HYDRO POWER CAVERNS | TECHNICAL
2. CAVERNS IN CHINA Total installed capacities in China, as reported in 2016, were 250GW for conventional plants, and 26GW for pumped storage plants, including semi- and full- surface plants. Trends in cavern sizes relating to total flow rate
replace the former trend for total turbine capacity. Furthermore, plants with and without separate transformer halls are distinguished. A reduction in blast volume is significant for conventional plants. Total length, cross-section area and excavation volume are given for Francis units (sizes for 4 units) with and without separate transformer halls, respectively: Cavern size trends for total flow volume are more
reliable for larger numbers of conventional plants, and are governed by a larger number of small total flow volume plants. In contrast to the increase in cavern height there is also strong indication that caverns will decrease in width. Total excavation volumes of pumped storage caverns,
with and without separate transformer halls, does not differ much on average. The former have smaller section areas but longer lengths, and the latter have larger section areas but shorter lengths. Consequently, one linear trend for cavern volumes is sufficient. Typical saving for other cavern dimensions, besides
the strongly controlled width, is 15% if a separate transformer hall is designed. Transformer cavern sizes were analysed for 230
plants. The volume may increase linearly with the total installed capacity: Volume = 9000m3
+ 27 * C total (MW)
Limited evaluation in Figure 4 yields in China: Volume = 3750m3
+ 37.5 * C total (MW) The decision to construct a separate transformer
cavern is becoming more frequent for larger installed turbine capacities. However, the saving is very strong for smaller power units, e.g. 15% of their dimension. Average values for the 370 Francis and 100 Pelton turbine plants investigated are v = 83,000m3 C = 4 * 100MW, Q = 300m3
700m, C = 3 * 80MW, Q = 40m3
/sec, and v = 32,000m3 /sec, respectively.
In Figure 1, and supported by reference 12, this
distribution of total hydropower plant capacities shows a wide range of sizes from 200MW to 6600MW. Numerous small capacity plants may not have been adequately reported.
3. HOST ROCK FORMATIONS AT CAVERNS Again, 165 strongly folded (katamorphic) rocks, 125 plutonic rocks, and 95 moderately folded (mesomorphic) rocks were found to be the most desirable host rocks in this ranking order. The least desirable, and more expensive to realise, were the 20 plants constructed in calcareous rocks other than limestone, and in slightly folded (epimorphic) rocks. An effort was made to present simplified host rock
formation groups from eight world regions, totalling 574, but this time disregarding power house shafts.
, h = 160m, , h =
A further sub-division of formation rock groups is
possible. The numbers from the 2015 edition of the Yearbook, including shafts, were 155 sedimentary, 285 metamorphic, and 185 magmatic formations. The almost 50% contribution of metamorphic cavern host rocks clearly prevails, apart from in China, where magmatic rock is more common. The Yearbook sees an additional 8 caverns sited in
sedimentary rock formations, 6 in metamorphic rock formations, and 10 in magmatic rock formations. Figure 2 shows the results of a study of 46 recently
constructed caverns, 49 older conventional caverns and 21 recently constructed pumped storage caverns12
,
and reveal that the most attractive choice are intrusive rocks, followed by calcareous rocks and, finally, effusive rocks. Less prominent choices are clastic sediments and
metamorphic rocks which are for this figure, and Figure 6, attached in the same quantities as calcareous and clastic rocks. Throughout history the role of Europe has remained
strong despite its much smaller water resources when studying the 247, 237, and 86 cavern plants in Europe, Asia, and the Americas even without considering the numerous Scandinavian plants constructed within metamorphic rocks which currently remain unreported due to the limited data available. Some differences are noted between north and mid-
America and south America where there are much more sedimentary rock formations and less metamorphic formations. Notably, much less sedimentary and more metamorphic/magmatic rock formations exist in Africa than in Asia. The most remarkable difference between mid and
south-east Asian plants, with almost 100 plants each, is 75% metamorphic but 55% magmatic rocks, respectively. Major contributions to these numbers come from two major countries, i.e. India (excluding Bhutan and Nepal) with 70-75% metamorphic formations, and China with ~52% magmatic formations and 38% sedimentary formations. The prevailing host rock type is listed in the
accompanying tables instead of combinations of formations. Average values represent preliminary evaluations of the past, and are by no means a prediction of cavern host rock formations in already distinguished regions.
4. TYPICAL ROCK PILLARS BETWEEN CAVERNS The evaluation of Figure 3 shows the close correlation of intact rock pillar of cavern groups with the main power cavern height. A maximum value of 50m rock pillar width seems to cover all risks of unwanted structural failure modes of cavern groups even for large cavern height of ~85m. Furthermore, the cavern width seems to not be a valid criterion for the required pillar. Considerable influences on the required rock pillar,
like height, rock type and rock mass compressive strength have been discussed in previous Yearbooks.
September 2023 | 17
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