New CIGRÉ Principles for DC Insulation Selection CHRIS ENGELBRECHT
Cigré WG C4.303 New CIGRÉ Principles for DC Insulation Selection Chris Engelbrecht: Convener WG C4.303
Overview • Contamination flashover • Differences between DC and AC • Overview of dimensioning process • Simplified method
Pollution Flashover Unit Gets Contaminated: - Dry Contamination non-conductive Unit becomes wet by condensation / absorption: -Wet Contamination conductive – current flows - Corona Occurs due to E-field Redistribution Dry Bands Form due to Localized Heating -Where current density is high, e.g. close to pin I I - Dry Bands can be quenched by high wetting V V Arcs bridge Dry Bands - Dry bands grow due to heating at arc roots - Arcs extinguish if dry band too large - If wetting critical entire unit flashes
Differences AC and DC: Insulation Coordination AC Systems DC Systems 8 8 1.8 p.u 2.6 p.u Insulation distance, m Insulation distance, m 7 7 6 6 5 5 Pollution performance is dominant for insulation design on DC 4 4 3 3 1.7 p.u Pollution 2 2 Pollution Slow-front Slow-front 1 1 Lightning Lightning 300 500 700 900 1100 1300 300 500 700 900 Maximum system Voltage, kV System Voltage, kV Pollution based on glass or porcelain
Differences AC and DC: Pollution accumulation • More pollution accumulate on Energized DC insulators – Static E-field – Ratio between 1 and 10 times more on DC, Typically below 4 12 Ratio DC/AC contamination level (K p ) Pacific Intertie Pacific Intertie 10 Big Eddy 8 Need special DC site severity assessment 6 Ludvika 1964 4 Gezhouba Sweden Yamashina Japan Guojiagang Matsuoka 2 Kyowa Noto Terai Tsuruga Akita Kawagoe Takeyama Huangdu Weishanzhuang 0 0.001 0.01 0.1 1 ESDD measured on AC energized or non-energized Insulators (mg/cm 2 )
Differences AC and DC: Flashover development No voltage zeros (no re-ignition necessary) • DC arc more mobile under thermal and electromagnetic forces • 120 AC 100 Leakage Distance Efficiency (in % of AC value at 280 mm) 80 DC AC arc AC arc DC arc DC arc 60 Need special DC insulator profiles 40 Shed Diameter: 254 mm Salt Deposit Density: 0.12 mg/cm 2 20 Leakge distance per unit increases 0 0.6 0.8 1 1.2 Under-rib Factor [K]
Differences AC and DC: Flashover Strength • DC energization on polluted insulators Log(FOV) is more onerous than AC energization AC – FOVac = K 1 (ESDD) -0.22 DC – FOVdc = K 2 (ESDD) -0.33 Log(Severity) 2.6 2.6 Cap & Pin Antifog Cap & Pin Antifog Ratio AC(rms)/DC(-) Withstand Voltage Ratio AC(rms)/DC(-) Withstand Voltage Clean Fog Salt Fog 2.4 2.4 Post Cap & Pin Std Equipment Longrod 2.2 2.2 Equipment Post 2 2 1.8 1.8 Cannot derive required DC creepage from AC test/service results 1.6 1.6 1.4 1.4 1.2 1.2 1 1 0.8 0.8 0.01 0.1 1 10 1 10 100 1000 Salt Deposit Density[mg/cm 2 ] Withstand Salinity S W [g/l]
Laboratory vs Field results AC Energization Results can be applied directly for dimensioning
Laboratory vs Field results DC Energization Results needs to be adjusted before it can be applied
Relevance of Laboratory Testing Parameter Natural conditions Laboratory test Same uniform pollution utilised for all type of Differences in shed profiles Accumulate different amounts of pollution sheds Non-uniform pollution: Often non uniformly polluted, especially disc Uniform Pollution Top to bottom insulators Non-uniform pollution: Often non uniformly polluted Uniform Pollution Axial variations Type of salt Various types of salt may be present Tests are performed with NaCl Various types of non-soluble materials may be Type of non-soluble material Kaolin, Tonoko or Kieselguhr present Amount of non-soluble Varies over time and for different locations 40g per 1000 g water. material Different for each insulator. Large diameter Sometimes considered for determining test Diameter effect insulators collect less pollution severity. Tests typically performed in vertical Installation orientation Can be vertical, horizontal or angled. configuration Tests typically performed at laboratories Altitude Relevant to sites at high altitude i.e. >1000 m located at sea level
CIGRÉ Guidelines: • Polluted insulators: A review of current knowledge Technical brochure 158, June 2000. • Polluted insulators: Guidelines for selection and dimensioning – Part 1: General principles and the a.c. case Technical brochure 361 (June 2008) – Part 2: The d.c. case Technical brochure 518 (Dec 2012)
New IEC 60815: Selection and dimensioning of high-voltage insulators for polluted conditions • Four parts: – Part 1: Definitions, information and general principles – Part 2: Ceramic and glass insulators for a.c. systems – Part 3: Polymer insulators for a.c. systems – Part 4: Ceramic, glass and polymer insulators for d.c. systems
Dimensioning Approach Availabl Candidate insulator Decreasing confidence Information from exisiting Extrapolation of data from a.c. Design severity determination for Test station data from d.c. d.c. installations in the area or or installations or test station or or Qualitative severity estimation the candidates energised insulators (or similar) pollution monitoring On the basis of existing On the basis of existing Evaluation by testing where Selection of creepage distance applicable insulator data or applicable insulator data or previous data is not from the field from laboratory available/applicable Prequalified by previous Agreement to use dimensional Agreement to use severity Qualification or Full scale test or or experience interpolation/extrapolation interpolation/extrapolation Note: Phases 1-3 may need to be iterated
Contamination Design Process Pollution Severity Pollution Flashover Strength Estimation Estimation Required Insulation design performance Verification
Contamination Design Process Pollution Severity Pollution Flashover Strength Estimation Estimation Required Insulation design performance Verification
Contamination Design: Pollution Severity Estimation Dust deposit gauges ESDD/NSDD ESDD/NSDD DC Energized (AC data) Conversion from AC to DC (Conversion Factor between 1 – 10) Topography and Pollution Severity Maximum ESDD nature of area Estimation Distribution on Insulator Number of “pollution events” Type of contaminants
Characterizing the Environment: What to collect? Pollution severity measurement Topography and nature of the area • • – ESDD/NSDD (Preferably DC – Meteorological data Energized) e.g. No. of foggy days, seasonal precipitation, etc. – Distrubution of pollution on insulator Estimated number of wetting events – Top to botttom • Type of industry in the area – • Along insulator length • Service Experience – Chemical analysis of pollution Type of Insulation and Dimensions – • Ongoing study (effect of Gypsum) Creepage distance, diameter • Number of outages due to pollution – Minimum voltage level above which – flashovers occur
Contamination Design Process Pollution Severity Pollution Flashover Strength Estimation Estimation Required Insulation design performance Verification
Contamination Design: Pollution Flashover Strength Estimation Laboratory test results “Laboratory” Service experience Strength Published test results Conversion from lab to service Conversion Pollution Flashover Strength Estimation of line insulators Dielectric strength of the insulators (in service environment) as function of pollution severity (U 50 &Standard deviation) IEC creepage CIGRE Curve recommendation
Pollution Flashover Strength Estimation 100 Non-HTM Insulators • CIGRE curve HTM Insulators USCD (mm/kV) – Collection of published test results • Application of correction factors for NSDD=0.1 mg/cm 2 – Non-soluble deposit density T/B=1 – Insulator diameter 10 0.001 0.01 0.1 1 – Non-uniformity of pollution distribution SDD (mg/cm 2 ) – Insulator assembly arrangement, i.e. I/V strings, etc. – Type of contamination (Chemical analysis) – Height above sea level – Non-linearity of flashover voltage with insulator length
Contamination Design: Performing the design Pollution Severity Pollution Flashover Strength Estimation Estimation Insulation design Based on Required performance Statistical Principles Verification
Design Process: Stress: probability density of occurrence • Use statistical principles Strength P( g ) Strength: probability of flashover Stress f( g ) g g f ( ) P ( ) Risk of flashover • Use Cigré Approach Pollution severity( g ) – Simplified statistic (use correction factors) • Use creepage distance guidelines – Will be included in IEC
Recommend
More recommend
