Vol 65, No 4 (2018)

Considerably complicated power-system problems that have to be solved by the Russian power-generating units and, consequently, the need to ensure the fully automated operation of the power-generating units under working loads necessitated improving the structure of the automatic frequency and power control systems (AFPCSs). Not all solutions adopted in the 1980s, in particular, the use of the classical variants of the automatic power control systems (APCS-1 and APCS-2), ensure the required quality of suppressing power-system disturbances. In this work, the authors performed comparative analysis of six AFPCS variants that correspond to both the classical APCS-1 and APCS-2 and their combinations. The research was carried out using a mathematical model under disturbances of characteristic types, viz., those caused by network frequencies, unit power settings, and boiler loads. The model was developed by approximating experimental step response curves of the 300-MW power-generating unit 3 equipped with a once-through boiler at Kashirskaya State District Power Station. The quality of control was analyzed by comparing the amplitude-frequency characteristics and the curves of the transient processes of the compared AFPCS variants and the integral quadratic criterion values and the maximum deviations of the controlled variables and control actions in the course of the above processes. The combined AFPCS and advanced APCS-2 proved to be the most preferable. The combined AFPCS variants offer advantages in terms of maintaining the steam pressure, while the advanced APCS-2 has an advantage as a means for controlling the unit power with the gain in the first case being somewhat greater than that in the second case. Both variants have been successfully used for a long time at numerous power-generating units of the country.

The progress that has been achieved in the development and manufacture of vacuum circuit breakers opens the possibility of using them for a wider range of applications at power plants, including as generator circuit breakers. Such characteristics of modern vacuum circuit breakers as increased breaking capacity and high switching life are factors that make them closer in competitiveness to SF6 circuit breakers for generators with capacities up to 400 MW. The article considers problem aspects relating to clearing of short-circuit faults in the generator voltage circuits and interruption of out-of-phase making currents and no-load currents of generator transformers. Conditions leading to a longer period of time to the moment at which the switched current crosses zero are considered. It is pointed out that, unlike the IEC/IEEE Standard 62271-37-013, GOST (State Standard) R 52565-2006 does not specify the requirements for generator circuit breakers in full. The article gives the voltage drop values across the arc for different design versions of vacuum circuit breaker contacts and shows the effect the arc in a vacuum circuit breaker has on the time delay to the moment at which the current crosses zero. The standardized parameters of transient recovery voltage across the generator circuit breaker contacts are estimated along with the contact gap electric strength recovery rates ensured by modern arc quenching chambers. The switching overvoltages arising when vacuum circuit breakers interrupt short-circuit currents and no-load currents of generator transformers are analyzed. The article considers the most probable factors causing the occurrence of switching overvoltages, including current chopping, repeated breakdowns of the circuit breaker contact gap, and virtual current chopping. It is found that, unlike repeated breakdowns and virtual current chopping, an actual current chopping does not give rise to dangerous switching overvoltages. The article also determines the vacuum circuit breaker application field boundaries in which dangerous switching overvoltages may occur that would require additional measures for limiting them.

The present paper considers the approach to easy and rapid evaluation of the residual heat generation in the reactor core of a pool type research reactor after shutdown taking into account the previous operation cycles at constant operating cycle parameters. The selected mathematical model of the process is well-founded. The frames of the operational parameters and assumptions of the study are defined. The operating power level of 2000 kW and operation/shutdown schedules of 4/20, 6/18, 8/16, 10/14 and 12/12 hours per day during the five working days of the week and shutdown for the weekend are considered. The calculations are prepared for the first 180 seconds and the first 120 minutes after the fission reaction is stopped by the reactor scram system. The calculation results for the power of the residual heat generation in the reactor core at different work schedule are presented graphically as a percentage of the nominal power. For each particular work schedule the number of previous working weeks taken into account, the value of the correction coefficient and the share of the previous operating cycles in the residual heat generation power are presented in tables. The calculation results are discussed from the point of view of the safety analyses assessment and the nuclear fuel cycle management. Some possible directions for further research works are outlined and defined as particular tasks.

A redistribution process of corrosion product (CP) deposits in the primary circuit of nuclear power plants (NPP) with propulsion service is examined. It is demonstrated that, after a shutdown of the NPP, the activity of activated corrosion products (ACP) and the concentration of stable corrosion products in the coolant increase considerably due to their entering the coolant from the deposits on surfaces of the primary circuit equipment. The corrosion products enter the coolant principally in the form of insoluble compounds, and the suspended solid particles above 0.4 μm in size account for a minimum of 90% of the total content of CPs in the coolant. This increases the CP concentration in the coolant by a factor of approximately 10, and the total activity of ACPs becomes more than 300 times the initial value determined during reactor operation. It was established that injection of hydrazine facilitated redistribution of the corrosion products and radionuclides associated with them between the surface of construction materials and the coolant: the corrosion products went into the coolant and the rate of their redeposition decreased by a factor of ten. This enables us to increase the fraction of highly active ACPs removed from the circuit by the normal cleaning system. In addition, a shift in the equilibrium on injection of hydrazine gives a higher activity of ACPs in the coolant after completion of the deposition process and removal of CPs that entered the circuit in the normal treatment system. The equilibrium activity of ACPs is approximately ten times the value specific for the installation that is cooled down without introduction of hydrazine. A decrease in this characteristic to stable low values is observed only after the reactor is put into operation.

The reliability and economy of thermal power plant equipment, including combined-cycle plants, are affected by corrosion of structural materials and deposit formation in the water-steam circuit. To reduce the rate of these processes, coolant treatment is used. Ammonia, hydrazine, phosphates, etc. have been traditionally used as reagents. Recently, much attention has been paid to the use of water chemistry with the addition of complex reagents based on film-forming amines, not only for the equipment protection during the established operation mode but also for start-stops and preservation. Such water chemistry regimes are used not only in Russia but also in other countries for steam turbine and combined-cycle TPP, including biomass TPPs. The International Association for the Properties of Water and Steam, uniting specialists from all over the world, developed a guideline in 2016 for the use of such reagents based on theoretical studies and years of operational experience in power equipment in 21 countries. The review of domestic and international guidelines containing conditions for the use of film-forming amines and amine-based reagents for the organization of TPP water chemistry regimes for steam turbines and combined-cycle plants is presented.

Seepage of flue gases into the environment through the chimney of thermal power plant smoke stacks often occurs during their operation, causing intensive damage to them. This phenomenon occurs due to the difference of pressure between the smoke stack inner and outer surfaces when the stack is streamlined by the flow of atmospheric air. The smoke stack operating conditions as a function of the stack load and wind conditions was studied. In particular, the excess pressure in the smoke stack shaft is estimated according to L.A. Rikhter’s criterion, and the static pressure along the stack height is calculated. The motion of combustion products in the smoke stack shaft and streamlining of the shaft by cross flow of atmospheric air are simulated in separate and combined statements using 3D models at different values of flue gas flowrate and wind velocity. It is shown from the study results that assessment of excess pressure conditions in a smoke stack according to Rikhter’s criterion is not sufficiently correct, because it does not take into account the windinduced underpressure outside of the smoke stack. Numerical investigations carried out using 3D models showed that quite extensive zones with underpressure exist at the smoke stack outer surface when the stack is streamlined by atmospheric airflow. The greatest underpressure is observed under such conditions on the smoke stack lateral surfaces with respect to the wind direction. At a certain wind velocity, the underpressure outside of the stack becomes greater than it is inside of the stack, as a result of which zones appear on the stack surface in which flue gases seep through the shaft into the environment. An analysis of the obtained data showed that the seepage intensity increases with the wind velocity. On the other hand, the area of these zones and the difference of static pressures in them decreases with decreasing the flue gas flowrate.

The current status of activities associated with the assessment of conditions and prospects for prolonging the service life of equipment operating for a long time at thermal power stations (TPS) is described. The history of the creation and development of an industry system for diagnosing and prolonging the lifetime of heat-generating and mechanical equipment is briefly outlined. The core of the system was the results of investigations into long-term strength and creep of heat-resistant steels, and a regulatory base governing effective application of the system was formed and continuously improved within the system framework. It is noted that more and more extensive investigations resulted in the attempts to extend the criterial strength base using the cracking resistance criteria, including local creep fracture criteria. The advantages and disadvantages of the most-used local control method, i.e., the replica method, are analyzed. It is proposed to use the fracture mechanics criteria and, in particular, the threshold stress intensity factor (SIF) below which creep cracking does not occur in analyzing the process of steel creep fracture. It is demonstrated that the threshold SIF for a specific material depends functionally not only on the temperature but also on the loading time. Considering this fact, it is proposed to account for the accumulation of local (at the crack tip) and total damageability of the metal in the concept of threshold SIF. A similar approach can also be used for describing the subcritical crack growth. Therefore, it is proposed to construct crack growth diagrams using isochronous curves for the specified test times. It is demonstrated that dimensions of allowable flaws that do not lead to crack propagation can be predicted for given operating conditions. The cases were analyzed that occurred while obtaining results of examination of a structural member operating under creep conditions for prediction of the size of allowable (nonpropagating) crack-like flaws. This analysis has revealed that the application of the threshold SIF criterion and isochronous creep crack growth rate diagrams can be useful for setting forth the substantiated frequency of examinations for high-temperature equipment and establishing the time by which the equipment service life can be prolonged.