Electrolytic Corrosion of Hardware of Porcelain & Glass Suspension Insulators on UHV DC Lines WANG LIMING
Electrolytic Corrosion of Hardware of 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
INTRODUCTION 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 Corrosion area area Iron cap corroded insulators Pin corroded porcelain and glass insulator 深圳研究生院 能源与电工新技术实验室
INTRODUCTION 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 Surrounding forests spring. The right figures are the Heavy fog surrounding environment of 407# tower on Chusui transmission line. 深圳研究生院 能源与电工新技术实验室
CORROSION MECHANISM Electrolytic plays a leading role in the corrosion process due to the polarity phenomenon of 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. DC power supply Flow direction of electrons To the pin From the iron of upper cap of lower V insulator insulator Flow direction Flow direction Current of electrons of electrons direction Moist insulator surface Pin Iron cap Flow direction of cation Fe 2 OH 深圳研究生院 能源与电工新技术实验室
CORROSION MECHANISM Ferrous ions are formed by the oxidation reaction of the iron cap when the insulator surface becomes damp. +2e 2+ F F e e H + and OH - exist in electrolyte as a result of water ionization. + DC power supply H O H +OH To the pin From the iron 2 of upper V cap of lower insulator insulator Reduction reaction occurs on the cathode side. Flow direction Flow direction 2H +2e H of electrons of electrons 2 Cation imigrates to the cathode side and anion Pin Iron cap Flow direction of moves to anode side under the applied DC voltage. cation Precipitates are formed by Fe 2+ and OH - . Fe 2 OH 2 F +2OH F (OH) e e 2 深圳研究生院 能源与电工新技术实验室
CORROSION MECHANISM Due to the existence of oxygen in the solution, further oxidation reaction can take place. DC power supply To the pin From the iron 4F (OH) O +2H O 4F (OH) of upper cap of lower V e 2 2 2 e 3 insulator insulator The constituents of rust are shown as below. Flow direction Flow direction of electrons of electrons FO+ F O + H O m n p e e2 3 2 The values of m, n and p vary under different Pin Iron cap Flow direction of cation temperature, pH value and oxygen content Fe 2 OH conditions. 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S MECHANICALAND ELECTRICAL CHARACTERISTICS Three pieces of iron cap corroded porcelain insulators ± 800 retrieved from 407# tower of kV Chusui transmission line are carried out contamination degree measurement test. The surface of insulator is divided Upper surface into three parts, one 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 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S MECHANICALAND ELECTRICAL CHARACTERISTICS The measurement results are listed as below. Area A Area B Area C No. ESDD NSDD ESDD NSDD ESDD NSDD (mg/cm 2 ) (mg/cm 2 ) (mg/cm 2 ) (mg/cm 2 ) (mg/cm 2 ) (mg/cm 2 ) 1 0.1072 0.1090 0.0078 0.0697 0.0113 0.1165 Upper surface 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 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S MECHANICALAND ELECTRICAL CHARACTERISTICS 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 XZP 2 -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 of 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. 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF IRON CAP CORROSION ON INSULATOR’S MECHANICALAND ELECTRICAL CHARACTERISTICS Test results of pollution flashover test are shown as below. Flashover voltage (kV) Corroded No. element 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 The test results illustrate that, the flashover voltage of iron cap corroded insulators is 20% lower than that of pin corroded insulators. 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF PIN CORROSION ON INSULATOR’S MECHANICAL AND ELECTRICAL CHARACTERISTICS The XZP 2 -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. 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF PIN CORROSION ON INSULATOR’S MECHANICAL AND ELECTRICAL CHARACTERISTICS 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. 深圳研究生院 能源与电工新技术实验室
INFLUENCE OF PIN CORROSION ON INSULATOR’S MECHANICAL AND ELECTRICAL CHARACTERISTICS In terms of XZP 2 -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 of 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. 深圳研究生院 能源与电工新技术实验室
SIMULATION TEST METHOD 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 on the pin and locking device for iron cap and pin tests respectively. Wire Pin Locking device 1cm 5cm Waterproof Waterproof Wire plastic plastic Copper Copper electrode Wire Wire electrod e Fix the metal wire Fix the electrode Fix the electrode Fix the metal wire (iron cap test) (pin test) (iron cap test) (pin test) 深圳研究生院 能源与电工新技术实验室
SIMULATION TEST METHOD 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-voltage regulator 2-test transformer 8 7 3-full-bridge rectifier system 10 2 4-protection resistor 5-filter capacitor 4 1 7 10 10 6-DC voltage divider 7-cable 9 9 8-insulator suspension device 5 6 3 9 9-spray water device 10-leakage current measurement system 11 11-accelerated corrosion test room. 深圳研究生院 能源与电工新技术实验室
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