The Case of Stator and Rotor Maintenance for Power Generator Unit 1 and 2 of Samcheonpo Thermal Power Plant

Transcription

1 The Case of Stator and Rotor Maintenance for Power Generator Unit 1 and 2 of Samcheonpo Thermal Power Plant Gwang-sik Son, Hyung-wook Oh, Myung-kyu Kim KOSEP (Korea South East Power Co., Ltd) KOREA

2 1. Introduction Korea South East Power Co., Ltd is a specialized power generating company separated from Korea Electric Power Corporation and newly established on April 2, 2001 according to the Restructuring Policy in the Electric Power Industry of Korean Government; and is currently operating power generating facilities of 8,396MW in 2012 accounting for 10.6% of the total power capacity of 79,213MW of Korea. Among the power stations are: Samcheonpo Thermal Power Site Division in Goseong, Gyeongsangnam-do; Bundang Combined Cycle Power Plant in Seongnam, Gyeonggi-do; Yeongdong Power Plant in Gangreung, Gangwon-do; Yeosu Power Plant in Yeosu, Jeollanam-do; and four large-scale coal-base Thermal Power Plant in Ongjin, which includes the first 870MW capacity in Korea. KOSEP (Korea South East Power Co., Ltd) boasts itself by, among competitors, highest capacity factor, lowest power generating cost, highest rate of operating profits, and highest productivity of labor; and shows more than 200 billion won of net profit for three consecutive years owing to high strength management efficiency plan after the inauguration of current president & CEO, Do-soo Chang in August of In addition, Korea South East Power Co., Ltd do not spare investments for incessant growth in new businesses and overseas businesses including coal Thermal Power Plant, biomass, wind power generation in Malaysia, Indonesia, Kazakhstan, Sri Lanka, China, and Thailand, and in multiple projects such as commissioning service and O&M in India, Bahrain and Vietnam using coal blending, combustion, operation, maintenance, and Repowering technologies.

3 1-1. Samcheonpo Thermal Power Site Division Samcheonpo Thermal Power Plant is located in Hai-myeon, Goseong-gun of Korea as a soft (bituminous) coal Thermal Power Plant, of which the construction of Power Generator Unit 1 and 2 started in 1978 when industrial power demand was rapidly growing. Since the completion of the Power Generator Unit 1 and 2 in 1984, Samcheonpo Thermal Power Site Division responded to the increase of power consumption through succeeding construction of additional power generators, and tried to enhance process control and power quality to supply high quality power stably. In addition, Samcheonpo Thermal Power Site Division exerts itself in stable operation of power generating facilities and environmental conservations by operating Flue Gas Desulfurizer, a facility removing contaminants from thermal power plants and by conducting eco-friendly programs including recycling of fly ashes from coal Thermal Power Plant; and is trying its best to realized community reliable eco-friendly power station leading the change by creating renewable energy (solar light, Ocean Small Hydropower Plant) with 20 year-old know-how. Location Year completed Facility Capacity Fuel 952 Deokho-ri, Hai-myeon, Goseong-gun, Gyeongsangnam-do, Korea # 1 : , # 2 : , # 3: # 4 : , # 5 : , #6 : ,245MW(#1 ~4: 560MW 4units, #5,6: 500MW 2 units) bituminous[soft] coal Recently, Samcheonpo Thermal Power Site Division is operating facilities efficiently through large-scale investments in life extension works, boiler tube replacement, draft system capacity increasing, 1 st stage expansion of electrical dust precipitator, etc. of the Power Generator Unit 1 and 2. Table 1. Specifications of Power Generator Unit 1 and 2 of Samcheonpo Thermal Description Unit 1~4 Unit 5, 6 Facility Capacity 3,245MW 560MW 4 units 500MW 2 units Boiler Turbine Generator Capacity 1,796 Ton/h 4 units 1,720 Ton/h 4 units vapor pressure 177.9kg/ cm2 255kg/ cm2 Fuel coal Low sulfur coal Type Reaction, reheating, Reaction, reheating, regenerative regenerative Type 3 phase, synchronous 3 phase, synchronous Generator Generator Voltage 22kV 22kV

4 1-2. Introduction of Generator Stator and Rotor Renovation Power Generator Unit 1 and 2 of Samcheonpo Thermal Power Plant are used for a long time (29 years) since they were constructed; the stators of which were drawing to their life limit after life evaluation; rotors were low in reliability showing layer short (#2, 06), etc. To solve these problems, stators and rotors were renovated to secure the reliability of the facility including insulation class enhancement (B F) coil rewinding, Wedge improvement, and Water Clip structure improvement for stators; and coil re-insulation and major components replacement like Retaining Ring for rotors. The following figure illustrates the inspection and maintenance periods of the power generators according to their term of operation. After 20 years, rapid deterioration of performance and problems occur, so maintenance work and precision inspection like Stator Coil Rewinding & Rotor Rewinding are needed to extend the service life of the power generator. <Figure 3. Period of inspection and maintenance of power generator depending on its service period>

5 2. Body 2-1. Introduction to power generator The Power Generator Unit 1 and 2 of Samcheonpo Thermal Power Plant were manufactured by American GE in 1982; the capacity is 660,000kVA and runs on hydrogen cooling system. Detailed technical data are shown in [Table 2]. Table 2. Specification of power generator No. Item Design 1 Capacity kva 660,000 2 Speed rpm 3,600 3 Type ATB-2 4 Power Factor Frequency Hz 60 6 Number of Poles 2 7 Number of Phases 3 8 Terminal Voltage kv 22 9 H2 Pressure psig Short Circuit Ratio Cooling Method hydrogen cooled 12 Insulation Class B 13 Manufacturing Year 1982 As explained in the introduction, the Power Generator Power Generator Unit 1 and 2 of Samcheonpo Thermal Power Plant were renovated like the following major improvements. Table 3. Main range of renovation of power generators Stator Coil replacement and insulation class upgrade(b F class) Water Clip improvement Wedge replacement (Top Ripple Spring applied) Hydrogen cooler replacement Partial discharge monitoring system application Rotor Retainer Ring improvement(18mn18cr) H 2 Seal Ring improvement (4 2 Segment) Collect Ring, Wedge replacement Rotor coil re-insulation (#2 Only) Layer short monitoring system application

6 2-2. Stator renovation sequence and scope of operation Stator renovation is largely divided into disassembly and assembly, and the order of work is as follows. <Disassembly> Preparation Cooling Water Drain Air Gap Separator Disassembly Core EL-CID Test Slot Wedge, Filler, Slide Disassembly Teflon Hose Disassembly End Winding Support Disassembly Stator Bar Disassembly <Assembly> Stator Core Cleaning Core EL-CID Test End Winding Support Assembly Bottom Bar Pre Assembly Bottom Bar Dimension Check Make Tension Member Bottom Bar Disassembly Bottom Filler Assembly Conforming Material Assembly Bottom Bar Under Tension Member Install Bottom Bar Assembly Slide Ripple Spring Assembly Conforming Space Block Insert Bottom Bar Hi-Pot Test Bottom Bar Tension Member Assembly Slot RTD & Filler Assembly Conforming Material & Radial Ring Assembly Top Bar Pre Assembly Top Bar Dimension Check Make Tension Member Top Bar Disassembly Top Filler Assembly Conforming Material Assembly Top Bar Under Tension Member Install Top Bar Assembly Slide Ripple Spring Assembly Conforming Space Block Insert Filler & Wedge Assembly Top & Bottom Bar Hi-Pot Test Phase Connection Ring Assembly Series & Phase Loop Brazing Liquid Pipe Connection Brazing Leak Test Series & Phase Loop Taping Conforming Space Block Insert and Top Bar Upper Tension Member Assembly Flooding End Winding Curing & TC/RTD Test Wedge Tightness Test TC Assembly Packing Gland Making & Gland Assembly Bump Test Core EL-CID Test Flow Test & Vacuum Dry Terminal Box Insulation Painting Final Test(Bump Test, Megger & PI, Winding Resistance, Final Hi-Pot, RTD Check, TC Check) [Figure 4] is the scope of replacement parts and reusable parts for the renovation of the Stator. Most components were replaced except for the Stator Frame that does not cause a problem when reused and the Current Transformer that was already replaced.

7 <Figure 4. Out Line of Generator Stator> 2-3. Stator coil replacement and insulation class upgrade According to the remaining life assessment of the Stator coil of the Power Generator Unit 1 and 2 of Samcheonpo Thermal Power Plant in July 2006, the remaining dielectric strength was shown to be 43%, which is very near to the limit (40%). In addition, any water leak in the coil means the service life of the Stator coil is finished, so prompt replacement of the coil is necessary. : Unit 1 : Unit 2 <Figure 5. Generator Residual Life>

8 Water leak in the coil may be diagnosed by Capacitance Map Test. [Figure 6] below demonstrates a typical result of the Stator coil Capacitance Map Test, which indicates possible water absorption in coils including TE Side #11,20 and two more coils in Power Generator Unit 1 of Samcheonpo Thermal Power Plant and CE Side #3 and other four coils in Unit 2. <Figure 6. #1, 2 Generator Water Absorption Diagnosis for Generator Stator Windings> Coils diagnosed water absorption suspicion need to be promptly replaced after preparing a maintenance plan. Continuous water leak may result in fatal accident due to Stator coil grounding fault. [Figure 7] below is an actual instance of coil grounding fault accident due to coil water absorption in another power plant. The root cause of the accident is the coolant leakage in the coil Water Box due to manufacturing defect; the leak continued to result in dielectric breakdown of Stator Slot, which caused long-term (some 50 days) service in the power generator. <Figure 7. Example of Generator Ground Fault>

9 To solve this problem, Water Clip Body was improved to all-in-one type as in [Figure 8], thus reducing the number of brazing from five points to three points, enhancing reliability owing to minimization of leakage. <Figure 8. Water Clip Up-grade (Brazing Point: 5 3)> In addition, the replacement of all Stator coils and upgrading of insulation material from B F class may prevent deterioration and enhance service life because of the higher allowable temperature of the insulator. The following Figure shows the new bar assembly and finished coil rewinding of the Stator. <Figure 9. Stator Re-Winding Process> 2-4. Stator coil Wedge System improvement (Top Ripple Spring application) Stator Wedge secures the Stator coil in the slot and restrains the coil vibration to prevent the abrasion of coil insulation. As operation time passes, the tightness of the wedge degraded, which is indicated by minute change in the size of core, coil, Wedge and Filler. This Wedge Looseness cause problems like Wedge Slack, Shift, Disappeared and abrasion of Main Insulation.

10 <Figure 9. Example of Problems in Wedge Loose> To improve this condition, Top Ripple Spring is applied to Stator Wedge System as in [Figure 10] so that the spring fastens the wedge when core, coil, Wedge or Filler deforms to prevent vibration. <Figure 10. Wedge System Up-Grade>

11 2-5. Rotor renovation sequence of operation and scope Rotor renovation is mainly divided into disassembly and assembly, and the sequence of operation is as follows. <Disassembly> Preparation Run-out check Preparation of disassembly of retaining rings Disassembly of retaining rings Removing of rotor coil wedges Disassembly of field coils Disassembly of terminal bolts and sealing parts Disassembly of collector rings, fan, balance ring Polishing of field coils Cleaning and finishing of the rotor shaft and rotor parts NDI for rotor shaft [Journal, Stepped portion NDI for rotor parts [Retaining rings, Wedges] Painting <Assembly> Preparation of rewinding Installation of field coils Hi-pot test and layer short check Preparation of heating and pressing Heating and pressing Cooling Disassembly of pressing jigs Hi-pot test and layer short check Installation of end spacer blocks Installation of rotor coil wedges Installation of rotor coil wedges Installation of retaining ring insulation Preparation of assembly of retaining rings Installation of retaining rings Baking Installation of collector rings, collector fan Assembly of terminal bolts and sealing parts Leak test Installation of balance ring Hi-pot test and run-out check Caulking of wedges Painting Final check [Figure 11] shows the scope of replacement parts and reusable parts in. Most components were replaced except for the Rotor Shaft and Coil that do not cause a problem when reused.

12 <Figure 11. Out Line of Generator Rotor> 2-6. Rotor Coil Re-insulation and Break Down during Rotor Hi-pot Test The insulation material of the power generator Rotor coil should have sufficient mechanical strength and heat resistance against high centrifugal force and heating of adjoining coil. If the insulation material is burnt or damaged, the deposit of copper dust increases that was produced from the wear of contact surfaces between copper conductors and such problems occur as layer short and Rotor ground fault. Because copper dust is produced from relative motion(friction) of upper/lower coils in a slot during Turning operation, inspection and counter measure of the copper dust shall be conducted for the power generator which is relatively high in the number of Turning operations and start/stop. <Figure 12. Copper Dust of Field Coil> A break down occurred during the high-pot test before removing the Rotor Coil from the power generator unit 2. The cause was the friction of slot armor and the coil moving up and down due to its weight during startup and stopping or turning gear operation. At this time, the corner (round portion) of the L -shaped slot armor structurally undergoes repeated bending and straightening, which results in fatigue of the slot armor. This repeated movement of the slot armor also cause friction against first layer coil producing copper dust, which aggravates the friction between coil and slot armor. The breakdown during the high-pot test of Unit 2 is believed to be from the damage of the slot armor due to this cause.

13 <Figure 13. Slot Armor & Coil Break Down > In addition, the rotor of the unit 2 generator was not equipped with Coil Layer Separator, which caused friction between coils and damaged surface with copper dust. <Figure 14. Damaged surface of the coil due to non-application of Layer Separator> To improve this problem, Layer Separator was applied between top and bottom of the coils, which reduced the generation of copper dust due to friction between coils. Deteriorated Slot Armors were completely replaced by brand-new components enhancing the reliability of the insulation. <Figure 15. Layer Separator application and Layer Brazing > Internal inspection revealed the Pole to Pole Connector Crack as in [Figure 16], so it was replaced by a brand-new part. It played a role in preventing failures like failure of power generator field grounding during this renovation.

14 <Figure 16. Pole to Pole Connector Crack and replacement > Besides, Glass Cloth Base Laminated Polyester material used for Turn Insulation minimized the friction with copper and reduced the generation of copper dust. Secondly, Spot Brazing was conducted between layers of field coil to prevent friction copper strip interfacing surface, and thirdly, Teflon coating was applied three times to insulation material between Slot Armor and Field Coil to reduce friction force between coils during Coil assembly. <Figure 17. Improved Slot Armor> 2-7. Retaining Ring Replacement (Apply New 18Mn-18Cr Material) Retaining Ring is a component installed at the Rotor and preventing breakaway of the coil from strong centrifugal force produced during operation; conventional, and Retaining Ring was made of 18Mn-5Cr material which is good in electrical and mechanical properties. However, 18Mn-5Cr steel has a high potential to cause Stress Corrosion Crack (SCC) due to corrosion and stress in humid and corrosive environment.

15 <Figure 18. Example of SCC Retaining Ring (18Mn-5Cr)> To prevent this SCC, the old Retaining Ring was replaced by 18Mn-18Cr Retaining Ring, which has low potential to cause SCC. <Figure 19. New Retaining Ring (18Mn-18Cr)> 2-8. Collector Ring Replacement Collector Ring supplies DC power supply to Field Coil of power generator Rotor via the Brush, so the surface should always make a good contact with the brush. The grooves on the Collector Ring surface are for brush contact and cooling; and if the depth of the grooves got decreased from long-term use, the Collector Ring itself should be replaced. In addition, when the Collector Ring was used for a long time, electrical wear occurs from the current and mechanical abrasion occurs from by brush friction; generally the electrical wear is bigger. This abrasion affects the supplied current density by causing the contact between brush uneven; and in severe case, causes problems like arc generation and increased brush consumption. Inspection of Collector Rings of Power Generator Unit 1 and 2 did not show wear above operational limit, but bad bumpy profile of the positive (+) surface, which can cause sparks during operation, so it was replaced to secure reliability.

16 <Figure 20. Collector ring abrasion > Table 4. Collector Ring abrasion of Power Generator Unit 1 and 2 Classification Limit Dimension P(+) mm Within 1.5mm N(-) mm 2-9. H 2 Seal Ring improvement (4 2 Segments) Hydrogen cooled power generator is equipped with a shaft Seal System to prevent air inflow into power generator and to prevent hydrogen gas leak. Attached afloat on the shaft of the Seal System is a Seal Ring which has bigger diameter than Journal by 0.1~0.2[mm] and prevent hydrogen leak. Production and maintenance of the Seal Ring of Power Generator Unit 1 and 2 is very difficult because it is composed of 4 segments. <Figure 21. H 2 Seal System>

17 As in [Figure 22], Seal Ring Type (4 2 Segments) and installation method (spring spring & bolts) were improved to improve maintainability and reduce power generator hydrogen consumption. <Figure 22. Seal Ring improvement (4 2 Segments)> Improvement Brush Holder Rigging Brush Holder Rigging is the equipment that supplies direct current to the field windings of the rotor by contacting carbon brush to the circumferential surface of the rotating collect ring attached to the rotor. It locates the brush to the center of the rotating collect ring and compresses the brush with a constant pressure spring. The old Box-Type Brush Holder was replaced with an improved Pin Type of excellent cooling effect during this renovation work, and the shared current margin was enhanced to 22% per one brush, enabling stable operation of the equipment. <Figure 23. Brush Holder improved (Box-Type Pin Type)>

18 2-11. Rotor High Speed Balancing EPRI data recommends conducting Rotor Balancing and Over Speed Test when re-winding is performed in the factory. The mass of the rotating body has changed due to the replacement of main components like Retaining Rings, All Wedges, Collect Rings, etc. except for the Coil during the Rotor re-winding of the power generator Unit 1 and 2. The change of mass like this causes high vibration of the rotor during normal operation, and impedes dynamic stability of the rotor, so High Speed Balancing was performed to remove these potential problems and secure the reliability of the rotor. <Figure 23. Rotor High Speed Balancing> 3. Conclusion The renovation work was conducted to prevent potential problems in the Stator and Rotor that may occur from 29 years of long-term operation of the power generator; to extend the service life of the generator; and to secure the reliability of the generator by applying new technologies and improving facilities. The result is that the reliability of the Stator was enhanced by Water Clip improvement to prevent water leak, upgrade of insulation class of the stator (B F class), and Stator coil Wedge System improvement. For the Rotor, potential breakdown was prevented through Layer Separator application and insulation improvement to prevent dust, collector ring replacement, and Seal Ring improvement. Especially, it contributed greatly to preventing unplanned stop to remove the potential cause of breakdown by timely maintenance owing to the identification of insulation damage during the high potential test of the Rotor of the Unit 2 generator.

19 <Figure 24. Finish of Generator Stator Renovation > The following is the summary of effects of the renovation of the Stator 1) Life extended of the generator stator The service life of the generator stator was extended by prevention of deterioration by temperature rise owing to the application of upgraded insulation (B F); and by replacement of water-absorbed coil with a brand-new coil. 2) Reliability enhanced by application of new technology 1 Improvement of Water Clip structure, reducing the number of Brazing Point (5 3). 2 Top Ripple Spring between Top Bar and Wedge of the stator coil. 3 Improved reliability through real time monitoring of the stator insulation during the operation by application of stator Partial Discharge System

20 <Figure 25. Finish of Generator Rotor Renovation > Finally, the following is the summary of effects of the renovation of the Rotors. 1) Life extended of the generator rotor Replacement of degraded winding insulation and damaged Slot Armor 2) Reliability enhanced by application of new technology 1 Copper dust reduced through the application of the Layer Separator 2 Stress Corrosion Crack prevented by replacing Retaining Ring with 18Mn-18Cr 3 Increase shared current per unit brush by improving Brush Holder to Pin-Type 4 Hydrogen leak prevention and maintainability improved by improving Seal Ring 5 Mechanical, electrical wholesomeness of the rotor confirmed by high-speed balancing of the rotor 6 Improved reliability through real time monitoring of interlayer shortage of the rotor by application of rotor Flux Probe System Samcheonpo Thermal Power Site Division of KOSEP (Korea South East Power Co., Ltd.) are trying incessantly to stably supply high-quality power, based on the successful maintenance practices of Stator and rotor of the generator Unit1&2, and is preparing for the Stator Renovation of the Power Generator Unit 3 which is aging in progress scheduled in the next preventive maintenance(overhaul).