With the rapid development of National Electric Power Construction and the increasing of installed capacity, the heat effect caused by energy conversion in the operation of generator stator intensifies. In order to ensure the safe operation of the electric generator, the generator must be cooled in a certain way. Depending on the type and capacity of the generator, external-cooled turbogenerators often use gas for cooling, such as hydrogen-cooled or air-cooled, while internal-cooled turbogenerators use gas (hydrogen) or liquid (water or oil) for cooling the rotor or stator, water Cooling is a common cooling method for stator. The stator coil contains a part of air-core wires, which are cooled by water, which can reduce the temperature and rise of each part of the generator and prolong the life of the generator. But the inner cooling water can cause the corrosion of the hollow copper conductor while cooling the stator. The hollow conductor of the generator stator coil is usually pure copper, and the inner cooling water is deionized water. The content of Soluble Ions in the inner cooling water is very low, and the corrosion of metal in the water is relatively slow. The inner cooling water system of generator stator requires high water conductivity. The corrosion of copper wire will not only worsen the water quality, but also affect the efficiency and safety of generator.

The corrosion factors of the stator cooling water environment are different from the cooling water in other equipments. The Electric Power Industry has very strict requirements on the quality of the stator cooling water, a clear standard DL/t 1039-2016“Generator internal cooling water treatment guidelines”has been developed. The electrical conductivity is an important index to evaluate the quality of the inner cooling water. The high or low electrical conductivity represents the content of dissolved ions in the inner cooling water, this may be due to poor sealing of the system, Ion-exchange Resin failure, or high Ph, resulting in a gradual increase in conductivity. Dissolved Oxygen (do) is a key index to influence the corrosion of pure copper in internal cooling water. Do is directly involved in the corrosion process of copper and is a CATHODIC depolarizer for copper corrosion in internal cooling water. The oxygen corrosion of pure copper conductor can be greatly reduced by maintaining the low dissolved oxygen of internal cooling water. The dissolved oxygen in the inner cooling water circulating in the closed pipeline is relatively low. In order to reduce the dissolved oxygen in the internal cooling water, Deoxidizer was used to treat the internal cooling water in some power plants. However, the addition of Deoxidizer can increase the conductivity of inner cooling water and the operation cost of power plant, so it is not widely used in inner cooling water.

The corrosion of stator windings the corrosion of stator windings in internal cooling water will cause safety accidents. Despite multiple measures to control copper corrosion, copper corrosion products are deposited in the inner cooling water system from time to time, which can cause damage to the generator stator and lead to the power plant’s forced outage. The deposits are concentrated in small channels and bends through which cooling water flows, some of which have a cross-sectional area of only a few square millimetres, where the deposition of small amounts of copper oxide or other corrosion products can clog the channels and cause the generators to overheat, this kind of problem has occurred in many power plants in China in recent years. QFSN-660-2-22 turbogenerator in a power plant in China has been operating for 7 years, the flow cross section of stator bar is seriously blocked, which causes the insulation of the bar to be damaged, causing the stator bar to be grounded and the generator to trip and shut down. The analysis of the accident shows that the cause of the bar clogging is that the sealing device of the stator inner cooling water system and the make-up water system is not perfect, the water quality is polluted by carbon dioxide, and the Ph value deviates from the set value. At the same time, under the action of dissolved oxygen, the copper corrosion is intensified, the copper oxide produced blocked the pipeline, caused the wire rod to overheat and expand and the insulation to be damaged under the stress, and caused the stator to trip and stop because of the grounding fault in operation. In order to remove copper corrosion products from internal cooling water systems, power plants are often equipped with filtration systems. For example, a particulate filter is used to remove particulate matter from internal cooling water, including copper oxide or Cuprous oxide that may be released from the copper surface. The image below shows the red sediment attached to the stator filter. Examination of the filter core revealed that the chemical composition of the red sediment on the filter core was mainly Cuprous oxide and copper, including black copper oxide particles. The adhesion of these deposits will result in the decrease of the cooling water flow of the stator coil and the increase of the temperature of the stator bar and the outlet water, which will affect the normal operation of the electric generator. In order to control the stator coil corrosion, it is necessary to analyze the factors that influence the stator coil corrosion and make reasonable protective measures. Influence factors of stator Coil Corrosion 1. The effect of dissolved oxygen on the stator inner cooling water system is usually sealed, but a little air will inevitably enter the system during the actual operation, causing the corrosion of copper conductor in the inner cooling water. The corrosion products of copper are related to the dissolved oxygen content. When the dissolved oxygen concentration was below 100 μg/l, copper was oxidized to Cuprous oxide, and when the dissolved oxygen concentration was 200 ~ 300 μg/l, copper oxide continued to be oxidized to Cuprous oxide, and the corrosion rate of copper was the highest under this condition. The results show that when copper is corroded, a double-layer film is usually formed on its surface, the inner layer is Cu2O, the outer layer is Cuo, and the solubility of CU2O and Cuo in water is affected by the content of dissolved oxygen, generally speaking, the corrosion rate of copper will be greatly reduced with the lower dissolved oxygen content. In addition, the stability of the oxide film is also affected by the content of dissolved oxygen. CuO Is stable when the content of dissolved oxygen in the inner cooling water is less than 20 μg/l, and Cuo is stable when the content of dissolved oxygen is more than 20 μg/l. The effect of 2.Ph value on the corrosion process of copper is significant. In Acidic medium, hydrogen ions can destroy the stability of copper surface oxide film. In neutral or alkaline medium, cuprous oxide film formed by copper surface oxidation has good stability and good protection to metal substrate. Ph also affects the rate of dissolution of the Cuprous oxide. To a certain extent, increasing the PH of the solution will keep the rate of dissolution of the Cuprous oxide at a low level, but higher Ph will promote the CU2O to CuO transition. In alkaline solution (PH = 7.5 ~ 8.5) , the corrosion product film on Copper Surface was observed by atomic force microscope (AFM) . It was found that the thickness of the oxide film formed in Ph = 7.5 was 30 ~ 60nm, the thickness of oxide film formed in PH8.5 solution increased to 400nm, and the oxide film completely covered with copper had higher roughness and better adhesion. 3. Once the inner cooling water system is not completely sealed, the carbon dioxide in the air will be dissolved in the inner cooling water, forming carbonic acid, carbonic acid will continue to ionize h + , thus reducing the Ph of the inner cooling water. The dissolved Carbon Dioxide also reacts with Cu2o, an oxide film on the copper surface, to form malachite. Therefore, when dissolved oxygen and carbon dioxide are present in the inner cooling water, the reaction with Cu2o will promote the oxygen-consuming corrosion of copper, and the formed malachite will not adhere to the inner wall of the hollow copper conductor, it is easy to be washed by water flow and flow with internal cooling water in the system, which further damages the oxide film on the copper surface and aggravates the corrosion of copper conductor. ACCELERATED CORROSION CAUSES CU2 + in Internal Cooling Water? The concentration of CO32 + and other impurity ions increased significantly, which improved the conductivity of the inner cooling water. The solid corrosion products may pile up on the inner wall of the copper conductor, which affects the heat transfer performance of the copper conductor and reduces the flow rate of the internal cooling water. 4. From the electrochemical point of view, the lower the conductivity is, the higher the solution resistance is, the higher the resistance to electrochemical corrosion is, and the lower the corrosion current density is. DL/t 1039-2016“Generator internal cooling water treatment guidelines”stipulated that the internal cooling water conductivity is not higher than 2μs/cm. However, it is also suggested that the dissolution rate of metals in water that is too pure will increase, for example, when the conductivity of inner cooling water is less than 1 ΜS/CM, the dissolution rate of copper will increase and the corrosion rate will accelerate, and when the conductivity of inner cooling water is less than 0.5 ΜS/CM, the corrosion rate of copper increased by 1.8 times, and that of copper increased by 35 times when its electrical conductivity decreased to 0.2μs/cm. Generally speaking, it is not strict enough to control corrosion only by reducing the conductivity of the inner cooling water, and other factors should be taken into consideration.

Control method of stator Coil Corrosion 1. To improve the quality of the cooling water in the stator and reduce the dissolved oxygen content of the cooling water at present, hydrogen is often used to cover the top of the cooling water in power plants to insulate the air and reduce the oxygen and carbon dioxide dissolved in the cooling water. In practice, before the water tank is filled with water, the generator is filled with hydrogen and pressurized to a certain pressure to remove the air from the cooling water system in the generator. The system is then filled with cooling water to replace the hydrogen gas. When the water tank is filled with water, start the pump with a certain pressure to make the cooling water into the stator winding coil, repeatedly filling the water tank to lower the water level, and then to the water tank filled with hydrogen. Through such rigorous water-air Displacement, oxygen and carbon dioxide are safely expelled and the system is insulated from the air. A small deaerator can also be designed and installed between the Turbo Generator stator windings and the return cooling water to the vacuum chamber to further tighten the deaerator, which can significantly reduce the stator coil corrosion. It is found that the corrosion rate of copper conductor in the water cooling system of stator windings decreases 2.1 times on average, and the oxygen consumption corrosion of stator windings is effectively prevented. This method can be used as a reference in the design of hydrogen water cooled Turbo generator. The domestic also has uses the nitrogen gas cover the way to deoxidize. The insulation is slightly less effective than a hydrogen blanket. Mainly because nitrogen density and air equivalent, once there is air into the system, not easy to remove clean. The low density of hydrogen can cover the top of the tank, so that the system can operate in a completely closed condition, without fear of water seal blowing, cooling water tanks and backwater pipelines can be filled with hydrogen, improve the reliability of equipment operation. The only disadvantage of using hydrogen seal is that hydrogen is flammable gas, there are safety risks, in the operation of equipment to be used carefully. 2. In general power plants, demineralized water or condensed water is used as internal cooling water for stator coils. The impurity ion content in the desalted water is very low and the conductivity is small, which is in line with the electric power industry standard about the cooling water in the stator. Because the conductivity of the inner cooling water is very low, it has little buffering, when the CO2 in the air is dissolved, the Ph of the inner cooling water will be lower than 7, and the corrosion of the copper conductor will be more serious under the condition of Lower Ph. Therefore, maintaining the Ph of the inner cooling water in a proper range is an effective method to alleviate the corrosion of copper conductor. In the past, the requirement of stator cooling water was low in our country. Usually, the condensate water was used and the ammonia-adding device was connected in the condensate system. Trace amount of ammonia in the inner cooling water can keep its Ph above 8.8, but adding ammonia will increase the conductivity of the inner cooling water, and the ammonia will react with copper ion to form a complex blocking the coil, if there is mixed bed water treatment equipment in the system will also cause resin pollution, so that the failure. In addition, Naoh is also used to adjust the Ph of the inner cooling water to 8 ~ 9, but other ions such as Cu 2 + , Cu + exist in the integrated inner cooling water. This treatment method can make the total conductivity of the inner cooling water exceed 2μs/cm, the conductivity of the inner cooling water increases continuously, even exceeds 5μs/cm. The high conductivity of the inner cooling water does not meet the insulation requirements of the stator coil. When alkaline substances are used to regulate the PH of internal cold water, both Ph and conductivity are difficult to meet the requirements. At present, a more reliable way to increase the Ph of the internal cooling water is to use special ion exchange to make the internal cooling water weakly alkaline, and to meet the requirements of the electrical conductivity of the internal cooling water, so as to alleviate the stator coil corrosion.

3. The aim of small mixed bed treatment is to remove impurity ions in cooling water. RNa-ROH resin mixed bed and RH-ROH resin mixed bed are commonly used in power plants. RNa-ROH Resin and RH-ROH Resin are often used together in many power plants. RH-ROH Resin was used to control the conductivity of cooling water, and RNa-ROH Resin was used to control the alkalinity of the internal cooling water, so that the conductivity and Ph of the internal cooling water could meet the requirements at the same time. The conductivity of the internal cooling water was controlled to 1-5 μs/cm and Ph was 7-9, the utility model not only satisfies the water quality requirements of the internal cooling water, but also reduces the corrosion of the stator coil to the greatest extent. 4. Adding corrosion inhibitor to inner cooling water is a traditional corrosion control method for stator hollow-core copper coils. The corrosion inhibitors developed earlier are 2-mercaptobenzothiazole (MBT) and Benzotriazole (BTA) , it is widely used because of its excellent corrosion inhibition effect on copper corrosion in internal cooling water. The two inhibitors have the same corrosion inhibition mechanism to pure copper, that is, the inhibitor molecules form an adsorption film on the surface of copper by chemical or physical adsorption, and the Copper Matrix is isolated from the corrosion medium to inhibit the corrosion of copper. But there are some defects in the use of these two corrosion inhibitors. If the concentration of corrosion inhibitor is strictly controlled in use, too low concentration will not act as corrosion inhibitor, but will accelerate copper corrosion, and too high concentration will reduce its corrosion inhibition effect, this is because of the way of forming film on the surface of copper: When the concentration of inhibitor is low, the inhibition rate increases with the concentration of inhibitor, and the inhibitor molecules tend to adsorb parallel to the surface of copper; After the inhibitor reaches the optimum concentration, increasing the inhibitor concentration will cause the repulsion force between the molecules, which will lead to the vertical or non-planar adsorption of the inhibitor molecules and decrease the inhibition rate. The concentration of BTA is much less than MBT, and the addition of BTA has little effect on the conductivity of solution. In addition, MBT must be dissolved with Alkali when used, which will increase the conductivity of the inner cooling water. Adding traditional corrosion inhibitor can slow down the corrosion of stator coil to a certain extent, but the water solubility of corrosion inhibitor such as MBT is poor, so it is easy to adhere to the inner wall of coil in insoluble state, which affects the cooling effect of Inner Cooling Water. According to the characteristics of the internal cooling water system and the requirement of environmental protection, it is necessary to design a more suitable corrosion inhibitor combined with the internal cooling water environment in order to control the corrosion of copper in the internal cooling water. Generally used for internal cooling water corrosion inhibitors need to meet the following three requirements: (1) low toxic environmental protection, with the improvement of environmental protection requirements and internal cooling water quality requirements, copper corrosion inhibitors are usually required to add a small amount of toxicity, pollution-free. (2) the corrosion inhibition performance is better, the corrosion inhibitor of copper mainly depends on the chemical adsorption of its organic group with the metal surface to insulate the corrosive ion in the medium. (3) the electric conductivity of the inner cooling water should not be affected greatly when it is used. It can not only have a good corrosion inhibition rate but also meet the low electric conductivity requirement of the inner cooling water when a certain amount is added. Some new copper corrosion inhibitors studied in recent years are mainly azoles, amino acids and Schiff base organic corrosion inhibitors. A series of novel azole inhibitors, such as 2-MBT, have been developed based on the modification of traditional inhibitors. The presence of nitrogen or sulfur in organic molecules can improve the corrosion inhibition performance by forming coordination bond with copper, the 2-MBT dissolved in the solution can react with copper to form a complex, which can protect the surface of copper. Tetrazole derivatives, such as 5-phenyl tetrazole and o-bromo-5-phenyl tetrazole, adsorbed on copper surface by Cu-N bond and inhibited the dissolution of anode copper and the reduction of cathode oxygen by potentiodynamic polarization, the adsorption film formed has high adsorption energy and coverage, excellent corrosion inhibition performance, even in a strong acidic solution can play its corrosion inhibition performance, it can be used as pickling inhibitor in the pickling process of stator copper conductor during shutdown. Amino acids and Schiff base corrosion inhibitors as corrosion inhibitors for copper have been studied for many years. There are Amino groups in the structure of amino acid corrosion inhibitors, which can be used as good adsorption sites to adsorb on the surface of copper. The amino acid type organic corrosion inhibitors studied earlier are L-Aspartic Acid, L-Cysteine, Met, Ala and so on. For slightly alkaline internal cooling water environment, L-cysteine also has good corrosion inhibition performance. In addition to the Amino Group, there is a-SH group in the L-Cysteine molecule, which can adsorb Cu (i) cys protective film on the surface of copper under alkaline condition. Schiff base is a condensation product of Amines, ketones or aldehydes containing R2C = NR group. The IMINO groups in its molecular structure are easy to coordinate with metals to form stable complexes, which have good corrosion inhibition effect. Although some new corrosion inhibitors have unique advantages over the traditional ones in corrosion inhibition performance or environmental protection, it still needs to be verified whether they can meet the requirements of internal cooling water for practical application.

5. Strict water quality monitoring in the internal cooling water system configuration of the corresponding import and export water monitoring devices, on-line monitoring of pressure, flow, temperature and conductivity and other parameters. In order to improve the monitoring level of internal cooling water quality, an on-line monitoring and leak detection device for water cut-off protection is also provided. At the same time, it should be noted that poor instrument cooling or improper setting of water quality parameters of stator inner cooling water system will lead to corrosion of hollow copper conductor. Sophisticated monitoring devices can help system engineers monitor water quality and keep parameters within design values. During the operation of the generator, if the water flow or pressure of the cooling water system and the inlet and outlet water temperature are abnormal, the water quality monitoring system will feed back the abnormal information immediately and provide sufficient time for the generator to resume normal operation, prevent further clogging of the cooling water system. Maintenance of stator Coil 1. The deposition of corrosion products in the inner cooling water system of the chemically cleaned stator on the inner wall of the Hollow Copper Conductor leads to the decrease of the inner cooling water flow, which affects the heat transfer and causes the local overheating of the stator coil and affects the normal operation of the stator, therefore, chemical cleaning agents are often used to clean the inner cooling water system during the generator shutdown to remove the corrosion products and other scale layers on the pipe wall. One of the key factors that affect the quality of chemical cleaning is to select suitable cleaning agent. The selection of cleaning agent usually needs to meet the following conditions: (1) can not cause greater copper conductor corrosion. (2) the removal rate of copper oxide is higher. At present, the main cleaning agents used in generator internal cooling water system are glacial acetic acid, citric acid, sulfamic acid acid, Hydroxyacetic Acid, etc. . Usually a combination of sulfamic acid and Hydroxyacetic Acid acid is used in combination with a certain amount of corrosion inhibitor such as BTA. The function of the corrosion inhibitor is to inhibit the corrosion of the cleaning agent to the copper wire in the cleaning process. The object of chemical cleaning is not only the stator coil, but also the inner surface of the generator hollow-core bar, the heat exchanger, the filter, the stator water pump of the inner cooling water tank and the connected pipes. The whole circulation circuit of the internal cooling water should be cleaned, especially the copper oxide deposited on the pipe wall or the bend, and the filters and sieves should be cleaned thoroughly to restore the flow of the internal cooling water. The cleaning order is first water wash then acid wash, and finally water wash again. In the process of pickling, the content of copper ion in the cleaning agent should be monitored in real time, and the pickling can be stopped when the concentration of copper ion reaches the balance. When washing, use the demineralized water that meets the requirements. When the PH of the outlet reaches 8 ~ 9 and the electrical conductivity is less than 2 ΜS/CM, the washing process is completed. After cleaning, the stator windings should be dried and the insulation of the stator windings should be tested to check whether the insulation of the stator meets the actual operation requirements. After chemical cleaning, the corrosion products deposited on the copper surface are removed, leaving the copper base surface bare. In order to prevent the new corrosion on the exposed copper surface, the copper surface is usually pre-coated after cleaning, and the new copper base surface is reoxidized to form passivation film by adding alkalizing agent, thus, the corrosion of Copper Matrix by corrosive ions is restrained. However, under weak acid condition, the passivation film can react with H + in water, and Cu 2 + and Cu + can be released into the inner cooling water, which increases the copper content in the inner cooling water and makes the electrical conductivity exceed the standard. To prevent the dissolution of the passivation film, a pre-film agent can be further added, usually a mixture of BTA and MBT in a certain proportion. Bta can react with Cu2o to form a stable complex film, which has good corrosion resistance, and MBT can enhance the protection of the film. 2. Dry maintenance of stator windings dry maintenance of stator windings is a common maintenance method adopted by many power plants in China during downtime. The specific measures are to blow dry air into the air-core coil during the long-term stator shutdown, so that the relative humidity in the inner cooling water system is less than 60% , and then vacuum to further remove the moisture in the coil and control the relative humidity inside less than 30% , finally, nitrogen is injected into the system to make it oxygen-free. In order to remove the moisture and oxygen in the dead corner of the stator coil structure, it is necessary to repeatedly vacuum to reduce the humidity in the coil. The practical application experience shows that the dry maintenance of the stator coil can greatly reduce the moisture and oxygen in the stator coil, thus effectively control the stator corrosion during shutdown. But in order to completely eliminate the water and air in the coil, it needs a lot of time and cost to carry out repeated vacuum purging. Conclusion the corrosion of stator coil is closely related to the water quality of stator inner cooling water. The inner cooling water environment plays a major role in the corrosion of hollow copper conductor. In order to control the internal cooling water quality, the electrical conductivity, Ph, dissolved oxygen and dissolved carbon dioxide should be taken into account. On the premise of meeting the internal cooling water quality standard, the research and development of new environmental protection and high efficiency corrosion inhibitor is also an important direction to deal with the stator corrosion. In addition, during the generator outage, the stator coil cleaning and maintenance is also an important means to control its corrosion. At the same time, it is necessary to establish an effective water quality monitoring system to prevent copper corrosion caused by deterioration of internal cooling water quality. In view of the special corrosion environment of the generator inner cooling water system, more detailed research work is needed in the future to analyze the influence of various factors on the stator copper coil corrosion, and put forward more effective methods to deal with the stator coil corrosion.

 Source: Corrosion & Protection, by Yu Huaqiang, Meng Xinjing, GE Honghua

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