Electric Characteristics of Arc Furnace & Considerations for Their Modeling

Arnaldo J. P. Rosentino Jr. (Ph.D. Visitor Student)

University of Alberta, Department of Electrical and Computer Engineering

arnaldo@ieee.org

1- Introduction

The pre-assessment or predetermination of

flicker produced by an AC electric arc furnace

(EAF), before the equipment is put into operation, is

a difficult but important exercise. The pre-

assessment is necessary to evaluate the place of

connection of the arc furnace and the need to install

compensation.

2 - EAF Electrical Characteristics

Figure 1 highlights a three-phase AC EAF as

well as the typical layout of installation.

A. Tap-to-tap time

Figure 2 highlights the EAF tap-to-tap time.

B. Electric Arc Behavior during EAF Operation

Initially the process has low voltage (arc

shortened). Once the arc is shielded by scrap,

voltage is increased (arc lengthened). At the begin

of melting point the electric arc current is very

unstable. However, when there is a complete metal

pool, it is more stable. Then, the arc is shortened.

C. EAF Operation Stages

• Striking Period (Initial Period): very unstable

process;

• Melting Period (Main Period): unstable process;

• Refining Period (Final Period): stable process.

Figure 3 highlights an actual system that

includes a 50-ton AC EAF (EAF 1).

Figures 4 – 9 present the instantaneous

current characteristics of EAF 1, respectively, for the

striking, melting and refining period.

3 – Considerations for Flicker Prediction

Figure 10 summarizes the algorithm for flicker

prediction due to EAF load.

A. EAF Model Considerations • Data Identification Process (Extract knowledge);

• Modelling Process (Time Series Approach);

• Checking Process.

Data Identification Process

Figure 11 shows the electric system, where is

located a 44 MW - AC EAF (EAF 2).

Figures 12 and 13 present the instantaneous

current characteristics of EAF 2 for melting period.

Figures 14 - 19 show the RMS current

characteristics of the both EAF for melting period.

Note: The identification process of actual EAF

data has been already done. At the moment, the

efforts have been directed to conclude the

modelling, which is based on time series analysis.

Then, the EAF model response will be checked in

such a way to validate it.

B. Flicker Evaluation Considerations

Flicker problem is evaluated according to the

IEC flicker meter described in IEC 61000-4-15,

which defines the short-term flicker severity (Pst) as

the fundamental parameter used to evaluate the

irritation. The IEC flickermeter is structured by 5

blocks, Figure 20.

C. System Model Considerations • Flicker at EHV or HV can significantly attenuate

when it propagates into the MV or LV systems;

• There is a compensation effect due to rotating

machines connected at system.

4 – Conclusions

Taking into account the work performed so far,

it has revealed the possibility to develop a

mathematical model based on time series analysis.

It is worthwhile to highlight that such model should

be feasible in such a way to be incorporated in a

future tool, which with the IEC flickermeter and a

correct system model will predict the flicker problem

before EAF installation at the electrical network.

Therefore, it will be verified the need of mitigation

device.

Figure 4 – Instantaneous current

waveform of EAF 1 for the

striking period.

Figure 5 – Current frequency

spectrum of EAF 1 for the

striking period.

Figure 6 – Instantaneous current

waveform of EAF 1 for the

melting period.

Figure 7 – Current frequency

spectrum of EAF 1 for the

melting period.

Figure 1 – EAF construction and layout.

Figure 2 – Typical EAF tap-to-tap time.

Power

System

45 MVAkV Y 4.11 / kV 161

33 MVAV 604 / kV 4.11

PCC

Electric Arc

Furnace

PQ

MeterEAF Bus

Figure 3 – Electric system where is located EAF 1.

Figure 8 – Instantaneous current

waveform of EAF 1 for the

refining period.

Figure 9 – Current frequency

spectrum of EAF 1 for the

refining period.

Figure 10 – Flowchart of algorithm for flicker prediction due to EAF load.

Figure 11 – Electric system where is located EAF 2.

Figure 12 – Instantaneous

current waveform of EAF 2 for

the melting period.

Figure 13 – Current frequency

spectrum of EAF 2 for the

melting period.

Figure 14 – RMS current

waveform of EAF 1 for the

melting period.

Figure 15 – RMS current

waveform of EAF 2 for the

melting period.

Figure 16 – RMS current

frequency spectrum of EAF 1 for

the melting period.

Figure 17 – RMS current

frequency spectrum of EAF 2 for

the melting period.

Figure 18 – RMS current

histogram plot of EAF 1 for the

melting period.

Figure 19 – RMS current

histogram plot of EAF 2 for the

melting period.

Figure 20 – IEC Flickermeter block diagram.

Algorithm for Flicker Prediction caused by EAF

EAF Model Flicker Evaluation System

Model

Tool:

EAF Flicker

Prediction

Simulynk;

PSCAD;

ATP;

Etc

Time Series AnalysisIEC Flickermeter

Model

Consider attenuation

of flicker effect.