STUDY OF TRANSIENTS IN MULTIPLE POWER FILTER CIRCUIT COMPRISING C-TYPE FILTER

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P O Z NA N UN I V E R S ITY O F TE C H N O LO GY A C A D E M IC J O U R N AL S

No 95 Electrical Engineering 2018

DOI 10.21008/j.1897-0737.2018.95.0011

__________________________________________

** AGH University of Science and Technology

** TAMEH Polska Sp. z o. o.

Jurij WARECKI

^*

, Michał GAJDZICA

^**

STUDY OF TRANSIENTS IN MULTIPLE POWER FILTER CIRCUIT COMPRISING C-TYPE FILTER

Passive filters are widely used to mitigate the effect of load harmonics on the supply power grid. Topology selection of the filters is based on the frequency bandwidth to be suppressed. Filter topologies comprising single-tuned filters and damped filters are the most popular in industrial power system applications. The article focuses on the influ- ence of the damped C-type 2^nd harmonic filter on the transients behavior under arc transformer energization in the arc furnace power supply system compensated by SVC. The examination results of transient currents and voltages across capacitors and reactors of the 3^rd and 5^th passive filters are compared for two topologies of the SVC filter circuits:

with and without C-type filter. The Matlab/Simulink Software was used to modeling filter behavior during unload transformer switching-on.

KEYWORDS: capacitor, reactor, C – type filter, single-tuned passive filter, switching transient, transient overvoltage, transient overcurrent.

1. INTRODUCTION

Due to the increase in the number of nonlinear loads in power networks, the utility grid voltage is diverted from its pure sinusoidal shape at the fundamental frequency, although that signal is repetitive at the fundamental frequency. Non- linear loads especially arc furnaces, arc welders, rolling mills, traction loads, adjustable drivers generate high harmonic currents and voltages. There are some advanced techniques such as: magnetic flux compensation, series or shunt active filters, harmonic current injection to reduce harmonic distortion problems in power systems. However, these techniques are so complex and cost too much, so they could not compete with currently passive harmonic filter applications.

In practice the most commonly used are single-tuned harmonic filters. That solution is used in simple and complex configurations of power supply systems and reasonable during filtering one, predominant harmonic in the spectrum. The problem with compensation and filtering occurs, while in power system is re- quired to lead a several harmonic filtration. It should be noted, that due to devia-

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106 Jurij Warecki, Michał Gajdzica

tion of single-branch filters reactor and capacitor bank parameters, as well the variation of parameters which can be derived from two major factors: tempera- ture and manufacturing tolerance, it is not possible to guarantee the full harmonic filtering of high frequencies. Besides, the filtering effect and its quality on that case strictly depends on the short-circuit power at the point of connected SVC system.

To eliminate these disadvantages in multiple filter circuits (FC) the C-type filter structure are widely used in industry application [1]. This high pass filter has good suppression at the tuned frequencies and efficiently damps the resonance instead of shifting it to lower harmonic order. A power supply systems with damped filter also offers lower active power losses, because the L-C2

branch (Figure 1) is tuned to the fundamental harmonic frequency. The current of fundamental harmonic is not passing through the bypassed, parallel resistance which avoid to large of power losses.

The purpose of this paper is to study the transient overvoltages and overcurrents which appear on the filter circuits during transformer energization in a typical industrial arc-furnace power system. The comparative transient analysis has been carried out between the results from the examined supply system whose filter circuit is equipped with two single-tuned and 2^nd harmonic C-type filter, with the case study which FC configuration based on three types of the passive filter circuits [2], [3]. The magnitude of long lasting currents and voltages of transformer and filter units on each implemented industrial applications, which are operated with all filters on SVC systems are presented. To analyze this phenomenon the Matlab/ Simulink Software has been chosen because of there are known limitations in the filled testing with respect to the circuits condi- tion and the number of times that the test can be carried out.

2. C – TYPE FILTER PARAMETERS AND CONFIGURATION

The C-type filter is included in the category of broadband (damped) filters.

Broadband filter damps commutation notches more effectively than traditional shunt harmonic passive filters and has a much broader bandwidth. They have good ability to eliminate inter-harmonic components generated by static frequency converters. Besides, the C-type filter has good suppression at tuned frequencies and more effectively damps the resonance that may occur. The main advantage of using damped filter is a considerable reduction in fundamental frequency losses [4], [5]. An example, for the 2^nd order C-type filter, there are at the same time good damping for the 2^nd harmonic. Figure 1 illustrates a branch circuit model of C-type filter.

The filter branch L-C2 is tuned by fundamental harmonic and branches L-(C1+C2) are tuned by filtered harmonic. The capacitive and inductive elements

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Study of transients in multiple power filter circuits … 107

of the 2^nd harmonic filter have been calculated respectively as [6, 7]:

Fig. 1. The configuration of the C-type filter

2 1 2

n F

U C Q

  (1)

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n r F

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The total capacitance of the main capacitor (C1) and auxiliary capacitor (C2) and inductance of reactor in C-type filter at fundamental frequency is the same as that of single branch filter. The values of damping resistance have been ob- tained for assumed quality factor as [6], [7]:

2 2

F r F T n

Q f q

R  U (4)

where: U_n– bus rated voltage, f_r – relative tuning frequency, q_F – filter quality factor

The damped filter has different behaviors with various categories of frequencies and acts as various types of passive filters. At fundamental frequency, it acts as a stand-alone capacitor (C₁), where the damping resistance is bypassed due to the tuned arm, series branch L-C₂. At the moment, when the frequency increases, the filter acts as a single-tuned filter with damping resistor, where the inductor starts to resonate with capacitance C₁ and C₂. At higher frequencies, the C-type filter acts as a first-order unit, where the inductance value has higher magnitude than that of C₁ and C₂ [4].

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3. CASE STUDY

This paper considers a comparative analysis of two electric arc furnace supply systems presented in Figure 2. The examined power supply system 20 kV is connected to HV 110 kV bus supplying by means of step down wye-delta connected power transformer TS, with the primary neutral solidly grounded. The 50 MVA arc furnace transformer TP of Yd1 windings and the Static Var Compen- sator (SVC) unit are connected to the same MV bus whence Alternating Current Electric Arc Furnace (AC-EAF) unit is supplied. The power supply system is equipped with three harmonic filters which are tuned for: 2^nd, 3^rd and 5^th current harmonics. The arc furnace and SVC units are connected to 20 kV bus through the appropriate vacuum circuit breakers Q_n. The arc furnace system is supplied by 80 MVA of power transformer unit and the examination has been carried out during all designed harmonic filters are connected to industrial installation.

System A System B

Fig. 2. One line diagrams of the comparative AC-EAF power supply systems

The filter circuit was implemented as three individual filters which are sized to supply 5, 22 and 15 MVAr for the single-tuned filters of 2^nd, 3^rd and 5^th harmonics respectively in the System A. The C-type 2^nd harmonic filter and the two single-tuned filters of 3^rd and 5^th harmonics of the same rated powers are used in the System B. Each of the filters in FC unit is tuned with ideal resonance fre- quency fr. The power system frequency has a constant rated value 50 Hz. The total reactive power of FC unit is the sum of the reactive power for each of them:

QFC = QF2 + QF3 + QF5. Parameters of individual harmonic filters depend on the sum of harmonic levels supplied from the arc furnace and Thyristor Controlled Reactor module (TCR). In the Table 1 and 2 are shown the FC parameters in-

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cluding 2^nd C-type filter and single-tuned passive filters of 3^rd and 5^th harmonics respectively.

Table 1. Specification of the C-type filter.

Filter

order Tuning frequency

Capacitance C1 [μF]

Capacitance C2 [μF]

Inductance L [mH]

Damping Resistance

RT [Ω]

F2 2.00 39.79 119.37 84.94 100.00

Table 2. Specification of the single-tuned passive filters.

Filter

order Tuning frequency

Capacitance C [μF]

Inductance L [mH]

Resistance R [Ω]

F2 2.0 29.86 84.94 0.24

F3 3.0 155.05 7.27 0.06

F5 5.0 114.64 3.54 0.03

4. SIMULATION RESULTS A. Transformer inrush current modeling

In the examined supply system arc furnace transformer (TP) energizing occurs several times a day and it is associated with a high magnitude of the inrush current [3, 6]. Simulations of the TP inrush currents have been carried out for worst case conditions, just like in paper [2, 3] where residual flux in A, B, C cores of the arc transformer unit was taken as: 0.6Ψ_nom; 0; -0.6Ψ_nom respectively and instant of switching provided highest magnitude of the inrush current. Fig- ure 3 shows the waveform of the simulated inrush current for phase A under energizing the arc furnace transformer from the system transformer (TS) of 80 MVA capacity.

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structure comprising C-type filter. It is observed, that during transformer energization, the nature of transient voltages are closely related to the current waveforms. The contribution between capacitive and inductive elements of filter units strictly depends on the value and frequency of transient currents. Besides, the application with C-type filter in FC system, relatively reduce the transient voltages generated on F3 and F5 units. The bypassed resistor effectively damps the transient oscillations and therefore reduces their duration in the other filter circuits.

As one can see the transient voltages across capacitors in the system B are lower than in the system A and the transient durations are shorter. Comparative analysis of magnitudes has shown that higher multiples (relative to the rated filter voltages for capacitors) are observed on single-tuned filters. In the case of transient voltages across reactors, there are observed the lower values on passive units of F3 and F5 filters operated with damped 2^nd harmonic filter than in the case from system A. Analysis of the maximum voltages generated across the implemented filter circuits has shown, that in the two cases A and B at relatively moderate overvoltage multiples (related to rated voltages for capacitors) across capacitor banks there are observed a higher multiples on filter reactors (related to rated voltages for reactors) insulation.

The comparative study shows that the damping resistor of the 2^nd harmonic filter reduces the transient duration on the capacitive and inductive elements of the F3 and F5 filter circuits. It should be noted, that in the case of any other power system topology and filtering system one can obtain the completely different relation of transient values.

The results of transient magnitudes of currents and voltages across the filters installed in FC, in the examined supply network are shown respectively in Ta- ble 3 – for the system A and in Table 4 – for the system B.

Table 3. Peak transient currents and voltages in FC, comprising single-tuned filters.

Filter Circuit F2 F3 F5

Peak current kA 1.75 2.39 2.48

p.u.^(*) 8.61 2.68 4.06

Capacitor Peak

voltage

kV 108.57 33.49 31.71

p.u.^(*) 4.73 1.79 1.85

Reactor Peak

voltage

kV 88.98 14.07 8.70

p.u.^(*) 13.42 5.85 11.01

(*) base value – rated filter current and voltages for capacitors and reactors

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Table 4. Peak transient currents and voltages in FC, comprising 2^nd damped filter.

Filter Circuit F2 F3 F5

Peak current kA 0.77 2.27 1.86

p.u.^(*) 3.77 2.55 3.04

Capacitor Peak

voltage

kV 46.37 34.25 26.52

p.u.^(*) 2.02 1.83 1.43

Reactor Peak

voltage

kV 35.62 12.09 5.46

p.u.^(*) 5.38 5.03 6.91

(*) base value – rated filter current and voltages for capacitors and reactors

5. CONCLUSIONS

Transients in a compensated arc furnace power supply were investigated by simulation. Comparison of transients in two filtering systems, built on all single-tuned filters and single-tuned filters comprising 2^nd harmonic C-type filter, under transformer energization transient is presented.

A general analysis has shown that switching-on arc furnace unit causes high peak magnitudes of transient voltages and current in the filter circuits. In the examined power supply systems, consisting damped second harmonic filter one can see decreasing the transient voltage and current magnitudes. At the same time there are observed the short duration of transient in each filter circuits on SVC system.

Using damped C-type filter in the power supply system improves the filter transient characteristics and decreases requirements on selecting ratings of the filter reactors and capacitor banks and so their cost reduction.

Badania były finansowane przez Ministerstwo Nauki i Szkolnictwa Wyższego (Grant AGH Nr 11.11.210.312).

REFERENCES

[1] Klempka R., Hanzelka Z., Varetsky Y.: Bank Harmonic Filters Operation in Power Supply System - Cases Studies, Power Quality Issues, p.202 - 230.

[2] Varetsky Y., Gajdzica M.: Analiza procesów podczas załączania transformatora pieca łukowego zasilanego z układu z filtrami wyższych harmonicznych, Zeszyty Naukowe Politechniki Poznańskiej. Elektryka. – 2014 no.79, s. 279 - 286.

[3] Warecki J., Gajdzica M.: Załączanie transformatora pieca łukowego w sieci z układem filtrów wyższych harmonicznych, Przegląd Elektrotechniczny - 2015 R.

91 nr 4, s. 64 – 69.

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[4] Mohamed Islam F., Shady H.E., Aleem A., Ibrahim Ahmed M., Zobaa Ahmed F.:

Optimal Sizing of C-Type Passive Filters under Non-Sinusoidal Conditions, Energy Technology & Policy: An Open Acess Journal, 2014, p. 35 - 44.

[5] Klempka R.: A New Method for the C - Type Passive Filter Design, Przegląd Elektrotechniczny, R.88 nr. 7a/2012, p. 277 – 281.

[6] Varetsky Y.: Damping transients in compensated power supply system. // Proc. of VI Sc. Conf. „Electrical power networks-SIECI 2008” Poland, Szklarska Poręba, September 10 – 12, 2008, p. 397 - 404.

[7] Xiao Y., Zhao J., Mao S.: Theory for the Design of C-type Filter, 11th International Conference on Harmonics and Quality of Power 2004, p. 11 – 15.

[8] Warecki J., Gajdzica M.: Praktyka doboru filtrów harmonicznych dla układów zasilania pieców łukowych, Zeszyty Naukowe Politechniki Poznańskiej, Elektryka 2015, s. 45 – 53.

(Received: 30.01.2018, revised: 12.03.2018)

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