U.S. patent application number 13/195950 was filed with the patent office on 2012-02-09 for power conversion systems.
This patent application is currently assigned to American Superconductor Corporation. Invention is credited to Douglas C. Folts, Arnold P. Kehrli.
Application Number | 20120033468 13/195950 |
Document ID | / |
Family ID | 43029845 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120033468 |
Kind Code |
A1 |
Folts; Douglas C. ; et
al. |
February 9, 2012 |
POWER CONVERSION SYSTEMS
Abstract
In a general aspect, a power conversion system includes a power
converter, a transformer, and a voltage adjustment device. The
power converter is configured to receive a variable DC power
generated by a power generation device and to convert the received
DC power to AC power at a first voltage. The transformer is
configured to receive the AC power from the power converter and to
deliver AC power at a second voltage to a utility power network.
The voltage adjustment device is configured to adjust the first
voltage to a target value determined on the basis of a voltage of
the DC power.
Inventors: |
Folts; Douglas C.; (Baraboo,
WI) ; Kehrli; Arnold P.; (Middleton, WI) |
Assignee: |
American Superconductor
Corporation
Devens
MA
|
Family ID: |
43029845 |
Appl. No.: |
13/195950 |
Filed: |
August 2, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12625093 |
Nov 24, 2009 |
7989983 |
|
|
13195950 |
|
|
|
|
Current U.S.
Class: |
363/95 |
Current CPC
Class: |
H02J 2300/24 20200101;
H02J 3/383 20130101; H02J 3/381 20130101; Y02E 10/56 20130101 |
Class at
Publication: |
363/95 |
International
Class: |
H02M 7/00 20060101
H02M007/00 |
Claims
1. (canceled)
2. An apparatus for conversion of time-varying DC power to AC power
at a specified voltage, said apparatus comprising: a power
converter configured to receive DC power having a time-varying
power level, said DC power being generated by a power generation
device, and to convert the received DC power to AC power at a first
voltage; and a voltage regulation system for adjusting said first
voltage to a target value based at least in part on a voltage of
said time-varying power level.
3. The apparatus of claim 2, wherein the voltage regulation system
includes a first transformer for stepping said AC power at said
first voltage to a second voltage.
4. The apparatus of claim 3, further comprising a second
transformer for stepping said second voltage to a third
voltage.
5. The apparatus of claim 4, further comprising a controller for
adjusting the second voltage in response to time-varying power
level.
6. The apparatus of claim 3, wherein said first transformer
comprises a variable voltage transformer for generating a variable
first voltage from said second voltage.
7. The apparatus of claim 4, wherein the second transformer
comprises a load tap changer.
8. The apparatus of claim 2, wherein said voltage regulation system
comprises a collector bus for receiving AC power originating from
said power converter, said collector bus receiving additional AC
power originating from additional power converters.
9. The apparatus of claim 8, wherein said voltage regulator system
further comprises a distribution voltage regulator on said
collector bus.
10. The apparatus of claim 9, wherein said voltage regulator
comprises a load tap changer connected to a transformer.
11. The apparatus of claim 9, further comprising a second
transformer coupled to a load tap changer, said second transformer
being is configured to transform said second voltage to a third
voltage.
12. The apparatus of claim 2, wherein the voltage regulation system
further comprises a controller that receives information from the
power converter and, based at least in part on said information,
provides control signals to a load tap changer controlled by the
voltage regulation system.
13. The apparatus of claim 12, wherein said information comprises a
DC voltage received by said power converter.
14. The apparatus of claim 10, wherein said distribution voltage
regulator is configured to connect to a utility grid.
15. The apparatus of claim 2, further comprising a solar array
connected to said power converter.
16. An apparatus for converting variable DC power to AC at a
specified voltage, said apparatus comprising: means for converting
variable DC power into a first AC voltage; means for converting
said first AC voltage into a target AC voltage based at least in
part on said variable DC power.
17. The apparatus of claim 16, further comprising means for
providing said variable DC power to said means for converting.
18. The apparatus of claim 17, wherein said means for providing
said variable DC power comprises means for converting sunlight into
electricity.
19. An apparatus for providing AC to an electric utility, said
apparatus comprising: a first solar array; a second solar array; a
first inverter connected to said first solar array; a second
inverter connected to said second solar array; a collector bus for
receiving AC power originating at said first inverter and at said
second inverter; and a voltage regulator for converting a first AC
voltage on said collector bus to a specified second AC voltage.
20. The apparatus of claim 19, further comprising a controller for
determining said specified second AC voltage based at least in part
on an output of said first and second inverter, and causing said
voltage regulator to output said specified second AC voltage.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Under 35 U.S.C. 120, this application claims the priority of
U.S. application Ser. No. 12/625,093 (now U.S. Pat. No. 7,989,983),
filed on Nov. 24, 2009, the contents of which are hereby
incorporated in their entirety.
FIELD OF THE INVENTION
[0002] This invention relates to power conversion systems.
BACKGROUND
[0003] Utility class solar photovoltaic (PV) inverters are used to
convert DC power generated by a PV array into AC power for delivery
to a utility power grid. The generated DC power has a variable
voltage that depends on factors such as temperature, amount of
loading, level of solar irradiance, and age. During normal
operation of a PV system, the inverter provides a loading condition
to the PV array that allows the PV array to operate at its maximum
power point (MPP).
SUMMARY
[0004] In a general aspect, a power conversion system includes a
power converter, a transformer, and a voltage adjustment device.
The power converter is configured to receive a variable DC power
generated by a power generation device and to convert the received
DC power to AC power at a first voltage. The transformer is
configured to receive the AC power from the power converter and to
deliver AC power at a second voltage to a utility power network.
The voltage adjustment device is configured to adjust the first
voltage to a target value determined on the basis of a voltage of
the DC power.
[0005] Embodiments may include one or more of the following.
[0006] The converter is an inverter. The voltage adjustment device
is electrically connected to the transformer. The voltage
adjustment device forms a part of the transformer. The voltage
adjustment device comprises an on-load tap changer. The voltage
adjustment device is further configured to adjust the first voltage
to within a predetermined range of voltage.
[0007] The power converter includes a converter configured to
convert the received DC power to AC power at a third voltage and an
intermediate transformer configured to convert the AC power at the
third voltage to AC power at the first voltage.
[0008] The power conversion system includes a first controller
configured to determine the target value of the first voltage. The
target value of the first voltage of the AC power is less than the
voltage of the DC power by at least a predetermined design margin.
The target value of the AC power is determined such that a current
associated with the AC power is minimized. The second voltage is
higher than the first voltage.
[0009] The DC power varies on the basis of a temperature of the
power generation device. The power generation device includes a
photovoltaic cell. The power generation device includes a wind
power harvesting device. The power conversion system includes a
second controller configured to control the power generation device
to generate a predetermined amount of DC power.
[0010] The power conversion system includes a plurality of power
converters, each power converter configured to receive DC power
from a corresponding power generation device. The power conversion
system includes a collector bus configured to receive the AC power
from each of the plurality of power converters. The voltage
adjustment device is configured to adjust the first voltage of the
AC power on the collector bus. The power conversion system includes
a plurality of collector buses, each collector bus configured to
receive the AC power from a subset of the plurality of power
converters.
[0011] In another general aspect, a method for controlling a power
converter configured to receive a variable DC power generated by a
power generation device and to convert the received DC power to AC
power for delivery to a utility power network includes determining
a target voltage of the converted AC power on the basis of a
voltage of the variable DC power generated by the power generation
device and controlling a voltage adjustment device to adjust the
voltage of the converted AC power to the target voltage.
[0012] Embodiments may include one or more of the following.
[0013] The target voltage is determined such that the target
voltage of the AC power is less than the voltage of the DC power by
at least a predetermined design margin. Determining the target
voltage of the AC power includes determining a minimum current
associated with the AC power. The target voltage can be calculated
in a variety of ways, including by a computer.
[0014] Controlling the voltage adjustment device includes
determining an operational parameter of the voltage adjustment
device. Determining the operational parameter includes using a
look-up table. The operational parameter is determined using a
feedback mechanism based on the voltage of the AC power converted
by the power converter. The voltage adjustment device includes an
on-load tap-changer (LTC) and the operational parameter includes a
tap setting of the LTC. The method includes receiving, from a
controller associated with the power generation device, a report of
the voltage generated by the power generation device.
[0015] The power converter is configured to receive DC power
generated by a plurality of power generation devices and the target
voltage is determined on the basis of a minimum voltage of the DC
power generated by the plurality of power generation devices. The
target voltage is determined on the basis of a ratio between the
voltage of the DC power received by the power converter and the
voltage of the AC power converted by the power converter.
[0016] Controlling the voltage adjustment device includes allowing
less than a predetermined number of adjustments in a given time
period. Controlling the voltage adjustment device is performed on
the basis of an expected change in the DC power. The method
includes determining the expected change in the DC power on the
basis of previous variations of the DC power.
[0017] In a further general aspect, a method includes receiving a
variable DC power from a power generation device; converting the
received DC power to AC power at a first voltage; increasing the
voltage of the AC power to a second voltage; and delivering the AC
power at a second voltage to a utility power network. The first
voltage is determined on the basis of a voltage of the received DC
power.
[0018] Embodiments may include one or more of the following.
[0019] Converting the received DC power to AC power includes using
a voltage adjustment device to adjust the first voltage of the AC
power. The voltage adjustment device includes an on-load
tap-changer (LTC). Adjusting the first voltage of the AC power
includes adjusting a tap setting of the LTC.
[0020] Among other advantages, the system and method described
herein improve the efficiency of a PV installation and reduce its
overall system cost. The AC output voltage of the inverter is
adjusted to an optimum level for the inverter, enabling the
magnitude and range of the AC output current from the inverter to
be reduced. The reduced current allows lower-rated and thus less
expensive inverters to be used. Furthermore, "clipping" of the
output power of the inverter does not occur at low temperatures,
minimizing the wasted power generated by the PV array but not
converted to AC power by the inverter and increasing revenue from
the PV array. In general, reduced current in the inverter also
allows current to be reduced in the collector bus, the cabling
between the inverter and the step-up transformer, and the bus-work
within the inverter, improving overall system efficiency.
[0021] Other features and advantages of the invention are apparent
from the following description and from the claims.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a schematic diagram of a grid-connected PV
system.
[0023] FIG. 2 is a graph showing the effect of temperature on the
current-voltage characteristics of a solar cell.
[0024] FIG. 3 is a graph showing the terminal characteristics of a
string of PV panels at constant temperature for various solar
irradiance levels.
[0025] FIG. 4 is a graph of the current-voltage characteristics of
a PV array and the power-voltage characteristics of the array for
various solar irradiance levels.
[0026] FIG. 5 is a schematic diagram of a utility scale PV
installation.
[0027] FIG. 6A is a table giving temperature coefficients of the
exemplary PV panel described in Table 1.
[0028] FIG. 6B is a graph illustrating the temperature dependence
of the short-circuit current, open-circuit voltage, and maximum
power of the PV panel described in Table 1.
[0029] FIG. 7 is a graph illustrating the temperature dependence of
the output power and voltage of an exemplary PV array.
[0030] FIG. 8 is a schematic diagram of a three-phase inverter.
[0031] FIG. 9 is a graph illustrating PV array power and inverter
output current versus temperature for a fixed AC output voltage
from the inverter.
[0032] FIG. 10 is a graph illustrating inverter output current
versus temperature for a fixed AC output voltage and a variable AC
output voltage from the inverter.
[0033] FIG. 11 is a schematic diagram of an on-load tap-changer
connected to a transformer with multiple taps.
[0034] FIG. 12 is a graph illustrating inverter DC bus voltage and
AC output voltage versus temperature.
DETAILED DESCRIPTION
[0035] Referring to FIG. 1, in a grid-connected solar photovoltaic
(PV) system 150, a photovoltaic array 100 includes a collection of
PV panels 102 connected in a series/parallel arrangement. Each PV
panel is composed of multiple interconnected PV cells ("solar
cells") 104. PV panels connected in series, known collectively as a
"string" 106, enable a high operating voltage to be achieved.
Multiple strings 106 are connected in parallel to achieve a target
generated power rating of the PV array 100. PV array 100 is
connected to a DC bus 108 of an inverter 110, which converts DC
power generated by PV array 100 to AC power. The voltage of the AC
power is increased by a step-up transformer 112 for delivery to a
medium-voltage (e.g., 34.5 kV) collector bus 114. In some
embodiments, PV array 100 generates a medium voltage and inverter
110 is a medium-voltage inverter that delivers converted
medium-voltage AC power directly to collector bus 114.
[0036] Referring to FIG. 2, the terminal voltage versus current
characteristic for a typical solar cell is shown for a fixed solar
irradiance (i.e., the amount of sunlight shining on the solar
cell). The open-circuit voltage, or the voltage across the solar
cell at 0 A of load, varies inversely with solar cell temperature
(region 200). The short-circuit current, or the current flow for
zero voltage across the cell, is relatively constant (region
202).
[0037] Referring to FIG. 3, the shape of the terminal voltage
versus current characteristic for a PV array is similar to that of
an individual solar cell. The voltage range is greater for the
array, reflecting multiple solar cells per PV panel and multiple PV
panels in series; the current range is also greater, reflecting
multiple solar cells and multiple PV panels connected in a
series/parallel arrangement as described above. In FIG. 3, the
temperature is held constant at 25.degree. C.; each curve
represents a different solar irradiance. A higher solar irradiance
generates more current in the PV array.
[0038] Referring to FIGS. 3 and 4, to maximize the power produced
by a PV array (and thus the value sold to a power utility), the PV
array is operated at its maximum power point (MPP). The MPP is the
point on the array's characteristic voltage-current (V-I) curve at
which the product of PV array voltage and current reach a maximum,
and depends on both solar irradiance and temperature. The MPP for
an exemplary PV array is identified in FIG. 3 for each plotted
solar irradiance level; an MPP curve 302 represents ideal operating
points that maximize the power output of the PV string. For
instance, for a solar irradiance of 1000 W/m.sup.2, the MPP occurs
at approximately 180 V and 6 A (point 300). Typical V-I curves
(thin lines) and power curves (thick lines) at various solar
irradiance levels for another exemplary PV array are shown in FIG.
4. The MPP for each solar irradiance level falls at the maximum
point of each power curve. For instance, a point 400 on the 1000
W/m.sup.2 power curve indicates the maximum power for that
irradiance, which corresponds to the MPP (point 300) illustrated in
FIG. 3.
[0039] Referring to FIG. 5, in a utility scale PV installation 500,
collector bus 114 collects the AC output power from multiple
inverters 110, each inverter connected to a PV array 100. The AC
power carried on multiple collector buses 114 is stepped up to a
transmission voltage (e.g., 138 kV) by a power transformer 116,
e.g., at a substation 118. The AC power at the transmission voltage
is delivered to a utility power transmission system 120.
[0040] A controller associated with inverter 110, known as the
maximum power point tracker (MPPT), adjusts the DC bus voltage of
the inverter to match the MPP of the PV array 100. The MPP varies
throughout the day (and by season) as the temperature and solar
irradiance change; the controller continuously maintains the DC bus
voltage at the MPP even as conditions change. The MPPT is generally
implemented in software by well-known algorithms. In other
embodiments, the MPPT is implemented in circuitry. In the context
of this description, the MPPT controls the electrical operating
point of the PV array. Mechanical MPPTs that physically adjust the
PV panels 102 to point directly at the sun as the sun moves across
the sky also exist. In some embodiments, the MPPT of a small-scale
PV system sets the DC bus voltage of the inverter to a
predetermined percentage, such as 76%, of the open-circuit PV
voltage at full sun conditions.
[0041] In order to maximize the efficiency and reduce the cost of a
typical PV installation, the open-circuit DC output voltage of the
PV array 100 is set as high as possible without exceeding each
panel's maximum system voltage rating. The electrical
characteristics of an exemplary PV panel are given in Table 1. For
this PV panel, the maximum system voltage rating is 1000V.
TABLE-US-00001 TABLE 1 Specifications for electrical
characteristics of an exemplary PV panel manufactured by Suntech
Power Holdings Co., Ltd., San Francisco, CA. STC conditions include
1000 W/m.sup.2 irradiance, 25.degree. C. module temperature, and
air-mass value (AM) = 1.5. Characteristics STP280-24/Vd
Open-circuit voltage (V.sub.OC) 44.8 V Optimum operating voltage
(V.sub.mp) 35.2 V Short-circuit current (I.sub.SC) 8.33 A Optimum
operating current (I.sub.mp) 7.95 A Maximum power at STC
(P.sub.max) 280 Wp Operating temperature -40.degree. C. to
+85.degree. C. Maximum system voltage 1000 V DC Maximum series fuse
rating 20 A Power tolerance +/-3%
[0042] Maximum system voltage occurs at open-circuit loading
conditions on the coldest morning (ambient temperature) reached at
the location of the PV installation. The temperature dependence of
a solar cell (illustrated in FIG. 2) is typically specified
numerically and graphically on PV panel data sheets. Referring to
FIGS. 6A and 6B, for the exemplary PV panel described in Table 1,
the temperature coefficient of the open-circuit voltage (V.sub.OC)
is -0.34%/.degree. C. As stated in Table 1, the electrical
characteristics of the panel are normalized to 25.degree. C. Thus,
if the minimum ambient temperature at the site of a PV system
including the exemplary panel is -15.degree. C., then the open
circuit voltage at that temperature will be ((-15.degree.
C.-25.degree. C.)*-0.34%/.degree. C.)=+13.6%, or 13.6% higher than
the open circuit voltage at 25.degree. C. Since, from Table 1,
V.sub.OC at 25.degree. C.=44.8 V, then V.sub.OC at 15.degree. C. is
(1.13*44.8) V, or 50.86 V.
[0043] In order to keep the voltage of the PV system below its
maximum voltage rating of 1000 V (at -15.degree. C. and under open
circuit conditions), 19 PV panels are connected in series, yielding
a total system voltage of 967 V. (One additional PV panel would
yield 1018 V, exceeding the 1000 V rating of the panels.) The
maximum voltage that will appear on the DC bus of the inverter when
the inverter is off (i.e., in a standby, non-operating state) is
thus 967 V.
[0044] In this example, with 19 PV panels per string, the number of
strings needed to generate a target PV array power of 1 MW at
25.degree. C. can be calculated. As given in Table 1, the nominal
power per PV panel is 280 W, meaning that each string generates
19*280 W=5.32 kW of power. Thus, a 1 MW PV array can be formed of
188 parallel strings.
[0045] Referring to FIG. 7, based on the temperature coefficient
calculations above and assuming that the MPP voltage of the PV
array has the same temperature coefficient as the open-circuit
voltage, the output voltage (curve 702) and power (curve 704) of
the exemplary PV array discussed above are calculated and plotted.
The PV array voltage and power increase with decreasing
temperature. In a typical PV panel, the solar cells are encased
behind protective glass. Within this glass casing, the solar cells
generally reach a temperature 25-30.degree. C. warmer than the
ambient temperature under full sun conditions on a calm, windless
day. The 85.degree. C. cell temperature may thus be selected as the
point at which PV array output voltage is at a minimum. Neglecting
voltage drops on the DC collector bus (108 in FIG. 1), the minimum
PV array voltage is the minimum DC bus voltage at which the
inverter is capable of operating.
[0046] To properly match an inverter to a PV array, the AC output
voltage of the inverter is set based on the minimum DC voltage
generated by the PV array, which occurs at the hottest expected
ambient temperature at the location of the PV installation. The AC
output voltage of the inverter and the rated power of the PV system
determine the current rating of the inverter. As the ambient
temperature decreases, a solar PV array generates more voltage, and
thus more power. Eventually, at a low enough temperature, the
inverter will meet its nameplate power rating. As the temperature
continues to fall and the output of the PV array continues to
increase, the inverter limits or "clips" the power to its nameplate
rating in order to stay within safety agency requirements, such as
the Underwriters Laboratories Inc. Standard for Safety for Static
Inverters and Charge Controllers for Use in Photovoltaic Power
Systems, UL 1741. Although more power may be available from the PV
array at these lower temperatures, the inverter will not deliver
power above its nameplate rating to the utility power grid. The
excess power generated by the PV array but not delivered to the
power grid represents lost revenue.
[0047] Even without the constraint of safety agency rating
requirements, at a low enough temperature, the inverter will
eventually reach its AC output current limit. The inverter is not
able to further increase its output current beyond its output
current limit to accommodate the increasing DC power generated by
the PV array. Thus, although more power may be available from the
PV array, the inverter will not deliver that power to the utility
power grid, again resulting in lost revenue. A larger inverter with
a higher current rating can be used to avoid clipping, but such an
inverter costs more.
[0048] Referring to FIG. 8, an inverter 900 is a typical
three-phase inverter with an "H-bridge." Inverter 900 receives DC
input from the PV array along DC bus 108 and outputs AC power to a
transformer 112 (as shown in FIG. 1 and FIG. 5) which steps up the
inverter output voltage to the higher voltage of the collector bus
114. The minimum DC bus voltage of the inverter imposes an upper
limit on the AC output voltage of the inverter beyond which the
inverter controller cannot maintain adequate control over the AC
output current. For inverter 900, the DC bus voltage is preferably
greater than the peak value of the line-to-line AC voltage by some
amount of design margin DM:
V.sub.DC.gtoreq.DMV.sub.AC {square root over (2)} (1)
where DM represents a design margin prudent to assure that the
inverter modulator maintains sufficient control of the output
current, V.sub.DC is the DC bus voltage, and V.sub.AC is the RMS
line-to-line AC output voltage from the inverter. DM=1.1 represents
a 10% design margin. For this discussion, it is assumed the
inverter incorporates a three-phase modulator capable of producing
a theoretical AC voltage peak equal to the DC bus voltage at
maximum modulation index (e.g. space vector modulation or
conventional pulse-width modulation [PWM] with optimum
third-harmonic reference injection).
[0049] As shown in FIG. 7, the minimum DC bus voltage (i.e., the
minimum PV array voltage) is 532.4 V (point 700). For a 10% design
margin, Eq. (1) gives the maximum AC voltage at the inverter output
as 339 V. In a typical solar PV system, the AC output voltage is
fixed, and the AC output current varies to accommodate the variable
DC power generated by the PV array. The AC output voltage is
generally selected to be several percent lower than the calculated
maximum value in order to accommodate other system design
constraints such as utility voltage variation, inverter efficiency,
or voltage drops in the collector bus.
[0050] Referring to FIG. 9, disregarding these design constraints
and using 339 V as the fixed AC output voltage of the inverter, the
AC output current of the inverter is plotted versus temperature
(curve 900) for the PV array illustrated in FIG. 1. At low
temperatures, the PV array generates more DC power (curve 902), and
the inverter thus produces a higher AC output current. As the
temperature continues to drop, the inverter approaches its rated
power limit or rated current limit. For instance, consider an
inverter with a maximum AC output current rating of 1800 A. At
temperatures below about 14.degree. C., the DC power generated by
the PV array would correspond to an output current from the
inverter of greater than 1800 A. However, the inverter cannot
accommodate this excess power and thus the AC output current from
the inverter saturates at 1800 A (represented by a dashed line
904). That is, the inverter is unable to process the incremental PV
array power produced as the temperature falls below 14.degree. C.
and instead "clips" the output at cold temperatures. Revenue from
the incremental PV power capability below the clipping temperature
is thus lost.
[0051] Referring to FIG. 10, by allowing the AC output voltage from
the inverter to vary, the inverter AC output current range can be
reduced, as shown in a curve 1002. As discussed above, PV array
voltage increases with decreasing temperature (curve 1000). With a
fixed inverter output voltage, as shown in curve 1004 and as
discussed above, the inverter output current correspondingly
increases up to a maximum output current rating of the inverter
(represented by a dashed line 1006). When the inverter output
voltage is allowed to vary, some of the increase in DC power
generated by the PV array is accommodated by an increase in
inverter output voltage. The inverter output current thus increases
more slowly and remains below the maximum output current rating of
the inverter (curve 1002). The reduction in AC output current
eliminates clipping of the output of a PV array at cold
temperatures, improving overall PV system efficiency. Furthermore,
because of the reduced AC output current, it may be possible to use
an inverter with a lower current rating, reducing the cost of the
PV installation.
[0052] Continuing the example given above and neglecting system
losses for simplicity of calculation, the AC output voltage of the
inverter is 339 V at 85.degree. C. and the AC output current is
1214 A. At -15.degree. C., the PV array voltage (and the inverter
DC bus voltage) rises to 760 V (as shown in FIG. 7). Following Eq.
(1), for a fixed inverter output voltage, the AC output current
would be 2032 A at -15.degree. C. (curve 1004 of FIG. 10). However,
given that the maximum AC output current rating of the inverter is
1800 A, clipping occurs and not all of the power generated by the
PV array is transferred by the inverter. In contrast, when the
inverter output voltage is allowed to vary, the output voltage is
increased to 483.5 V at -15.degree. C., which corresponds to only
1424 A of AC output current from the inverter (curve 1002 of FIG.
10). This reduction in current allows the inverter to fully process
the 1192 kW of PV array power (483.5 V*1424 A* {square root over
(3)}=1192 kW, where {square root over (3)} reflects the three-phase
AC output of the inverter). That is, at -15.degree. C., the output
current from an inverter with a variable output voltage is about
30% less than the output current from an inverter with a fixed
output voltage, and more of the DC power generated by the PV array
is transferred by the variable voltage inverter. An inverter with a
lower current rating can thus be used. Since the cost of an
inverter is related to its current rating, a variable output
voltage allows the use of a lower-current-rated and less expensive
inverter.
[0053] To generate a variable AC output voltage from the inverter,
any number of variable voltage transformer technologies (e.g., a
Variac or Powerstat.RTM.) may be used on the low-voltage side of
the step-up transformer (112 in FIG. 1) connected to inverter 110.
However, Variacs are prohibitively expensive and would more than
offset the savings in inverter cost. In general, any device that
can be controlled to adjust the AC voltage may be used.
[0054] Referring again to FIG. 5, an on-load tap-changer (LTC) 122
associated with the main power transformer 116 that connects the
solar PV installation to the utility grid 120 is used to vary the
inverter AC output voltage. For large (e.g., greater than 20 MW)
solar PV installations, it is common to have a dedicated power
transformer (e.g., transformer 116) feeding the medium-voltage
collector buses 114 that combine the collective output of multiple
PV arrays 100 and inverters 110. That is, there are no distribution
circuits feeding industrial or residential loads off the medium
voltage. Thus, the voltage on collector bus 114 can be varied over
a wide range without damaging loads designed to accept a fairly
constant voltage.
[0055] Generally, a power transformer with an LTC accepts a varying
voltage on the high-side (transmission voltage primary) and
provides a regulated voltage on the low side (medium voltage
secondary). For instance, regulating a +/-10% input voltage to
within +/-0.625% on the output (medium voltage) side is a common
application for a power transformer with an LTC.
[0056] Referring to FIG. 11, each phase of the three-phase power
transformer (116 in FIG. 5) is wound to include several voltage
taps 1100. LTC 122 connects to one of the taps 1100 and is able to
change from one tap to another without interrupting the load
current or shorting out the transformer. An LTC typically has 10-22
tap positions for linear taps (such as shown in FIG. 11). In other
embodiments, an LTC is used with additional transformer windings in
a coarse/fine or plus/minus switching scheme that yields 15-35
effective tap positions.
[0057] Rather than using a power transformer with an LTC to
regulate the low-side voltage, as an LTC is typically used, the LTC
122 is used to vary the low-side collector bus (114 in FIG. 1 and
FIG. 5) AC voltage. Referring to FIG. 12, in order to accommodate
changes in the DC voltage generated by the PV array (curve 1200),
the AC voltage on the collector bus varies from 339 V to 483.5 V
(curve 1202). An LTC with a +/-15% range is used to regulate this
30% variation in collector bus AC voltage. Optionally, the use of a
+/-20% LTC would allow a +/-5% variation in transmission voltage on
the transmission lines (120 in FIG. 5) to also be accommodated. A
+/-20% LTC is well within the capability of power transformer
manufacturers.
[0058] A power transformer with a standard +/-10% LTC generally
costs about 25% more than a power transformer without an LTC. A
wide range LTC (e.g., +/-20%) may cost an additional 10%. For a 100
MVA transformer costing $2M, and for a wide range LTC that may add
35% to the cost of the transformer, the incremental cost of the
transformer in order to achieve a variable AC voltage on the
collector bus would be $700 k. This cost is offset by the roughly
30% savings achieved by using an inverter with a lower current
rating as noted above. For instance, for a 100 MW solar PV
installation with one hundred 1 MW inverters, each costing $250 k,
the inverter savings is 30% of $250 k for each of the one hundred
inverters, for a total of $7.5M in savings. That is, the inverter
savings may be over ten times the cost for the LTC used to generate
a variable AC voltage to the inverter. In general, for large
utility-scale PV installations (e.g., >20 MW) with a dedicated
collector bus and a power transformer, the variable AC voltage
mechanism lowers the overall cost of the PV installation.
[0059] For smaller PV installations, a distribution voltage
regulator may be used on a medium voltage collector bus dedicated
to several PV inverters. Distribution voltage regulators employ an
LTC connected to an auto-transformer, which is a tapped transformer
without a secondary. This design is preferable for small PV
installations in which the point of connection (POC) to the utility
grid is at the distribution voltage (i.e., the medium voltage)
and/or in which the medium voltage feeds other loads. The
combination of an LTC and an auto-transformer can be designed for a
wide voltage range.
[0060] An LTC accepts contact closure commands to signal it to
switch up or down by one tap on the main transformer. A solar PV
installation such as shown in FIG. 5 has a microprocessor-based
central master controller 124 that communicates with each of the
individual PV inverters 110 via communication lines 126. Each
inverter includes a MPPT controller that regulates the DC bus
voltage of the inverter to the MPP of the PV array 100 feeding the
inverter. The MPP is a function of solar cell temperature, solar
irradiance, and other factors, and varies throughout the day. Each
inverter reports its DC bus voltage to the central master
controller on a periodic basis (e.g., once per second). The master
controller executes instructions that select the minimum value of
all reported inverter DC bus voltages, compute a target AC voltage
based on the minimum DC bus voltage using Eq. (1), and issue the
appropriate up or down commands to an LTC controller 128, which
controls the LTC to connect to the tap closest to the target AC
voltage.
[0061] In some embodiments, the master controller computes the
target AC voltage based on an average DC bus voltage across all
inverters of the PV installation or based on a minimum DC bus
voltage plus a fixed percent. In other embodiments, the master
controller computes the ratio of the measured DC bus voltage to the
measured AC output voltage for each inverter and, based on the
lowest ratio, the average ratio, or another metric, determines
whether an up or down tap change is needed to bring the ratio
within a target window. Other characteristics of the DC bus
voltages may also be used as a basis for determining the target AC
voltage.
[0062] Any of a number of methods may be used to determine the tap
setting of the LTC once the target AC voltage is determined. An
open-loop control scheme uses a look-up table of tap setting versus
target voltage range. The voltage range of each tap step spans a
0.5% to 2% voltage range, depending on the number of effective LTC
tap positions in the transformer. A closed-loop control scheme uses
the AC output voltage reported by each inverter to the master
controller to determine if an up or down tap change is needed to
bring the measured inverter AC voltage within a predetermined
target window of the target AC voltage. The target window is based
on the target AC voltage and takes into account the granular
resolution (i.e., voltage per tap step) of the LTC.
[0063] The algorithms implemented by the master controller include
filtering and anticipatory algorithms to minimize how often the LTC
is commanded to change taps. Since the mechanical and contact life
of an LTC are typically in the range of 500,000 operations or more,
the controller preferably commands no more than 68 LTC tap changes
per day in order to allow a 20 year minimum life for the LTC. To
minimize tap changes, the window for the target AC voltage may be
widened and/or hysteresis may be added to the calculation for
determining when a tap change is needed. That is, the PV
installation operates, on average, at an AC voltage lower than that
predicted by Eq. (1), which translates to a slightly higher
inverter current. Practically, this situation has little negative
impact on the power conversion efficiency, as additional inverter
current is generally available on all but the coldest days (see,
e.g., FIG. 10).
[0064] In some embodiments, the master controller stores the daily
inverter DC bus voltage versus time information. Using the
resulting daily profiles, the inverter `learns` that the DC bus
voltage changes quickly in the morning as temperature and solar
irradiance increase, remains fairly constant throughout the hot
midday period, and changes quickly again as the sun sets. With this
learned profile, the master controller allocates more of the tap
change budget to the early morning and the late afternoon, knowing
that relatively few tap changes will be needed during the longer
midday period. This learning algorithm may be further enhanced by
providing the outside temperature, the PV module temperature,
and/or readings from solar irradiance sensors as inputs to the
algorithm. Historical meteorological and climate data for the site
of the solar PV installation may also be pre-loaded and used by
such an adaptive algorithm.
[0065] If the PV installation includes only one single large energy
source and one inverter, the functions of the centralized master
converter may be performed by a controller within or forming a part
of the inverter controller.
[0066] The variable inverter output voltage concept has been
described above in the context of a solar PV installation. However,
a variable output voltage inverter can also be applied to other DC
energy sources that generate a variable output voltage and/or a
variable output power with temperature. More generally, a variable
output voltage inverter can be applied to other DC energy sources
that generate a variable output voltage based on another factor
other than temperature. For instance, the conversion efficiency of
a wind power installation can be improved by use of such an
inverter.
[0067] In some embodiments, the temperature coefficient of the
open-circuit voltage is positive and the output of the DC energy
source increases with temperature. The same principles described
above also apply to these systems.
[0068] It is to be understood that the foregoing description is
intended to illustrate and not to limit the scope of the invention,
which is defined by the scope of the appended claims. Other
embodiments are within the scope of the following claims.
* * * * *