POWER quality (PQ) has become an important issue over the past two decades due to the relentless integration of sensitive loads in electrical power systems, the disturbances introduced by nonlinear loads, and the rapid growth of renewable energy sources. Arguably, the most common PQ disturbance in a power system is voltage sags, but other disturbances, such as harmonic voltages and voltage imbalances, may also affect end user and utility equipment leading to production downtime and, in some cases, equipment terminal damage.
The dynamic voltage restorer (DVR) is one of the most efficient and economic devices to compensate voltage sags . The DVR is basically a voltage-source converter in series with the ac grid via an interfacing transformer, conceived to mitigate voltage sags and swells . For low-voltage applications, DVRs based on two-level converters are normally used  and, therefore, much of the published literature on DVRs deals with this kind of converter. Nevertheless, for higher power applications, power-electronic devices are usually connected to the Medium-voltage (MV) grid the use of two-level voltage converters becomes difficult to justify owing to the high voltages that the switches must block.
Power Quality Improvement Using 5-Level Flying Capacitor Multilevel Converter Based Dynamic Voltage Restorer For Various Faults Matlab Project report