# Smart Grid Protection, Automation and Control: Challenges and Opportunities
## 3 Evolution of Substation Architectures and Digital Modernizations for Smart Substations
### Comparasion of a Conventional P&C Systems vs. CPAC Approach  
A traditional protection and control system implements numerous distributed devices for the protection and control of different substation sections. This can bring certain operational and maintenance difficulties into the application [19; 20]. In contrast, under the Centralized Protection Automation and Control approach, by centralizing all the critical intelligence into one system, the engineering, commissioning, and maintenance become easier by providing a single view for the whole substation [18]. This approach gives improved synchronism between protection and control schemes via Precision Time Protocol (PTP) and redundancy via protocols such as RSTP (Rapid Spanning Tree Protocol), MRP (Multiple Spanning Tree Protocol), HSRP (Hot Standby Router Protocol) and PPP (Point-to-Point Protocol) [2]. The CPAC concept allows the centralized configuration of multiple protection and control functions, as shown in Figure 8. It implements complex schemas and reduces lifecycle costs due to the redundancy of spare parts and the faster reconfigurations that are allowed by evolving protection requirements. On top of that, CPAC allows better flexibility in both engineering and operations, enhancing the efficiency, reliability, and adaptability of electrical substations. Figure 6: Typical IEC 61850 P&C architecture using GOOSE messages and sampled values. Figure 7: Evolution from conventional to centralized substation. The fundamentals of the CPAC allow for consolidating critical protection intelligence into one device, which permits the redundancy within to lower risks and systematically provides segregation to the sections controlled by software, allowing more flexibility in the deployment of protection and control systems, as represented in Figure 9.  
Moreover, the CPAC can reduce substation lifecycle costs by as much as 15% compared to conventional ones [19]; this is mainly due to the reduced maintenance and operational costs [20]. The CPAC architecture may be implemented in different ways, i.e., centralized, hybrid and decentralized architectures. The choice depends on the protection philosophy of the substation, real-time application needs, and requirements for redundancy (Figure 10). Table 1 comprehensively compares the conventional P&C systems with the CPAC approach in aspects of flexibility, costs, scalability, security, and operational efficiency. In the case of redundancy, standard protocols like PRP [21] and HSR [22], under the IEC 62439-3 [23], allow a communication network that is not only resilient but also minimizes recovery time in case one fails. These are inclusive of IEEE 1686 [24], IEC 62351 [25] and NERC CIP [26], among others, prescribing the protection and control measures of a system against the cyber threat. There are several recommendations, such as role-based  
Figure 8: Simplified diagram of a CPAC system. Figure 9: SoC design principle with CPAC concept. access control, secure protocols for communication, and remote asset management, among others, regarding the safety and security of the critical electrical infrastructure systems. The centralized architectures concentrate all the important protection functions within one device, hybrid architectures integrate centralized intelligence with localized elements, and decentralized architectures distribute protection among several NPRs. From the point of view of synchronization, the CPAC systems must implement technologies such as IEEE 1588v2 [27] and IEC 61850-9-3 that reach an accuracy of 1 us, which is of paramount importance in time-critical applications.