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Electrolytic Corrosion of Hardware of Porcelain & Glass Suspension Insulators on UHV DC Lines

WANG LIMING

深圳研究生院 能源与电工新技术实验室

Electrolytic Corrosion of Hardware

• f Porcelain & Glass Suspension

Insulators on UHVDC Lines

WANG Liming Tsinghua University, China

深圳研究生院 能源与电工新技术实验室

• In recent years, the phenomenon of electrolytic corrosion on porcelain and

glass insulators’ hardwares has occurred on many UHVDC transmission lines in China such as the ±800 kV Chusui, ±800 kV Xiangshang lines.

Corrosion area Corrosion area

Iron cap corroded insulators Pin corroded porcelain and glass insulator

• INTRODUCTION

深圳研究生院 能源与电工新技术实验室

• The insulators corroded seriously

mostly locate at the forests region, where the humidity is high and continuous heavy fog appears in the late autumn, winter and early

• spring. The right figures are the

surrounding environment of 407# tower on Chusui transmission line.

Surrounding forests Heavy fog

• INTRODUCTION

深圳研究生院 能源与电工新技术实验室

• Electrolytic plays a leading role in the corrosion process due to the polarity phenomenon
• f corroded hardware. The electrolytic loop is consisted of DC power supply, hardware

and electrolyte.

• For the iron cap corroded insulator, the iron cap connected to the positive side (ground

side) of DC power supply is anode, the pin connected to the negative side is cathode.

Current direction Flow direction of electrons Moist insulator surface V

 

DC power supply

OH

Flow direction

• f electrons

2

Fe 

Flow direction of cation Pin Iron cap To the pin

• f upper

insulator From the iron cap of lower insulator Flow direction

• f electrons
• CORROSION MECHANISM

深圳研究生院 能源与电工新技术实验室

Ferrous ions are formed by the oxidation reaction of the iron cap when the insulator surface becomes damp.

V

 

DC power supply

OH

Flow direction

• f electrons

2

Fe 

Flow direction of cation Pin Iron cap To the pin

• f upper

insulator From the iron cap of lower insulator Flow direction

• f electrons

2+ e e

F F +2e 

+ 2

H O H +OH



2

2H +2e H

  



2 e e 2

F +2OH F (OH)

  



• CORROSION MECHANISM

H+ and OH- exist in electrolyte as a result of water ionization. Reduction reaction occurs on the cathode side. Cation imigrates to the cathode side and anion moves to anode side under the applied DC voltage. Precipitates are formed by Fe2+ and OH-.

深圳研究生院 能源与电工新技术实验室

V

 

DC power supply

OH

Flow direction

• f electrons

2

Fe 

Flow direction of cation Pin Iron cap To the pin

• f upper

insulator From the iron cap of lower insulator Flow direction

• f electrons

Due to the existence of oxygen in the solution, further oxidation reaction can take place.

e 2 2 2 e 3

4F (OH) O +2H O 4F (OH)  

e e2 3 2

FO+ F O + H O m n p

• CORROSION MECHANISM

The constituents of rust are shown as below. The values of m, n and p vary under different temperature, pH value and oxygen content conditions.

深圳研究生院 能源与电工新技术实验室

Three pieces of iron cap corroded porcelain insulators retrieved from 407# tower

• f

±800 kV Chusui transmission line are carried out contamination degree measurement test. The surface of insulator is divided into three parts,

• ne

is corrosion by-products accumulation area on the upper surface (area A), another is the non corrosion by-products area on the upper surface (area B), the other is the lower surface (area C).

Lower surface

• INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S

MECHANICALAND ELECTRICAL CHARACTERISTICS

Upper surface

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No. Area A Area B Area C ESDD (mg/cm2) NSDD (mg/cm2) ESDD (mg/cm2) NSDD (mg/cm2) ESDD (mg/cm2) NSDD (mg/cm2) 1 0.1072 0.1090 0.0078 0.0697 0.0113 0.1165 2 0.0998 0.2744 0.0061 0.0759 0.0079 0.1166 3 0.0513 0.4802 0.0053 0.0500 0.0083 0.0858

The results indicate that, the ESDD and NSDD of area A are much higher than those of area B and area C. Namely, the contamination degree of rust channel area is higher than that of other area.

Lower surface Upper surface

The measurement results are listed as below.

• INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S

MECHANICALAND ELECTRICAL CHARACTERISTICS

深圳研究生院 能源与电工新技术实验室

The pollution flashover tests are carried out in a 26m×26m×30m fog room. The maximum output voltage of DC power supply is 1000 kV. Four groups of XZP2-300 type porcelain insulators on Chusui transmission line are used to conduct tests. Group 1 and 2 are retrieved in March, 2013, groups 3 and 4 are retrieved in March

• 2014. Groups 1 and 3 are iron cap corroded insulators, groups 2

and 4 are pin corroded insulators. Every group includes 14 pieces

• f insulators. They are hung in V-string (76°), every side includes

7 pieces. Boost voltage method is used to conduct flashover test, the flashover time of each group is 6.

• INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S

MECHANICALAND ELECTRICAL CHARACTERISTICS

深圳研究生院 能源与电工新技术实验室

No. Corroded element Flashover voltage (kV) 1 2 3 4 5 6 Ave 1 Iron cap 276 255 263 256 253 260 260.5 2 Pin 317 321 316 312 326 315 317.8 3 Iron cap 272 224 229 250 266 265 251.0 4 Pin 340 335 331 292 306 299 317.2

Test results of pollution flashover test are shown as below. The test results illustrate that, the flashover voltage of iron cap corroded insulators is 20% lower than that of pin corroded insulators.

• INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S

MECHANICALAND ELECTRICAL CHARACTERISTICS

深圳研究生院 能源与电工新技术实验室

The XZP2-300 type porcelain insulators on 407# tower of ±800 kV Chusui transmission line are taken for simulation calculation by means of FEM method. These porcelain insulators on this tower were hung in the double V-strings (76°) and each string owns 69 pieces of insulators. The transmission lines is six division LGJ-630/45 type aluminium conductor steel reinforced.

• INFLUENCE OF PIN CORROSION ON INSULATOR’S MECHANICAL

AND ELECTRICAL CHARACTERISTICS

深圳研究生院 能源与电工新技术实验室

The calculation results show that, for the pin, the part exposed to the air and the cement- zinc sleeve interface has great strain under the stress. If the cross-section of pin reduces due to corrosion, the portion exposed to the air is easy to pull off. If the adhesive strength between pin and cement decreases due to the expansion of corrosion by-products, the whole pin may be pulled out of the iron cap.

• INFLUENCE OF PIN CORROSION ON INSULATOR’S MECHANICAL

AND ELECTRICAL CHARACTERISTICS

深圳研究生院 能源与电工新技术实验室

In terms of XZP2-300 type porcelain insulators, of which the rated mechanical failure load is 300 kN, the failure load of new production insulators is mainly in the range

• f 380~410 kN. But for the pin corrosion insulators with

less than 4 year’s operation, their final destruction load is mainly in the range of 340~370 kN. The test results indicate that, for pin corrosion insulators, even the zinc sleeves have not been penetrated through and the cross section of pins have not reduced, the mechanical strength

• decreases. That is because the hoop stress at the

cement-zinc sleeve interface weakens the bonding strength between them.

• INFLUENCE OF PIN CORROSION ON INSULATOR’S MECHANICAL

AND ELECTRICAL CHARACTERISTICS

深圳研究生院 能源与电工新技术实验室

Copper electrod e Wire Waterproof plastic

5cm

Wire Pin

Copper electrode Wire Waterproof plastic 1cm

Wire Locking device

The spray water method is used to simulate the corrosion process of hardware. Before test, the copper electrode should be pasted and fixed onto the surface of insulators. The distances between electrode and insulation element are about 5 cm and 1cm for iron cap and pin tests

• respectively. The metal wire connected to the other end of copper electrode should be fixed
• n the pin and locking device for iron cap and pin tests respectively.

Fix the electrode (iron cap test) Fix the metal wire (pin test)

• SIMULATION TEST METHOD

Fix the metal wire (iron cap test) Fix the electrode (pin test)

深圳研究生院 能源与电工新技术实验室

During the test, NaCl solution is sprayed to the surface of insulator to form electrolyte. For iron cap test, the voltage applied on pin is in the range of -0.8~-1.5kV and iron cap is grounded, the conductivity of NaCl solution and its spray velocity are 8~10 mS/cm and 8~10 L/h respectively. For pin corrosion test, the voltage applied on pin is in the range of +0.4~+0.8 kV and iron cap is grounded too. The conductivity and spray velocity are 2~3 mS/cm and 2~3L/h respectively.

1 2 4 5 6 7 8 3 11 9 10 10 10 9 9 7

1-voltage regulator 2-test transformer 3-full-bridge rectifier system 4-protection resistor 5-filter capacitor 6-DC voltage divider 7-cable 8-insulator suspension device 9-spray water device 10-leakage current measurement system 11-accelerated corrosion test room.

• SIMULATION TEST METHOD

深圳研究生院 能源与电工新技术实验室

Iron cap Zinc ring Zinc ring Iron cap Iron cap Zinc ring

For the new produced insulators, the zinc ring that attached to the iron cap is useful to suppress its electrolytic corrosion. The structure and size of zinc rings were optimally designed according to the simulation test results of specimen insulators.

Specimen inslator A Specimen inulator B Specimen inulator C

• SOLUTION OF IRON CAP CORROSION OF NEW PRODUCED

INSULATOR

深圳研究生院 能源与电工新技术实验室

Integrated boundary of slice

Boundary

• f iron cap

Boundary

• f zinc ring

The respective sections of the area with the biggest corrosive depth in the iron caps with and without a zinc ring are shown as below.

• SOLUTION OF IRON CAP CORROSION OF NEW PRODUCED

INSULATOR

Without zinc ring With zinc ring

Test results show that the corrosive depth of the iron cap without a zinc ring is quite large. However, after the installation of the zinc ring, the section of iron cap is almost un-corroded. For new produced insulator, it is effective to suppress electrolytic corrosion of iron caps by installing a zinc ring.

Position of slice Iron cap Cement

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The U-shaped zinc ring is designed to suppress electrolytic corrosion of iron cap

• f operating insulator.
• SOLUTION OF IRON CAP CORROSION OF OPERATING INSULATOR

Installation U-shaped zinc ring

深圳研究生院 能源与电工新技术实验室

In order to verify the suppressing effectiveness of the U-shaped zinc ring, XZP2-300 type porcelain insulators are used to carry out accelerated corrosion test with the spray water method.

Integrated boundary of slice Slice of uncorroded area Slice of corrosion deepest area Corrosion area

• SOLUTION OF IRON CAP CORROSION OF OPERATING INSULATOR

Test results show that the corrosive depth

• f the iron cap without a zinc ring is quite
• large. However, after the installation of the

zinc ring, the section of the area with the deepest corrosion is almost the same as that

• f the un-corroded area. Therefore, it is

effective to suppress electrolytic corrosion of iron caps by installing a U-shaped zinc ring.

Cement Cement Interface of cement and air Pin Zinc sleeve Original corrosion part Organic sleeve Present corrosion part

• SOLUTION OF PIN CORROSION OF OPERATING INSULATOR
• According to the previous research results, the mechanical

strength of the operating insulators decreases even their pins are not corroded and become thin. The expansion caused by corrosion by-products reduces the bonding strength between the cement and zinc sleeve. Thickening the zinc sleeve is no use for this condition.

• The recommendation that installing organic material sleeve
• nto the zinc sleeve is proposed. The organic sleeve can alter

the corrosion part of zinc sleeve, which will shift the corrosion area from the interface of cement-zinc sleeve to the exposed portion of zinc sleeve where does not bear hoop stress. This method can avoid the decrease of bonding strength between cement and zinc sleeve.

Half package type sleeve Flush type sleeve Whole package type sleeve

• SOLUTION OF PIN CORROSION OF OPERATING INSULATOR

Based on the XZP2-300 type porcelain insulators, the samples installed half package type, flush type and whole package type organic material sleeves are designed and manufactured.

A B C D 200 220 240 260 280 300 320 340 360 380

Tensile failure load（ kN） Insulator types

• SOLUTION OF PIN CORROSION OF OPERATING INSULATOR

After tests, 1000 kg of cement block has been hung on these insulators for six months. Six months later, the tested insulators are carried out tensile load test in accordance with Chinese National Standard GB/T19443. The tensile failure load are shown as below.

Note: A stands for without organic sleeve insulators, B stands for Flush type sleeve insulators, C stands for whole package type sleeve insulators, D stands for half packed type sleeve insulators. Every value is the average of 3 pieces of insulators' test results.

The test results verify the effectiveness of half package type organic sleeve.

深圳研究生院 能源与电工新技术实验室

• CONCLUSIONS
• 1. The hardware corrosion phenomena occurred on the ±800 kV UHVDC

transmission lines is mainly caused by electrolytic corrosion.

• 2. The iron cap corrosion can lead to the contamination accumulated on insulator's

surface and decrease the flashover voltage of entire insulator string.

• 3. The pin corrosion can reduce the mechanical strength of insulator.
• 4. Zinc ring and U-shaped zinc ring can be used to suppress the iron cap corrosion
• f new produced and operating insulators respectively.
• 5. Installing organic material sleeve can solve the problem of decreasing of the

mechanical strength caused by the expansion of corrosion by-products.

深圳研究生院 能源与电工新技术实验室

Thank you！