# A New Transformer-Less Single-Phase Photovoltaic Inverter to Improve the Performance of Grid-Connected Solar Photovoltaic Systems

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## Abstract

**:**

## 1. Introduction

- Mitigates the ripples in the stray capacitor voltage;
- Eliminates the extra leakage current;
- Keeps the common-mode voltage constant;
- Reduces the harmonics in the grid voltage and current, as well as the losses;
- Increases the inverter performance, even if the solar energy intensity is scarce.

## 2. Proposed Single-Phase Transformer-Less Inverter

#### 2.1. Structure of the Proposed Transformerless PV Inverter

_{1}–D

_{6}) and an IGBT switch (S

_{7}). The diodes D

_{1}–D

_{4}form another bridge circuit along with two diodes D

_{5}and D

_{6}, which form the freewheeling branch. These diodes are forward biased in the freewheeling modes for both positive power region operation and negative power region operation. In the positive half cycle, switches S

_{1}and S

_{4}have the same switching sequence. Likewise, in the negative half cycle, switches S

_{2}and S

_{3}have the same switching sequences. Switches S

_{5}and S

_{6}conduct in both conduction modes and are disconnected in both of the freewheeling modes. The common-mode voltage is constant and clamps half of the input DC voltage. As a result, voltage in the stray capacitor does not fluctuate and the leakage current flow is reduced.

#### 2.2. Modulation Technique

_{1}and S

_{4}have the same switching sequences. Similarly, switches S

_{2}and S

_{3}have the same switching sequences. The switches S

_{1}and S

_{4}are activated in the positive half cycle, whereas the switches S

_{2}and S

_{3}are activated in the negative half cycle. During the positive half cycle, the switching pulses of the switches S

_{5}and S

_{6}are as similar as the gate pulses of the switches S

_{1}and S

_{4}. Similarly in the negative half cycle, the switching pulses of S

_{5}and S

_{6}are as similar as the gate pulses of the switches S

_{2}and S

_{3}. The switch S

_{7}has the opposite pulses compared to S

_{5}and S

_{6}; i.e., when S

_{7}is ON, S

_{5}and S

_{6}are OFF and vice versa.

#### 2.3. Control Scheme

_{p}and K

_{I}.

_{Filter}is the filter capacitance, r

_{capacitor}is the series resistance of the filter capacitor, and T

_{v}is the time constant of the voltage controller. The proportional and integrator terms for the current controller are shown below:

_{Filter}is the filter inductance, r

_{inductor}is the series resistance of the filter inductance, and T

_{c}is the time constant of the current controller.

_{inv}, G

_{filter}, G

_{PI}, and G

_{PWM}represent the gains of the inverter, the filter, the PI controller, and the pulse width modulator.

## 3. Conduction Modes of the Proposed Transformer-Less Inverter

#### 3.1. Conduction Modes in Unity Power Factor Operation

_{5}, S

_{1}, S

_{4}, and S

_{6}. In this situation, inverter output voltage (V

_{AB}) becomes equal to the input DC voltage (V

_{DC}), where V

_{AN}= +V

_{DC}and V

_{BN}= 0. Therefore, the common-mode voltage becomes:

_{g}is the grid voltage.

_{7}, four diodes (D

_{2}, D

_{3}, D

_{5}, and D

_{6}) are forward biased. Circulating freewheeling current flows via the path shown in Figure 7b. In this mode, inverter output voltage (V

_{AB}) becomes zero, decreasing V

_{AN}to half of the DC input voltage (V

_{DC}/2) and increasing V

_{BN}to half of the DC input voltage (V

_{DC}/2). Thus, the common-mode voltage in freewheeling mode remains the same as that of the active mode, and stray capacitor voltage fluctuations are avoided. As a result, the leakage current is reduced. The common-mode voltage yields:

_{AB}) becomes equal to the input DC voltage again, but the polarity is negative (−V

_{DC}). This is because V

_{AN}decreases to 0 and V

_{BN}increases to the DC input voltage (+V

_{DC}). Meanwhile, the common-mode voltage stays the same as the previous modes, V

_{DC}/2. The load current flows through the switches S

_{5}, S

_{3}, S

_{2}, and S

_{6}. It was observed that the switches S

_{5}and S

_{6}conduct in both the active modes. Thus, finally, the common-mode voltage yields:

_{AB}) becomes zero in this mode too. This is because V

_{AN}increases to V

_{DC}/2 and V

_{BN}decreases to V

_{DC}/2. Along with the switch S

_{7}, four diodes (D

_{1}, D

_{4}, D

_{5}, and D

_{6}) are forward biased. Circulating freewheeling current flows via the path shown in Figure 7d. It can be observed that the switches S

_{5}and S

_{6}are open circuited in both freewheeling modes. Common-mode voltage stays constant in this mode too, which yields:

#### 3.2. Conduction Modes under Non-Unity Power Factor

_{DC}and 0. Likewise, in positive power region 2, modes 3 and 4 simultaneously generate −V

_{DC}and 0 again. Modes 2 and 4 are freewheeling modes and act to remove overflow of the leakage current. Here, modes 1 and 2 are the active modes.

_{DC}and 0. Likewise, in negative power region 2, modes 7 and 8 simultaneously generate −V

_{DC}and zero again. In the negative power region, power flows from the grid to PV system, which means the inverter consumes power in this mode.

## 4. Simulation Results

_{PV}). Two 3 mH series inductors and a 2.2 uF capacitor were utilized as the LCL filter.

_{AN}and V

_{BN}are depicted in Figure 10a,b, respectively. Figure 10c,d depict the waveforms of the common-mode voltage V

_{C}

_{M}and differential-mode voltage V

_{A}

_{B}, respectively. Figure 10c also provides a closer look of the V

_{CM}. The oscillation of the common-mode voltage is not excessively high, which inhibits unnecessary leakage current flow. Even at 0 output voltage, it remains constant. The grid voltage waveform is depicted in Figure 10e, where a THD of 1.25% was measured. As illustrated in Figure 10f, the grid current THD was recorded as 0.94%. The common-mode voltage was maintained at half of the DC input voltage in a steady manner. As a result, an 8.03 mA leakage current was measured, which is illustrated in Figure 10g.

_{IGBT}is the voltage of the IGBT switch in conduction mode. The individual average voltage waveforms of the IGBTs, as well as the diodes used in the inverter topology, are depicted in Figure 13. Figure 13a–g represent the stress voltages of the IGBTs. Figure 13h–m illustrate the individual voltage waveforms of the diodes that are used in the proposed topology.

## 5. Experimental Results

_{1}, D

_{2}, D

_{3}, and D

_{4}. For diodes D

_{5}and D

_{6}, RHRP15120 hyper-fast diodes with soft recovery characteristics were utilized. The input DC voltage was generated by programming an AMETEK TerraSAS PV Simulator ETS 1000/10 at 1000 W/m

^{2}irradiance and 30 °C. A parasitic capacitor was employed to emulate the capacitance between the PV module and ground. An LEM CT 0.2-P current sensor was utilized for the leakage current measurement, which usually works on the flux gate principle. The presented controller was designed using the dSPACE MicroLabBox-based embedded platform. The onboard FPGA in the MicroLabBox was programmed with the real-time interface FPGA programming block sets. KEYSIGHT N2791A differential voltage probes and Agilent N2781A differential current probes were utilized to collect the voltage and the current signals, respectively. To observe the voltage and current signals, a KEYSIGHT InfiniiVision DSOX4024A digital oscilloscope was employed.

_{A}

_{N}, V

_{BN}, and V

_{AB}are depicted in Figure 15a, and the waveforms of the grid voltage (V

_{g}), grid current (i

_{g}), common-mode voltage (V

_{C}

_{M}), and leakage current (i

_{Leakage}) are depicted in Figure 15b. Here, the grid voltage and grid current are in the same phase.

_{AB}was constant because of the voltage controller. Furthermore, the current controller also held the grid current (i

_{g}) flow at a constant point. As a result, it is clear that the changes in input voltage did not affect the grid voltage or current as the inverter output voltage remained constant.

## 6. Comparison with Other Existing Topologies

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 7.**Conduction modes under unity power factor: (

**a**) positive half-cycle active mode, (

**b**) positive half-cycle freewheeling mode, (

**c**) negative half-cycle active mode, and (

**d**) negative half-cycle freewheeling mode.

**Figure 9.**Conduction modes under non-unity power factor: (

**a**) mode 1 (V

_{A}

_{B}= +V

_{DC}), (

**b**) mode 2 (V

_{AB}= 0), (

**c**) mode 3 (V

_{AB}= −V

_{DC}), (

**d**) mode 4 (V

_{A}

_{B}= 0), (

**e**) mode 5 (V

_{AB}= +V

_{DC}), (

**f**) mode 6 (V

_{AB}= 0), (

**g**) mode 7(V

_{AB}= −V

_{DC}), (

**h**) mode 8 (V

_{AB}= 0).

**Figure 10.**Simulation results for the proposed transformer-less inverter: (

**a**) V

_{AN}, (

**b**) V

_{BN}, (

**c**) V

_{CM}, (

**d**) V

_{AB}, (

**e**) grid voltage V

_{g}and its THD, (

**f**) grid current ig and its THD, and (

**g**) leakage current i

_{Leakage}.

**Figure 11.**Grid voltage and current waveforms with the proposed transformer-less inverter and its harmonic counting after varying the current reference in the current controller: (

**a**) grid voltage and its harmonic counting, (

**b**) grid current and its harmonic counting.

**Figure 12.**Grid voltage and current waveforms with step change of DC input voltage in grid-connected mode: (

**a**) input DC voltage variation, (

**b**) grid voltage V

_{g}, (

**c**) grid current i

_{g}.

**Figure 13.**Stress voltage waveforms of the IGBTs and diodes used in the proposed transformer-less inverter: (

**a**) S

_{1}, (

**b**) S

_{2}, (

**c**) S

_{3}, (

**d**) S

_{4}, (

**e**) S

_{5}, (

**f**) S

_{6}, (

**g**) S

_{7}, (

**h**) D

_{1}, (

**i**) D

_{2}, (

**j**) D

_{3}, (

**k**) D

_{4}, (

**l**) D

_{5}, and (

**m**) D

_{6}.

**Figure 15.**Steady-state response of the proposed inverter: (

**a**) V

_{AN}, V

_{BN}, and V

_{AB}; (

**b**) grid voltage (V

_{g}), grid current (i

_{g}), common-mode voltage (V

_{CM}), and leakage current (i

_{Leakage}).

**Figure 16.**Dynamic response of the proposed transformer-less inverter with the step change in the DC input voltage.

Parameter Name | Symbol | Value |
---|---|---|

Input DC voltage | V_{DC} | 400 V |

Grid voltage | V_{g} | 230 V/50 Hz |

Maximum output power | P_{o}_{ut} | 2 kW |

Switching frequency | f_{s} | 20 kHz |

DC link capacitor | C_{DC}_{1} and C_{DC}_{2} | 940 uF |

Parasitic capacitor | C_{pv}_{1} and C_{pv}_{2} | 75 nF |

Output filter inductor | L_{1} and L_{2} | 3 mH |

Output filter capacitor | C_{0} | 2.2 uF |

Topology Name | H5 [11] | oH5 [12] | HERIC [13] | H6 [14] | HBZVR [14] | HBZVR-D [14] | Proposed |
---|---|---|---|---|---|---|---|

Number of IGBTs | 5 | 6 | 6 | 6 | 5 | 5 | 7 |

Number of diodes | 0 | 0 | 0 | 2 | 5 | 6 | 6 |

PWM pattern | Unipolar SPWM | Unipolar SPWM | Unipolar SPWM | Unipolar SPWM | Unipolar SPWM | Unipolar SPWM | Unipolar SPWM |

CMV | Floating (~200 V) | Constant | Floating (~200 V) | Constant | Semi-floating (~200 V) | Constant | Constant |

Leakage RMS current (mA) | 45 | 18 | 48.8 | 15.5 | 74.4 | 42.7 | 8.03 |

Grid current THD (%) | 1.5 | 1.5 | 1.5 | 1.5 | 1.9 | 1.9 | 0.94 |

Efficiency (%) | 97.59 | 97.55 | 98.16 | 97.24 | 97.65 | 97.88 | 97.93 |

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**MDPI and ACS Style**

Biswas, M.; Biswas, S.P.; Islam, M.R.; Rahman, M.A.; Muttaqi, K.M.; Muyeen, S.M. A New Transformer-Less Single-Phase Photovoltaic Inverter to Improve the Performance of Grid-Connected Solar Photovoltaic Systems. *Energies* **2022**, *15*, 8398.
https://doi.org/10.3390/en15228398

**AMA Style**

Biswas M, Biswas SP, Islam MR, Rahman MA, Muttaqi KM, Muyeen SM. A New Transformer-Less Single-Phase Photovoltaic Inverter to Improve the Performance of Grid-Connected Solar Photovoltaic Systems. *Energies*. 2022; 15(22):8398.
https://doi.org/10.3390/en15228398

**Chicago/Turabian Style**

Biswas, Mohua, Shuvra Prokash Biswas, Md. Rabiul Islam, Md. Ashib Rahman, Kashem M. Muttaqi, and S. M. Muyeen. 2022. "A New Transformer-Less Single-Phase Photovoltaic Inverter to Improve the Performance of Grid-Connected Solar Photovoltaic Systems" *Energies* 15, no. 22: 8398.
https://doi.org/10.3390/en15228398