# Future Renewable Energy Communities Based Flexible Power Systems
## 3 Power Converter Systems for Energy Sources  
In smart grids, power electronics-based power conversion systems (PCSs) play a vital role to realize the commands ordered by upper-level controllers. In order to develop a PCS for an Energy Source (ES) or an Energy Storage System (ESS), different factors must be considered:  
1. The input voltage, which could be regulated/unregulated DC or AC;
2. The quality of the drawn current from the input;
3. The type and value of the output voltage;
4. The efficiency;
5. The size and volume;  
Figure 4: Dynamic pricing of kWh. Figure 3: Solar power density, continuous, and discretized model. The cost. Considering the input and output voltages, single-stage and multi-stage PCSs can be employed. Typically, the efficiency/cost of single-stage PCS is higher/lower than multi-stage PCS. However, multi-stage PCS offers more options such as controlling the quality of input current, higher voltage gain, increasing the system reliability with paralleling input side and output side converters. Figure 5 shows the practical structures of PCSs. According to Figure 5a, single-stage PCSs are employed in case of having a common DC bus. On the other hand, two-stage PCSs must be used for the common AC bus to handle the requirements of the source and the AC bus side at the same time (Figure 5b). As an example, a combination of a DC/DC converter and an inverter is normally used to connect a PV system to the grid. The DC/DC converter can be as a buck or boost converter depending on the required voltage gain which is used to perform the maximum power point tracking (MPPT). Besides, the inverter injects power to the grid with standard AC current in terms of total harmonic distortion (THD) and power factor. New architectures must be used for complex electric systems such as smart grids, microgrids, and hybrid energy systems. In general, three types of architectures can be used to interlink the components: (1) AC link, (2) DC link, and or (3) Hybrid AC/DC link. The AC architecture depicted in Figure 6 has been used as the primary solution to perform the combination of these ESs and EESs and connection to conventional power systems (AC). However, the nature of the generation portfolio and the load have been changed over the years. Therefore, other architectures must be developed to provide more hands for power system planners and operators in smart grid technologies. The output voltages of most ESs and EESs (PVs, fuel cells and batteries) are DC which must be converted to AC by DC/DC converters and additional DC/AC converters in order to integrate into the AC bus. Meantime, many consumers are accepting the AC as well as DC voltage such as electric vehicles (EVs), light-emitting diode (LED) lights, and modern home appliances (e.g., TV, computer, stove, and air conditioner). Therefore, DC distribution systems are recently taken into consideration due to the widespread development and deployment of DC power sources and loads. According to Figure 7, DC architecture has been suggested to reduce the number of PCSs among components in small scale power systems like remote areas, islands, shipboards and electrical vehicles. Conversely, in DC grids, AC sources must be converted to DC for connecting to the DC bus. Furthermore, extra DC/AC converted are demanded to feed classic AC loads. It can be concluded that multiple PCSs are expected in both individual AC or DC systems which may add additional cost and loss to the system. Figure 5: Practical power conversion system, (**a**) DC bus based, (**b**) AC bus based. Therefore, it makes sense of employing both hybrid AC and DC buses in the ongoing and future smart grids. Figure 8 displays a typical hybrid microgrid architecture, which is proposed to reduce multiple PCSs in exclusive AC or DC architectures and to promote the connection of various AC/DC sources/loads as a multi-energy career system. In contrast to AC or DC grids, control, system management and operation of a hybrid microgrid are more complex. The following services can be provided by this architecture:  
1.