# A Review of Flywheel Energy Storage System Technologies and Their Applications
## 2 Description of Flywheel Energy Storage System
### Structure and Components of FESS
#### 2.2.3 Power Electronics
In high power and high voltage applications, the two level DC-AC inverters are limited to operating at a high frequency, due to switching losses and device rating constraints [44]. Multilevel inverters, with an additional DC link capacitor, can be used to generate higher voltages with a less transient d\(\upsilon\)/d\(t\) and reduced THD [45]. The common multilevel converter topologies in industry are diode-clamped converters or neutral-point-clamped (NPC) converters, cascaded H-bridge (CHB) converters, and flying capacitor (FC) converters. A three-level twelve-pulse NPC converter topology for FESS is proposed in [46]. Two NPC converters are connected in BTB combination to connect the FESS to the PCC between the electrical grid and wind generation. An AC to AC or matrix converter (MC) is an array of nine bidirectional switches arranged in a way to allow the connection of any of the output phases of the converter to any of the input phases. MCs are used to modify the amplitude, frequency, and phases of the waveforms between the two asynchronous ac systems [44]. During its early stages, the AC-AC power converter was termed a Forced Commutated Cycloconverter, as it mainly relied on forced commutated thyristors [45]. These converters became a viable alternative to BTB converters after the emergence of BJTs. A comparison of MCs and BTB converters is presented in [47]. MCs are compact in size and have a lower weight, since there are no large capacitors or inductors required for energy storage. However, the lack of an energy storing capacitor limits their maximum voltage transfer ratio to 86%. This limit can be exceeded, but with the cost of unwanted low frequency components in the input and output waveforms [45]. AC-AC converters are used with FESS in dynamic voltage restorer applications to mitigate voltage sags [48; 49]. The choice of the matrix converter improves the reliability of the system and increases the power density [48]. A cascaded DC-DC and DC-AC converter configuration can also be used for FESS applications connected to a DC microgrid. A three-phase full bridge circuit in series with a bidirectional Buck-boost converter is proposed in [50]. This combination will be used in more applications with the emergence of DC MG in the future. In cases where the discharging speed of the FESS is low, a DC-DC boost converter is connected at the DC link between the BTB converters, to regulate the output voltage. The boost converter can be bypassed by a switch during the charging [51]. In [52], a Z-source inverter topology is used as an alternative to a boost converter. In this configuration, inverter output waveform distortion is reduced and the reliability of the system is greatly improved, because of the short circuit which can exist across any phase leg of the inverter. A higher switching frequency for the inverter and rectifier reduces current ripples and increases control bandwidth, but switching losses will be increased. In addition, fast switching of the power converter reduces higher order harmonics to produce an improved sinewave. Harmonics can be further reduced by the introduction of AC filters on the converter AC side. Filters can reduce current ripples, winding deterioration, and losses [53; 54].