Simulation of Active Power Generation During Operation of a Steam Turbine Stage in Low-Steam and Motor Modes

The operation mode of the steam turbine stage in low-steam and motor modes is considered. The features of the operation of the steam turbine stage in the motor mode are given. It is shown that the condition accepted in earlier studies of hydrodynamic processes in the turbine stage during its operation in the motor mode, that the cooling steam flows through the stage without expansion, i.e. without generating active power, is valid if the entire flow part of the turbine is under pressure in the condenser and there is no pressure drop in the stages. The conditions under which the stage operates in the ventilation mode or in the mode of generating active power are determined. The technique developed with this condition in mind allows, with known parameters before the stage and the pressure ratio before and behind the stage, to determine the operating mode of the stage.


Introduction
The report is devoted to modeling the operating modes of a steam turbine stage when it operates in a deeply uncalculated low-steam mode -steamless, motor, idle rotation with a nominal frequency, in the mode of its own needs, etc.The low steam mode is also characteristic of heating turbines when they operate in the heating mode with a closed diaphragm with the passage of a small amount of steam into the condenser for cooling the last stages of the low pressure cylinder (LPC).The purpose of the simulation is to determine the operating mode of the steam turbine stage, whether the stage generates active power at a given flow rate and steam parameters or rotates in the power consumption mode.A important significance of the problem under study is due to the fact that the limited possibilities of load regulation in power systems due to the absence of special maneuverable power plant, as well as the impossibility of controlling energy losses in power systems have led to deep unloading of all types of thermal power plants, including stations with steam-gas installations.[1][2][3].At the same time, with deep gaps of energy consumption schedules for a long time on Sunday, the equipment shutdown mode with subsequent start-up becomes economically advantageous compared to the unloading mode.However, with the existing structures of generating capacities in power systems and sharply variable schedules of energy consumption, the redundancy of the capacities of thermal power plants (TPP) units through the use of stop-start modes (SSM) is associated with a deterioration in efficiency and a decrease in the reliability of the equipment.In addition, due to the increased speeds of the power consumption schedule, when exiting a gaps in the power systems, high demands are placed on TPP units for their maneuverability and mobility.These requirements must be taken into account when choosing the optimal operating mode of power units in conditions of variable load schedules.It is especially difficult to prevent a decrease in the reliability and efficiency of turbine units during fast starts necessary to regulate the load of the power system, and the existing practice of switching power units to a minimum load mode within the adjustment range is not always justified both in terms of efficiency and reliability [4,5].An alternative way to put power units in reserve with complete removal of the active load is the motor mode (MM) -the operation mode of the turbine unit without supplying fresh (working) steam to the turbine without disconnecting the generator from the grid.In this case, the generator switches to the engine mode with synchronous speed, consuming active power from the grid, necessary to cover mechanical and ventilation power losses in the turbine and electrical and mechanical losses in the turbo generator.Experimental studies and accumulated operational experience have shown that the conversion of turbine units to MM for periods of night gaps of the electrical load schedule has a number of operational advantages compared to the start-stop mode, which significantly increase the maneuverability and reliability of equipment when operating in the load control mode of the power system.The following technological advantages of MM can be distinguished: • the turbine unit in the MM is a hot rotating power reserve for the power system and power plant and, if necessary, can be loaded quickly, within 10-15 minutes to rated power; • during the reverse descent, the following stages are excluded from the MM state: preparatory operations, push and set of revolutions, idling, synchronization and inclusion of the generator in the grid; • in the process of redundancy, the generator can serve as a powerful source of reactive power for the power system (synchronous compensator mode).The features of the operation of the steam turbine stage and the group of stages in this mode are due to the following factors: • when operating in steamless motor mode without supplying cooling steam to the turbine, due to the small flow rate of steam sucked into the flow part of the turbine through the end seals and a slight drop in its pressure, there is no expansion of steam in the stages and it is in idle rotation mode; • the flow part of the turbines (with the exception of turbines with intermediate steam overheating) is under steam pressure in the condenser, i.e. small quantities of superheated steam enter the condenser; • when the steam turbine is operating in the MM with the supply of cooling steam, some stages in the places of supply of cooling steam can operate in the mode of generating active power; • unavoidable power losses during idle rotation of the stage (for friction, ventilation, etc.) in the absence of cooling steam lead to heating of steam and metal of the stage; • heat losses to the environment through the external surfaces of the turbine housing, which are usually ignored when calculating turbine stages in design modes due to their smallness (compared to the amount of heat of the steam flow through the stage), in low-steam modes are comparable to heat losses on friction and ventilation (especially for stages of high pressure cylinder (HPC)) and can affect the temperature state of the metal of the guide vanes and the housing [6-9].

Simulation of operating modes of a steam turbine stage in low-steam mode
In previous studies of hydrodynamic processes in the turbine stage, when operating in motor mode, the condition was assumed that the cooling steam passes through the stage without expansion, i.e. all stages operate in ventilation mode with power consumption.As stated above, this condition is valid if the entire flow part of the turbine is under pressure in the condenser and there is no pressure drop in the stages.The need to determine the generation of active power during the operation of a steam turbine in the MM is due to the fact that the amount of power consumed by the generator from the grid, as one of the main indicators of the efficiency of the use of the MM, will be less by the total amount of the active power generated by the stages, while it is not excluded that under certain conditions the generator can switch to the generator mode with the output of power to the grid.Such a mode of operation of the turbine unit is possible, but economically unprofitable mode, since at the same time the costs of cooling steam significantly increase, which leads to additional costs for their preparation.
The power developed by the steam turbine stage in variable mode, according to [10], can be written as: (1 where G is the steam flow rate through the stage; -the available heat transfer (from the braking parameters) of the stage; -the full efficiency of the stage without taking into account the friction losses of the disk and humidity; -humidity losses; -cumulative friction power loss.Considering that during the operation of the stage in MM, the friction losses of the disk are compensated by the power consumed by the generator from the grid, and the losses from humidity are zero, since the flowing steam is always overheated, we write the expression (1) as: (2) To calculate the efficiency of the stage, you can use the expression obtained by the [10] for variable modes of its operation, including power consumption modes: (3) где -the maximum efficiency value of this stage (according to factory calculations); -the relative value calculated by the expression: (4) here u is the rate of steam flow; is a fictitious rate, calculated from the adiabatic expansion of steam in a stage.The value it is determined by the actual parameters of the stage under consideration, and -by the following expression [1]: (5) where -total losses from leaks in the stage; -the degree of reactivity of the stage.The available heat transfer in the stage with known parameters before and after the stage is calculated according to the h-s diagram, but for superheated steam you can use the expression: and (6) where -the heat transfer of the stage is assumed and calculated according to static parameters; -the ratio of pressures in the stage according to static parameters and taking into account braking.In some cases, in the absence of experimental data, only the static parameters before the step are known.Then you can use the dependency

(
) In the motor and low-flow modes, when the stage operates in a deeply uncountable mode, the degree of reactivity is calculated by the formulas: , Obviously, at G > 0, the condition corresponds to a mode with a negative value of the stage efficiency, since the heat transfer is determined by the absolute value.
Consider the conditions for constant steam consumption and assume that , i.e. = 0.
From (3) it is not difficult to obtain that for low steam consumption this condition corresponds to the mode at x = 2,09740.Let 's denote this relation , and the relation at which this value is achieved is .It will be determined in this case from the ratio ( 9) From where we get (10) where -fictitious speed and heat transfer at which the condition is met .
On the other hand (12) When calculating for the control stage, it can be assumed that , i.e. , and for the remaining stages, the temperature is determined by the actual steam parameters.

Conclusions
Based on the expressions obtained, the condition can be written as Or after substitution ( ) As can be seen from ( 14), all other things being equal, the fulfillment of the condition depends on the ratio of vapor pressures before and after the stage.From ( 14) we can write (15) in this case, the condition will be written as ( 16) Thus, the control calculations carried out for the characteristic mode of operation of the three-cylinder steam turbine K-200-130 in the motor mode when the cooling steam is supplied with the corresponding parameters in the middle pressure cylinder (MPC) for the 17th stage and in low pressure cylinder (LPC), MPC up to the 18th stage and the last three stages of the LPC worked in ventilation mode with power consumption, and the remaining stages of the MPC and the first stage of the LPC are in the power output mode [6].The total generation of active power was 290 kWt with a generator power of 2.65 MWt.Thus, the developed technique allows, with known parameters before the stage and the pressure ratio before and behind the stage, to determine the operating mode of the stage, which is very important when modeling the temperature state of both this and subsequent stages when operating a steam turbine in low-steam and motor modes.

References
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