I. Introduction With the widespread application of Distributed Control Systems (DCS) in power plant control systems and the gradual formation of the power system market mechanism, the concept of a plant-level monitoring information system (SIS) aimed at "economic operation" is gradually being understood and accepted by power plants, design institutes, and control system manufacturers. However, because the concept of SIS is artificially proposed, it does not have the clear boundaries of a unit-level DCS system, especially in terms of advanced application software. Due to this uncertainty, many specific problems have been encountered in practice. This article discusses some preliminary views on this issue. II. The Inevitability and Necessity of the Emergence of SIS Systems Modern large-scale power enterprises invariably adopt an "unit control, plant-wide management" operation model, with production process control and enterprise management being two important aspects. In terms of improving overall enterprise efficiency, the two are intertwined and inseparable; however, in terms of the timeliness of their information, there is a significant difference between them, making it impossible to directly link them using existing information systems. Therefore, there is a need to add an information system between the DCS and MIS systems, building an information bridge to organically connect unit production information and enterprise management information, thereby further reducing power plant operating costs and improving overall efficiency. This led to the concept of the SIS system. Before the emergence of the SIS concept, the common method for plant-level real-time production management was to add equipment management modules to the DCS system or real-time production data to the MIS system. Practice has shown that both solutions have brought new problems to power plants. On the one hand, adding too many complex analysis, optimization, and diagnostic functions to the DCS system makes the DCS structure increasingly complex, inevitably leading to data blockage or software "crashes," reducing the reliability of production process control. On the other hand, analysis, optimization, and diagnostic functions require the support of a large amount of historical operating data from unit units. Adding massive amounts of second-level data to the "static" MIS database complicates the structure of the MIS database and greatly increases the difficulty of implementing the entire MIS system project. Therefore, it is essential to add an independent integrated control system between the "static" plant-wide MIS system and the dynamic unit DCS system. This system should take into account both the "dynamic" real-time production information of the entire plant and the "static" historical operating information of each unit, with the goal of optimizing plant-wide operation and management. This system is the SIS system. III. Challenges in Implementing SIS Systems and the Necessity of DCS/SIS Integration The SIS system should first and foremost possess the ability to display and store real-time production information for the entire plant. This is undisputed, acceptable to all parties, and not particularly difficult to implement. Essentially, it is an expansion of the DAS and HIS functions of the DCS system in terms of scope and extent, which will not be discussed in detail here. The more controversial aspect is the analysis, optimization, and diagnostic functions of the SIS system. These functions can be further divided into basic and extended functions. Basic functions include: plant-level performance calculation, unit performance calculation, unit economic index analysis, optimized operation and operation guidance, equipment status monitoring (excluding diagnostics), and online unit performance testing. Regardless of the appropriateness of this classification, there are significant problems even in terms of basic functions. Firstly, there's the conflict between the basic functions of a SIS system and the advanced functions of a DCS system. Should the online performance calculation and consumption difference analysis of unit cells belong to the DCS system or the SIS system? For a long time, people have been accustomed to considering the online performance calculation and consumption difference analysis of unit cells as a sub-item of the DAS function of a DCS system. Now, classifying them under the SIS system inevitably leads to confusion. Secondly, analysis, optimization, and diagnostic software cannot function without DCS measurement points, but the conventional DCS measurement point layout often cannot meet the requirements of these software functions. This necessitates that the measurement point requirements of analysis, optimization, and diagnostic software be considered during the implementation of DCS projects. Third, most analysis, optimization, and diagnostic software (including online performance calculation and consumption difference analysis software) are "expert-level" software. This means the software itself rarely provides definitive conclusions, but rather offers a basis for further judgment by "experts." The most authoritative software often relies on the software developers themselves as "experts." Therefore, to achieve optimized operation of power plants, a SIS system with closed-loop control aimed at optimized operation is a necessary requirement. In summary, since both DCS design practices and SIS technical requirements include advanced application software functions aimed at "economic operation," this reflects that both the DCS and SIS systems should play a role in economic operation. Separating them is not entirely appropriate; a more accurate term would be an integrated DCS/SIS system. In such a system, advanced application software serves as the functional link connecting the DCS and SIS systems. IV. Xinhua Company's Practice with Advanced Application Software 1. Xinhua Company's Thermal Power Plant Performance Analysis System XPAS-400 Xinhua Company's thermal power plant performance analysis system XPAS-400 is an advanced application software system developed by Xinhua Company based on its core system software platform technology-400, which possesses completely independent intellectual property rights. Unlike other software of this type, XPAS-400 utilizes software platform technology consistent with Xinhua Company's DCS system and SIS system, effectively solving the problem of functional software configuration for operation in the DCS and SIS systems. Because XPAS-400 is developed based on the XDPS-400 software platform technology, it is very convenient for power plant personnel to master. The functional software packages in the XPAS-400 system have the following characteristics: ● Performance analysis and calculation are performed entirely using the measurement points of the unit's DCS system; ● Developed on the XDPS software platform, advanced analysis software functions are implemented through configuration; ● Each functional analysis and calculation software package is both relatively independent and a unified whole; ● Functional software packages can be configured in the DCS system or SIS system according to the power plant's application requirements and the structural needs of the SIS/DCS hardware platform; 2. Advanced Application Software Solution in Xinhua's Simple DCS System for 300MW Units According to the unit DCS system specification, the advanced application software in the DCS system includes unit performance calculation functions and consumption difference analysis functions. In Xinhua's simple DCS system using XDPS, this function is implemented using an independent performance calculation station (PFS) within the system, as shown in Figure 1. The advanced application software uses configuration-based performance analysis software based on XDPS's virtual DPU technology. During real-time calculation, it belongs to the DCS layer network, and its network access points are combined with the network access points in a general DCS into a point directory file, with the same status. 3. Advanced Application Software Solution in Xinhua DCS/SIS Integrated System of Xingtai Power Plant The advanced application software of the Xingtai Power Plant's SIS system includes performance calculation and consumption difference analysis for each unit (a total of six 200MW units), as well as multi-unit load optimization and allocation based on performance calculation and consumption difference analysis. Since the DCS of all six units adopts Xinhua's XDPS system, the final system is the Xinhua DCS/SIS integrated system. In this system, two plant-level performance calculation stations are used to implement the functions of the advanced application software, as shown in Figure 2. The advanced application software adopts a configuration-based performance analysis software based on XDPS's virtual DPU technology. During real-time calculation, it belongs to the SIS layer network, and its network access point is combined with other network access points in the SIS to form a point directory file. 4. Discussion of Advanced Application Software in DCS Layer and SIS Layer ● Xinhua's configuration-based advanced application software, based on XDPS technology, can belong to either the DCS layer network or the SIS layer network during real-time calculation. ● For DCS system security considerations, advanced application software at the DCS layer should employ mature and reliable software. Currently, only performance calculation and consumption difference analysis software can be considered, and it should be put into operation during the DCS system construction process as much as possible. ● The measurement point requirements for advanced application software should be considered during DCS system construction. Typically, the number of measurement points related to performance calculation and consumption difference analysis software should be no less than the number of measurement points required for performance testing. ● Due to the limitations of the DCS system construction cycle, advanced application software at the DCS layer cannot be adjusted extensively based on actual operating data, which will affect the accuracy of the software calculations. To ensure the software matches actual operating data, a DCS/SIS integrated system should be adopted, with the advanced application software placed at the SIS layer. ● Verification issues for advanced application software at the DCS and SIS layers. Because the advanced application software at the DCS and SIS layers are at different levels, the verification standards for the same function should also differ. Typically, the criteria for performance calculation and consumption difference analysis results in the DCS system can be based on the main equipment's design calculations, while the criteria for performance calculation and consumption difference analysis results in the SIS system should be adjusted based on the actual unit's operating data. The Electric Power Industry Thermal Automation Standardization Technical Committee should organize experts as soon as possible to formulate the inspection standards for advanced application software of SIS system. 5. Closed-loop control problem for optimized operation In order to achieve the optimized operation of power plants, a DCS/SIS integrated system with the goal of closed-loop control for optimized operation is necessary and is also the development goal of SIS system. Reference [3] only stipulates the hardware conditions for achieving closed-loop control: "SIS and DCS must be set up with independent networks, and the information flow should be designed in a unidirectional manner, allowing only data to be sent from DCS to SIS. When certain functions configured in the SIS in the project require SIS to send control commands or set value commands to DCS, hard-wiring should be used to achieve this, and necessary data correctness judgment functions should be set up on both the SIS side and the DCS side respectively." However, this is far from enough. The closed-loop control of a large system involves much more complex issues than simple data transmission issues. Achieving this goal from the aspects of software, system debugging and engineering organization requires a process. At present, it is only appropriate to conduct small-scale pilot projects in power plants with suitable conditions to gain experience. 6. Conclusion and Outlook ● Since Mr. Hou Ziliang proposed the concept of SIS systems in 1998, it has generated considerable buzz in the industry, demonstrating its strong vitality and market potential. However, as SIS systems are a man-made concept, their boundaries will remain blurred for a considerable period. Therefore, each vendor's SIS will exhibit distinct characteristics. Xinhua's XSIS system software technology, based on the XDPS platform, is unique precisely because we possess the XDPS software platform and the configurable advanced analysis application software XPAS embedded within it. ● The inherent requirements of advanced application software in SIS systems necessitate an integrated DCS/SIS system. In such an integrated DCS/SIS system, advanced application software serves as the functional link between these two system layers. ● In developing integrated DCS/SIS systems, the advanced functions of a pure DCS system still have their necessity and room for development. Where the advanced application functions of the DCS system and the basic application functions of the SIS system overlap, appropriate distinctions and differentiations should be made in terms of software analysis depth. ● Testing standards for advanced application software in SIS systems should be developed as soon as possible to guide the development and application of such software and improve its reliability and usability. ● Optimized closed-loop control is the development goal of SIS systems. Achieving this goal requires a process in terms of software, system debugging, and engineering organization. Currently, it is only advisable to conduct small-scale pilot projects in suitable power plants to gain experience.