Grid-connected reactive-load compensation and harmonic control are becoming a central topic as photovoltaic (PV) grid-connected systems diversified. This research aims to produce a high-performance inverter with a fast dynamic response for accurate reference tracking and a low total harmonic distortion (THD) even under nonlinear load applications by improving its control scheme. The proposed system is expected to operate in both stand-alone mode and grid-connected mode. In stand-alone mode, the proposed controller supplies power to critical loads, alternatively during grid-connected mode provide excess energy to the utility. A modified variable step incremental conductance (VS-InCond) algorithm is designed to extract maximum power from PV. Whereas the proposed inverter controller is achieved by using a modified PQ theory with double-band hysteresis current controller (PQ-DBHCC) to produce a reference current based on a decomposition of a single-phase load current. The nonlinear rectifier loads often create significant distortion in the output voltage of single-phase inverters, due to excessive current harmonics in the grid. Therefore, the proposed method generates a close-loop reference current for the switching scheme, hence, minimizing the inverter voltage distortion caused by the excessive grid current harmonics. The simulation findings suggest the proposed control technique can effectively yield more than 97% of power conversion efficiency while suppressing the grid current THD by less than 2% and maintaining the unity power factor at the grid side. The efficacy of the proposed controller is simulated using MATLAB/Simulink.

Environmental concerns, the limitation of traditional resources, and the rising demand for electrical energy, particularly of high quality and reliability, have resulted in the creation of distributed energy resources (DER) that typically used renewable energy (RE) sources [

The main demerit of nonlinear load connected with DER system is the excessive current harmonics in the grid is one of the most serious power quality challenges [

The DER system in this paper is using PV as its generation source. PV systems feature nonlinear I-V and P-V characteristics, with a single optimum working point known as the maximum power point (MPP), which varies alongside fluctuating environmental circumstances [

This paper’s main contribution is to produce a high-performance inverter with a fast dynamic response for accurate reference tracking with high-efficiency power conversion and manage to suppress excessive current harmonics in the grid even under nonlinear load applications yet can be functional during both stand-alone and grid-connected modes. In addition, the proposed controller also was designed to regulate the grid current compensates, minimize the current harmonics generated by the local nonlinear loads and as well as adjusting the grid current phase to meet the local load reactive power demand in such a way that the final grid-connected current harmonic content and the grid-connected power factor can comply with the IEEE. Std. 929–2000 and IEEE Std. 1547–2003 requirements [

To integrate power conversion and MPPT function in one stage, a single-stage full-bridge inverter is used for converting DC to AC power and also for feeding the local loads and the utility grid, while tending MPP voltage regulation across the storage capacitor.

The full-bridge single-phase inverter consists of four power switches. There are two operating modes for this topology. When switches S1 and S3 are closed (S2 and S4 are open), a positive voltage is applied across the output terminals. The second operating mode happens when switches S2 and S4 are closed (S1 and S3 are open) and the voltage is reversed, allowing reverse operation at the output circuit. To prevent shoot-through, switches on the same leg cannot be turned on simultaneously. The inverter switching scheme is using current control approach with the DBHCC method. In this control scheme, the measured load current of the inverter is compared with the generated reference current of the modified PQ theory. The current error is confined within a fixed hysteresis band (HB) which produces switching pulses for the inverters.

To achieve a lower switching frequency, the properties of the unipolar symmetrical pulse width modulation (PWM) technique and the typical HCC technique are merged in the DBHCC method. The proposed modified DBHCC switching control scheme is shown in

It can be stated that the switching frequency calculated using

The derivation of the sinusoidal command signal (I_{REF}) implemented in the DBHCC switching scheme is based on the utilization of the PQ theory by applying the instantaneous load current and grid voltage. The maximum reference power value (P_{MPP}), which is obtained from the VS-InCond algorithm, is used as the reference power P_{MPP}, such that the DC-link voltage across C_{DC} is constantly monitored based on the maximum power supplied by the PV array. Then the active power from the grid side is combined with P_{MPP} as an active power component. Moreover, the reference reactive power Q_{REF} supplied to the load is a part of the power produced by the PQ theory block and conditioned by the single-stage inverter in such a way that both active and reactive current components are formed individually to accommodate active and reactive power requirements as shown in

A single-phase system modification can be implemented to obtain four-sided symmetry. These were achieved by setting up a lag or lead of π/2 rad to both single-phase grid voltage and load current as represented by

The transformation of current and voltage in terms of α-β configuration to the p-q configuration is made using

Next, the acquisition of the reference current using ^{*} is the summation of P_{αβ} and P_{MPP.} This reference current, as shown in

MPPT is a key control algorithm for determining the maximum input power in response to changes in the source of voltage and current [

For the conventional InCond method, when the system’s operating point is to the right of the MPP, it may be quickly turned to the left of the MPP owing to an excessively high step size, causing the output power to oscillate near the MPP [_{ref} is calculated based on the following

_{MPP} is modified by the controller.

To validate the proposed system, simulation modeling is performed in the MATLAB/SIMULINK environment. The simulations mainly analyze the proposed system performance in terms of active and reactive power compensation and also the THD of grid current during nonlinear load operations with various irradiance fluctuations. The design parameters used are shown in

Parameters | Values |
---|---|

PV maximum power | 465 W |

PV voltage at MPP | 375 V |

PV current at MPP | 1.24 A |

PV open-circuit voltage | 22.53 V |

PV short-circuit current | 1.5 A |

Temperature coefficient of V_{OC} |
−0.35%/°C |

Temperature coefficient of I_{SC} |
0.05%/°C |

Temperature of normal operation cell | 25°C |

Grid voltage | 230 V_{rms}, 50 Hz |

DC capacitance | 900 uF |

LCL filter | 30 mH, 140 uF |

Grid inductance | 60 mH |

Nonlinear load 1 | Bridge rectifier |

R = 170 Ω, C = 32 uF | |

Nonlinear load 2 | Bridge rectifier |

R = 200 Ω | |

Nonlinear load 3 | Bridge rectifier |

R = 170 Ω, L = 320 mH | |

Switching frequency | 10k Hz |

Following standard test conditions (STC), the reference irradiance is set at 1000 W/m^{2} and 25°C temperature. The measured active powers are 231.81 W for nonlinear load 1, 264.46 W for nonlinear load 2 and 230.55 W for nonlinear load 3 respectively.

In this section, the capability of the proposed controller is tested under varying solar intensities. ^{2} at 4 s, while the temperature is maintained at 25°C with nonlinear load 1 at 231.81 W. During irradiance 800 W/m^{2}, the inverter (2.64 A peak) supplies the nonlinear load and the active power is also delivered to grid side (0.85 A peak). It can also be seen that the inverter current is severely affected by the nonlinear load, while the grid current is not affected at all due to the effective regulation of the proposed controller. At irradiance 200 W/m^{2}, due to the low irradiance condition, the nonlinear load 1 draws power from the grid (−0.88 A peak) since inverter power (0.63 A peak) is not able to completely meet the load demand.

Meanwhile, ^{2}, the inverter supplies the load with 2.26 A peak current and the surplus is delivered to the grid at 0.64 A peak. At irradiance 200 W/m^{2}, the grid complements the remaining load power at −1.08 A peak current and the inverter only manages to deliver 0.55 A peak current to the load. This is due to the low irradiance on the DC supply side. ^{2}, the inverter supplies the load with 2.64 A peak current and the surplus of 0.86 A peak current is delivered to the grid. During low irradiance at 200 W/m^{2}, the inverter only manages to supply the load with 0.63 A peak current, while the remaining current is drawn from the grid at −0.88 A peak. As can be observed, whether resistive, capacitive or inductive nonlinear loads, the proposed controller manages to maintain the unity power factor level at the grid side. The grid side is also not affected by the nonlinear loads. While the inverter current is affected due to the nonlinear load conditions. The proposed controller stabilizes within less than 0.5 s during the transients.

^{2}, the inverter power supplied by the PV array is about 462 W, while the surplus power delivered to the grid is 230.19 W. When the sunlight intensity changed to 800 W/m^{2}, the inverter power supplied by the PV array is reduced to 370 W with the surplus power to the grid at 138.19 W. During low sunlight intensity condition which occurs at 200 W/m^{2}, a grid power of −143.81 W is taken to supply the nonlinear load 1 (231.81 W). This happens since the load draws higher power than the available inverter power at 88 W. ^{2}, the inverter power supplied by the PV array is 460 W, while the surplus delivered to the grid is 195.54 W. When sunlight intensity is changed to 800 W/m^{2}, the inverter power supplied by the PV array is 368 W with the surplus power to the grid being 103.54 W. ^{2}, the inverter power supplied by the PV array is about 460 W. The grid receives 229.45 W and the load absorbs 228.75 W. When the sunlight intensity changed to 800 W/m^{2}, the inverter power supplied by the PV array is about 370 W and the grid takes about 139.45 W. During low sunlight intensity condition which occurs at 200 W/m^{2}, the grid power is at −142.55 W which indicates that the grid partially supplies power to the nonlinear load 3 (at 230.55 W). While the inverter makes up another 88 W to the load. Based on the results, the spike of sudden irradiance changes is only less than 0.5 s for the proposed controller to full operation. These results prove that the proposed controller manages to regulate and control the active and reactive powers between the inverter, nonlinear load and grid.

From FFT analyses, the THD values of the grid current are analyze. The THD values are 1.6% for the nonlinear load 1, 0.95% for nonlinear load 2 and 1.5% for nonlinear load 3 respectively. The THD values indicate that the grid current is not affected by the nonlinear load. THDs are well within the specified limits of IEEE 519 standards. The active power compensation and grid current THD comparison for the different type of load and irradiance is presented in

Irradiance, Temperature | Type of nonlinear load | Inverter power | Load power | Grid power | THDi grid |
---|---|---|---|---|---|

1000 W/m^{2}, 25°C |
Load 1 | 462 W | 231.81 W | 230.19 W | 1.6% |

Load 2 | 460 W | 264.46 W | 195.54 W | 0.95% | |

Load 3 | 460 W | 230.55 W | 229.45 W | 1.5% | |

800 W/m^{2}, 25°C |
Load 1 | 370 W | 231.81 W | 138.19 W | 1.1% |

Load 2 | 368 W | 264.46 W | 103.54 W | 0.85% | |

Load 3 | 370 W | 230.55 W | 139.45 W | 1.12% | |

200 W/m^{2}, 25°C |
Load 1 | 88 W | 231.81 W | −143.81 W | 1.2% |

Load 2 | 89 W | 264.46 W | −175.46 | 1.0% | |

Load 3 | 88 W | 230.55 W | −142.55 W | 1.25% |

^{2}. When the solar intensity is changed from 1000 W/m^{2} to 800 W/m^{2} at 2 s, the steady state oscillation amplitude is about 372 W reached within 1 s. At 5 s, the light intensity drops to 600 W/m^{2} and the modified algorithm can track the maximum power within less than 0.5 s, the oscillation amplitude is around 184 W. At 7 s, the light intensity drops to 200 W/m^{2}, and it takes 2.5 s for the system to track the MPP again at 90.21 W. The modified VS-InCond algorithm also proves that it remains unchanged after the MPP is reached during irradiance variation.

Efficiency is another crucial measure in any power conversion system. The proposed modified VS-InCond is evaluated for its efficiency under tested loads and irradiance changes. Based on P_{MPP} and P_{INVERTER} (as observed in ^{2}, the efficiency of power conversion for nonlinear load 1 is 99.35%. While, for nonlinear load 2 and load 3, the efficiency of power conversion are 98.92%. Then, at irradiance 800 W/m^{2}, the efficiency of power conversion for nonlinear load 1 and load 3 are 99.46% and for nonlinear load 2 is 98.92%. Whereas, at 600 W/m^{2}, the efficiency of power conversion for nonlinear load 1 and load 3 are 98.91% and for nonlinear load 2 is 97.82%. Lastly, at irradiance 200 W/m^{2}, the power conversion efficiency is 98.75% for nonlinear load 2 and 97.64% for nonlinear load 1 and load 3.

Irradiance, temperature | Type of nonlinear load | MPP power | Inverter power | Conversion efficiency |
---|---|---|---|---|

1000 W/m^{2}, 25°C |
Load 1 | 465 W | 462 W | 99.35% |

Load 2 | 460 W | 98.92% | ||

Load 3 | 460 W | 98.92% | ||

800 W/m^{2}, 25°C |
Load 1 | 372 W | 370 W | 99.46% |

Load 2 | 368 W | 98.92% | ||

Load 3 | 370 W | 99.46% | ||

600 W/m^{2}, 25°C |
Load 1 | 184 W | 182 W | 98.91% |

Load 2 | 180 W | 97.82% | ||

Load 3 | 182 W | 98.91% | ||

200 W/m^{2}, 25°C |
Load 1 | 90.12 W | 88 W | 97.64% |

Load 2 | 89 W | 98.75% | ||

Load 3 | 88 W | 97.64% |

In order to verify the capability of the modified VS-InCond algorithm, comparisons are carried out with the conventional InCond algorithm developed by [^{2} and the oscillation amplitude is range from 465 W to 463 W. whereas, at 800 W/m^{2}, the MPP is track within 0.6 s and the oscillation amplitude is unsettle between 465 to 463 W between 359 to 348 W. Then, when irradiance changes from 800 to 600 W/m^{2} at 5 s, the new MPP is track within 0.55 s and the oscillation amplitude is range between 183.75 to 183.3 W. At 200 W/m^{2}, the new MPP is track within 1.2 s and the oscillation amplitude is range between 83.7 to 83 W. From the figures, it is observed that when irradiance changes and the oscillation at the MPP is high and difficult to stabilize at the maximum theoretical peak. Comparing the results in

This paper presents the performance of a grid-connected single-phase inverter for nonlinear load applications in PV renewable energy systems. As justified by the simulation findings, the improved current control scheme consisting of modified PQ theory and DBHCC can effectively reduce the voltage distortion of the inverter caused by several types of nonlinear loads, minimize the grid current harmonics within 5% specified limits of IEEE 519 standards and regulate unity power factor at the grid side. The proposed control scheme is also able to deliver a power conversion efficiency of more than 97% based on several irradiance changes modeling. Also by comparing the results of the proposed modified VS-InCond MPPT algorithm with the conventional InCond MPPT algorithm, it is found that the proposed modified VS-InCond MPPT algorithm has superior performance in delivering a stable active power from the PV arrays to the load and the grid network during various irradiance conditions.

This research was funded by Geran Galakan Penyelidik Muda GGPM-2020-004 Universiti Kebangsaan Malaysia.

The authors declare that they have no conflicts of interest to report regarding the present study.