New CIGR Principles for DC Insulation Selection CHRIS ENGELBRECHT - - PowerPoint PPT Presentation

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

I V I V

Dry Bands Form due to Localized Heating

• Where current density is high, e.g. close to pin
• Dry Bands can be quenched by high wetting

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

8 7 6 5 4 3 2 1 300 500 700 900 1100 1300

Maximum system Voltage, kV Insulation distance, m

1.8 p.u 2.6 p.u

8 7 6 5 4 3 2 1 300 500 700 900

System Voltage, kV Insulation distance, m

1.7 p.u

Pollution Slow-front Lightning Pollution Slow-front Lightning

AC Systems DC Systems

Pollution based on glass or porcelain

Pollution performance is dominant for insulation design on DC

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

2 4 6 8 10 12 0.001 0.01 0.1 1

Ratio DC/AC contamination level (Kp) ESDD measured on AC energized or non-energized Insulators (mg/cm2)

Big Eddy Gezhouba Ludvika 1964 Huangdu Guojiagang Noto Akita Takeyama Kyowa Weishanzhuang Terai Matsuoka Yamashina Kawagoe Pacific Intertie Sweden Japan Pacific Intertie Tsuruga

Need special DC site severity assessment

Differences AC and DC: Flashover development

• No voltage zeros (no re-ignition necessary)
• DC arc more mobile under thermal and electromagnetic forces

AC arc DC arc AC arc DC arc

20 40 60 80 100 120 0.6 0.8 1 1.2

Under-rib Factor [K] Leakage Distance Efficiency (in % of AC value at 280 mm)

Leakge distance per unit increases Shed Diameter: 254 mm Salt Deposit Density: 0.12 mg/cm2 AC DC

Need special DC insulator profiles

Differences AC and DC: Flashover Strength

• DC energization on polluted insulators

is more onerous than AC energization

– FOVac = K1 (ESDD)-0.22 – FOVdc = K2 (ESDD)-0.33

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 0.01 0.1 1 10

Salt Deposit Density[mg/cm2] Ratio AC(rms)/DC(-) Withstand Voltage

Cap & Pin Antifog Post Equipment 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 1 10 100 1000

Withstand Salinity SW[g/l] Ratio AC(rms)/DC(-) Withstand Voltage

Cap & Pin Antifog Cap & Pin Std Longrod Equipment Post

Clean Fog Salt Fog

Cannot derive required DC creepage from AC test/service results

Log(Severity) Log(FOV) AC DC

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 Differences in shed profiles Accumulate different amounts of pollution Same uniform pollution utilised for all type of sheds Non-uniform pollution: Top to bottom Often non uniformly polluted, especially disc insulators Uniform Pollution Non-uniform pollution: Axial variations Often non uniformly polluted Uniform Pollution Type of salt Various types of salt may be present Tests are performed with NaCl Type of non-soluble material Various types of non-soluble materials may be present Kaolin, Tonoko or Kieselguhr Amount of non-soluble material Varies over time and for different locations 40g per 1000 g water. Diameter effect Different for each insulator. Large diameter insulators collect less pollution Sometimes considered for determining test severity. Installation orientation Can be vertical, horizontal or angled. Tests typically performed in vertical configuration Altitude Relevant to sites at high altitude i.e. >1000 m Tests typically performed at laboratories 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 Design severity determination for the candidates Information from exisiting d.c. installations in the area (or similar)

• r

Test station data from d.c. energised insulators

• r

Extrapolation of data from a.c. installations or test station or pollution monitoring

• r

Qualitative severity estimation Selection of creepage distance On the basis of existing applicable insulator data from the field

• r

On the basis of existing applicable insulator data from laboratory

• r

Evaluation by testing where previous data is not available/applicable Qualification Prequalified by previous experience

• r

Full scale test

• r

Agreement to use dimensional interpolation/extrapolation

• r

Agreement to use severity interpolation/extrapolation Note: Phases 1-3 may need to be iterated

Contamination Design Process

Pollution Severity Estimation Pollution Flashover Strength Estimation Insulation design Verification Required performance

Contamination Design Process

Pollution Severity Estimation Pollution Flashover Strength Estimation Insulation design Verification Required performance

Contamination Design: Pollution Severity Estimation

Pollution Severity Estimation ESDD/NSDD (AC data)

Conversion from AC to DC (Conversion Factor between 1 – 10)

ESDD/NSDD DC Energized Topography and nature of area

 Maximum ESDD  Distribution on Insulator  Number of “pollution events”  Type of contaminants

Dust deposit gauges

Characterizing the Environment: What to collect?

• Pollution severity measurement

– ESDD/NSDD (Preferably DC Energized) – Distrubution of pollution on insulator

• Top to botttom
• Along insulator length

– Chemical analysis of pollution

• Ongoing study (effect of Gypsum)
• Topography and nature of the area

– Meteorological data e.g. No. of foggy days, seasonal precipitation, etc. – Estimated number of wetting events – Type of industry in the area

• Service Experience

– Type of Insulation and Dimensions

• Creepage distance, diameter

– Number of outages due to pollution – Minimum voltage level above which flashovers occur

Contamination Design Process

Pollution Severity Estimation Pollution Flashover Strength Estimation Insulation design Verification Required performance

Contamination Design:

Pollution Flashover Strength Estimation

Pollution Flashover Strength Estimation of line insulators (in service environment)

Service experience

Conversion

CIGRE Curve Laboratory test results

Dielectric strength of the insulators as function of pollution severity (U50 &Standard deviation)

IEC creepage recommendation Published test results “Laboratory” Strength

Conversion from lab to service

10 100 0.001 0.01 0.1 1

USCD (mm/kV)

SDD (mg/cm2)

Non-HTM Insulators HTM Insulators

NSDD=0.1 mg/cm2 T/B=1

Pollution Flashover Strength Estimation

• CIGRE curve

– Collection of published test results

• Application of correction factors for

– Non-soluble deposit density – Insulator diameter – Non-uniformity of pollution distribution – 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 Estimation Pollution Flashover Strength Estimation Insulation design Required performance Based on Statistical Principles Verification

Design Process:

• Use statistical principles
• Use Cigré Approach

– Simplified statistic (use correction factors)

• Use creepage distance guidelines

– Will be included in IEC

Stress f(g) Strength P(g) Risk of flashover Pollution severity(g) Stress: probability density of occurrence Strength: probability of flashover

) ( ) ( g g P f 

Cigré Simplified method: Part 1

• For site severity

Measurements from DC test site/station or existing nearby

• r similar installations

Measurements from AC installations or non-energized insulators as per IEC 60815-1 Conversion of values to equivalent DC energized Correct for:

• electrostatic attraction,
• Climate data (wind rain)
• Nature of pollution sources

Measured ESDDDC NSDD* CUR* Pollution composition Estimated ESDDDC NSDD‡ CUR ‡ Pollution composition * Should be measured ‡ Preferably measured,

• r else use default values

Correct for the type of salt◊

Kp Kc

◊ No agreed upon method available at this time Correct for NSDD to a reference value of 0,1 mg/cm2

Kn

Site DC severity

Cigré Simplified method: Part 2

• For each Insulator Type

Correct the severity for non- uniformity of the pollution layer

KCUR

Site DC severity Correct the severity for the effect of diameter on pollution accumulation

Kd

Statistical data correction

Ks

Number of insulators Number of pollution events Required/Design DC severity Preliminary estimation of the required USCDDC for the candidate type and material Correct the USCDDC for the effect of diameter on flashover

Cd

Correct the USCDDC for the effect of altitude

Ca

Required/Design USCDDC

Creepage distance Requirement

Ceramic and glass insulators

10 100 0.001 0.01 0.1 1

USCD (mm/kV)

Design DC severity (SDD: mg/cm2)

Non-HTM Design Values Good Performance Flashovers

CUR = 1 NSDD=0.1 mg/cm2

Creepage distance Requirement

Hydrophobicity Transfer Material (HTM) Insulators

10 100 0.001 0.01 0.1 1

USCD (mm/kV)

Design DC severity (SDD: mg/cm2)

Design curve non-HTM insulators Design curve HTM insulators Design Values Good Performance No Flashovers, but deterioration

CUR = 1 NSDD=0.1 mg/cm2

Conclusions

• Contamination is the dimensioning parameter for

external insulation on DC

• Becomes increasingly critical as system voltage goes up.
• “Have” to use hydrophobic insulators to obtain practical

designs.

• Dimensioning process defined

– Accuracy of method is very dependent on quality of input data