CASE STUDY OF INGULA AND LIMA PUMPED STORAGE SCHEMES

Transcription

1 CASE STUDY OF INGULA AND LIMA PUMPED STORAGE SCHEMES Frans Louwinger Engineering Department (Hydro), Eskom Enterprises Division, PO Box 1091, Johannesburg, 2000 SOUTH AFRICA, ABSTRACT Due to the anticipated high growth in peak demands, Eskom has commenced the construction of the 1332MW Ingula Pumped Storage Scheme which is scheduled for completion in 2012 and has completed a feasibility study of a second scheme, namely Lima. This paper describes the key design criteria and the process that was followed in selecting these schemes. The Ingula scheme is located 23km north-east of Van Reenen, within the little Drakensberg mountain range. It consists of a CFRD upper dam and an RCC lower dam. The powerhouse complex consists of a 23m wide machine hall and transformer hall. The machine hall houses four pump-turbines which will generate at a rated head of 441m. The powerhouse complex is linked to the upper and lower reservoirs by underground waterways. The Lima scheme is located near Roossenekal in the Limpopo province and has a capacity of 1500 MW and a rated head of 636m. The upper reservoir is an off-channel cut-to-fill CFRD and the lower reservoir an earth fill embankment with clay core. Lima is earmarked for completion in KEYWORDS Pumped storage, pump-turbines, rated power, rated speed, rated head, hydraulic stability, headrace, tailrace, penstocks, draft tubes, powerhouse complex. 1. INTRODUCTION The prime objective in planning the capacity expansion of an electricity supply system is to produce a plan which enables anticipated future power and energy demands to be met as economically as possible. Such a plan is prepared by Eskom on a regular basis, called the Integrated Strategic Electricity Plan (ISEP). The ISEP consists of three groups. One group focuses on possible supply side options, the other on demand side management options and the third on demand forecasting. Various supply side options, such as coal fired, gas fired, nuclear (PBMR), imported power and pumped storage schemes, are considered in this plan; each fulfilling a different function in the total supply mix. Pumped storage schemes, for instance, play an important role in supplying power during peak demands, improving the power factor of the system, providing black start facility, and smoothing the load demand curve to be supplied by coal fired and traditional nuclear stations. Due to the anticipated high growth in peak demands, the ISEP has identified the need for a number of these schemes in the future, with the first one required for commissioning in In order to comply with the need for new pumped storage schemes, Eskom started a comprehensive search in the mid 1980 s. In this process, ninety potential sites were identified and systematically reduced to two. This paper discusses the process and selection criteria which were followed in filtering out the best sites and describes the selected Ingula scheme which is currently under construction. The second scheme, named Lima, is also introduced. 1

2 2. PUMPED STORAGE TECHNOLOGY A pumped storage scheme is, in effect, a large storage battery. Energy is stored in the form of water during off-peak periods and released during peak electricity demands. The scheme normally involves the construction of two adjacent water containment reservoirs, one at a significantly higher elevation than the other. During periods of low demand, normally overnight and over weekends, excess energy from the transmission grid is used to pump water from the lower reservoir to the higher reservoir. During periods of peak electricity demand, most usually during the early to mid morning and early evenings of weekdays, the process is reversed. The stored water is allowed to flow back to the lower reservoir through hydraulic turbines driving generators, thus enabling peak demand to be met. Normally, the pumping and generating modes at modern pumped storage plants use the same turbomachinery and generator-motor equipment; their direction of rotation simply being reversed depending on which role is required. Hence the term pump-turbines or reversible turbines. To avoid cavitation damage to these machines, particularly during the pumping process, they are located in deep underground caverns or, in some cases, deep shafts. The schemes investigated by Eskom are some meters below the low water level of the lower reservoir. Underground water tunnels, some 6-9 m in diameter, link the pumpturbines to the lower and upper reservoirs. A typical layout of this arrangement is illustrated below. Fig. 2.1 Diagrammatic section of the Drakensberg Pumped Storage Scheme A pumped storage plant is actually a net consumer of energy: it returns approximately 3 kilowatt-hours (kwh) of electricity for each 4 kwh required for pumping. However, it offers the following important benefits: The energy generated during peak periods has a higher monetary value than the energy required for pumping during off-peak periods. It permits continuous operation of the highest efficiency plants in the total system. Pumped storage generation, by its use of large quantities of off-peak power for the pumping process, enables Eskom to operate its coal fired power stations under much more stable loading conditions. It provides rapid and flexible response to system load changes. Very large load swings can typically be accommodated. The overall fuel consumption of the total system is reduced as thermal plants can operate more efficiently. 2

3 It could fulfil an important role in water transfer schemes. Due to its high pumping efficiency (between 85% and 90%) and the fact that off-peak energy is used during pumping, a pumped storage scheme could also be combined very effectively with a water transfer scheme, by pumping more water than required for generation of electricity. Both Drakensberg and Palmiet pumped storage schemes in fact fulfil this dual function. The Drakensberg pumped storage scheme, for instance, pumps an additional quantity of water for the Department of Water Affairs and Forestry (DWAF) from the lower located Tugela catchment to the higher located Sterkfontein dam, which supplements Vaal dam. 3. KEY DESIGN CRITERIA The following design criteria are essential in identifying possible pumped storage schemes. Reservoir capacity In the past, the live (operating) volume of the reservoirs was set to an equivalent of 28 hours continuous generation, starting with a full upper reservoir. This volume allows 50 hours of generation per week, resulting in a 30% utilisation factor. As the demand of electricity has become more peaky, a different approach is now followed. Firstly the storage volume is set to allow a minimum utilisation factor of 20% plus 4 hours of emergency operation. This results in an equivalent generating period of approximately 38 hours per week. Once this volume has been established, an optimisation study is carried out which compares the additional cost of a larger dam with the associated saving in system production cost. A larger storage volume increases the utilisation of the scheme and reduces the operating cost of other more expensive plant, but it cost more to construct. Waterways - Horizontal distance between upper and lower reservoir The distance between the upper and lower reservoir is normally limited to m. Long waterways are costly and result in higher hydraulic losses. - Lining system Reinforced concrete is normally used as tunnel linings the maximum internal water pressure under transient conditions is less than the minimum rock stress by a safety margin of 1,2. Steel linings are used minimum rock stresses are inadequate to resist the internal water pressures. - Design velocities Concrete lined tunnels Steel lined tunnels: 5,0 5,5 m/s 8,5 m/s Machine characteristics - Rated power Rated power is calculated as: P g = ρ. η g.q g.h g /1000. w P g = rated generating power in kw η = efficiency (approximately 0,90) ρ w = density of water in kg/m 3 3

4 g = gravitation constant in m/s 2 Q g = rated generating flow in m 3 /s H g = rated generating head - Pumping power P p = ρw. g.q p.h p /1000/η P p = pumping power in kw Q p = pumping flow in m 3 /s H p = pumping head The pumping power is approximately the same as the generating power. - Specific speed for pumping Each turbine is characterised by a constant, called the specific speed and is defined as: n q = n.q p 0.5.H p n = rated speed in r.p.m. n q = specific speed - Pump-turbine selection k=n q.h p 0.75 n and P p are selected so that k is maximum To ensure that practical machine configurations are selected, N q and H p are plotted and compared with previously used pump-turbines - Rated speed The rated synchronous speed can be calculated once the specific speed, rated power and head are known. n r = 50x60/n p (in a 50 Hz system) n r = rated speed n p = number of pairs of poles in the generator (i.e. 4,5,6,7 etc. - certain numbers such as 9, 11, 13 are not preferred) - Rated head The rated head is the head under which the pump-turbine can just produce rated power with the guide vanes fully opened. At lower heads than rated, the output will be lower than the rated generating power. The criteria used to select rated head is: H r = 0,3(H max H min ) + H min H r = rated head in m 4

5 H max = maximum generating head H min = minimum generating head - Turbine submergence In order to prevent cavitation in pumping mode, the turbine should be placed a certain depth (submergence) below the minimum downstream pressure. An initial submergence can be calculated as follows: H s = H b -H v -H.σ H s = submergence in m (negative value) H b = barometric pressure in m H v = vapour pressure in m σ can be derived from model tests but a number of empirical formulae may be used in the initial design stage. One of these formulae is: σ = 0,00195n 1,14 q n q = specific speed in pumping - Head variation Ratio of the maximum pumping head to the minimum generating head is limited to 1,2 to 1,3. The 1,3 limit is applicable for a pumping head of approximately 400 m and 1,2 for approximately 700 m. Hydraulic stability If the momentum of the water dominates the inertia of the machine, no equilibrium is reached between the driving force and the power output. In this case the flow has to be adjusted continuously in order to maintain a constant power output. To prevent this, a ratio of at least 3,0 is used between the mechanical and water starting time. The mechanical starting time is the time during which the machine increases its rotational speed from standstill to rated speed when the hydraulic driving force is applied, and is normally expressed in hydro technology as: t m 2 2 GD.n = P n = rated speed in r.p.m. P = power in kw 2 GD = inertia of rotating masses expressed in kgf. m 2. The water starting time is the time in which the water reaches the rated velocity from standstill, and is expressed as: m i l. vi i= 1 t w = g.h = length of i th waterway in m l i 5

6 v i = velocity in i th waterway in m/s g = gravitation constant in m/s 2 H = head in m m = number of waterways The summation is carried out for waterways between the free water surfaces (reservoirs and/or surge shaft/chamber). In general, the shorter the length between the free surfaces, the higher (better) the ratio. The inertia of the rotating masses may also be increased to improve the situation, but this will result in higher machine costs. 4. SITE SELECTION PROCESS 4.1 Site identification Eskom initiated an extensive programme in mid 1980 s in the search for possible pumped storage sites throughout South Africa. In order to identify as many sites as possible, no limitations (such as capacity and location from demand centres) were set in this search. This process resulted in the identification of ninety sites in total, of which - 21 sites in Mpumalanga and Limpopo Province; - 21 sites in KwaZulu-Natal; - 40 sites in the Eastern Cape; and - 8 sites in the Western Cape Apart from the key design criteria discussed under point 3 above, no other restrictions were followed in this study. As a result of this, potential capacities of the schemes varied from approximately 400MW to 2 000MW with heads ranging from 220m to 610m, and in the case of a few sites bordering Lesotho, as high as 1 050m. 4.2 Preliminary site selection Once the 90 sites were identified, more appropriate sites were filtered out using the following criteria: Potential capacity The increasing need for pumped storage schemes indicated that each scheme should be at least 1000 MW. Location from main demand and generating centres In order to prevent excessive transmission integration cost and transmission losses, it is important that a scheme is located in the vicinity of the generating centres and near the main national grid. Water availability Although a pumped storage scheme does not consume water (except for a small quantity lost to seepage and evaporation), the water source must be sufficient to allow priming within a period of 2 to 3 years. Head conditions The higher the head, the less flow is required to generate the same power, resulting in smaller waterways and reservoir sizes. An operating head criterion between 400m and 700m was decided on. No single stage pump-turbines with pumping heads in excess of 700m had been implemented at the time. Accessibility 6

7 The site must be reasonably accessible from existing infrastructure as access roads could contribute a substantial impact and cost to a scheme. Costs Capital cost of the scheme and transmission integration and Operation and Maintenance costs. Multi-purpose potential Perfect examples of multi-purpose facilities are the current Drakensberg and Palmiet pumped storage schemes. Multi-purpose potential has not only a cost advantage but could also have environmental benefits, especially dams can be shared. The following 7 top schemes were eventually selected: Impendle located in KwaZulu-Natal Ingula located in KwaZulu-Natal Mutale located in Limpopo Province Lima located in Limpopo province Strijdom located in Mpumalanga Waayhoek located in KwaZulu-Natal Hogsback located in the Eastern Cape Mutale Steelpoort Lima Strijdom Braamhoek Ingula Waayhoek Impendle Hogsback Fig. 4.1 Location of 7 potential sites 4.3 Final selection Pre-feasibility studies were successively conducted on these seven sites between 1987 and 1995 followed by a comparative study and ranking process. This resulted in the selection of three top schemes, namely Ingula, Lima and Mutale. 7

8 In order to confirm the ranking of the three selected sites in merit order, feasibility studies were conducted. These studies focused on Environmental Impact Assessments, more detailed geotechnical investigations supported by some 1200m of drilling, updating of the preliminary designs and re-costing of the schemes. Detailed geotechnical studies concluded that the Mutale scheme was unsuitable for pumped storage. The potential slope instability of the upper reservoir basin, the relatively high seismic risk and the occurrence of geological joints at the site, shifted the risk factor beyond acceptable limits. The Mutale scheme was therefore discarded. The Ingula scheme was selected to be developed first. The Basic Design started in 2004, followed by the Tender Design a year later. When the DWAF decided to develop the De Hoop Dam, approximately 20km downstream the proposed Lima site, a supplementary feasibility study was conducted to investigate the possibility of supplying water from this dam and moving the lower reservoir out of the Steelpoort River and closer to the escarpment. This resulted in a revised Lima scheme. 5. THE INGULA PUMPED STORAGE SCHEME 5.1 Location The Ingula pumped storage scheme is located about 23km north-east of Van Reenen, within the little Drakensberg mountain range. The upper reservoir site is located in the Free State and the lower in KwaZulu-Natal. The escarpment forms the border between these provinces. The distance between the upper and lower reservoirs is in the order of 6km and the elevation difference is approximately 470m. 5.2 Description General The scheme consists of the following basic components: An upper reservoir A lower reservoir An underground power house complex with access tunnels and associated waterways that link the two reservoirs 4 pump-turbines coupled directly with generator-motors Ancillary works that include building works, roads, transmission lines and temporary and permanent infrastructure The rated generating capacity is 1332MW and the energy storage capacity MWh (15,8 generating hours) Environmental The upper reservoir is located in a very sensitive environment. It consists of natural grasslands and wetlands extending downstream of the reservoir, both critical habitats for many species, including the critically threatened Whitewinged Flufftail. One of the most important and challenging mitigation measures is to release water from the dam at flow rates as near as possible to the inflow hydrograph. These releases should also be distributed evenly into the downstream wetland to avoid erosion. As all inflows into the upper reservoir have to be released, priming of the system and make-up water will be affected from inflows into the lower reservoir only. 8

9 5.2.3 Geology The area is underlain by sedimentary rocks of the Ecca and Beaufort Groups, which have been intruded by dolerites of the Karoo Dolerite Suite. The sedimentary rocks comprise mudrocks, claystones, siltstones and sandstones. In the vicinity of the machine and transformer halls, thermal effects have altered the mudrock into a very dark grey massive rock with high uniaxial compressive strengths. In view of this reasonable quality of rock it is not necessary to construct a separate valve hall as in the case of the Drakensberg Pumped Storage Scheme. The rather large span of 23m for the machine hall is possible, but systematic rock support, comprising pattern bolting with tensioned anchors, is required Machine characteristics Station capacity 1332 MW (4 Units of 333 MW each) Static heads Maximum 480,6m Minimum 450,0m Rated head 441m Pump-turbine synchronous speed 428,6 r.p.m. Maximum transient pressure upstream of turbine 726m Dams (a) Upper Dam The upper dam is a concrete faced rockfill type with a length of 810m and a maximum height of approximately 41m. An outlet facility with radial gate is provided to simulate floods of up to 1:10 year occurrence. Data: 40m high intake tower and a 60m long access bridge Outlet/river diversion conduit beneath the embankment Outlet house and control works Emergency spillway, with a 100m sill crest length Maximum discharge capacity - 70 m 3 /s. Minimum release under full head 10 litres/s Flood Safe Evaluation Flood (SEF) - 301m 3 /s Hydrograph volume 7,17 million m 3 Equivalent over-pumping duration 8 hours (b) Lower Dam The lower dam is a roller compacted concrete structure with a length of 310m and a maximum height of approximately 39m. Data: Uncontrolled 40m ogee crest length Maximum discharge capacity 74 m 3 /s Flood Safe Evaluation Flood 1550m3/s Hydrograph volume 17,44 million m Waterways (a) Headrace Intake and Approach Channel 9

10 The headrace intake structure is located at the reservoir s edge at the end of an 850 m long, partially concrete lined rock cut channel that will connect it to the deeper parts of the reservoir. The single intake structure serves two headrace tunnels. (b) Low Pressure Headrace Tunnels The scheme will have two, 6,6 m diameter, 1030 m long, circular, fully reinforced concrete lined low pressure headrace tunnels. These tunnels will connect the intake to the two surge shafts. A waterproof membrane will be provided required. (c) Headrace Surge Shafts There will be two, vertical, 15 m diameter, post-tensioned concrete lined headrace surge shafts. These will be located above the pressure shafts. The surge shafts will be connected to the headrace tunnels and the pressure shafts by 6,35 m diameter reinforced concrete lined riser shafts. (d) Headrace Pressure Shafts The headrace will have two, 390 m deep vertical pressure shafts. These shafts will convey the water from the low pressure headrace to the high pressure headrace, which will be at lower level. These pressure shafts will be 5,1m in diameter and steel lined. (e) Headrace Pressure Tunnels The upper part of the pressure tunnels will be 5,1m in diameter and steel lined. (f) Steel Lined Penstocks There will be two steel lined penstocks each bifurcating into two branches and so feeding the four pumpturbine units. The length of penstock to the main inlet valves varies from approximately 320 m to 324 m. Up to the bifurcation the 5.1 m diameter steel liners will be 45mm thick with 260 mm by 25 mm thick stiffeners. Downstream of the bifurcations the 3.6 m diameter steel liners will be smooth and 33 mm thick. The steel penstock liners will be concreted in to rock tunnels. (g) Draft Tubes and Draft Tube extension tunnels The steel lined draft tube tunnels will convey the water from the pump-turbines to the two surge chambers. They will be circular, in section and 135 m long. (h) Tailrace Surge Chambers Each of the two tailrace surge chambers will serve two draft tubes. The draft tube gates, one for each draft tube, will be accommodated in the surge chambers. Their lifting hoists will be located at the top of the surge chambers. The surge chambers will be vertical, circular and 20 m in diameter. They will be lined with reinforced concrete. (i) Low Pressure Tailrace Tunnel The single tailrace tunnel will collect the water from the two surge chambers and convey it to the lower reservoir (the flow will be in the opposite direction during pumping). The tailrace tunnel will be circular, fully reinforced concrete lined with a waterproof membrane and 9.4 m diameter. It will be 2450 m long. (j) Tailrace Outlet Structure The reinforced concrete tailrace outlet structure will be of the tower type and will include steel screens and stoplog gates. It will have a top deck that can be accessed from a short reinforced concrete bridge. The outlet structure will be located at the edge of the lower reservoir. 10

11 (k) Tailrace Outlet Channel The tailrace outlet channel will connect the tailrace outlet structure to the deeper parts of the lower reservoir. It will be excavated in rock and soil and will be partly lined in concrete and partly lined in riprap. It will be approximately 1250 m long and have a base width of 25 m Powerhouse complex (a) Powerhouse Cavern There will be a single underground powerhouse cavern in rock to accommodate the main inlet valves, the pump-turbine, motor-generator units and their auxiliary equipment. The powerhouse will be served by two, 265 tonne overhead cranes with auxiliary hooks and a small 8 tonne crane below the larger cranes. The powerhouse will have a central erection bay, three main floors (pump-turbine floor, motor-generator floor, and an operating floor). The floors, unit supports structure, inlet valve support structure, etc will be in reinforced concrete. The powerhouse will include a control room, ablutions, battery room, workshops, etc. (b) Transformer Cavern The transformer cavern will be an underground rock cavern with a reinforced concrete floor and two intermediate floors. The four main generator transformers, the two static variable frequency converters, and the two station service transformers shall be located in the transformer cavern. The transformer cavern will be located next to and parallel with the powerhouse cavern. (c) Construction adits Construction adits are provided to allow access to the operating floor, the pump-turbine floor and drainage gallery for excavation purposes. Adits for access to the bifurcations and penstock anchorage gallery are also provided Access tunnels The Exploratory Tunnel (ET) will be used for initial access to the underground caverns during construction. Access will be improved once the Main Access Tunnel (MAT) is completed. The MAT will provide access of the electro-mechanical equipment. The ET will also house the ventilation duct and the 400 kv cables from the transformers to the surface switch yard Ventilation Fresh air is forced into the inlet ventilation duct and distributed to the various floors of the Machine hall. The discharge is divided into three routes, namely the ET for cable cooling, the MAT to force out exhaust gasses and the smoke shaft. 5.3 Contract strategy The construction of the project involves a number of separate, but inter-dependent contracts. Sixteen contracts will be awarded for the development of the scheme, namely: No Contract Civil & Building 1 Access Roads to lower and upper sites 2 Main Access Tunnel 3 Infrastructure for construction accommodation and laydown areas 4 Quarry in lower reservoir basin for aggregates 5 Upper dam 6 Lower dam 7 Underground Works 11

12 8 Visitor Centre, Admin. Building 9 Houses for relocation of labour tenants Electro-Mechanical 10 Turbines & Generators 11 Main & Auxiliary Crane 12 Transformers 13 Mechanical Auxiliary Plant 14 Electrical Auxiliary Plant Power Supply 15 Distribution power supply 16 Transmission integration 6. THE LIMA PUMPED STORAGE SCHEME 6.1 Location The Lima scheme is located on the escarpment between the Nebo Plateau and the Steelpoort River valley, near the town Roossenekal in the Limpopo Province. The distance between the upper and lower reservoir is approximately 2km and the elevation difference is approximately 646m. 6.2 Description The scheme consists of the following basic components: An upper off-channel cut-to-fill CFRD reservoir A lower earthfill reservoir with clay core An underground power house complex with access tunnels and associated waterways that link the two reservoirs 4 pump/turbines coupled directly with generator/motors Ancillary works that include building works, roads, transmission lines and temporary and permanent infrastructure The main differences compared with the Ingula scheme are: Good quality igneous granitic and gabbroic rock is present as opposed to sedimentary rock at Ingula An off-channel upper reservoir as opposed to an on-channel reservoir at Ingula A lower reservoir located on a small triburary. Water for filling and losses is planned to be supplied by a pipeline from the proposed De Hoop Dam located approximately 20km downstream on the Steelpoort River Installed capacity of 1500MW. Rated head 636m. Synchronous speed 500 r.p.m. Distance between the two reservoirs is approximately 2km, as opposed to 6km for Ingula. Powerhouse has five floor levels as opposed to three. The loading bays and access to them is from the sides as opposed to a central loading bay. The additional excavation provides more space for equipment which had to be limited at Ingula due to poorer rock conditions No requirement for a tailrace surge chamber due to the relative short distance from the powerhouse to the outfall. 12

13 7. CONCLUSIONS The selection process resulted in two feasible schemes which comply with South Africa s system requirements. The Ingula scheme has been selected to be developed first, as the larger operating volume allows more flexibility and provides better backup to the national electricity grid during emergency conditions Due to the high growth in peak demand, other pumped storage schemes, after Ingula, may be required as well. While Lima is earmarked to be one of them, further studies are currently in progress in the search for a third scheme. In this process, previously identified schemes are being dusted off and reassessed. 13