Three Phase Four-Wire Inverter for Grid-Disconnected Operation

In this work, the modeling of a three-phase four wires inverter and the design of two control schemes for its grid-disconnected operation are presented. The advantages of the four-wire topology are that by means of a coordinate transformation (Park transformation) it is possible to obtain a decoupled linear model, as well as feeding single-phase and three-phase loads in this operating mode. In the model, two control loops are taken into account: a current loop (controller slave) and a voltage loop (controller master) with the aim that this inverter can regulate its output voltage and when it is supplied to a local load.. The contributions of this work is present in the inverter modeling, the method for obtaining the transfer functions and in the controllers design obtained by this method. The inverter model allows to easily obtain the transfer functions of both loops; this is very important for the control area, considering that using classical control techniques and the methodology proposed in this work, the gains can be obtained directly and not adjusted to trial and error. The main advantage of this proposal is that when the kp and ki gains are obtained through the Bode diagrams, we can obtain generalized integrators; applying the coordinate transformation method to the integral gain that was obtained by the design of the PI controller in direct current (dc); This method is described in detail in the presented work and allows obtaining a new controller called proportional controller plus generalized integrator (P+GI). This controller presents a good performance, since it can follow sinusoidal references, in addition to presenting a better disturbances rejection due to the high gains it presents at the grid frequency and harmonic frequencies of the load.