A New Single-Source Nine-Level Quadruple Boost Inverter (NQBI) for PV Application

Multi-level inverters (MLIs) with switched capacitors are becoming popular due to their utilization in AC high-voltage applications as well as in the field of renewable energy. To achieve the required magnitude of output voltage, the switched capacitor (SC) technique employs a lesser number of DC sources in accordance with the voltage across the capacitor. Designing an efficient high-gain MLI with fewer sources and switches needs a rigorous effort. This paper introduces a prototype of a nine-level quadruple boost inverter (NQBI) topology powered by one solar photo-voltaic source using fewer capacitors, switches, and diodes when compared to the other SC-MLIs topology. The suggested NQB inverter produces nine levels of voltage in its output by efficiently balancing the voltages of the two capacitors. The various SC-MLIs are compared in order to highlight the benefits and drawbacks of the proposed nine-level quadruple boost inverter (NQBI) topology. To validate the efficacy of the proposed solar photovoltaic based NQBI without grid connection, detailed experimental results are presented in a laboratory setting under various test conditions.


I. INTRODUCTION
Multi-level inverters (MLIs) are the most popular means for voltage conversion from DC to AC power, and are used in fuel cell and photo-voltaic (PV) based sustainable or renewable energy systems (RES). The core advantages of MLIs include, higher voltage output with minimal switch stress voltage, lowered dv/dt, and a better output voltage harmonic profile. Over the last few decades, the three most common forms of MLIs have been proposed: the flying capacitor (FC) topology, the diode clamped neutral point clamped (NPC) topology, and the cascaded H-bridge (CHB) topology. In order to maintain the voltage balance of capacitors, external hardware circuits and complex control algorithms are needed in the case of the FC-MLI and NPC-MLI topologies. In CHB-MLI, a large number of DC sources are employed, which results in a higher number of output voltage levels. While implementing the MLI with RES, voltage boosting is expected to achieve the higher output voltage. A dc-dc boost converter or transformer is required to achieve the desired higher output voltage. These components results in complex, large, and costly inverter The associate editor coordinating the review of this manuscript and approving it for publication was Alon Kuperman .
systems [1]- [5]. Multilevel inverters based on the SC principle have been identified as a recent advancement to address the aforementioned shortcomings [6]- [18].
The SC-MLIs boost the input voltage along with an increased number of output voltage levels. In [6], a single isolated DC source was proposed without the need for an H-bridge. The individual switches in [6] are subjected to less voltage stress, but as the voltage level rises, the number of capacitors and power electronics switches also increases.
The SC-MLI topologies proposed in [7], [8] have inherent capacitor voltage balancing and boosting abilities and are suggested for renewable energy conversion due to low voltage stress. Though both topologies has same number output voltage levels and switch count, [7] uses one additional capacitor than [8] to develop better voltage boost.
A nine-level H-bridge based switched capacitor inverter and its three-phase application is also explored in [17]. The inverter topology has self capacitor voltage balance capacity but lacks the boosting ability. Three nine-level SC-based inverters with a single source topology have been suggested in [9], [10], [18], in which capacitor voltage balance is performed without complicated circuits or auxiliary power. On the other hand, the voltage multiplication factor for them is two. The converter with a voltage multiplication factor of four is suggested in [11]. This topology uses fewer power electronics switches, but the number of capacitors and diodes is more. In [12], the SC-MLI uses a single isolated DC source for a quadruple boost in the output voltage, but the topology requires more switching devices. Several topologies having a wide range of higher voltage applications are mentioned in [13]- [16]; however, they require more components to produce a (9-L) output voltage. The cost of inverter topology and losses increases with an increase in the number of components. This work proposes a nine-level quadruple boost inverter (NQBI) topology to resolve the shortcomings of the traditional topologies established in previous research. The significant benefits of the proposed topology are mentioned below: • In the NQBI topology, the amplitude of the output voltage can be attained up to four times the voltage magnitude of the DC source.
• The NQBI topology has self-voltage balancing capability. Hence, no extra hardware circuits are required to control the capacitor voltage.
• The NQBI topology can be connected with a load having a standard range of power factor.
• The NQB inverter is designed with a significantly reduced component count. The structure of this paper is as follows: In Section II, the design and operation of the proposed inverter are discussed. Section III compares the performance of the proposed topology with the other states of SC-MLI. The experimental results are presented in Section IV. Section V gives the conclusion of the paper.

II. PROPOSED NQBI TOPOLOGY BASED PV SYSTEM
The proposed system using NQBI for an off-grid solar photovoltaic application is shown in FIGURE 1. The DC-DC buck converter is connected to the PV output, and the maximum power point tracking (MPPT) method is implemented to obtain the maximum output power. The suggested NQBI configuration is depicted in FIGURE 2. The proposed MLI can produce a 9-L output voltage with a quadruple boost in the output voltage and requires only ten power electronics switches. This inverter topology has a level-generating part, which can provide four voltage levels by quadruple boosting and employing two capacitors, C 1 and C 2 with a voltage rating of V dc and 2V dc , respectively, as seen in Figure 2. The switches  T 5 and T 6 are configured as reverse blocking switches to prevent the unnecessary discharge of the capacitors C 1 and C 2 . The alternating polarity can be generated by the polarity generating part composed of four switches H 1 , H 2 , H 3 and H 4 . Table1 shows the possible switching states and modes of capacitors to achieve the nine-level AC output voltage.

A. ANALYSIS OF SELF VOLTAGE BALANCE AND CAPACITANCE OF C 1 AND C 2
The proposed NQBI has self voltage balancing ability as it implements serial-parallel linking technique for charging and discharging of capacitors C 1 and C 2 . The capacitor C 1 gets connected to DC source and charged upto V dc when T 1 and T 5 are simultaneously in on-state during ±0, ±V dc and ±3V dc output voltage level as shown in TABLE 1. The capacitor C 1 is connected in series with DC source and the capacitor C 2 gets connected in parallel when T 2 , T 3 and T 6 are in ON state. The capacitor C 1 discharges while capacitor C 2 charges upto 2V dc during ±V dc output voltage level. Both capacitors (C 1 & C 2 ) and DC source are connected in series when T 2 , T 4 are simultaneously turned on and ±4V dc voltage level is attained. FIGURE 3 displays the charging and discharging patterns of C 1 and C 2 for one cycle of output voltage. The capacitor voltage of C 1 and C 2 starts to reduce from the nominal value under discharging mode of operation. The overall capacitance value C 1 and C 2 needed for an appropriate ripple of voltages ( V), peak load current (I o, peak ), frequency of output voltage and corresponding maximum discharge time  of T 1 and T 2 are as follows: where T 1 = t 5 -t 4 is the interval of the voltage level 4V dc and T 2 = t 6 -t 3 is the interval of the voltage level of (3V dc and 4V dc ) [11].

B. PULSED-WIDTH MODULATION TECHNIQUE
The NQBI is being controlled with sinusoidal pulse-width modulation, in which a sinusoidal reference signal (v ref ) of required amplitude (v m ) is simultaneously compared with eight level-shifted carrier waves of equal amplitude (A C ) as depicted in FIGURE 4. The generated gate-pulses for the proposed NQBI is shown in the FIGURE 5. The magnitude of the reference signal varies according to the modulation index to obtain the desired magnitude of output voltage. The modulation index is expressed as follows: (2)

C. POWER LOSS ANALYSIS OF THE PROPOSED NQBI TOPOLOGY
The losses in the inverter occur due to the existing non-idealities in the inverter components. Three types of major losses occurring in the proposed NQBI are switching losses (P sw ), conduction losses (P c ) and ripple losses (P ripple ) considered. Hence, total power loss (P loss ) occurring in the NQBI is During each switching transition, the voltage and current overlap due to intrinsic delays in the switching of IGBT, which leads to switching loss. It can be expressed as: (4) where, V on is the voltage across the IGBT in the pre-ON state. The current that flows through the power switch after it has been turned on is referred to as I on . T on is the time span during which the ON state is in transition. The voltage across the power switch, the current that runs through a power switch before the transition to the OFF state, and the transition duration of the OFF state are V off , I off , and T off respectively. The switching frequency and the frequency of the fundamental output voltage are denoted by f 0 and f fsw respectively. Conduction losses are power losses that occur as a result of the internal resistance provided by the switch while the switch is in the conduction mode, and can be expressed as In which I switch is the amount of current that flows through a switch with an internal resistance of R on . The ripple losses caused by the charging and discharging of the capacitors are also a significant component in the overall power loss of the SC-MLI. It is possible to lose power when the parallel capacitor connected to the DC source is charged. The charging current flows through the capacitor and, as a result of a difference between the input source and the capacitor voltage, the ripple voltage V C results, which causes the power loss. The ripple power loss of a capacitor can be computed using the following formula: The component-wise power loss for the proposed inverter at an output power of 0.5 kW has been listed in Table 2. The proposed NQB inverter exhibits 97.3 efficiency at 0.5kW output power.

III. COMPARATIVE ANALYSIS
The voltage boosting capability of the proposed topology, along with nine voltage levels at the output terminal, makes it a suitable candidate for injecting high power quality into the load/grid.
A comparison is carried out with the other MLI topologies that can produce quadruple voltage at their terminals, and the result is shown in the TABLE 3. As evident from the table, topologies suggested in [11] and [15] requires fewer active switches, but the number of diodes is greater than the proposed NQBI topology. Furthermore, the proposed topology has two capacitors of V dc and 2V dc rating for quadruple voltage increase, while the topology of [11] requires two capacitors of 2V dc rating and one capacitor of V dc rating. Having a greater number of capacitors in an MLI leads to an increase in cost and difficulty managing charging and discharging. The topology suggested in [12] requires two capacitors but at the expense of twelve active switches, as against ten active switches required in the proposed inverter.
Another important benchmark for comparison of SC topologies is the number of conducting power electronic components in the loading of capacitors, i.e. the N SC . In order to accommodate the charge current of the capacitor, equipment in the charging loop must be of greater current rating. The proposed topology would need only four devices with higher current ratings. Topologies proposed in [6], [9], [11]- [16] requires more devices in the loading loop, resulting in higher costs and losses as compared to the proposed topology in this paper. The total number of power electronic devices conducting (TCD) at each voltage level is also considered for comparison. Furthermore, from TABLE 3, it can be inferred that during the inverter's operation of the proposed NQBI, a minimum number of conduction devices is required at any stage compared to other state-of-the-art topologies. The total average standing voltage per unit (TSV pu ) of the proposed inverter is greater than that of other MLIs, except for [15]. The proposed NQBI's cost function (CF) can be defined as: (7) where weight coefficients of TSV pu and TCD avg are α and β respectively. The CF is regulated by the expense of the power electronic switches, capacitors, and gate driver circuits. TABLE 3 enlist CF of various typologies for different values of weight coefficient α and β. The CF value of the proposed NQB-MLI is found to be less than that of other MLIs. Different topologies are modelled in PLECS programme with specification: V dc = 30V , C = 4700µF and carrier frequency=5kHz. FIGURE 6 shows the efficiency of different topologies with various loading conditions. The maximum efficiency of the suggested topology is 98.2 %, as evident from FIGURE 6. Moreover, because of the lower component count of lower voltage ratings and lesser conduction losses, the performance of the suggested NQB-MLI is found to be better than all other topologies.

IV. HARDWARE IMPLEMENTATION AND RESULTS ANALYSIS
A laboratory prototype of the proposed NQBI-based standalone solar PV system was developed to validate its operations as shown in FIGURE 7.
The specifications of the components utilised to design the proposed NQBI are listed in The R-C based circuit is designed to introduce a dead time of 0.002 ms between the switches in the same leg to avoid the resulting shoot-through current through them.
The switching pulse generated at unity modulation index (MI) under level shifted PWM is fed to the respective switches of NQBI and corresponding voltage and current       To further verify the operation of proposed NQB inverter at lower modulation index (say 0.5), the voltage and current waveform of the DC/DC converter, switched capacitors (C 1 and C 2 ) and the output of inverter is obtained and represented in FIGURE 9. The voltage and current waveform of the VOLUME 10, 2022  capacitors show that only C 1 is engaged actively. The reduced output voltage (v o (t)) with five levels and respecive current (i o (t)) waveform for resistive-inductive load (Z L = 80 + 200mH ) is shown in FIGURE 9 (d).
FIGURE 10 illustrates the inverter output waveforms for dynamic source and load conditions at unity modulation index. FIGURE 10(a) shows the voltage (v o (t)) and current (i o (t)) waveform when the resistive-inductive load is changing from Z L1 = 160 + 200 mH to Z L2 = 80 + 200 mH . It is observed that the magnitude of load current increases as the impedance decreases. The phase difference between v o (t) and i o (t) also varies accordingly. In addition to that FIGURE 10 (b) shows the output voltage (v o (t)) and current (i o (t)) waveforms for resistive load (Z 1 = 160 ) when the input DC voltage is varied from 30 volts to 18 volts corresponding to the variation in irradiates. The voltage appearing across the switches of the NQBI are shown in FIGURE 11. The maximum blocking voltage across switches T 1 , T 2 , T 3 , T 4 , T 5 are 30V, 60V, 30V, 60V and 30V respectively. The maximum blocking voltage across all four switches H 1 , H 2 , H 3 and H 4 are 120 V as they are operating as polaritygenerating parts.
It is evident from the experimental results that the operation of the proposed NQBI-based PV system is effective and reliable. The output of the NQB inverter is found to be stable under a variety of operating conditions, including changes in load type and load impedance.voltage across both capacitors is also balanced to provide a nine-level output voltage under load variation without any capacitor voltage balancing technique.

V. CONCLUSION
A reduced-switch, nine-level, switched-capacitor-based quadruple boost inverter topology is proposed in this paper. The NQBI is suggested for PV applications by using a solar PV panel as the only source. Though the inverter topology employs two capacitors of unequal voltage rating, the capacitor voltage equilibrium is established without employing an additional voltage balancing strategy. The comparative study shows that the proposed nine-level inverter topology has several advantages over other popular nine-level switched-capacitor inverter topologies, such as performance, total component count, and manufacturing cost. Finally, the results from the experiments show that the proposed NQB inverter-based PV system can work.