U.S. patent application number 13/872979 was filed with the patent office on 2013-11-28 for photovoltaic array systems, methods, and devices with bidirectional converter.
This patent application is currently assigned to Ideal Power Converters, Inc.. The applicant listed for this patent is Ideal Power Converters, Inc.. Invention is credited to William C. Alexander, Paul Bundschuh.
Application Number | 20130314096 13/872979 |
Document ID | / |
Family ID | 46172543 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130314096 |
Kind Code |
A1 |
Bundschuh; Paul ; et
al. |
November 28, 2013 |
Photovoltaic Array Systems, Methods, and Devices with Bidirectional
Converter
Abstract
Devices, systems and methods for operating, monitoring and
diagnosing photovoltaic arrays used for solar energy collection.
The system preferably includes capabilities for monitoring or
diagnosing an array, under some circumstances, by using a
bidirectional power converter not only to convert the DC output of
the array to output power under some conditions, but also, for
diagnostic operations, applying a back-converted DC voltage to the
array.
Inventors: |
Bundschuh; Paul; (Austin,
TX) ; Alexander; William C.; (Spicewood, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ideal Power Converters, Inc.; |
|
|
US |
|
|
Assignee: |
Ideal Power Converters,
Inc.
Spicewood
TX
|
Family ID: |
46172543 |
Appl. No.: |
13/872979 |
Filed: |
April 29, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13308356 |
Nov 30, 2011 |
8461718 |
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13872979 |
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61418144 |
Nov 30, 2010 |
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61480048 |
Apr 28, 2011 |
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Current U.S.
Class: |
324/500 ;
307/151 |
Current CPC
Class: |
H02J 3/383 20130101;
H02S 50/00 20130101; Y02E 10/56 20130101; H02S 50/10 20141201; H02M
7/66 20130101; G01R 31/2603 20130101; G05F 1/67 20130101; H01L
31/02021 20130101; H02J 1/00 20130101; H02J 2300/24 20200101; H02J
3/381 20130101 |
Class at
Publication: |
324/500 ;
307/151 |
International
Class: |
G01R 31/26 20060101
G01R031/26; G05F 1/67 20060101 G05F001/67 |
Claims
1.-19. (canceled)
20. A method for operating a photovoltaic array, comprising: in
daylight conditions, providing DC current from photovoltaic cells
to a bidirectional power converter, and operating said
bidirectional power converter to provide an AC power output onto an
AC connection; in dark conditions, operating said bidirectional
converter in reverse, to apply a reduced DC potential, which is
less than half of the maximum voltage provided by said cells under
daylight conditions, to said string of photovoltaic cells;
measuring a DC current through the string of photovoltaic cells;
and initiating an alert if the measured DC current indicates that a
photovoltaic cell in the string has been disconnected.
21. A solar energy system which implements the methods of any of
the preceding claims.
22. A photovoltaic power generation system comprising: a plurality
of photovoltaic cells; and a bidirectional power converter which is
operatively connected to groups of said photovoltaic cells;
wherein, in a first mode of operation, said bidirectional power
converter draws current from at least some ones of said groups of
photovoltaic cells, and provides an AC power output onto an AC
connection; and wherein, in a second mode of operation, said
bidirectional power converter draws power from one or more sources
other than said photovoltaic cells, and accordingly applies a DC
potential to at least some ones of said photovoltaic cells.
23. The system of claim 22, wherein, in said second mode, said
bidirectional converter is powered from said AC connection.
24. The system of claim 22, wherein said second mode is initiated
automatically, from time to time, during normal operation.
25. The system of claim 22, wherein said second mode is initiated
automatically, from time to time, during dark conditions.
26. A photovoltaic power generation system comprising: a plurality
of photovoltaic cells; and a bidirectional power converter which is
operatively connected to groups of said photovoltaic cells;
wherein, in a first mode of operation, said bidirectional power
converter draws current from at least some ones of said groups of
photovoltaic cells, and provides an AC power output onto an AC
connection; and wherein, in a second mode of operation, said
bidirectional power converter draws power from one or more sources
other than said photovoltaic cells, and accordingly applies a
variable DC potential to at least some ones of said photovoltaic
cells; and wherein one or more diagnostic tests are performed
during said second mode.
27. The system of claim 26, wherein, in said second mode, said
bidirectional converter is powered from said AC connection.
28. The system of claim 26, wherein said second mode is initiated
automatically, from time to time, during normal operation.
29. The system of claim 26, wherein said second mode is initiated
automatically, from time to time, during dark conditions.
Description
CROSS-REFERENCE
[0001] Priority is claimed from U.S. Provisional Patent
Applications 61/418,144, filed Nov. 30, 2010, and 61/480,048, filed
Apr. 28, 2011, both of which are hereby incorporated by
reference.
BACKGROUND
[0002] The present application relates to photovoltaic arrays for
collecting solar energy to provide useful power, and more
particularly to improved devices, systems and methods for
diagnosing and monitoring such arrays.
[0003] Note that the points discussed below may reflect the
hindsight gained from the disclosed inventions, and are not
necessarily admitted to be prior art.
[0004] Photovoltaic systems are one of the fastest growing sources
of energy and are increasingly cost-effective. The cost and
reliability of photovoltaic systems and their components, including
photovoltaic modules and photovoltaic converters, continue to
improve.
[0005] Photovoltaic modules and components are often in service for
many years. An array or its components may be warranted for more
than 20 years. While modules and other components are generally
reliable, problems do arise. Over their service life, components
are subject to malfunctions and/or performance degradation.
[0006] Various power monitoring systems are used to determine if
actual performance meets projections. These power measurements can
be: AC power at the output of the converter, DC power at the input
of the converter, DC power at the string level, and DC power at the
individual module. Each of these monitoring systems provides
certain useful data for determining if the system is operating
properly. However, these power monitoring systems are less useful
for diagnosing issues when performance is inadequate.
[0007] Photovoltaic modules in the laboratory or manufacturing
environment are often measured to provide a current-voltage curve
("IV curve") that determines the module performance across the
range of operating current and voltage points. The IV curve also
varies with the amount of solar insulation and ambient temperature.
IV curves provide a detailed insight to the internal performance of
the photovoltaic module, which cannot be obtained easily once the
system is installed in the field.
[0008] More recently, IV curve trace tools have been developed to
enable photovoltaic maintenance personnel to diagnose issues in the
field with installed photovoltaic systems. IV curve trace
measurements can identify the cause of low performing modules such
as soiling or shading, but they require highly-trained maintenance
staff to use and interpret the measurements.
[0009] Published US application 2010/0071744 describes a
Photovoltaic Installation with Automatic Disconnect Device. The DC
disconnect device described therein uses a single DC disconnect
with a control mechanism to disconnect in case of fire or other
emergency. The single DC disconnect is located at the typical
manual DC disconnect location between the string combiner and the
converter. However, this system does not appear to offer any
capability for string monitoring or diagnostics.
SUMMARY
[0010] The present application discloses novel systems, methods,
and devices for a photovoltaic array with improved monitoring and
diagnostic capabilities.
[0011] In some embodiments, an array comprises a bidirectional
power converter capable of applying a DC potential to the array or
to a portion of the array to perform monitoring and diagnostic
operations. In some embodiments the bidirectional converter can
bias the photovoltaic array to variable DC voltage of either
polarity, and under any solar condition including day or night.
These capabilities, when combined with existing DC current
measurements from conventional power monitoring, can allow
measurements such as an I-V curve trace to be created without
additional equipment.
[0012] In some embodiments, an array comprises multiple strings of
photovoltaic modules. The array is configured so that individual
modules, strings or sets of strings may be selectively connected
and disconnected from an converter, allowing monitoring and
diagnostic operations on a module, string, or sub array level. The
array can also include a safety mode with no power output.
[0013] The disclosed innovations, in various embodiments, provide
one or more of at least the following advantages. However, not all
of these advantages result from every one of the innovations
disclosed, and this list of advantages does not limit the various
claimed inventions. [0014] Allows diagnostics during daylight and
nighttime or shaded conditions; [0015] Allows higher level
diagnostic operations without additional expensive equipment;
[0016] Monitoring and diagnostics may be performed without
technicians; [0017] Provides for string or module level monitoring
and diagnostics; [0018] Provides for regular monitoring during
operation without significant power loss; [0019] Provides no output
safe mode for maintenance or emergencies; [0020] Improves array
performance and efficiency; [0021] Allows early detection of
problems; [0022] Reduces costs associated with warranty obligations
of the system manufacturer or installer; [0023] Reduces maintenance
costs; [0024] Allows for diagnostics on module, string or sub array
level without separate equipment at each unit; [0025] Allows for
better visibility into the long-term life expectancy, and
in-service degradation, of a solar array, and hence allows for
lower implicit risk allowance in financing of solar array
systems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The disclosed inventions will be described with reference to
the accompanying drawings, which show important sample embodiments
and which are incorporated in the specification hereof by
reference, wherein:
[0027] FIG. 1 schematically shows an embodiment of a PV system.
[0028] FIG. 2 schematically shows possible measurement locations
for a PV system.
[0029] FIG. 3 is a flow chart illustrating a procedure for
monitoring power output of a PV array.
[0030] FIG. 4 shows an embodiment of a PV array with one string
disconnected.
[0031] FIG. 5A is a flow chart illustrating a procedure for
diagnosing problems in a PV array.
[0032] FIG. 5B is a flow chart illustrating an improved procedure
for diagnosing problems in a PV array.
[0033] FIGS. 6A-6F illustrate several operating and diagnostic
scenarios for a PV array.
[0034] FIGS. 7A-7C shows an embodiment of a PV array with one
string connected.
[0035] FIG. 8 shows an example of an I-V curve for a properly
operating PV module.
[0036] FIG. 9 shows examples of I-V curve abnormalities.
[0037] FIG. 10 shows an example of an I-V curve for a properly
operating PV module at night time.
[0038] FIG. 11 shows an example of a bidirectional power converter
topology which can be used with the disclosed inventions, and is
especially advantageous.
[0039] FIG. 12 shows a more comprehensive illustration of a solar
power system.
[0040] FIGS. 13A-13D show example states of an embodiment of a PV
string combiner.
[0041] FIG. 14 shows a PV system with string shorting to eliminate
output voltage on for safety.
DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS
[0042] The numerous innovative teachings of the present application
will be described with particular reference to presently preferred
embodiments (by way of example, and not of limitation). The present
application describes several inventions, and none of the
statements below should be taken as limiting the claims
generally.
[0043] The present application describes photovoltaic power systems
with a number of new features. In the following description, it
should be remembered that the various inventions can be used in
other combinations or subcombinations, or separately, and do not
have to be combined as described.
[0044] The disclosed inventions focus on improving system
performance by enabling low-cost string-level DC monitoring and
diagnostics with no reduction in peak efficiency. Improved DC
monitoring and diagnostics will enable PV system owners and
operators to easily identify and cost-effectively correct
non-optimal array conditions.
[0045] Several of the new functions will now be described briefly,
and a more detailed description follows later.
[0046] System and String DC Power Monitoring
[0047] AC power monitoring from the converter output is common in
the industry, but DC power monitoring is less frequent due to
higher implementation cost of measuring DC current. One group of
disclosed inventions allows a single DC current and voltage power
monitoring system in the converter. The converter can measure
combined string DC current and voltage (continuously or with
sampling). This data will be stored temporarily in the converter
and downloaded to a remote internet data server.
[0048] In the preferred implementation, the DC power monitoring
system will regularly disconnect individual strings and measure the
change in DC current and voltage. The difference in power will be
the power lost from the disconnected string. This only requires
short disconnect time (approximately 1 second per string) with the
rest of the system active. As result there is no significant energy
loss in this procedure. This function would be performed regularly
(such as once every few hours) or as desired via remote operator
control.
[0049] In a sample implementation, the string power measurements
are compared to power measurements from other strings from the same
point in time and previous measurements from the same string to
determine if the power is lower than predicted. This would be
determined by the remote data server or by the converter. If the
difference between predicted string power and measured string power
is higher than a set point, then the system may be programmed to
trigger a string alarm at the converter or by sending email or text
message(s) to maintenance personnel.
[0050] Some industry products support string alarms, but with
higher system costs. These systems use string level DC current
meters, an expensive component, on every string. These provide
continuous DC current measurements on every string, but practically
these provide no more useful system information than the regularly
sampled string monitoring achieved with this group of
inventions.
[0051] String Diagnostics
[0052] String diagnostics would occur when a string alarm is
received or during infrequent maintenance (perhaps monthly). String
diagnostics can be performed on all strings simultaneously, if
appropriate current sensors are provided for each string, or can be
performed one string at a time using string disconnects with one
current sensor at the central converter. When combined with
advanced DC monitoring capabilities in the converter, a wide
variety of string measurements can be taken including open circuit
voltage (Voc), Short Circuit Current (Isc), and current-voltage
(I-V) performance curve. Furthermore this could be implemented
during both daylight and night conditions.
[0053] This data will be provided to the remote data server which
can be used to determine a wide variety of string problems
including shading, soiling, degrading/out of specification PV
modules, and string wiring shorts/open conditions. This alerts
maintenance personnel to specific problems and corrective action
requirements.
[0054] String Disconnect for String Maintenance
[0055] Disconnecting the individual string can allow maintenance to
work on a specific string while the remainder of the array is
active. Replacing a PV module or looking for wiring shorts/opens is
an example of maintenance work that can be simplified with this
system.
[0056] Array Disconnect for Converter Maintenance
[0057] The system can provide DC array disconnects near the PV
array while the converter may be located some distance away. By
disconnecting all PV strings, the converter is disconnected from
the DC power source. This can simplify converter maintenance or
replacement procedures.
[0058] Array Disconnect for Safety
[0059] Fire departments normally shut off the main AC power
disconnect when responding to fires to ensure the electrical lines
in a building are de-energized. Since PV modules generate power
whenever the sun is shining, disconnecting the main AC alone does
not shut off DC power lines associated with the PV system. Many
fire codes (including NEC) require DC disconnects on the roof near
the PV array. The string disconnects in the preferred
implementation of the disclosed inventions will have a local manual
switch as well as a remote converter controlled array shutdown for
safety.
[0060] PV Power Architectures with Bidirectional Power
Converter
[0061] For many applications, a Photovoltaic power system will
include connection to local AC load, power line, battery,
photovoltaic array, and sometimes also to other local loads (e.g.
DC, or multiple AC voltages). As described in published PCT
application WO2008/008143, which is hereby incorporated by
reference, a Universal Power Conversion architecture is useful for
such applications. This architecture provides extremely versatile
conversion of voltage, current, impedance, frequency, and/or phase.
The present inventors have realized that this architecture opens
the door to even more capabilities in PV architecture.
[0062] FIG. 11 shows a simple example of a current-modulation PV
converter topology, which can be used as the bidirectional
converter in the various system implementations which will be
described below. The example shown in this drawing is a bipolar
array, where a common ground is used with two photovoltaic arrays
which provide separate positive and negative voltage outputs.
However, the same converter topology can also be used in a
single-ended configuration, as shown in the examples of FIG. 1 and
other drawings. This example shows conversion from a bipolar
grounded array of photovoltaic cells to three-phase 480V AC power
(e.g. at 60 Hz). The link reactance is an inductor-capacitor
parallel combination, and is operated in a current-modulation mode.
In this example, five identical phase legs are used, each of which
includes two AC switches. Two phase legs are used for the DC lines
to the PV array, and three are used to connect to the three phases
of the AC output lines. No other power semiconductors are
needed.
[0063] In addition to the active components in the phase legs the
converter uses a central Link Inductor (LI) with a Link Capacitor
(LC) in parallel, and input and output capacitive filters. All
power transfer occurs through the LI such that input and output
lines are never connected together, which is why this topology does
not need a transformer to achieve common mode isolation between the
solar array and the utility lines. The LC allows for zero voltage
turn-off (ZVT), almost eliminating switches losses.
[0064] At full power, the link frequency is 7,000 Hz. The converter
is controlled by an FPGA, which only needs line and link voltage
sensing to operate. This simple example shows a minimal
configuration with only two ports (line and photovoltaic array),
but practical implementations would typically include more
connections.
[0065] Many of the described embodiments are single-ended, where
only two wires (power and ground) connect the converter to the PV
array. However, this figure illustrates a center-grounded
embodiment, where three wires (positive, ground, and negative)
connect the converter to the PV array.
[0066] FIG. 12 is a more complete example, showing how a
bidirectional converter would be connected to multiple portals in a
complete PV power system. In this example, four portals are
provided, namely "House" (local AC load), "Utility" (power line),
"Backup" (battery), and photovoltaic array. However, other portals
can be added. In addition, polyphase connections can be provided
instead of or in addition to the single-phase connections
shown.
[0067] String Shorting
[0068] As photovoltaic array designs have migrated to higher DC
voltage levels, safety has become an increasingly important
concern. (Older designs might use e.g. 48V DC peak daylight
voltage, but newer designs commonly stack PV modules to achieve a
600V or 1000V DC voltage, for reduced loss.) The present
application provides a new approach to safety: when total shutdown
is desired, two identical (or comparable) strings can be connected
together with opposite senses, i.e. positive to negative (or
ground) and negative (or ground) to positive. This avoids the
danger of a high voltage appearing when illumination changes. This
connection will cause a current loop, so preferably this connection
is made between elements or subarrays which will not exceed the
current rating of the switches. Alternatively, a positive terminal
of a string is shorted to the negative terminal of the same string.
An embodiment of a PV system in safe mode with shorted strings is
shown in FIG. 14.
[0069] Apply DC Voltage to Array for Monitoring/Diagnostics
[0070] One important class of teachings uses the bidirectional
converter to apply a selected DC voltage back onto the PV array.
This can be used in a number of ways, as described below, to
provide the system with additional capabilities.
[0071] In a system with battery backup, the converter can also be
used to draw power from the backup battery to apply a back voltage
onto the array. However, this is less preferred.
[0072] Method of Diagnostics by Applying Variable DC Voltage
[0073] One important class of teachings uses the bidirectional
converter to apply a variable DC voltage back onto the PV array. By
ramping or stepping the applied voltage, while monitoring current,
IN curve characterization can be used to assess the PV array.
[0074] In a system with battery backup, the converter can also be
used to draw power from the backup battery to apply a back voltage
onto the array. However, this is less preferred.
[0075] Method of Diagnostics by Applying DC Reverse Voltage
[0076] Another important class of teachings uses the bidirectional
converter to apply a negative DC voltage (opposite to the voltage
which is generated in daylight) back onto the PV array at
nighttime. The negative voltage permits the bypass diodes to be
tested.
[0077] In a system with battery backup, the converter can also be
used to draw power from the backup battery to apply a back voltage
onto the array.
[0078] Theft Prevention
[0079] Another important class of teachings uses the bidirectional
converter to apply a small DC voltage onto the PV array at
nighttime, even when no diagnostic tests are being run. This
permits changes in the impedance of the array to be measured, to
thereby detect theft of elements or modules.
[0080] Implications for Risk Assessment and Bankability
[0081] Solar energy is an area of great interest, but the scale of
solar installations is still relative small compared to other
energy sources. As this industry rapidly grows and matures, there
are some important issues beyond the basic scientific and
engineering technologies. One of these is the financial risk of a
large solar power system, when considered as a major asset.
[0082] One known issue for photovoltaic arrays is degradation over
time. Sealing can degrade, leakage currents can grow worse, and the
semiconductor material itself can develop increased internal
leakage (and possibly even short circuits). Connections can degrade
too, leading to open circuits or short circuits. The physics of
these changes is fairly well understood, but these issues present
some uncertain financial risk over the assets' lifetime.
[0083] For example, the initial performance of a $100 solar power
kit for enthusiasts might be sufficient to justify its price, even
if the unit only lasts a few years under outdoor conditions.
However, if we are considering a $10M capital investment, for an
array which may put out megawatts of power, more careful analysis
is needed. For such an investment to be practical, it needs to be
"bankable," i.e. predictable enough (in its future contribution to
revenue) to support financing. PV module vendors provide warranties
for maximum degradation, but it is difficult to measure module or
array degradation accurately once the system is installed. To the
extent that long-term degradation is not accurately measurable in
the installation, there is an additional risk from investors. If
the degradation can be accurately and easily measured, the
financial risk is reduced and so will the interest cost of
financing.
[0084] The disclosed inventions have a major impact on this
uncertainty, since degradation can be monitored accurately and
easily in-service. Thus a photovoltaic power system with monitoring
capabilities as described herein will not only have a predicted
initial lifetime, but that lifetime will become less uncertain as
time goes by: not only will the in-service monitoring show where
individual components are, within their lifetimes, but results from
similar but older arrays will give an increasingly accurate picture
of expected lifetime and maintenance cost.
[0085] FIG. 1 schematically shows a photovoltaic (PV) system 10 for
collection of solar energy. PV system 10 generally comprises a PV
array 110, a string combiner 120, and a converter 130. PV array 110
preferably comprises a plurality of photovoltaic modules 112. A PV
module is typically a generally planar device comprising a
plurality of PV cells.
[0086] Several PV modules 112 are combined in series to form
strings 114. String 114 preferably comprises between 8 and 15 PV
modules 112. However, the number of PV modules 112 in a string 114
can vary depending on the output voltage of each PV module 112 and
the desired maximum DC operating voltage of PV system 10. Common
maximum DC operating voltages are 600V DC and 1000V DC. Strings 114
are preferably combined in parallel at string combiner 120.
[0087] String combiner 120 preferably includes a switch 122 for
each string 114 in PV array 110. Switch 122 is configured to
selectively connect or disconnect string 114 from PV array 110.
Each switch 122 is separably operable, so that one or more switch
122 can be opened (disconnecting one or more string 114 from PV
array 110) while other switches 122 remain closed (other strings
114 remain connected). Most preferably, switch 122 is controllable
by a machine or user at a remote location. String combiner 120 also
preferably comprises a fuse 124 for each string 114. While switches
122 are useful for practicing certain embodiments of disclosed
inventions, switches 122 are unnecessary for several innovations
disclosed herein.
[0088] Under normal operating conditions, DC power from string
combiner 120 is fed into PV converter 130. PV converter 130
converts the DC power to AC power, which can be used onsite or
distributed over an AC distribution system. PV converter 130 is
preferably a bidirectional PV converter, such as the PV converter
described in WO2008/008143. PV converter 130 can alternatively be
operated in reverse, so that PV converter 130 draws power from an
AC power distribution system, converts the AC power to DC, and
delivers a DC potential to PV array 12. PV converter 130 is
preferably configured to be able to provide either a forward
potential--that is, a DC voltage tending to induce current in the
normal direction of current flow of the PV modules, or a reverse
potential, tending to induce a current in the opposite direction of
normal current through the PV modules or tending to retard the flow
of current in the normal operating direction.
[0089] FIG. 2 illustrates potential measurement locations for a PV
system. Measurements preferably include at least voltage and
current. Position A represents array-level measurements taken at
the PV converter on the AC side. Position B represents array-level
measurements taken at the PV converter on the DC side. Position C
represents string-level measurements. Position D represents
module-level measurements.
[0090] Certain aspects of the disclosed invention can be performed
using measurements taken at any of locations A, B, C, or D. Other
disclosed inventions are preferably combined with measurements at
particular locations. In some embodiments, measurement of one
parameter, such as current, can be taken at one location while
measurement of other parameters, such as voltage, can be taken at
other locations.
[0091] FIG. 3 is a flow chart showing one process for monitoring PV
array 110. In step 302, real-time power output data from PV array
110, or a portion of a PV array, is measured. In step 304,
real-time weather data is determined. In step 306, the measured
real-time power output is compared to historical power output data
for similar weather conditions. If the power output is within a
predicted range, then PV array 110 continues to operate normally.
If the power output is not within the predicted range, then, in
step 308, an alert condition is initiated.
[0092] Referring again to step 302, the real-time power output data
measured for use in comparison step 306 can be data for the array,
a string, or a module. Power output data for the array is
preferably measured at locations A or B of FIG. 2. Power output
data for a string is preferably measured at location C. Power
output data for a module is preferably measured at location D.
[0093] Alternatively, power output data for a string can be
indirectly measured at locations A or B if the PV system is
equipped with automatic switches 122. In that case, one string is
preferable disconnected using automatic switch 122, while other
strings continue to provide power, as illustrated in FIG. 4. Power
output data from the connected strings is compared to historical
power output data from the connected string and/or historical or
recent power output data for the entire array. If the power output
from the connected strings is within a predicted range, the
disconnected string is automatically reconnected and the next
string is tested. If the power output from the connected strings is
outside of a predicted range, an alert condition is initiated.
Testing strings using the method described in this paragraph allows
for string-level monitoring of a PV array without requiring
expensive monitoring equipment for each string.
[0094] The response to an alert condition depends on the
capabilities of the PV system. FIG. 5A illustrates a procedure for
use in a PV system without automatic diagnostic capabilities. In
step 502, maintenance personnel are dispatched to the PV system
location. In step 504, maintenance personnel perform on-site
diagnostics procedures as necessary to determine the problem. In
step 506, maintenance personnel correct the problem.
[0095] Disadvantages of the procedure shown in FIG. 5B are that
maintenance personnel responding to the alert will have limited
information about the problem before arriving on site. Rather,
maintenance personnel will need to perform on-side diagnostics,
which takes additional time and cost. The requirement of on-site
diagnostics also means more experienced and qualified staff must
respond to the alert, even though the problem may be relatively
simple to fix. Additionally, the lack of information increases the
likelihood that maintenance will not have the necessary equipment
to fix the problem and that a second visit will be required. These
issues increase maintenance costs.
[0096] FIG. 5B illustrates a procedure for use in a PV system with
automatic diagnostic capabilities. In step 552, after an alert
condition is initiated, diagnostic procedures are performed
automatically by the PV system or by a remote server or operator.
Certain useful diagnostics procedures are discussed elsewhere
herein. In step 554, results of the diagnostics procedures are
preferably transmitted to a remote server or operator. In step 556,
the remote server or operator determines an appropriate responsive
action. In step 558, a maintenance visit is scheduled. In step 562
maintenance personnel correct the problem. The procedure
illustrated in FIG. 5B can greatly reduce maintenance costs because
information about the problem and potential causes will be
available before an on-site visit. This information increases the
likelihood that the right personnel and equipment are dispatched.
The additional information can also be utilized by maintenance to
better understand the severity of the problem. In some case it may
be more financially attractive to delay dispatching maintenance and
combine the site visit with normally schedule maintenance.
[0097] In the event an alert condition is initiated or detailed
diagnostics is desired for other reasons, certain inventions
disclosed herein provide systems and methods for such
diagnostics.
[0098] In one class of embodiments, PV converter 130 provides a DC
potential to the connected strings. PV converter 130 is preferably
configured to provide a variable positive or negative
back-converted DC potential to connected strings using power from
an AC distribution system. FIGS. 6A-6G illustrate a number of
potential operating and diagnostic scenarios.
[0099] FIG. 6A shows a PV array under sunlight conditions with a DC
voltage Vpv being generated by PV strings. Current is conducted in
the normal direction. Current is preferably measured, in this
example, using a DC current meter 602 at each string. Energy is
transferred through PV converter 130 to the AC distribution system.
A graph of voltage versus time for this condition is also shown.
Some random variation may be present, due to variation in cloud
cover etc.
[0100] FIG. 6B shows a PV array under nighttime conditions with no
back-converted DC voltage applied. With negligible Vpv and Vpc, no
current is conducted in the system, and no energy transfer
occurs.
[0101] FIG. 6C shows a variable back-converted DC voltage (from
Vpc=0 to Vpc>Voc) applied in daylight.
[0102] FIG. 6D shows a variable back-converted DC voltage applied
at nighttime in the negative and positive directions. Again Vpv is
negligible.
[0103] FIG. 6E shows a small constant back-converted DC voltage
(e.g. 24V) applied at nighttime in the forward direction through
all strings for theft-detection purposes. The accompanying graph
shows current versus time. A change in current as shown by the
dotted line potentially indicates tampering; for example, if one
string is removed by thieves, the leakage current contribution that
string will disappear. Net energy transfer is from the converter
into the array, but is typically very small.
[0104] FIG. 6F shows a negative back-converted DC voltage applied
at nighttime to test bypass diodes.
[0105] Alternatively, to measuring current using DC current meters
602 on each PV string, string-level diagnostics can be performed
using switches 122. In this case, all switches 122 are open with
the exception of the switch associated with the string being
analyzed, as illustrated in FIG. 7A. Diagnostic functions can then
be performed such as the examples shown in FIG. 7B (daytime I-V
curve trace of string 1) and FIG. 7C (daytime I-V curve trace of
string 1).
[0106] FIGS. 13A, 13B, 13C and 13D illustrate another embodiment of
a PV string combiner in several states of operation. The string
combiner illustrated in FIGS. 13A-13D divides the strings (not
shown) into positive and negative subarrays. For purposes of
string-differentiation power monitoring, as discussed above in
connection with FIG. 4, a string pair (consisting of one positive
and one negative string) is preferably disconnected in this
embodiment, as shown in FIG. 13C. For purposes of individual string
diagnostics, as discussed in connection with FIG. 7, only one-half
of a string pair is preferably connected, as shown in FIG. 13D.
[0107] For diagnostic purposes, measurements of current through the
string are taken at multiple voltages, as shown in FIGS. 6C and 6D.
Preferably, current measurements are taken at a sufficient number
of voltages to create an I-V curve trace. An I-V curve trace can
provide valuable information about the functioning of a PV
string.
[0108] FIG. 8 shows an example of an I-V curve trace of a PV string
taken during daytime with significant insolation. At V0, the PV
converter is not providing any load on the PV string and the
current through PV string is at the maximum current, also called
the short-circuit current (Isc). For a normally-operating string,
as the applied voltage increases, the current through the string
decreases. At lower applied voltages, the current is not sensitive
to changes in voltage and the current decreases gradually from Isc,
giving a shallow slope. At higher voltages, the current decreases
rapidly as the applied voltage increases, providing a steep slope.
Around the transition point is a maximum power voltage (Vmp), which
is located at the point on the I-V curve where I*V is greatest. As
the voltage increases beyond Vmp to its maximum (open-circuit)
value Voc, the current falls rapidly to zero.
[0109] FIG. 9 is an I-V curve illustrating a number of problems
that can be identified using I-V curve analysis. Solid line 902
shows an I-V curve for a normally-operating string. The dashed
lines illustrate portions of hypothetical IV curves corresponding
to string malfunctions. Line 904 has a steeper slope near V0,
potentially indicating shunt losses. Shunt losses can be caused by
malfunctions such as a resistive path within a cell, possibly
caused by cracks or other physical damage to a cell or module.
[0110] Line 906 shows a notch in the I-V curve near Vmp,
potentially indicating a mismatch error in the string. A mismatch
error can be caused by shading, uneven soiling, mismatched modules,
or shorted bypass diodes.
[0111] Line 908 shows a curve with an unexpectedly shallow slope
near V.sub.0C. This can indicate series losses. Series losses can
be caused by problems such as a corroded connector.
[0112] Other features of I-V curves that can reveal potential
problems are reduced I.sub.SC and reduced V.sub.0C. Reduced
I.sub.SC can indicate uniform soiling or module degradation.
Reduced V.sub.0C can indicate a high module temperature,
potentially caused by poor air circulation.
[0113] FIG. 10 shows an example of an I-V curve trace (solid line)
taken at nighttime, when the PV modules are producing zero or
negligible power. The curve shown in FIG. 6 is preferably created
by applying a variable voltage using PV converter 130. The applied
voltages preferably range from a negative voltage (i.e. a potential
applied in the direction opposing the normal direction of current
in the array) below a V.sub.t times the number bypass diodes to a
positive voltage above V.sub.t times the number of PV cells being
traced. Under normal conditions, a negative current will be
measured at applied voltages below V.sub.t times the number of
bypass diodes. A positive current will be measured at applied
voltages above V.sub.t times the number of PV cells. Between these
two points the current through the string will be negligible.
[0114] Examples of dark-condition curve traces that can indicate
potential problems through a shift in these points. This can
indicate an open bypass diode, which is a leading cause of arc
faults in PV systems.
[0115] Alternatively to constructing an I-V curve trace, the system
can take current measurements at key voltages associated with the
PV string. For example, if V.sub.mp is known for a particular
string, the system can measure current through the string at
V.sub.mp, and the V.sub.mp current can provide useful diagnostic
information.
[0116] Another use for the system is to provide highly accurate
performance measurements. This can be useful for initial system
commission and annual performance reviews. The highly accurate
performance measurements can be usefully in monitoring long term
panel degradation. Currently PV systems have relatively high
interest charges, in part due to the financial risk in PV array
degradation. By providing highly accurate PV array degradation
measurements this system may reduce financial risk and interest
costs.
[0117] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A method for operating a
photovoltaic array comprising: in a first mode, providing DC
current from photovoltaic cells to a bidirectional power converter,
and operating said bidirectional power converter to provide an AC
power output onto an AC connection; in a second mode, operating
said bidirectional power converter in reverse, to use electrical
power from sources other than said photovoltaic cells to apply a DC
potential to at least some ones of said photovoltaic cells.
[0118] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A method for operating a
photovoltaic array, comprising: in a first mode, providing DC
current from photovoltaic cells to a bidirectional power converter,
and operating said bidirectional power converter to provide an AC
power output onto an AC connection; in a second mode, operating
said bidirectional converter in reverse, to apply a DC potential to
said string of photovoltaic cells; and performing a diagnostic test
on said string using the applied DC potential.
[0119] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A method for operating a
photovoltaic array comprising: in a first mode, providing DC
current from photovoltaic cells to a bidirectional power converter,
and operating said bidirectional power converter to provide an AC
power output onto an AC connection; in a second mode, operating
said bidirectional converter in reverse, to apply a variable DC
potential to said string of photovoltaic cells; and varying said DC
potential, while measuring electric current through said string of
photovoltaic cells, to thereby obtain an I-V profile; and using
said I-V profile to evaluate the string of photovoltaic cells.
[0120] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A method for operating a
photovoltaic array comprising: in a first mode, providing DC
current from photovoltaic cells, with a first voltage polarity, to
a bidirectional power converter, and operating said bidirectional
power converter to provide an AC power output onto an AC
connection; in a second mode, operating said bidirectional
converter in reverse, to apply a DC potential to said string of
photovoltaic cells, with a voltage polarity which is opposite to
said first voltage polarity; and varying said DC potential, while
measuring electric current through said string of photovoltaic
cells, to thereby obtain an I-V profile; and using said I-V profile
to evaluate the string of photovoltaic cells.
[0121] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A method for operating a
photovoltaic array, comprising: using a bidirectional AC-DC
converter to apply a DC potential to a string of photovoltaic cells
under dark conditions; measuring a DC current through the string of
photovoltaic cells; initiating an alert if the measured DC current
indicates that a photovoltaic cell in the string has been
disconnected.
[0122] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A photovoltaic power
generation system comprising: a plurality of photovoltaic cells;
and a bidirectional power converter which is operatively connected
to groups of said photovoltaic cells; wherein, in a first mode of
operation, said bidirectional power converter draws current from at
least some ones of said groups of photovoltaic cells, and provides
an AC power output onto an AC connection; and wherein, in a second
mode of operation, said bidirectional power converter draws power
from one or more sources other than said photovoltaic cells, and
accordingly applies a DC potential to at least some ones of said
photovoltaic cells.
[0123] According to some but not necessarily all disclosed
inventive embodiments, there is provided: A photovoltaic power
generation system comprising: a plurality of photovoltaic cells;
and a bidirectional power converter which is operatively connected
to groups of said photovoltaic cells; wherein, in a first mode of
operation, said bidirectional power converter draws current from at
least some ones of said groups of photovoltaic cells, and provides
an AC power output onto an AC connection; and wherein, in a second
mode of operation, said bidirectional power converter draws power
from one or more sources other than said photovoltaic cells, and
accordingly applies a variable DC potential to at least some ones
of said photovoltaic cells; and wherein one or more diagnostic
tests are performed during said second mode.
[0124] According to some but not necessarily all disclosed
inventive embodiments, there is provided: photovoltaic power
systems which perform any of the innovative methods described
above.
[0125] According to some but not necessarily all disclosed
inventive embodiments, there is provided: Devices, systems and
methods for operating, monitoring and diagnosing photovoltaic
arrays used for solar energy collection. The system preferably
includes capabilities for monitoring or diagnosing an array, under
some circumstances, by using a bidirectional power converter not
only to convert the DC output of the array to output power under
some conditions, but also, for diagnostic operations, applying a
back-converted DC voltage to the array.
[0126] Modifications and Variations
[0127] As will be recognized by those skilled in the art, the
innovative concepts described in the present application can be
modified and varied over a tremendous range of applications, and
accordingly the scope of patented subject matter is not limited by
any of the specific exemplary teachings given. It is intended to
embrace all such alternatives, modifications and variations that
fall within the spirit and broad scope of the appended claims.
[0128] For example, while the preferred embodiment uses silicon
photovoltaic diodes, other semiconductor materials can be used,
e.g. Si.sub.0.9Ge.sub.0.1 or other SiGe alloys, or III-V or other
semiconductor materials. Amorphous, polycrystalline, or
single-crystal materials can be used.
[0129] A variety of structures have been proposed to gather solar
energy, including structures using concentration, wavelength
conversion, and multicolor (multi-bandgap) structures, and the
disclosed inventions are applicable to all of these.
[0130] As noted above, the Universal Power Converter (UPC)
topologies of WO2008/008143 are particularly advantageous, since
they provide great flexibility in conversion of power from any
portal to any other portal, in a multi-portal converter. However,
this specific family of topologies is not required for most of the
disclosed inventions. Many converter topologies can provide
bidirectional (or multidirectional) transfer of power, even if the
full flexibility of the UPC topologies is not present, and some of
these bidirectional-power-transfer topologies can be used for some
of the claimed inventions. (For example, a simple isolated
buck-boost topology can be operated to provide bidirectional power
transfer, e.g. as in starter/generator systems for aircraft
engines.) Some of the disclosed inventions are useful even if a
bidirectional converter is not present. Of course, a great variety
of variations are possible within the basic UPC topology too.
[0131] The particular requirements of different applications can
also be accommodated by appropriate customization of different
portals of a single converter. For example, in some applications it
might be useful to have a 12V DC output for standard automotive
accessories, as well as a 48V DC output for connection to a battery
bank, a 120V 60 Hz output for standard consumer or office
appliances, and/or a 5V DC output for deicing through exposed
connections.
[0132] None of the description in the present application should be
read as implying that any particular element, step, or function is
an essential element which must be included in the claim scope: THE
SCOPE OF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED
CLAIMS. Moreover, none of these claims are intended to invoke
paragraph six of 35 USC section 112 unless the exact words "means
for" are followed by a participle.
[0133] Additional general background, which helps to show
variations and implementations, as well as some features which can
be synergistically with the inventions claimed below, may be found
in the following publications and applications: Balakrishnan et
al., Soft switched ac link buck boost converter, in Applied Power
Electronics Conference and Exposition 2008, pp. 1334-1339;
Balakrishnan et al., Soft switched ac-link wind power converter, in
IEEE International Conference on Sustainable Energy Technologies
2008, pp. 318-321; Toliyat et al., Soft switched ac-link AC/AC and
AC/DC buck-boost converter, in Power Electronics Specialists
Conference 2008 pp. 4168-4176; and U.S. application Ser. No.
13/205,243 (pending), which is a continuation of Ser. No.
12/479,207 (pending, and published as US 2010-0067272), which is a
continuation of Ser. No. 11/759,006 (now issued as U.S. Pat. No.
7,599,196), which is a nonprovisional extension of 60/811,191 filed
Jun. 6, 2006 (and now expired); WO 2008/008143; and U.S. Pat. No.
7,778,045). All of these applications have at least some common
ownership, copendency, and inventorship with the present
application, and all of them are hereby incorporated by
reference.
[0134] The claims as filed are intended to be as comprehensive as
possible, and NO subject matter is intentionally relinquished,
dedicated, or abandoned.
* * * * *