## 5 Conclusions  
The complex integration of power flow and information flow has presented challenges to the analysis of operational characteristics in CPPS. A hardware-in-the-loop co-simulation scheme of CPPS was put forward in this paper, and the effectiveness of the platform was demonstrated by a load control case. Through the formation and measurement of communication delay, the paper discusses the influence of real-time performance and reliability on the operation stability of CPPS, verifying that the platform has high processing capacity for data transmission and analysis performance. Moreover, the platform is applicable for multiple CPPS services such as coordination control and safety analysis.  
The proposed platform possesses a distributed framework, which has a flexible structure and wide adaptability. Through block configuration utilizing its modular architecture, various kinds of simulation time scales and precision can be obtained in all distributed simulators. Moreover, through the integration of digital and hardware simulation, the advantages of both simulation types could be balanced to truly reflect the dynamic, static, and nonlinear characteristics of CPPS, and provide experimental support for the verification of the relevant CPPS theory.  
With the large-scale access of distributed generation and micro-grid systems, the stability control of the power grid will be more complicated. To deeply utilize and improve the current platform, further research will be devoted to the following areas:  
1. The power communication network modeling requires improvement. At the functional level, the IP network simulation should be established for the scheduling data network, the SDH/OTN network simulation should be established for the power transmission network, and the EPON/GPON simulation should be established for the distribution communication network.
2. Different information collection, transmission, and control processes of multi-CPPS services require research, and the real-time simulation mode needs to be analyzed according to the different precision and speed requirements of power system services.
3. The cyber-attack against the CPPS service, application, and safety operation requires analyzing. At the communication level, various attack behaviors (e.g., communication interruption, DOS attack, and fault data injection) can be simulated. At the power and control level, security systems can be deployed to detect and defend the potential attack behavior. Therefore, an attack-defense platform can be established to improve cyber-security in CPPS.  
This work is supported by the National Natural Science Foundation of China (Grant No. 51577030), the Project of State Grid Corporation of China under Grant No. SGRIXTKJ [2016] 454 (Research on Architecture and Several Key Technologies for Grid Cyber Physical System), and the National Key Research and Development Program of China under Grant 2017 YFB0903000 (Basic Theories and Methods of Analysis and Control of the Cyber Physical Systems for Power Grid).  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] contributed in developing the ideas of this research. [PERSON] conceived and designed the experiments; [PERSON] analyzed the data and wrote the paper. [PERSON] and [PERSON] provided guidance for practical engineering applications. All the authors read and approved the final manuscript.  
The authors declare no conflict of interest.