Preface: A little bit about harmonics…

1. Case StudyCNP Investigation of Stray Triplen HarmonicsPamela MendozaDistribution System ReliabilityCenterPoint EnergySWEDE 2014

2. Preface: A little bit about harmonics… • Triplenharmonics are multiples of the 3rd harmonic component • Comprised of zero sequence vectors • Sum in the neutral • Can cause excessive heating, damage equipment • What is stray current? • “A current resulting from the normal delivery and/or use of electricity that may be present between two conductive surfaces that can be simultaneously contacted by members of the general public and/or their animals.” • Potential Causes of Stray Current: • Improper grounding techniques • Breaks in the neutral path • Induced voltage from high voltage lines • There is a possibility that capacitor bank(s) may resonate at harmonic frequency to amplify condition.

3. How RR Track Circuits Work • RR Tracks have electrical circuits for train detection and crossing predictors • Inject a signal between a section of tracks • Isolating joints separate track circuits • Transmitter/Receiver used to detect train location • Crossing Predictors measures magnitude of track impedance and dividing the rate of change of the impedance. (Time = Distance/Velocity) • Stray current will interfere with the Crossing Predictor calculations

4. How RR Track Circuits Work • RR Tracks have electrical circuits for train detection and crossing predictors • Inject a signal between a section of tracks • Isolating joints separate track circuits • Transmitter/Receiver used to detect train location • Crossing Predictors measures magnitude of track impedance and dividing the rate of change of the impedance. (Time = Distance/Velocity) • Stray current will interfere with the Crossing Predictor calculations Isolation Joints Isolation Joints

5. How RR Track Circuits Work • RR Tracks have electrical circuits for train detection and crossing predictors • Inject a signal between a section of tracks • Isolating joints separate track circuits • Transmitter/Receiver used to detect train location • Crossing Predictors measures magnitude of track impedance and dividing the rate of change of the impedance. (Time = Distance/Velocity) • Stray current will interfere with the Crossing Predictor calculations Isolation Joints Isolation Joints Transmitter

6. How RR Track Circuits Work • RR Tracks have electrical circuits for train detection and crossing predictors • Inject a signal between a section of tracks • Isolating joints separate track circuits • Transmitter/Receiver used to detect train location • Crossing Predictors measures magnitude of track impedance and dividing the rate of change of the impedance. (Time = Distance/Velocity) • Stray current will interfere with the Crossing Predictor calculations Isolation Joints Isolation Joints Receiver Transmitter

7. How RR Track Circuits Work • RR Tracks have electrical circuits for train detection and crossing predictors • Inject a signal between a section of tracks • Isolating joints separate track circuits • Transmitter/Receiver used to detect train location • Crossing Predictors measures magnitude of track impedance and dividing the rate of change of the impedance. (Time = Distance/Velocity) • Stray current will interfere with the Crossing Predictor calculations Isolation Joints Isolation Joints Receiver Transmitter

8. How RR Track Circuits Work • RR Tracks have electrical circuits for train detection and crossing predictors • Inject a signal between a section of tracks • Isolating joints separate track circuits • Transmitter/Receiver used to detect train location • Crossing Predictors measures magnitude of track impedance and dividing the rate of change of the impedance. (Time = Distance/Velocity) • Stray current will interfere with the Crossing Predictor calculations Isolation Joints Isolation Joints Receiver Transmitter

9. The Case: Stray Current on the RR Tracks

10. The Case: Stray Current on the RR Tracks • RR company called CNP to report that they were getting stray voltage on RR tracks. • Voltage differential across isolating joints measured by RR • Stray current is causing crossing arm malfunction at two intersections • RR had to have a manned traffic control at two road crossings • Problem worsened when it rained...

11. The Case: Stray Current on the RR Tracks • Surrounding area: • Distribution lines run parallel to RR tracks on same side of road • Buried phone lines in same utility easement • Transmission line crossing (perpendicular to RR tracks) at one location

12. Question #1: Who is Causing the Stray Current? • Used EPRI“Power System & Railroad Electromagnetic Compatibility Handbook” for the investigation • Measure dominant frequency on RR tracks using a Fluke Harmonic Analyzer • RR and Phone Company have different frequency signatures than Utility • Measurements taken on each rail across isolation joint,near large customer on circuit, “Customer X”

13. Question #1: Who is Causing the Stray Current? • Dominant frequency measured to be 180Hz (triplen harmonic) • Next step: install monitors at Substation, three phase customers near cap banks, at RR bungalow. • Block cap banks offline until they could be fully evaluated

14. Question #1: Who is Causing the Stray Current? • Found large customer “Customer X” operating outside of IEEE 519-1992 harmonic limits. • Customer allowed maximum 5% Current TDD • Actually measured between 10%-17% Current TDD • CNP began working with Customer X to install harmonic filters • RR Temporary fix – move filter location • Downside, train speed reduced

15. Graphs: At the SubstationV(THD) should be below 5%

16. Graphs: At the SubstationI(TDD) should be below 8%

17. Graphs: At “Customer X” POCCV(THD) should be below 5%

18. Graphs: At “Customer X” POCCI(TDD) should be below 5%

19. Question #2: How is Stray Current Getting onto Tracks? • And…is CNP contributing to the path of the stray current? • Stray current can travel through multiple mediums • 3rd harmonic voltage measurements increase when ground is wet. • Test 1: inspect neutral conductor and ground rods • Especially around wire splices • An infrared inspection was conducted for all major equipment • Focused on all splices in primary and neutral • Results came back normal for all splices • Ground rod measurements came back with normal results as well

20. Question #3: What About the Cap Banks? • CNP cap banks blocked offline during first part of investigation. • Evaluation of cap banks: • Blocked each cap bank on for 24hr period, then both banks blocked on for 24hr period • Assessed harmonic trending during each test at all 3 monitoring locations • CNP cap banks were not amplifying the harmonic levels

21. Conclusions • Harmonic issue found to be caused by Customer X • Customer told to evaluate all grounding connections • Customer also given written notice about IEEE 519-1992 limitations • Customer installed harmonic filters to address compliance with IEEE limits • First round of filters reduced harmonic levels to below 10% ITDD but were still outside allowed level of 5% ITDD. • Customer is currently in Phase 2 of harmonic filter installation • A full investigation of CNP-owned equipment was conducted to ensure CNP was not contributing to or amplifying stray harmonic currents • Reference: • Power System and Railroad Electromagnetic Compatibility Handbook Revised First Edition

22. Any Questions?