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1 Generator, Unit Hydrogen, Seal Oil, Stator Coolant, Demin Water Plant Operations Training Huntly Power Station: Units 1-4 Assessment ID: 8014, 8012, 8009, 8015, 8016 NZQA Unit Standard: Assessment Marking Schedule /107 marks

2 Questions 1. List 4 possible causes of heat generation within the generator. (4 marks) Friction bearings, seals etc Windage air circulation due to rotating components Copper losses resistance in the windings Iron losses eddy currents, hysteresis 2. Explain the terms hot and cold gas temperature. (2 marks) The hot gas temperature is the temperature of the hydrogen gas in the generator before it is cooled by the hydrogen coolers. The cold gas temperature is the temperature of the hydrogen gas in the generator after it is cooled by the hydrogen coolers. 3. What is the normal operating pressure of hydrogen in the generator? (1 mark) a) 3.5 bar b) 3.0 bar c) 2.5 bar d) 4.0 bar 4. What is the normal operating pressure at the seals of the seal oil? (1 mark) a) 3.5 bar b) 3.0 bar c) 2.5 bar d) 4.0 bar 5. What system directly cools the hydrogen in the generator? (1 mark) a) Auxiliary cooling water b) Demin water c) Stator coolant d) Cooling water 6. What is the explosive range for hydrogen? (1 mark) a) 20% - 75% b) 4% - 95% c) 20% - 95% d) 4% - 75% 8014 Written Assessment Marking Schedule.doc Genesis Energy 2

3 7. Seal Oil / Hydrogen differential pressure is normally? (1 mark) a) 1.0 bar b) 0.6 bar c) 0.5 bar d) 1.5 bar 8. What are the advantages of using hydrogen as the cooling medium in a generator? (5 marks) Low windage losses. Improved heat transfer coefficient. Hydrogen has excellent thermal conductivity. Using hydrogen as the coolant also reduces the possibility that corrosive acids will form due to corona in the generator. With the use of air, and so oxygen, nitric acid can form which will attack the insulation. With no oxygen being present the fire risk is eliminated. Gas tight enclosure ensures the no dust or air can get in. Quieter running Please assign 1 mark for each correct answer above up to a maximum of 5 marks 9. What are the disadvantages of using hydrogen? (3 marks) Risk of explosion. A CO2 system has to be installed so that it can be used to displace the hydrogen in the casing before the casing is opened for inspection or for maintenance. Additional plant is necessary. Seal oil to maintain a gas tight enclosure, hydrogen coolers, cooling water pump, alarms and instrumentation Stator frame built to withstand pressure and possible explosion Please assign 1 mark for each correct answer above up to a maximum of 3 marks 10. When purging the generator from hydrogen to CO 2, where is the CO 2 admitted and why? (2 marks) CO2 is admitted in the bottom of the alternator because it is heavier than the H 2 and will push the H 2 up and out of the alternator 11. What pressure is the generator depressurised to during a de-gas, before purging to CO 2 commences. Why is this pressure maintained during the purge process? (2 marks) The generator is depressurized to 0.5 Bar for a degass. This pressure is maintained during the purge process to ensure positive pressure is maintained in the generator hence not allowing any air from outside to mix with the gas in the generator. 12. What is the function of the electric generator heaters, and when are they turned on? (1 mark) 8014 Written Assessment Marking Schedule.doc Genesis Energy 3

4 The function of the generator electric heaters is to stop the formation of moisture in the generator when the unit is shutdown. 13. Why is it important to maintain hydrogen purity? (1 mark) It is important to maintain hydrogen purity so that the hydrogen in the generator has no change of entering its explosive range. 14. What is the function of the hydrogen dryer? (1 mark) The function of the hydrogen drier is to keep the hydrogen in the generator moisture free. The reason for this is that if moisture is present in the hydrogen then insulation and corrosion problems will occur in the generator windings. 15. What is the purpose of the lute on the side of the hydrogen dryer? (1 mark) The purpose of the lute on the side of the hydrogen dryer control cabinet is that it acts as a water seal for the cabinet which is pressurised with air to stop the ingress of any flammable gas. 16. Why is the auto hydrogen make-up system not normally used? (1 mark) The auto hydrogen system is not used because if the unit develops a leak the amount of H2 being made up to the unit is not known and could go unnoticed Written Assessment Marking Schedule.doc Genesis Energy 4

5 17. Referring to the diagram of the seal oil system (Figure 1); (15 marks) a) Discuss overall system function and operation. b) Name all key components (Use the encircled alphabetic reference callouts in your answer to refer to system features) A B C D E F G H J K L M Figure 1 A.C. Seal Oil pump A.C. Seal Oil pump Pressure Control valve D.C. Seal Oil pump Seal Oil Loop Seal Backup supply from Lub Oil system Seal Oil Cooler Filters Bearing 7 hydrogen seal Bearing 8 hydrogen seal Generator Detraining Chamber Gas Trap Oil is drawn from the seal tank by the A.C. seal oil pump (A) and passed into the system through non-return valve SO6. Pressure control valve SO52 (B) on the outlet side of the pump maintains the discharge pressure at approx. 8.6 bar. The oil then flows through isolating valve SO8, differential pressure throttle control valve SO9, isolating valve SO14, the seal oil cooler (F) and is discharged through valve 8014 Written Assessment Marking Schedule.doc Genesis Energy 5

6 SO15 into one of the two filters 'A' or 'B' (G), (service and standby), which are situated downstream of the cooler. A relief valve SO27 operates as a bypass should the pressure drop across the cooler and filter become excessive. From here the oil is passed to the hydrogen seals (H, J). After passing through the hydrogen seals most of the oil discharges outwards from the seals into separation chambers. This oil then flows through sight glasses and is returned to the seal oil tank via a loop seal chamber (D) which is provided with a vapour extractor that evacuates any gas entrained in the oil. Oil which discharges inwards towards the hydrogen side of the seals is led through sight flow glasses into the hydrogen detraining chambers (L). Hydrogen released from the oil in the detraining chambers is returned to the stator casing. An internal drain line is led from the bottom of each detraining chamber through float operated valves into the hydrogen gas trap (M) where any hydrogen left in the oil vents to atmosphere. The oil then flows through the loop seal chamber (D) into the seal oil tank Written Assessment Marking Schedule.doc Genesis Energy 6

7 18. Referring to the diagram of the Seal Oil Gas detraining chambers (Figure 2), briefly describe its operation. (5 marks) Figure 2 A chamber is provided for each hydrogen seal. This serves as a vessel where any hydrogen entrained in the oil, which has drained from the seal, is released and returned to the generator stator casing. The oil level in this tank is normally controlled by the float. The pressure inside the chamber (frame pressure) forces oil out to the gas trap as the level in the chamber rises due to the continual drainage from the gas side returns of the hydrogen seal. There is a back up to this float valve in the form of an air operated, solenoid controlled by-pass valve which opens if the high level mobrey switch is operated and will close when the low level mobrey switch is operated. This prevents oil backing up into the frame in the event that the float valve jams shut. The high and low mobrey switches also initiate high and low level alarms when they are operated Written Assessment Marking Schedule.doc Genesis Energy 7

8 19. What is the purpose of the seal oil system? (1 mark) The purpose of the seal oil system is to provide oil to the generator hydrogen seals in order to prevent any hydrogen escaping from the generator at a controlled pressure and temperature under all operating conditions. 20. What does the oil flow from the seal sides tell you and why is it important. State the implications of too little or too much flow in this sight glass. (4 marks) The flow of oil from the seal sides tells you how much seal oil is passing through the seal to the hydrogen side of the generator. It is essential that the correct amount (3mm diameter trickle max) does flow past the sight glass so that you can be sure the seal is operating correctly and that there is also sufficient lubrication. If there is too little or no flow past the sight glass hydrogen may leak out and the seal may begin to overheat. If this occurs first check that the seal oil to hydrogen diff pressure is not low as this will put more hydrogen pressure on the seal and reduce the flow of seal oil back into the hydrogen side of the seal. If there is too much seal oil flow past the sight glass the seals may be starting to fail or have failed. Also an increase in the seal oil to hydrogen diff pressure will cause more seal oil to pass to the hydrogen side of the seal. 21. Explain the seal oil back-up Oil supply and how it works. (2 marks) The seal oil back up supply is taken from the lubricating oil discharge of the shaft driven oil pump. If the discharge pressure from the AC seal oil pump goes low (6.5 Bar) the back up supply control valve (SO28) opens supplying seal oil to the system. 22. When does the D.C. seal pump cut-in? (1 mark) The Seal Oil DC pump cuts in when selected to remote on the switch gear and auto on the CCR panel, if the seal oil / H2 diff drops below 0.34 Bar 23. What are the implications of the D.C. seal oil pump running for an extended time? (2 marks) The implications of the DC seal oil pump running for an extended time are that when the DC pump is running the seal oil is supplied directly from the pump to the seals by passing the cooler and filters, this means the oil is not cooled or filtered which could in turn overheat the seal or allow impurities to come in contact with the seal faces. 24. Explain the importance of the differential between seal oil pressure and hydrogen pressure. (1 mark) The important of the differential between the seal oil pressure and the hydrogen pressure is that the seal oil pressure needs to be higher to stop any hydrogen escaping from the generator Written Assessment Marking Schedule.doc Genesis Energy 8

9 25. What is the Stator Coolant normal flow? (1 mark) a) 16.5 litre/minute b) 18.6 lites/minute c) 20 gallons/minute d) 5 gallons/minute 26. What is the Stator Coolant flow value that will generate the alarm Stator Coolant Flow Low? (1 mark) a) 14.2 litres/minute b) 14.2 gallons/minute c) 16.1 litres/minute d) 20 litres/minute 27. What is the Stator Coolant flow value that will start the standby pump? (1 mark) a) 14.2 litres/minute b) 14.2 gallons/minute c) 16.1 litres/minute d) 20 litres/minute 28. What is the trip associated with Stator Coolant flow? (1 mark) a) 5.2 litres/minute b) 16.2 gallons/minute c) 8.1 litres/minute d) 14.0 litres/minute 8014 Written Assessment Marking Schedule.doc Genesis Energy 9

10 29. Referring to the diagram of the stator coolant system (Figure 3). (14 marks) e) Discuss overall system function and operation f) Name all key components g) Describe in detail the function and purpose of all labeled equipment items. (Use the encircled alphabetic reference callouts in your answer to refer to system features) A B C D E F G H Figure 3 Coarse strainer Fine strainer Demineralisation plant Flow measurement Stator Coolant Head tank Supply from WTP Gas Alarm and Automatic release chamber Gas Detraining chamber 8014 Written Assessment Marking Schedule.doc Genesis Energy 10

11 The system is a closed circuit maintained under a constant static pressure by a make-up header tank with an internal float operated valve. The header tank (E) is supplied from the demineralised water system (F) through a strainer at the tank inlet. Two 100% duty a.c. motor driven pumps service and standby are provided to circulate the coolant around the system, via the stator coolant coolers and coolant strainers, through the resistance columns at the exciter end of the casing to the generator stator windings. Having passed through the stator windings the coolant is discharged through the resistance columns at the turbine end of the casing to the suction side of the pump via the gas detraining chamber. A branch pipe is provided in the line between the stator coolant coolers and the coolant strainers to enable approximately 3% of the flow to be passed through the demineralisation plant (C), which maintains purity of the coolant. Equipment is provided to indicate the presence of gas in the system and to allow it to be released via the gas alarm and automatic release chamber (G). Gas release pipes are run from various parts of the system to the connecting pipe between the gas detraining chamber and the gas alarm and automatic release chamber. Any gas entrained in the coolant which passes through the gas detraining chamber (H) is released into the gas alarm and automatic release chamber. The gas is then automatically discharged from the release chamber through a solenoid operated valve and an orifice into a hydrogen trap. From the hydrogen trap the gas is vented to atmosphere. Three flow transmitters are fitted in the outlet pipework at the turbine end of the stator winding. These transmitters are set to sequentially start the stator coolant pumps. Further contacts are provided to initiate a unit trip on coolant flow failure and a 'Coolant Flow Failure' alarm. To prevent the flow transmitters causing a unit trip in the event of one becoming unserviceable, two out of the three transmitters must register low flow before the tripping sequence is initiated. 30. What types of coolers are used on the stator coolant? (1 mark) The types of cooler used on the stator system are 100% capacity plate type heat exchangers. 31. Identify which strainers can be bypassed and for what reason. (1 mark) The strainer that can be bypassed is the fine strainer so that it can be cleaned with the plant in service. 32. Explain why the stator coolant system operates at a slightly lower pressure than the hydrogen system. (1 mark) The Stator coolant system operates at a slightly lower pressure than the hydrogen system so that in the event of a leak in the system the hydrogen would leak into the stator coolant Written Assessment Marking Schedule.doc Genesis Energy 11

12 33. Referring to the diagram of the Stator Coolant gas release chamber (Figure 4), describe operation including alarms and operating timers by completing the sentences provided. (6 marks) Figure 4 The Gas Release Chamber has three mobrey float switches which are operated as the stator coolant level lowers (due to gas collecting). The mobreys control the automatic gas release and initiate alarms as follows. When the top level switch is operated a 20 minute timer is started. As level continues to drop due to the gas leak, one of two situations will occur. Either a) The timer times out before middle float switch is reached in which case the gas will be released and a gas in coolant alarm initiated b) Or if middle float is reached within the timer period and gas in coolant and gas in coolant excessive alarms will be initiated and gas will be released. (If the "gas in coolant excessive" alarm is initiated it means that more than 50m³/24 hours is leaking and an inspection should be carried out). If the bottom Mobrey switch is operated then the Gas in coolant alarm inoperative will be initiated which means either:- a) The solenoid valve has not opened. b) Or the gas leakage is too much for the solenoid valve to cope with. The solenoid valve shuts when the level rises above the top Mobrey switch Written Assessment Marking Schedule.doc Genesis Energy 12

13 34. Explain the implications of the following alarms:- a) Stator coolant conductivity High (1 mark) Because the stator coolant comes in contact with the electrical connects in the generator the water must be pure so that it dos not conduct electricity. Alarm 600 µs/m and generator unloaded if 1000 µs/m b) Gas in coolant (1 mark) Means the system has a small leak and should be monitored c) Gas in coolant excessive (1 mark) Means the system has a leak of 50 cubic metres / 24 hours and inspection should be carried out. d) Gas in coolant alarm inoperative (1 mark) Leak is too much for the gas release chamber to cope with. e) Stator coolant make-up tank level low. (1 mark) The tank has a low level and if not topped up the system will not maintain its head causing loss of coolant to the system (system has a leak.) 35. What unit trip is associated with the stator coolant make-up tank? (1 mark) The unit trip associated with the stator coolant make up tank is a high level trip on the tank Written Assessment Marking Schedule.doc Genesis Energy 13

14 36. Referring to the diagram of the Generator Demin system (Figure 5); (13 marks) a) Discuss overall system function and operation b) Name all key components c) Describe the function and purpose of all labeled equipment items. (Use the encircled alphabetic reference callouts in your answer to refer to system features) A B C D E F G H J Figure 5 Demineralised Water Pumps Demineralised Water cooler Seal Oil Cooler Stator Coolant coolers Hydrogen dryer reactivation cooler Inlet manifold or waterbox Outlet manifold or waterbox Demin head tank Makeup from WTP 8014 Written Assessment Marking Schedule.doc Genesis Energy 14

15 The Demin water system is a closed circuit cooling water circuit which operates under a static head pressure provided by the demin water make-up/head tank (H). Demineralised water from the water treatment plant (J) and 1000ton tank common discharge supplies the make-up water to the demin make-up tank. The outlet from the make-up tank connects to the system at the demin pumps inlet. Water is discharged from the i/s pump through a NRV and discharge valve to the i/s cooler (B). From the cooler s common discharge pipe the demin water travels in parallel paths; through the seal oil cooler (C), through the i/s stator coolant cooler (D), through the hydrogen dryer reactivation cooler (E), and through the four hydrogen coolers via the hydrogen coolers inlet (F) and outlet (G) manifolds. The seal oil cooler, the stator coolant coolers and the demin coolers all have a bypass control valve for their systems temperature control. The four parallel paths meet back up at the demin pump s suction pipe. 37. Where does the make-up to demin system come from? (1 mark) The make up for the demin system comes from the RFW (WTP) system 38. Explain how the make-up tank level is controlled (1 mark) The level in the make up tank is controlled by a ball cock on the tank inlet Written Assessment Marking Schedule.doc Genesis Energy 15