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Edited by: Umashankar Subramaniam, Prince Sultan University, Saudi Arabia

Reviewed by: Ali Q. Al-Shetwi, Universiti Tenaga Nasional, Malaysia; Chiranjit Sain, Siliguri Institute of Technology (SIT), India

This article was submitted to Process and Energy Systems Engineering, a section of the journal Frontiers in Energy Research

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Solar photovoltaic-powered water pumping systems are becoming very successful in regions where there is no opportunity for connecting the electric grid. The photovoltaic technology converts solar energy into electrical energy for operating direct current (DC) or alternating current (AC) motor-based water pump. In the case of a solar AC motor water pump, it engages two energy conversion stages (DC–DC and DC–AC) in the power conditioning unit. This usually resulted in increased size, cost, and complexity and decreases the efficiency of the entire system. The existing two-level inverter (DC–AC) stage generates higher harmonics in output voltage and higher electromagnetic interference that deteriorates directly the AC motor performance. Therefore, as a solution to the addressed problems, in this study, an innovative seven-level inverter with five power semiconductor switches for the operation of 0.5 HP single-phase induction motor pump had been investigated. The proposed multilevel inverter has the capability of providing lesser harmonic voltage that reduces filter requirements; along with this, other part components are used lesser when compared to other conventional multilevel inverters. To provide better insight into the work performance of this proposed topology, the simulation is executed in the MATLAB/Simulink environment, and hardware implementation of the same is depicted. From the results, it is observed that the proposed multilevel inverter topology obtained a total power loss of 1.6034 W and efficiency at 98.11%. This quality output voltage acquired from the multilevel inverter had been fed to drive the induction motor water pump; it pumped the water at the desired flow rate accordingly.

Globally, solar photovoltaic-powered water pumping systems (SPVWP) are progressively used in the areas where there is plentiful sunshine and scarce power generation systems. Besides, photovoltaic (PV) system hardware prices declined to 80% in the last two decades. In the last 10 years, the levelized cost of electricity (LCoE) measure has lowered by 75% to USD 69/MWh (Jäger-Waldau,

In SPVWP, the power conditioning unit (PCU) comprises of all electronic conversion units. Therefore, in the PCU unit, if the system employs a DC–DC converter or DC–AC inverter alone, this is regarded as a single-stage system. Else, if the system employs both converters, this is considered as a double-stage system (Karampuri et al.,

The renewable energy-based systems usually take the foremost cascaded H-bridge type since it involves more isolated DC sources (Amamra et al.,

In this study, the operation of a seven-level inverter with five switches inverter had been tested experimentally with 0.5 HP single-phase induction motor water pump with the help of d-SPACE RTI-1104 platform. This topology uses an insulated-gate bipolar transistor (IGBT), as power switches and gate signals are given through a sinusoidal pulse width modulation technique. This proposed system eliminates the need for the DC–DC conversion stage in SPVWP and provides a single-stage solution through the MLI topology. The observed results show that the induction motor pump stayed its operation smoothly by receiving quality output voltage with lesser THD and also with reduced switching losses from seven-level five-switch topology. The speed and flowrate are regulated at the desired value with the help of pulses obtained from the proposed MLI topology.

The solar PV standalone water pumping system framework is depicted in

A framework of the proposed system.

The photovoltaic cells are made of customized PN junction diode that converts the visible light into DC, and this process is referred to as photovoltaic effect. The PV modules combined in parallel or series to generate higher voltage and currents (Aliyu et al.,

It represents the individual PV cell; a PV module consists of many cells or an array that includes many modules together. The mathematical equation describing the PV system is expressed in Equation (1).

where _{pv} is the current produced by incident light (A), _{o} is the leakage current of a diode (A), ^{−19}C), ^{−23} J/k), α is the diode ideality constant (1 < α < 1.5), _{s} is the equivalent PV array series resistance (Ω), _{p} is the equivalent PV array parallel resistance (Ω), _{ser} is the number of cells in series, _{par} is the number of cells in parallel, _{t} is the PV array thermal voltage (V).

The seven-level inverter with five-switch topology representation is given in _{m}” and frequency of “_{m}” is taken to generate the gate signal where comparison is made in between reference and the carrier waves. The signals produced would be equal to the number of carrier signals. The modulation index (_{a}) can be represented by

Initially, the first leg of switches S_{t1}, S_{t2}, and S_{t3} had to be fetched with 2 kHz switching frequency followingly the second leg of switches S_{t4} and S_{t5} that should be fed with 50 Hz frequency. The gating signal pattern of the first leg of the switches must be unidirectional; otherwise, the output waveform may get distorted. The second leg of inverter assures the polarity reversal pattern. The switching pattern for the proposed topology is presented in

Photovoltaic (PV)-fed seven level inverter with five switches.

Switching pattern of seven-level inverter with five switches.

_{t1} |
_{t2} |
_{t3} |
_{t4} |
_{t5} |
||
---|---|---|---|---|---|---|

1 | OFF | OFF | ON | OFF | ON | +Vdc |

2 | OFF | ON | OFF | OFF | ON | +2Vdc |

3 | ON | OFF | OFF | OFF | ON | +3Vdc |

4 | OFF | OFF | OFF | OFF | OFF | 0 |

5 | ON | OFF | OFF | ON | OFF | −Vdc |

6 | OFF | ON | OFF | ON | OFF | −2Vdc |

7 | OFF | OFF | ON | ON | OFF | −3Vdc |

In a wider range, induction motor is applied in domestic and industrial applications since they are of lower cost and need lesser maintenance. In general, induction motors are of symmetrical rotor cage and two non-symmetrical stator winding (main winding and auxiliary winding with starting or running capacitor), supplied with an equivalent sinusoidal voltage source. In this context, the second type had been chosen for water pumping applications. For modeling and analyzing the single-phase induction motor, two methods are available: one is the double field revolving theory, and the other is the cross-field theory. Of those, the first one is preferred often since it is more akin to the three-phase induction motor theory. In context to the theory, a part of alternating quantity can be resolved into two axes that are able to rotate in relatively opposite directions. Meanwhile, the axes of both stator windings are orthogonal as illustrated in

Perspective of dq model representation for single-phase induction motor.

The current flow through the stator is expressed as

If the current drawn by the rotor as _{2}, then the no-load current can be expressed by

The voltage across the rotor inductance is given by

The airgap power developed by the induction motor is given by

The motor torque is given by

The load balance equation is expressed by

whenever the load increases, the rotor current of the induction motor tends to increases. Furthermore, the energy generated from an induction motor is converted into kinetic energy in the liquid flow by triggering the liquid revolution. The centrifugal pump is used for pumping operation due to easy operation. It is particularly intended for fixed head applications, and the pressure difference generated increases along with the speed of the pump. These pumps are a rotating impeller category that radially throws water at a casing to transform water momentum into useful pressure.

Simulations conducted with MATLAB/Simulink environment to verify and confirm whether the proposed multilevel inverter topology can be practically suitable to solar photovoltaic-powered water pumping system or not. Using PV modeling, the IV and PV characteristics obtained for the Solyndra SL-001-200 PV panel, taken at different irradiance conditions, is represented in _{oc}) of 99.7 V is obtained at the standard irradiance condition of 1,000 W/m^{2}. At minimum irradiance of 200 W/m^{2}, _{oc} is developing at the range of 84 V. At some point of combination of the current and voltage, the power reaches a maximum value, which locates as _{mp} and _{mp}. In this specific point, the maximum power produced is referred to as maximum power or MPP. The solar PV panel specifications are given in

IV characteristics of Solyndra SL-001-200 photovoltaic (PV) panel.

Photovoltaic (PV) characteristics of Solyndra SL-001-200 PV panel.

Solyndra SL-001-200 (200 W) solar photovoltaic (PV) panel specifications.

STC power rating | 200 W |

Peak efficiency | 10.18% |

Power tolerance | −4%/+4% |

_{mp} |
2.55 A |

_{mp} |
78.3 V |

_{sc} |
2.78 |

_{oc} |
99.7 V |

Temp. coefficient of power | −0.38%/K |

Temp. coefficient of voltage | −0.289 V/K |

Series fuse rating | 23 A |

Maximum system voltage | 600 V |

Following that, using POD technique, the gate pulses are generated for power switches of proposed MLI topology. Based on the pulse pattern, the resulted seven-level output voltage and current attained from multilevel inverter topology is depicted in

where _{1rms} denotes the root mean square (RMS) value of the fundamental component, which is slightly high because of lower switching losses obtained from the proposed topology. The total power loss in an inverter is the combination of conduction (_{C}) and switching loss (_{Sw}). The expression of total power losses occur in an inverter is given by

By considering the losses, the efficiency of the inverter can be calculated using the following expression,

Hence, considering both the losses calculated for each switch of the proposed inverter is represented in

Seven-level output voltage from seven-level inverter with five switches.

Output current from seven-level inverter with five switches.

Fast Fourier transform (FFT) analysis of seven-level inverter with five switches.

Power loss analysis for seven-level inverter with five switches.

Comparison of part count in proposed multilevel inverter (MLI) and others.

Dynamic performance of single-phase induction motor fed by seven-level inverter with five switches.

The hardware prototype for the proposed seven-level five switch inverter fed with a single-phase induction motor water pump had been tested experimentally. The gate signal generated through POD technique had been brought out with the help of digital I/O ports in the d-SPACE RTI-1104 platform. Then, it is passed to the TLP 350 gate driver board and to the power IGBT switches. The triggering of switches could happen only if the gate driver board enhances the voltage from 5 to 15 V. A switching frequency of 2 kHz had been applied to the first leg of three switches, and 50 Hz had been given to the other two switches. This kind of pulse pattern helps to reduce the switching losses significantly. To emulate or approximate the PV source equivalence, a small resistance is connected between the programmable DC supply and the seven-level inverter five switches. The input RMS voltage of 78.3 V and current of 1 A were given as input to the four-DC source input side for the topology from programmable DC supplies, which is fixed to operate at constant voltage mode. As a result, the final output of the seven-level voltage generated at the output side with lesser harmonics, as shown in

Output voltage and current of seven-level inverter with five switches.

Harmonics generated in seven-level inverter with five switches.

The seven-level inverter with five switches had been applied for the investigation of a 0.5 HP single-phase induction motor water pump. The reduced switch count, lesser requirement of gate driver circuits and DC sources, and increased voltage levels in the output are the main features of the proposed multilevel inverter topology. As a promising source, the photovoltaic system could be able to give constant voltage to the reduced switch multilevel inverter input DC source side that guarantees the standalone operation of a water pump. In summary, the simulation results reveal that the switching losses calculated across each switch are less and the THD content is also very low without any filter compared to other conventional multilevel inverter topologies. The reduced harmonic content in the output voltage enabled the induction motor to deliver its striking dynamic performance, and it pumped water at the desired flowrate with the control in PWM technique. The hardware implementation of the same setup matched with the simulation outcomes, which validate the system, although when implementing practically, thermal resistance and leakage current in the circuit affects the power quality, which differentiates the simulation and practical results. The internal resistance of the auxiliary circuit (gate driver board) used to drive the IGBT also tends to create a difference among the simulation and practical results. The overall system performance found to be satisfactory for operating induction motor water pump, thereby validating the system model.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

TP and MK conceived of the presented idea. TP executed simulation and experiment analysis. MK encouraged TP to investigate and supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.