Control of Static Converter to Balance Perturbations on ARC Furnaces

Copyright © IFAC Control of Industrial Systems. Belfon. France. 1997

CONTROL OF STATIC CONVERTER TO BALANCE PERTURBATIONS ON ARC FURNACES

B. Gollentz.*, M. Kratz**, J.L. Pouliquen***, A. Berthon****

• Centre de Recherche en Electrotechnique et Electronique de Belfort, 1 rue Morimont 90008 Belfort. BP 479, FRANCE. ··EDF DER BP 408. 92141 Clamart, FRANCE. ••• CEGELEC. 3 avenue des Trois Chenes. 90018 Belfort, FRANCE. •••• lnstitut de Genie Energetique. Parc technologlque, 2 avenue Jean Moulin, 90000 Belfort, FRANCE. Resume : L'article presente un Compensateur d'energie reactive de 55MVAR pour fours a arc connectes sur un reseau de llkV, afin de reduire les fluctuations de tension du reseau. Nous presentons un nouveau controle base sur le concept du modele interne pour obtenir une compensation efficace de flicker. La regulation est base sur le controleur FDPS. Nous concluons cet article en decrivant la reduction de flicker obtenue sur site. Abstract : This paper describes a 55MVAR Static Var Compensator (Fixed Capacitors Thyristors Controlled Reactors: FC!fCR) for Arc Furnaces connected to llkV network, to reduce voltage variations on bus line. We present a new control based on internal model control concept for high performance dynamic compensation of flicker. The electronic control is based on Fully Digital Processor System (FOPS). We conclude this paper by describing the flicker reduction obtained on site. Keywords: Static Var Compensators, Internal Model Control, Flicker, Power Factor.

I.

percent magnitude range at these frequencies. The FCrrCR or Statocompensator'N is used for A.C. arc furnaces to compensate reactive power and to reduce flicker level. We present a description of arc furnace and a static converter (Statocompensator) with its control algorithm which reduces delays for a better reduction of flicker. Finally we present results of flicker levels obtained with the Statocompensator on site.

INfRODUCTION

The main function of an arc furnace, whether supplied with alternating or direct current, is to melt down solid scrap iron, transforming it into raw liquid steel. The productivity is typically lOOt scrap per hour. However, phenomena of arc instability and reactive power fluctuations are the underlying cause of disturbances transmitted into electric power network. So, loads as A. C. arc furnaces with rapid fluctuation of reactive power consumption disturb the bus voltage and flicker occurs on the power line system. It has been found in tests that the human eye is most sensitive to modulating frequencies in the 5 to 20Hz range with voltage variations in the 0.3 to 0.4

2. DESCRIPTION OF AN ARC FURNACE

2.1

Steel elaboration

The arc furnace (figure I) is composed by a steel panel, covered with refractory wall, and the electric

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2.3

power is transmitted to the scrap with graphite electrodes, where electric arcs are created.

-

1Ut"'R0Da

I

I

•

I •

Flicker evaluation

A working group of the International Union for Electroheat (VIE) has developped an internationally agrees instrument to measure flicker, and criteria for evaluation of flicker severity. The specifications are published as an I.E.C. publication. The flickermeter consists of several blocks, which includes an input voltage adapter, a quadratic demodulator, weighting filters, and a statistic evaluation. The flickemeter gives the measurement results Pst(short time) every 10 minutes and Plt(1ong time) every 2 hours. VIE gives the following compatibility levels:

COOUP VAllLT

Table 1: limits of flicker compatibility, recommended by VIE Figure 1: Diagram of an arc furnace.

The main elements of an arc furnace are: -the electric supply, -the furnace, which can contain scrap with low density (so it has necessary an important volume), and reduced steel between 1600°C and 17OO°C, -auxiliary installations, such as the smog treatment. There are typically three steps of steel elaboration:

HT 0.79 0.58

The following graph shows us the limits of flicker compatibility, caused by sinusoidal voltage fluctuations, as a function of the frequency:

•

I till

l~~"

,

1. the start-up phase: this is the first period of the melting. The electrode are digging a hole into the scrap. The electric power is generally.reduced.

...

mm:

m

11 III

........

'"

1

2. the melting: during this phase, the electric arcs are surrounded by the scrap, and the heat transfert is essentially made by thermic radiations.

I

/

3. the refining: the temperature of the steel is raised until 1700°C. During this period, the electrical power is adapted to the refractory consumption.

2.2

Mf 1.00 0.74

BT 1.00 0.74

PST PLT

I

- ..--

Figure 2: Values of 8UIU for PST=1.

Perturbations ofan arcfumace 3. PRINCIPLE OF TIlE STATOCOMPENSATOR

Due to the rapid variations of the arc length and so, the reactive power, the voltage level of the network fluctuate, according to the following eqution: 8U 8Q -=(1) U Sce Where: 8UIU represents the relative voltage fluctuation, 8Q represents the reactive power variation, Sce represents th short-circuit pwer of the network

The principle of the Statocompensator ( Boidin ,Compas, Henry et Rouget ,1980), (W.Horger, D. Schrooer ,1991) is to control the reactive current in a reactor with a thyristor switch. There are two anti-paralleled thyristors, connected in series with a reactor, as shown in figure 3. The reactor and the thyristors constitue an arm of the Statocompensator, which is connected phase to phase. The reactive current in each arm is regulated to compensate the variations of reactive current of the arc furnace, so that the total reactive current created by the arc furnace and the Statocompensator can be compensated by capacitor banks.

This voltage fluctuations are responsible for luminance changes in incandescent lamps, called flicker. In the range of 9Hz of voltage fluctuation, the human eye is most sensitive to flicker. In fact, voltage changes should not pass 0.3%.

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between the voltage command [u] and the current in positive and negative sequence of the load (furnace+filters) [ldi_load].

Th1 u(t) =vsinlli

Figure 4: Model of the complete system.

The inputs of the system are the voltage command of the TCR [u] given by u =

2~ - sin(2~)

in each

1t

phase and the symmetrical components of the load current (furnace+filters) [ldi_load]. The outputs of the system are the observed symetrical components of the line [ldi_line_obs] and of the load currents [ldi_load_obs]. G(z) is given by the following matrix: -1 G(z) = V • 2 (4) 3 [ -1/2 -1/2 1

Figure 3: Principle ofa Statocompensator.

The reactive current at firing angle a. is represented by the following equation: U I(t) = Leo (cos(a.) - COS(Cilt»

.!.. -.J3 /

i~2 ~I]

(2) 4.1.1 Calculation ofactive and reactive power:

The fundamental component of the reactive current can be calculated, and is a function of a.. IF =~(2~ - sin(2~» Leo 1t (3)

The calculation of active and reactive power with the aid of their symmetrical components is described below in figure 4:

~=1t-a.

Associated with the capacitor banks, the reactive power can be suppressed. The voltage level of the network is then regulated. The objectif of the Statocompensator is then to compensate the rapid fluctuations of reactive power (5 to 20Hz) in each phase and so, to attenuate the flicker level.

CaIculali...

oflhepowen

Notcbfikln lad

[pdU·odJ

low .... m....

Figure 5: Calculation of the powers 4. CONTROL SECTION 4.1

A further transformation procedure enables the three phase voltage and current systems to be represented by a two phase voltage and current systems, so [v] and (Uoad] represents the voltage and the load current in the a.~ system. The transformation procedure is given by:

Control Concept

The implemented control is based on the internal model control concept, using a model of the SVC system including its disturbances. The employed model is a robust model, based on active and reactive currents in positive and negative sequence, in load, line and TCR, and it is shown in figure 3. The model includes the dynamics of the observers Obs""p(z) and Obsj'(z). G(z) represents the transfer function between the voltage command and the current in positive and negative sequence of the TCR, and H(z) represents the transfer fonction

[~;]=~.[~ ~}t ~J~H~}5) The purpose of vector identification is to produce the rotational

vector

X(t) = Xej(wt+
variable X(t) = Xcos(wt +
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the

calculated from the model with required references and measured disturbances (it is the previsible part, in open loop), and one term of correction from the difference between system and model (closed loop). To avoid creation of even harmonic currents, thyristor conduction periods for positive and negative thyristors must be equal, so the control ensures the balancing ofthyristors on each arm.

x

Figure 6: Vector identification

-.-..

This vector identification gives a shift between Xd and Xq (90°) independant on the frequency. The gain factors Kd and Kq are to be tuned to ensure an unit gain at 50Hz,

so

1+S.TII s. T

Kd =

and

11

Figure 7: Control structure of the Statocompensator

Kq = 111 + s· TII where s=j.l007t. In order to achieve short regulation times, the vector identification should take only a minimum of time so T was choosen equal to 1. 84ms. The calculation of active and reactive power is given by following equations:

The complete structure of the control is represented in figure 8. The inputs of the system to be controled are: • the voltage command of the TCR [u], • the disturbance load current [P], • and the unknown disturbance [d) which represents errors of modelisation The estimated outputs of the system are: • the active and reactive currents in line (y] • the active and reactive currents in load (furnace+filters) [ps] • the active and reactive currents in load (furnace+filters) [Pp]. The current [Pp] is used, through the phase lead network for the predictive compensation. The currents [ps] and (y] are used in the regulation in a internal model structure. The current (Pp] measurement must be fast, whereas [ps] and (y] measurements must be synchronous. So there are two distinct estimations of active and reactive load current.

Vex • lex = (Vad • lad + Vcxq • lcxq) Vex • l~ = (Vad • l~d + Vcxq • l~q)

V~ • lex = (V~d • lad + V~q ·lcxq)

(6)

V~ .l~ =(V~d *l~d +V~q ·l~q)

= (Va * la + V13 * 113 ) 12

Pd

Qd=(Va *113- VI3*l a )/2 Pi

= (Va

Qd

• la - V13 * 113 ) I 2

=(- Va

(7)

* 113 - V13 • la) I 2

where Pd, Qd, Pi, Qi represent respectively the active power and the reactive powers in the positive sequence, and the active and reactive powers in the negative sequence. The filters network are made of notch filters at 50Hz, 100Hz, 150Hz, 300Hz followed by a low pass Butterworth filter. Transfer function of one notch 2 2 wo +bl*s+s filter is given by: 2 2 wo + 2 • ~. wo • s + s Because the dynamic of the system is given by the observers on active and reactive powers, the blocks Obs.J)(z) and Obs....Y(z) will be represented by an equivalent 2nd order filter of the whole calculation system with filters network included.

Op(z) represents the dynamics of the observer Obspp(z) whereas Oy(z) represents the dynamics of the observers Obs-ps(z) and Obs-y(z). Oy(z) is a linear function obtained by identification procedure. Op(z) can't explicitly be seen on the diagram. Da(z) represents the phase lead network on the symetrical components of the load current [pp). and Ga is the gain factor of the phase lead network Da(z). Da(z) is given by: 1+ s * Tl

Da(z)

= l+s*T2 •

2

+ 2 • ~. wo • s + 2 * s 2 2 s +2*~*wo .s+wo

wo

2

(8)

4.2

Control Structure Tests with exact model showed that the load power measurements are dependent on the voltage command [u]. Even if this dependency is quite low, it is a nuisance because of possible resonance due to

The control structure principle is shown on figure 6. This represents a command with an approach by internal model control. In this structure, the voltage command consists on two parts: one command

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filters on line. This dependency is represented by the transfer H(z). G and Gh are the matrix static gain of G(z) and H(z). Dr(z) represents filter for the control robustness. Transfer H(z)*Da(z)*Op(z) is obtained by identification procedure: • a static gain matrix [Gh]*Ga, • a 2nd order dynamic identical for all arms Dh(z), • a distinct pure delay in each ann. 5. DESCRIPTION OF AN INDUSTRIAL STATOCOMPENSATOR 'DNA arc imlI:B

The StatocompensatornA has been installed in order to suppress the flicker generated by two 20 MVA arc furnaces, to compense the reactive power, to reduce hannonics caused by the arc furnace and the StatocompensatornA himself and to reduce the negative phase sequence current by balancing unsymmetrical furnace loads.

:DNA

arc Mule

Figure 9: Single line diagram of the electrical system.

5.1

Simulations

In order to confirm the calculations, multipurpose SABER and MATLAB simulation softwares are used. This allows us to simulate the complete electrical network including impedance, transformers, arc furnace, filters, SVC system and its regulation. The simulated system performance studies have been carried out as follows: • Voltage and flicker studies • Transient network disturbances analysis • Hannonic analysis.

Figure 8:Internal compensation.

The simulations permits us to optmuse the performances of the Statocompensator. In fact, the effects of the modifications on the parameters are difficult to estimate on site, because of the chaotic functionnement of the arc furnaces;the modifications are usually observable after two days at least.

model control with predictive

5.2

The StatocompensatornA comprises a 45MVAR hannonic filter and a 55MVAR thyristor controlled reactor, and is connected on a llkV AC bus, which has a short circuit power of 300MVA. The level of flicker is controlled on a 66kV AC bus, where the short circuit power comprised between 620MVA (fault configuration) and 1l00MVA (normal configuration). The structure is shown in figure 9.

Operating results

The performances obtained in flicker reduction have been mesured' with a flickermeter according to the LE.C. specifications. The measure of severity is based on an observation period of 10 minutes and is designated by Pst (short time severity). The compatibility levels of flicker are still in study, but it seems that the Pst95% (i.e. the probability that a Pst value is not exceeded in 95% of cases) is a correct criterion and should be less than 1.(M. Kratz ,1995) The figures 10 and 11 show the flicker level without the Statocompensator (figure 10) and the flicker level compensated with the Statocompensator (figure

1121

11), while the arc furnaces work. The tension level on the 11kV bus is regulated, and the rapid variations of the load current are also effectively reduced to a compatible level. Pst

Pst with arc fumace

2.5,--_,..-_-,-_-,-_--.-_--,.._----._----,

..

..

..

.

:~ 1~lf!\\~~l'~\~~ijL

REFERENCES Publication n0868 CEI 1988 et 1990.Flickermetre, specifications fonctionnelles et de conception

o L...-_..i..._----l._ _L...-_-'--_....L._ - - - - l . _ - - J 7.5

8

8.5

9

9.5

10

Statocompensator to balance these effects has been shown in this paper. Simulations of the whole structure of the Statocompensator with Saber software have been made and the reliability of the method is demonstrated by implementation on a 55MVAR Statocompensator connected in 11kV network.

10.5

11

time (days)

M.Boidin et Drouin (1984). Les systemes de compensation statique rapide dans les reseaux industriels. RGE.

number of measures 40 ,--_ _~ - - - - - . - ~ - - - , 437 menures 348 witlllumaces

30

Pst-m ax: 2.28 Pst-99 : 2.05 Pst-95 : 1.80

20

Boidin ,Compas, Henry et Rouget (1980). Le statocompensateur, un moyen pour limiter les perturbations sur les reseaux electriques. DIEG. W.Horger, D.SchrOder (1991). Contro10fthyristor controlled reactors for optimal flicker reduction EPE Florence.

Figure 10: Measured Statocompensator

flicker

without

Pst with arc furnace

Pst

G.de Preville, O. Lapierre, G. Bomard (1995) Control of Static VAR Compensator for optimal flicker reduction. EPE.

1.2,----,....---.-----.-------,_----, 1

;

0·'1

;

1

: I

1

:

t:

::A~" llfl'I. ·1

.

J t~,

M. Kratz (1995) Guide de choix des alimentations electriques pour fours a arc de siderurgie. Publication EDF.

J

DIE (1988) Connection to fluctuating loads.

o2'--_ _L..:--'-_L...---'---'-L...-_----''---_--' 18.5

19

20.5

21

~measlXeS

193 witll furnaces Pst-max: 1.02 Pst·99 : 0.96 Pst-95 : 0.89

30

20

10

o o

n

1.5

0.5

D.O'Kelly, K.H.Salem, B.Singh (1992). Reduction of voltage flicker of a simulated arc furnace by reactive compensation. EPSR

Pst

Figure 11: Measured flicker with Statocompensator 6. CONCLUSION Arc furnaces are an important source of flicker effects, because of the rapid variations of arc length, and so, of reactive power. Dynamic control of

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