DESC:Form 2
The Patent Act 1970
(39 of 1970)
AND
Patent Rules 2003
Complete Specification
(Sec 10 and Rule 13)

Title Transformer-less Solar Photovoltaic Gird Connected Inverter
Applicant(s) National Institute of Technology Karnataka Simlife Electric Private Limited
Nationality India India
Address Srinivasnagar PO, Surathkal, Mangalore - 575025, Karnataka, India. 10/2, 3rd floor, 9th Cross, Sarakki, JP Nagar 1st phase, Bengaluru – 560078, Karnataka, India.

The following specification, particularly describes the invention and the manner in which it is to be performed.

DESCRIPTION
FIELD OF INVENTION
Embodiments of the present disclosure relate generally to a transformer-less inverter and more specifically to a transformer-less solar photovoltaic gird connected inverter using wave shaping control.
RELATED ART
Photovoltaic (PV) systems are being widely used to meet energy demands of rapidly increasing human population and industrialization. There are two types of grid-connected PV systems viz., with transformer and transformer-less systems. In transformer-based grid-connected PV systems, the bulky size of transformers makes the system heavy and expensive that compromises with efficiency of the entire system. To overcome these limitations, transformer-less PV systems are developed which are smaller, lighter, lower in cost and highly efficient.
However, safety has become a major concern in using the transformer-less PV systems due to their high leakage current. Further, the leakage current increases the grid current ripples, system losses, and electromagnetic interference.
In grid connected photovoltaic systems, voltage at input of an inverter is expected to be greater than peak value of the grid voltage. This requires several series connected PV modules in order to achieve a required voltage at an input (a DC bus). When the operating point is at a maximum power point (MPP) of the whole PV system, maximum power is not extracted from all the PV modules in case of partial shading or module mismatch. In order to address this problem, a converter may be provided for each module that takes care of the MPP tracking in the respective module, and these converters are further connected to a common bus.
At high duty ratios, the efficiency of a converter reduces drastically thereby making it impossible to obtain high voltage gains which makes conventional boost converters not useful. Though transformers along with high turns ratios, coupled inductors and cascaded boost converters may help in obtaining a high output to input voltage ratio, these too have low conversion efficiencies.
Followed by the high voltage gains, the converter may act as an inverter that converts the voltage at the DC bus to AC voltage in synchronous with the grid. Conventionally, various inverters are there being used for grid connected PV systems which are described in detail in the patents US8612058, US20140217827, EP2634909, US8085564, CN103532408, US9621073, KR20130105002 and CN107742899. Conventional low or medium power grid-connected inverters are either two or three level inverters which requires large filter components to reduce the total harmonic distortion of the current injected into the grid. The filter inductance and capacitance requirements may be reduced by employing multi-level inverters.
However, the existing multi-level inverter topologies achieve increase in number of levels only with an increased number of switches and other components thereby increasing losses in the system. Hence it is required to provide an efficient and reliable system and apparatus without using a transformer to reduce losses in the grid connected PV system.
SUMMARY
According to an aspect of the present disclosure, a photovoltaic gird connected inverter system (301) comprising a photovoltaic module (302) to generate a DC voltage, a high gain converter (304) embedded with a controller (309), a couple of switches S1 and S2 enabling in converting the DC voltage into a half sine waveform in that the controller (309) controls the switches S1 and S2, a H-bridge inverter (306) to convert the half sine waveform voltage into a full sine waveform with fundamental switching loss, a filter inductor (308) to filter out unnecessary frequencies from the full sine waveform, and a single-phase utility grid (310) to receive a maximum alternating current, wherein the controller automatically controls the switches S1 and S2 to track a maximum power in the system and to deliver the maximum alternating current to the utility grid with fundamental switching losses of the inverter, in that the inverter is operated at a switching frequency equal to a fundamental frequency of the grid.
Several aspects are described below, with reference to diagrams. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the present disclosure. One who skilled in the relevant art, however, will readily recognize that the present disclosure can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the features of the present disclosure.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram illustrating conventional grid connected PV power generation system.
FIG. 2 is a block diagram illustrating conventional circuit configuration involved in grid connected PV systems.
FIG. 3A is a block diagram illustrating a topology of the transformer-less grid connected PV circuit in an embodiment of the present disclosure.
FIG. 3B is a diagram illustrating the change in voltages throughout the circuit in an example.
FIG. 3C is a diagram illustrating an automatic control system of an embedded controller to operate the switches of the converter in an embodiment of the present disclosure.
FIG. 3D is a table illustrating switching states of high-gain buck-boost converter in an embodiment of the present disclosure.
DETAILED DESCRIPTION OF THE PREFERRED EXAMPLES
FIG. 1 is a block diagram illustrating conventional grid connected PV power generation system. As shown there, the grid connected PV power system comprises a solar panel (or PV module) 110, a PV inverter 120, a utility grid 130, a load 140 and a battery 150.
A photovoltaic array of a photovoltaic system in the PV module 110 uses photons in the sunlight and generates electricity from the absorbed solar energy. In an example, a plurality of PV modules may be coupled to each other in a series or parallel and the total energy absorbed by the PV modules is computed and transferred to a common bus where the cumulative solar energy is fed to a PV inverter (or a PV converter) 120 for further processing. The PV inverter 120 draws and processes a DC input obtained from the PV modules into an AC voltage required for transferring it either to the utility grid 130 or the load 140 or the battery 150. In an example, plurality of PV inverters may be connected to each PV module wherein the output is connected to a common bus where the power distribution unit such as the utility grid 130, the load 140 and the battery 150 are connected. The utility grid 130 and the battery 150 draws power from AC through charger circuit in the PV inverter 120 only if the demand of current at load 140 is met wherein the excess power generated from the PV modules is fed to the utility grid 130 and the battery 150 for power storage and distribution purposes.
FIG. 2 is a block diagram illustrating conventional circuit configuration involved in grid connected PV systems. As shown there, conventional circuit configuration 201 comprises a PV module 210, a boost converter 220, a PV inverter 230, a filter 240, a utility grid 250, a maximum power point tracking (MPPT) control 260, a PV inverter control 270 and a load 280. The PV module 210 is connected to the boost converter 220 and to the PV inverter 230 in series. The DC voltage from the PV modules 210 is first boosted by using the boost converter 220 and then fed to the PV inverter 230. The boost converter 220 comprises an energy storage element such as a capacitor or an inductor, at least two semiconductors such as a diode and a transistor. The boost converter 220 steps up the DC voltage from the PV modules 210 and outputs a higher DC voltage to the PV inverter 230. The PV inverter 230 then draws the DC power from the boost converter 220 and process it to convert to the AC power. In an example, the PV inverter 230 uses a maximum power point tracking (MPPT) to obtain maximum possible power from the PV array. Conventionally, there are PV inverters which utilize plurality of transformers as well as without using any transformer. The transformers employ a multistep process of converting the DC from the PV modules to high-frequency AC and then back to DC which is then finally converted to AC output voltage. The AC voltage generated by the PV inverter 230 may be fed directly to the load 280 comprising any electronic equipment or the utility grid 250 or through a charger to a battery for storage of electricity. In an example, a variety of filters 240 may be used to filter and process a regulated AC voltage from the PV inverter 230. As shown in the figure, the MPPT controller 260 and the PV inverter controller 270 are used to control, detect and regulate the flow of power through various elements of the circuit configuration 201. The circuit configuration 201 may employ plurality of capacitors (290A and 290B) and inductors to store energy at various steps in order to avoid voltage ripples and frequent loss of energy.
In conventional PV grid connected systems, it is necessary to use a large filter 240 if the Total Harmonic Distortion (THD) of the grid current has to be maintained within desired standards as it is not suggested to use inverters of level greater than three for low and medium power applications. Further, maximum power extraction becomes difficult in case of series connected PV modules 210 due to partial shading and module mismatches.
FIG. 3A is a block diagram illustrating a topology of the transformer-less grid connected PV circuit in an embodiment of the present disclosure. As shown there, a multi-level two-stage solar grid connected circuit 301 comprises a PV module 302, a high gain converter 304, a single-phase H-bridge inverter 306, a filter inductor 308, and a single-phase grid 310. The PV module 302 generates the solar energy comprising DC voltage of Vpv and transmits to the high gain converter 304. The DC voltage Vpv from the PV module 302 is converted into a half sine waveform by controlling a plurality of components within the high gain converter 304. The converter with a wide range of gain (for example, 0-6 in case of a 230V grid) is required to convert the DC voltage from the PV module 302 into the half sine waveform since a single PV module 302 is used at the input side. In an embodiment, a common grounded Z-source buck-boost high-gain converter is used as the high gain converter 304. This converter 304 comprises two switching devices S1 and S2 with three states, t0, t1 and t2, in each switching period T, plurality of diodes (312A and 312B), plurality of capacitors (314A, 314B and 314C), inductors (316A, 316B and 316C) and a resistor 318. In an example, the capacitor 314A and the inductor 316A connected in series within the converter 304 may store as well as discharge energy in the circuit to achieve a maximum output voltage in the single-phase grid 310 based on controlled switching mechanism of the switches S1 and S2. The capacitor 314B and the inductor 316B within the converter 304 also functions similar to that of the capacitor 314A and the inductor 316A. The half sine waveform generated from the converter is then fed to the single-phase H-bridge inverter 306 which generates a complete sinusoidal wave with fundamental switching loss of voltage passing through it. The generated voltage in sinusoidal waveform is then may be fed directly to a load or else to the single-phase grid 310 for utility purposes. In an embodiment, the voltage from the inverter 306 is fed to the filter inductor 308 to filter out the harmonics before sending it to the single-phase grid 310. Thus, the sinusoidal output voltage is obtained by varying the DC link voltage, as a full-wave rectified sine, such that the inverter 306 is operated at a switching frequency (say fsw) equal to a fundamental frequency of the grid (say fg).
FIG. 3B is a diagram illustrating the change in voltages throughout the circuit 301 in an example. As shown there, 303 represents the voltage generated at the PV module 302 which is fed to the converter 304 for generating a half sine wave form. The converter 304 generates the half sine waveform (as shown in 305) and inputs to the inverter 306. The inverter 306 then converts the half sine waveform into a complete sinusoidal form and transmits to the grid 310 as shown in 307. In an embodiment, the variation in the voltage from DC input of the PV module to the sinusoidal waveform in the grid is controlled by varying the magnitude or voltage gain of the converter. The manner in which the switches S1 and S2 in the converter 304 are controlled is further explained in the following figures.
FIG. 3C is a diagram illustrating an automatic control system of an embedded controller to operate the switches of the converter in an embodiment of the present disclosure. The control system 309 is embedded within the converter 304 and is capable of obtaining an output current of comparatively low total harmonic distortion with the H-bridge electronic circuit of the high gain converter, operating at the switching frequency equal to the fundamental frequency of the grid. This eliminates the requirement of additional transformers providing the transformer-less grid connected PV system of the present disclosure. The embedded automatic control system 309 comprises a control algorithm which converts the low voltage to the high voltage with fundamental switching loss in the system. The control system automatically operates the switches S1 and S2 to track the maximum power and to deliver the same to the grid without harmony. As shown there, the control system employs, a PI controller 320, a phase-locked loop (PLL) 322 and a signal generator (324A & 324B) along with plurality of adders.
The voltage at the maximum power point Vmpp in the PV system is compared with voltage of the PV module Vpv to generate a peak reference current I*L(peak). The phase-locked loop 322 generates a reference sine waveform based on the grid voltage Vg. The peak reference current I*L(peak) is multiplied with variables k1 and k2 and are fed to the adders (326A & 326B) respectively where the fundamental reference frequency from the PLL 322 is added to it. In an embodiment, the variables k1 and k2 are determined based on the reference grid voltage Vg, the peak reference current I*L(peak) and a peak voltage of the capacitor Vz(peak). The variables from the adders 326A and 326B are then fed to the signal generators 324A and 324B respectively wherein in each signal generator, the variable is multiplied with a sine block to obtain a sine waveform. The sine waveform from the signal generator 324B is then compared with the peak voltage of the capacitor Vz(peak) before adding it to the capacitor voltage Vz to create a switching pulse for S2 by detecting the voltage hysteresis. Similarly, the current iL passing through the circuit creates a switching pulse for S1 by detecting the current hysteresis. In case of voltage hysteresis, the voltage Vz of the capacitor 314B is added to the adder 326C where the reference peak voltage of the capacitor Vz(peak) is compared to operate the switch S2. In case of current hysteresis, the current passing through the inductor 316C i.e., iL is fed to the adder 326D where the reference current peak value i*L is compared to operate the switch S1. Thus, a full-wave rectified sine waveform is generated by operating the inverter 306 at a switching frequency (say fsw) equal to a fundamental frequency of the grid (i.e., fg=?t).
FIG. 3D is a table illustrating switching states of high-gain buck-boost converter in an embodiment of the present disclosure. Due to the presence of diode 312A, the working of converter 304 during a switching state S1=1 and S2=1 is same as that of the state during S1=0 and S2=1. The voltage-gain of the converter, M, under continuous conduction mode is expressed as:
M=t_1/(t_1-t_2+t_0 )=(d_1-d_2)/(1-2d_2 )=d_1d/(1-2d_2 ) (1)
where d1, d2 and d1d represents duty ratio of S1, S2 and S1(S_2 ) ¯ (combined duty ratio of S1 and S2) respectively and t0, t1 and t2 represents three states in each switching period T wherein d2 is always less than 0.5. In an example, a conventional proportional-integral controller is used to maintain Vpv at the voltage corresponding to maximum power point, Vmpp and hysteresis control is used to obtain switching pulses for both the switches S1 and S2. In an embodiment, the current hysteresis control for iL is achieved by means of switch S1 and voltage hysteresis control for Vcz by means of switch S2.
Maximum energy harvesting is not possible in case of series connected PV modules under non-uniform conditions. The proposed PV grid connected system provides a viable solution for energy generation from PV at module level using a high-gain converter. Further the converter is controlled to obtain a varying DC link voltage such that the inverter operating at fundamental frequency could produce a sinusoidal output thereby considerably reducing the switching losses of inverter, THD of inverter output and the size of filter components. The system is used for integration of PV at module level to the grid. Thus, this system 301 reduces filter inductance, harmonics content and switching losses improving overall efficiency.
Thus, grid integration of PV system at module-level is made possible ensuring maximum power is extracted under all conditions including partial shading conditions by using the high-gain converter of the present disclosure. This system enables real power injection to grid with reduced filter size and improved efficiency.
While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-discussed embodiments but should be defined only in accordance with the following claims and their equivalents.
,CLAIMS:CLAIMS
I/We Claim,
1. A photovoltaic gird connected inverter system (301) comprising:
a photovoltaic (PV) module (302) to generate a DC voltage from solar irradiance;
a high gain converter (304) embedded with a controller (309);
a couple of switches S1 and S2 enabling in converting the DC voltage into a half sine waveform in that the controller (309) controls the switches S1 and S2;
a H-bridge inverter (306) to convert the half sine waveform voltage into a full sine waveform with fundamental switching loss;
a filter inductor (308) to filter out unnecessary frequencies from the full sine waveform; and
a single-phase utility grid (310) to receive a maximum alternating current, wherein the switches S1 and S2 are automatically controlled to maximize the power in the system (301) and delivers the maximum alternating current to the utility grid (308) with fundamental switching losses of the inverter (306) by converting a DC voltage into a full-wave rectified sine waveform, in that the inverter (306) is operated at a switching frequency equal to a fundamental frequency of the grid (310).
2. The photovoltaic gird connected inverter system as claimed in claim 1, wherein activation of the switch S1 controls a current hysteresis while activation of the switch S2 controls a voltage hysteresis in the system (301) to achieve the maximum current delivery to the grid (310).
3. The photovoltaic gird connected inverter system as claimed in claim 2, wherein the controller activates the switch S1 by comparing the current (iL) passing through the inductor (316C) within the converter (304) with a reference current peak value (i*L) obtained from the PI controller (320).
4. The photovoltaic gird connected inverter system as claimed in claim 3, wherein the controller activates the switch S2 by comparing the voltage (Vz) of the capacitor (314B) within the converter (304) with a reference peak voltage (Vz(peak)) of the capacitor.
5. The photovoltaic gird connected inverter system as claimed in claim 4, wherein a phase-locked loop (322) present in the controller generates a reference sine waveform based on the grid voltage Vg.
6. The photovoltaic gird connected inverter system as claimed in claim 5, wherein the controller employs at least two signal generators (324A and 324B) to generate a sine waveform from an input DC voltage of the PI controller (320) and rectifies the input DC voltage with at least one variable (k1 and k2) to generate a rectified sine waveform for controlled operation of the switches S1 and S2.
7. The photovoltaic gird connected inverter system as claimed in claim 6, wherein the converter (304) with automatically controlled switches S1 and S2 reduces Total Harmonic Distortion of the inverter output without need of additional filter components.
8. A method, system and apparatus providing one or more features as described in the paragraphs of this specification.

Date: 11-10-2018 Signature………………………
OMPRAKASH S.N
Agent for Applicant
IN/PA-1095