An Improved Real-Time Digital Feedback Control for Grid-Tie Multilevel Inverter M. Trabelsi, L. Ben-Brahim, K.A Ghazi College of Engineering, Electrical Department Qatar University Doha, QATAR
[email protected] Abstract—This paper proposes a weighted Real-Time Digital Feedback Control for a grid connected Distributed Generation (DG) system (gas turbines, fuel cells, photovoltaic, wind…). The DG source is connected to the grid through a Power Conditioning Unit (PCU) consisting of a multilevel Flying Capacitors Inverter (FCI), an inductor, and a step-up transformer. The proposed controller is predictive type offering the possibility of furthering the capacitor voltages tracking, load current tracking, or a tradeoff between all state variables tracking (capacitor voltages should be controlled in order to ensure a proper operation of the FCI). The Weighted Predictive Controller (WPC) transfers the power to the grid, even under grid voltage variation (sag or swell) due to its Voltage Ride Through (VRT) capability by controlling the current with unity power factor. Theoretical analysis, simulation and experimental results are described in this paper. Keywords— Weighted Predictive Controller, Voltage Ride Through Capability, Grid connection, Unity power factor.
I. INTRODUCTION Renewable Energy (RE) technologies play a crucial role in producing energy with low or zero Greenhouse Gases (GHGs). These alternative sources of energy are being explored for different applications [1-5]. Furthermore, the use of DC/AC inverters in Distributed Generation (DG) systems (gas turbines, fuel cells, photovoltaic, wind…), loosely defined as small-scale electricity generation systems at the beginning of the century, is progressively becoming the standard. Hence, there are a lot of power converter topologies employed in interconnecting the DG systems to the grid which are characterized by two-stage or single-stage and 2-level or multilevel inverter [6-9]. Currently, multilevel inverters are a good solution for power applications since they can achieve high power using mature mediumpower semiconductor technology. The most common multilevel inverter topologies are Neutral-Point Clamped (NPC), Flying Capacitors Inverter (FCI), and Cascaded HBridge (CHB) [10]. Several surveys on multilevel converters have been published to introduce these topologies [11]. FCI and CHB present different characteristics compared with NPC, such as lower number of components, higher modularity, lower control complexity, higher flexibility in the multilevel voltage synthesis, and higher fault tolerance [10]. Furthermore, the FCI doesn’t require increasing the number of dc sources when the number of level is high and is capable to feed loads with DC
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level component (supports loads of diverse nature). Thus, the multilevel FCI is one of the suitable power architectures for the grid-tie DG systems. This multi-cell structure is composed of n cells separated by flying capacitors of which the voltages have to be controlled in order to ensure a proper operation of the whole system. However, the FCI is inherently affected by grid voltage fluctuations like any voltage source inverter. Voltage sags are probably the most common power quality problem affecting sensitive loads and grid-tied inverters causing current overshoot and distortions. Therefore, control techniques for inverters should achieve high performance not only during normal operating conditions of the grid, but also under fault conditions. In this paper, a 4-level single-phase FCI is used as a power conditioning unit connected to the grid through a step-up transformer. A weighted predictive control with high VRT capability is proposed to provide power to the grid with a unity PF. All control algorithms are implemented using a dS1104 microcontroller. Experimental results obtained on a 1 kW prototype show high performance of the proposed control on allowing system to continue operating even under grid voltage variations. Theoretical analysis, simulation and experimental results are presented in this paper. II. PROPOSED SYSTEM Fig. 1 shows the circuit of the proposed system. A 4-level FCI is used as a power conditioning unit connected to the grid through an inductor L and a step-up transformer. The FCI consists of three cells separated by two flying capacitors C1 and C2. Depending on the capacitor voltages references E1ref and E2ref and the control states uj (j=1,2,3), the level of the output voltage is determined. Since there are two switching states for each cell, eight different switching patterns are obtained ((Ui(t), i = 1,………8)). Unlike other types of inverters, the grid current ig(t) and the capacitor voltages E1(t) and E2(t) must be jointly controlled for ensuring proper operation of the FCI. Furthermore, E1 and E2 should be 0
