V.: ABC Reference Structure-Based Control

V. PROPOSED CONTROL SCHEME

The proposed iUPQC control arrangement is an ABC reference structure based control, where the SAF and PAF are controlled in an independent way.

In the suggested control scheme, the power calculation and harmonic abstraction are not required since the harmonics, unbalances, disturbances, and displacement would be compensated.

The SAF has a current loop in order to make sure a sinusoidal grid current synchronized with the grid voltage. The PAF has a voltage loop in order to ensure a balanced controlled load voltage with small harmonic distortion. The SAF and PAF control loops are independent from each other since they act independently in each active filter. The dc link voltage control is prepared in the SAF, where

Frequency response including the open-loop transfer function (OLTFd), controller transfer function (Hdsf), and compensated loop transfer function (OLTFd+Hdsf).

Fig.12. Single-phase equivalent circuit of SAF

The current control scheme contains of three equal current loops, except for the 120 phase displacements from the references of each other. The current loops have a fast response to track the sinusoidal references, agreeing the decoupling analysis in relation to the voltage loop. The current loop transfer function is acquired through the analysis of the single-phase equivalent circuit shown in Fig. 12.

The voltage source represents the voltage on the coupling transformer. The dynamic model is obtained through the circuit analysis using average values associated to the switching period. Under these conditions, the voltages Vs(t) and VL(t) are constants. Through small signal analysis and by using Laplace, the current loop transfer function is given by

Gifs(s)=(Isc(s))/(D(s))=Vb/(sA1+n^2∙(Rs+Rlg)) (11)

Where

A1=n^2∙(Ls+Llg)+Lsf