# 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  
The energy conversion in a FESS is accomplished by the electrical machine and a bi-directional power converter. The power electronic converter topologies that can be used for FESS applications are DC-AC, AC-AC, and AC-DC-AC, or a combination of these. The switching devices of the power converters are selected, based on their operational characteristics and application. These include a bipolar junction transistor (BJT), metal oxide semiconductor field effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), and thyristor (SCR, GTO, MCT) [38]. The commonly used switches are a silicon controlled rectifier (SCR), gate turn off thyristor (GTO), and IGBT. SCR and GTO have been traditionally used for variable frequency power converters. However, IGBT has been greatly adopted in recent years, due to its higher power capability and higher switching frequencies [9]. The widely used configuration of power converters in FESS is the back-to-back (BTB) or AC-DC-AC configuration, connected to a DC link capacitor. The converters in the BTB topology are three-phase bridged semiconductor switches, often controlled by the pulse width modulation (PWM) technique [18]. The PWM uses rectangular pulses and modulates the width of these pulses to produce a variable waveform. The pulses are applied to the power electronics converter to produce a sinusoidal AC current from a DC input [9; 39]. The grid side converter maintains the DC link voltage, where the machine side converter is used to control the operation of the MG and the flywheel. A DC to AC converter, usually known as an inverter, produces an AC output of a desired magnitude and frequency from a fixed or variable DC input. Variable output waveforms are achieved by either varying the DC input or adjusting the gain of the inverter [40]. The widely used industrial \begin{table}
\begin{tabular}{c l l l} \hline \hline
**Machine** & **Asynchronous** & **Variable Reluctance** & **Permanent Magnet Synchronous** \\ \hline Size & 1.8 L/kW & 2.6 L/kW & 2.3 L/kW \\ \hline Tensile strength & Medium & Medium & Low \\ \hline Torque ripple & Medium (7.3\%) & High (24\%) & Medium (10\%) \\ \hline Maximum/base speed & Medium (>3) & High (>4) & Low (<2) \\ \hline Demagnetization & No & No & Yes \\ \hline Cost & Low (22 €/kW) & Low (24 €/kW) & Low (38 €/kW) \\ \hline \multirow{8}{*}{Advantages} & Low cost & Robustness of temperature overheat & Low loss, high efficiency \\ \cline{2-4}  & Simple manufacture & Overcurrent capability & High power density \\ \cline{2-4}  & Technology-matured & Excitation coil can repeat adjustment & High load density \\ \cline{2-4}  & Adjustable power factor & Lower loss at starting torque & High torque density \\ \cline{2-4}  & No demagnetization & Easy to dissipate heat & Small volume, light quality \\ \cline{2-4}  & High energy storage & Lower loss, higher efficiency & low rotor resistance loss \\ \cline{2-4}  & No running loss & High power density & No field winding loss \\ \cline{2-4}  & & Flexible shape and size \\ \cline{2-4}  & & Simple control mode \\ \hline \hline \multirow{8}{*}{Disadvantages} & High slip ratio of rotor & Complex structure & poor robustness of temperature \\ \cline{2-4}  & Limited speed & Difficult to manufacture & Demagnetisation \\ \cline{1-1} \cline{2-4}  & Larger volume & Low power factor & High cost \\ \cline{1-1} \cline{2-4}  & Low power to quality ratio & Torque ripple, vibration and noise & Materials fragile \\ \cline{1-1} \cline{2-4}  & High losses, low efficiency & More outlet from machine & Difficult air gap flux- \\ \cline{1-1} \cline{2-4}  & Difficult to regulate speed & field adjustment \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._applications of inverters for FESS are variable-speed ac motor drives, renewable energy, transportation, and uninterruptible power supplies (UPS) [40; 41]. In wind power applications, an inverter is used to connect the FESS directly to the grid or to the DC link of the wind generator [42; 43].