## 2 Outlook for Smart Grid Integration  
### The Challenges of a Conventional Power System  
The current AC power grid has been dominated by synchronous generators for over 100 years. The system architecture is hierarchical, which is composed of four parts: generation, transmission, distribution, and consumption, with unidirectional power transmission. In the power system, a set of generators in parallel regulate the system's stability and global energy balance. In this way, the system reliability is also protected against failure or the scheduled maintenance of one generator or part of the grid. When the optimal operation point of the synchronous generator is near its full load, a system operation schedule is created. Due to the large kinetic energy stored in rotors, synchronous generators have large inertia and a slow response, which benefits the grid in the transient process.  
However, due to the increasing penetration of power electronic-based renewables and other units, power systems has been through intrinsic changes. In terms of energy sources, the irreversible transition from fossil energy to renewable energy has deeply changed the power system. Part of renewable energy generation is centralized through renewable power plants in remote areas, such as hydro plants, wind farms, or solar plants. More renewables have been introduced as DGs and microgrids have been integrated into distribution networks to enhance the reliability and power quality, as well as pursue profits for the owners. Due to the low-energy density of renewables, the quantity of integrated DGs needs to be large in comparison to traditional power systems.  
Hence, the topology of the distribution network has been greatly changed. First, the morphology of the power system has been transformed from centralized generation with relatively few major power plants to a system that is highly decentralized and distributed. From the view of the dispatching center, it is sometimes difficult to regulate the specific node that regulates the self-governed renewables, which has a local impact. Second, renewable energy--together with the energy storage or generation backup established in the form of pumped storage power plants, electric vehicles, and advanced energy storage devices--converts power transmission direction from unidirectional to bidirectional or multidirectional. Moreover, the interfaces of renewables, energy storage, electric vehicles, AC adjustment, and direct current (DC) consuming loads are commonly converter-based. As a result, future power systems will be electronic-based, rather than based on electrical machines [3]. As the energy conversion interfaces in power systems evolve from magnetic fields between rotors and stators to high-frequency power-switching devices, future power systems based on electronic converters benefit from more flexibility, controllability, and a faster response. At the same time, the system inertia also decreases significantly.  
At the power system level, when renewable penetration is relatively low, the system stability and overall energy balance can be regulated through spinning reserves, which compensates for generation disturbance and frequency fluctuation. However, if the penetration reaches a certain level, the system stability is severely degraded. In the distribution network, the challenge of renewable integration is more complex, due to the large quantity of renewable units with different profiles. Data acquisition and supervisory control are also needed for optimal operation and resilient control [3]. The efficient coordinative regulation of massive DGs and analytical methods of computing and utilizing a massive amount of data for performance enhancement challenge the distribution network.