1、 Item No. 24254 NACE International Publication 05114 This Technical Committee Report has been prepared by NACE International Task Group (TG) 023,*“High-Voltage Direct Current (DC) Transmission: Effects on Buried or Submerged Metallic Structures.” High-Voltage Direct Current Interference May 2014, NA
2、CE International This NACE International (NACE) technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using product
3、s, processes, or procedures not included in this report. Nothing contained in this NACE report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by letters patent, or as indemnifying or pro
4、tecting anyone against liability for infringement of letters patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictable circumstan
5、ces may negate the usefulness of this report in specific instances. NACE assumes no responsibility for the interpretation or use of this report by other parties. Users of this NACE report are responsible for reviewing appropriate health, safety, environmental, and regulatory documents and for determ
6、ining their applicability in relation to this report prior to its use. This NACE report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operations detailed or referred to within this report. Users o
7、f this NACE report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any existing applicable regulatory requirements prior to the use of this repor
8、t. CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACE reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice. NACE reports are automatically withdrawn if more than 10 years old. Purchasers of NACE reports may r
9、eceive current information on all NACE International publications by contacting the NACE FirstService Department, 15835 Park Ten Place, Houston, Texas 77084-5145 (telephone +1 281-228-6200). Foreword The purpose of this technical committee report is to present information and review data on the oper
10、ation of high-voltage direct current (HVDC) transmission systems and their effect on underground/underwater or surface metallic structures, such as pipelines, telephone, electric power, and cable television signal transmission (CATV) cables, railways, reinforced concrete structures, etc. It is inten
11、ded for individuals associated with the pipeline, water, cable, railway, and electrical transmission and distribution industries. Stray direct current (DC) associated with HVDC transmission systems can result from normal operation of monopolar transmission systems or unbalanced currents in bipolar t
12、ransmission systems. Bipolar transmission systems can also generate significant levels of stray DC when operating in monopolar operation using the earth as a return circuit as a result of equipment faults or during planned maintenance of the system or converters. This technical committee report was
13、prepared by NACE Task Group (TG) 023, “High-Voltage Direct Current (DC) Transmission: Effects of Buried or Submerged Metallic Structures.” TG 023 is administered by Specific Technology Group (STG) 05, “Cathodic/Anodic Protection,” and is sponsored by STG 03, “Coatings and Linings, ProtectiveImmersio
14、n and Buried Service,” and STG 35, “Pipelines, Tanks, and Well Casings.” This report is issued under the auspices of STG 05, “Cathodic/Anodic Protection.” *Chair Peter Nicholson, Markdale, ON NACE International 2 NACE technical committee reports are intended to convey technical information or state-
15、of-the-art knowledge regarding corrosion. In many cases, they discuss specific applications of corrosion mitigation technology, whether considered successful or not. Statements used to convey this information are factual and are provided to the reader as input and guidance for consideration when app
16、lying this technology in the future. However, these statements are not intended to be recommendations for general application of this technology, and must not be construed as such. Table of Contents Introduction 2 Definitions 4 Stray Current Interference . 4 Underground/Underwater Pipelines 4 Teleco
17、mmunication Cables . 8 Buried and Aerial Structures . 8 Electric Power Cables and Facilities . 9 Mitigation . 9 Procedures 9 Equipment . 9 Long-Term Operation of HVDC Mitigation Systems . 10 Monitoring 10 New HVDC Transmission Systems . 10 Commission and Design of HVDC Systems . 11 References 12 Bib
18、liography . 12 Appendix A: General Considerations for HVDC Systems . 13 Appendix B: Typical Test Procedures for HVDC Systems 13 FIGURES Figure 1: Monopolar Earth-Return HVDC Transmission . 3 Figure 2: Bipolar HVDC Transmission 3 Figure 3: Graph Depicting HVDC Test Fault Current Magnitude and Pattern
19、 5 Figure 4: Effect of HVDC Faults on a Pipeline 6 Figure 5: Pipe-to-Soil Potential on a Pipeline as a Result of Fault Current . 6 Figure 6: Current Flow on a Pipeline Affected by the HVDC Tests Fault Current . 7 Figure 7: HVDC Test Fault Current on the Pipe-to-Soil Potential of a Pipeline. 7 TABLES
20、 Table 1: Typical Log of HVDC Faults 8 Introduction HVDC transmission is used to carry electrical energy over long distances or to interface two alternating current (AC) power systems that might not be synchronized. HVDC transmission can be performed using monopolar systems, which are typically eart
21、h-return systems, or bipolar wire-return systems. In monopolar systems, power is transmitted through a metallic conductor in one direction; see Figure 1. (IG refers to current flow in the Earth). Monopolar earth return systems use the earth as a conductor, and they typically use the sea as the earth
22、 return because of its low resistance and its ability to conduct large currents for a sustained period of time. Continuous operation of monopolar HVDC transmission systems is prohibited in some countries. NACE International 3 Figure 1: Monopolar Earth-Return HVDC Transmission Back-to-back HVDC syste
23、ms are generally used to solve synchronization problems or where different operating frequencies exist. These stations generally use a metallic return conductor rather than earth return. Monopolar metallic return systems do not use the earth as a conductor or earth return circuit. Bipolar HVDC syste
24、ms generally transmit power over a two-wire system where one wire is positive and the other wire is negative to ground (see Figure 2). Current through the earth is the unbalanced current between the two wires. In the case of a fault or equipment failure, bipolar systems generally revert to monopolar
25、 operation. Figure 2: Bipolar HVDC Transmission Both the operation of bipolar HVDC transmission systems that use the earth as a conductor of transmission currents and monopolar systems that use earth return currents can have serious repercussions on underground metallic structures. Whenever stray DC
26、 interference current discharges directly into the ground, corrosion occurs. The following is a list of underground or underwater metallic structures that may be affected by HVDC stray currents. This list is not exhaustive. Oil pipelines, Gas pipelines, Slurry pipelines, Chemical products pipelines,
27、 Water transmission pipelines, Water distribution pipelines, Telephone cables, Pipe-type cables, CATV cables, Railways, Electrical transmission and distribution systems, and Any other underground/underwater metallic plant. NACE International 4 Definitions Anode: The electrode of an electrochemical c
28、ell at which oxidation occurs. (Electrons flow away from the anode in the external circuit. It is usually the electrode where corrosion occurs and metal ions enter solution.) Bipolar Transmission: The transmission of DC power over wires; only unbalanced current and fault current are carried in the e
29、arth. Cathode: The electrode of an electrochemical cell at which reduction is the principal reaction. (Electrons flow toward the cathode in the external circuit.) Cathodic Protection (CP): A technique to reduce the corrosion rate of a metal surface by making that surface the cathode of an electroche
30、mical cell. Corrosion: The deterioration of a material, usually a metal, that results from a chemical or electrochemical reaction with its environment. Current Injection: The flow of current into the earth from an electrode system in an HVDC transmission system. Current Pickup: Current gained from t
31、he earth onto a system or network as the result of current flowing in the earth. Deep Groundbed: One or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earths surface in a drilled hole for the purpose of supplying cathodic protection current. Earth Electrode: An
32、 electrode system in the earth from which current discharges or is picked up from the earth circuit. Electrolyte: A chemical substance containing ions that migrate in an electric field. Embrittlement: Reduction of ductility, or toughness, or both, of a material (usually a metal or alloy). Monopolar
33、Transmission: The transmission of power in one direction through a metallic conductor, typically using the earth as the return conductor. Stray Current: Current flowing through paths other than the intended circuit. Stray Current Interference HVDC-generated stray currents can be picked up by metalli
34、c structures in direct or indirect contact with soil or water such as ground rods, grounding grids, bare concentric cable neutrals, and bare metallic support structures associated with the electric power transmission and distribution systems. Stray currents follow all paths of resistance; the magnit
35、ude is inversely proportional to the resistance of the path. The grounding network can include buried cable sheaths or neutrals, overhead messenger wires, pipe-type cable pipes, and any other systems such as water, sewer, or communication systems that might be bonded to the electrical grounding syst
36、em for safety reasons. The potential of a steel pipeline becomes more negative in locations in which unbalanced currents or fault currents are picked up. Underground/Underwater Pipelines A metallic structures potential shifts in the negative direction at the point of stray current pickup. This shift
37、 can be beneficial for a pipeline, provided that the pipe steel is not susceptible to hydrogen embrittlement. Sustained high negative voltages can cause hydrogen disbondment of pipeline coatings and embrittlement of high-strength steel and can damage amphoteric metals (e.g., aluminum, lead). Corrosi
38、on of underground or underwater metallic pipelines can occur if current discharges from the metallic structure into the electrolyte unless the pipeline remains protected to its criterion. According to Faradays Law, 9.5 kg (21 lb) of steel is lost for each ampere-year of current discharge. This metal
39、 loss is particularly critical on well-coated pipelines where the current discharge is concentrated at holidays in the coating. Perforation of the pipe wall can occur rapidly at a holiday as a result of the high density of current discharge from the pipeline to the electrolyte. Compared to their ele
40、ctrolytes, pipelines are good conductors of electricity. Because of their long length, low electrical resistance, and electrical continuity, metallic pipelines can pick up and discharge large amounts of current. NACE International 5 HVDC Fault Tests Figures 3 through 7 show the effect of 1,200 amper
41、es of HVDC fault current on a pipeline located approximately 70 km (44 mi) from the south earth electrode of a 2,700 megawatt bipolar HVDC transmission system. These staged fault tests were undertaken at the request of the utilities and pipeline operators to confirm that the effect of HVDC faults ha
42、d not significantly changed. Figure 3 shows the current flow through the earth circuit during a staged HVDC fault test. Each fault test had an eight-minute cycle, where during the first four minutes the southern electrode was positive with the DC injected into the earth to travel through the earth c
43、ircuit to the north electrode. The test cycle used entails (1) DC fault current four minutes south electrode positive, (2) two minutes zero fault current, and (3) two minutes south electrode negative, repeated hourly until sufficient data have been gathered to accurately analyze the effect of HVDC f
44、ault current. Figure 3: Graph Depicting HVDC Test Fault Current Magnitude and PatternFigure 4 shows the effect on the pipe-to-soil potential of a pipeline during staged HVDC fault current injection. The fault current was approximately 1,200 amperes, which is less than 50% of the maximum fault curren
45、t that can occur in the case of a major fault in which one line is temporarily lost and the earth is used as the return conductor. This figure depicts the change in pipe-to-soil potential of a pipeline because of HVDC faults of approximately 1,200 ampere magnitude from 09:15 hours (Coordinated Unive
46、rsal Time UTC) to 17:00 hours on June 16, 2007. The data recorded show the effect of cyclic fault current on a coated and cathodically protected pipeline. It can be readily seen that current pickup is occurring at this location when the south electrode is injecting current into the ground circuit. T
47、he pipe-to-soil potential is less negative when the south electrode is receiving current, indicating current discharge from the pipeline. NACE International 6 Figure 4: Effect of HVDC Faults on a Pipeline Figure 5 depicts the effect on the pipe-to-soil potential of a coated and cathodically protecte
48、d pipeline as a result of fault current recorded during the 09:30 hour fault test on June 16, 2007. The test cycle includes four minutes of positive fault current followed by two minutes with zero fault current, then two minutes of negative fault current. This cycle was repeated hourly for approxima
49、tely 12 hours to allow pipeline operators to observe the effect on their pipelines. Figure 5: Pipe-to-Soil Potential on a Pipeline as a Result of Fault Current NACE International 7 Figure 6 depicts the current flow in a coated and cathodically protected pipeline as a result of fault current recorded during the 16:30 hour fault test on June 16, 2007. The test cycle is four minutes of positive fault current followed by two minutes with zero fault current, then two minutes of negative fault current. This cycle was repeated hourly for approximately 12 hours, to allo
