A novel digital-controlled single-phase transformer-based inverter for non-linear load applications

The paper presents a new digital-controlled single-phase transformer-based inverter for non-linear load applications. A capacitive full-bridge circuit with high bandwidth is added to provide instant current under non-linear load condition and thereby reducing the voltage harmonics significantly to meet the harmonic standard, IEEE 519-1992. The redundant capacity, cost, size and weight of line frequency transformer can therefore be dramatically reduced. Moreover, an integrated controller is proposed to eliminate both DC current component and steady state error even under heavy load condition. The proposed integrated controller consists of a proportional (P) controller, DC offset canceller, an RMS compensator and non-linear load compensator. The P-type voltage controller is used to generate the main voltage control force. DC offset canceller and RMS compensator are utilized to mitigate the effect of DC component generated by output voltage of the DC-AC module and reduce the steady-state error between voltage command and its feedback, respectively. The non-linear load compensator is added to generate instant current such that both the voltage harmonics and redundant capacity of transformer can be reduced for non-linear load application. Experimental results are carried out on a 1.5 kVA and 45~400Hz single-phase inverter controlled by digital signal processor (TMS320F2809). It will be shown that both redundant capacity of line frequency transformer and voltage harmonics are significantly reduced even with non-linear load. It will also be demonstrated that the DC current component and steady state error are eliminated even under heavy load condition. These experimental results confirm the superb performance of the proposed inverter and control techniques.

I. Introduction

In recent years, single phase inverter [1]–[3] has been widely used in various applications, such as UPS [4]–[8], renewable energy conversion [13], power source, etc. The circuit structure [1] of the single-phase inverter can be divided into half-bridge, full-bridge, and three-level structures. The main function of the inverter is to provide an AC output voltage with less voltage distortion [3] while meeting the codes of safety and harmonics (power quality) under both linear and non-linear loads [6], [14], [15]. The inverter can be divided into transformer-based and transformer-less types [17], [18] to fulfill the requirement of safety standards and the field applications [16]–[19]. Although the transformer-less inverter, without line-frequency transformer installed in front of load for isolation, provides the merits of compact size, lower cost, less voltage harmonics yielded by the saturation of line frequency transformer [19], and fast response [17].