Transcript Voltage Reduction Study

Case Study
CNP Investigation of Stray Triplen
Harmonics
Pamela Mendoza
Distribution System Reliability
CenterPoint Energy
SWEDE 2014
Preface: A little bit about harmonics…
 Triplen harmonics 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.
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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
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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
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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
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
Transmitter
Isolation
Joints
Receiver
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
Transmitter
Isolation
Joints
Receiver
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
Transmitter
Isolation
Joints
Receiver
8
The Case: Stray Current on the RR Tracks
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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...
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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
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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”
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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
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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
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Graphs: At the Substation
V(THD) should be below 5%
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Graphs: At the Substation
I(TDD) should be below 8%
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Graphs: At “Customer X” POCC
V(THD) should be below 5%
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Graphs: At “Customer X” POCC
I(TDD) should be below 5%
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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
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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
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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
o First round of filters reduced harmonic levels to below 10% ITDD but were still
outside allowed level of 5% ITDD.
o 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
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Any Questions?
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