Abstract
The simple circuit based on DC-DC converters is the main attractive feature of the differential inverter topologies. It has a single-stage and provides modularity and scalability. However, the Negative Sequence Harmonic Component (NSHC) generated at the output terminal may hinder its practical applications. This paper presents a single-stage three-phase isolated differential inverter based on three High-Frequency Link (HFL) transformer-based DC-DC SEPIC converters. The utilized SEPIC converters perform voltage step-up/ down capability with galvanic isolation, which is essential for Renewable Energy Sources (RES). It mitigates the Common-Mode Voltage (CMV) and Electro-Magnetic Interference (EMI). Moreover, this paper proposes a two-loop based d-q synchronous frame grid-current control to mitigate its NSHC. A Type-II compensator and simple NSHC detection circuit are proposed to enhance the inverter's stability and compensate phase-delay of the utilized SEPIC converters. NSHC detection is developed using three cascaded Low Path Filters (LPFs). A 1.6kW inverter prototype was set to validate the performance of the proposed inverter and its control. The control is implemented by the MWPE3 C6713A Expert III DSP board. The proposed topology has a maximum efficiency of 89.744 at 700W output power and 86.4% at full power. The proposed control decreases the NSHC from 40.6 % to 1.614%, which shows its accuracy and precision. Furthermore, THD is reduced from 35.61% to 4.087% and satisfy the recent grid codes (<; 5%). The simulation results using PSIM software, power loss distribution, and a comparison study of the proposed inverter with similar topologies are also presented.
Highlights
The fast penetration of Renewable Energy Sources (RES), such as Photovoltaics (PVs), increases the popularity of DC-AC power converters at grid-connected applications [1]–[5]
The output current of the differential inverter is distorted with Negative Sequence Harmonic Component (NSHC) even at static linearization that outlined in equation (3)
Case 1: This case discusses the effect of NSHC on the SEPIC converters, DC source, and grid
Summary
The fast penetration of RES, such as Photovoltaics (PVs), increases the popularity of DC-AC power converters at grid-connected applications [1]–[5]. For low-order harmonic components, higher than NSHC, the differential inverters’ variable duty cycle was achieved using a sinusoidal duty cycle in many traditional PWM strategies [43] Using this modulation introduces a mismatch between the actual output voltage and its reference signal for the utilized DC-DC converters. Each SEPIC converter has two MOSFET switches (main switch and synchronous switch), one input inductor, one HFL transformer, and two film capacitors (coupling capacitor and output capacitor) This low number of components enhance the power density and decrease the size of the proposed inverter. The output voltages of the SEPIC converters vou, vov, vow as shown in Fig 1 and 2, have two different components; a line-frequency sinusoidal component em, synchronized with the grid side and a DC offset component vdo, which generated due to the variable duty cycle and the unipolar operation of the SEPIC converters They are expressed as follows: vou vov (t ) (t ). It worth noticing that, according to equation 6, vdovdo equals em and, the voltage gain of the differential inverter depends on static gain M and the turn’s ratio of HFL transformer n
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