SYSTEMS ANALYSIS OF AN IRRIGATION AND CASCADE HYDROPOWER SCHEME

Abstract

The planning, sequencing, operation and scheduling of complex water resources systems become a formidable task for decision makers due to possibilities of large number of feasible alternative strategies, which involve huge cost and time factor It has been a widely accepted fact that systems analysis techniques that emphasize systematic approaches towards multidisciplinary decision-making goals could be the best bet for dealing with such complex systems. The National Water Policy of India emphasizes that the special multi-disciplinary comprehensive plans should be prepared taking into account not only of irrigation but also various water uses so that the available water can be optimally utilized. India is having an enormous small hydropower sites with a potential of about 6295 MW in natural streams in Himalayan region and in irrigation canal drops. These sites are very much suitable for run-ofriver hydropower schemes because of their simplicity and being environmental friendly, therefore, this available hydropower potential should be thoroughly tapped and properly developed. The Central Water Commission under the Ministry of Water Resources, the main authority to accord technical sanctions to water resources projects in India, has recommended for adopting the optimization methods for project planning. To accomplish these, the reservoir yields of different reliabilities from an existing reservoir should be re-estimated to meet the ever-changing demands of various water uses. Allocation of available irrigation water through canal networks with minimum seepage and evaporation losses would be of immense importance for efficient irrigation management. A study should be carried out for selecting suitable number of turbines and their capacities to be provided at a hydro-plant to achieve IV maximum annual power generation. Planning for capacity expansion and sequencing of power plants for meeting power demand is essential for phasing of power plants during the construction period. A long-range operational scheduling of the power plants by unit commitment of the power plants sequenced would also be of immense importance. Therefore, looking at the above objectives the existing Harabhangi irrigation project has been chosen as the study area, which is an inter-basin irrigation project in Orissa state (India), where water from river Harabhangi, a tributary of river Vanshadhara in Vanshadhara basin (an inter-state basin, between Orissa and Andhra Pradesh), is transferred to Padma river in Rushikulya basin (a basin in Orissa state). The river Padma is currently serving as a carrier for the transferred water. This water is being utilized for irrigation purposes through a canal network emerging from the barrage at the downstream (at Gokulpur) constructed over river Padma. For utilizing the geo-physical advantage of the project location, it is now proposed first, that the quantum of irrigation based reservoir regulated transferred water to Padma river be bypassed by diverting it through a diversion power channel as per the withdrawal rate of power requirement and establish seven number of small hydropower schemes in series in the form of a cascade, that will utilize natural total gross water head of 230m with a proposed total installed capacity of 24.75 MW, second, the power discharge after generation is to be re-regulated by routing it through a proposed balancing pond to be located on the river Padma at the downstream of the power channel, before it is delivered for irrigation supplies through the Gokulpur barrage. To accomplish the above-mentioned objectives, for the Harabhangi project the following studies were carried out. An optimal annual reservoir yield for irrigation from the Harabhangi reservoir has been determined at 75 percent annual project dependability, with a provision to supply a predetermined partial amount of irrigation water during failure years to take care of the minimum food requirements of the cultivators. An implicit stochastic linear programming yield model is used for this purpose. The quantum of rrigation water to be released from the reservoir is suitably converted to the equivalent power withdrawal rates and is presented in the form of aflow-duration curve. These power discharges are diverted through the proposed canal (power channel) on which seven proposed power plants forming acascade are optimally designed as run-of-river schemes using a dual model system, i.e., a combination of non-linear and dynamic programming models. The objective was to maximize the annual energy generation by maximizing the use of efficiencies of the turbines during the power generation. For the proper capacity expansion and sequencing, and thereafter unit commitment of these power plants, plans for their optimal sequencing and scheduling, respectively, using a dynamic programming model have been proposed. The required irrigation discharge to be diverted for irrigation is obtained by routing the power discharge after the power generation, through the proposed balancing pond. The irrigation water thus released through the pond is then optimally distributed (allocated) through the complex network of irrigation canals using a dynamic programming model in order to minimize the seepage and evaporation losses from canals. Based on the above studies, the following are the findings in respect of the Harabhangi project. Two types of optimal annual reservoir yields from the reservoir capacity of 14.125 Tham are obtained, i.e., (i) 18.787 Tham, for irrigation planning with 75 percent annual reliability, and also to supply aprescribed amount of irrigation water to the farmers during the failure years based on the requirement of minimum food needs of the farmers, and (ii) 21.758 Tham, for hydropower planning, based on the maximum probable annual reservoir yield. On VI the basis of the annual yields from the reservoir, the balancing pond was designed for a capacity of 30.0 ham to maintain almost a uniform power discharge very near to the power channel capacity in a given month. At each of the hydropower plants three turbine units are finally selected as the desired numbers for installation, with total installed capacity of 25.650 MW. This will generate an average annual energy of 104.828 million KWH. The first power plant (KAM) in the cascade system is taken as mandatory to be sequenced &the beginning of the capacity expansion period of 20 years. An annual energy consumption growth rate of 5 percent, and 4 years as the unit time period for sequencing are finally selected. The longrange unit commitment of the power plants is done for a period of 16 years, and it is found that the month of May will face the maximum numbers of energy deficits during the operation period. The values of the optimal water allocations of available irrigation water through the canal network are presented in tabular forms and can serve as ready rekoners to the canal operators during the canal operations.