Harmonics Problems in Distribution SystemsCase StudyAEP Texas Central CompanyScott Lee, Reliability Engineer

SYMPTOMS OF A PROBLEM

Residential customer calls complaining of problems with electronics: TV, thermostat – AC, motors making strange noises

Serviceman and Operations Tech troubleshoot and have transformer replaced because of unusual humming and arcing sounds from coil

New transformer makes the same sound, customer still has problems – call the engineer

Tech goes to new pipe plant down the road and finds 2000kVA pad transformer making same noise

Houston based company built a pipe coating and storage plant in George West to better serve the Eagle Ford Shale area

Two 1000kW induction furnaces to heat and coat 2 inch to 24 inch diameter casing with fusion bonded epoxy

Two solid state 6-pulse rectifiers and step up transformers provide variable frequency 4kV power to induction coils

For more info . . . see texisle.com and ajaxtocco.com

THE SOURCE

INSTALLATION

Previously during the plant planning stages, AEP provided system, line, and transformer impedances to drive manufacturer for design. Distance and PQ were discussed.

3 miles of distribution feeder was upgraded with 477MCM aluminum, with a total length of 6.4 miles

Drive manufacturer wanted two 1500kVA or one 3000kVA transformer, we supplied one 2000kVA. They were also very insistent on our transformer meeting their specs. Actual peak is 1380kW.

A 1000kVA pad was installed for other plant power that peaks at 600kW

AEP standard for three phase padmounts is wye-wye IEEE-519 requirements were not discussed, customer

did not tell us the inverter was a 6-pulse drive

George West Sub 138/12kV 9.4 MVA

Plant *

Residence

Feeder upgraded to 477, 6.4 circuits miles from sub to customer

1000kW 500 Hz Variable

Frequency 6-pulse rectifier No filtering Water cooled

PMI recorders installed at plant transformer and at residential customer. Set to record harmonics and capture waveforms.

Voltage THD at pipe plant on secodary, 14%

Voltage THD at residential secondary, 5.1%

PMI stands for Power Monitors Incorporated, see www.powermonitors.com, iVs-3 and Eagle 440

MEASUREMENTS

3rd12.4% THD-Voltage

7th11th 13th

5th

23.4% THD-Amps

2000 kVA Padmount6-Pulse Drive

5th7th

11th

Unfiltered, running at 1550kVA

12.4% THD-V23.4% THD-A

Unfiltered, 2000kVA Padmount, 6-Pulse Drive

THE FIX Met with pipe company and drive manufacturer to request

they install filtering and comply with IEEE-519 (5% THD limit on voltage)

Move other distribution customers in area to another feeder

Drive manufacturers said they would install passive filters (reactors and capacitors) but parts would have a 14 week lead time plus engineering and installation time

I countered to pipe company that this was unacceptable, and offered three filter manufacturers with on the shelf parts that offered active filtering and engineering support.

Recomended MTE Matrix Pure Sine active filters, Schaffner EMC ECOsine active filters, and TCI H5 active filters.

There was a debate over passive versus active, but after some research, I pressed the customer to go with active filters because of the varying load and the possibility of hitting resonance with a passive unit, also recommend a 5% line reactor which is standard for 6 pulse drives

The customer chose a vendor, and we had several phone conferences about what to install, and what would be acceptable with AEP

The vendor chosen was Schaffner EMC and the ECOsine FN3420 filter with additional 5% reactors

For more info . . .List location or contact for specification (or other related documents)

THE FIX cont.

Schaffner EMC ECOsine filter injects up to 300 amps of current

Compensates for harmonic currents

Compensates for reactive current

Can choose percentage of overall current dedicated to each harmonic

Two installed with 5% reactor ahead of each drive

Before at 1850 amps of load: Voltage THD 12.6% Current THD 23%

After at 2150 amps of load: Voltage THD 5.5% Current THD 8.8%

After at 1320 amps of load: Voltage THD 7% Current THD 5% Adjacent Transformer Voltage THD

3.5%

Total Load 1800 ampsHarmonic Amps Amps with Filter

5th 427 497th 225 6011th 113 5213th 60 59

THD-V from 12.2% to 8.1%

2300A rms5.9% THD-V9.6% THD-A 8.4% THD-V

24.2% THD-A

Neutral volts and amps

Testing, filter on and off

---------Filter off

Waveforms showing commutation notching

6.9% THD-voltage5% THD-current1300 amps

UG Residence, one mile from pipe plant, plant running at 7% THD-V

3.5% THD-V3rd, 7th, and 5thV

I

Not below 5% THD on voltage at meter Filtering has done all it can do Work with drive manufacturer to

control commutation notching using saturating reactors on SCRs to speed gate closing

Should have required drive manufacturer to meet IEEE 519 standard

Get a written proposal from drive manufacturer on how they will meet the standard

Use delta primary transformer to trap Triplen harmonics, this is 2.7% of total

Recommend active filtering Recommend 12 pulse drives or technology

to reduce commutation notching

IN RETROSPECT