grid short circuit

power is relatively low compared to the EAF power, i.e. when the maximum flicker allowed by the grid operator is low. Future flicker limits are expected to be lower than in today's utility grids because more renewable energy sources are expected to be used. Thus, for production of the “green steel”, possible only if the local grid codes can be met during all process phases in electric steelmaking, DF EAF supply systems with adaptable energy storages are highly advantageous.

## Conclusion

To further increase EAF productivity and grid power quality, a scalable MV multilevel converter system for a direct EAF power supply has been developed, manufactured and installed in a pilot plant. Thanks to the HMMR1 converter topology 33% (PF=0.95) to 50% (PF=1) of the rectifier submodules can be replaced by simple diode stacks, with positive impact on size, efficiency and cost. The direct converter supply enables current controlled supply of the EAF with higher rates of energy transfer and better flicker reduction than achievable with a DSVC or STATCOM. The expected benefits include more stable EAF operation, reduced electrode consumption, the possibility to control the voltage and frequency of the EAF supply in real-time, no need for separate compensation systems or harmonic filters, and reduced dynamic stresses on the entire power supply.
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