Voltage Source Inverters (VSIs) are an important part for power flow control in electrical vehicles, motor/generator drives, flexible power transmission, and integration of renewable generation units (photovoltaic panels, fuel cells, batteries, etc.) into the grid, among others. In the last two decades, the Z-source inverter (ZSI) and Switched boost inverters (SBI) began to appear in scientific literature for solving some drawbacks of VSIs. One of such drawbacks is that AC output voltage cannot be higher than the available DC source and thus extra power stages are needed for applications demanding higher voltage levels [1]. In 2002 and 2003 Peng [2], proposed the ZSI as a robust alternative to the VSI with the capacity to increase the output voltage. Some important advantages of ZSI include the ability to boost-buck voltage in a single-stage power conversion, allow tolerance to one-leg short circuit, minimize component count and reduce costs. Also, in ZSI converters, the output waveform presents less distortion, mainly because both power devices in a leg can turn on at the same time. Other authors [3] have proposed alternative topologies based on ZSIs. For example, [4] proposes a bidirectional ZSI (BZSI), in which a bidirectional switch replaces the input diode. The BZSI admits bidirectional power exchange between AC and DC. Meanwhile, an improved ZSI (IZSI) [5] uses the same elements found in the original ZSI, but in different positions, thus allowing a decrease in the capacitor’s voltage stress. On the other hand, the quasi-resonant soft-switching Z-source inverter (QRSSZSI) [6] adds a quasi-resonant network for soft-switching and it increases the overall efficiency of the converter. Other extended topologies use switched inductor and/or capacitor circuits that increase the boost capacity and power density, which results in increased complexity. The ZSI converter has been the basic topology for other converters [3] that always aim at single stage conversion and buck-boost characteristics. Another objective of ZSI-based converters is to decrease the size of passive components, reduce leakage current in the system, and improve output voltage quality, among others. On the other hand, the switched boost inverter (SBI) proposed in [7] uses less passive components than the ZSI and one more active switch. Besides, it fits better in low power applications such as photovoltaic (PV) systems and micro grids.

Photovoltaic (PV) solar energy use has experienced an exponential increase in the last decade. The growing results of PV applications are mainly due to changes in energy and environmental policies, optimization of manufacturing processes of PV panels and power electronic devices, and new research on materials and electronic devices, among others. In PV systems, mismatching produced by shadows, aging and tolerances in manufacturing, result in large power losses. Recently, panel reconfiguration and distributed power processing (DPP) systems have shown promising results to overcome the limits imposed by mismatching [8]. DPP systems use power electronics solutions dedicated to small portions of the PV array, usually a string or single panel (microinverter). In PV microinverters, there is a panel associated with a power converter. The converter must have a high boost factor because the voltage range in commercial PV panels is low, typically between 20V to 40V. An advantage of inverter-per-panel DC-AC power conversion is the capacity to extract the maximum available power from each panel by using a MPPT algorithm in the DC-to-DC stage. Moreover, PV microinverters allow for plug-and-play connectivity to the grid. Therefore, PV systems based on this technology are scalable and the impact of faults is lower than in centralized or string configurations. Additionally, the decrease in power management reduces the demands on the DC wire [9]. However, a disadvantage of PV microinverters is the installation cost, as they require as many microinverters as panels. Also, the number of power converters with non-unitary efficiency affects the installation’s power harvesting. On the other hand, string configurations use a DC-to-DC converter for a group of PV panels, thus reducing the boost voltage factor in comparison with PV microinverter technology. The main weakness of string configuration is the reduced robustness under mismatching conditions. Additionally, typical string configurations provide a single phase solution on the AC side. A distribution of the control means that more power converters with higher efficiency and voltage boost are needed. For this type of applications, the previously mentioned ZSI and SBI converters seem to be suitable solutions.

The Quasi-Switched Boost Inverter (qSBI) belongs to the class of SBI topologies. The qSBI has low capacitor voltage in a dc-linked type, with high boost voltage and continuous input current [1]. Previous studies by other authors present complete analyses of the qSBI [1],[10]. This work implements a modulation technique for space vector PWM to increase the boost voltage factor of the qSBI converter, which makes it suitable to be used in PV microinverters. This paper is organized as follows: Section two explains the general performance of a qSBI. Section three details the PWM control techniques and describes the experimental prototype. Section 4 presents the experimental results using a fixed DC source as well as a DC source based on a single PV panel and string configuration. The paper ends with conclusions and future works.

The topology of a qSBI converter has higher efficiency and boost gain than common boost converters (Fig.1) [1]. Such topology uses one more active switch in comparison with others boost inverters, such as the ZSI, but it requires less passive components. Additionally, the qSBI can provide a continuous input current.

Fig. 1. Fig. 1. One-line diagram of the Three-Phase quasi-Switched Boost Inverter. Source: authors.

As shown in Fig.1, a qSBI converter is essentially formed by two cascaded power stages, a boost DC/DC stage and a Voltage-Source Inverter (VSI) stage. When the VSI outputs space vectors zero ( or in Fig.2), the voltage in the DC bus ( V_pn ) has no effect on these vectors’ value. This opens the possibility of using the VSI to establish a path for energy storage in the boost inductance L_a , an operating condition known as shoot-through state. While the VSI puts out space vectors zero, the shoot-through state uses any VSI leg while switching S₀ to feed the boost inductance from the PV generator. This stores energy in inductance L_a , as described above, and at the same time capacitor C_a discharges into the load. The other state (non-shoot-through state) occurs when switch S_{0 opens while removing the VSI short-circuit. During the non-shoot-through state, inductor La transfers energy from the DC voltage source (Vpn )to the load and to capacitor Ca .}

Fig. 2. Fig. 2. Space vectors in the voltage source inverter (VSI). Source: authors.

Fig.3 shows the equivalent circuits for the two qSBI’s stable states, where r_s is the power devices’ resistance; r_L and r_C , the parasitic resistance of the inductor and the equivalent series resistance (ESR) of the capacitor; i ₀ , the current in the load; R, the load; and r_D , the diode’s resistance. Diodes D₁ and D₂ do not conduct during the shoot-through state, while switch S₀ turns on and the VSI closes the path by turning on all its (high side and low side) power devices, which results in the equivalent circuit shown in Fig.3a. On the other hand, diodes D₁ and D_{2 conduct during the non-shoot-through state, while the VSI does not close the path and switch S0 is off, which results in the equivalent circuit shown in Fig. 3b.}

Fig. 3. Fig. 3. qSBI stable operating states. Source: [1]

The two stable operating states provide the equations describing the circuit's operation and the capacitor’s voltage V_ca as a function of the input voltage V_ph and duty cycle D [1][10]:

The voltage ratio between V_ca and V_pℎ indicates the converter's voltage gain. Since the overall circuit operation, shoot-through and non-shoot-through states depend on the duration of the VSI’s zero state, the duty cycle in (1) bounds the VSI modulation index M. Therefore, the time allotted for the non-zero space vectors plus the time for the shoot-trough state should not exceed the modulator’s carrier period. According to (1), the ideal converter's gain becomes undetermined for D = 0.5. This imposes a practical low limit for the modu-lation index value of M_BL = 0.5.

(1)

This work uses the triangular comparison method for implementing the generalized space vector modulation [11],[12],[13]. Fig.4 shows an example of the PWM pulses generated by test duty cycle demands in phases a,b and c, where V_cc is approximately equal to the capacitor’s voltage v_c . The zero vectors in Fig.4 appear spread in three different regions. From the start of the switching period to t₁ , they are associated with zero vector ; from t₃ to t₄ , with ; and from t₆ to the end of the switching period, with . It is clear from Fig. 4 that nonzero space vectors are active between instants t₁ and t₃ and between t₄ and t₆ . The extreme duty cycles D_max and D_min define the previous time slices, employed later by the qSBI modulation method.

Fig. 4. Fig. 4. PWM pulses generated using the triangle comparison method. Source: [13].

Fig. 5 shows the time distribution of the PWM carrier signal. The duty cycle requires sinusoidal modulation for the three phases of the VSI and the duty cycle, for the shoot-though states (D_L and D_H ). The references of the duty cycle for the shoot-through states also provide a visual representation of the VSI’s maximum modulation index.

Fig. 5. Fig. 5. Modulation and Carrier signals in PWM by triangle comparison for sinusoidal modulation and boost references. Source: authors.

Fig.9 shows the experimental prototype used in this work. The power stage uses seven switches and two diodes mounted on four heat-sink columns. The driver for the IGBT switches sits on top of the heat-sinks to minimize the cable’s length. The DC link is located in the middle of the equipment, with the capacitor placed on the left. The sensor board, placed in the lower part of the setup, allows to record the control variables for the inverter and quasi-boost converter.

Fig. 9. Fig. 9. Experimental prototype for the qSBI. Source: authors.

The processing unit is based on a digital signal processor (DSP) that provides the necessary processing power to execute complex computational tasks. The test rig in this work uses an Analog Devices DSP of the SHARC family (ADSP-21369).

This section shows the qSBI experimental validation for three practical cases. The first test uses a DC source to show the qSBI’s basic performance. The other two tests use a PV source controlled with an MPPT algorithm. One uses a single PV panel, which reveals the need for high boost factor and high efficiency in distributed applications. The other PV test uses a PV string feeding the qSBI and has the advantages of using SVPWM instead of the common SPWM.

In this paper a qSBI experimental prototype has been implemented. Space Vector Pulse Width Modulation (SV PWM) with triangular comparison has been proposed to increase the total boost factor of the converter, which is needed for recent applications such as PV microinverters in distributed PV systems where a single or few PV panels feed the converter. The qSBI prototype employed in this work uses a DSP and FPGA technology that enables the synchronization needed by the modulation method. In PV applications, when MPPT algorithms are used for harvesting power, some oscillations are present in the output voltage. Therefore, more complex MPP algorithms are required to mitigate this undesirable behavior. That will be the aim of future works.