Initial Pressure Optimization of Thermal Power Unit under Deep Variable Operating Conditions

Aiming at the problem of how to adjust the main steam pressure under the load instruction from the frequency modulation of the power grid to make the operation safer and more economical in the process of peak regulation, we take that 650 MW reheating condensed steam turbine as the research object. This paper introduces some domestic scholars’ research status of initial pressure optimization. The model is established by using EBSILON Professional thermodynamic system simulation software and combining it with the heat balance diagram of the power plant under VWO working conditions. The initial pressure optimization under deep variable working conditions is studied, then introduces the operation mode of the steam turbine under variable working conditions during peak shaving. The mathematical relationship between heat consumption rate and initial pressure was established to determine the feasible range of initial pressure. The exhaustive method traverses the feasible pressure interval under a particular working condition. The heat consumption rate under each pressure was calculated to screen out the optimal initial pressure and optimize the initial pressure curve.


Introduction
Coal consumption in China is mainly applied in thermal power generation, the chemical industry, and other aspects.At present, coal-based thermal power plant units occupy a dominant position.To meet the demand for peak load regulation of the power grid, many scholars have done a lot of optimization research on the parameter of initial pressure, which greatly influences thermal efficiency.
Test optimization is to directly obtain the thermal economic indicators of power plant equipment through the test and then use these indicators to determine the main steam pressure when the heat consumption rate is the lowest and connect the main steam pressure under each working condition into a line, which is the initial pressure optimization curve.
In the method of theoretical optimization, some parameters are calculated by establishing a formula describing the working performance of the unit, and the relationship between the initial pressure and the heat consumption is established by using an optimization algorithm to find the optimal initial pressure within the allowable pressure range under a certain working condition.Li [1] established the online heat rate prediction model using the IGSA-LSSVR hybrid modeling method and determined the optimal initial pressure value by searching the corresponding main steam pressure value at the lowest heat rate.Taking a 200 MW unit as an example, Wang et al. [2] conducted experimental research on the relationship between energy consumption and initial pressure change under different unit loads and obtained the optimal initial pressure under each load.Based on this, the initial pressure optimization curve guiding the unit peak shaving was drawn.
Zhang et al. [3] established a mathematical model for online determination of the optimal initial operating pressure, calculated with the operating data of a steam turbine in a power plant, determined the feasible pressure range, optimized by an exhaustive search method, and analyzed the shortcomings of the original sliding pressure operation curve.Hu et al. [4] calculated the pressure parameter optimization method of a steam turbine under variable working conditions by using the principle of steam turbine, and used particle swarm optimization (PSO) to obtain the pressure value, and then took a 1000 MW steam turbine as an example to analyze the optimal operating pressure under different loads.
Based on the ALSSVR and IGSA algorithms, Liu et al. [5] proposed a new method for online prediction of heat rate and optimization of operating initial pressure under off-design operating conditions, which can reflect the characteristics of off-design steam turbines and make the optimization of initial pressure more accurate.
This paper studies the main steam pressure optimization of 65024.2/566/566intermediate reheat condensed steam turbine thermal power units to improve the economy of thermal power units under deeply variable operating conditions.By using EBSILON Professional software simulation, the optimal initial pressure of power plant operation was found out by exhaustive method under various working conditions, and the initial pressure optimization curve of the steam turbine under the condition of depth variable load was made, to make appropriate correction guidance for power plant operating parameters.

Establishment of the model under VWO condition
The simulation object of this study is a 650 MW power plant.The unit is characterized by supercritical, intermediate reheating, condensing type, three cylinders, four rows of steam, and a single shaft.The main parameters are shown in Table 1 and Table 2 In the power plant studied in this study, the high-pressure and intermediate-pressure cylinders have two steam extraction ports, and the low-pressure cylinder has four steam extraction ports and one steam exhaust port.One steam generator, condenser, deaerator and generator, seven heaters, one small steam turbine, two water pumps, and several diverter and return valves.
In addition, the upper-end difference of the No.1 heater is -1.7℃, the upper-end difference between No. 2, No. 3, and the deaerator is 0℃, and the upper-end difference between other heaters is 2.8℃.The lower upper difference of all heaters is 5.6℃, and the lower upper difference of the deaerator is 0℃.The pipe pressure drop is 3%.On this basis, the shaft seal cooling system is added, and the power of the small turbine and feed pump, generator power, and boiler flow are matched through the controller.The final model is shown in Figure 1.
Heat rate of power generation: Standard coal consumption rate of the whole plant: where P e is the generator power, Q cp is the plant heat consumption, and B is the boiler coal consumption.
1 q is the low calorific value of standard coal, 1 29270 / q k J k g  .

Verification of heat consumption rate
Combined with the unit model under the VWO condition established above, the controller module component in EBSILON software is used to simulate the thermodynamic model under different conditions.The heat rate under each condition is calculated to verify the accuracy of the model.After calculation, the simulation parameters affecting heat rate under different loads and the relative error of heat rate are shown in Table 3. From the design drawing of the power plant, it can be seen that although the simulated values of several parameters affecting the heat rate have a slight deviation from the design values, the relative error of the heat rate under each working condition is small.The relative error under 100% THA, 75% THA, and 50% THA conditions is less than 1%, within the allowable error range.The model is relatively accurate, and further analysis and research on initial pressure optimization can be carried out.Relative error -0.39% -0.08% -0.58% -0.41%

Operating characteristics of sliding pressure
In sliding pressure operation, the pressure varies greatly with the load.Under sliding pressure operation mode, the state of regulating the control valve of the steam turbine unit is unchanged.The power is regulated by changing the main steam pressure and flow.

Advantages of sliding pressure operation
(1) We improve the reliability of unit operation and prolong the service life of equipment.During sliding pressure operation, the main steam temperature is kept unchanged to avoid strong thermal stress and thermal deformation caused by excessive temperature differences of each unit component.During peak shaving, the steam pressure and steam humidity decrease with the load reduction, which can not only improve the efficiency of the last stage of the steam turbine but also reduce the erosion of wet steam to the blades.
(2) We adapt to variable load caused by peak regulation requirements.By controlling the steam pressure of the boiler to change the load, the change rate of the steam turbine load of the coal-fired boiler can reach 5%~8% of the rated load per minute.In addition, when the load is low, the temperature of the main steam is mild, and the temperature of the reheated steam is unchanged.After the shutdown, the temperature of the equipment is relatively high, which is beneficial to the rapid start.
(3) We improve the economy of the unit.The stage loss and moisture loss are also small at low loads [6].The lower the power consumption of the feed pump with the load.However, the power consumption of the feed pump is basically unchanged during constant pressure operation, so the economy of the sliding pressure operation unit can be improved [7].

Types of sliding pressure operation
Sliding pressure operation includes three regulating modes: pure sliding pressure, throttle sliding pressure, and composite sliding pressure [8].
(1) Pure sliding pressure operation: The governing valve is fully open to adjust the boiler outlet pressure to adapt to the load change [9].
(2) Throttling and sliding pressure operation: The governing valve is not fully opened during normal operation.When the load suddenly increases, we open the speed regulating valve, and when the steam pressure increases to a certain degree, we close the valve until the original opening.Because the governing valve is often in a throttling state, the thermal economy of the unit is reduced.
(3) Combined sliding pressure operation: The regulation mode combines constant pressure operation with sliding pressure operation.Constant pressure operation is adopted under high and low loads, and sliding pressure operation is adopted under medium loads.It can meet the need for frequency regulation at high loads, improve thermal efficiency at medium loads, and has good load adaptability and reliability.

Seeking for optimal initial pressure curve of steam turbine
During the operation of the generator unit, a certain load corresponds to multiple allowable pressures.Each value in the pressure range composed of these pressures can meet the load under specific conditions, but the heat rate differs.The optimal initial pressure is the heat rate with the best economy within this pressure range.

Figure 2 Pressure range analysis chart
Figure 2 shows that the ordinate P Od represents the rated pressure, and the abscissa N gd represents the rated load.As can be seen from the straight line Od, when the single valve is fully opened, the controllable load range is up to 40% of the rated load; as can be seen from the straight line Oc, the maximum controllable compliance range is 80% of the rated load when the two valves are fully opened; according to the straight line Ob, the maximum controllable load range is 100% of the rated load when the three valves are fully opened; it can be seen from the straight line Oa that the maximum controllable load range is 120% of the rated load when the four valves are fully opened.
Any point in Figure 2 can indicate certain pressure and load state, and the valve opening and closing status can be judged according to the position of the point.If point E is below Oc and above Ob, under the main steam pressure corresponding to point E, to make the unit output reach N gE , at least two valves must be fully opened [10].Otherwise, it is impossible to meet the load requirements.Point M is the minimum pressure value for maintaining load N gE , and the load will drop below this pressure value.Point N is the rated pressure, which is generally the main steam pressure under design conditions.
In Figure 2, the triangle OMN gE is similar to the triangle Oa1.2N gd , from which the minimum main steam pressure (P Omin ) can be deduced.The formula is as follows: where N gd is the rated load, and P Od is the rated pressure.

Optimal initial pressure model
Based on the study of the thermal system of the thermal power unit, the heat rate model of the unit is established.There are many factors affecting the heat rate, which can be expressed in Formula (5): HR f p D N T T X  ( 5 ) In the above formula,   is the heat rate,  0 is the main steam flow,  0 is the main steam pressure,  e is the load,  zr is the temperature of the hot steam,  0 is the main steam temperature, and X is other factors affecting heat rate.
When operating with deep variable load,  0 and  zr in Formula (5) do not change, but  0 and  0 can be adjusted.And  0 and  0 correspond one-to-one, knowing one can find the other.Therefore, Equation ( 5) can be simplified as follows: The range of steam turbine loads is as follows: ,min ,max e e e N N N   .(7) By integrating ( 5), (6), and ( 7), the relational expression (8) between heat rate and initial pressure can be obtained: In Formula (8):  t represents the heat rate value in kJ/(kWh), and X represents other influencing factors;  0 represents the main steam pressure of the unit, and  d represents the rated main steam pressure of the unit, and the pressure unit is MPa.
refers to the unit load,  .max refers to the unit-rated maximum load,  .min refers to the unit-rated minimum load,  d refers to the unit-given load, and the load unit is MW.

Searching for optimal initial pressure by exhaustive method
When operating under variable conditions in the model established by EBSILON, the initial pressure value of the minimum heat rate scenario can be achieved without changing the load output, which is the optimal initial pressure [11].
As shown in Figure 2, the relationship between pressure and load is not a one-to-one correspondence.The same pressure can correspond to different operating conditions, and the same operating condition can also correspond to different pressures.Equation (8) shows that heat rate is closely related to pressure and load values.In contrast, other factors affecting heat rate are affected by the operation status of multiple thermal equipment.It can be considered a fixed value during optimization, which has little impact on heat rate, to reduce calculation difficulty.
Under various working conditions, we change the initial pressure within the feasible range, calculate the heat consumption rate under various initial pressures, and select the most effective initial pressure using the exhaustive method.The specific values are shown in Table 4.We take 30%THA working condition as an example, calculate the heat rate of all allowable pressures within the feasible pressure range at 0.1 MPa intervals under the condition of deep variable load, and select the main steam pressure corresponding to the lowest heat rate that is the optimal initial pressure under 30%THA working condition.Table 5 shows the corresponding relationship between main steam pressure and the heat rate under 30% THA condition.At 7 MPa, the heat consumption rate reaches the minimum value.That is, the optimal initial pressure at 30%THA is 7 MPa.In the model established by EBSILON, the main steam pressure corresponding to the minimum heat rate is obtained under the operating conditions of 100% THA, 75% THA, 50% THA, and 40% THA by the same method and connected into a line to draw the optimal initial pressure curve of thermal power unit under deep variable operating conditions, as shown in Figure 3. (2) The mathematical model of initial pressure optimization aiming at the minimum heat consumption rate of the unit is established.The initial pressure optimization of the target unit under the depth-varying operating conditions from 100% THA to 30% THA is performed by using the exhaustive method, and the optimal initial pressure curve of the unit is obtained.

Figure 1
Figure 1 Thermal model diagram of power supply unit under VWO Condition 2.2.Accuracy verification of the model 2.2.1.Thermal economy index There are three thermal economic indicators for power plants: plant thermal efficiency cp  , heat rate of

Figure 3
Figure 3 Optimal initial pressure curve of thermal power unit under deep variable working conditions4.Conclusion(1) This paper takes the 650 MW intermediate reheat condensing steam turbine unit as the research object, uses EBSILON to build the power plant model, and verifies the error of the unit heat rate.The maximum relative error is about 0.58%, which does not exceed the allowable error.(2)The mathematical model of initial pressure optimization aiming at the minimum heat consumption rate of the unit is established.The initial pressure optimization of the target unit under the depth-varying operating conditions from 100% THA to 30% THA is performed by using the exhaustive method, and the optimal initial pressure curve of the unit is obtained.

Table 3
Error comparison between simulation results and design values of simulation parameters under different loads

Table 4
Optimal initial pressure and corresponding heat rate under various conditions

Table 5
Corresponding relationship between main steam pressure and the heat rate under 30%THA