7/28/2019 IEEE_04039473

1/3

732 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007

Predictive Method for Improving SVC Speed inElectric Arc Furnace Compensation

Haidar Samet and Mostafa Parniani, Senior Member, IEEE

AbstractThe ability of static VAr compensators (SVC) to re-duce the flicker caused by electric arc furnaces and other variableVAr loads depends on their speed, which is limited by delays in re-active power measurement and thyristor ignition. In this paper, apredictive method is proposed to compensate the delay time and,hence, to improve the SVC performance. Previous samples of theload reactive power are used to predict its future values. PredictedVAr is then applied as a reference for reactive power compensa-tion.

Index TermsElectric arc furnace, flicker, prediction, static VArcompensator (SVC), variable reactive power.

I. INTRODUCTION

AN AC electric arc furnace is an unbalanced, nonlinear, and

time-varying load that can cause various power-quality

problems. Electric arc furnaces and other variable VAr loads

produce voltage fluctuation or flicker due to reactive power vari-

ation. A widely used method for flicker reduction is to em-

ploy static VAr compensators (SVCs) comprising thyristor-con-

trolled reactors (TCRs) or thyristor switched capacitors (TSCs)

[1]. However, the ability of SVCs in flicker reduction is lim-

ited by delays in reactive power measurement and thyristor ig-

nition [2]. The maximum ignition delay is a half cycle of line fre-

quency. Due to these inherent delays, the performance of SVC

in flicker reduction depends on the frequency of reactive powerchanges. With the load variation frequency of up to about 6 Hz,

its performance is very good, but at higher frequencies, it cannot

effectively compensate the load. The synchronous static com-

pensator (STATCOM) is a faster device to remedy this problem

[2]. It is, however, more expensive than SVCs. So, in this paper,

we offer a predictive method to compensate the delay time and,

hence, to improve performance of SVC in flicker reduction.

II. EFFECT OF COMPENSATOR DELAY TIME ON

ITS PERFORMANCE

The effect of delayed reactive power compensation can beshown using the following equations [3]:

(1)

(2)

(3)

Manuscript received June 1, 2006. Paper no. PESL-00035-2006.H. Samet is with the Electrical and Computer Engineering Department, Is-

fahan University of Technology, Isfahan 84156, Iran (e-mail: samet@ec.iut.ac.ir).

M. Parniani is with the Department of Electrical Engineering, Sharif Univer-sity of Technology, Tehran, Iran (e-mail: parniani@sharif.edu).

Digital Object Identifier 10.1109/TPWRD.2006.886768

where , , and are the load, compensator, and sourcereactive powers, respectively. is the frequency of reactive

power variation, and is the compensation delay. Then, the

variable VAr reduction factor is equal to

(4)

For an ideal compensator, is zero. With and in-

creasing, increases too, and above a specific frequency, this

factor becomes greater than one. In this case, the compensator

will have a negative impact on flicker, and causes it to grow.

The frequency limit is inversely proportional to the compensa-

tion delay time.

III. IMPROVING SVC PERFORMANCE WITH ESTIMATING LOAD

REACTIVE POWER IN FUTURE TIMES

To alleviate the negative impact of delay time, we use a

method to estimate the load reactive power in future times.

Predicted values will then be used as a reference for SVC, to

compensate the delay. VAr prediction is carried out using the

past samples of load reactive power. Laplace transform of the

lead time prediction function, can be approximated as

(5)

where s are constant coefficients to be calculated. One

simple way of calculating these coefficients is

(6)

It is straightforward to calculate s by expanding the right-

hand side (RHS) of (6), and approximating the series with its

first terms. Fig. 1 shows the amplitude and phase of the ap-

proximate functions for ms. In this figure, is shown

on each curve and the curve without any number is the exact

function. For , (6) is valid if

(7)

So, knowing the desired prediction horizon, the maximum

compensation frequency can be found from (7). For instance,

with ms, the maximum theoretical compensation fre-

quency is 16.6 Hz. A better approximation for to increase

this range is obtained as follows:

(8)

0885-8977/$20.00 2006 IEEE

7/28/2019 IEEE_04039473

2/3

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007 733

Fig. 1. Amplitudeand phase of theapproximation functionswith a = 1 . (Colorversion available online at: http://ieeexplore.ieee.org).

Fig. 2. Amplitude of approximation functions for large frequencies with a =

1

.

Maximum compensation frequency with (8) is

(9)

A sufficiently large simplifies (9) to . In-

creasing the number of terms raises the accuracy of finite

approximation of(8). However, it has a negative impact in am-

plifying noise and harmonics at specific frequencies. This phe-nomenon is shown in Fig. 2 for and 10 ms. Studies

show that harmonic amplification tends to lessen with higher

values of .

Resonance may occur if the amplification frequency coin-

cides with the line frequency or its harmonics. In that case,

at the RHS of (5) needs to be replaced with . For ex-

ample, this is the case when using (8) with ms in a

50-Hz (314-rad/s) system (see Fig. 2 ). Choosing ms

then yields

(10)

Inspection of the frequency characteristics of (10) does not re-

veal any resonance condition.

IV. OPTIMUM APPROXIMATION

The approximation introduced in the last section is not nec-

essarily the optimum one. As already discussed, the sampling

delay time can be different with the prediction horizon. So with

the following general expression:

(11)

the optimum coefficients are found by solving

(12)

The choice of and depends on the desired compensa-

tion frequency range. Flicker frequency is normally between 4

Hz and 14 Hz. For convenience, let and .

Then, (12) can be rewritten as

(13)

Also, see (14)(15) at the bottom of the page.

The following set of equations needs to be solved to find the

optimum coefficients:

(16)

Table I shows constants for ms, ms, and

.

V. CONCLUSION

Inherent delays of SVCs limit their capability of compen-sating fast varying reactive loads and flicker reduction. To im-

prove the speed of compensation, a predictive method based on

the estimation of load reactive power in the future using its past

(14)

(15)

7/28/2019 IEEE_04039473

3/3

734 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 1, JANUARY 2007

TABLE I

OPTIMUM COEFFICIENTS FOR T = 1 0 ms AND T 1 = 1 4 ms.

samples was presented. It was shown how to choose the esti-

mation parameters to extend the compensation frequency and

to avoid resonance conditions. Optimum parameters were cal-

culated with due consideration of flicker frequency.

REFERENCES

[1] E. Acha, V. Agelidis, O. Anaya, and T. J. E. Miller, Power ElectronicControl in Electrical Systems. Oxford, U.K.: ButterworthHeine-

mann, 2002.[2] A. G. Cerrada et al., Comparison of thyristor-controlled reactors and

voltage-source inverters for compensation of flicker caused by arc fur-naces, IEEE Trans. Power Del., vol. 15, no. 4, pp. 12251231, Oct.2000.

[3] T. J. E. Miller, Reactive Power Control in Electrical Systems. NewYork: Wiley, 1982.