Case Studies Flicker Problems in Steel Plant Caused by Interharmonics

7/28/2019 Case Studies Flicker Problems in Steel Plant Caused by Interharmonics

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Case study

Michle De WitteLaborelec

December 2007

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Power Quality

Flicker problems in a steel plantcaused by Interharmonics


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http://www.leonardo-energy.org

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Introduction

A steel manuacturing plant was experiencing a ficker problem. Lights in the o ce areas o the plant were oten

fickering, aecting productivity and creating an irritating nuisance or employees.

A Power Quality consultant was called in to investigate the case. He carried out measurements to gain a better

understanding o the problem and to enable him to identiy the source o the ficker phenomenon and the operating

conditions in which it occurred. Once this inormation in hand, he was able to propose an appropriate solution.

1. Problem description

1.1 Flicker

Flicker is a particular deviation rom the sinusoidal waveorm o an electric voltage. It is usually experienced through

the fickering o lights. Two types o ficker exist:

1) Normal ficker is caused by repeated rms voltage variations. It can be measured using a fickermeter1.

2) Flicker provoked by interharmonic voltages2

. An interharmonic voltage has a requency that is a non-integermultiple o the undamental requency o 50 Hz. This kind o ficker cannot be measured by means o

a normal fickermeter.

Not all interharmonic voltages necessarily cause visible ficker in all types o lighting equipment. Some equipment

is more sensitive to certain interharmonic requencies than others. Sensitivity curves or various types o equipment

can be drawn based on experiments. An example representing the curves o fuorescent and compact fuorescent

lamps is given in Figure 1.

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Figure 1: Sensitivity curves or various types o lighting equipment

1 IEC 61000-4-15 electromagnetic Compatibility (EMC) Part 4: Testing and measurement techniques Section 15: Flickermeter Functional and design specications2 Light ficker caused by interharmonics; 8th International Power Quality Applications Conerence; 1998; Cape Town, South-Arica


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1.2 Problems encountered

Normal ficker

The fickermeter indicated a relatively high level o

normal ficker on the 30 kV bus bar. This ficker was

traced and ound to originate at loads at the higher

voltage level (150 kV), which included an electric

urnace. However it was demonstrated that this

normal ficker measured on the 30 kV busbar resulted

in a ficker that was hardly visible at the lower

voltage to which the o ce lights were connected.

Consequently, the normal ficker provoked by the

electric urnace could not be the sole reason or

the fickering o the lights in the o ces.

Flicker by interharmonics

The 30 kV bus bar also connected a rolling mill.

To improve the cos, capacitor banks and their

corresponding lters are installed on the 30 kV

net. There were lters or the th, seventh, and

eleventh harmonic currents. These were passive

lters that can be operated separately. Such lters

create a low-impedance circuit or one specic

requency, but also create anti-resonance at

requencies adjoining the tuning requency.

This act made it likely that the cause o the ficker

problem was in the high interharmonic voltages provoked by the interaction between the rolling mill and the various

lter passes. In other words, high harmonic impedance in the lters at requencies that also show up in the current

harmonics o the load proved to be the origin o this ficker problem.

2. Measurements and simulations

2.1 Theory

When a capacitor bank is installed to improve the power actor o the load, a combination o lters and inductors

is oten installed to attenuate certain harmonics. The lter creates a series resonance that attenuates a certain

harmonic. The inductor, installed in series with the capacitor, creates a parallel resonance at a lower requency than

the ltered harmonic requency. The combination o the series resonance rom the lter and the parallel resonance

obtained by the inductor, leads to the curve shown in the gure entitled Series + parallel resonance.

()

Zgh

Xh

Xch

Xc

X

h1 hr h h

()

Xgh

XfhXgh

Xh

Xsh

Xch

()

Zgh

Xch XIc

R

hr h

Figure 3: a) Series resonance b) Parallel resonance c) Series + parallel resonance

I several lters are installed, each tuned to another harmonic requency, there is a cascade o these types o curves.

There are several requencies or which the harmonic impedance is (nearly) zero and several corresponding requencies

or which the impedance tends to innity. An example o such a curve is shown in Figure 4.

Figure 2: Electrical scheme o the in-stallation

45/66 MVAUsc = 14%

150 kV/32.86 kV

4800 kvar

R = 10 Ohm

L = 0.063 mH

45/66 MVAUsc = 14%

150 kV/32.86 kV

4800 kvar

R = 10 Ohm

L = 0.063 mH

Rolling

millFinishing train with 6 passes

M1 M2 M3 M4 M5 M6

Filter

H5

Filter

H7

Filter

H11


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h

Xgh

Xh

Xh

Figure 4 : Diferent lters in parallel

2.2 Network modelling and simulations

To veriy the theory o the anti-resonances, a simulation o the network was set up to examine the harmonic impedance

o the system. The simulation was carried out or three confgurations:

1) Filters or h5, h7, and h11 switched on (hereater called frst flter confguration)

2) Filters or h5 and h7 switched on (second flter confguration)

3) Filter or h5 switched on (third flter confguration)

The principal results can be seen in the ollowing graphs.

0 200 400 600 800 1000 1200

Hz

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

Zpu

Base: 100 MVA / 30 kV

Zohm = Zpu

9

Harmonic impedance with the flters or H5, H7 and H11 in service

Figure 5: Harmonic impedance with three lters (h5, h7 and h11) switched on

The frst graph (Figure 5) shows the harmonic impedance o the 30 kV bus bar when the three flters are switched on.

One can observe that impedance is (nearly) zero at three requencies: 250 Hz (h5), at 350 Hz (h7), and at 550 Hz (h11).

These are the tuning requencies o the flters. The flters behave as short-circuits or currents at these requencies.

An inherent characteristic o a tuning requency is that it provokes an anti-resonance requency just below the tuning

requency. In this case these anti-resonance requencies are 180 Hz, 330 Hz, and 485 Hz respectively.


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0 200 400 600 800 1000 1200

Hz

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5

4

3

2

1

0

Zpu

Base: 100 MVA / 30 kVZohm + Zpu

9

Harmonic impedance with the flters or H5 and H7 in service

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6

5

4

3

2

1

0

Base: 100 MVA / 30 kV

Zohm = Zpu

9

Harmonic impedance with the flter or H5 in service

Zpu

Hz

Figure 6: Harmonic impedance Figure 7: Harmonic impedance with two lters

with one lter (h5) switched on (h5 & h7) switched on

Figure 6 shows the harmonic impedance with two flters switched on. The anti-resonance requencies in this case are

185 Hz and 330 Hz. This shows that the anti-resonance can shit depending upon the number o flters switched on.

Figure 7 shows the harmonic impedance with only the flter or the fth harmonic switched on. The anti-resonance

lies at 190 Hz.

The dierence o 10 Hz in the anti-resonance requency can be signifcant. I any o the interharmonics produced by

the rolling mill should coincide or almost coincide with one or more o these anti-resonance requencies, this

will result in rather high voltage components at these requencies. This can cause icker and its consequent nuisance.

Thus, when setting anti-harmonic flters on or o, the harmonic impedance o the entire system changes and icker

can be provoked in one or more o the confgurations. Attention must be drawn to the act that the presence o other

signifcant loads or other changes in the network confguration can also impact harmonic impedance and lead to

icker.

2.3 Measurements

During the frst measurement campaign, the flter or the fth harmonic was switched on while those or the seventh

and the eleventh harmonics were o. Under normal operating conditions, the flter or the seventh harmonic would

be on. When this flter was switched on, an immediate rise in the icker level was observed. When this icker was

perceived, the flters also produced a severe and entirely abnormal noise. This confrmed the suspicion that the

observed icker originates rom interharmonic voltages combined with an anti-resonance requency o one or more

passive flters.

Measurements were carried out with the rolling mill and various combinations o flters in service. The purpose was to

see which o the confgurations was provoking icker.

3. Results

3.1 First measurement: 5th, 7th and 11th harmonic flters on

First we examined interharmonic content o the 30 kV supply voltage during a given period o unctioning o the

rolling mill and six passes (ater the rolling mill, the steel plate goes through six dierent passes to urther smooth

the surace o the steel). Only the maximum values or each interharmonic were recorded. The analysis period

is 500 seconds, and the interharmonics are calculated at 5 Hz steps.


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1.5

1.4

1.3

1.2

1.1

1.0

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0.5

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0.1

00 2 4 6 8 10 12 14 16 18 20

Harmonic order

%o

fUfundamental

100th percentile o URS with the flters or H5, H7 and H11 in service

Figure 8: Voltage interharmonic spectrum in the rst lter conguration (maximum values)

We can see that the resonances appear at the ollowing requencies:

! Rank 2,5 or 125 Hz




Comparing these requencies with the anti-resonance requencies o the frst flter confguration, we can see that two

requencies correspond: 180 Hz (or 185/190 Hz) and 330 Hz. A little slack is observed at the 485 Hz component, but the

dierence stays very small. There is consequently a serious risk o resonance at those requencies. The corresponding

evolution over time o the currents (50 Hz values) drawn by the rolling mill and the six passes, is shown in Figure 9.

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600

500

400

300

200

100

00 50 100 150 200 250 300 350 400 450 500

Duration (sec)

Ifund

(A)

Currents in the rolling mill and the dierent passes with the flters or H5, H7 and H11 in service

pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

rolling mill

Figure 9: 50 Hz currents o the rolling mill and six passes rst lter conguration

The various passes that start up one ater another can clearly be observed.

When we now look at the content o the dierent harmonics in each o these 7 currents, we obtain the results given

in Figure 10 to Figure 13 :


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7

6

5

4

3

2

1

00 50 100 150 200 250 300 350 400 450 500

Duration (sec)

IH2,5

(A)

Harmonic current o order 2,5 (or 125 Hz) in the rolling mill and the diferent passeses with the lters or H5, H7 and H11 in service

pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

rolling mill

Figure 10: Interharmonic current at 125 Hz rst lter conguration

5

4

3

1.0

1

00 50 100 150 200 250 300 350 400 450 500

Duration (sec)

IH3,6

(A)

Harmonic current o order 3,6 (or 180 Hz) in the rolling mill and the diferent passes with the lters or H5, H7 and H11 in service

pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

rolling mill

Figure 11: interharmonic current at 180 Hz rst lter conguration

2.5

2.0

1.5

1.0

0.5

00 50 100 150 200 250 300 350 400 450 500

Duration (sec)

IH6,6

(A)


pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

rolling mill



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4

3.5

3

2.5

2

1.5

1

0.5

00 50 100 150 200 250 300 350 400 450 500

Duration (sec)

IH9,2

(A)


pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

rolling mill


Two major conclusions can be drawn rom these fgures:

! The rolling mill provokes by ar the greatest part o the interharmonics

! The currents o the passes have higher interharmonics at 125 Hz and 330 Hz when they are not unctioning

than when they are in service!

Looking at the harmonic currents running through the various flters, the ollowing can be observed:

! Along with the 50 Hz component, each flter obviously has a signifcant harmonic component corresponding

to the tuning requency it was designed or

! Each flter also has current components at other characteristic harmonic requencies (rank11, rank13, etc.),

mostly those higher than the tuning requency (this is particularly true or higher tuning requencies)

! Interharmonic currents can also be observed in the three flters:

o Rank 2,5

o Rank 3,6

o Rank 6,6

o Rank 9,2

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0 2 4 6 8 10 12 14 16 18 20

0 2 4 6 8 10 12 14 16 18 20

30

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10

0

20

15

10

5

0

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30

20

10

0

100th percentile with the lters or H5, H7 and H11 in service

A

A

A

Harmonic order

Current in the lter or H5



Figure 14: interharmonic currents in the three lters rst lter conguration


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3.2 Second measurement: 5th and 7th harmonic flters on

Again, we frst examined the interharmonic voltages during a given period o unctioning o the rolling mill and the

six passes. The analysis period was 750 seconds this time.

1.5

1.4

1.3

1.2

1.1

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

00 2 4 6 8 10 12 14 16 18 20

Harmonic order

%o

fUfundamental

100th percentile o URS

with the flters or H5 and H7 in service

Figure 15: voltage interharmonic spectrum in the second lter conguration (maximum values)

The resonances appear on the ollowing requencies:




Rank 9.2 was not taken into account this time, since there is no anti-resonance impedance expected or this rank.

Regardless o the impact o this interharmonic, it will be smaller than in the frst flter confguration.

The corresponding evolution over time o the currents (50 Hz values) o the rolling mill and the dierent passes

is as ollows:

900

800

700

600

500

400

300

200

100

00 100 200 300 400 500 600 700

Duration (sec)

Ifund

(A)

Currents in the rolling mill and the dierent passes with the flters or H5 and H7 in service

pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

quatro

Figure 16: 50 Hz currents o the rolling mill and six passes second lter conguration

Again, the various passes that start up one ater another can clearly be observed.

When examining the content o the harmonics in each o the seven currents, the observations are similar, as in the

frst measurement:

! The rolling mill provokes by ar the greatest part o the interharmonics

! The currents o the passes have higher interharmonics at 125 Hz and 330 Hz when they are not

unctioning than when they are in service


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Looking at the harmonic currents running through the various flters, the ollowing can be observed:

! The eleventh harmonic is higher in the two remaining flters o this confguration (this is obvious since the

flter or h11 is not in service)

! The ninth harmonic is much lower than in the frst confguration

! The interharmonics o rank 9.2 have disappeared

0 2 4 6 8 10 12 14 16 18 20

0 2 4 6 8 10 12 14 16 18 20

30

25

20

15

10

5

0

15

10

5

0

100th percentile with the flters or H5 and H7 in service

A

A

Harmonic order

Current in the flter or H5

Current in the flter or H7

Figure 17: Interharmonic currents in the three lters second lter conguration

3.3 Third measurement: 5th harmonic flter on

We examined the interharmonic voltages during a given period o unctioning o the rolling mill and the six passes or

the third time. The analysis period was again 750 seconds.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

00 2 4 6 8 10 12 14 16 18 20

Harmonic order

%o

fUfundamental

100th percentile o URS

with the flters or H5 in service

Figure 18: voltage interharmonic spectrum in the third lter conguration (maximum values)

The resonances appear at the ollowing requencies:



The resonance or rank 6,6 has almost completely disappeared.

The corresponding time evolution o the currents (50 Hz values) o the rolling mill and the six passes is as ollows:

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800

700

600

500

400

300

200

100

00 100 200 300 400 500 600 700

Duration (sec)

Ifund

(A)

Currents in the rolling mill and the diferent passes with the lters or H5 in service

pass 1

pass 2

pass 3

pass 4

pass 5

pass 6

rolling mill

Figure 19: 50 Hz currents o the rolling mill and six passes third lter conguration

Again, the various passes that start up one ater another can clearly be observed. When again examining the contento the harmonics in each o the seven currents, observations are similar to those in the frst measurement:

! The rolling mill still produces interharmonics o rank 2.5 and 3.6

! The passes provoke mainly interharmonics o rank 3.6

4. Lines of enquiry

Based on these measurement results, one can start seeking the origin o the interharmonic currents. A new

measurement campaign was carried out to record an incident. The aim was to determine where the interharmonics

come rom and how they provoked the incident.

18

16

14

12

10

8

6

4

2

017:00 17:05 17:10 17:14 17:19 17:25 17:30

Duration (hour: minute)

I(A)

180 Hz currents (200ms values)

Rolling mill

Transformer 13

cold rolling mill

Figure 20: 180 Hz currents in the main rolling mill, a cold rolling mill, and their eeding transormer

As shown in Figure 20, the 180 Hz component is clearly present in the currents o the eeding transormer (called

transormer 13 in the fgure), the rolling mill, and in a cold rolling mill that is also connected to this transormer.

It is consequently certain that this 180 Hz current is linked to the loads connected to this transormer. Looking into

detail at the six passes, it appears that passes 1, 2, and 6 also provoke this 180 Hz component, while the other three

passes do not, as shown in the ollowing fgure:

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0 1 2 3 4 5 6

50

25

0

100th percentile

A

0 1 2 3 4 5 6

50

25

0

A

0 1 2 3 4 5 6

50

25

0

A

0 1 2 3 4 5 6

50

25

0

A

0 1 2 3 4 5 6

50

25

0

A

0 1 2 3 4 5 6

50

25

0

A

Harmonic order

Cage 1

Cage 2

Cage 3

Cage 4

Cage 5

Cage 6

Figure 21: maximum o the 180 Hz component o the current in the passes

The fnal conclusion was that passes 1, 2, and 6 provoke the interharmonic currents. The 180 Hz component is present

in those three passes, and only when they are unctioning. The 180 Hz component is also present in the main rolling

mill (and also in a cold rolling mill), but without correlation with the unctioning o this mill.

Note: in Figure 22, the transormer current was measured at the high voltage side (primary voltage).

800

700

600

500

400

300

200

10017:00 17:05 17:10 17:14 17:19 17:25 17:30

Duration (hour: minute)

I(A)

50 Hz currents (200ms values)

Rolling mill

Transformer 13

Cold rolling mill

Figure 22: 50 Hz currents in the main rolling mill, a cold rolling mill, and their eeding transormer

5. Conclusions and solutions

5.1 Conclusions

The measurements clearly show the presence o interharmonics on the voltages as well as on the currents. Their

requencies are 125 Hz (rank 2.5), 180 Hz (rank 3.6), 330 Hz (rank 6.6), and 460 Hz (rank 9.2).

Those our interharmonics are produced by the rolling mill and its passes.

The three flters create high harmonic impedance or the requencies 180 Hz, 330 Hz, and 485 Hz respectively. These

are the anti-resonances o the tuning requencies o the flters.

The levels o the various interharmonics are high enough to provoke perceptible icker.

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Whether the flter or h11 is in service or not, the interharmonics in the two other flters remain at the same level.

However, they do not present a danger to the flters themselves.

5.2 Solutions

In so ar as the presence o interharmonics on the voltage is not aecting the state o the equipment connected to the30 kV busbar, the easiest solutions to this icker problem are either changing the lighting devices, or eeding them via

another transormer (separation o the electrical circuits).

I malunctioning or even damage is observed on the equipment connected to the 30 kV busbar, other solutions must

be investigated. Those can include:

! A deep study o the mechanism inside the drives that lies at the origin o the interharmonic currents

(e.g. a study o the regulation loop/o the closed loop o the drive control), in order to eliminate them

! Detuning the flters, with the risk o aecting their e ciency in fltering the harmonics

The malunctioning mentioned above can or instance be a vibration problem, which can provoke the detachment o

connections and consequently flter damage. It is a good idea to check the connections regularly.

This publication is subject to copyright and a disclaimer. Please refer to the Leonardo ENERGY website.

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Case Studies Flicker Problems in Steel Plant Caused by Interharmonics

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