DESC:HOTSPOT MITIGATION CIRCUIT FOR PHOTOVOLTAIC MODULE

Field of invention:

The present invention relates to a photovoltaic module and more particularly, to a hotspot mitigation circuit for the photovoltaic module.

Background of the invention:

During partial shading of photovoltaic module or array the shaded cells gets reverse biased and reverse breakdown of the shaded cells occur. The shaded cells start acting as load to the illuminated cells connected in series therewith and instead of producing power the shaded cells starts dissipating power and generates hotspot temperature. This phenomenon causes accelerated aging, loss of output and permanent damage of the cells and/or fire in extreme cases.

The conventional method to protect the modules from hotspot temperature under partial shading is to connect a bypass diode (BPD) across one third of the series connected cells in a module or sub-array as shown in figure 1a. In this arrangement in the event of partial shading on any cell, the bypass diode goes to conduction mode and the current is bypassed through the BPD. However, as illustrated in figure 1b during conduction of the bypass diode reverse voltage which gets applied to the shaded cell can be expressed as below:
Maximum reverse bias voltage,
-V_(R (max))=?_(n=1)^((N-1))¦V_(OC,n) +V_D … (1)
Where,
N=Number of cells in a sub-array
VOC= Open circuit voltage of PV cells
VD= Forward voltage drop in the bypass diode

Hence, with the existing arrangement the reverse bias voltage is not eliminated completely but restricted based on the number of cells across which a BPD is connected which still causes reverse break down of the shaded PV cells.

From Equation (1) maximum reverse voltage which gets applied to the shaded PV cell can be quantified as:
-VR(max)= (20-1)x0.6+0.5= 11.9 V … for 20 V(NOM) module (60 cell Module)
= (24-1)x0.6+0.5= 14.3 V … for 24 V(NOM) module (72 cell Module)
= (32-1)x0.6+0.5= 19.1 V … for 32 V(NOM) module (96 cell Module)
Considering VOC=0.6 V, and VD=0.5 V

The above voltages are large enough to cause reverse break down of the photovoltaic cells which may lead to accelerated aging and eventually permanent damage due to hotspot creation. Due to hot spotting the temperature of the affected cell may increase up to 150°C which is far beyond the safe operating limit of 85°C.

The modules of the elevated temperature cause detrimental impacts on the module reliability in two ways. Firstly, due to secondary break down or thermal break down in which the reverse current flows through one directional narrow channel. The local temperature in secondary breakdown may reach well above 400 °C leading to irreversible damage of the PV cells. Secondly, under prolonged operation at elevated temperature and repeated thermal cycle the electrical joints weaken over time and eventually disconnect causing electrical arc.

The protective glass cover shatters due to the high temperature produced due the above phenomena in some cases which allow the atmospheric oxygen to enter inside the module. At the elevated temperature and under the presence of oxygen the highly inflammable encapsulating material-ethyl vinyl acetate (EVA), used in solar modules, catches fire. Hence, hot spotting not only causes accelerated aging, but also poses serious safety and reliability concern for the PV power plants which the existing bypass arrangement is not able to address.

Accordingly, there exists a need to provide a hotspot mitigation circuit for a photovoltaic module that overcomes the abovementioned drawbacks of the prior art.

Objects of the invention:

An object of the present invention is to reduce reverse bias voltage applied across a shaded cell to prevent break down of the photovoltaic cells.

Another object of the present invention is to maintain temperature of the affected cell within safe operating temperature.

Yet, another object of the present invention is to reduce accelerated aging of the photovoltaic cells.

Summary of the invention:

Accordingly, the present invention provides a hotspot mitigation circuit for a photovoltaic module. The photovoltaic module comprises of a plurality of subpanels, wherein each subpanel has a plurality of photovoltaic (PV) cells connected in series. The hotspot mitigation circuit is connected with each subpanel. The hotspot mitigation circuit comprises a bypass diode and an insulated-gate bipolar transistor (IGBT). The IGBT is connected in series with the subpanel along with the bypass diode.

During normal operation, the voltage generated by the PV cells in the subpanel (i.e. n*V) is applied to gate-emitter voltage (VGE) of the IGBT and turns on the IGBT that conducts a photo current generated by all the PV cells connected in series thereto thereby offering negligible on state resistance and on state power loss.

During partial shaded condition, the gate voltage reduces and the IGBT is pushed in to a nonlinear region of operation. In this condition, the voltage across the collector and the emitter (VCE) increases. The reverse bias voltage which gets applied across the shaded cell gets reduced by the voltage drop across the IGBT. This prevents the shaded PV cell from avalanche breakdown and hotspot temperature.

Brief description of drawings:

The objects and advantages of the present invention will become apparent when the disclosure is read in conjunction with the following figures, wherein

Figure 1a shows a conventional circuit for hotspot mitigation, in accordance with the prior art;

Figure 1b is a circuit diagram showing a reverse breakdown voltage applied to shaded cell in the conventional circuit for hotspot mitigation of figure 1a; and

Figure 2 shows a hotspot mitigation circuit for a photovoltaic module, in accordance with the present invention.

Detailed description of the invention:

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments.

Accordingly, the present invention provides a hotspot mitigation circuit for a photovoltaic module. The hotspot mitigation circuit reduces the reverse bias voltage applied across a shaded cell thereby preventing break down of the photovoltaic cells as a result temperature of the affected cell remains within safe operating temperature.

The present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate corresponding parts in the various figures. These reference numbers are shown in bracket in the following description and the table given below.

Table:

Component Name Component Number
Hotspot Mitigation Circuit 100
Plurality of photovoltaic cells (PV cells) 10
Bypass diode 20
Insulated-gate bipolar transistor (IGBT) 40
Subpanel of a photovoltaic module 50

Now referring to figure 2, a hotspot mitigation circuit (100) for a photovoltaic module in accordance with the present invention is shown. The photovoltaic module comprises of a plurality of subpanels (50) arranged in parallel. Each subpanel (50) includes a plurality of photovoltaic cells (10) (hereinafter “the PV cells (10)”) connected in series therein. The hotspot mitigation circuit (100) is connected with the each subpanel (50). The hotspot mitigation circuit (100) comprises a bypass diode (20) and an insulated-gate bipolar transistor (40) (hereinafter referred as, “the IGBT (40)”).

The bypass diode (20) is connected in series with a first end (not numbered) of the subpanel (50). The bypass diodes (20) of the each subpanel (50) are connected in series with each other.

The IGBT (40) is connected in series with the subpanel (50) along with the bypass diode (20). The IGBT (40) includes a gate terminal (not numbered), a collector terminal (not numbered) and an emitter terminal (not numbered). The gate terminal is connected to the first end of the subpanel (50). The collector terminal is connected to the bypass diode (20). The emitter terminal is connected to the second end (not numbered) of the subpanel (50).

In an embodiment, the IGBT (40) can withstand temperature of up to 150°C. In a specific embodiment, the IGBT (40) along with the bypass diode (20) can be physically installed in a junction box (not shown) of the photovoltaic module. The junction box is an external component and mounted on a backside of the photovoltaic module.

During normal operation, the voltage generated by the PV cells (10) in the subpanel (i.e. n*V) is applied to gate-emitter voltage (VGE) of the IGBT (40) and turns on the IGBT (40) which conducts a photo current generated by all the PV cells (10) connected in series thereto thereby offering negligible on state resistance and on state power loss.

During partial shaded condition, the gate voltage reduces and the IGBT (40) is pushed in to a nonlinear region of operation. In this condition, the voltage across the collector and the emitter (VCE) increases. The reverse bias voltage which gets applied across the shaded cell gets reduced by the voltage drop across the IGBT (40). The reverse bias voltage which gets applied across the shaded cell under partial shading condition can be expressed by Equation (2):
-VR(MAX) = V x (N-1)+VD-VCE

This prevents the shaded PV cell (10) from avalanche breakdown and hotspot temperature. However, the power dissipation in the IGBT (40) increases the temperature of the IGBT (40). But the IGBT (40) can withstand temperature of up to 150°C and shall be physically installed in a junction but along with the bypass diode (20). The junction box is an external component and mounted on a backside of the photovoltaic module. Hence the power dissipation component is moved to a part which is external to the module hence reliability of the module increases.

Advantages of the invention:

The hotspot mitigation circuit (100) reduces the reverse bias voltage across the shaded cells below its avalanche breakdown voltage as a result break down of the PV cells does not occur and temperature of the affected cell remains within safe operating temperature.

The hotspot mitigation circuit (100) increases reliability and prevents accelerated aging without significant increase in the cost.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the scope of the claims of the present invention. ,CLAIMS:We claim:

1. A hotspot mitigation circuit (100) for a photovoltaic module, the photovoltaic module having a plurality of subpanels (50), each subpanel (50) including a plurality of photovoltaic cells (10) connected in series, wherein the hotspot mitigation circuit (100) is connected with each subpanel (50), the hotspot mitigation circuit (100) comprising:
a bypass diode (20) connected in series with a first end of the subpanel (50); and
an insulated-gate bipolar transistor (40) connected in series with the subpanel (50) along with the bypass diode (20), the insulated-gate bipolar transistor (40) including,
• a gate terminal connected to the first end of the subpanel (50),
• a collector terminal connected to the bypass diode (20), and
• an emitter terminal connected to the second end of the subpanel (50);
wherein, during normal operation, voltage generated by the plurality of photovoltaic cells (10) in the subpanel (50) is applied to gate-emitter voltage (VGE) of the insulated-gate bipolar transistor (40) and turns on the insulated-gate bipolar transistor (40) that conducts a photo current generated by all the photovoltaic cells (10) connected in series thereto thereby offering negligible on state resistance and on state power loss, and
wherein, during partial shaded condition, the gate voltage reduces and the insulated-gate bipolar transistor (40) is pushed in to a nonlinear region of operation so that the voltage across the collector and the emitter (VCE) increases and the reverse bias voltage applied across the shaded cell gets reduced by the voltage drop across the insulated-gate bipolar transistor (40), thereby preventing the shaded photovoltaic cells (10) from avalanche breakdown and hotspot temperature.

2. The hotspot mitigation circuits (100) for the photovoltaic module as claimed in claim 1, wherein the bypass diodes (20) of each subpanel (50) are connected in series with each other.

3. The hotspot mitigation circuits (100) for the photovoltaic module as claimed in claim 1, wherein the insulated-gate bipolar transistor (40) is physically installed in a junction box of the photovoltaic module along with the bypass diode (20), wherein the junction box is an external component and is mounted on a backside of the photovoltaic module.

Dated this 20th day of December 2019

Madhavi Vajirakar
(Agent for Applicant)
(IN/PA-2337)