Mitigation Of Power Quality in a Grid Connected PV System Using UPQC under condition of 3 phase fault

Abstract

A unified power quality conditioner or UPQC is a device that is similar in construction to a unified power flow conditioner also known as UPFC . The UPQC, just like a UPFC, employs two voltage source inverters (VSIs) that are connected to a common dc energy storage capacitor. One VSIs is connected in series with the ac line while the other is connected in shunt with the same line. An UPFC is employed in a power transmission system to perform shunt and series compensation at the same time. Similarly, a UPQC can also perform both the tasks in a power distribution system. However, at this point the similarities in the operating principles of these two devices end. Since a power transmission line generally operates in a balanced, distortion (harmonic) free environment, a UPFC must only provide balanced shunt or series compensation. A power distribution system, on the other hand, may contain unbalance, distortion and even dc components. Therefore a UPQC must operate under this environment while providing shunt or series compensation.
The UPQC is a relatively new device and not much work has been reported on it yet.It has been viewed as a combination of series and shunt active filters. It can be used to attenuate current harmonics by inserting a series voltage proportional to the line current. Alternatively, the inserted series voltage is added to the voltage at the point of common coupling such that the device can provide a buffer to eliminate any voltage dip or flicker. It is also possible to operate it as a combination of these two modes. In either case, the shunt device is used for providing a path for the real power to flow to aid the operation of the series connected VSI. Also included in this structure is a shunt passive filter to which all the relatively low-frequency harmonics are directed.
The study is done using MATLAB/SIMULINK software for two cases- one without UPQC and another with UPQC. . Further a 3 phase fault is introduced in the system.The voltage and current waveforms are compared for both the cases with and without UPQC to compare the difference. THD analysis is also done for both the cases. It was observed that with UPQC , the performance of the system was improved considerably.