## 4 Application Scenarios and Platform Extensions  
### Case Study: Wide Area Intelligent Load Control  
An intelligent load case was proposed to verify the effectiveness of the proposed control scheme and platform. As an ultra-high voltage direct current transmission (UHVDC) receiving system, when several AC/DC lines are out of order at the same time, a huge power deficiency will cause a serious frequency decline in the power system. Therefore, through the quick load shedding function of the system, some interruptible loads should be cut within 650 ms to help recover system stability.  
In RT-LAB, a power system of four generators and 12 nodes was set up as shown in Figure 7, with a total load of 1446 MW. A short circuit fault caused a disconnection between both main lines, forming two isolated systems: districts A and B. Before the fault occurred, district B belonged to the receiving area, so the fault resulted in a power shortage and frequency decline, which required emergency control. N10 of district B contained 30 MW of intelligent interruptible load.  
At the same time, the communication network between the control center and control subsystem was built in the OPNET. N8 was the control center, and was connected with the control platform through a SITL module, and N10 was the control substation, which was connected with RT-LAB by the other SITL module; the other control substations were connected with the background traffic generation modules to simulate the network data transmission. A 2M fiber channel was applied for the data transmission.  
The implementation effects of the proposed CPPS real-time simulation platform were the focus in our research. The control strategy was as follows: the control center station sent the instructions to N10 when the frequency dropped to 49.6 Hz, which required 20 MW of intelligent load to be removed automatically.  
To verify the simulation effectiveness, basic Environments 1 and 2 were set. Environment 1 represented the case without control strategies, and Environment 2 was the ideal situation without time delay. Moreover, two additional kinds of communication environments were set for the information channel in OPNET. Environment 3 was the normal state. Each substation sent measurement and status information to the control center at a relatively low frequency, while receiving regulation information from the control center. Environment 4 was the emergency state (e.g., frequency fall). The data communication between the substations and control center surged, resulting in a long queuing delay at the control center, which led to a lag in data upload and instructions. All scenes were simulated and compared, and the frequency results after the fault are shown in Figure 8.  
Comparing the frequency curves of Environments 1 and 2, it was derived that the wide area intelligent load control strategy could remarkably alleviate the frequency decrease after faults, and increase the minimum frequency from 49.13 Hz to 49.53 Hz. Environments 3 and 4 were compared  
Figure 7: Model of a 4-bus power system with its corresponding communication system.  
with Environment 2, which showed that the time transmission delay affected the action time of the control strategies. Due to the different link attributes and data rates of the communication links, Environments 3 and 4 caused a delay for 81.7 ms and 161.6 ms, respectively, and thus the minimum frequency dropped by 0.0112 Hz and 0.0243 Hz. Therefore, the proposed platform could accurately reflect the communication impact on the power service simulation results.