STATOR WATER SYSTEM MONITORING FOR LARGE TURBO- GENERATOR- A USER S PERSPECTIVE. A.K. Gupta Dinkar Devate D.K. chaturvedi* NTPC Ltd.

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1 21, rue d Artois, F PARIS A1-107 CIGRE 2012 http : // STATOR WATER SYSTEM MONITORING FOR LARGE TURBO- GENERATOR- A USER S PERSPECTIVE A.K. Gupta Dinkar Devate D.K. chaturvedi* NTPC Ltd. Noida, INDIA SUMMARY In the developing countries like India, the primary focus of utility is to make the units available to its maximum and minimising the maintenance duration. Simultaneously, these units have to run continuously at base load ensuring the high reliability. The present IEC standards for Electrical Rotating Machines are referred for testing to establish the performance and quality of the large electrical machine, however they do not specify limits or trending of various operating parameters of generator Stator Water System, which needs to be continuously monitored. The availability of different designs of turbo-generators Stator Water systems calls for understanding the basic ingredients of the system, their function and how the system can perform its best. The paper covers a brief on the types of Generator Stator Water systems, their advantages and disadvantages, design requirements & monitoring of critical parameters along with permissible associated limits. The process of corrosion in hollow strands, crevice corrosion on clip to strand joint and various steps to check corrosion right at incipient stage have been discussed. A few designs experienced by the user for stator water system have also been summarised. KEY WORDS Base Load, Trending, Parameters, Monitoring, Turbo-generator, Stator, hollow Strands, Crevice Corrosion, Incipient. *dkchaturvedi@ntpceoc.co.in

2 DESCRIPTION Requirements for directly Water cooled generator The directly cooled generators are provided with roebel bars hollow conductors connected to common water header at inlet and outlet of the winding. The conductors as well as the water headers are generally of copper, however in some cases stainless steel hollow conductor, cooling water header and piping are being provided. The direct cooled machine are designed generally above 500MW, to ensure quick removal of majority of heat by the water flowing in direct contact of the winding conductor, in addition to the removal of heat by hydrogen from the surface of hot winding insulation. The winding insulation besides having good dielectric strength, has excellent thermal conductivity to ensure fast removal of heat. All strands of the roebel bar are brazed together (with BCuP) at the water box connecting to header (Fig.-1) [1]. The flexible connection between the high voltage (e.g. up to 15kV L-N ) conductors to water header is through Teflon tube. Understanding of the water chemistry is very important for longevity of the machine. Here some important term and the relationships in the chemical process are described; Fig-1 Connection of Water Header to Stator Bar Corrosion in Copper Conductor: The copper does not react with pure water (DO - less than 5ppb, ph= 7). As the water enters in the winding, the hollow conductor Copper comes in contact with the water and form cuprous oxide Cu2O /cupric oxide CuO depending on the electrochemical potential (ECP), which varies with the operating temperature and the DO present. The copper oxide forms a stable passive layer on the hollow copper conductor inner

3 surface. This stability of passive layer increases with the increase of stator water ph. As shown in the figure-2, the corrosion rate of copper in contact with the cooling water mainly depends upon the ph and Dissolved Oxygen (DO). The makeup water entry in low DO type stator water system, can lead to copper release in excess of the solubility limits at the operating temperature (say 85 degree Celsius conductor temperature on rated load). One part of the copper oxides forms a coating on the conductor surface, second part dissolved in water and the other moved in the circuit, leading to deposit in critical area of the winding. The solubility of Copper oxides[2] in water does follow V-curve relation (see fig.3) depending upon the ph of water and the operating temperature. The re-deposition of copper oxides in the hollow strands also cause uneven blockage. Figure-2 shows the corrosion rate of copper bar with dissolved oxygen Fig-3: Solubility of Copper in Water with temperature[2]

4 The leakage in stator water system requires frequent makeup, which unless due care is taken, allows change of DO from low to high and vice versa due to formation of copper oxides. The air ingress (presence of CO2) at suction side of the water pump due to aged gaskets can also results in lowering of the ph value to acidic zone[3] from neutral zone and subsequent increase of corrosion rate. Deioniser: The process of deionising the water is ensures not only removal of soluble ions, but also filters the insoluble oxides and other particles. The deioniser controls the conductivity of water, which should be maintained below 2µS/Cm. The conductivity is determined at reference temperature of 25 C, which increases with temperature, 2% per degree Celsius[4] of temperature rise. The conductivity increases suddenly as NaOH dosing is done in elevated ph type system. The conductivity depends upon the presence of free ions in the water system. The deionised pure water has poor conductivity of µs/cm. The deioniser can be mixed bed type, where water is continuously pass through mixed bed having both resins in the same vessel. The other system two-bed deioniser provides larger capacity, where two vessels are used having separate cation and anion resin beds. THE TYPES OF STATOR COOLING WATER SYSTEM The stator water systems generally available are of three types [5] : a) Elevated ph (ph 8.5) low Dissolve Oxygen type b) High dissolve oxygen (>2000ppb) neutral ph type and c) Low dissolve oxygen (<50ppb) neutral ph type. High Oxygen and high ph system is not in use as it cause clip corrosion [5]. The key issues to be address while designing the stator water system are: a) Low corrosion rate in water cooled hollow stator bars b) Elimination of crevice corrosion in clip to strand brazed joint c) Maintenance of conductivity lower than 2µS/Cm. d) Quantum of dissolved oxygen and e) Regulation of alkali dosing in elevated ph type systems. ELEVATED PH TYPE SYSTEM The elevated ph low dissolved oxygen results in formation of Cuprous Oxide Cu 3 O layer inside the conductor as the water enters and protects the copper bar from corrosion. The curve shows that Global corrosion rate at Elevated ph is much lower that the value at neutral ph. The lower corrosion rate has the advantage of ensuring better cooling of the stator bars. However, the issue of crevice corrosion at clip to strand brazing joint needs to be addressed here considering the formation of acidic solution from the braze material (BCuP & Cu 3 P) at clip to strand location.

5 Figure-4 : shows a typical Scheme of Elevated ph type System DEARATED TYPE STATOR WATER SYSTEM The dearated type stator cooling water system was introduced by GE to eliminate the issue of crevice corrosion at clip to strand joint of the stator bars. The corrosion initiated in brazed metal (BCuP-1) due to formation of low ph phosphate solution. The chemical reaction attacks both the brazing alloy BCuP and Cu 3 P. The development of cracks results in leakage of hydrogen into water. The leakage of hydrogen for long duration allows moisture penetration to the insulation (by capillary action) causing swelling and severe deterioration. Figure-5 Showing the configuration of a typical Neutral ph and Low Dissolved Oxygen Type Stator Water System

6 The system is designed for low dissolved oxygen and therefore, it is mandatory to have following features in this type of system[6]: Detraining Tank The detraining tank is provided in the scheme to remove the dissolved gases from the cooling water by passing the primary water on a set of baffles. It is important to ensure that the vent pipe is not closed. Stator Water Storage Tank-Special features The tank is of stainless steel provided topping over water with hydrogen blanketing or nitrogen scavenging. The nitrogen scavenging is preferred considering the safety aspects despite requirement of additional system with nitrogen cylinders. The water in storage tank has to be continuously stirred by suitable sparging equipment. Hydrogen blanketing has advantage of effective use of catalyst palladium to control the DO, as and when it increases to more than 50ppb. Gasketing Arrangement Requirements The gaskets for stator water system operating under low oxygen regime have to be designed for long life while working in contact with water, hydrogen and air. The flange joint surfaces shall have smooth profile to ensure proper sealing and no ingress of air especially at water inlet side of the pump. The hydrogen leakage can also take place on high pressure side of the water circuit by Venturi effect (where the difference of water pressure and hydrogen pressure is low). Concern in Low DO Type system: The initial filling is with neutral ph water having normal value of DO i.e. 1500ppb. As the water is filled up and passes through the circuit, the dissolved oxygen is consumed forming cuprous oxide Cu2O layer on the hollow copper conductor inner surface. The DO consumption results in reduction of DO level to about 10ppb, i.e. low DO regime. In case of makeup with same water at later stage, the DO suddenly increases and can cause severe corrosion. The following precautions may be taken in the low DO type system: a) Sparger along with nitrogen or hydrogen capping in the stator water storage tank may be installed. Necessary vent open to atmosphere has to be provided. b) Oxygen removal system may be used in the makeup line. In case of low DO system catalyst palladium[5] is used in the deioniser to form water quickly from the dissolved oxygen. Such provision shall be done with Hydrogen blanketing in the stator water storage tank to make the dissolve hydrogen present in the cooling water. The ph can also be slightly elevated as emergency measure by NaOH dosing to check the global corrosion rate, but the solution is to be worked out in next overhaul of the unit. Keeping elevated ph in old system originally designed for low DO neutral ph will lead to severe crevice corrosion in clip to strand joints. The continuous trending of Dissolved Oxygen and conductivity is must for neutral ph operation.

7 HIGH DISSOLVED OXYGEN TYPE SYSTEM[6] The figure-6 shows a typical scheme of the high dissolved Oxygen type of stator cooling water scheme. Figure-6: High Dissolved Oxygen type of Stator Cooling Water Scheme The high DO system operates at Oxygen Concentration above 2ppm and neutral ph. The ph of stator water system has to maintain always at 7 considering the risk of crevice corrosion at clip to strand joint at elevated ph. Clip to strand joint The fig-1 shows the water box connection from the water header to the winding bars. On one side of the water box the Teflon tube connection is there with washer and nut to ensure perfect sealing of the flexible joint with the water box, on the other side the water box is connected to copper conductor strands[1]. These strands are brazed with the water box. The brazing is done with phosphorous rich braze alloy. If the brazing is not proper, water may travel to outside the water box due to capillary action damaging the insulation. PRECAUTIONS The aerated as well as deareated type of system have their own benefits, however deviation from specified requirement can cause severe damage to the winding conductor as well as the insulation. All the designs discussed above have their own advantages. However, as seen from figure-2, the operation close to neutral ph with dissolved oxygen of ppb can cause severe plugging of the stator bars requiring Ethylene-Diamine-Tetra acetic Acid (EDTA) cleaning. CHANGE OF SYSTEM AT LATER STAGE The change of system from one type to other is not advisable rather the operation has to be ensured as per OEM recommendation for longevity of the units. NTPC has so far no experience of change of system from one type to other. However, reports

8 suggest that change of system at later stage requires utmost care and understanding of the associated process, else results in severe damage to the machine. A few cases has been summarised below: a) Elevated ph instead of Low Dissolved Oxygen (DO) System The change of neutral ph Low DO system to elevated ph is good as long as the process of corrosion is not initiated. This change at later stage can lead to severe crevice corrosion at clip to strand brazed joints and damage to the winding subsequently. This occurs if the winding prior to change has experience or initiated process of crevice corrosion or Global corrosion. It has been experience that NaOH dosing at later stage can lead to increase in crevice corrosion and subsequent winding insulation failure. b) Change from Dearated to Areated These systems worked under neutral ph. As per EPRI report, a large fleet of EDF units have been converted from low DO to High DO system[8]. The dearated system was having nitrogen blanket in the stator water storage tank. EDF have problems of plugging in all of their dearated systems due to ingress of air from joint to the stator water. This resulted in dissolved oxygen higher than the desired level leading to plugging of the tubes. Even after the conversion to high DO type system, the plugging continued due to: Operation of the units in Intermediate range DO and Presence of residual of global corrosion occurred earlier. After thorough cleaning of the system and taking due precaution the system operated successfully. c) Change from Areated to Deareated One of the nuclear power stations of Europe was having the generator to operate with high DO level but actually operated as low DO type system. The system was provided with only conductivity monitoring at inlet & outlet of the deioniser[8]. The clip to strand leakage resulted in large quantity of dissolved hydrogen in the stator water system, accumulated at the top of water surface in stator water tank. The capping of hydrogen also not allowed the air contact to water, which is mandatory to achieve 2000ppb dissolved oxygen level. The dissolved oxygen was consumed in corrosion process of copper, leading to operation in low oxygen regime. Due to hydrogen leakage from clip to strand the DO level reached below 10ppb (instead of >2ppm). After thorough cleaning of the system, the system was converted to dearated type by taking suitable measures and thereafter run successfully. LEAK REPAIR It has been experienced that even in new machine cracks may appear at clip to strand joint initiating leakage of hydrogen into the primary water. The cause can be one of the following: Severe end winding vibration of effected bar causing development of the crack Any kind of impact the bar subjected during the transportation or installation.

9 Development of crevice corrosion and Poor brazed joint with presence of voids. One of the following methodologies is adopted by leading manufacturer: a. Complete bar replacement (for top bar) b. Epoxy injection repair c. Braze/TIG repair. In case the leaks are at many places, Global Epoxy Injection or complete[1] stator rewinding is done. THERMAL MONITORING OF STATOR COOLING WATER SYSTEM The thermal profile of cooling water is very important for healthy operation of the machine. The most critical measurement in this system is the water temperature at the bar outlet, and hence the same has to be continuously monitored for each bar individually or common for top & bottom bars. The Embedded temperature Detectors are placed on the designed hot spot locations and thus have temperature higher than the water temperature flowing in the conductors. Considering the availability of excellent insulation tapes of high electric strength, good thermal conductivity and coefficient of expansion close to the copper, the temperature at ETD locations shall not exceed in any case more than 95 degree Celsius[7]. As we know that water boiling point slightly rises with increase of operating pressure (in this case water pressure in the conductor) and operation close to those limits shall cause bubble formation and subsequent blockage of cooling water flow. The hot water temperature at outlet shall generally be kept below 75 degree Celsius. COMMON FEATURES The stator water system should have the following to ensure healthiness of the system: a) Monitoring of every single or two(top & bottom) bar water outlet temperature b) Monitoring of Online Dissolve Oxygen c) Measurement of ph d) Conductivity Monitoring (should be less than 2µS/Cm.) e) Differential pressure across stator bars f) Differential pressure across deioniser g) Primary water inlet pressure and h) Regulated alkali dosing for elevated ph (maintaining the conductivity). In line samples are also taken periodically to check and monitor the values of b), c) and d) above. The monitoring of differential pressure across stator winding is on continuous basis. The trend shall clearly show the corrosion or blocking of the bars. The table-1 shows the suggested values of parameters of stator water system: S.No. Parameter Elevated ph, Low DO Low DO, Neutral ph High DO, Neutral ph 1 ph 8.5 to Dissolve Oxygen < 50 ppb <50ppb >2000ppb 3 Conductivity to µS/Cm*

10 change resin(generally to be done with increasing trend) 4 Conductivity Max µs/cm (alarm) 5 Conductor temp. 95 C 6 Water pressure at 3.5kg/sqcm. inlet to conductor 7 Max. Permissible 1.5Kg/Cm2 diff. Pr. (DP) Across Conductor 8 Recommended Hot 75 C (max.90 C) water temperature 9 Differential with 1-2 Kg/Cm2 Hydrogen pressure 10 Hot Gas Max. 75 C Temperature 11 Cold Gas Max. 46 C temperature 12 Cooling Water Max. temperature 40 C EPRI report[5] suggest change of resin at (a)conductivity of 0.5µS/Cm (b) every 18 to 20 months interval or (c) pressure drop across deioniser resin exceeds 15psi. NTPC experience is change of water as conductivity reaches max. 10µS/Cm (BHEL, ALstom, Elektrosila suggest max. 10µS/Cm for alarm) NTPC EXPERIENCE The NTPC has more than 50 nos. water cooled generator of 200MW and 500MW ratings in operation for more than 5 years. The available designs provide us with experience of stator water system of following types: - Low Dissolved Oxygen and Elevated ph (500MW units) - Low Dissolved Oxygen and Neutral ph (500MW units) and - High Dissolved Oxygen and Neutral ph (200MW units). The experience of these units is generally satisfactory and suggests: - Due care shall be taken to maintain desired Dissolved Oxygen level by having On-Line monitoring. - Conductivity shall also be ensured within limits, establishing machine safety and healthiness of deionizer. - ph should be maintained at the designed level to check initiation of crevice corrosion. - The parameters of stator water system in NTPC generally comply to table- 1 above. In two of NTPC units with low DO and neutral ph, severe corrosion was experience requiring ETDA cleaning of the winding. As large numbers of units in NTPC are with Elevated ph type system, even at

11 the same project, the criticality of Maintaining Dissolve Oxygen was not rightly accessed initially. The initiation of corrosion process in the winding lead to review the cause. Necessary corrective actions (as mentioned above) have been taken to maintain the DO well within 20ppb along with neutral ph. - The resin is generally changed in duration not exceeding six (6) months. The alarm and trip value have been specified as 13 and 20 µs/cm for high DO type 200MW (15.75kV machines) and 10 µs/cm as alarm setting has been specified in 24kV 500MW machines. No tripping has been considered based on stator water conductivity in 500MW units. As practice, samples of stator water are taken to monitor the conductivity at specified interval (normal value 2.0 µs/cm). In case of increasing trend of stator water conductivity value, the resin is to be replaced. CONCLUSION As discussed above maintaining various parameters of stator water as per system design is of utmost importance. Any variation in DO, ph, conductivity, DP and bar outlet water temperature can damage the machine severely. The increase in DO in low DO neutral ph system and decrease of DO in neutral ph high DO system can cause severe corrosion and subsequent deposition in critical areas like clip to strand joint. The operation under such condition for long can result in insulation failure and subsequent ground fault. Change from aerated to deareated and vice versa shall be done only after carrying out a careful detailed analysis. The change from deareated to elevated ph by implementing ammonia dosing can be done only as short term measure necessitating implementation of long term measures for running the system as low-do type, in next overhaul of the unit. It is suggested that the parameters tabulated above for stator water system can be reviewed for inclusion in relevant international standards. REFERENCES: 1. Understanding, Diagnosing, and Repairing Leaks in Water-Cooled Generator Stator Windings- GE Power Systems 2. Behaviour of Copper in Generator Stator Cooling Water System by Robort Svoboda and Donald A. Palmer, Alstom Power 3. Forgotten Water: Stator Water Chemistry by David G Daniels 4. Deionised Water Application Myron L Company 5. Generator Cooling System Operating Guidelines - EPRI 6. Handbook of Large turbo-generator Operation and Maintenance by Geoff Klempner 7. IEEE Requirement for Cylindrical Rotor Synchronous Generators 8. Conversion to Dearated Stator Cooling Water in Generators Previously Cooled Areated Water EPRI 9. Electrochemical Corrosion Potential (ECP) of Hollow Copper Strands in Water Cooled Generators EPRI 10. Design and Experience Feedback of Large turbo-generator Upgrade & Retrofit Philippe Machard, Paul Lapotre, Wynand Pienaar, Marc Peltier