Interharmonic Levels at a DC Arc Furnace Installation

Erich W. Gunther

Electrotek Concepts, Inc.

Load and Supply Characteristics

• DC Arc Furnace

• Contract demand of 100 MW

• 345 kV PCC, 7811 MVA SC

• SVC on 34.5 kV Bus– 2nd, 3rd, 5th, 7th, 12th (HP) filters

Load Current Statistics

RMS Current (Amps)

0 50 100 150 200 250 0

100

200

300

400

500

0

20

40

60

80

100

Irms (A)

Cou

nt

Cu

mu

lati

ve P

rob

ab

ilit

y (

%)

Samples: Min: Max:

Range: Mean: Std Dev:Variance: Std Err:

Median:Skewness:

Kurtosis:

1902 31.0437 244.711 213.667 101.045 56.4328 3184.66 1.29398 95.7066 0.247029 -1.49685

Load Current Waveforms

100.0A

0.0A

-100.0A

295KV

0.0V

-295KV1.67 ms/div0.00ns 33.33ms

Snapshot Waveform Model 9010/9020 8V/8I

BMI Meter 04/02/98 01:06:34.00 PM

Three Phase Wye

2

4

6

250.0A

0.0A

-250.0A

303KV

0.0V

-303KV1.67 ms/div0.00ns 33.33ms

Snapshot Waveform Model 9010/9020 8V/8I

BMI Meter 04/02/98 11:55:34.00 AM

Three Phase Wye

2

4

6

Light Load Heavy Load

Arc Furnace Generated Waveform Distortion

• Difficult to classify and separate phenomena– Attributes of both transient and steady state phenomena

– Waveform distortion characteristics change rapidly

– Observed interharmonic levels may be due to actual interharmonic injection, a side effect due to the changing load, or both

• Use appropriate analysis method based on expected impact to system– Harmonic, Interharmonic, Transient, Flicker, Unbalance

Interharmonic Impacts

• Makes filter design more difficult

• Adverse impact on control systems

• Can excite low frequency oscillations in mechanical systems

• Communications interference

• Light flicker

IEC Measurement Method

300

345335

340325

320315310

305

330

355

360

350

295365

290 370

Used for calculating I 56Used for calculating I 5 Used for calculating I 6

Harmonic Current SpectrumSampled Waveform Every 12 Cycles

Frequency Resolution 5 Hertz

350

310

256

ffII

365

355

26

ffII

305

295

25

ffII

12 Cycle Snapshot

250.0A

0.0A

-250.0A

298KV

0.0V

-298KV10.00 ms/div0.00ns 200.00ms

Snapshot Waveform Model 9010/9020 8V/8I

04/26/98 01:35:14.00 AM

Three Phase Wye

Ia

Spectrum of Snapshot

Interharmonic Current

0

2

4

6

8

10

0 30 60 90 120

150

180

210

240

270

300

330

360

390

420

Frequency (Hertz)

Cu

rren

t (A

mp

s)

Phase A

Fundamental = 194 Amps

Interharmonic Metrics

• Total Interharmonic Distortion (TID)– RSS of IEC based interharmonic components– Normalized by fundamental for voltages– Normalized by peak demand for currents– IEC statistical approach used - report 95% CP

• Individual Interharmonic Interval Levels

Results for Currents

Quantity DistortionValues

CP95

ITID - STD 11.36

ITID - IEC 11.12

Current% of IL

H H CP95 H CP950-1 0-1 13.51 20-21 0.341-2 1-2 8.42 21-22 0.372-3 2-3 7.84 22-23 0.553-4 3-4 1.54 23-24 0.494-5 4-5 0.81 24-25 0.365-6 5-6 0.85 25-26 0.346-7 6-7 0.49 26-27 0.317-8 7-8 0.42 27-28 0.308-9 8-9 1.07 28-29 0.29

9-10 9-10 0.56 29-30 0.3010-11 10-11 0.36 30-31 0.2911-12 11-12 0.34 31-32 0.2912-13 12-13 0.38 32-33 0.3013-14 13-14 0.41 33-34 0.3114-15 14-15 0.35 34-35 0.3215-16 15-16 0.33 35-36 0.3416-17 16-17 0.31 36-37 0.3217-18 17-18 0.31 37-38 0.3218-19 18-19 0.31 38-39 0.3319-20 19-20 0.32 39-40 0.33

More Current Results

Current Interharmonic H03-H04 (B and C Only)

0 1 2 3 4 5 0

20

40

60

80

100

120

140

0

20

40

60

80

100

IH0304i (% Peak)

Cou

nt

Cu

mu

lati

ve P

rob

ab

ilit

y (

%)

Samples: Min: Max:

Range: Mean: Std Dev:Variance: Std Err:

Median:Skewness:

Kurtosis:

366 0.0829874 4.73872 4.65573 0.724425 0.541289 0.292994 0.0282936 0.5883 2.98482 14.5441

Results for VoltagesVoltage% of V1

CP95 CP95H BC H BC

0-1 0.36 20-21 0.091-2 0.38 21-22 0.132-3 0.17 22-23 0.263-4 0.09 23-24 0.284-5 0.08 24-25 0.215-6 0.09 25-26 0.186-7 0.08 26-27 0.097-8 0.07 27-28 0.068-9 0.07 28-29 0.069-10 0.06 29-30 0.06

10-11 0.06 30-31 0.0711-12 0.06 31-32 0.0712-13 0.06 32-33 0.0613-14 0.06 33-34 0.0514-15 0.05 34-35 0.0515-16 0.05 35-36 0.0516-17 0.05 36-37 0.0517-18 0.05 37-38 0.0518-19 0.05 38-39 0.0519-20 0.06 39-40 0.05

Quantity Distortion Values

(Phases B and C Only)

CP95

VTID – STD 1.03

VTID – IEC 0.69

More Voltage Data

IEC Voltage Interharmonic THD (B and C Only)

0.00 0.25 0.50 0.75 1.00 1.25 0

20

40

60

80

100

0

20

40

60

80

100

TID2v (%)

Cou

nt

Cu

mu

lati

ve P

rob

ab

ilit

y (

%)

Samples: Min: Max:

Range: Mean: Std Dev:Variance: Std Err:

Median:Skewness:

Kurtosis:

244 0.281093 1.08742 0.806327 0.482795 0.126441 0.0159872 0.00809453 0.457559 1.35618 2.93133

More Voltage Data

Voltage Interharmonic H00-H01 (B and C Only)

0.0 0.2 0.4 0.6 0

20

40

60

80

100

120

0

20

40

60

80

100

IH0001v (% Fundamental)

Cou

nt

Cu

mu

lati

ve P

rob

ab

ilit

y (

%)

Samples: Min: Max:

Range: Mean: Std Dev:Variance: Std Err:

Median:Skewness:

Kurtosis:

244 0.0892301 0.577014 0.487784 0.195936 0.0776163 0.00602429 0.00496888 0.174201 2.23601 5.99988

Conclusions

• IEC measurement methods are relatively easy to apply - similar level of effort as for harmonic measurements

• Measured levels are low– Difficult to measure accurately (A/D resolution)

– Statistical techniques required (many samples)

• Low levels can cause observable impacts• More work on relating impacts to measured levels

required