In this article we will discuss about the methods of loading the interconnected hydroelectric power stations.
There are large possibilities that interconnected hydro power stations are of different types such as run-off river power plants with small pondage, hydro plants with a forebay large enough to keep the turbines running at full load for hours together and high head power plants with storage capacity large enough to operate continuously at full load for weeks/ months together.
For economical operation of such an interconnected system the best way is to utilize the rainfall in the catchment area to the maximum possible and this can be done by running power stations supplied by long open channels and rivers during heavy rainfall. The noteworthy point is that during heavy rainfalls open channels and rivers get considerable amount of surface water that drains into the streams and this inflow may in certain cases exceed what is required for power generation by turbines operating on full load.
The practice is, therefore, to make the forebay down to a low level just before heavy rains set in and reduce the load on stations with high head plants and ample storage capacity. This arrangement results in high water levels of storage reservoirs at the beginning of the dry season and generates maximum possible electrical energy before the rainy season commences. The interconnection of different types of hydro power stations provides more effective utilization of water for generation of electrical energy.
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Methods of Loading of Hydro Power Plants:
The performance characteristics of different units in hydro power stations may vary, and, therefore, determination of division of load among the various units, considering the incremental costs becomes essential. The required information for the economic scheduling of hydro power stations is as follows- the efficiency curve of each unit, the input-output curve indicating the total rate of flow of water against the load, and the incremental rate curve showing the change in the rate of flow required per unit change in load.
Methods for very accurate determination of the incremental costs from hydro plants have not yet been well developed. Each hydro power station has a small reservoir near to it for short-term regulation of the water required under various loading conditions. The reservoirs for long-term regulation may be situated at a greater distance from the power stations and may not affect the head at the power station to an appreciable extent, even with change of load on the power stations.
The losses in the conduits and water turbines increase with the increase in required rate of flow of water, and the incremental cost of power from a hydro plant is higher during peak generation compared to that when supplying a smaller load. The incremental cost also varies with the water head at the power station, different load conditions, and the time of their occurrence.
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Long transmission lines are usually necessary in case of hydro plants, and the effect of transmission losses is to be included in economic schedule studies. The incremental cost of power supplied from a hydro plant may be determined by the analysis of losses owing to short-term regulation.
For large power plants with large reservoirs, the incremental cost of water at power stations, with long-term regulation is required to be determined. Thus, this is very involved study and needs estimates based on probability calculations of the flow of water available from the expected rain fall etc., and details of storage in reservoirs at different times during the period under review.
When a number of identical machines having identical input-output curves are installed in a hydro power station, then the most economical method of operation can be had in the following manners:
1. One unit may carry the load variations, while all the other units operating at their points of maximum efficiency.
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2. The more preferable method is loading of all the machines equally and operating at points of equal slope on their input-output curves.
If there is a fall in load equal to capacity of one unit, it is best for economy to shut-off that machine and redistribute the load among the remaining units.
If in a power plant, the machines are provided with individual penstock, the improved efficiency is obtained by adding units in addition to those already in operation. This is due to the fact that the leakage loss in the standby units is either reduced or eliminated.
However, when all the machines are provided with one common pipeline, then the performance will be as depicted in Fig. 10.4. As the additional units are put in operation, the leakage loss is reduced but the pipe friction loss is increased. A stage will be reached when the gain due to decrease in leakage loss is offset by the increased pipe friction loss. Thus, as obvious from Fig. 10.4, the plant will operate at the highest efficiency with only two units in operation.
