Optimization and Performance Improvement of Automatic Generation Control (AGC) for 330 MW Subcritical Thermal Power Unit

With the continuous assessment of the “Two Detailed Rules”, various power generation groups are paying increasing attention to AGC. A 350 MW subcritical unit has been in a poor performance state under AGC-R mode since the 2022 heating season, with a K1 value below 1.2 and a K2 value not reaching the qualified value. It is facing the cancellation of the qualification for AGC-R mode. In the face of the problem with the unit, this article analyzes three reasons. Firstly, the speed setting exceeds the requirements, but the actual unit action does not meet the standards. There is a deviation in power and a significant deterioration in control quality during the turning point and continuous climbing of AGC instructions. Based on the above analysis, this article proposes three renovation plans to optimize the instructions through nonlinear correction functions and achieve the elimination of instruction deviations. It calculates and judges the difference between the corrected AGC load command value and the actual power value of the unit. It calculates the deviation and sends the deviation to a saturated nonlinear link to generate the feedforward of the main turbine control, improving the output of the main turbine control. By setting a variable integral function, the main control output of the boiler is improved. After this technical transformation, the AGC performance of the unit has been significantly improved.


Introduction
Automatic Generation Control (AGC) is an important function in the energy management system EMS, which controls the output of frequency modulation units to meet the constantly changing power needs of users and keep the system in an economical operating state.In the joint power system, AGC controls the output of generators within the region on a regional system basis [1][2].We must ensure the safety of power generation equipment in power plants.Stable operation, high-quality service, and economical operation [3][4] are proposed.Social development is more in line with the requirements of electricity generation.Production is uninterrupted in order to ensure safe operation.After the implementation of the "Two Detailed Rules", the assessment efforts have been continuously strengthened.As the load regulation quality of the unit increases, the defect of poor quality gradually [5] becomes apparent.It affects not only the safe and stable operation of the unit but also the assessment of the power grid.The unsatisfactory outcome also affects the company's economic interests.Therefore, it is urgent to optimize the coordinated control system of the unit, improve the steady-state operation performance [6][7] and variable load performance [8] of the unit, and also improve the profitability of the "Two Detailed Rules".

Active Load AGC Command
Master Command Load Command

Problem description
A certain unit is a 350 MW subcritical unit, and its performance has been in a poor state under AGC-R mode since the heating season of 2022.After the heat supply decreased in February 2023, the power plant organized multiple disciplines, such as thermal engineering and operation, to conduct selfanalysis, but no feasible technical solution was found.Not only does it seriously affect the market revenue of auxiliary services for power plants, but it is also on the brink of being disqualified from investing in the AGC-R model.
The operation data of the power plant is analyzed, and the main parameters are shown in Figure 1.It was found that there are many problems with the unit, mainly concentrated in the following points:  Although the AGC regulation rate of the unit is set at 7 MW/min, which is higher than the 1.5% Pe/min specified in the "Two Detailed Rules", the assessment index did not meet the expected 1.2. There is a power deviation in actual load control, and the adjustment accuracy of the unit has not reached the specified 1.0 qualified value. When the unit is at the turning point of AGC command and continuously climbing, there is a significant deterioration in control quality, and the response time index is also lower than the majority of units in the province.
Figure 1.AGC regulation curve of a certain 330 MW unit.

Transformation and improvement plan
At present, the AGC assessment of Shandong Power Grid mainly involves three main indicators: regulation rate, regulation accuracy, and response time.
Regulation rate K 1 refers to the rate at which the unit responds to the set point command, which can be divided into the rising rate and falling rate.Adjustment accuracy K 2 refers to the difference between the actual output and the set point output after the unit response stabilizes.Response time K 3 refers to the time taken by the EMS (Dispatching Energy Management System) to reliably cross the regulation dead zone consistent with the regulation direction, based on the original output point, after issuing instructions.
A certain unit is a 350 MW subcritical unit.In response to the quality non-compliance of its AGC operation control results in the assessment and evaluation of dispatch, three directions of transformation and improvement work have been carried out around the three main indicators of the assessment.

AGC power signal correction
By judging the real-time changes in the load of the unit, feedforward optimization and adjustment are carried out on the main control of the steam turbine of the unit, and line optimization is carried out on the load change rate of the unit.This ensures that the unit can adjust the rate index in the PROPR mode while effectively improving the regulation accuracy and response time.

AGC deviation correction of power command.
The AGC power command is controlled by EMS through RTU to DCS [9][10], which involves multiple conversions.The provincial dispatch is contacted for command comparison, and the AGC load command value issued by EMS is analyzed for deviation.The deviation at the specified load point is found, and the deviation is added to the original command value.The instruction is optimized through a non-linear correction function, as shown in Table 1

Verification and correction of actual power
The power signal used for AGC assessment is transmitted from one power transmitter of the power plant through RTU, while the power signal used for CCS control of the unit is the signal obtained by taking three out of different power transmitters [11].There is a deviation different signal sources.By utilizing the advantages of the network source platform's provincial dispatching assessment and complete power plant operation data, a signal correction method based on dynamic weighted residual correction is adopted to automatically calculate the dynamic residual and use it for on-site power correction.

Logic optimization and parameter adjustment of steam turbine main control and boiler main control
Based on the AGC assessment regulations, renovation technologies such as nonlinear compensation for steam turbine main control and dynamic adjustment of boiler main control integral parameters are proposed.At the same time, the feedforward signal of the steam turbine main control is optimized to reduce the interference of primary frequency regulation on AGC.

Optimization treatment of steam turbine main control
The difference between the corrected AGC load command value and the actual power value of the unit is calculated and judged.The deviation is calculated and sent to a saturated nonlinear link to generate the feedforward of the main control of the first steam turbine.The difference between the corrected AGC load command value and the actual power value of the unit is calculated and judged.The deviation is calculated, and the load change rate is switched by controlling the selector switch of the switch module based on whether the deviation exceeds the set limit value, as shown in Figure 2. At the same time, the feedforward of the steam turbine main control is adjusted from the original load command given by the unit after primary frequency regulation to the load command given by the unit before primary frequency regulation.This is to reduce the invalid power jitter caused by transient fluctuations in primary frequency regulation.

Boiler master control optimization processing
When operating in AGC mode, the unit is usually in CCS mode, where the turbine side regulates power, and the boiler side regulates the pressure.However, due to the inherent hysteresis of boiler-side combustion, coupled with the prolonged combustion cycle caused by denitration and low nitrogen combustion modifications in recent years, there is a common occurrence of AGC load command changes in units.The cycle time from boiler control command changes to coal quantity changes and combustion work is much longer than the time required for turbine side load adjustment.In other words, during a certain adjustment process, there may be AGC load command changes.If the adjustment direction has been reversed, the boiler side only releases the energy required for the last load command change, forming a reverse adjustment between the boiler side and the turbine side.Due to the development of large-scale remote transmission of AC and DC, the power grid has higher requirements for AGC regulation rate, requiring a faster load regulation rate on the turbine side of the unit, resulting in increased fluctuations in main steam pressure.It will further exacerbate the problem of mismatched time matching between the boiler side and the turbine side.It has a certain negative impact on the stability of the coordinated operation of the unit and the accuracy of load control.Further, it affects other assessment indicators, such as the assessment of the dispatch and control department, as shown in Figure 3.When the load command of the unit changes rapidly beyond the set range, that is, when it is in fast load regulation mode such as AGC, the system output is a fixed and large integral time constant to the main controller of the boiler.When the load command of the unit changes slowly within the set range, that is, when tracking the planned output curve of the power grid for conventional load regulation mode, the system outputs the integral time constant of the boiler main controller generated by the load command after passing through the segmented function F (x).

Transformation effect
In mid to late March, the technical research team of the grid source coordination room of the power grid technology center conducted remote diagnosis based on the AGC function module of the grid source platform.It processed the AGC regulation circuit and operation data of the unit on site, successfully solving on-site technical problems, as shown in Figure 4.

Conclusion
Compared with the previous AGC performance indicators, the adjustment indicators and depth have significantly improved.From Table 2 and Figure 7, it can be clearly seen that after optimization and transformation by our technical experts, the AGC assessment indicators of the unit have significantly improved.Compared with the assessment data in February, the adjustment rate K 1 has increased from unqualified 0.96 to the upper limit 1.20, the adjustment accuracy K 2 value has increased from unqualified 0.93 to 1.17, the response time K 3 has increased from 1.64 to 1.76, and the comprehensive indicator K p has increased from around 1.49 to 2.45.The daily compensation income of AGC increased by nearly 10000 yuan in the assessment of the "Two Detailed Rules" by the power plant.

Figure 2 .
Figure 2. Logic diagram of nonlinear compensation for steam turbine main control under coordinated mode.

Figure 3 .
Figure 3. Logic diagram of boiler optimization control system based on dynamic adjustment of integral parameters.

Table 1 .
. Load command deviation correction calculation table.