Condition Assessment of High Voltage Insulation in Power System Equipment. R.E. James and Q. Su. The Institution of Engineering and Technology

Transcription

1 Condition Assessment of High Voltage Insulation in Power System Equipment R.E. James and Q. Su The Institution of Engineering and Technology

2 Contents Preface xi 1 Introduction Interconnection of HV power system components Alternating voltage systems Direct-voltage systems Insulation coordination High-voltage test levels Power-frequency voltages Lightning-impulse voltages Switching surges Very fast transient tests (VFTT) Direct-voltage tests Power system developments Reliability requirements Condition of present assets Extension of power system life New systems and equipment Future insulation monitoring requirements Summary References Problems 18 2 Insulating materials utilized in power-system equipment Review of insulating materials Gases Vacuum Liquids Solids 27

3 vi Condition Assessment of High- Voltage Insulation 2.2 Characterization of insulation condition Permittivity (e) and capacitance (C) Resistivity (p) and insulation resistance (IR) Time constants Dielectric dissipation factor Partial discharges (PD) Physical and chemical changes Modes of deterioration and failure of practical insulating materials Dielectric losses Partial discharges - sources, forms and effects Ageing effects Electrical breakdown and operating stresses Development of insulation applications Summary References Standards related to insulating materials Problems 54 Introduction to electrical insulation design concepts Overview of insulation design requirements Electrical requirements Physical limitations Working environment Mechanical requirements Thermal conditions Processing Reliability Electric stress distributions in simple insulation systems Multiple dielectric systems Edge effects Multiple electrode configurations Electric stress control Summary References Problems 70 Insulation defects in power-system equipment: Part Suspension and post insulators Suspension (string) insulators Post insulators High-voltage bushings High-voltage instrument transformers Oil-impregnated current transformers Dry-type current transformers Capacitor-type voltage transformers - CVT 81

4 List of contents vii High-voltage power capacitors High-voltage surge arresters High-voltage circuit breakers Gas-insulated systems (GIS) High-voltage cables Oil-paper cables Extruded cables Summary References Standards related to Chapter 4 Problems Insulation defects in power-system equipment: Part Electrical rotating machines Low-voltage motors High-voltage machines Possible insulation failure mechanisms in rotating machines OGRE summary of expected machine insulation degradation Future of machine insulation 5.2 Transformers and reactors Windings Transformer insulation structures Summary References Problems Basic methods for insulation assessment Generation and measurement of test high voltages Power-frequency voltages High-frequency voltages Very-low-frequency voltages (VLF) Direct voltages Hybrid test circuits Lightning impulse voltages Switching surge voltages High-voltage equipment for on-site testing Non-destructive electrical measurements Insulation resistance (IR) measurements Measurements of the dielectric dissipation factor (DDF) Measurement of partial discharges by electrical methods Dielectric response measurements 147

5 viii Condition Assessment of High- Voltage Insulation 6.3 Physical and chemical diagnostic methods Indicators of in-service condition of oil-paper systems Analysis of SFö samples from GIS Surface deterioration of composite insulators Water treeing in XLPE cable insulation Ultrasonic methods for detection of partial discharges Miscellaneous techniques Summary References Standards related to basic test methods Problems 158 Established methods for insulation testing of specific equipment Overhead line and substation insulators Porcelain and glass insulators (overhead lines) Ceramic and glass insulators (post type - indoor and outdoor) Composite insulators for overhead lines (string and post units) Overhead line and substation hardware Surge arresters Switchgear Circuit breakers Self-protected switchgear Disconnectors (isolators) Metal-enclosed switchgear Transformer tap changers Bushings High-voltage instrument transformers Current transformers Inductive voltage transformers Capacitor voltage transformers High-voltage power capacitors High-voltage rotating machines Stator bars Assembled machine High-voltage cables Oil-impregnated cables Extruded cables Distribution and power transformers Power-frequency overvoltage withstand tests Partial-discharge tests Summary of transformer HV test requirements 180

6 List of contents ix Additional tests Dielectric testing of HVDC equipment Miscellaneous items Summary References Standards related to Chapter Problems 188 Sensors for insulation condition monitoring Ultra-high-frequency sensors Optical-fibre sensors Basic physics of optical-fibre sensing Optical-fibre PD sensors Optical-fibre temperature sensors Advantages and disadvantages of optical-fibre sensors Directional sensors for PD measurements Directional coupler sensor Directional field sensor Summary References Problems 205 Online insulation condition monitoring techniques The main problems with offline condition monitoring Noise-mitigation techniques Noise gating Differential methods Noise identification by signal waveform analysis Multiple terminal PD measurements Non-electrical online condition monitoring Temperature monitoring of the insulations Online DGA Acoustic-based techniques for PD detection Online acoustic/electric PD location methods for transformers Acoustic transducers and winding terminal measurements Application of internal combined acoustic and VHF/UHF transducers Electrical online condition monitoring Online dielectric dissipation factor and capacitance measurements Online leakage current measurement Electrical online PD detection Summary 236

7 x Condition Assessment of High- Voltage Insulation 9.7 References Problems Artificial-intelligence techniques for incipient fault diagnosis and condition assessment Database for condition assessment A computer database and diagnostic program A combined method for DGA diagnosis Fuzzy-logic fault diagnosis The conventional methods A fuzzy-logic method Asset analysis and condition ranking Equipment ranking according to the insulation condition Insulation health index Membership functions of fuzzy set Example of fuzzy logic condition ranking Summary References Problems 264 Appendix 1 List of Abbreviations 265 Appendix 2 Major standards organizations 266 Appendix 3 Answers to problems 267 Index 270