The central inverter topology based on three-level converters is widely used in large-scale photovoltaic (PV) power plants because of its simple and reliable structure and high efficiency. However, when two PV arrays are connected in series for higher dc-link voltage, the single maximum power point tracking (MPPT) operation cannot function properly when there is partial shading between the arrays. This paper presents a configuration where the two PV arrays are connected in series together with an auxiliary power converter feeding a neutral-point-clamped central inverter. This configuration ensures maximum power production from both PV arrays during partial shading conditions, achieving the MPPT operation for both PV arrays. The proposed converter together with their controls are verified by simulation and experiment.

Maximum Power Point Controller for Large-Scale Photovoltaic Power Plants Using Central Inverters Under Partial Shading Conditions

This article presents a novel asymmetrical 21-level multilevel inverter topology for solar PV application. The proposed topology achieves 21-level output voltage without H-bridge using asymmetric DC sources. This reduces the devices, cost and size. The PV standalone system needs a constant DC voltage magnitude from the solar panels, maximum power point tracking (MPPT) technique used for getting a stable output by using perturb and observe (P&O) algorithm. The PV voltage is boosted over the DC link voltage using a three-level DC-DC boost converter interfaced in between the solar panels and the inverter. The inverter is tested experimentally with various combinational loads and under dynamic load variations with sudden load disturbances. Total standing voltage with a cost function for the proposed MLI is calculated and compared with multiple topologies published recently and found to be cost-effective. A detailed comparison is made in terms of switches count, and sources count, gate driver boards, the number of diodes and capacitor count and component count level factor with the same and other levels of multilevel inverter and found to be the proposed topology is helpful in terms of its less TSV value, devices count, efficient and cost-effective. In both simulation and experimental results, total harmonic distortion (THD) is observed to be the same and is lower than 5% which is under IEEE standards. A hardware prototype is implemented in the laboratory and verified experimentally under dynamic load variations, whereas the simulations are done in MATLAB/Simulink.

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A Novel Asymmetrical 21-Level Inverter for Solar PV Energy System With Reduced Switch Count

This paper proposes a general control scheme for the dual-input three-level inverter, which comes from a neutral point clamped three-level inverter. It can be used for photovoltaic (PV) systems or battery storage systems with two dc sources. The dual-input three-level inverter can be connected with two PV arrays, which are able to realize maximum power point tracking independently. First, the dynamic model of the dual-input three-level inverter is established, which describes the relationship between the dual-input voltages and the duty cycle. After discussing the control theory of the dual-input voltages, a typical control scheme is proposed. The design of a control loop is introduced in details. Afterward, modulation and restriction of the proposed method are discussed. Finally, experiment on a 125 kW dual-input three-level PV inverter is implemented to verify the validity and feasibility of the proposed method.

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General Control Scheme for a Dual-Input Three-Level Inverter

This article presents a control strategy for a dual-input neutral-point-clamped (NPC) inverter-based grid-connected photovoltaic (PV) system to asymmetrically control the PV arrays without incorporating any additional external circuit. This is achieved by employing an analytically developed linearization block, which helps in generating the required neutral point (NP) current to counter for the difference in the PV currents. The analysis also provides the maximum NP current compensation limits of the NPC inverter. If the required NP current reference is less than the maximum limit, both PV arrays operate at their maximum power points. However, if the required current exceeds the limit, one of the PV arrays operates at its maximum power point, while the output power of the other PV array is reduced to satisfy the NP current limit. Experimental tests are conducted on a laboratory prototype to validate the performance of the proposed control strategy.

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A Control Strategy for Dual-Input Neutral-Point-Clamped Inverter-Based Grid-Connected Photovoltaic System

Neutral-point-clamped multilevel converters are currently a suitable solution for a wide range of applications. It is well known that the capacitor voltage balance is a major issue for this topology. In this paper, a brief summary of the basic topologies, modulations, and features of neutral-point-clamped multilevel converters is presented, prior to a detailed description and analysis of the capacitor voltage balance behavior. Then, the most relevant methods to manage the capacitor voltage balance are presented and discussed, including operation in the overmodulation region, at low frequency-modulation indexes, with different numbers of AC phases, and with different numbers of levels. Both open- and closed-loop methods are discussed. Some methods based on adding external circuitry are also presented and analyzed. Although the focus of the paper is mainly DC–AC conversion, the techniques for capacitor voltage balance in DC–DC conversion are discussed as well. Finally, the paper concludes with some application examples benefiting from the presented techniques.

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A Survey on Capacitor Voltage Control in Neutral-Point-Clamped Multilevel Converters

In a conventional grid connection of Photovoltaic (PV) system, a number of PV modules are connected in series and integrated to grid through a conventional two level inverter. Due to the impact of partial shading and module mismatch, such systems have a much lower yield compared to their maximum potential. To address this problem, direct connection of PV source to a three-level diode clamped inverter is considered in this paper, resulting in a single stage power conversion. A novel auxiliary converter is proposed that allows the two split halves of the PV source to be directly connected to the two dc-link capacitors, with each half operating at it's respective Maximum Power Point (MPP). This auxiliary converter can be rated for less power converter compare to the inverter capacity. The unequal MPP voltage of these split PV panels may cause detrimental effect on output current of inverter. To address this power quality issue, a modified switching algorithm is discussed to achieve voltage equalization across the split DC-Links of the multilevel inverter. Though the case of a split PV source (with two halves) has been considered, the proposed scheme is equally valid for a PV array split into more than two parts that may be suitable for a higher level Neutral Point Clamped (NPC) inverter. The proposed auxiliary converter and algorithm are evaluated and verified by simulation in MATLAB-Simulink.

Tejash Naik

Papers

Three-level NPC inverter with novel voltage equalization for PV grid interface suitable for partially shaded conditions