Abstract: This paper combines the practice of simulation control technology of process industrial systems such as thermal power generation at home and abroad, and fully defines the introduction . The widespread application of distributed control system (DCS) in process industries such as power, petrochemical and metallurgy is one of the greatest achievements in the field of international automatic control technology in the 1990s. After more than ten years of rapid development and fierce competition, several major automatic control system suppliers in the United States, Germany, Japan and other countries have launched three generations of DCS products. China has also produced DCS products with independent intellectual property rights. At present, the application of DCS has become mature. DCS is undergoing technical restructuring and adjustment, but its technological progress has not stopped [1]. With the promotion of computer information network technology, the industry has put forward the demand for "networking", "informatization" and "digitalization". As the foundation of digitalization, DCS will play an irreplaceable role. The application of DCS system will further develop towards control and management integration, while the full-range simulation of DCS is developing towards the so-called "virtual" technology. This article, based on relevant international standard drafting content and the latest information provided by foreign software development organizations, and combined with years of technical practice in process industrial system simulation such as thermal power generation both domestically and internationally, fully defines the concept of "virtual DCS" technology, and provides a detailed classification and discussion. This article particularly emphasizes the development and application of "virtual DCS" technology on open computer software and network platforms, comprehensively discussing the technical paths and implementation schemes of open platform "virtual DCS," for reference and discussion by domestic and international peers. I. Definition of Virtual DCS Virtual DCS is relative to the real DCS running in a process industrial system. "Virtual DCS" is the reproduction of a real DCS in some form on a non-DCS computer system. "Virtual" is a widely used high-tech concept today, such as "virtual reality" for visual simulation, "virtual instruments" using CRT interaction, and "virtual conferencing" for building remote multimedia two-way communication, etc. Of course, virtual technology is entirely built upon today's high-performance computer hardware, software, and network systems. Unlike other virtual technologies, the virtual DCS is a computer system, not a physical system. The purpose of virtual DCS is to reproduce a computer system on a computer system. Specifically, it is to reproduce a distributed control computer system as realistically as possible in a computer information management system, which is usually an open platform. Virtual DCS is one of the foundations of process industry digitalization. In practical applications, in order to achieve system application goals such as design and debugging, personnel training, and testing and diagnosis, it is necessary to reproduce the real DCS in a non-DCS computer system. There are currently three forms, which are implemented by acquiring system resources according to different stages of the DCS life cycle, such as control design, offline configuration, and the formation of the operating system, as shown in Figure 1. (1) Stimulation DCS - usually a simplified input/output board and peripherals, using the appropriate or minimum configuration of the hardware, software and network system of the real DCS to reproduce the DCS. Stimulation DCS has the highest hardware and software fidelity, but the hardware and software implementation cost is very high, it is difficult to connect with the object model system, and it cannot complete complex simulation application functions. (2) Virtual DCS – After completing the DCS configuration, the DCS platform is transferred and reproduced by intelligently compiling and converting the DCS network downloaded files. The virtual DCS should have extremely high software function realism, low implementation cost, and be able to complete complex simulation application functions. (3) Simulation DCS – As long as the DCS completes the control function and logic design, simulation can be performed according to the design drawings. Simulation DCS has been the common form of training simulation systems for many years. Although the implementation cost is low and it can complete complex training simulation application functions, the software function realism and reliability are relatively low, and it is difficult to track and modify. It can hardly complete advanced application functions other than personnel training functions. The characteristic of virtual DCS is that the control parameters and algorithms are completely derived from the downloaded files. It uses the same algorithms, modules, time slices, tag numbers, etc. as the DCS, and can be modified and updated synchronously. The software function realism is very high. It can be said that virtual DCS can be truly effective, economical and widely used in personnel training and online detection and diagnosis, meeting the "digitalization" needs of process industries such as thermal power generation. II. Classification of Virtual DCS Between fully stimulated DCS and fully simulated DCS, there are various types of virtual DCS. Since DCS is mainly composed of Distributed Process Units (DPUs) and Human Machine Interfaces (HMIs) at operator stations, there are corresponding classifications such as "virtual DPUs" and "virtual HMIs". The various permutations and combinations of virtual DPUs, virtual HMIs, and partially virtual ones constitute the classification of virtual DCS. A detailed classification of virtual DCS is helpful for unifying concepts, clarifying planning and application development, and is very necessary for research and engineering practice. The classifications listed here, except for (1) and (8), all belong to the scope of virtual DCS. (1) Fully stimulated DCS - The number of DPUs and their running software, the number of HMIs and their running software are completely consistent with the real DCS, and only the interface software for connecting the process model needs to be developed. At present, most of the simulation systems used abroad and the simulation systems of large imported units in China adopt this kind of completely realistic technical implementation scheme. (2) Minimum configuration DPU + real HMI – The originally scattered DPU software is concentrated on one DPU to run. The DPU hardware takes the minimum stimulus configuration, while the DPU software needs to undergo some virtual modification to develop real-time data sharing interface software. A real HMI is used, which has complete realism for the operation training of operators. (3) Minimum configuration DPU + simulation HMI – Similarly, the DPU hardware takes the minimum stimulus configuration and the DPU software is virtually modified, but the HMI is developed using a third-party human-machine interface configuration tool and implemented on an open computer software and network platform. It has complete functional realism for online use by management and maintenance personnel. (4) Virtual DPU + real HMI – In order to avoid the development of complex DPU process model interface software, and at the same time obtain rich reproduction DCS functions and high realism, this type of virtual solution can be adopted. (5) Virtual DPU + virtual HMI – This is a completely virtual DCS type. Whether it is DPU or HMI, its virtual software is obtained by intelligent compilation and conversion of DCS download files. This is an ideal technical solution to save investment, shorten the development cycle, obtain the highest realism and the most application functions. (6) Virtual DPU + Simulated HMI – When HMI software cannot achieve intelligent compilation and conversion, a third-party human-machine interface configuration tool is used for development. The HMI is transferred to an open computer software and network platform. The DPU is virtualized. This is a very practical technical solution for personnel training and online detection and diagnosis. (7) Configuration Diagram Virtual DPU + Simulated HMI – If neither the DPU nor the HMI software can achieve intelligent compilation and conversion, the virtualization method is to simulate the configuration diagram software. A set of graphical configuration software with an appearance and operation mode that is very close to the real DCS is developed separately on an open computer software and network platform. The system personnel or maintenance personnel perform synchronous configuration and modification of the real DPU and the virtual DPU. The configuration diagram virtual DPU generation software can output virtual programs and connect with the HMI of the process model to realize the power distribution of the virtual DCS. (8) Full Simulation DCS – This is the technical solution commonly used in domestic personnel training simulation systems. Various virtual DCS classifications fall between fully stimulated DCS and fully simulated DCS, with their fidelity, application functionality, and investment savings conforming to the distribution shown in Figure 2. Currently, virtual DCS exhibits a relative comprehensive advantage due to its near-stimulated DCS fidelity, near-simulated DCS application functionality, and minimal investment. III. Technical Implementation of Virtual DCS The key to implementing virtual DCS lies in developing intelligent compilation and conversion software and establishing a virtual operation technology platform. The intelligent compilation and conversion software and algorithm module software are developed using general-purpose tools such as Visual C++ 6.0, enabling them to run on the open Microsoft Windows platform. This allows the virtual DCS to reproduce DCS functions on platforms such as UNIX within a management network connecting the plant-level SIS, MIS, and simulation systems. The virtual DCS, along with the process mathematical model, becomes a readily accessible detection and diagnostic tool for operation analysis and maintenance management personnel on their desktop computers. The software technology development of virtual DCS requires a comprehensive and unified software approach to process the configuration information of various real DCS systems; its substantive work is no less demanding than developing several DCS software operating system programs. There are various technical means and methods for realizing virtual DCS, and the software packages of different research and development institutions are also different. The author of this article believes that the core software technology of virtual DCS consists of the following components: (1) DCS technical data - Since virtual DCS is to develop the underlying simulation program of DCS, it is necessary to refer to the complete DCS underlying technical data, have a deep understanding of the hardware and software system structure, configuration method, algorithm module, system management, database management, file management and communication protocol of DCS, and receive technical training from DCS manufacturers, and even obtain technical support from DCS manufacturers to solve the problems that may occur in the virtualization process. (2) Intelligent scanning and interpretation - The intelligent compilation and conversion software of virtual DCS first scans and interprets the downloaded file code after the real DCS configuration or modification is completed. Since these codes do not have a standard and unified pattern, but are different special forms of assembly language defined by various DCS manufacturers, their scanning and interpretation need to have a certain intelligent judgment function, and intelligently interpret and process special contents such as code connection, communication, redundancy, fault tolerance, exception, system management, etc. After the scan interpretation is completed, a complete virtual DCS configuration semantic library can be established to provide source data for unified code conversion and output. (3) Automatic code generation - The problem faced by virtual DCS is that the downloaded files before conversion will come from different DCS systems, and the converted code must be suitable for different simulation or analysis calculation program requirements. The best solution is to automatically generate standard and uniformly formatted Visual C++ 6.0 program code files based on the configuration semantic library established by the scan interpretation results, and then consider special processing in the interface program module. The automatically generated content includes database definition, initialization definition, signal connection definition, instantiation definition between algorithm module classes, module group definition, interface definition, etc. Automatic code generation requires no manual intervention, no manual modification, and continuous batch completion. (4) Program compilation and debugging - The program compilation system of virtual DCS is based on a standard C++ compiler and has strict syntax and link checking functions. If semantic logic errors, data type errors, or missing link definitions occur, the system will submit alarm or debugging information. (5) Virtual Parameter Database – The parameters of the virtual DCS include input/output variables, controller parameters, network variables, human-machine interaction variables, etc. Data types include analog quantities, switch quantities, integrated quantities, character quantities, etc. It is required to be able to add, delete, modify, and define Chinese descriptions, etc. (6) Virtual Algorithm Module Library – The DPU control algorithm module of the virtual DCS should be developed entirely based on the modules defined in the real DCS. There are about one hundred module types for each type of DCS, including input/output modules, analog control modules, logic control modules, sequential control modules, and special calculation modules, etc. It is required to use object-oriented real-time control system module programming technology to create virtual DCS algorithm classes, reflecting the advanced software technology characteristics of encapsulation, inheritance, and polymorphism. (7) Human-Machine Interface Component Library – If a simulation HMI solution is adopted, a third-party human-machine interface drawing and configuration software should be used for development. If a virtual HMI solution is adopted, various dynamic components of the human-machine interface should be developed, including data display, status display, buttons, graphs, bar charts, and soft handheld devices, etc. The component library calling program should also be automatically generated or generated with minimal manual intervention. Special attention should be paid to generating embedded logic programs or script programs with the same functions as the real DCS to ensure the realism of the virtual HMI. (8) Process model interface - The virtual DCS should retain all the DCS input and output measurement point numbers, and the connection with the process object mathematical model can also simulate the on-site installation measurement points and debugging process. (9) Real-time scheduling function - The virtual DCS is also a real-time program, and real-time scheduling needs to be performed during runtime. It is required to retain the same time slice and real-time scheduling combination as the real DCS during compilation and conversion to improve the realism of the virtual HMI. (10) Virtual simulation application function - The virtual DCS is developed and run on a general and open computer software platform. Therefore, it can be connected and run with the real DCS system, or run in a closed loop with the process mathematical model system, or networked with the SIS system real-time data or historical data to realize complex virtual simulation application functions such as fast storage, initial conditions, interface, check, return, replay, comparison, statistics, prediction, simulation, optimization, etc. IV. Current status of virtual DCS application For many years, foreign simulation systems have used the method of stimulating DCS, while domestic simulation systems have adopted the method of simulating DCS. Recently, the research and development of real-time computer simulation and control system technology and intelligent compilation software technology have been able to provide virtual DCS solutions. The corresponding research and development in China has started, while some simulation control system research and development institutions in the United States and Germany have provided complete virtual DCS software packages [2]. As one of the application examples of virtual DCS technology, the software design was carried out based on the above core technical concept. The author of this paper was responsible for organizing and completing the virtual development of a certain DCS, realizing the full set of control functions and logic of DAS, MCS, SCS, FSSS, electrical, network control and other functions of large thermal power units. There are a total of 6151 interface points and more than 60000 state and parameter points. The total amount of the final execution module program is 8.667MB, which can be put into actual operation together with simulation and mathematical model software. In the ANSI/ISA-77.20 American National Technical Standard "Technical Specification for Thermal Power Plant Simulation System" [3] revised in 1994, the American Instrumentation, Systems and Automation Society (ISA) made technical descriptions of excitation DCS and simulation DCS, but did not directly involve virtual DCS. In order to standardize the research, development and application of virtual DCS, ISA established a working group for drafting the ANSI/ISA-77.21 technical standard in 2001. Technical experts from major US power companies and internationally renowned simulation system companies are working on drafting standard content specifically for stimulating DCS and virtual DCS. The author of this article also participated in the technical discussions of the working group. It is believed that after the official release of the ANSI/ISA-77.21 standard, it will become a de facto international standard and will play a great role in promoting the development and application of virtual DCS technology. The simulation of thermal power generating units is already quite popular in China. Recently, the plant-level monitoring information system (SIS) of the power industry has also developed significantly [4][5]. One application of SIS is to perform online detection and diagnosis. Due to various constraints, this application has not yet been truly developed. The basis for this application of SIS is to have an accurate process mathematical model, and the premise of verifying the process mathematical model is to adopt the virtual DCS technical solution. After the mathematical model has been verified, it forms a closed loop with the virtual DCS, providing data reference and process reproduction. It is compared with the real data from the field to obtain reliable detection and diagnosis results. V. Conclusion The "virtual DCS" technology, developed based on today's high-performance computer hardware, software, and network systems, reproduces a real DCS in some form on a non-DCS general-purpose and open computer software and network system platform. The current state of research, application, and standardization of virtual DCS technology both domestically and internationally demonstrates that virtual DCS is a new simulation control system technology that has received attention and investment from user units, development institutions, standard setting, and management, and has a significant role in promoting the "digitalization" process of process industries such as thermal power generation.