WO2020144696A1

WO2020144696A1 - A dc-shift method and device for reducing potential-induced degradation (pid)

Info

Publication number: WO2020144696A1
Application number: PCT/IL2020/050045
Authority: WO
Inventors: Ilya NEMENMAN
Original assignee: Vigdu V Technologies Ltd
Priority date: 2019-01-13
Filing date: 2020-01-12
Publication date: 2020-07-16

Abstract

The present invention relates to an apparatus for reducing PID in photovoltaic solar systems comprising: a converter, connected at its output to the DC bus of the inverter comprising: (a) a switching element; (b) at least one coil; and (c) a control unit for harvesting energy from the modules of the solar systems and adding the energy to the input of the inverter and shifting the voltage of said DC poles in order to reduce the PID phenomenon.

Description

A DC-SHIFT METHOD AND DEVICE FOR REDUCING POTENTIAL-INDUCED DEGRADATION (PID)

Technical Field

The present invention relates to photovoltaic solar energy fields. More particularly, the present invention relates to a method and apparatus for reducing Potential-Induced Degradation (PID) in photovoltaic solar systems.

Background

As of today, Photovoltaic solar power generation systems, i.e. solar farms, are typically made up of many solar modules each comprising many photovoltaic "cells". Photovoltaic cells are semiconductor devices that convert light into energy. When light shines on a module, a voltage develops across the module, and when connected to a load, current flows. Practically, the solar modules are mounted on a racking system and are typically connected in series. A number of solar modules connected in series is referred to as a "string", which has negative and positive DC poles.

It is typically desirable to connect as much modules as possible, in a string, in order to increase the voltage at the expense of the current, as the higher the voltage is - the less the energy loss. This is especially significant in large scale systems which typically have high current densities and longer cables for carrying the produced power. Nevertheless, the permitted maximum output voltage, of a single string, is typically limited by standards and state rules, due to the hazardous nature of a very high voltage. Since the Solar cells generate DC power, while the electricity grid is typically AC power, an“inverter” has to be connected. An inverter may be connected to many parallel-connected strings, i.e. an array of strings, for converting their DC power to AC power, for feeding the electricity grid or local consumers. The components; module frames, racking, switch boxes, rails, tubes, conduits, inverter, etc.— are typically required to be properly grounded in order to reduce the risk of electrification which can be caused by leakages or lightning.

Solar modules can generally be divided to N-Type or P-Type depending on their solar cell technology, where P-Type forms a current of negative charges and N- Type form a current of positive charges. In one example, When P-Type modules are connected in a string, the voltage between negative DC pole and ground may go up to -400V. This creates an electric field which may pull charges from the system outwards, e.g. from the frame, glass, racking etc., which leaves fewer charges in the modules themselves, causing a lower output voltage and thus lower power capacity for the affected modules. This phenomenon is widely known as Potential Induced Degradation (PID). On N-type modules, the same electric field is formed between the positive DC pole and the ground, which may disrupt the positive charges of the modules.

For the sake of brevity an explanation of the PID phenomenon is described for a P-type module, although the PID phenomenon applies for N-type modules as well. Since the main problem of the PID phenomenon is the voltage that forms between the ground and the negative pole of the string, one prior art solution is to connect the negative pole to the ground - negative grounding. This is possible if the inverter is galvanically isolated, e.g. has a transformer. However, with a transformer-less inverter this can only be done with a grounding kit that comprises a resistor, typically in a scale of KW. However, this poses a safety problem as it prevents the inverter, e.g. RCDU of the inverter, form detecting electrification.

Another prior art solution is“nighttime charging”— connecting a device to the system during the night when the solar power generation systems is not expected to generate power. This solution calls for applying a positive voltage between ground and either the positive or the and negative pole, of the string, which brings back the charges to the areas of the system that have lost them.

US 10, 103,689 discloses a power supply system, which includes: a photovoltaic panel string, an inverter, and a transformer, where the input end of the inverter is connected to the output end of the photovoltaic panel string, the output end of the inverter is connected to the input end of the transformer, and the output end of the transformer is configured to the output of a supply voltage, which is controlled by a voltage controller. The disclosed control unit is configured to determine a duty cycle of a pulse width modulation signal and to measure peak amplitudes of the alternating current voltage. The disclosed control unit can then control the energy storage circuit to charge/discharge, in order to change the voltage of the neutral point of the alternating current side of the inverter. However, the disclosed solution has safety problems.

It would therefore be desired to propose a system void of these deficiencies.

Summary

It is an object of the present invention to provide a method and apparatus for reducing Potential-Induced Degradation (PID) in photovoltaic solar power generation systems. It is another object of the present invention to provide a method and apparatus that has a combined“daytime” and“nighttime” solution for reducing PID in photovoltaic solar systems.

It is still another object of the present invention to provide a method and apparatus for reducing PID in photovoltaic solar systems that does not require an external energy source.

Other objects and advantages of the invention will become apparent as the description proceeds.

The present invention relates to an apparatus for reducing PID in photovoltaic solar systems comprising: (a) at least one string of solar modules, capable of providing solar DC power, where said modules have frames; (b) a PV+ bus, connected to the positive DC pole of said string; (c) a PV- bus, connected to the negative DC pole of said string; (d) a transformer-less inverter, connected, at its input, to said PV+ bus and PV- bus, for converting said solar DC power, from said string, to AC power; and (e) a converter, connected at its output to said PV+ bus, of said inverter and said string, comprising: a switching element; at least one coil; and a control unit for harvesting energy from said modules and adding said energy to the input of the inverter and shifting the voltage of said DC poles in order to reduce the PID phenomenon, by shifting the potential of one of the poles closer towards the ground thereby decreasing the potential between said modules and their frames for reducing the effect of the PID phenomenon.

Preferably, the switching element is a MOSFET gate.

Preferably, the apparatus is also used for the nighttime mode. In one embodiment, the converter may be embedded in the inverter.

In one embodiment, the solar modules are p-type modules.

In one embodiment, the solar modules are n-type modules.

The present invention also relates to an method for reducing PID in photovoltaic solar systems comprising: (a) providing at least one string of solar modules, capable of providing solar DC power, where said modules have frames; (b) providing a PV+ bus, connected to the positive DC pole of said string; (c) providing a PV- bus, connected to the negative DC pole of said string; (d) providing a transformer-less inverter, connected, at its input, to said PV+ bus and PV- bus, for converting said solar DC power, from said string, to AC power; and (e) providing a converter, connected at its output to said PV+ bus, of said inverter and said string, and connected at its input selectively to earth or to an isolated power generator, where the PV- is the reference point for the input and the output of the converter, said converter comprises: a switching element; at least one coil; and a control unit for shifting the voltage of said DC poles in order to reduce the PID phenomenon, by shifting the potential of one of the poles closer towards the ground thereby decreasing the potential between said modules and their frames for reducing the effect of the PID phenomenon.

Brief Description of the Drawings

The accompanying drawings, and specific references to their details, are herein used, by way of example only, to illustratively describe some of the embodiments of the invention. In the drawings:

Fig. 1 is a schematic depiction of a typical prior art solar system.

Fig. 2 is a graphical representation of the desired“daytime” DC shift towards the positive pole, for a P-type solar system, with a numeric example, according to an embodiment of the invention.

Fig. 3 schematically depicts a P-type solar system having a converter, for reducing the PID, according to an embodiment of the invention.

Fig. 4 schematically depicts some of the inner parts of the converter 11, during daytime mode, according to an embodiment of the invention.

Fig. 5 schematically depicts a flow chart, during daytime mode, according to an embodiment of the invention.

Fig. 6 schematically depicts some of the inner parts of the converter 11, during nighttime mode, according to an embodiment of the invention.

Detailed Description

Hereinafter, parts, elements and components that are depicted in more than one figure are referenced by the same numerals.

The terms“ground”,“earth”, and“grounded”, may be used interchangeably within the description below and are meant to include a potential reference point of the system from which voltages are measured.

Fig. 1 is a schematic depiction of a typical prior art solar system. The solar system comprises a number of solar strings, such as strings 8, connected in parallel to a transformer-less inverter 3. The inverter 3 receives the DC power from the strings and converts the DC power to AC power for feeding to the electricity consumer grid 12. The solar modules’, such as modules 2, frames and the inverter 3 are typically connected to the ground 1 to avoid electrification which can be caused by leakages or lightning. Each module, in the strings, typically has a high inner resistance (Ri) and a low inner parasitic capacitance (Ci) in relation to the earth, which are typically minor to have any significant effect on the individual modules, however, for a large system, the total parasitic capacitance (Ct) of all the modules can accumulate to significant values. In contrast, the total inner resistance (Rt) of all the modules can be low due to the fact that the inner resistors are connected in parallel. These known inner parasitic influences are not physical resistors and capacitors, however, for the sake of the explanation, an equivalence of them is depicted and referred to. For the sake of brevity, the accumulated inner resistances, of the modules, have been symbolized by resistors 14 and the accumulated inner parasitic capacitances, of the modules, have been symbolized by capacitors 15. Essentially, the values of the resistances and the capacitances depend on environmental conditions and vary throughout the day, thus, the voltage that falls across them may vary as well.

In P-type solar systems the voltage on the negative pole, in relations to the earth, of the array of strings, may cause PID, as there may be a significant negative potential between the modules and their grounded frames, which may pull the negative charges, from the modules, leaving fewer charges in the modules. In other words, this system forms a voltage potential between the DC poles and the ground, where the negative half, of the DC+AC voltage of the modules and inverter, is responsible for the PID. The AC voltage components 9 and 10, in Fig. 1, are depicted in order to visualize the added AC components that came from the inverter’s operation to the DC components of the strings, to simulate how the circuit behaves when taking everything into account. Fig. 2 is a graphical representation of the desired“daytime” DC shift towards the positive pole, for a P-type solar system, with a numeric example, according to an embodiment of the invention. One of the goals of the present invention is to“shift” the voltage of the DC component of the system in order to reduce the PID phenomenon. In a P-type solar system, the desire is to shift the negative pole PV- towards the ground thereby decreasing the negative potential between the modules and their frame and thus reducing the effect of the PID phenomenon. The present invention may create virtually a voltage divider that shifts the DC power towards the positive pole, as will be explained later in greater detail in relation to Fig. 3. For a numerical example, instead of having the system float between -400V to +400V the system may be shifted to work between -10V and +790V where the actual voltages can differ, from system to system, and change between different times of the day. In addition, the amplitude of the negetive AC component may be lowered which can help stabilize the voltage and further reduce the electric field. Furthermore, the present invention may make use of the residual energy, that is wasted on the inner-resistance of the modules, and use it to elevate the voltage of the system, thereby saving energy, while keeping leakage current under allowed threshold.

Fig. 3 schematically depicts a P-type solar system having a converter, for reducing the PID, according to an embodiment of the invention. The depicted solar string 12 refers to one or more strings connected in parallel to the buses PV+ and PV-. As depicted, the converter 11 is connected at its input to earth and PV-, and the output is connected to PV+ and PV-. The converter 11 may be connected in parallel to one of the resistors 14 in order to adjust the DC voltage component on the negative PV- pole to be as close as possible to the ground in order to reduce the PID effect. The converter 11 may include a DC/DC converter that takes the recovered energy from being consumed by resistor 14 and converts it to the momentary string voltage PV+ and adds all of it to the energy produced by the array 12 which ultimately goes to the inverter. By this operation, the converter 11 not only reduces PID, by reducing the negative voltage causing it, but also harvests energy that is normally wasted, on the inner resistors of the modules, and adds it to the input of the inverter and thus increases the output of the system. Thus, the converter 11 does not require an external energy source for reducing PID in photovoltaic solar systems.

Fig. 4 schematically depicts some of the inner parts of the converter 11, during daytime mode, according to an embodiment of the invention. As described in relations to Figs. 2-3 the converter 11 main task is to“shift” the voltage of the DC component of the system in order to reduce the PID phenomenon. In a P- type solar system, the desire is to shift the negative pole PV- towards the ground thereby decreasing the negative potential between the modules and their grounded frames and thus reducing the effect of the PID phenomenon. At first it is required to connect the voltage between earth to the converter’s 11 input (-750V to -10V) together with large output voltage spread (+300V - + 1450V). In some embodiments, the conversion process should be proportional and slow in order to discharge the parasitic capacitance of the system without effecting the leakage detection sensors and inverter operation. In addition to wide input range the converter 11 should limit the sinking leakage current to some defined level, for example to 300mA, or to a higher limit for larger systems. The sinking leakage current that the converter 11 draws has to be less than a defined level for security reasons, thus, if the leakage current is higher than the defined level the shift of the voltage of the DC component can be lowered in order to decrease the leakage current. The control Unit 30 can control the switching element 20, which may be a Mosfet, transistor, or any other switching element, and may sample the Vin, Vout, and the input current of the system. Coils 21 and 22 may be wind together, to form a transformer, that has inductive coupling, where both windings of transformer coils 21 and 22 voltage can be divided in between Vout to Vin on the coils 21 and 22 windings, proportional to the winding ratio. For the sake of brevity coil 21 value is referred to as LI where coil 22 value is referred to as L2. For example, the ration between the winding of the coils may be 1:3 where LI = ImH and L2 = 9mH.

In order to implement all requirements of the system it is important to start operation in small steps with limited portion of energy and increase it gradually by increasing the switching on Time (Ton) of switching element 20 periodically every portion of cycles. Switching element 20 ON will charge the primary winding 21 of the transformer with current by (VinxTon)/Ll. During the off Time (Toff), when switching element 20 is switched OFF, the Transformer, comprising coils 21 and 22, will discharge current by ((Vout- Vin)xToff)/(Ll+L2) through both windings of coils 21 and 22, of the transformer, which will divide voltage in between Vout to Vin on coils 21 and 22 windings proportional to the winding ratio.

Fig. 5 schematically depicts a flow chart, during an exemplified daytime mode, according to an embodiment of the invention. In this example, at the start, the control unit checks if Vin>1000v. If Vin>1000v maybe the VP+ is shorted to the ground, or there may be a short in the system somewhere else, therefore, the system stops at this point. Then, the control unit checks if Vin<10v, if Vin<10v then the system has already reached the desired outcome, therefore, the system stops at this point as well. If not, at this stage, the Pulse width modulation (PWM) is configured at a default start point for example: Ton = 1.4usec and Tcycle =Ton+Toff = 52usec. At this stage the control unit induces cycles of Ton and Toff at a predefined time delay, for example delay = lsec. Then, the control unit checks if 10v<Vin<15V. If not, if Vin<10 then the system has already reached the desired outcome. If Vin>10 and VinxTon<M, then the time of Ton may be increased, by lOOnsec for example, by the control unit until Vin<10v or until VinxTon<M, where M=N^cDF is the Constant parameter that set by chosen Core Magnetic Flux and design windings. Such a protection is necessary to eliminate operation in saturation of Transformer. If Vin>10v and VinxTon>M then Ton is set and frozen. At this stage, the control unit induces cycles of Ton and Toff at a predefined time of delay, for example delay = lsec. If l_in>l_Lim the Ton is decreased, by lOOnsec for example, until VinxTon<M. However, if l_in<l_Lim, then Tcyc>Tmin and Vin > Vin_min are checked. If Tcyc>Tmin and Vin > Vin_min then the Toff is decreased, by lusec for example, which causes the decrease of the Tcyc, by lusec for example, if not then 10v<Vin<15V is checked. If Vin does not reach 10v<Vin<15V then the Tcyc is set and frozen. In both cases, the control unit induces cycles of Ton and Toff at a predefined delay time, for example delay = lsec, and checks if Vin>10. If Vin voltage is more than lOv then the control unit can increase Ton, for example by lOOnsec, however if Vin voltage is less than lOv the control unit can hold to decrease Ton, for example by lOOnsec, in order to pull Vin to operate in the area close to lOv. If, at this stage l_in>l_Lim then is Tcyc is increased, by lusec for example, until Tcyc>Tmin. Thus the control unit is designed to operate in the area close to Vin = lOv. In any case the Ton has a max limit of 26 usee.

In one embodiment, the converter operation can be stopped any time by the inverter or any other entity in order to produce leakage tests on the system. Charge of parasitic capacitors can be fast by self-charging or slow by implementation of in versed process of discharge that has been described above.

In one embodiment, the converter can detect a sudden change that can be Leakage currents that flow through a human body to ground resulting in a risk of electric shock. In one embodiment, there may be 2 trip thresholds for the device monitoring detection as required by the DIN VDE 0126-1-1 standard or similar standard. A low threshold is used to protect against sudden change in leakage typical of direct contact by people. A higher threshold is used for slowly rising leakage currents, to limit the current in grounding conductors for fire safety. The default value for higher speed personnel protection is 30mA, and 300mA or higher for larger systems for lower speed fire safety. The detection of sudden change can be monitored by detecting fast AV/At on Vin or by detecting a sudden drop in operation input current of invention AI/At. Over Voltage protection made by measuring output voltage that stop Immediate switching in case of over voltage when output rise above 950V for 1000V systems and 1450V for 1500V systems.

In one embodiment the control unit 30, as depicted in Fig. 4, requires a few hundred mW of power in order to operate. In one embodiment the control unit 30 may be powered by using the power retrieved from switching the coils 21 and 22, i.e. the transformer, that operate the feeding of power from the inner resistance of the modules. In another embodiment the control unit may be powered by an external power source. In yet another embodiment the control unit may be powered by a combination of the power retrieved from the inner resistance of the modules and an external power source.

Fig. 6 schematically depicts some of the inner parts of the converter 11, during nighttime mode, according to an embodiment of the invention. The same converter 11 discussed above can be implemented for“nighttime” as well, while the inverter and the system do not produce solar energy. In this period of time, when the voltage on the photovoltaic system drops, PID can be reversed on the modules by suppling high voltage against earth to the positive pole, or to both positive and negative poles, of the photovoltaic system, against earth. In the “nighttime” mode, power may be supplied from an isolated AC/DC or DC/DC source. In this mode the input of converter 11 is shifted from the earth to a power generator 33, such as an isolated AC/DC or DC/DC source, and one of the output connections of the converter 11 is shifted from PV- to the earth. Thus, the power from the generator 33 can create a positive potential on PV+, compared to the earth, during the night, for pulling negative charges back to the system effectively reversing the PID phenomenon that has happened through the day.

In order to implement both solutions, with the same design, the“nighttime” mode - of continuous voltage supply against earth and the“daytime” mode - of DC Shift of Negative or Positive pole against earth, it is required to reconfigure connection polarity of input and output. In case of “nighttime” mode operation, the stabilization pass to output voltage and device is close the negative loop on output of the converter in between PV+ to earth. Vast option of converter control loop can be implemented in order to stabilize output voltage.

In one embodiment, the converter may be embedded in the inverter.

Although the above description refers to a system with P-type modules. The present invention works with N-type modules as well, where the input may be connected to positive DC pole and ground. In this embodiment, where the n- type accommodates the same technic of operation as the described p-type in respect to inversed polarity of connection.

While the above description discloses many embodiments and specifications of the invention, these were described by way of illustration and should not be construed as limitations on the scope of the invention. The described invention may be carried into practice with many modifications which are within the scope of the appended claims.

Claims

1. An apparatus for reducing Potential-Induced Degradation (PID) in photovoltaic solar systems comprising:

at least one string of solar modules, capable of providing solar DC power, where said modules have frames;

a PV+ bus, connected to the positive DC pole of said string;

a PV- bus, connected to the negative DC pole of said string;

a transformer-less inverter, connected, at its input, to said PV+ bus and PV- bus, for converting said solar DC power, from said string, to AC power; and

a converter, connected at its output to said PV+ bus, of said inverter and said string, comprising:

a switching element;

at least one coil; and

a control unit for harvesting energy from said modules and adding said energy to the input of the inverter and shifting the voltage of said DC poles in order to reduce the PID phenomenon, by shifting the potential of one of the poles closer towards the ground thereby decreasing the potential between said modules and their frames for reducing the effect of the PID phenomenon.

2. An apparatus according to claim 1, where the switching element is a MOSFET gate.

3. An apparatus according to claim 1, where the apparatus is also used for the nighttime mode.

4. An apparatus according to claim 1, where the converter may be embedded in the inverter.

5. An apparatus according to claim 1, where the solar modules are p-type modules.

6. An apparatus according to claim 1, where the solar modules are n-type modules.

7. A method for reducing PID in photovoltaic solar systems comprising:

providing at least one string of solar modules, capable of providing solar DC power, where said modules have frames; providing a PV+ bus, connected to the positive DC pole of said string;

providing a PV- bus, connected to the negative DC pole of said string;

providing a transformer-less inverter, connected, at its input, to said PV+ bus and PV- bus, for converting said solar DC power, from said string, to AC power; and

providing a converter, connected at its output to said PV+ bus, of said inverter and said string, and connected at its input selectively to earth or to an isolated power generator, where the PV- is the reference point for the input and the output of the converter, said converter comprises:

a switching element;

at least one coil;

a control unit for shifting the voltage of said DC poles in order to reduce the PID phenomenon, by shifting the potential of one of the poles closer towards the ground thereby decreasing the potential between said modules and their frames for reducing the effect of the PID phenomenon.

8. A method according to claim 7, where the method is also used for the nighttime mode.

9. A method according to claim 7, where the converter may be embedded in the inverter.

10. A method according to claim 7, where the solar modules are p-type modules.

11. A method according to claim 7, where the solar modules are n-type modules.