# Future Renewable Energy Communities Based Flexible Power Systems
## 10 Cutting-Edge Multilevel Converters Powered Smart Grid Systems
It also requires external circuitry to pre-charge the capacitors to their nominal voltage values plus an additional control scheme to keep them balanced [48]. FC converters offer redundant switching states and equal switching conduction stress across the switches. A BTB current source converter (CSC) PEI is shown in Figure 15. The generator-side output is connected to a Current Source Rectifier (CSR) while the grid-side is connected to a Current Source Inverter (CSI). Even though the DC side of the CSR and CSI can be connected through a single inductor/choke, a common-mode choke is used. This prevents any three-phase common-mode currents from flowing between the CSI side and the CSR [49]. This feature, along with the voltage boosting ability, makes such power electronics structure very interesting for transformerless motor drive applications, and even better for generators connected to high voltage AC grid. The low cost and high reliability, due to the elimination of electrolytic capacitors, makes this PEI a very promising technology for multi-megawatt wind turbine systems, particularly for grid-connected future energy flexible communities applications. Figure 14: Multimegawatt wind turbine grid-connected systems with DC link, (**a**) two BTB-VSC connected in parallel, (**b**) Back-to-back voltage source converters with separate individual dc-links. A CSC STATCOM is shown in Figure 16, which is used by utilities to help with compensating voltage fluctuations during renewable energy lack, surplus of generation, load increase, and voltage fluctuations caused by transmission lines with high activity as well as the connection of electric-vehicle plug-ins and battery storage. Figure 16 shows six unidirectional switches, required by any CSC topology, with a DC inductor. Although GTOs or IGCTs can be used as unidirectional switches, monolithic Reverse Blocking IGETs (RB-IGBTs) or IGBTs connected in series with diodes are preferred due to their higher switching frequency capabilities. In order to accommodate high current applications, several CSC STATCOM modules can be connected in parallel as portrayed in Figure 17. Using a proper modulation technique, these modules can operate as a Multilevel CSC (MCSC) STATCOM [43]. The number of CSC modules determines the number of levels an MCSC STATCOM can produce at the output. An MCSC STATCOM that has \(M\) CSCs can produce a waveform that has a \(2M+1\) levels. The PWM technique used in the individual CSC modules is the same as the so-called \"tri-logic\" PWM proposed in [50]. To synthesize multilevel waveforms, the Phase-Shifted Carrier PWM (PSC-PWM) is used [51], where the phase-shift between any two adjacent carrier waveforms is set to \(2\pi/M\). Figure 16: Grid Current-source based STATCOM for grid-connected utility for flexibility and ancillary services. Figure 15: Grid connected wind turbine system with a back-to-back current source converter with multilevel based control. In [50], the Current-Source Modular Multilevel Converter (CSMMC) is introduced as the current-source dual of the voltage-source Modular Multilevel Converter (MMC). A CSMMC-based STATCOM is studied in detail in [50]. Each phase leg of the CSMMC is split into an upper arm and a lower arm. Each arm has a capacitor connected in parallel to \(2M\) half-bridge inductor Sub-Modules (SMs). The SMs are composed of two unidirectional switches and an inductor. The two switches in the SMs, \(S\) and \(\overline{S}\), work in a complementary manner to produce current outputs of 0 and \(I_{L}\). The three-phase CSMMC STATCOM requires \(12M\) submodules, for a \(2M+1\) level STATCOM, 6 arm capacitors, and a main DC inductor. The main DC inductor should be rated for the full load current. The submodule inductors are sized to accommodate energy variations at the fundamental and double frequencies. Compared to the MCSC STATCOM, the CSMMC STATCOM requires sized inductors and a few more components [50].