نشریه سد و نیروگاه برق آبی ایران
پیاپی 31 (زمستان 1400)

Transformers in power plants act as intermediaries between the power plant and the power grid, which are affected by electrical stresses such as switching, thermal stresses such as overload and arc, and chemical reactions in the dielectric oil. Acid and moisture components cause deterioration of transformer oil and paper insulation. Condition assessment of transformers is a method to predict and prevent spreading of faults in power transformers. The purpose of this paper is to evaluate the health condition of sample transformers of Dez and Karkheh dam hydro power plants using the proposed probabilistic method. The voltage levels of these transformers are 410 and 230 kV and the possible first-order and second-order reliability methods used to determine failure in materials or structures are based on the data of the transformers of Dez and Karkheh dam power plants to determine the reliability assessment. The results are compared and the efficiency of the proposed methods is demonstrated.

A power plant can be considered a workplace with a high level of risk.For this reason in order to ensure sustainable Performance in Power Plants risk management is essential.  In this study the AHP method for Classification of risks And the FMEA method for risk Rating and SAW method was used to rank the risks. Finally, the ranking was done by FMEA&SAW method.According to the result s of the AHP method among the main Criteria the highest weight is related to un expected events with a weight of /403.The results of FMEA method showed that the risk of explosion and fire with 600 RPN is at an unbearable risk level. In the ranking with SAW method Also explosions and fires, earthquakes, lightning and human error they were in the first priorities of this ranking. Also risk Prioritization with FMEA&SAW integrated model also showed that the risks of explosion and fire*earthquake*human error*lightning and system defects are in the top 5 Priorities ranking.So we can say using this method provides amore accurate analysis of first.As a result more efficient actions will be in herited and will achieve and maintain amore desirable degree of confidence.

In this research, a deep explosion in the reservoir of a concrete gravity dam has simulated by the coupled Euler –Lagrange finite element method. The validity of explosion hydrodynamics pressure has compared with Taylor empirical formula.  The surface sloshing not considered due to deep explosion. The dam and its retained water excited by detonating large explosive charges in deep water upstream from the dam. The damage plastic model of Lee has applied for concrete and damage pattern detection of dam. The dam responses to the underwater explosion were recorded as crest displacement time history. The result of explosion simulation of one ton TNT has compared with actual damage of Koyna dam due to 6.5 Richter earthquakes in 1976. The crack pattern was similar in two cases, but in underwater explosion test, the crack propagation is stopped before reaching to downstream of dam. Evolution of result was shown that controlled explosion test in the reservoir of a dam can be used as a method for study of hydrodynamics effects on dam body safety.

Pervasive construction of elevated liquid tanks as the most important sources for urban hygienic water, and their failure under earthquake, highlighs the necessity for research toward new passive control devices. Design and use of base-isolation systems for elevated liquid tanks is propounded as a novel seismic engineering technology. To date, no special examination has been discussed in relation with the workability of such systems in reducing the seismic response of elevated liquid tanks considering tank wall flexibility. In this study, the seismic performance of a steel elevated water tank with/without isolator was analyzed and outcomes were evaluated using lead-rubber and elastomeric bearings by performing time-history analyses. To this end, finite shell elements for tank wall and internal finite elements for liquid, and bilinear hysteretic elements for the base-isolation systems are elected, and the seismic analysis of flexible liquid tank is performed in the time domain under real earthquake. The results show that the lateral control device despite excessive structural displacement could offer a substantial benefit for reducing the system’s seismic response which depends on the earthquake characterestics.

The choice of concrete consumption and high strength in the structure of concrete dams is of particular importance. In this laboratory research, slag alkaline concrete containing 0, 4 and 8% nanosilica and 1 and 2% polyolefin fibers has been produced in 5 mixing designs. A design of control concrete containing Portland cement was prepared to compare with the test results of reinforced concrete. Modulus of elasticity, impact resistance of falling hammer and scanning electron microscope images were performed on concrete samples at 90 days of processing age. Addition of nanosilica to reinforced concrete improved (Module 4 compared to Scheme 2) in the modulus of elasticity test and the energy absorbed in the impact test by 13.42% and 36.36%, respectively. This superiority was increased by 7.5% and 8.26 times by adding fibers to concrete, respectively. The results of the SEM test overlap with other results.

Spillways are responsible for draining floods in dams. However, morning glory spillways are considered as free spillway type. In floods, a submerged flow is formed in their crest. This factor leads to severe pressure fluctuations in the shaft, suction and vibration of the structure and also reduces the spillway efficiency. Insufficient knowledge of the hydraulics of the submerged flow will be a warning sign for failure and overtopping of the dam. Studies show that the use of ancillary structures in morning glory spillways will improve the flow pattern. In this experimental research, an attempt has been made to investigate the effect of deflector and flow aeration by constructing a physical model and also the parameters affecting the discharge coefficient of submerged flow of morning glory spillways are expanded. Finally, by applying the weight effect of all geometric and hydraulic factors, a nonlinear relationship was introduced to estimate the discharge coefficient. The proposed relation with the RMSE of 0.091, compared to other previous relations (which have errors of 0.122 and 0.118) is able to better predict the discharge coefficient of submerged flow of spillway. The results also show that the discharge coefficient, unlike previous research, is highly dependent on the deflector.

Sedimentation in dam reservoirs is known as an effective factor of reducing dams’ life-time and reservoir capacity, disruption of dam operation, blockage and corrosion in intake structures, drain valves and other sediment venting structures and consequently increase costs of maintenance and other related damages. This proves the importance of understanding the hydraulic behavior of density current in dam reservoirs. In this study, the trend of density current moving in Lorestan Roudbar reservoir has been investigated by Mike 3 model. The aim of this study is to withdraw maximum gravity current concentration in flood time period from bottom outlets in order to decrease sedimentation in reservoirs active volume. Therefore, three-dimensional hydrodynamics simulation of density current in this reservoir has been done. The modeling scenarios have been designed to calculate how much mass can be vented from bottom outlet to save more capacity in reservoir when flood with various return time period occur. The results show that increasing the return period of the flood the sediment discharge efficiency, increases. Sediment venting efficiency is increasing with a two-year return period and reaches a maximum in floods with a 20-year return period when the maximum efficiency of sediment venting is roughly 17%.