## 3 Establishment of CPPS Real-Time Hardware-in-the-Loop Co-Simulation Platform
### Simulation Requirements and Overall Structure for the Platform  
One of the most important features of a CPPS is that complete and detailed information can be obtained by means of advanced sensor and high-speed communication networks. This includes various types of measured data such as frequency and voltage collected by the phasor measurement unit (PMU) and remote terminal unit (RTU), and external data such as meteorological information and intelligent load information. Plenty of information is conveyed to the control center to support monitoring, regulation, and protection.  
To investigate the interactive coupling characteristics between power flow and information flow, an accurate wide area load control case in the source-network-load system was put forward that can promote wide area protection effects. However, there is currently no related research on this kind of scheme. In a traditional power system, the source follows the change of load in one way. The source-network-load system is a flexible system where the power supply, transmission network, and intelligent load respond to and reserve each other. The status information of three layers is gathered to obtain the optimal control strategy, which requires fast and accurate awareness of multi-layer information. As shown in Figure 2, the type and value of load terminal information should be accurately and rapidly informed by the control layer. When a security risk occurs in the system, an optimal load shedding strategy is executed according to the priority of load to maintain thevoltage and frequency stability of the grid. Therefore, this kind of power service demands that the power simulator can support the real-time simulation of a large scale network, and an information simulator can realize the physical and logical construction of control devices and a measurement terminal mapped to the power nodes. The communication network can support a special power communication protocol and real-time link simulation, which can transfer the load information and control instructions among the control center, control substation, and the terminal in a timely fashion as well as responding quickly to changes in the power grid situation.  
Load information is gathered from the control terminals in the bottom layer to control centers in the top layer. According to the control strategy and algorithm, control instructions are generated by the control center and are issued by the control substation. This typical multi-level stability control service goes through the process of data acquisition, transmission, processing, transmission, and execution.  
For data processing procedures among multiple data transmissions, an independent control module is established in the platform, which receives and handles data from the information simulator and can return control instructions. As shown in Figure 3, a three-layer co-simulation platform was established, including a power system layer, communication network layer, and a control center layer. The power layer and communication layer were established by the digital simulators RT-LAB and OPNET, respectively, which are responsible for the real-time simulation of the power and communication environments. Meanwhile, the control center layer was established by hardware devices to simulate the control master station and control execution station, thus forming a closed control loop of the whole platform.  
Figure 3: Three-layer design of the CPPS real-time co-simulation platform. ARM: advanced RISC machines.  
Figure 2: Overall structure of wide area load control.