Performance of unified power quality conditioner using pi and fuzzy logic controllers

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Power electronics is playing an important role in transmission and utilization of electrical power due to its capability of processing electric power in most efficient and cost-effective way. However, the nonlinear characteristics of power electronic devices give rise to two important limitations; they generate harmonics and draw lagging current from the utility. In recent years unified power quality conditioner (UPQC) is being used as a universal active power conditioning device to compensate both harmonics as well as reactive power.

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UPQC is an advanced version of unified power flow controller (UPFC). The performance of UPQC mainly depends upon how quickly and accurately compensation signals are derived. The UPQC mitigates harmonics and provides reactive power to the power systems network so as to improve the power factor close to unity.

The UPQC is a combination of shunt active and series active power filters connected through a dc bus. The shunt active filter of UPQC acts as a current source for injecting compensating current through a shunt transformer, whereas, the series active filter acts as a voltage source for feeding compensating voltage through a series transformer. The aim of the dissertation work is to study the control strategies of UPQC based on PI controller and fuzzy logic controller in detail.

In the case of PI controller, the dc link voltage is sensed at regular intervals and is compared with a reference value. The error signal thus derived is processed in a PI controller. A limit is put on the output of the controller to ensure that the shunt active power filter supplies active power of the load through the series active power filter.

The fuzzy logic controller is basically nonlinear and adaptive in nature. This gives a robust performance in the cases where the effects of parameter variation of controller are also taken into consideration. It is a well established fact that the fuzzy logic controller yields results that are superior to those obtained as compare to those obtained through conventional controllers such as PI and PID because of the fact that it is based on linguistic variable set theory and does not require a mathematical model. Generally, the input variables are error and rate of change of error. If the error if coarse, the fuzzy controller provide coarse tuning to the output variable and if the error is fine it provides fine tuning of the output variable.

The present thesis investigates PI controller and fuzzy logic controller as concerned to UPQC application for power quality improvement. The UPQC is studied and its advantages over conventional APFs and UPFC are discussed in detail. The relevant mathematical models and equations to explain the working of UPFC are derived for both the cases (PI controller and fuzzy logic controller).The relevant simulations are carried out using MATLAB/Simulink.

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