PFC Units Sizing in Steel Factory Harmonics

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PFC units sizing in steel factory harmonics

environment: a case studyThe installation of non linear loads determines the presence of current and voltage harmonics

and interharmonics. Components particularly sensitive to this kind of phenomena are powerfactor correction equipments. This paper describes the design process of a centralized PFC

system under harmonic pollution conditions pointing out the key role of measurement

campaigns.

Angelo Baggini

Faculty of Engineering

University of Bergamo

Dalmine (BG), Italy

[email protected]

Franco Bua, Francesco Buratti, Alan Ascolari

ECD Engineering Consulting and Design

Pavia, Italy

[email protected], [email protected],

[email protected]

Abstract This paper deals with PFC units design related

problems in case of installation in harmonics rich environments:

in particular the paper contains an overview of sizing approach

used in a real case of PFC units design for installation in a steel

factory where new PFC units installation has been necessary

after a fault with catastrophic consequences for the existing ones.

The document contains a report on stresses calculation on PFC

units due to harmonic currents generated by non linear loads

installed in the above mentioned plant. Plant load, estimated to

be around 80 MVA, is formed by a big amount of small loads fed

by power converters: this is a big disturbance source for the

installation in terms of harmonic content.

In particular, due to the usual lack of real harmonics contents

data availability, main issue has been the process of definition of

a generic harmonic distortion for installed loads, based on power

converters installed and literature available data on power

converters spectra and its on field verification for PFC optimal

design, with three measurement campaigns aiming to verify real

harmonic distortion and series and parallel resonance risk.

This kind of problem is really actual and, most of all, is

constantly growing also in other industries, not only in steel

factories where harmonics content has always been a key

problem. Now, due to the continuous increase of power converter

usage and equipment power consumption, and mostly to the

increase of equipment disturbances sensitiveness, to avoid

potentially tragic faults, a deep measurement campaign is the key

solution at PFC units design stage, also considering the usual lack

of data on installation harmonic distortion.

The experience described in this paper constitutes the first

detailed example of on-field verification of harmonic

disturbances effects on two 8.4 Mvar, 13 kV PFC units in a

harmonics critical environment aiming first of all at a detailed

fault analysis (referred to pre-existent PFC Units) and then to a

correct new unit design.

In the paper the different design stages and related issues are

described, including: installation network analysis, load analysis,harmonic distortion evaluation, PFC units stresses analysis, PFC

units sizing, on field stresses verification.

This work highlights the strong need for deep measurements

campaigns in harmonics rich industrial environment as the only

way, in addition to a correct distortion pre-analysis aiming to a

careful PFC design strategy to prevent potentially catastrophic

faults.

Keywords- PFC, Harmonics, Case study, Non linear loads,

Measurements

Authorized licensed use limited to: UNIVERSIDADE ESTADUAL DE CAMPINAS. Downloaded on November 27, 2008 at 13:41 from IEEE Xplore. Restrictions apply.


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I. INTRODUCTION

The presence of harmonics in industrial plants nowadays isconstantly increasing and therefore its becoming a key issuefor designers when sizing PFC systems. In particular, ameasurement campaign should be carried out before the designstage in order to simplify design activity: in this situation,component sizing is performed on the basis of reliable data andnot on literature available data which usually may not becorresponding to the real conditions of the system.

In particular, this paper contains an overview of sizingapproach used in a real case of PFC units design for installationin a steel factory where new PFC units installation has beennecessary after a fault with catastrophic consequences for theexisting ones.

II. DESCRIPTION OF THE SYSTEM

The plant serving a steelworks is fed by an overhead HV

(220kV) line through two HV/MV 220/10.95 kV transformers,with rated power equal to 30/40 MVA (ONAN/ONAF).TheMV network has a radial scheme and cable connections.

Figure 1. Scheme of the plant

Several non linear loads are installed inside the plant, most of

them being six-pulse and twelve-pulse power converter havingdifferent rated power and feeding the production plant

machineries.

Production line has been designed and realized for processingproducts with different dimensions, which means that its

possible to have different process types each one producing a

different harmonic spectrum.

Since this is a semiautomatic production line, the number of

products and their positions are random factors. The direct

consequence is a continuous and unforeseeable variation of the

harmonic spectrum in the plant.

III. DESIGN OF THE POWER FACTOR CORRECTION SYSTEM

On the basis of calculations performed in order to determine

the required reactive power system, the buyer asked for the

installation of two three phase units, each one with rated

power equal to 8400 kvar at 13kV.The choice of a double unit is related to the fact that the plant

usually foresee one day per week programmed stops, thus

varying the absorption of reactive power in comparison with

working days.

Among the project data the buyer supplied for PFC system

design only the plant single-wire scheme and a list of main

transformers feeding the existing loads in the steel factory.

A. Calculation of thermal and dielectric stresses

Since no data related to harmonic pollution was available,

the waveform to be used for calculations has been assumed on

the basis of:

type of converter;

literature data for installed converters typical spectra [2].

Since the load is formed by several small power VSDs and

since it was not possible to know phase angles, the overall

current calculation has been made through the sum of the

RMS values of the components with the same frequency

according to two methodologies:

arithmetical sum, as if the harmonics had identical phaseangle (limit and worst situation);

as a square root of the sum of square values, assuming thatsome components were compensated by others because of

the phase angles. It is important to remember that thismethod, though without any theoretical basis, is used incommon practice and largely adopted in technicalliterature [8].

Considering the nature of plant loads it has been decided to

adopt a detuned PFC system with three single phase series

reactors. The choice of the unit characteristics has been made

by comparing stresses resulting on the capacitors due to

voltage harmonics coming from non linear loads, calculated

considering three different tunings which are commonly used

for this type of installations.

189 Hz (Case A);

204 Hz (Case B);

210 Hz (Case C).

Calculation of thermal and dielectric stresses on the PFC

unit has been performed with a mathematical model (based on

circuit in Figure 2. ) of the whole system for each one of thetuning frequencies.



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Figure 2. Circuit model.

Parameters shown in Figure 2. are:- IhL is h

thload harmonic current;

- LN ; RN are network parameters at PCC;

- IhN is hth

harmonic current;

- L, C are PFC unit parameters;

- Ihf is hth

harmonic in PFC unit.

B. Calculation of the resonance frequency

When designing a PFC system a preliminary verification of

the absence of potential resonance phenomena is alwaysnecessary. In this case short circuit power of the network hasbeen calculated on the basis of the available information,assuming that transformer short circuit voltage is referred to therated power highest value among the ones stated for thedifferent refrigeration systems, as specified by the standard EN60076-1 [3].

The value of short circuit power equal to 273.25 MVA hasbeen calculated through the equation (1) on the basis of datashown in TABLE V.

lcc

T

lcc

T

cc

Z

V

V

S

Z

V

V

S

S2

2

+

= (1)

where:

ST is transformer rated power;

Vcc is transformer short circuit voltage (in p.u.);

Vis system rated voltage;

Zl: line impedance between transformer and PFC units.

The resonance frequencies calculated for the differentconfigurations assumed are shown in TABLE I.

TABLE I. RESONANCE FREQUENCY

Data Case A Case B Case C

Units

connected1 2 1 2 1 2

Resonance

frequency

(Hz)

169.5 155.0 179.9 162.7 183.9 165.6

The resonance frequencies calculated have valuescomprised between the 3

rdand the 4

thharmonic order. On the

basis of the harmonic spectrum of six pulse and twelve pulse converters found in literature [2] it has been possible toverify the absence of resonance phenomena, because the

harmonics generated by these converters have usually onlyfrequency components higher than 250 Hz.

C. Choice of the parameters of the PFC system

On the basis of simulation results reported in TABLE I. andafter evaluation of related stresses on the units, as shown inTABLE II. TABLE IV. for one unit energized, case C,referring to the previous considered scenarios, the decisionadopted has been the realization of the PFC system tuned on210 Hz. This choice gives a higher safety margin to avoidresonance phenomena, even if its consequence is a slightincrease in the sizing of the components. This solution anywayallows the adoption of smaller reactors, with a benefit in termsof reduced losses.

Also for reactor thermal sizing, considering the uncertaintyon the real harmonic contents of the system, oversizingapproach has been adopted in order to obtain a higher safetymargin.

TABLE II. PFC UNIT TOTAL CURRENTS (2 UNITS) RESULTS REFER TO 1UNIT.

Case CCurrent components

PFC Network

Fundamental (A) 317.6 IN5th (A) 33.5 32.4

7th (A) 15.7 33.0

11th (A) 9.3 26.1

13th (A) 6.8 20.1

Total (A)* 320 57 + IN* Calculated as quadratic sum of all contributions for e ach frequency.

TABLE III. PFC UNITS TOTAL CURRENTS (1 UNIT).

Case CCurrent componentsPFC Network

Fundamental (A) 317.6 IN5th (A) 50.6 48.9

7th (A) 20.7 43.6

11th (A) 11.7 33.0

13th (A) 8.6 25.2

Total (A)* 322.6 77.5 + IN* Calculated as quadratic sum of all contributions for e ach frequency.

TABLE IV. PFC UNIT VOLTAGE STRESSES (IN CASE OF 2 UNITS, RESULTSREFER TO 1 UNIT).

Capacitor voltage

components

Case C

Number of units 1 2

Fundamental (kV) 11,66 11,66

5th (kV) 0,214 0,142

7th (kV) 0,063 0,048

11th (kV) 0,022 0,018

13th (kV) 0,014 0,011

Total Algebric Sum (kV) 12,20 12,04

Total Quadr. Sum (kV) 11,67 11,67

On the basis of calculations made, it has been proposed toinstall a PFC system divided into two units, each one as doubleunbalanced star (3+2) with the possibility to easily modify thetuning frequency to approximately 225 Hz, through theadoption of a further capacitor per phase and per section andthe addition of a second couple of units tuned on a frequencybetween the 11th and the 13th harmonic. The main data are

shown on TABLE V.



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TABLE V. MAIN UNIT DATA

Units 2

Rated voltage (kV) 13

Rated frequency (Hz) 50

Rated power (kvar) 8400

Capacity (F) 150.21

Inductance (mH) 3.83

Inductance thermal current (A) 600

Unit type Double Y

Tuning frequency (Hz) 210

It should be pointed out that the choice of this PFC systembrings several advantages. Besides advantages coming from thedetuning inductance being a filter for the harmonics present in

the network, it limits remarkably the inrush current avoidingthen component stresses.

The problem of inrush current should always be consideredwhen sizing PFC units mainly in situations where frequentswitching are foreseen (in this plant it has been estimated tohave at least 52 switches per year).

IV. MEASUREMENT CAMPAIGNS

Because of the uncertainty of the harmonic spectrum due tocontinuous load variations, and then on the related stresses of

the PFC system, some measurement campaigns have been

carried out on the installation in order to verify the most

important system parameters, i.e.:

voltage on the units (RMS and peak values);

currents on the units (RMS and peak values);

voltage and current harmonics (odd and even ones, up tothe 15th component);

active, reactive and apparent power.

Before system energization, visual controls have been madeaiming to monitor s ystem installation activities and to avoidunbalance generated by improper installation of cables. Duringthe installation of PFC units, measurements of inrush transientshave been carried out in order to verify the effectiveness, interms of current limitation, of the inductance installed in serieswith the PFC unit. All parameters have been monitoredcontinuously through the use of a three phase network analyzer,for the whole duration of the measurement campaigns.Therefore it has been possible to monitor and record theharmonic spectrum absorbed by the PFC units, both withprocess plant idle and with plant working on different products.Measurements have been carried out either with a single groupand with both groups connected.

A. Analysis of the results

The duration and the peak values of the inrush currentsmeasured during the first switch-on resulted to be lower than

the values estimated at design stage. However it has to be

pointed out that calculations made for inrush current pick valueevaluation have been performed with reference to the worstcase in terms of switching time, because it is not possible toknow supply voltage phase angle at the switching time whenthe measurement is performed.

As expected, monitoring has shown a continuous variationof the electrical parameters observed, not only for whatconcerns harmonics, but also in terms of supply voltage (Figure3. ). A variation of the total harmonic distortion (Figure 4. ) andof the various harmonic components has been observed (Figure5. and Figure 6. ), either in case of different products or duringmanufacturing of identical items but with different positions inthe production line.

It is important to point out that a not negligible thirdharmonic component has been found: this component had notbeen foreseen at design stage because the harmonic spectrumof this type of loads usually contains only components from thefifth order upwards.

Figure 3. Typical phase to earth voltage shape measured on the three phases

Figure 4. Typical THDi measured on the three phases



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Figure 5. Typical 7th current harmonic component on the three phases

Figure 6. Typical 11th current harmonic component on the three phases

B. Stresses verification

On the basis of measurement campaign results obtained,

stresses on capacitors have been evaluated in the following

scenarios:

spectrum corresponding to the highest THD valuemeasured for each process type;

spectrum obtained by considering each harmonic

component highest value.

It is however necessary to specify that the combinations of

harmonic components used in scenario 2 have never been

observed contemporaneously.

Values related to scenario 1 are shown in TABLE VI.Through the values of the harmonic currents measured, the

voltages on the capacitors for three different products

monitored have been then calculated.

TABLE VI. HIGHEST VALUES OF VOLTAGE HARMONICS MEASURED WITH

ONE OR TWO PFC GROUPS ENERGIZED

Harmonic component (%)1

PFC

unit THD 3a

5a

7a

9a

11a

13a

15a

P1 26.36 6.6 25.3 6.6 0.9 5.5 3.4 0.7

P2 26.51 13.1 23.2 7.0 0.7 5.0 3.5 0.5

P3 27.06 7.5 25.7 7.4 0.7 5.0 3.7 0.4

Max 27.06 13.1 25.7 7.4 0.9 5.5 3.7 0.7

Harmonic component (%)2

PFC

units THD 3a 5a 7a 9a 11a 13a 15a

P1 21.87 16.3 18.5 5.3 0.8 5.3 3.7 0.5

P2 42.6 41.5 16.6 5.3 0.6 5.2 3.9 0.4

P3 32.4 31.4 18.6 6.1 0.8 4.9 4.1 0.3

Max 42.6 41.5 18.6 6.1 0.8 5.3 4.1 0.5

Var.(%)

58 217 -28 -18 -10 -5 10 -22

The highest dielectric stresses calculated on the basis of

measurements results proved to be aligned with the capacitorsizing calculations performed under the statistical assumption

of phase angle random differences among the harmonic

components (modelled by algebraic sum). The spectrum with

the highest values of each harmonic component measured onthe different product manufacturing processes monitored is

shown in Figure 7.

Figure 7. Current harmonic spectrum related to the highest values measured

In terms of thermal stresses it has been possible to observethat the highest current (RMS value) measured showed to bealigned with the value defined for thermal sizing of capacitors.

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V. CONCLUSIONS

The presence of harmonics in industrial plants nowadaysmust be taken for granted, therefore it is necessary to take itinto consideration especially when sizing PFC systems. Inparticular, mainly in existing installations as in the casedescribed in this paper, a measurement campaign carried outbefore the design stage should simplify considerably the designactivity because the component sizing is performed on the basisof reliable data and not on literature data which, as valid asthey can be, however may not be corresponding to the realconditions of the system. On this issue its important to recallthat in the system described here, a third harmonic componentthat had not been foreseen at design stage, has been found: itspossible the to assume that it is not produced by facility loads,but by the network. Through an adequate preliminarymeasurement campaign this not negligible issue would havebeen taken into consideration. However it is important tohighlight that by having tuned the filter on a rather lowfrequency it has been possible to obtain a relevant mitigation ofthis third harmonic component in the system.

ACKNOWLEDGMENT

This article has been prepared with the support of LPQIproject (www.lpqi.org.). LPQI project has been funded by the

European Commission and ICA (International CopperAssociation Ltd.). The authors point out does not necessarilyreflect the position of the European Community, nor does itinvolve any responsibility on the part of the EuropeanCommunity.

REFERENCES

[1] J. Arillaga, N. R. Watson: Power System Harmonics. Wiley, 2004.

[2] IEEE 519-1992 - Recommended Practices and Requirements forHarmonic Control in Electrical Power Systems. Institute of Electricaland Electronics Engineers, 01-May-1992.

[3] IEC 60076-1 - Power transformers Part. 1 General.

[4] S. Fassbinder: Capacitors in Harmonic-Rich Environments. LPQIApplication Guide #3.1.2, http://www.lpqi.org.

[5] S. Fassbinder: Passive filters. LPQI Application Guide #3.3.1,http://www.lpqi.org.

[6] D. Chapman: Harmonics - Causes and effects. LPQI Application Guide#3.1, http://www.lpqi.org.

[7] F. Bua, G. Tacchi: Criteri di dimensionamento di batterie di rifasamento

in impianti caratterizzati dalla presenza di armoniche. Article availableon http://www.lpqi.org.

[8] R. C. Dugan, M. F. McGranaghan, S. Santoso, H. Wayne Beaty:Electrical Power Systems Quality. McGraw Hill, 2002.

PFC Units Sizing in Steel Factory Harmonics

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