Outdoor Insulation in Polluted Conditions: Guidelines for Selection and Dimensioning Part 2: The DC Case PDF

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518 2 Outdoor Insulation in Polluted Conditions: Guidelines for Selection and Dimensioning Part 2: The DC Case Working Group C4.303 December 2012 OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING PART 2: THE DC CASE WG C4.303 Members    Main Authors:  C.S. Engelbre...

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Outdoor Insulation in Polluted Conditions: Guidelines for Selection and Dimensioning Part 2: The DC Case

Working Group C4.303

December 2012

OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING PART 2: THE DC CASE

WG C4.303 Members    Main Authors:  C.S. Engelbrecht, Convenor (NL), J.P. Reynders, Secretary (ZA),   I. Gutman (SE), K. Kondo (JP), C. Lumb (FR), A. Pigini (IT), V. Sklenicka (CZ), D. Wu (SE).     Contributions have been made by:  A.C. Britten (ZA), R. W. Garcia (BR), C. Kovacs (DE), N. Mahatho (ZA), R. Matsuoka (JP),  T. Nakachi (JP), S. Nishimura (JP), A.J. Phillips (US), W. Schwardt (DE), E. Solomonik (RU),   N.J. West (ZA), M. Yamarkin (RU), X. Liang (CN), R. Znaïdi (TN). 

Copyright © 2012 “Ownership of a CIGRE publication, whether in paper form or on electronic support only infers right of use for personal purposes. Are prohibited, except if explicitly agreed by CIGRE, total or partial reproduction of the publication for use other than personal and transfer to a third party; hence circulation on any intranet or other company network is forbidden”.

Disclaimer notice “CIGRE gives no warranty or assurance about the contents of this publication, nor does it accept any responsibility, as to the accuracy or exhaustiveness of the information. All implied warranties and conditions are excluded to the maximum extent permitted by law”.

ISBN : ISBN: 978- 2- 85873-211-1

OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

ISBN : (To be completed by CIGRE)

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OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING PART 2: THE DC CASE

Table of Contents MEMBERS ............................................................................................................................ 0 EXECUTIVE SUMMARY ........................................................................................................ 4 DEFINITIONS ....................................................................................................................... 5 ABBREVIATIONS ................................................................................................................. 6 Chapter 1: Introduction ........................................................................................................................... 7 Background............................................................................................................................................. 7 Aim with this document ......................................................................................................................... 8 Chapter 2: Scope Methodology ........................................................................................................... 9 Differences between DC and AC ....................................................................................................... 9 Principles of dimensioning.................................................................................................................... 9 Insulator selection Flow Chart ...........................................................................................................10 Chapter 3: Identify Candidate insulators..........................................................................................13 Introduction ...........................................................................................................................................13 Choice of insulation material............................................................................................................. 14 Choice of insulator profile .................................................................................................................17 Chapter 4: Assessment of Environmental and System Stresses ..................................................... 21 Introduction ...........................................................................................................................................21 Influencing factors ...............................................................................................................................22 How to determine the pollution severity .........................................................................................24 Chapter 5: Determination of the insulator characteristics and dimensions .................................28 Introduction ...........................................................................................................................................28 Influencing factors ...............................................................................................................................28 Characterisation of insulator performance .................................................................................... 35 Available artificial pollution test methods for HVDC ................................................................... 35 Chapter 6: A simplified method to determine the required USCD .............................................. 38 Introduction ...........................................................................................................................................38 Determining the site DC Severity .....................................................................................................39 Determining the Required USCD ......................................................................................................40 Other considerations ...........................................................................................................................42

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OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

Chapter 7: Design Verification ............................................................................................................43 Chapter 8: Discussion and validation .................................................................................................44 Substations with Non-HTM insulators ...............................................................................................44 Overhead lines with Non-HTM insulators ....................................................................................... 46 HTM insulators ......................................................................................................................................46 References ...............................................................................................................................................48 Annex A: Derivation of correction factors .........................................................................................53 Ratio of DC to AC (or non-energised) pollution accumulation, Kp ............................................53 Correction for the amount of calcium ions present in the pollution layer, Kc ...........................55 Correction for NSDD, Kn..................................................................................................................... 56 Correction for the non-uniform pollution distribution (top to bottom ratio), KCUR. ...................56 Correction for diameter on the pollution accumulation, Kd. ........................................................ 58 Correction for diameter on the flashover performance ..............................................................59 Statistical co-ordination factor .........................................................................................................60 Performance of different insulator types .......................................................................................60 References ............................................................................................................................................63 Annex B: Influence of profile on the DC pollution flashover performance .................................65 General .................................................................................................................................................65 Influence of the creepage factor .....................................................................................................65 Creepage distance versus clearance (l/d)..................................................................................... 68 Influence of distance between sheds ...............................................................................................69 Spacing versus shed overhang .........................................................................................................70 References ............................................................................................................................................71 Annex C: Service experience............................................................................................................... 73 Survey of applied USCD at existing HVDC schemes ................................................................... 73 Outages at converter stations........................................................................................................... 76 Pollution Performance of insulation ..................................................................................................78 References ............................................................................................................................................79 Annex D: Typical parameters to record when performing ESDD/NSDD measurements .........81 References ............................................................................................................................................83 Annex E: The Statistical Approach ......................................................................................................84 References ............................................................................................................................................87

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OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

EXECUTIVE SUMMARY For a variety of important technical reasons, high‐voltage direct current (HVDC) is an increasingly desirable option  for the transmission of bulk electrical energy. With 50 years of experience, utilities are entering the ultra‐high  voltage (UHV) arena with HVDC schemes of 800kV and above.  With the important need to contain costs and  improve reliability, it was deemed appropriate for CIGRE to commission a working group (WG) with the task of  reviewing and analysing the practice of the past 50 years and draw up guidelines which will enable designers to  select the most appropriate insulation systems for HVDC lines and substations, taking into account system  requirements, environmental conditions and modern insulator technology.     In contrast to high‐voltage alternating current (HVAC) systems, where switching and lightning performance are  the dominant factors influencing the overall length of insulation, under HVDC the ability to deal with  environmental pollution on the insulator surfaces is the defining stress the insulation designer has to address. The  static electrostatic field along the length of an insulator, in conjunction with the prevailing wind, lead to a steady  build‐up of pollutants on the insulator surface which may, typically, range between 1‐ 4 times, but possibly as high  as 10 times more severe than that on comparable HVAC insulation in the same environment. The situation is  exacerbated by the fact that the leakage current in the pollution does not experience natural current‐zeros and as  a result, the dry‐band arcing is very aggressive.    An accurate assessment of site severity is, therefore, the starting point for any insulation design. The guideline  explores a range of options for doing this, starting with data from insulators energised at HVDC through  information from insulators energised at HVAC to a survey of likely pollution sources, coupled with wind and rain  data. The further the assessment is from that on live candidate HVDC insulators the less confidence there will be  in the outcome. The site severity is determined in terms of both the concentration of salts (ESDD) which  contribute to electrical conduction and the concentration of non‐soluble material (NSDD) which contributes to  water retention.     Bearing in mind the difficulty of doing an accurate assessment of the site severity using energized HVDC  insulators, the document surveys publications related to large number of HVDC sites around the world and  presents a range of correction factors for adjusting site severity information from HVAC insulation or from the  analysis of source, wind and rain conditions to determine the HVDC site severity.      By using the two primary site severity parameters, ESDD and NSDD, the document addresses the selection of  material, profile and creepage length.     The final stage in the process is the validation of the chosen insulation and different options are considered.  Where possible a live test in the actual environment is the most desirable, allowing sufficient time for the natural  accumulation of pollution. It is rare for this to be feasible and a means of performing a representative laboratory  test is thoroughly discussed. Although the confidence level may be low, the option of a pencil‐and‐paper  validation using comparable insulation in a comparable environment, elsewhere in the world, can be undertaken.    The need and application of maintenance and palliative measures should already be considered at the design  stage so that cost‐effective steps can be taken to secure reliable service of the system.           

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OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

DEFINITIONS Composite insulator: An insulator made of at least two insulating parts, namely, a core and a housing equipped with metal fittings. Note that composite insulators, for example, can consist either of individual sheds mounted on the core, with or without an intermediate sheath, or alternatively, of a housing directly moulded or cast in one or several pieces on to the core. Composite Insulators are a subset of Polymeric insulators. Contamination Uniformity Ratio (CUR), is the ESDD level on the bottom surface of the insulator divided by that of the top surface. It should be noted that many papers quote the Top to Bottom ratio (T/B) which is the inverse of the CUR. Creepage Factor (CF), is a global check of the overall density of creepage distance and is equal to l/A where: l is the total creepage distance of the insulator and A is the arcing distance of the insulator. For disc insulators the CF is determined for a string comprising at least 5 insulator units. DC system voltage, is the highest mean or average operating voltage to earth, excluding harmonics and commutation overshoots (IEC Standard 61245 pollution test of HVDC insulator). Equivalent Salt Deposit Density (ESDD): The amount of sodium chloride (NaCl) that, when dissolved in demineralised water, gives the same volume conductivity as that of the natural deposit removed from a given surface of the insulator divided by the area of this surface; generally expressed in mg/cm². Hybrid insulator: An insulator made of at least two insulating parts, namely a core equipped with metal fittings and a housing. In the case of a hybrid insulator the core can be made from either porcelain or glass and the housing is made of a polymeric material. Hydrophobicity Transfer Materials (HTM): In this document polymer materials that exhibit hydrophobicity and have the capability to transfer hydrophobicity to the layer of pollution on their surfaces are referred to as Hydrophobicity Transfer Materials (HTM). It should be noted that hydrophobicity may be lost in certain conditions, either temporarily or in some cases permanently. IEC 62073 gives guidance on the measurement of wettability of insulator surfaces [76]. Non- Hydrophobicity Transfer Materials (non-HTM): Materials which do not exhibit hydrophobicity transfer are referred to as non- Hydrophobicity Transfer Materials (non-HTM). Non Soluble Deposit Density (NSDD): The amount of the non-soluble residue removed from a given surface of the insulator divided by the area of this surface; generally expressed in mg/cm². Polymeric insulator: An insulator whose insulating body consists of at least one organic based material. Coupling devices may be attached to the ends of the insulating body. Salt Deposit Density (SDD): The amount of sodium chloride (NaCl) in an artificial deposit on a given surface of the insulator (metal parts and assembling materials are not included in this surface) divided by the area of this surface; generally expressed in mg/cm². Site Equivalent Salinity (SES): The salinity of a salt fog test according to IEC 60507 that would give comparable peak values of leakage current on the same insulator as produced at the same voltage by natural pollution at a site, generally expressed in kg/m³. Top to Bottom ratio (T/B), is the ESDD level on the top surface of the insulator divided by that of the bottom surface. It should be noted that many papers quote the Contamination Uniformity Ratio (CUR) which is the inverse of the T/B. Unified Specific Creepage Distance (USCD) is the creepage distance of an insulator divided by the maximum operating voltage across the insulator (for AC systems usually Um/√3). It is generally expressed in mm/kV. Note that this definition differs from that of Specific Creepage Distance where the phase-to-phase value of the highest voltage for the equipment is used. For phase to earth insulation, this definition will result in a value that is √3 times that given by the definition of Specific Creepage Distance in IEC 60815 (1986).

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OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

ABBREVIATIONS CF:  CUR:  EPDM:  ESDD:  HTM:  NSDD:  SDD:  SES:  SIR:  SPS:  T/B:  USCD: 

Creepage Factor  Contamination Uniformity Ratio  Ethylene‐Propylene‐Diene Monomer  Equivalent Salt Deposit Density  Hydrophobicity Transfer Material  Non Soluble Deposit Density  Salt Deposit Density  Site Equivalent Salinity  Silicone Rubber  Site Pollution Severity  Top to Bottom Ratio  Unified Specific Creepage Distance 

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OUTDOOR INSULATION IN POLLUTED CONDITIONS: GUIDELINES FOR SELECTION AND DIMENSIONING

Chapter 1: Introduction Background As society has become increasingly dependent on a continuous supply of electrical energy, more attention has, in recent years, been given to the reliability and cost of each component in the electricity supply system, including the insulation of power lines and substations. The integrity of outdoor insulation is crucial in maintaining the reliability and cost-effectiveness of a modern electricity supply utility. In service, the outdoor insulation should withstand all voltage and environmental stresses that it may be subjected to. The pollution performance of the insulation is, therefore, one component of the overall insulation coordination design and the final solution will be chosen taking due cognisance of all the aspects of insulator performance. In June 2000, CIGRE published an important review of ...