RELIABILITY EVALUATION OF A HYDROPOWER STATION

Abstract

Electric energy is the backbone of modern civilization and national economy. It has been thought as one of the safest and cleanest forms of energy. The consumption of electric energy both at homes and at industries in almost all parts of the world has been increasing proportionally to the advancement of civilization. Modern society, because of its pattern of social and working habits, has come to expect the electric supply to be continuously available on demand, which is not an easy task for designer, planner, operation and management personal of the power supply system. Power supply system comprises generation, transmission and distribution systems. All three are equally important for a reliable power supply. Hydropower stations are one of prime contributors to the generating systems in most of the counties. Therefore, its role is crucial in supplying continuous and reliable power on customer's demand. If the nation is serious about improving the reliability of the electric grid, engineers and concerned professional must be ready to face the uncertainties in generating capacity of the hydropower stations. The uncertainties arising in generating capacities of hydropower stations are mainly because of flow variations in the river and forced outages of electromechanical equipments. It is therefore essential to estimate the overall reliability of existing and planned hydropower stations so that it can be assured a certain amount power to the grid for a desired and specified duration. The trend of reliability analysis in defense sector, aerospace sectors, nuclear reactors, communication sectors have been increasing day by day all over the world, but hydro professionals of developing countries have not yet given significant attention towards reliability analysis of hydropower stations. As a result, unplanned interruptions in power supply and load shedding are most common in most of the developing countries. Reliability analysis is essential in decision making for operation, control, maintenance and uprating of existing hydroelectric stations. Thus, risk and reliability analysis of hydropower stations has become an area of extensive research. The present study is an endeavor precisely in this direction. Abstract W..07.0417/APYGT4274,/.07.0VIPM.V.W.V;10/415%/11.47/.2.74neViee.6%770/07,0%Bytlacr/.47.4•40:40/■62.40/41%4MCW:47/47/./..feasZICOZOleitarAlrhr/ACCVASAW Reliability of a hydropower stations basically depend on the availability of individual generating units and water availability for its operation. Availability of individual turbine-generating units depends on the availability of different components and subcomponents. Time taken to repair after occurrence of sudden and catastrophic failures of different equipments is a major parameter in reliability evaluation. Based on the number of occurrence of failures and total repair time, mean time to repair (MTTR), mean time between failures (MTBF), mean time to failure (MTTF), failure rate, probability of occurrence can be estimated. Collection of failure details of all components in a systemic order for a complex system like hydropower station is not an easy task. It needs generation of a reliability database. Reliability database in its broad sense is an ordered and constantly updating filling system of data. The database generated is based on the operational history of different components. It contains various observed causes that lead to the outage (forced as well as scheduled) of each generating unit. The observed causes are equipment failures, water unavailability, failure and disturbance in the power evacuation system, etc. The database describes the date and time at which these causal events have occurred to put the individual turbine-generating units into shut down mode and the date and time at which they have got recovered to put the turbine-generating units back into operation. The present study involves a case study of Chilla Hydropower. Station (CHPS). As explained above, a reliability database has been generated for CHPS. The database has been further used to calculate MTTR, MTBF, MTTF, failure rate, probability of occurrence of each basic event. Failure of any hydropower station causes effects on society and nation as well. The effects may be social, economical, environmental, etc. Therefore, risks involved in the operation and management of any hydropower station is usually high. Equipments' failure in any hydropower leads to unplanned outages and significant expenses. Therefore, analyzing or assessing of a hydropower station from failure side is more important than analyzing the system from success side. Failure probability, different causes of system failures, different fault paths, interaction and importance of different components causing system failure can be easily identified during failure analysis. In this -IV- Abstract OKIF/AP41•100YXYLVAV.07.1r/AMFAIWAVAW14,70207,10/047//2046,41:50%614f1.01"9/47/arld7/029/.17%.4%.9,212.6±47763/47/0.W10,64,47/0/0/.17/A1/274MP11,4s7,0/4:27=0,1750%97.47/47/.A76f/ar/A9/0, study, Fault Tree Analysis (FTA) has been carried out for failure assessment of CHPS. The fundamental concept of FTA is the translation of the failure behavior of a physical system into a visual diagram and logic model. The diagram segment provides a visual model that very easily portrays system relationships and root cause fault paths. The logic segment of the model provides a mechanism for qualitative and quantitative evaluation. FTA is based on reliability theory, Boolean algebra and probability theory. There are mainly four steps viz. system definition, fault tree construction, qualitative analysis and quantitative analysis to be carried out under FTA. For smaller systems, quantitative analysis can be done manually but when the number of components in a system increases, thereby increasing system complexity, it becomes difficult to carry out such computations manually. Computer software(s) have been developed to carry out such computations efficiently. CARA Fault Tree (2000) version 4.1 has been used in this study. The probability of occurrence of TOP event (Forced outage of Turbine-generating unit) obtained from quantitative analysis of FTA gives the electro-mechanical unreliability of each unit of CHPS. The undesired event (e.g. accident or failure) in a particular system to be analyzed is normally called the TOP event. Further the electromechanical reliability of each generating unit has been calculated. As discussed above, overall reliability of hydropower station depends not only on the electromechanical equipments but also on the water availability to operate individual units. So, based on a typical power duration curve, operation of either one unit or two units or three units or all the four units or operation of none of the units have been estimated from the common concept of probability by combining the individual electromechanical reliabilities of different generating units with the percentage of time the power available for their operation. Further, based on the observed water availability evaluated by FTA, probability of operation of either one unit or two units or three units or all four units or none of units have been evaluated from the same concept of probability. FTA of each generating unit is carried out with an assumption that the operation of individual unit at partial capacity will not be caused due to the partial failure of equipments. It is affected only due to water unavailability and demand variations. -V- Abstract ,W,2222747/6"/ZW/4749/4*211;a72571.07,745:2,/,0/.60,711/1201.7.6,Z,V1aZtrams'AP/A,Z4W4r47/07.0/.0,-WW/Ar4MVA7.40,0%,052r/if./.6%17/AVACVIra3/.107.0)27/IF/AVIVIAWN74370/..W.CYLAV.1%Mrar/0741%/0:, Even though the installed capacity of CHPS is 144 MW (4*36 MW), the results obtained from reliability analysis are reliability of generating 36 MW of power is 0.992473, reliability of generating 72 MW of power is 0.920231, reliability of generating 108 MW of power is 0.66069, and reliability of generating of 144 MW is 0.246954. The unreliability of CHPS, i.e., unavailability of all the generating units simultaneously is 0. 007527. On the basis of these results, one can conclude that the reliability of grid on CHPS for 36 MW of power is the highest and reliability for 144 MW is the least. But as the reliability of grid on CHPS for 72 MW and 108 MW is not so worse, it may depend on CHPS for 72 MW and 108 MW of power with reliabilities 0.920231 and 0.660692 respectively, meaning thereby, if the grid expects 72 MW of power from CHPS, it will be in danger of losing 72 MW of power by 7.9769% and if it expects 108 MW of power, the danger of losing will be 33.9308%. Thus, on the basis of these results, effective and efficient grid operation can be managed. As FTA of hydropower stations can be easily understood, it is recommended that every hydropower station has its fault tree diagram like a suitable maintenance plan so that one can identify the critical components, different paths for occurrence of TOP event, various possible causes of failures, etc. Operation, maintenance scheduling and allocation of spares can be decided on the basis of frequency of occurrence of failures. In addition, affects on the overall performance or availability of the individual generating unit due to failure of a particular item/component in it can be compared with the cost of unavailability with the cost of providing redundancy for more failure prone components like pumps and other auxiliaries.