Analysis on electric corrosion government of generator stator bar in the near Wake Island Hydropower Station

In view of generator stator bar corrosion problems in the near Wake Island Hydropower Station, performed causal analysis and technical innovation. The effect of cleaning and repairing measures were analyzed and verified. In view of the security hidden danger still exist after processing, put forward the stator bar transformation ideas and carried it on. The reconstruction effect was test verification as well. The results of the study can provide reference for power plant generator to handle similar stator bar corrosion problems.


Introduction
Electric corrosion is defined as the high-energy capacitive discharge caused by the loss of electric contact between generator slot, stator bar surface and slot wall, then resulting in thermal and mechanical effects on the stator bar surface [1][2]. Electric corrosion usually occurs between the insulation surface of generator stator bar slot and the slot wall, or between the corona proof coating and the main insulation [3]. It causes the burning and corrosion of corona proof coating on stator bar surface, main insulation, slot wedge and spacing strip. The consequences varies from change color to corona proof coating become crisp. Then the major insulation pitting appears, accompanies by the odor of ozone [4][5][6]. The electric corrosion of stator bar was found in the near Wake Island Hydropower Station during maintenance, then the cause analysis and repair treatment were carried out, and the modification technology of stator bar was put forward.
The near Wake Island Hydropower Station is located at near Wake Island Town, the junction of Hengnan, Changning and Qidong counties. It is a runoff power plant, the seventh level in Xiangjiang mainstream development plan. Three SV628/80-155 bulb tubular units were installed. They were discharged in 1996 and put into operation in 2000. By the end of 2014, the equivalent operating hours of Unit 1, 2 and 3 were 59529.7h, 58950.6h and 57603.9h respectively. Main parameters of units are shown in Table 1.
In the station, each unit has 480 slots of stator core and 960 stator bars. The original design used the filling method with N189 semiconducting silica gel. The interlayer strip was installed within the upper and lower stator bar. No other filler between stator bar and iron core except silicone rubber.  Figure 1) were found fallen off, and the main insulation of the upper stator bar was partly damaged (Figure 2 and Figure 3). 56 stator bar ends had different degrees of electric corrosion on the upper and lower layers. All the electric corrosion happened on the high potential position on the upstream side ( Figure 4) [7]. When the wedge of No.48 slot was punched out, it was found that the semiconductor silica gel in the slot was uneven. The semiconductor layer in the slot of the upper stator bar was burned out in large area due to electric corrosion and the interlayer strip was damaged.    The stator bar was fixed in the slot and corona proof by filling and injecting semiconductor silicone rubber [8]. Practice proved that it was difficult to fill the gap between the stator bar and the slot wall, to fill the gap between the stator bar, the partition and the slot wedge completely, even if the highpressure pump was used to inject according to the manufacturer's process. Because the single-side gap in the slot was only 0.2 mm, and the viscosity of the silicone rubber is relatively high and the fluidity is poor, meanwhile the air in the slot was difficult to overflow during the injection process. Therefore, there must be a large number of gap between the surface of the stator bar, the slot wall, the partition and the slot wedge (this was fully verified in the process of transformation in the later stage). Due to the air gap and bubbles produced by the injection process defects, the potential gradient of the unit was not uniform in these parts during operation, and corona discharge was generated, gradually corroded the slot wedge and insulation layer. The slot wedge became smaller and deteriorated due to electric corrosion and eventually fell off. At the same time, the thickness of interlayer strip of this unit were up to 4mm (generally 2mm in China), which led to the increase of discharge gap and further aggravated the corona corrosion.
After reviewing the electromagnetic program of the original manufacturer, it was found that the thermal load, stator electric density, stator temperature rise, rotor temperature rise, air gap flux density and other parameters of the unit were all very high. The calculated value of stator temperature rise was about 73K, and the value of air gap flux density was close to 1T, which was beyond the allowable range of this model. At the same time, in order to reduce the magnetic flux density, the ventilation slot which is usually used in the iron core was cancelled. The current-carrying capacity per unit area and the magnetic flux density of the iron core were close to the allowable limit value, which led to the increase of temperature rise of the stator and rotor, and aggravates the electric corrosion of the stator bar insulation.

Repair treatment
In order to avoid long-term outage of generators and reduce the economic loss, temporary repairs were carried out for No. 1 generator according to the technical scheme provided by the original manufacturer: 1. Replace the No. 48 upper stator bar in the main insulation. 2. Repair other parts with electric corrosion: Clean the damaged point and contact point; repair the corona proof coating; repair the corona injury parts of stator core area; repair the stator bar end; repair the high and low stopband; brush the insulation paint in the whole repair area ( Figure 5).
After the Unit 1 had been repaired, the same inspection was carried out on Unit 2 and Unit 3. The end of No. 122 slot winding of Unit 2 was electro-corroded. No falling of slot wedge or arching was found. The end of No. 69 slot winding of Unit 3 was electro-corroded. No falling of slot wedge or arching was found.
The test data of the three units were analyzed, the result is as follows: 1. The voltage of PD test was set according to rated voltage. Partial discharge of Phase B of Unit 3, Phase B and Phase C of Unit 2 exceeded the standard. Phase A of three units was relatively good. The stator insulation and DC voltage withstand test were carried out for the three generating units respectively, and the AC voltage withstand of the Unit 1 were qualified. DC voltage withstand of Unit 1 and Unit 2 was 2.0 Un(nominal voltage) according to Maintenance Standard B, and Unit 3 was 1.5 Un according to Maintenance Standard C. The ozone concentration of Unit 1 was the highest, and there was ozone odor in operation; Unit 2 and 3 had lower ozone concentration, no obvious ozone odor in operation. This is consistent with the severity of electric corrosion.
2. Although the PD test data exceeded the standard, the insulation and DC leakage test and AC voltage withstand test of Unit 1 were all qualified because of the long failure reaction of the whole insulation system. Due to the design and manufacture, completely eliminate the air gap between the stator bar and the iron core by filling and injecting the semiconductor silicone rubber by the repairing technology was difficult. The unit could only operate for a short time after electric corrosion was  In order to systematic understand the operation of this type of unit in other hydropower stations in China, special investigations were carried out on Nanjindu hydropower station in Hunan Province and Wangfuzhou hydropower station in Hubei Province, where similar units were installed. The findings are as follows: Nanjindu Hydropower Station and the near Wake Island Hydropower Station belong to a river basin. The capacity of the power station was basically close, and the equipment layout was basically the same. There had been many stator earth protection actions caused by electric corrosion. From external inspection, serious corona corrosion occurred in high potential area. At the later stage of the corrosion, the bulb heads of the three units all had strong ozone odor, and gas masks were needed when inspecting the equipment. Serious electric corrosion and frequent stator earth protection actions had brought serious hidden dangers, huge economic losses and high processing costs of imported equipment. Therefore, the three units had been reformed locally by replacing domestic stator bar. After the government, the actual output, operating temperature and vibration of the unit were excellent, and could run steadily at full load for a long time.
In Wangfuzhou hydropower station, there was an interphase short-circuit and earth protection action of stator bar slot opening in Unit 3. The treatment was carried out by hanging out stator, replacing part of stator bar and repairing electric corrosion of slot opening. The operation is normal at present, but ozone smell still exist. Comparing the treatment methods between the two power plants, the new stator bar technology can completely eliminate the hidden danger of electric corrosion caused by the design and manufacture, and improve the safety and reliability of the unit equipment. But the cost is high, the construction period is long and the preparation work is numerous. The repair treatment cost is relatively small, the construction period is short, but stator bar electrical corrosion and hidden dangers cannot be completely eliminated.
In July 2015, Unit1 was installed on-line PD monitoring device. Monitoring data show that the trend of partial discharge data had been on the rise.
In 2016, the on-line PD data of Phase B and C of Unit 1 increased rapidly, and the maximum QM+ of Phase C in July was 500 mV (about 3 times that of one year ago), which was close to 95% of the empirical database provided by the manufacturer (525 mV), indicating that the development of electric corrosion of Unit 1 presented a rapid aggravation trend ( Figure 6). In order to completely eliminate the hidden danger of stator electric corrosion and avoid more losses, it was advisable to reform the stator bars of the generator through comprehensive analysis.

Modification idea of stator bar
According to the analysis of stator bars with corona corrosion in near Wake Island Hydropower Station, the lowest voltage was about 1.5 kV, meanwhile the corona voltage was low. In order to increase the initial voltage of spark discharge between slots, the distance between the surface of corona proof coating and the contact point of slot wall of iron core should be shortened as far as possible.
The fundamental cause of the corona at the stator bar end was the concentration of electric field at the slot opening [9,10]. Therefore, the electric field should be homogenized to effectively inhibit the occurrence of the end corona. The electric field at the stator bar end would be homogenized and the voltage drop near the slot opening would be decreased by reducing the surface resistivity. In addition, in order to improve the potential gradient of the whole corona proof system, semiconductor insulating At present, the technology of laying two kinds of semiconductor layers with different resistance at the stator bar end are commonly used in stator bar manufacturing in China. The semiconductor layers with lower resistance values are used near the slot opening and the layers with higher resistance are used far away from the slot opening. Thus, the potential gradient distribution is changed and electric field concentration at the slot exit has been reduced. In addition, the problem of assembly gap in the height and width direction of the stator bar slot can be solved by the stator bar insertion technology of large domestic generator manufacturers.

Requirements for new stator bar transformation
The modification of stator bars in the near Wake Island Hydropower Station required that the stator core should not be changed and only the stator bars should be redesigned and produced. In order to eliminating the hidden danger of the unit completely, the transformation requirements are as follows: 1. Only the stator bars and insertion technology should be modified. The stator core and other key structure remain unchanged, and the generator capacity and performance parameters remain unchanged. The temperature rise of the generator bar should be reduced or unchanged.
2. The transformed generator should meet the requirements of GB/T7894-200l "Fundamental Technical Specifications for Hydro Generators". Mainly including: a single stator bar should not be coronated at the 1.5 times of rated voltage. When the whole machine is under voltage, at the 1.0 times of rated voltage, there is no obvious golden bright spot and continuous corona at the end. The leakage current of stator winding at 3 times rated DC voltage should not increase with time, the difference of leakage current in each phase should not exceed 50% of the minimum value, and the time of DC withstand voltage can last for 1 minute. The whole machine can last for 1 minute at 2 times rated voltage + 3000V AC voltage according to the standard of factory test, without insulation breakdown.
3. The electrical performance of a single stator bar should meet the requirements of DS/ZJ011-2002 "Product Quality Grading of Large Hydro Generators", and a single stator bar should be tested for partial discharge.
4. The slot potential of stator bar should meet the requirements of GB/T8564-2003 "Specification Installation of Hydraulic Turbine Generator Units": After insertion, the slot potential or slot resistance should be measured at rated phase voltage. The slot potential is generally less than 10V, or the slot resistance meets the manufacturer's requirements (<5000Ω).
5. The fixing of the coil end should ensure that the coil would not sink in various operation conditions after long-term operation, and can prevent the vibration and deformation of the generator in the most serious short-circuit situation. The whole end support system should have sufficient strength, stiffness and good ventilation, and can be easy to check the end winding of the stator and measure vibration. The off-line PD measurement should be carried out after the stator bar is inserted, to retain the data and facilitate the subsequent unit status evaluation.

Design and manufacture of stator bar transformation
The gap between the bar and slot is 0.4 mm (total on both sides). In order to make it easy to insert, the new bar interlayer strip was designed to be thinned properly. The thickness of a single electromagnetic wire was increased; the width of a single electromagnetic wire was reduced; the cross section of the original electromagnetic wire was kept unchanged, and the thickness of the main insulation was adjusted properly. It was designed that in the same area the thickness of single strands increased by 0.02mm and the width decreased by 0.05 mm. The width of double strands could be reduced by 0.1 mm, and the gap between bar and slot wall could be increased from 0.4 mm to 0.5 mm when the total insulation thickness remains 2.4 mm, to meet the requirements of insertion. Good heat conductivity between the bar and core had been met similarly.
The new stator bar adopts the insulation structure of internal shield + major insulation + low resistance corona proof coating; the material is made of single-sided reinforced mica tape. The