Pfc Converters

ABSTRACT

In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor. A high power factor is generally desirable in a transmission system to reduce transmission losses and improve voltage regulation at the load. It is often desirable to adjust the power factor of a system near to unity. PFC schemes may be applied by an electric power transmission

The PFC schemes can be done by using different converters viz,.buck, boost, buck-boost, Cuk, SEPIC and Sheppard - Taylor converters. The implementation of buck converters is explained in [1] and the use of buck converter[8] has the limitation of performing only buck operation[9]. Another disadvantage is that isolation cannot be provided by the converter. PFC scheme using boost converter is detailed in [2] and [3] and the disadvantage of using boost converter is that it cannot perform buck operation. [4] gives the PFC scheme based on buck-boost converters and the main demerit in using buck-boost converter is polarity inversion, i.e, the output will be in opposite polarity to the input. The Sheppard-Taylor topology is explained in [5] and the demerit of using Sheppard-Taylor is the control of the two synchronized switches is difficult. Cuk converters given in [6] also have the disadvantage of Polarity inversion. The comparison of different schemes is done in

Figure 4 Equivalent circuit when switch is OFF

IV.DERIVATION OF STEADY STATE MODEL OF SEPIC CONVERTER The transfer function using steady state analysis is given by the formula The input matrix is given by = AX + BU

When switch is ON the input matrixis given by When switch is OFF the input matrix becomes The ouput matrix equation is given by

Y = CX + DU

Y=

Where D = [0]

And A=A1d + A2 (1-d)

B = B¬1d + B2 (1-d)

After computation the matrices are given as, A = ; B = ; C =

By substituting these values in the formula T.F=〖C[SI-A]〗^(-1) B+D the transfer function can be obtained. It is given by

T.F=

V. TUNING OF A PID CONTROLLER USING TYREUS-LUYBEN METHOD

The steps to tune the PI controller using Tyreus-Luyben method [7] are given as follows Determine process transfer function Implement proportional control and introduce set point by setting Ki and Kp at minimum values Increase the value of Kp until sustained periodic oscillations are obtained Record ultimate gain and period Evaluate control parameters as prescribed by TYREUS-LUYBEN method as