# Coordinated Control of Virtual Power Plants to Improve Power System Short-Term Dynamics
## 4 Case Study
### Impact of Communication Delays  
As mentioned in Section 3, there exists a delay when the frequency signal \(\Delta\omega_{\mathrm{POC}}\) and active power \(p_{\mathrm{inj}}\) transmitted through the communication network. In this section, the communication delay with respect to three levels of communication networks, namely high-speed, middle-speed, and low-speed communication networks are considered. All the remote signals are considered to be 100 bytes PMU data with a reporting rate of 25 frames per second. UDP/IP protocol is adopted to avoid the data re-transmission for reducing the communication delay, where the physical link layer of the communication network is a Point-to-Point link. The background traffic consists of both 500 bytes Remote Terminal Unit (RTU) data with a reporting rate of 2 frames per second, and 1024 bytes video streams,  
Figure 8: Trajectories of the frequency of the COI using: (**a–c**) proportional gain; (**d–f**) lead-lag controller; and (**g–i**) PI controller.  
200 frames per second, respectively. Table 2 shows the parameters of the communication network for each scenario.  
Most real-world communication networks used in power system applications are somewhere in between the high-speed and medium-speed communication networks depicted in Table 2. However, low-bandwidth communication networks are cheaper and, thus, the low-speed communication network is also considered here. Clearly, the lower the speed (bandwidth) of the communication network, the higher the delays of the control signals.  
Figure 9 shows the impact on the frequency of the COI of communication delays as obtained with the three communication networks of Table 2. The system undergoes the same three-phase fault considered in Section 4.1. Only Modes 4, 5 and 6 are compared as these are the modes that require a communication network. Simulation results, which were obtained using the proportional controller, indicate that the proposed coordinated control approach, especially Modes 5 and 6 are particularly impacted by communication delays.  
When the communication network has a low bandwidth with a large time-varying delay of around 200 ms, the approach that only coordinates the ESS gives rise to an oscillation on the system frequency (see the interval between 25 and 35 s in Figure 9a). Mode 6 sharply increases this oscillation. Then in the medium-speed communication network with a medium time-varying delay of around 100 ms, the performances for Modes 4 and 6 are acceptable; however, for Mode 5, the frequency oscillation is still significant. Finally, the delays of the high-speed communication network are around 55 ms. This delay has only a slight impact on the overall frequency behavior. Based on the simulation results, it appears, thus, that communication delays should be kept below 100 ms when the proposed coordinated control approach is adopted.  
It is important to note that due to the nonlinearity of the model of the power systems, it is not possible to draw general conclusions on the impact of delays. However, based on our knowledge and experience with several systems and scenarios, communication delays in control loops can be expected to worsen the dynamic response of the system and reduce its stability margin (see for example [18; 38]).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Levels** & **Bandwidth** & **PMU Data Rate** & **Background Traffic** \\ \hline High Speed & 20 Mbps & 25 frames/s & RTU, Video Stream \\ Medium Speed & 5 Mbps & 25 frames/s & RTU, Video Stream \\ Low Speed & 1 Mbps & 25 frames/s & N/A \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters of the communication networks.