# Future Renewable Energy Communities Based Flexible Power Systems
## 10 Cutting-Edge Multilevel Converters Powered Smart Grid Systems  
There are tremendous advantages in implementing back-to-back (BTB) multilevel current source converters (MSCs) as power electronics interfaces for wind energy conversion systems (WECS). BTB MSCSCs are scalable, eliminating the need of parallelizing switches, because the system DC current-source is split among several CSC modules. MSCSCs operate at a low switching frequency because they synthesize multilevel current waveforms. Such operation reduces switching losses and increases overall efficiency for WECS. The MSCSC structure eliminates the need for electrolytic capacitor. It is typically used in voltage-source converters (VSCs). Its well-documented electrolytic capacitors, which are subject to frequent failure, could lower the middle-time between failures (MTBF), increase downtime of wind generation, and rise costs of maintenance. The overall reliability of a BTB MCSC WECS is higher, particularly if several wind turbines are coupled in a wind farm installation at higher sub-transmission utility voltage level. The most common power electronics interface for wind turbine is the two-level BTB VSC, used by 90% of applications with rated power of 0.75 MW or less [43]. Two VSCs are connected in a back-to-back fashion, as shown in Figure 13, each converter can control active and reactive power independently. The DC bus voltage can be regulated by the grid-side VSC whereas the generator-side VSC has an instantaneous vector control for MPPT of the turbine aerodynamics. For higher power applications, Figure 13(a) shows a parallel connection with two converters on each side of the back-to-back with a common dc-link, while Figure 13(b) shows BTB VSCs with separate individual dc-links. The advantage of separate individual dc-links is the connection with a utility substation transformer with separated power-flow routing, and the parallel connections might be used for high-reliability applications. In wind farms, the rated voltage typically ranges from 3 kV to 5 kV, and multilevel converters can be better choices over two-level VSCs, because they minimize the need for connecting transistors in series. By producing multilevel voltage waveforms, the Electromagnetic Interference (EMI) and THD of the voltage and current waveforms are reduced. This reduces the size of the output filter. Currently, the most common multilevel converter-based power electronics interface is the BTB Neutral Point-Clamped (NPC) [43]. It is manufactured in scale by ABB [44] and Ingeteam [45]. Their DC bus composes of two series-connected capacitors where current can flow in or out of the neutral point, depending on the current angle. However, the DC voltage across the capacitors tends to drift away from the nominal value, requiring some balancing. In addition, the switching stress is not split equally across all switches in a single leg, eventually, a higher kVA installation is made with derating in the converter to consider such effects. Figure 13: Grid-connected wind turbine system with a back-to-back (BTB) DC-link double-PWM voltage source converter (VSC). To overcome the unbalanced losses and switching stresses in the switches, overheating, and also voltage drifting in the capacitors, the clamping diodes in the NPC can be replaced by active switches. Such a topology is referred to as the Active NPC (ANPC) [46]. It has been reported that replacing an NPC converter by an ANPC converter can increase the power rating of the power electronic interface (PEI) by 32% and the maximum switching frequency by 57% [47]. However, the use of additional active switches increases the cost and size of the PEI. Another modification to the NPC is to replace the clamping diodes with this topology which is referred to the Flying-Capacitor (FC) converter. Compared to the NPC converter, it is obvious that an FC converter requires more capacitors, which may affect reliability.