Important Aspects of Small Hydro Development. Presentation by Jawahar Lal Khosa

Transcription

1 Important Aspects of Small Hydro Development Presentation by Jawahar Lal Khosa

2 Intakes for Hydro Power station 2

3 Intakes for Hydro Power station Problems: Excessive trash clogging intake screen and trashracks. Erosion of river bed, downstream of gates. 3

4 Tetrapods as energy dissipaters A tetrapod is a fourlegged concrete structure, installed loosely on sea shores, as in Marine Drive, Mumbai, India Terapods have been installed downstream of gates to reduce erosion. 4

5 Run-of-the-river schemes Balancing Reservoirs Location wherever possible Can be near to forebay. Usable volume from say 1 hour to many hours of turbine operation, based on economic considerations. Gates provided, for outflow. 5

6 Reservoir at end of power canal For run-of-the-river projects: The canal is generally open, with tunnels and cut-and-cover at some locations (to suit topography and eliminate any risk of landslides. Photo shows the balancing reservoir combined with forebay. Intake gates and trashracks are provided. 6

7 Large scale Balancing Reservoir Beas-Sutlej Link: - Tunnels and canal - Balancing Reservoir - Surge Tank - Valve House - Penstocks - Power Station (6x165 MW) - Tailrace connected to Sutlej river 7

8 Large scale Balancing Reservoir 3.7 million cu.m volume Balancing Reservoir at Sunder Nagar, Himachal Pradesh, India Feeds 6x165 MW, 282 m rated head Francis Turbines, at Dehar. 8

9 Forebay Pond like civil structure at end of power canal. Effective capacity for few minutes, as a minimum, to ensure any fluctuations in flow due to the operation of hydro-sets. Silt can settle at the bottom. Drainage sump should be provided, with gravity drainage. Water should not spill over the banks, in case of trip of all units in the power station. Includes Gates, Trash racks, connection to penstocks and instrumentation. 9

10 Canal & Forebay for Tummel Bridge Power Station, Scotland, UK Approx. 4 mile long canal, from main reservoir, gates at intake of canal, ending at forebay. Level at forebay drops by approx. 1.2 m wrt reservoir level, to feed 2x17 MW Turbines (Originally 20 MW). 10

11 Gates, Trash Racks & Small Forebay Gates are electric-hoist operated at this installation. Trash racks are located immediately d/s of the gates. Small forebay is formed between location of trash racks and start of penstock (Rectangular water passage) feeding 2x15 MW Kaplan Turbines. Spiral casing at this installation is in concrete for 15 m head operation. 11

12 Trash Rack Head loss across trash rack is dependent on the shape of the bar. USBR general practice is to limit the head loss to 1/8*(V^2)/(2*g) 12

13 Trash Rack With low-head plants with integral intakes, the trashrack may provide significant head losses, and all attempts to reduce it should be implemented. Structurally the trash rack assembly should withstand full differential pressure, in case of total choking. Trash racks have collapsed, resulting in forced shut down of the power station extending to even 2 months, requiring full repair or replacement. Larger area rack should be provided, where blockages are expected to be frequent. Trash removal and trashrack cleaning system should be automated, to reduce running costs. 13

14 Gates Gates are kept in fully open position, during operation of a hydroelectric scheme. Vertical lift gates generally preferred, and are suspended and operated by electric-operated hoist. Designed to close under emergency conditions, to protect downstream civil works and hydro-sets. Head losses due to eddy flow currents in gate slots must be controlled. Scouring of gate slots: By compressed air, with embedded pipes and nozzles. 14

15 Pergau Hydroelectric scheme, Malaysia Hydraulic Trip & Emergency closing of intake Gates Underground Power Station. Hydraulic Trip due to: Rupture of Penstock or Spiral casing, Flooding of Valve floor and/or turbine floor. Action: All Units trip. Spherical Valves close and Intake Gates close. 3 intake gates at inlet to head race tunnel. Closing in 30 seconds for gate stroke of 7 m, and operated by hydraulic cylinders. 15

16 Vortex Prevention For low head installations and bulb type units: Uniform flow conditions must be provided at the inlet to turbine spiral casing. If vortex is present, the intake is exposed to a swirling flow, which may be carried into the penstock. Vorticity affects flow uniformity and reduces capacity at intakes. Loss of turbine efficiency and cavitation may result, particularly for short penstocks. If air content is high, the operation of turbine will be rough, with high vibrations. 16

17 Vortex Prevention Vorticity can be prevented By: - Adequate submergence at intake. - Improvements to approach conditions - Ant-vortex devices Submergence Deep enough approach minimises the surface velocity and the potential for vortex formation. Model studies are often conducted, to check the intake performance, for flow stability. 17

18 Vortex Prevention Gordon (1970) formula, for minimum submergence: S=k*V*SQRT(D) S: Submergence above top of intake gate, m V: Velocity at intake gate, m/s D: Height if intake gate, m k: coefficient 0.3 for symmetrical approach flow, 04 for unsymmetrical approach flow. 18

19 Atmospheric Air availability at inlet to Penstocks Penstocks, d/s of intake gates, should never be subjected to vacuum conditions, resulting from fast loading of the generating-sets. Therefore atmospheric air entry should be provided. Some methods are: - Vertical rectangular opening in the intake civil structure. - Pipe embedded in intake concrete structure. - Kinetic air valves 19

20 Intake - Experience Narrow vertical slots, for gates or air entry act as mini surge tanks. Under certain conditions, the water level surges upwards, and water splashes out. Vertical cylindrical intake, located within the reservoir Vortex formed approx. 2 m above minimum reservoir water level, for generation, causing air entry into the penstock, and bad turbine operation. Action: Minimum water level increased by approx. 3 m, on a 90 m rated head turbines. 20

21 21

22 Surge Tanks

23 Surge Tanks Old thumb rule: If L/H 6, then provide either surge tank or pressure relief valve. Old practice was to limit speed rise and pressure rise to low values. New practice: Higher speed rise and pressure rise allowed. Thereby no need of either surge tank or pressure relief valves, if possible. Long Headrace tunnel Surge tank usually required. 23

24 Surge Tanks If surge tank is required, its location is generally dictated by local topography and geological conditions. Surge tank can be built as a civil structure on a stable ground or can be fabricated as a non-spilling vertical pipe. Both options are good for small hydro. 24

25 Tongland Surge Tank, Scotland, UK Tongland, Scotland, UK Surge tank & Power Station 25

26 Tongland Surge Tank, Scotland, UK Steel Surge Tank, with integral Spillway. Governed by economical design Loss of water through spillway (has its independent connection, to tailrace) 26

27 Surge Tanks Surge Tank for Botocon Small Hydro Power Station, Philippines, Installed in

28 Surge Tanks Surge tank shortens the distance between Turbine and free-water surface (provided by surge tank). Pressure transient (water-hammer) effect is reduced. Water-hammer effect in the tunnel, from intake to surge tank, are reduced to very minimum, and can be neglected. Surge tank acts as a relief opening Partial or whole water mass is diverted, due to load rejection, or fast unloading. Water level in surge tank rises. For load acceptance, the surge tank acts as a reservoir, and water level falls, to provide necessary flow to Turbine. 28

29 Surge Tank Performance, for Load Rejection At any steady-state operation of Turbine, the water level in surge tank is lower than the reservoir, taking into account the head loss in tunnel (conduit). Water level increases, in surge tank, due to load rejection or trip. Turbine cuts-off flow in a very short time. The level oscillations are a characteristics of surge tank, and reduces over a time period for stable performance. 29

30 Surge Tank performance for Load Acceptance For Load acceptance, water level in surge tank reduces. Level oscillations result, which reduce over a period of time, for stable operation. Surge tanks often cater for the operation of two or more turbines, and the worst cases are analysed, to ensure satisfactory operation of surge tank. 30

31 Clunie Surge Tank, Scotland, UK The surge tank was excavated in the Highland Schist and has reinforced concrete lining. The surge tank is located over tunnel bend and is connected by 6 ports. 31

32 Clunie Surge Tank & Power station Owner: SSE (Scottish & Southern Energy), Scotland, UK 32

33 Clunie Hydro Power station, Scotland, UK Head: 51 m Station Output: 61 MW Commissioned:

34 Dehar 6x165 MW Hydro Power Station, BBMB, India Surge tank is of differential type. Caters for the operation of 6 Francis Turbines, with rated flow of 6x67.3 cu.m/s. Height: m Diameter: m Riser shaft: 7.62 m dia. Construction: reinforced concerete Performance Test: Fast 150 MW load acceptance of one unit, in 6 secs, resulted in 23% pressure drop. 34

35 Surge Tanks Important factors Surge tank should be located close to power station. Surge tank should be of sufficient height to prevent overflow. Otherwise provide expansion gallery, overflow area at top, additional surge tank, or overflow spillway. Surge tank should be low enough, to prevent complete drain of water, and prevent air to enter the penstock(s). Cross sectional area of surge tank should be optimised, to ensure stability. 35

36 Surge Tanks Important factors Increased oscillations if governing is based on frequency vs. power relationship. Better performance with frequency vs. gate (Guide vanes) relationship. Above conditions are dictated by Permanent speed droop, and its value should be optimised. Loading and unloading rates should be optimised. Start of units and loading should be checked with surge tank level, to avoid super-surge. 36

37 Grid-Connected & Isolated Load Operation

38 Grid-connected Operation Thumb-rule for a grid capacity (Pg), related to unit capacity (Pu): Pg 25 x Pu The frequency of the grid is dictated to a lesser extent by the generating unit, to be connected, under above condition. Under this condition, once the generating set is synchronised, the speed of the generator-turbine is dictated by the grid frequency. Load can be increased or decreased through governor control or PLC, for any steady-state operation. 38

39 Grid Characteristics Stiffness (MW/Hz) characteristic may be low for small grids. Examples: 1. In a grid of 45 MW with largest hydro unit of 11 MW, stiffness: approx. 3-4 MW/Hz. 2. Grid capacity: Approx. 60 MW (Diesel-sets & Gas turbines). 25 MW Hydro unit was synchronised, with stiffness: 4 MW/Hz. The hydro unit was too big and controlled the grid frequency. Matching of demand and supply forced slow loading/unloading of hydro unit. Higher MW/Hz is better for the Grid and Generating-sets. 39

40 Grid Characteristics Example for large grid: UK grid of approx. 50,000 MW. 150 MW was loaded on Dinorwig 300 MW pump/turbine unit in 5 seconds approx. The frequency increased from 50 Hz to Hz, and after few seconds the frequency returned back to 50 Hz, due to automatic corrective action (through Governors)by other generating power stations. Grid dynamic characteristic = 150/0.03 = 5000 MW/Hz Extremely stable Grid 40

41 Grid-connected Operation UK GRID: For small hydro - Regulation capability is not required up to 20 MW, as per Grid Code. Units above 20 MW should have regulation capability, therefore use of Governors. PLC control, to enable synchronising, followed by loading to rated or best efficiency load can be adopted. Therefore Governor is not required. Power dead band, say ±1-2% helps in stable operation. Provide frequency dead band, for stable operation. Grid-connected operation may be specified by Clients/Consultants. 41

42 Grid-connected Operation In many Grids (Generating sets + transmission lines + Consumers) balancing (generation vs. demand) is controlled artificially by manual or automatic load shedding. Transmission lines are tripped at specified low frequencies. Above method prevents collapse of the grid. Tripping of large generation, can result in total blackout of the grid. Examples: a. Malaysia (main land) b. North zone, India c. USA-Canada: North-East (Famous black-out, 1965). 42

43 Why Total Black-out? Failure (Tripping) of large generating power station or tie-line creates cascade tripping of transmission lines and generating sets, due to automatic operation of trip devices. Over-loading of transmission lines is prevented. High cost to Industry and residents. Robust monitoring may help. Response from fast acting hydro power stations will help. Example: Pergau (4x150 MW) hydro power station has capability to load MW in 12 secs., to prevent total black-out of Malaysian grid. 43

44 Grid-connected Operation Governors PID control, Frequency dead band, Load dead band, Permanent speed droop based on Power vs. Frequency or Gate vs. Frequency relationship. Optimise operating (opening and closing) times. Fast loading results in pressure drop and frequency drop, therefore loading has limitations. DEFINE acceptable frequency variations, say ±5%, as an example, for loading/unloading of a Unit. 44

45 Isolated Operation Danger: Collapse of Unit speed (Frequency) Example: 15 MW Pelton Unit. Isolated grid load: 4.5 MW (80% resistive, all switches ON.). Unit started and synchronised on dead-bus, resulting in race between needle opening vs. drop in frequency. Unit tripped, through mechanical governor at lowspeed (Approx. 37 Hz). Solution: Unit was operated at 52 Hz, and synchronised on dead-bus. No tripping resulted. Frequency dropped then recovered to 50 Hz. 45

46 Isolated Operation Example: First-time synchronising of 4.5 MW Francis turbine- Generator set, with 1 MW resistance (heater) at receiving station. Solution: Unit was operated at 52 Hz, and synchronised on dead-bus. Frequency dropped, less than 2 Hz, then recovered to 50 Hz. 46

47 Isolated Operation Example: Incident at Power Stations, in Andean Mountains, South America Several Impulse unit power plants supply a city and a few small industrial loads, in high Andean mountains of South America. The system was not stable when subjected to a major load change, such as that caused by loss of generation at one of the plants due to a fault. However, the system appeared to be operating correctly. Checks made in 1968, revealed that when one of the power plant drops off the system, that the system operating problems became apparent. On loss of generation, the whole system shuts down because under frequency relays trip out at sub-stations. ½ to 2 hours required for re-connection to grid. 47

48 Isolated Operation - Example Example: North-Western Canada. 2X5.6 Kaplan units added to existing diesel generation. Local hospital converted boiler from oil to electricity. Whenever the 4 MW boiler started up, the sudden load demand caused frequency to drop, and the underfrequency relay initiated trip (Disconnect the generators from the town), resulting in temporary blackout. Solution: Boiler controls changed, so that the load was added in 1 MW steps, with time delay between steps. Higher generator inertia, at project stage can be considered for such incidents. 48

49 Isolated Operation Optimise Generator Inertia (Impacts cost of equipment) Optimise water time constant, Tw Tw=(L*V/g*H) Define allowed speed (Frequency) variation. Load connections: Many power lines controlled from power station or from receiving station. For example each line load is 0.1 to 0.2 pu. Each line switched on or off to suit loading and unloading of a Unit. 49

50 Isolated Operation Speed Variation (N2/Ns)^2=[1-T/Tm*(2P2-(P1+P2)(1-hw)^1.5] 1 Output corresponding to P P, kw Step Loading kw Initial Turbine Output P Final Turbine Output P Operating time for P2 T Pressure Drop, due to fast loading of P2 hw Machine Time Constant at P Tm Machine time constant at P2 Tmp Rated speed Ns, pu N2^2 N2^ Speed at end of load change N Speed Variation N2-Ns Rated Frequency Hz Speed Variation, Hz Hz

51 Initial Load = 0, Opening time for 100% stroke = 15 secs., Pressure Rise = 0 (assumed) 51

52 Data of USA Small Hydro Projects S. No: Name of Project Capacity Tw Tm Tw/Tm MW secs sec 1 North Hartland Mill 'C' Cadyville Pembroke Conemaugh Garland canal West Delaware Tunnel Sepulveda - Impulse Turbine Note: Grid connected Power Stations 52

53 Sepulveda Pelton Turbine, USA Needle closing time is 4950 seconds 53

54 Pressure Relief Valve

55 Pressure Relief Valves Long Penstock results in high L/H ratio, and results in high pressure rise. High pressure rise may not be acceptable. Solution: Provide Surge Tank or Pressure Relief Valve. A Relief Valve is designed to discharge full or partial turbine rated discharge, thereby reducing the pressure rise, following load rejection of trip, to a negligible value. Pressure Relief Valve does not improve the stability or response of a Unit. Pressure Relief Valve is connected to the spiral casing of a Francis Turbine. Water wasting device. 55

56 Pressure Relief Valves Servomotor Runner Pressure Relief Valve Spiral Casing Draft Tube Design based on Model Tests 56

57 Pressure Relief Valve - Operation Guide Vanes close rapidly due to Load rejection or trip. Simultaneously the Pressure Relief Valve opens, to a corresponding stroke dictated by guide vanes. Pressure Relief Valve (PRV) releases water to tailrace. So Qturbine - Qprv = almost zero. Therefore no pressure rise. Practically Qprv cannot be matched with Qturbine. Therefore a small pressure rise results, which is acceptable. 57

58 Pressure Relief Valve - Operation Guide vanes close fully. From this instance, the PRV starts closing, slowly, to fully closed position. Let us assume guide vane closing time = 3 secs. PRV will open in 3 secs., then close in say 50 secs. Slow closing of PRV ensures that undue pressure rise does not result. Reliability: PRV must operate 58

59 Pressure Relief Valve Operation using combined Guide vane & PRV Servomotors 59

60 Pressure Relief Valve Operation using combined Guide vane & PRV Servomotors 60

61 Pressure Relief Valve Operation using separate Guide vane & PRV Servomotors 61

62 10.09 MW Load Trip Pressure Relief Valve Performance 62

63 Pressure Relief Valve Poppet type PRV Discharge through submerged Energy Dissipator Piping to tailrace. Flow water mass Design based on Model Tests 63

64 Pressure Relief Valve Perforated Cylinder type Totally submerged in water. Discharge to tailrace (parallel to draft tube discharge) Hydraulic servomotor operated perforated cylinder. Headloss in each orifice (taper design). Cavitation occurs away from cylinder, in water mass. Residual head may correspond to approx. 20 m head. 64

65 Pressure Relief Valve - Reliability For indirect-operated PRV s Additional safety features like: - Rupture disc on penstock. - Slow closing of guide vanes, initiated by high pressure switch (located on penstock). 65

66 Thank you Jawahar Lal Khosa Chief Technical Mentor B Fouress (P) Limited Plot No: 7 KIADB Industrial Area HOSKOTE Bengaluru, Karnataka, India Tel: