## 1 Introduction
Compared with the independent analysis of these attributes, unified modeling can more effectively describe the internal connection among the multilayer physical and information objects [10; 11; 12]. In order to analyze the interaction characteristics among the physical environment, communication environment, and control flow, typical CPPS services and applications are simulated and analyzed [13]. Mainstream research interests include the contradiction between fast response demand and limited network bandwidth, the interaction between distributed computing modules and the centralized control center, and the interactive optimization algorithm among power sources, networks, and loads [14]. To analyze the impact of communication quality on the power grid operation, there must be guarantees of the smooth interconnection and intercommunication between the power grid and communication grid. The key lies in how to unify the information system and power system fully. The modeling, analysis, and control methods matching CPPS are essential in the design and guidelines of specific power system applications. However, there is a gap in the traditional research on the power and information systems. The power system is a time-varying continuous system where the power flow information is calculated by differential algebraic equations; while the information system is discrete and event-driven, where the key event point can be derived by the finite state machine. Therefore, the split representation features between both systems hinder the unified theoretical modeling of CPPS, which remains to be further researched. Before the breakthroughs of theoretical research on CPPS, software simulations based on the prototype feature of CPPS played an ever-increasing role in simulation, testing, and verification support of theoretical and practical research [15]. Therefore, a platform that simulates the multi-state and multi-fineness characteristics of CPPS needs to be established. Typical power system simulatorsapply the discrete step to approximate the continuous process, while typical information system simulators use the discrete-state model to describe the network behavior. There is no unified software simulating both systems simultaneously. Instead, the co-simulation scheme first simulates power and communication situation alone, and then realizes the data sharing and collaborative work through an interactive interface. This creates a combination of several simulators, which allows participants to interact, coordinate, and cooperate with each other, while conserving their own advantages. Its easy implementation and high reliability make co-simulation a feasible scheme. In recent years, with increasing attention and support of CPPS, several co-simulation tools have been developed, and are summarized in detail in Section 2. The focus of current co-simulation platforms is the data synchronization and coordinated control between both simulators. Based on the simulation speed, the data synchronization methods include real-time and non-real-time methods; and according to the location of control unit, the control methods include internal control and independent control. The main applications of the current platforms are to analyze the impact of the parameters (e.g., delay, throughput, bit error rate of the power communication protocol) on the power service operation. Due to the lack of real-time performance and model accuracy in a digital simulation platform, the process of control loop and the time sequence of data interaction in CPPS cannot be accurately constructed and analyzed. Therefore, a real-time hardware-in-the-loop platform was put forward in our paper, where real-time simulators were utilized to make up for the simulation speed, and hardware control devices were utilized to make up for the model authenticity. As a result, a complete, closed-loop and credible data flow in CPPS was built to simulate and verify the real-world power-communication interaction case.