Note: Descriptions are shown in the official language in which they were submitted.
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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.
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[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
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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
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[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
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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.
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[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
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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
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the predefined "isolation termination time." If, however, the answer at step
26
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
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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
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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.
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