CA2883843A1

CA2883843A1 - System and method for providing periodic electrical isolation in a power system, such as a solar power generation system

Info

Publication number: CA2883843A1
Application number: CA2883843A
Authority: CA
Inventors: Daniel Francis Opila
Original assignee: GE Energy Power Conversion Technology Ltd
Current assignee: GE Energy Power Conversion Technology Ltd
Priority date: 2012-09-18
Filing date: 2013-09-16
Publication date: 2014-03-27
Also published as: BR112015005896A2; WO2014045125A2; WO2014045125A3

Abstract

A method of automatically electrically isolating a source of current of a first type from a load on a periodic basis comprising a plurality of intervals that employs a power converter portion structured to convert current of the first type to current of a second type, a first selectively operable electrical isolation device structured to provide selective electrical isolation between the source and the power converter portion, and a second selectively operable electrical isolation device structured to provide selective electrical isolation between the power converter portion and the load. The method includes, for each of the intervals: (i) determining, based on a predetermined schedule, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval, and (ii) causing the determined scheduled isolation device to move to an electrically isolating condition during the interval.

Description

SYSTEM AND METHOD FOR PROVIDING PERIODIC ELECTRICAL
ISOLATION IN A POWER SYSTEM, SUCH AS A SOLAR POWER
GENERATION SYSTEM
BACKGROUND OF THE INVENTION
1. Field of the Invention [01] The present invention pertains to power systems wherein power is converted from one form to another, and, in particular, to a system and method for providing periodic electrical isolation in a power system, such as a solar power generation system, in a scheduled manner.

2. Description of the Related Art [02] Solar power generation systems convert sunlight into electricity using photovoltaics. More specifically, a photovoltaic (PV) cell, also known as a solar cell, is a device that converts light into direct current (DC) electrical current using the photovoltaic effect. In a typical solar power generation system, multiple PV cells are connected together to form a PV module, and multiple modules are connected together to form a PV array. Since PV cells produce DC power, that DC power must be converted to alternating current (AC) power before it is provided to the commercial electrical grid. In addition, the DC power output by a PV array fluctuates with the intensity of the light received by the PV array. Thus, in a typical typical solar power generation system, the DC output of the PV array is provided to a solar inverter, which is an electrical power conversion device that converts the variable DC output of the PV array into an AC current that can be provided to the commercial electrical grid. Solar inverters are well known and are manufactured and sold by a number of companies, such as, without limitation, the assignee of the present invention.

[03] In addition, solar power generation systems are typically provided with devices such as AC and/or DC circuit breakers, contactors or switches to provide electrical isolation in the case of a fault condition or in situations where maintenance must be performed. Such devices typically include built in logic for the fault protection functionality and manual actuators for the maintenance functionality.

[04] Furthermore, in solar power generation systems, the PV array is typically electrically isolated from the electrical grid when not generating power (usually overnight). Thus, in a typical solar power generation system, it will be necessary to electrically isolate the PV array from the electrical grid according to a regular schedule, such as once each day. This will thus typically require that an isolating device (e.g., a DC or AC breaker, contactor or switch) be opened and closed at least once a day. Most PV arrays and solar inverters have a life of about 25 years or so. However, most large (1000 A and up, especially the 4000 A class) DC or AC breakers, contactors and switches required in solar power generation applications (e.g., 1 MW and up) do not have sufficient cycle capability (with a required opening and closing at least once a day as just described) to match the 25 year life of the PV array and/or inverter. This forces undesirable design choices, including the use of both contactors and breakers, or contactors and fuses.
SUMMARY OF THE INVENTION

[05] In one embodiment, a power conversion apparatus for use with a source of current of a first type (e.g., DC, wherein the source is a PV cell array) is provided, wherein the power conversion system is structured to automatically electrically isolate the source of current from a load (e.g., an electrical grid) on a periodic basis including a plurality of intervals. The apparatus includes a power converter portion (e.g., a solar inverter) structured to convert current of the first type to current of a second type (e.g., AC), a first selectively operable electrical isolation device structured to be provided between the source and an input of the power converter portion to provide selective electrical isolation between the source and the power converter portion, a second selectively operable electrical isolation device structured to be provided between an output of the power converter portion and the load to provide selective electrical isolation between the power converter portion and the load, and a control unit operatively coupled to the first selectively operable electrical isolation device and the second selectively operable electrical isolation device. The control unit is structured to, for each of the intervals, determine, based on certain control logic, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval and cause the determined scheduled isolation device to move to an electrically isolating condition during the interval.

[06] In another embodiment, a method of automatically electrically isolating a source of current of a first type (e.g., DC, wherein the source is a PV cell array) from a load (e.g., an electrical grid) on a periodic basis comprising a plurality of intervals is provided. The method employs a power converter portion (e.g., a solar inverter) structured to convert current of the first type to current of a second type (e.g., AC), a first selectively operable electrical isolation device structured to provide selective electrical isolation between the source and the power converter portion, and a second selectively operable electrical isolation device structured to provide selective electrical isolation between the power converter portion and the load. The method includes, for each of the intervals: (i) determining, based on certain control logic, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval, and (ii) causing the determined scheduled isolation device to move to an electrically isolating condition during the interval.

[07] These and other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS

[08] FIG. 1 is a block diagram of a solar power generation system according to an exemplary embodiment of the present invention; and

[09] FIG. 2 is a flowchart illustrating a method of providing electrical isolation for the solar power generation system of FIG. 1 according to one particular, non-limiting exemplary embodiment.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[10] As used herein, the singular form of "a", "an", and "the" include plural references unless the context clearly dictates otherwise. As used herein, the statement that two or more parts or components are "coupled"
shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs. As used herein, "directly coupled" means that two elements are directly in contact with each other. As used herein, "fixedly coupled" or "fixed" means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other.

[11] As used herein, the word "unitary" means a component is created as a single piece or unit. That is, a component that includes pieces that are created separately and then coupled together as a unit is not a "unitary" component or body. As employed herein, the statement that two or more parts or components "engage" one another shall mean that the parts exert a force against one another either directly or through one or more intermediate parts or components. As employed herein, the term "number"
shall mean one or an integer greater than one (i.e., a plurality).

[12] Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, upper, lower, front, back, and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein.

[13] FIG. 1 is a block diagram of a solar power generation system 2 according to an exemplary embodiment of the present invention. Solar power generation system 2 includes a PV cell array 4 comprising a number of PV
modules, wherein each PV module includes a number of interconnected PV
cells. PV cell array 4 is structured to generate DC power by converting sunlight into DC electrical current using the photovoltaic effect. Solar power generation system 2 also includes a solar inverter 6. Solar inverter 6 is structured to convert the DC electrical current generated by PV cell array 4 into AC power that is suitable for provision to a commercial electrical grid 8.
Solar inverters are well known in the art, and thus will not be described in detail herein. One suitable example solar inverter is described in United States Patent No. 8,184,460. It should be noted, however, that solar inverter 6 may includes multiple conversion stages or bridges in series or parallel, or multiple conversion modules in serial or parallel.

[14] In addition, as seen in FIG. 1, solar power generation system 2 also includes a selectively operable DC isolation device 10 that is positioned in between PV cell array 4 and solar inverter 6. As used herein, a "selectively operable DC isolation device" shall mean an electrical apparatus that is structured to provide selective DC electrical isolation between two electrical components by isolating single or multiple conductors, and shall include, without limitation, a DC circuit breaker, a DC contactor or a DC switch.
Selectively operable DC isolation device 10 is able to provide electrical isolation between PV cell array 4 and solar inverter 6 as needed. In the exemplary embodiment, selectively operable DC isolation device 10 includes internal logic for automatically providing isolation when certain fault conditions are detected, and a manual actuator mechanism for providing isolation upon manual actuation, such as when maintenance needs to be performed on solar power generation system 2.

[15] Solar power generation system 2 further includes a selectively operable AC isolation device 12 that is positioned in between solar inverter 6 and electrical grid 8. As used herein, a "selectively operable AC isolation device" shall mean an electrical apparatus that is structured to provide selective AC electrical isolation between two electrical components by isolating single or multiple conductors, and shall include, without limitation, an AC circuit breaker, an AC contactor or an AC switch. Selectively operable AC
isolation device 12 is able to provide electrical isolation between solar inverter 6 and electrical grid 8 as needed. In the exemplary embodiment, selectively operable AC isolation device 12 includes internal logic for automatically providing isolation when certain fault conditions are detected, and a manual actuator mechanism for providing isolation upon manual actuation, such as when maintenance needs to be performed on solar power generation system 2.

[16] Furthermore, solar power generation system 2 includes a control unit 14, which comprises a controller, such as, without limitation, a microprocessor, a microcontroller, a field programmable gate array (FPGA), or some other suitable processing device, that is coupled to (or includes) a suitable memory for storing software instructions in the form of one or more routines that are executable by the controller. Control unit 14 is operatively coupled to both selectively operable DC isolation device 10 and selectively operable AC isolation device 12, and includes routines for causing selectively operable DC isolation device 10 and selectively operable AC isolation device 12 to be opened on a regular, periodic basis according to a predetermined schedule. In particular, as noted elsewhere herein, in the exemplary embodiment of solar power generation system 2, PV cell array 4 (the DC
source) must be electrically isolated from electrical grid 8 according to a regular schedule, typically overnight when PV cell array 4 is not generating power. According to an aspect of the present invention, selectively operable DC isolation device 10 and selectively operable AC isolation device 12 share the responsibility for this isolation function, wherein only one of those two devices is used to provide the regular, periodic electrical isolation (i.e.., not for fault detection or maintenance) at any one time, and wherein the particular one of those two devices that is for that purpose is determined in a predefined, scheduled manner. This scheduled use of selectively operable DC isolation device 10 and selectively operable AC isolation device 12 allows those units, which each have cycle lives that may not allow them to individually serve this periodic isolation function (along with the other isolation functionality (i.e., fault and maintenance) they must provide) for the full life of solar power generation system 2 (e.g., 25 years), to service the solar power generation system 2 for the full life thereof.

[17] FIG. 2 is a flowchart illustrating a method of employing selectively operable DC isolation device 10 and selectively operable AC
isolation device 12 to share the responsibility for providing electrical isolation for solar power generation system 2 according to one particular, non-limiting exemplary embodiment wherein isolation is provided once a day, overnight, when PV cell array 4 is not generating power. The method illustrated in FIG.
2 is, in the exemplary embodiment, implemented in one or more routines stored in control unit 14 such that control unit 14 is able to control (i.e., open and close) selectively operable DC isolation device 10 and selectively operable AC isolation device 12 using certain control logic as dictated by the method.

[18] Referring to FIG. 2, the method begins at step 20, wherein control unit 14 determines whether the current time is equal to a predefined "isolation commencement time", which is the time at which the isolation of PV
cell array is to begin. As will be appreciated, the "isolation commencement time" can be a non-changing value, such as 9:00 PM, or may be configured to change periodically as lighting conditions change (e.g., as the time of the year (season) changes). If the answer at step 20 is no, then the method returns to step 20 and in effect waits for the current time to equal the predefined "isolation commencement time." If, however, the answer at step 20 is yes, then the method proceeds to step 22, wherein control unit 14 determines which one of selectively operable DC isolation device 10 and selectively operable AC isolation device 12 is the current "scheduled isolation device."
As noted elsewhere herein, this will be determined based on a particular, predefined schedule wherein the two devices are used in some alternating fashion. Any of a number of different parameters/criteria may be used to establish the particular schedule, and a number of particular example embodiments are described elsewhere herein. Regardless of which manner is chosen to establish the schedule, the schedule will result in one of selectively operable DC isolation device 10 and selectively operable AC
isolation device 12 being established as the current "scheduled isolation device" in step 22.

[19] Next, at step 24, control unit 14 automatically actuates the current "scheduled isolation device" to cause it to be in an electrically isolating condition. Then, in step 26, control unit 14 determines whether the current time is equal to a predefined "isolation termination time", which is the time at which the isolation of PV cell array is to end. As will be appreciated, the "isolation termination time" can be a non-changing value, such as 6:00 AM, or may be configured to change periodically as lighting conditions change (e.g., as the time of the year (season) changes). If the answer at step 26 is no, then the method returns to step 26 and in effect waits for the current time to equal the predefined "isolation termination time." If, however, the answer at step is yes, then the method proceeds to step 28, wherein control unit 14 automatically actuates the current "scheduled isolation device" to cause it to be in a non-electrically isolating condition. The method then returns to step

20, to wait for the next "isolation commencement time" to occur.
[20] Thus, the method illustrated in FIG. 2 will result in selectively operable DC isolation device 10 and selectively operable AC isolation device 12 being used to provide regular, periodic isolation for PV array 4 in some alternating fashion based on a particular, predefined schedule.

[21] In certain embodiments, specific operating conditions may dictate which isolation device (selectively operable DC isolation device 10 or selectively operable AC isolation device 12) is operated at a given time. For example, depending on the desired functionality, some isolation events are accomplished with selectively operable DC isolation device 10, with the specific intent to keep the AC side connected (e.g., some overnights may require keeping the AC connected but isolating DC).

[22] In one particular exemplary embodiment, the schedule that is employed in step 22 is based on the expected cycle lifetime of each of selectively operable DC isolation device 10 and selectively operable AC
isolation device 12. In particular, the use of one isolation device relative to the other may be based on the ratio of the cycle lifetime of one to the other. For example, if selectively operable DC isolation device 10 has 10K cycle lifetime and selectively operable AC isolation device 12 has 5K cycle lifetime, a ratio of 2:1 may be used such that selectively operable AC isolation device 12 will only be used every third day (with selectively operable DC isolation device 10 being used the other days).

[23] In another particular exemplary embodiment, the schedule that is employed in step 22 is based on the past use history of each of selectively operable DC isolation device 10 and selectively operable AC isolation device 12 and the ratio of their expected cycle lifetimes. For example, if selectively operable DC isolation device 10 has a 10K cycle lifetime and selectively operable AC isolation device 12 has a 5K cycle lifetime, and selectively operable DC isolation device 10 has been used 4000 times while selectively operable AC isolation device 12 has been used 1800 times, selectively operable AC isolation device 12 will be used to bring the usage ratio closer to the desired 2:1 based on the ratio of their expected cycle lifetimes. This technique/control logic also allows for optimal device scheduling even if outside factors affect the number of cycles on each device. In contrast to a simple "every third day" isolation scheme or similar, if other device operations cause the cycle count to deviate from desired, this method will correct them.

[24] In still another particular exemplary embodiment, the schedule that is employed in step 22 is based on the past use history of each of selectively operable DC isolation device 10 and selectively operable AC
isolation device 12, their expected cycle lifetimes, and their expected future use patterns. Other system operations may require isolation device operations independent of the isolation function, or system conditions may require a particular isolation method. The schedule of this embodiment will consider these expected future effects and past history of the devices, and will control the devices using control logic such that they stay within their rated lifetimes. For example, if selectively operable DC isolation device 10 has a 10K cycle lifetime and selectively operable AC isolation device 12 has 5K
cycle lifetime, and selectively operable DC isolation device 10 has been used 6K times while selectively operable AC isolation device 12 has been used 3K
times, and other system operations will require an extra 1K cycles of selectively operable DC isolation device 10, selectively operable AC isolation device 12 will be used more often to account for the expected future use of selectively operable DC isolation device 10. As another example, system considerations may randomly require DC isolation rather than AC isolation. In this case, the schedule would heavily bias usage of selectively operable AC
isolation device 12 initially to ensure sufficient cycle life is available on selectively operable DC isolation device 10. This may be as extreme as only using selectively operable AC isolation device 12 until it comes close to its maximum cycle limit, and selectively operable DC isolation device 10 from then onward.

[25] Moreover, while the above description illustrates the present invention in the context of a solar power generation system, it will be appreciated that the present invention may be applied to other type of power systems wherein power is converted from one form to another and two electrical isolation devices may be employed. For example, the present invention may also be employed in connection with wind turbines, motor drives, DC-DC converters, DC-AC converters, AC-AC conversion, variable frequency drives, etc.

[26] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" or "including" does not exclude the presence of elements or steps other than those listed in a claim. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware.
The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In any device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain elements are recited in mutually different dependent claims does not indicate that these elements cannot be used in combination.

[27] Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.

Claims

What is Claimed is:

1. A power conversion apparatus for use with a source of current of a first type, wherein the power conversion system is structured to automatically electrically isolate the source of current from a load on a periodic basis comprising a plurality of intervals, the apparatus comprising:
a power converter portion structured to convert current of the first type to current of a second type;
a first selectively operable electrical isolation device structured to be provided between the source and an input of the power converter portion to provide selective electrical isolation between the source and the power converter portion;
a second selectively operable electrical isolation device structured to be provided between an output of the power converter portion and the load to provide selective electrical isolation between the power converter portion and the load; and a control unit operatively coupled to the first selectively operable electrical isolation device and the second selectively operable electrical isolation device, the control unit being structured to, for each of the intervals, determine, based on control logic, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval and cause the determined scheduled isolation device to move to an electrically isolating condition during the interval.

2. The power conversion apparatus according to claim 1, wherein the control logic is such that, the control unit is structured to, for each of the intervals, determine, based on a predetermined schedule, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval and cause the determined scheduled isolation device to move to an electrically isolating condition during the interval.

3. The power conversion apparatus according to claim 1, wherein the periodic basis is once a day, wherein in each interval comprising a day the isolation of the source from the load will occur at a certain time for that day, and wherein the control unit is structured to, for each of the intervals, cause the determined scheduled isolation device to move to the electrically isolating condition at the certain time for that day.

4. The power conversion apparatus according to claim 1, wherein the first type is DC, wherein the source of current of the first type is a DC
source, wherein the second type is AC, wherein the power converter portion comprises an inverter, wherein the first selectively operable electrical isolation device is a selectively operable DC electrical isolation device, and wherein the second selectively operable electrical isolation device is a selectively operable AC electrical isolation device.

5. The power conversion apparatus according to claim 4, wherein the selectively operable DC electrical isolation device comprises one of a DC
breaker, a DC contactor, or a DC switch, and wherein the selectively operable AC electrical isolation device comprises one of an AC breaker, an AC
contactor, or a, AC switch.

6. The power conversion apparatus according to claim 4, wherein the load is an electrical grid.

7. The power conversion apparatus according to claim 6, wherein the DC source comprises a number of PV cells, and wherein the power converter portion comprises a solar inverter.

8. The power conversion apparatus according to claim 1, wherein the control logic is based on a first expected cycle lifetime of the first selectively operable electrical isolation device and a second expected cycle lifetime of the second selectively operable electrical isolation device.

9. The power conversion apparatus according to claim 8, wherein the control logic is based on a ratio of one of the first expected cycle lifetime and the second expected cycle lifetime to the other of the first expected cycle lifetime and the second expected cycle lifetime.

10. The power conversion apparatus according to claim 8, wherein the control logic is also based on a past usage history of each of the first selectively operable electrical isolation device and the second selectively operable electrical isolation device.

11. The power conversion apparatus according to claim 10, wherein the control logic is also based on an expected future usage pattern of each of the first selectively operable electrical isolation device and the second selectively operable electrical isolation device.

12. The power conversion apparatus according to claim 1, wherein the power converter portion includes multiple conversion stages or bridges in series or parallel, or multiple conversion modules in serial or parallel.

13. A method of automatically electrically isolating a source of current of a first type from a load on a periodic basis comprising a plurality of intervals using a power converter portion structured to convert current of the first type to current of a second type, a first selectively operable electrical isolation device structured to provide selective electrical isolation between the source and the power converter portion, and a second selectively operable electrical isolation device structured to provide selective electrical isolation between the power converter portion and the load, the method comprising:
for each of the intervals: (i) determining, based on control logic, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval, and (ii) causing the determined scheduled isolation device to move to an electrically isolating condition during the interval.

14. The method according to claim 13, wherein the control logic is such that, for each of the intervals, the method includes determining, based on a predetermined schedule, which one of the first selectively operable electrical isolation device and the second selectively operable electrical isolation is to be a scheduled isolation device for the interval and cause the determined scheduled isolation device to move to an electrically isolating condition during the interval.

15. The method according to claim 13, wherein the periodic basis is once a day, wherein in each interval comprising a day the isolation of the source from the load will occur at a certain time for that day, and wherein the method includes, for each of the intervals, causing the determined scheduled isolation device to move to the electrically isolating condition at the certain time for that day.

16. The method according to claim 13, wherein the first type is DC, wherein the source of current of the first type is a DC source, wherein the second type is AC, wherein the power converter portion comprises an inverter, wherein the first selectively operable electrical isolation device is a selectively operable DC electrical isolation device, and wherein the second selectively operable electrical isolation device is a selectively operable AC
electrical isolation device.

17. The method according to claim 16, wherein the selectively operable DC electrical isolation device comprises one of a DC breaker, a DC
contactor, or a DC switch, and wherein the selectively operable AC electrical isolation device comprises one of an AC breaker, an AC contactor, or an AC
switch.

18. The method according to claim 16, wherein the load is an electrical grid.

19. The method according to claim 18, wherein the DC source comprises a number of PV cells, and wherein the power converter portion comprises a solar inverter.

20. The method according to claim 13, wherein the control logic is based on a first expected cycle lifetime of the first selectively operable electrical isolation device and a second expected cycle lifetime of the second selectively operable electrical isolation device.

21. The method according to claim 20, wherein the control logic is based on a ratio of one of the first expected cycle lifetime and the second expected cycle lifetime to the other of the first expected cycle lifetime and the second expected cycle lifetime.

22. The method according to claim 20, wherein the control logic is also based on a past usage history of each of the first selectively operable electrical isolation device and the second selectively operable electrical isolation device.

23. The power conversion apparatus according to claim 22, wherein the control logic is also based on an expected future usage pattern of each of the first selectively operable electrical isolation device and the second selectively operable electrical isolation device.