White Paper: Arc Furnace Power Supply

Power Controller Applications –

Arc Furnace Power Supply Stephan Risse-Gerke, Dr.-Ing. Peter Wallmeier

AEG Power Solutions GmbH, Belecke

Abstract—This contribution explains the Thyristor Power

Controllers Application as power supply for arc furnaces with the advantage of a digital and dynamic current source with high system efficiency, high power factor (PF) and low harmonic distortion (THD) without electro-mechanically adjustable transformers.

Keywords—Power Controllers, Arc furnace, Current source

I. INTRODUCTION

The arc furnaces can be found e.g. in the Ceramic production, in the manufacture of Quartz Crucible for the pulling of semiconductor and PV silicon with the Czochralski-Procedure. The typical industry plant electrical diagram is shown on Fig.1. Every power supply, an AEG PS Thyrobox

TM-

F, is powered by a medium voltage transformer and has a pure capacitive Var compensation on the secondary side of the transformer.

Fig.1. A typical system configuration.

One power supply feeds two arc furnaces alternately with a maximum output current up to 2500Amps and a maximum output voltage up to 400V. The operating area with the maximum voltage and current ratings for the load is shown on Fig.2. The power supply has an interlock function for the two arc furnaces.

Fig.2. Operating area Thyrobox-F

II. POWER SUPPLY ARCHITECTURE

The demands for such a power supply are numerous. Beside a high power factor and a high efficiency, a low harmonic distortion is an important challenge. A power supply for an arc furnace with the typical strong nonlinear load curve (Fig.3), where a small voltage change results a large current change, requires a fast dynamic current source.

Fig.3. A typical strong nonlinear arc curve

One System consists of one control cabinet for the measurement acquisition and the control of the arc furnace as

White Paper: Arc Furnace Power Supply

well as three power cabinets with the power electronics. An overview of the three power cabinets, the control cabinet and the Var compensation is shown on Fig.4.

Fig.4. System overview

In every power cabinet a choke is used with the four thyristors forming a fullbridge. The choke takes the role of

energy storage according to

and impresses the

current in the DC pass of the fullbridge. By precise phase angle control performed by Thyro-P digital controller devices the choke current and in turn the output current is adjusted with the thyristors. The power supply takes over the function of a high dynamic current source. So the need of electromechanically adjustable transformers is eliminated by this application. This can only be achieved through a fast measured value acquisition of the electrical parameters and a fast dynamic control. The measurement of the electrical parameters and the control of the current source are effected by the Thyro-P Control Unit (Fig.5).

Fig.5. Thyro-P Control Unit

The arc voltage and current values are measured with a 14 bit measurement resolution. All 3,3ms (60°), the full digital control device records the current of the arc furnace and gives the pulse patterns via fiber optics to the water cooled thyristor stacks for all three phases. This guarantees a quick response to changes of the arc.

III. VAR COMPENSATION

With this architecture of the arc furnace power supply as high dynamic current control, sinusoidal current consumption with low harmonic distortion (THD) and flicker are possible (Fig.6).

Fig.6. Sinusoidal current consumption without compensation

The best power factor, which is defined as the ratio of the real power (P) to the apparent power (S), will be achieved at the maximum current of the Thyrobox

TM-F. If the arc furnace

operates not at full load, the power factor is lower as current shift relative to voltage. To increase the power factor in all operating conditions over 0.9, the use of an adjustable var compensation is necessary. The var compensation consisting of thyristor switched capacitor banks control the power factor on a maximum level by the reactive power (Q) compensation (Fig.7).

Fig.7. Phasor diagram power triangle

The reaction time is very fast by using SCR switches. A circuit diagram of the var compensation is shown on Fig.8. The var compensation system is installed on the secondary side of the medium voltage transformer.

White Paper: Arc Furnace Power Supply

Fig.8. schematic var compensation

With a Simplorer simulation model of the ThyroboxTM

-F (Fig.9) the plant was simulated and the var compensation topology optimized and parameters of the built unit determined.

Fig.9. Simulation model Thyrobox

TM-F with var compensation

The results of the power factor measurements on a field unit are shown on Fig.10. With the capacitive var compensation the power factor is increasing to be > 0.9. These values have been seen over a wide operating range in the field.

Fig.10. Waveforms showing low THD and phase shift between voltage

and current

IV. BENEFITS

A result of this architecture is the high system efficiency through the use of the lowest loss semiconductor elements thyristors. With the ability to alternately supply two electric arc furnaces with a power supply, the manufacturing capacity is increased, at the same time the space of the industry plant is optimized. Using a current source topology having sinusoidal input current an extreme high power factor of > 0.9 can be achieved by using simple capacitive var compensation. Finally the Thyrobox

TM-F and the var compensation can be seen on

figure 11 and 12.

Fig.11. AEG Power Solutions Thyrobox

TM-F

Fig.12. Var Compensation