## 3 Demand-Side Resources in Active Distribution Systems
### IEC61850 Standard for Coordination and Control of Active Distribution Systems  
In recent times, the smart grid, which incorporates several clean energy resources, has become a core of electricity and communication, so that the electric network will always be available, live, interactive, interconnected, and tightly coupled with communications in a complex energy and information real-time network. Consequently, this leads to more efficient power systems developed capable of regulating rising energy demand, providing fault resilience, and smoothly integrating distributed renewable sources, such as wind and solar [63]. The exponential growth of DG has resulted in the bidirectional flow of power instead of the old unidirectional one. This growth of DG and interconnection to the distribution system poses challenges in power flow and requires monitoring and control. This challenge that is associated with DG in management for high efficiency and stability needs is solved by Information and Communication Technologies (ICT) [64]. Information sharing is a linchpin of the efficient and effective operation of energy systems in real time. In the 1930s, this was executed telephonically where line loads were communicated to the control center and operators, therefore, did the switching at substations [65]. The advancement in digital communication in the early 1960s was little matured to the level that data could be acquired automatically from the substation. Further developments in digital communication space require that energy systems can utilize intelligent electronic devices (IEDs) to control and monitor the electrical network automatically [66]. An electrical substation is a link between the distribution network and the transmission system. They have devices where operators can monitor and control power flow and ensure the safety of the network [67]. There has been a spike in substation automation system (SAS) investments in recent times to reduce power interruptions, identify faults speedily, and restore power flow timely to consumers. SAS was vendor proprietary until 2007 due to the lack of standards, which resulted in closed systems with limited range. They also had their specific serial communication, thereby requiring the employment of protocol converters to incorporate a third-party device. The IEC61850 was then introduced to provide guidelines to enable the interoperability of IEDs between different manufacturers. This resulted in a smooth data transfer between several IEDs, and that was used to execute different functions in a coordinated way [68]. This standard does not specify how the control logic within IED should work nor how the communication must be but deals with the external behavior of control devices [69]. IEC 61850 was originally conceived and developed as a communication standard within a substation. As this technology started to gain momentum, it metamorphosed into a standard capable of covering other parts of the power system, such as electrical protection and control function models, device configuration languages, physical process data models, and electromagnetic compatibility [56]. The automation control of modern substations employs several standards, technologies, and protocols such as MODBUS, IEC60870, and IEC61850 protocols and are managed through an ethernet-based network [70]. The manufacturers established proprietary communication protocols for their IEDs, which led to challenges in interoperability between devices. This resulted in the development of IEC61850 to standardize data models in electrical systems equipment and provided operational specifications and engineering standards for required applications in electrical networks so as to mitigate the interoperability challenge amongst devices [67; 71]. In traditional substations, the communication signal would travel a long distance from the substation to the control room, which resulted in a high frequency of failures, expensive maintenance, and operational costs [71].