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Transcript
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. 2 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 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 Isolation Joints Isolation Joints 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 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 9 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... 10 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 11 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” 12 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 13 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 14 Graphs: At the Substation V(THD) should be below 5% 15 Graphs: At the Substation I(TDD) should be below 8% 16 Graphs: At “Customer X” POCC V(THD) should be below 5% 17 Graphs: At “Customer X” POCC I(TDD) should be below 5% 18 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 19 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 20 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 21 Any Questions? 22