A Single-Stage Single-Phase Non-isolated Microinverter for Residential Rooftop Photovoltaic Applications

Abstract

ABSTRACT A Single-Stage Single-Phase Non-isolated Microinverter for Residential Rooftop Photovoltaic Applications The present invention relates to a single-stage single-phase non-isolated photovoltaic microinverter (100) for converting available DC voltage at the output of photovoltaic (PV) into a single-phase AC voltage. The inverter (100) comprises of two inductor coils (Li, Lo), two capacitors (Ci, Co) and four active switches (SA to SD). For a half cycle of AC output voltage only two active switches operate at high frequency (switching frequency) and rest two active switches are completely off for that period. This adds to an advantage of improved efficiency. The inverter is capable of stepping up or stepping down the low input DC voltage, which eliminates the need of bulky power frequency transformer, reduces the overall size and weight of system and makes it suitable for the integration of small-scale intermittent DC output renewable sources. Moreover, the proposed inverter can be mounted on the PV panels, providing a modular plug and play solution. The inverter has a common ground terminal between the input and the output ports, assisting in the elimination of common mode noise which is one of the most critical problems associated with a grid connected PV system. [figure 1]

Specification

Description:FIELD OF THE INVENTION
This invention relates to a single-stage single-phase non-isolated photovoltaic microinverter for converting available DC voltage at the output of photovoltaic (PV) into a single-phase AC voltage. More particularly the proposed inverter is a buck-boost inverter for residential rooftop photovoltaic application.

BACKGROUND OF THE INVENTION
In a way to meet the increasing energy demand and to mitigate environmental impacts of conventional sources of energy, solar photovoltaics have become a popular choice. The variable low voltage DC output available at the terminals of the solar PV usually needs to be converted to grid AC voltage level at specified frequency in order to make it usable for most of the loads. One solution to meet such a specification is to use conventional H-bridge inverter. However, this solution involves multiple power conversion stages and makes the system bulky due the involvement of the power frequency transformer to meet the AC voltage specifications. To reduce the size and increase in the power density elimination of the power frequency transformer without the compromise in the AC voltage specification is necessary. This can be achieved by employing microinverters in the job. These inverters have the ability to step up and step down the variable DC voltage available at the PV output to the grid AC voltage level without the need of power frequency transformer. Being small in volume such inverters can be mounted at the back of the solar panels making the whole system modular.
Anurag A. et al., 2018, proposed an inverter that has a two-stage topology and uses 9 active switches.
Dutta S. et al, 2018, proposes a multi-input inverter, however, it is a two-stage topology and uses 8 active switches.
Husev O. et al, 2019, presented family of inverters, though they are two stage inverters. One among them uses coupled inductor which increases the size of the magnetic circuit and hence size of the inverter.
Xu S. et al, 2018, presented a single stage differential mode inverter, which requires two same circuits connected together producing opposite polarity output voltages. The inherent problem with these inverters is self-power dissipation among the two circuits. Also, common grounding is absent.
G T N. et al, 2023 presented a circuit that uses differential topology and incorporates coupled inductors, thereby increasing the overall size of the magnetics.
IN355651 discloses a single-stage transformer-less photovoltaic inverter.
IN376931 discloses a single stage buck-boost photovoltaic micro-inverter.
The prior arts above discuses single stage inverters, though they use coupled inductors, increasing the size of the magnetic circuit.
Problem in existing arts
The conventional inverters which use H-bridge configuration for power conversion requires a power frequency transformer to match the output voltage to the level of grid voltage as the peak of the output AC voltage for such configuration is always less than the maximum input DC voltage. For rooftop photovoltaic applications such a solution not only increases the weight of the whole inverter system but also has lesser efficiency and increased cost.
Also, most of the microinverters existing in literature have several problems including a greater number of switches, more internal power conversion stages, and most critical is that many do not have common grounding between the input and the output stages.
How does the present invention overcome the existing problems
The inverter proposed here is panel mountable device which converts the DC voltage (which can vary with the environmental condition) available at the PV terminals to the grid level AC output voltage without the need of the power frequency transformer.
Therefore, with this inverter mounted on the PV panel, makes the overall system modular and can connect several such inverters in series or in parallel in plug and play mode. Hence, such inverters are ideal for rooftop solar installations.
Additionally, it supports inherent elimination of the common mode noise which is one of the major sources of electromagnetic interference. Also, it offers single stage power conversion i.e. direct conversion of DC to AC. It has less number of switches and decoupled magnetics which reduces the volume and ultimately increases the power density of the system.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose an inverter with single stage solution which uses only four switches out of which only two to operate at high frequency (switching frequency) in a half cycle of the output AC voltage.
Another object of the invention is to use common ground terminal between input and output port for elimination of common mode noise.
Another object of this invention is to propose an inverter wherein the inductor coils are not coupled, thereby occupies less volume. Hence, the inverter is better in terms of efficiency and size.
These and other objects and advantages of the invention will be apparent from the ensuing description when read in conjunction with the accompanying drawing.
SUMMARY OF THE INVENTION
The proposed inverter is a single stage solution which uses only four switches out of which only two to operate at high frequency (switching frequency) in a half cycle of the output AC voltage.
In an aspect, it has common ground terminal between input and output port which has an additional advantage of elimination of common mode noise.
In an aspect, in terms of passive count, it uses two inductor coils and two capacitors. The inductor coils are not coupled, thereby occupies less volume. Hence, the inverter is better in terms of efficiency and size.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: Proposed inverter topology in (a) Standalone mode (b) Grid connected mode (c) Intended output AC voltage from the inverter.
Figure 2: Power flow for positive half (a) When SA = ON and SB = SC = SD = OFF (b) When SA = SB = SD = OFF and SC = ON (c) Positive half of output AC voltage for T = 20ms.
Figure 3: Power flow for negative half (a) When SB = ON and SA = SC = SD = OFF (b) When SA = SB = SC = OFF and SD = ON (c) Negative half of output AC voltage for T = 20ms.
Figure 4: Simulated results (a) Output AC voltage waveform (b) Output inductor (Lo) current.
DETAIL DESCRIPTION OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS OF PREFERRED EMBODIMENTS
In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to one skilled in the art that embodiments of the present disclosure may be practiced without some of these specific details.
Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention.
The terms and words used in the following description and claims are not limited to the bibliographical meanings but are merely used to enable a clear and consistent understanding of the invention.
As used in the description herein and throughout the claims that follow, the meaning of "a," "an," and "the" includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of "in" includes "in" and "on" unless the context clearly dictates otherwise.
The present invention relates to a single-stage single-phase non-isolated photovoltaic microinverter (100) for converting available DC voltage at the output of photovoltaic (PV) into a single-phase AC voltage.
The Fig. 1a and 1b shows the proposed inverter circuit in standalone and grid connected mode respectively. The inverter comprises of two inductor coils (Li, Lo), two capacitors (Ci, Co) and four active switches (SA to SD). However, for a half cycle of AC output voltage only two active switches operate at high frequency (switching frequency) and rest two active switches are completely off for that period. This adds to an advantage of improved efficiency.
The intended output of the circuit is shown in the Fig. 1c. The step-up and step-down capability of the inverter can be understood from Fig. 1d, where for the time instants T1, T3 and T5 of period T, the output AC voltage is less than the input DC voltage, therefore for these time instants the inverter steps down the voltage and for time instants T2 and T4 of period T, the output AC voltage is greater than input DC voltage and hence for these instants the inverter steps up the voltage. Due to this property, it is also called buck-boost inverter.
The detailed working of the inverter is explained below with the help of figures.

Working:
For the positive half of the AC voltage switches SA and SC are turned on and off in complimentary fashion at switching frequency with switch SA being the primary switch.
Fig. 2a and 2b shows the working of the circuit for the positive half of the AC voltage.
Fig. 2c shows the generated positive half of the output AC voltage for this sequence of operation of the inverter switches. For this duration switches SB and SD are completely off. While for the negative half cycle switches SA and SC remain completely off, and switches SB and SD are turned on and off in complimentary state at switching frequency to produce the remaining half of the output AC waveform with SB being the primary switch. A switch designated as primary will turn on first and after it turns off the other appropriate switch will turn on in that half cycle of the output AC voltage.
Fig. 3a and 3b shows the power flow in the negative half cycle of the output AC voltage and Fig. 3c shows the generated negative half of the output AC voltage for this sequence of operation of the inverter switches.
Following this switching sequence the complete cycle of the output AC voltage is produced. The capacitor C works as a power decoupling interface between the input DC and output AC voltage. The grid current is controlled by controlling the voltage across the inductor coil Lgrid when operated in grid connected mode.
Referring to figure 1a, for the standalone mode of operation the positive terminal of the PV is connected to node 1 while its negative terminal is connected to the ground (G) node. The inductor coil Li is connected between nodes 1 and 2.
The switch SA is connected between node 2 and the ground (G) node. The positive terminal of the capacitor Ci is connected to node 2 while its negative terminal is connected to node 3. The switch SB is connected between nodes 1 and 3. The inductor coil Lo is connected between node 3 and the ground (G) node. The switch SD is connected between nodes 3 and 4 while switch SC is connected between nodes 4 and 5.
The capacitor Co is connected between node 5 and the ground (G) node and the load is connected in parallel with capacitor Co i.e. between node 5 and the ground (G) node. The power decoupling capacitor C is connected between node 1 and the ground (G) node where its positive terminal is connected to node 1. When configured in grid connected mode (referring to figure 1b) all the connections remain same except the load is replaced by the grid, which can be accomplished by connecting inductor coil Lgrid between nodes 5 and 6 and finally the grid is connected between node 6 and the ground (G) node.
The input DC voltage being connected between nodes 1 and G and the output AC voltage being obtained across the output capacitor Co, wherein the load being connected between nodes 5 and G, when configured to operate in standalone mode.
In the similar fashion as above when configured in grid connected mode all the connection remains same i.e. The input DC voltage in connected between nodes 1 and G. Li is connected between nodes 1 and 2. Switch SA is connected between nodes 2 and G. The positive terminal of capacitor Ci is connected to node 2 while its negative terminal is connected to node 3. The inductor coil Lo is connected between nodes 3 and G. The switch SB is connected between nodes 1 and 3. The switch SD is connected between node 3 and 4 while the switch Sc is connected between nodes 4 and 5. The output capacitor is connected between nodes 5 and G, expect Lgrid is required to control the grid current which is connected between nodes 5 and 6 and lastly the grid is connected between nodes 6 and G.
The inverter is capable of stepping up or stepping down the low input DC voltage, which eliminates the need of bulky power frequency transformer, reduces the overall size and weight of system and makes it suitable for the integration of small-scale intermittent DC output renewable sources. Moreover, the proposed inverter can be mounted on the PV panels, providing a modular plug and play solution. The inverter has a common ground terminal between the input and the output ports, assisting in the elimination of common mode noise which is one of the most critical problems associated with a grid connected PV system.

Results
The feasibility of the circuit is tested by simulating a 400W system with an input voltage of 60V, and an output AC voltage of 110V at power frequency of 50Hz (i.e. T = 20ms). The simulation waveforms demonstrate that the inverter can provide peak output AC voltage (110√ 2 in this case) greater than the input (60V in this case) DC voltage. Hence, the inverter can provide the required gain without the need of the power frequency transformer.
The Fig. 4a shows the output voltage of the proposed inverter. The smooth transitions at zero crossings, which is an essential for such type of inverters, proves the viability of the design. The Fig. 4b shows the variation of current through the inductor coil Lo for sinusoidal variation of the output.
It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein.
Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms "comprises" and "comprising" should be interpreted as referring to elements, components, or steps in a non- exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.
The present disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the present disclosure when combined with information and knowledge available to the person having ordinary skill in the art and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

ADVANTAGES OF THE INVENTION
i. The active switch count of the inverter is 4, a lower count than the prescribed number of 6 active switches for such inverters.
ii. Out of the 4 active switches only 2 operate at high frequency in one full cycle of the output AC voltage, hence has better efficiency.
iii. As the inverter has inherent common grounding, the common mode noise is practically eliminated.
iv. The inverter has symmetric DC voltage gain for both half of the output AC voltage.
v. It is a single stage system i.e. there is no intermediate power processing stage involved 
, Claims:WE CLAIM:

1. A single-stage single-phase non-isolated photovoltaic microinverter (100) for converting available DC voltage at the output of photovoltaic (PV) into a single-phase AC voltage, said microinverter (100) comprising:
a pair of capacitors (Ci, Co) where the positive terminal of the capacitor Ci being connected to node 2 and its negative terminal to node 3 while the output capacitor Co being connected between node 5 and the ground (G) node,
a pair of inductor coils (Li, Lo), where Li being connected between positive terminal of the PV and the positive terminal of the capacitor Ci i.e. between nodes 1 and 2 and Lo being connected between negative terminal of capacitor Ci and the ground (G) node i.e. between nodes 3 and G, and
two pairs of active switches, a first switch (SA), a second switch (SB), a third switch (SC), a fourth switch (SD) where the active switch SA being connected between node 2 and G node, SB being connected between the nodes 1 and 3, SC being connected between nodes 4 and 5 and SD being connected between nodes 3 and 4,
wherein the input DC voltage being connected between nodes 1 and G and the output AC voltage being obtained across the output capacitor Co, wherein the load being connected between nodes 5 and G, when configured to operate in standalone mode,
wherein a grid inductor Lgrid BEING connected between nodes 5 and 6 and a grid being connected between nodes 6 and G, when configured to operate in grid connected mode,
characterized in that for a half cycle of AC output voltage only two active switches operate at high frequency (switching frequency) and rest two active switches are completely off for that period, wherein
(i) for the positive half cycle of the AC voltage the first and the third switches (SA and SC) being turned on and off in complimentary fashion at switching frequency with the first switch SA being the primary switch, wherein for this duration the second and the fourth switches (SB and SD) being completely off,
(ii) for the negative half cycle of the AC voltage the first and the third switches (SA and SC) remain completely off, and the second and the fourth switches (SB and SD) being turned on and off in complimentary state at switching frequency to produce the remaining half of the output AC waveform with the second switch SB being the primary switch
wherein the switch designated as primary (SA and SB) will turn on first and after it turns off the other appropriate switch will turn on in that half cycle of the output AC voltage.
2. The microinverter as claimed in claim 1, wherein the inverter being a buck-boost inverter.
3. The microinverter as claimed in claim 1, wherein inductor coils (Li, Lo) avoid coupling.
4. The microinverter as claimed in claim 1, wherein capacitor C being for acting as a power decoupling interface between the input DC and output AC voltage.
5. The microinverter as claimed in claim 1, wherein the grid current being controlled by controlling the voltage across the inductor coil (Lgrid) when operated in grid connected mode.
6. The microinverter as claimed in claim 1, wherein a common ground terminal being in between the input and the output ports.

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