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

Load Current Waveforms Light LoadHeavy 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

12 Cycle Snapshot 250.0A 0.0A A 298KV 0.0V -298KV ms/div0.00ns200.00ms Snapshot WaveformModel 9010/9020 8V/8I 04/26/98 01:35:14.00 AM Three Phase Wye Ia

Spectrum of Snapshot

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

More Current Results

Results for Voltages

More Voltage Data

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