U.S. patent application number 13/452528 was filed with the patent office on 2012-08-09 for electric power distribution methods and apparatus.
Invention is credited to Alan McDonnell.
Application Number | 20120200155 13/452528 |
Document ID | / |
Family ID | 40756143 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120200155 |
Kind Code |
A1 |
McDonnell; Alan |
August 9, 2012 |
Electric power distribution methods and apparatus
Abstract
A plurality of end-user locations are served by a commercial
utility grid. More than one and less than all of the end-user
locations are themselves interconnected by a feeder, the feeder not
metallically connected to the utility grid. The end-user locations
each have a local AC bus that is not metallically connected to the
utility grid or to the feeder, but that is linked by a coupler to
both the utility grid and to the feeder. None of the local AC buses
or the feeder is required to have the same phase or frequency as
the utility grid. Locally generated electric power may be passed by
means of the feeder to other end-user locations that are on the
feeder. Each local AC bus has two or more inverters powering the
bus.
Inventors: |
McDonnell; Alan; (Merrimack,
US) |
Family ID: |
40756143 |
Appl. No.: |
13/452528 |
Filed: |
April 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12519507 |
Jun 16, 2009 |
8183714 |
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PCT/US08/86686 |
Dec 12, 2008 |
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13452528 |
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60996954 |
Dec 12, 2007 |
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Current U.S.
Class: |
307/18 ;
307/82 |
Current CPC
Class: |
H02H 7/26 20130101; H02J
3/34 20130101; H02J 3/38 20130101 |
Class at
Publication: |
307/18 ;
307/82 |
International
Class: |
H02J 1/00 20060101
H02J001/00; H02J 3/00 20060101 H02J003/00 |
Claims
1. A system for use with AC power at a frequency, the frequency
defining a period, the system comprising: an AC feeder; a control
path, the control path being separate from the AC feeder, the
control path communicating control information with a latency no
greater than one-tenth the period of the AC power; at least two
inverters, each inverter receiving DC power from a respective DC
power source, each inverter having an AC output metallically
connected to the AC feeder, the metallic connection free of any
electrical transformer, the respective DC power sources isolated
from each other; each inverter disposed in a first state to
generate AC power unsynchronized with any other AC phase, and in
said first state to measure its AC output phase and to communicate
that phase external to the inverter by means of the control path;
each inverter disposed in a second state to receive an AC phase
signal from external to the inverter by means of the control path,
and to generate AC power synchronized with the received AC phase
signal, said AC power provided to the AC feeder; control means
responsive to a predetermined event causing the first one of the at
least two inverters to take said first state and causing any other
inverters to take said second state; said means after said
predetermined event responding to failure of the first one of the
at least two inverters by causing an inverter other than the first
one of the at least two inverters to change to said first state;
said changes of state occurring within an interval less than
one-half the period of the AC power; the AC feeder having loads
drawing AC power therefrom.
2. The system of claim 1 wherein the number of inverters is at
least three.
3. The system of claim 1 wherein each inverter generates wye
three-phase electrical power.
4. A method for use with a power system, the power system
comprising a utility grid providing AC electric power connectivity
and extending geographically to a plurality of first end-user
locations and to a plurality of second end-user locations, the
first end-user locations comprising more than one and less than all
of the end-user locations of the utility grid, each of the first
end-user locations comprising a local bus providing AC electric
power connectivity, each local bus having associated with it at
least one AC load metallically connected thereto and drawing power
therefrom, each local bus coupled by means of a respective first at
least one coupler to the utility grid, the at least one coupler
comprising a first power-supply-inverter and a second
power-supply-inverter, each power-supply-inverter having an AC
interface and a DC interface and a controller, each
power-supply-inverter disposed in a first mode to receive DC power
received at the DC interface and to generate AC power delivered at
the AC interface, and disposed in a second mode to receive AC power
received at the AC interface and to generate DC power delivered at
the DC interface, the DC interface of the first
power-supply-inverter electrically connected to the DC interface of
the second power-supply-inverter, whereby the at least one coupler
has a first AC interface defined as the AC interface of the first
power-supply-inverter and a second AC interface defined as the AC
interface of the second power-supply-inverter, the controller of
the first power-supply-inverter and the controller of the second
power-supply-inverter coupled so as to prevent the first
power-supply-inverter and the second power-supply-inverter from
being in the first mode simultaneously for extended intervals, and
so as to prevent the first power-supply-inverter and the second
power-supply-inverter from being in the second mode simultaneously
for extended intervals, each power-supply-inverter disposed when in
its first mode to generate its AC power delivered at its AC
interface consistent in voltage and phase and frequency with any AC
power present external to said AC interface, the power system
further comprising a feeder providing AC electric power
connectivity and extending geographically to the plurality of first
end-user locations, each local bus coupled by means of a respective
second at least one coupler to the feeder, the feeder being
unsynchronized with the utility grid, the method comprising the
steps of: at a first time, powering each of the local buses by
means of a respective coupler drawing power from the utility grid;
at a second time, generating locally generated electrical power at
one of the first end-user locations, and passing some of said
locally generated electrical power to others of the first end-user
locations by means of the feeder.
5. A method for use with a system, the system for use with AC power
at a frequency, the frequency defining a period, the system
comprising an AC feeder; a control path, the control path being
separate from the AC feeder, the control path communicating control
information with a latency no greater than one-tenth the period of
the AC power; at least two inverters, each inverter receiving DC
power from a respective DC power source, each inverter having an AC
output metallically connected to the AC feeder, the metallic
connection free of any electrical transformer, the respective DC
power sources isolated from each other; each inverter disposed in a
first state to generate AC power unsynchronized with any other AC
phase, and in said first state to measure its AC output phase and
to communicate that phase external to the inverter by means of the
control path; each inverter disposed in a second state to receive
an AC phase signal from external to the inverter by means of the
control path, and to generate AC power synchronized with the
received AC phase signal, said AC power provided to the AC feeder;
the AC feeder having loads drawing AC power therefrom; the method
comprising: at a first time, causing the first one of the at least
two inverters to take said first state and causing any other
inverters to take said second state; in response to failure of the
first one of the at least two inverters, causing an inverter other
than the first one of the at least two inverters to change to said
first state, said changes of state occurring within an interval
less than one-half the period of the AC power.
6. A system comprising: a three-wire AC power grid; at least one
four-wire wye inverter connected to the AC power grid and serving
as a voltage source therefor, the at least one four-wire wye
inverter having a respective first power output, any power on the
AC power grid having a power quality, the power on the AC power
grid defining a voltage and frequency for each phase thereof; a
three-wire inverter connected to the AC power grid and serving as a
current source therefor, the three-wire inverter having a
respective power output; the four-wire inverter disposed to sample
the voltage on each phase of the AC power grid, and to source or
sink power on each phase of the AC power grid so as to dynamically
maintain frequency and voltage regulation and improve the power
quality of the power on the AC power grid.
7. A system comprising: a three-wire AC power grid; at least one
four-wire wye inverter connected to the AC power grid and serving
as a voltage source therefor, the at last one four-wire wye
inverter having a respective first power output, any power on the
AC power grid having a power quality, the power on the AC power
grid defining a voltage for each phase thereof; an induction
generator connected to the AC power grid and serving as a current
source therefor, the induction generator having a respective power
output; the four-wire inverter disposed to sample the voltage on
each phase of the AC power grid, and to source or sink power on
each phase of the AC power grid so as to dynamically maintain
frequency and voltage regulation and improve the power quality of
the power on the AC power grid.
8. A method for use with a system comprising a three-wire AC power
grid, at least one four-wire wye inverter connected to the AC power
grid and serving as a voltage source therefor, the at least one
four-wire wye inverter having a respective first power output, any
power on the AC power grid having a power quality, the power on the
AC power grid defining a voltage for each phase thereof, an
additional AC current source connected to the AC power grid and
serving as a current source therefor, the additional AC current
source having a respective power output, the method comprising the
steps of: at the four-wire inverter, sampling the voltage on each
phase of the AC power grid, and sourcing or sinking power on each
phase of the AC power grid so as to dynamically maintain frequency
and voltage regulation and improve the power quality of the power
on the AC power grid.
9. A system comprising: a three-wire AC power grid; a generator
connected to the AC power grid and serving as a voltage source
therefor, any power on the AC power grid having a power quality,
the power on the AC power grid defining a voltage for each phase
thereof; a four-wire wye inverter connected to the AC power grid,
the four-wire wye inverter also connected to a DC bus, the DC bus
having DC power storage connected thereto; the four-wire inverter
disposed to sample the voltage on each phase of the AC power grid,
and to source or sink power on each phase of the AC power grid so
as to dynamically maintain frequency and voltage regulation and
improve the power quality of the power on the AC power grid.
10. A method for use with a system comprising a three-wire AC power
grid, a first generator connected to the AC power grid and serving
as a voltage source therefor, any power on the AC power grid having
a power quality, the power on the AC power grid defining a voltage
for each phase thereof, a four-wire wye inverter connected to the
AC power grid, the four-wire wye inverter also connected to a DC
bus, the DC bus having DC power storage connected thereto, the
method comprising the steps of: at the four-wire inverter, sampling
the voltage on each phase of the AC power grid, and sourcing or
sinking power on each phase of the AC power grid so as to
dynamically maintain frequency and voltage regulation and improve
the power quality of the power on the AC power grid.
11. A method for use with a first bus and a second bus, the first
bus and the second bus connected by a breaker, and with a plurality
of inverters, each inverter providing AC power to the first bus at
a first voltage, each inverter defining some maximum current
associated therewith, the method comprising the steps of: sensing a
fault on the second bus by means of voltage sensing; in the event
of a sensed fault, releasing each inverter to feed its maximum
possible current to the first bus.
12. The method of claim 11 further comprising the step, performed
after the releasing step, of detecting a return of the first
voltage, and thereafter, ceasing the feeding of maximum possible
current to the first bus.
13. A system comprising a first bus and a second bus, the first bus
and the second bus connected by a breaker, and a plurality of
inverters, each inverter providing AC power to the first bus at a
first voltage, each inverter defining some maximum current
associated therewith, each inverter responsive to a sensed fault
sensed by means of voltage sensing by delivering its maximum
possible current to the first bus.
14. The system of claim 13 wherein each inverter is further
responsive to detection of a return of the first voltage by ceasing
the delivery of the maximum possible current to the first bus.
15. An inverter, the inverter providing AC power at a first
voltage, each inverter defining some maximum current associated
therewith, each inverter responsive to a sensed fault sensed by
means of voltage sensing by delivering its maximum possible
current.
16. The inverter of claim 15 wherein the inverter is further
responsive to detection of a return of the first voltage by ceasing
the delivery of the maximum possible current.
17. A system comprising a first DC bus, a second DC bus, a feeder,
a first AC bus, a second AC bus, and first and second transformers
each connected with the feeder; the system further comprising a
first inverter coupling the first AC bus with the first DC bus, a
second inverter coupling the second AC bus with the second DC bus;
a third inverter coupling the first DC bus with the first
transformer; a fourth inverter coupling the second DC bus with the
second transformer; a first load connected with the first AC bus; a
first generator connected with the feeder, the first generator
having a power output level; a second load connected with the
feeder; a second generator connected with the second DC bus and
having a power output level; the second load consuming power
amounting to at least the power output of the first generator; the
first load consuming power amounting to at least a portion of the
power output of the second generator.
18. A method for use with system comprising a first DC bus, a
second DC bus, a feeder, a first AC bus, a second AC bus, and first
and second transformers each connected with the feeder; the system
further comprising a first inverter coupling the first AC bus with
the first DC bus, a second inverter coupling the second AC bus with
the second DC bus; a third inverter coupling the first DC bus with
the first transformer; a fourth inverter coupling the second DC bus
with the second transformer; a first load connected with the first
AC bus; a first generator connected with the feeder, the first
generator having a power output level; a second load connected with
the feeder; a second generator connected with the second DC bus and
having a power output level, the method comprising the steps of:
generating power at the first generator at a respective power
output level; generating power at the second generator at a
respective power output level; at the second load, consuming power
amounting to at least the power output of the first generator; and
at the first load, consuming power amounting to at least a portion
of the power output of the second generator.
19. A system comprising a first DC bus, a second DC bus, a feeder,
a first AC bus, a second AC bus, and first and second transformers
each connected with the feeder; the system further comprising a
first inverter coupling the first AC bus with the first DC bus, a
second inverter coupling the second AC bus with the second DC bus;
a third inverter coupling the first DC bus with the first
transformer; a fourth inverter coupling the second DC bus with the
second transformer; a first load connected with the first AC bus; a
first generator connected with the feeder, the first generator
having a power output level; a second load connected with the
feeder; a second generator connected with the second DC bus and
having a power output level; the second load consuming power
amounting to at least the power output of the first generator; the
first load consuming power amounting to at least a portion of the
power output of the second generator; the first and second AC buses
not synchronized with each other; the feeder not synchronized with
either of the first and second AC buses.
20. A method for use with system comprising a first DC bus, a
second DC bus, a feeder, a first AC bus, a second AC bus, and first
and second transformers each connected with the feeder; the system
further comprising a first inverter coupling the first AC bus with
the first DC bus, a second inverter coupling the second AC bus with
the second DC bus; a third inverter coupling the first DC bus with
the first transformer; a fourth inverter coupling the second DC bus
with the second transformer; a first load connected with the first
AC bus; a first generator connected with the feeder, the first
generator having a power output level; a second load connected with
the feeder; a second generator connected with the second DC bus and
having a power output level, the method comprising the steps of:
generating power at the first generator at a respective power
output level; generating power at the second generator at a
respective power output level, the power generated at the second
generator not synchronized with the power generated at the first
generator; at the second load, consuming power amounting to at
least the power output of the first generator, the power consumed
at the second load not synchronized with the power generated at the
first generator and not synchronized with the power generated at
the second generator; and at the first load, consuming power
amounting to at least a portion of the power output of the second
generator.
21. A system comprising a first DC bus, a second DC bus, a feeder,
a first AC bus, a second AC bus, and first and second transformers
each connected with the feeder; the system further comprising a
first inverter coupling the first AC bus with the first DC bus, a
second inverter coupling the second AC bus with the second DC bus;
a third inverter coupling the first DC bus with the first
transformer; a fourth inverter coupling the second DC bus with the
second transformer; a first load connected with the first AC bus; a
second generator connected with the second DC bus and having a
power output level; the first load consuming power amounting to at
least a portion of the power output of the second generator.
22. A method for use with system comprising a first DC bus, a
second DC bus, a feeder, a first AC bus, a second AC bus, and first
and second transformers each connected with the feeder; the system
further comprising a first inverter coupling the first AC bus with
the first DC bus, a second inverter coupling the second AC bus with
the second DC bus; a third inverter coupling the first DC bus with
the first transformer; a fourth inverter coupling the second DC bus
with the second transformer; a load connected with the first AC
bus; a generator connected with the second DC bus and having a
power output level, the method comprising the steps of: generating
power at the generator at a respective power output level; at the
load, consuming power amounting to at least a portion of the power
output of the generator.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a divisional of U.S. application
Ser. No. 12/519,507, filed Dec. 12, 2008, which claims the benefit
of U.S. application No. 60/996,954 filed Dec. 12, 2007, which
applications are hereby incorporated herein by reference for all
purposes.
BACKGROUND
[0002] Much attention has been given in recent times to energy
policy and energy conservation generally, and to electric power
generation and transmission in particular. Traditional models for
power generation and distribution may once have been the only
workable ways to deliver power to end users. But a variety of
factors including political events, rising energy costs,
technological progress, and concern for the environment have drawn
attention to the need for new paradigms and approaches.
[0003] It is apparent that one important goal is to make it
possible to draw upon distributed energy resources as a source of
electric power for end users. There are, however, a number of
forces seemingly conspiring to limit or frustrate the use of
distributed energy resources. One problem is that the operators of
commercial power grids refuse to permit large amounts of power to
be fed into the grid from end-user locations. Typically the
operator of a commercial power grid will permit feeding of such
power only up to a very small percentage of the distribution
capacity of the grid. This means that if one end-user location
happens to have a large amount of locally generated power
available, it is likely to be difficult or impossible to use the
commercial power grid as a way to distribute that power to other
end user locations.
[0004] It would thus be very desirable if a workable approach could
be found for passing large amounts of power from one end-user
location to another, despite the lack of cooperation on the part of
the operator of the commercial power grid.
[0005] As will be discussed in more detail below in connection with
the invention, experience reveals that moving electric power from
one part of an end-user location to another, or from one end-user
location to another, in an efficient and reliable way, is not easy.
Traditional ways of passing power from one grid to another have
many drawbacks. In more recent times, inverters have been developed
that convert DC to AC in an efficient fashion and with improved
quality of alternating current. But the inverters, taken singly, do
not serve the end users as well as might be desired.
[0006] It would be very helpful if an approach could be found for
coordinating the frequency and phase of generated AC so as to avoid
conflicts among the two or more inverters that might be connected
to a given AC power bus.
[0007] Patents of possible background interest include U.S. Pat.
No. 7,145,266 to Lynch, et alia entitled Parallel-connected
inverters with separate controllers having impedance current
regulators, U.S. Pat. No. 7,116,010 to Lasseter et alia, entitled
Control of small distributed energy resources, and U.S. Pat. No.
6,693,409 to Lynch, et alia entitled Control system for a power
converter and method of controlling operation of a power
converter.
SUMMARY OF THE INVENTION
[0008] A plurality of end-user locations are served by a commercial
utility grid. More than one and less than all of the end-user
locations are themselves interconnected by a feeder, the feeder not
metallically connected to the utility grid. The end-user locations
each have a local AC bus that is not metallically connected to the
utility grid or to the feeder, but that is linked by a coupler to
both the utility grid and to the feeder. None of the local AC buses
or the feeder is required to have the same phase or frequency as
the utility grid. Locally generated electric power may be passed by
means of the feeder to other end-user locations that are on the
feeder. Each local AC bus has two or more inverters powering the
bus. The inverters for a particular local AC bus are linked by
out-of-band signaling with a latency far shorter than the period of
the AC power, and at any given moment one of the inverters is a
master and the others are slaves, so far as voltage, frequency, and
phase are concerned. The DC buses powering the inverters are
themselves isolated from each other. The linkage from each inverter
to its AC bus is free from any transformer.
DESCRIPTION OF THE DRAWING
[0009] The invention will be described with respect to a drawing in
several figures, of which:
[0010] FIG. 1 shows a small-town distribution system;
[0011] FIG. 2 shows a plurality of inverters powering a local AC
bus;
[0012] FIG. 3 shows a detail of a bank of inverters, with
particular attention to out-of-band control links;
[0013] FIG. 4 shows detail of an inverter;
[0014] FIG. 5 shows an internal power circuit for a building;
[0015] FIG. 6 shows a prior-art standard interconnection;
[0016] FIG. 7 shows an installation with an independent grid for a
single building; and
[0017] FIG. 8 shows an installation with multiple independent
grids, as well as an unsynchronized feeder.
[0018] Where possible, like reference numerals have been used among
the figures to denote like elements.
DETAILED DESCRIPTION
[0019] FIG. 1 shows a small-town distribution system. In an
exemplary embodiment, AC power is transmitted to the area by a
utility company omitted for clarity in FIG. 1. The power is
transmitted to the area by line 11 which may be 115 kilovolts (kV)
three-phase AC power. This power is reduced in voltage at a
substation 24 operated by the utility company for distribution on a
distribution line or grid 12 which may carry 13 kV. This
distribution grid is likewise three-phase AC power. Residences 13
are served by the distribution grid 12, as is a medium industrial
plant 16, small industries 17, 18, and 19, a shopping mall 25A.
[0020] It may be that the utility company will place a strict limit
on the amount of electrical power that any one customer or end user
is permitted to generate locally under circumstances that might
lead to power being fed back into the utility grid. Such a strict
limit may in part be justified by legitimate engineering concerns,
and may be in part caused by mindsets dating from earlier decades
when all aspects of the utility grid were centrally planned and
controlled.
[0021] Thus for example in a prior-art system, if the shopping mall
25A were to possess a source of on-site power 21, and if the
connection of the on-site power through line 15 were such that
power could be fed back into the utility grid 12, there might be
strict limits on the permitted generation capacity of the on-site
power 21. The limit may be only a small percentage of the power
distribution capacity of the local grid 12. In such a prior-art
environment, there are many social, environmental, and economic
benefits that might have been available if only the on-site power
21 could have been larger in its power generation capacity than the
capacity permitted by the utility or regulatory environment. Such
social, environmental, and economic benefits are, however,
completely forgone in a prior-art system.
[0022] In one embodiment 41 of the invention, feeders 14 are
provided. Independent area 42 is defined by a respective feeder 14
which connects end-user locations 16 and 17. Note that the end-user
locations 16 and 17 represent more than one but less than all of
the end-user locations served by the utility, and represent more
than one but less than all of the end-user locations served by the
distribution grid 12. As will be described in more detail below,
the presence of the feeder 14 permits a local distribution of power
from an independent power plant 22, 23. The feeder 14 is not
metallically linked to the distribution grid 12, and need not be
operating at the same phase, or at the same frequency, as the power
provided by the utility via grid 12.
[0023] Each end user 16, 17 has a respective local AC grid omitted
for clarity in FIG. 1. At each local AC grid is a coupler coupling
the grid 12 thereto, and a coupler coupling the feeder 14 thereto;
the couplers are likewise omitted for clarity in FIG. 1.
[0024] Independent area 43 is also defined by a respective feeder
14 which connects end-user locations 25A, 19, and 18. Note that the
end-user locations 25A, 19 and 18 represent more than one but less
than all of the end-user locations served by the utility, and
represent more than one but less than all of the end-user locations
served by the distribution grid 12. As will be described in more
detail below, the presence of the feeder 14 permits a local
distribution of power from on-site power generators 20, 21. Just as
was described in connection with independent area 42, the feeder 14
of independent area 43 is not metallically linked to the
distribution grid 12, and need not be operating at the same phase,
or at the same frequency, as the power provided by the utility via
grid 12.
[0025] Each end user 25A, 19, 18 has a respective local AC grid
omitted for clarity in FIG. 1. At each local AC grid is a coupler
coupling the grid 12 thereto, and a coupler coupling the feeder 14
thereto; the couplers are likewise omitted for clarity in FIG.
1.
[0026] As will be described in more detail below, the embodiment 41
offers many benefits when compared with prior-art power
distribution systems. A local power source 22, 23, 21, 20 can be of
much greater power generation capacity than would be feasible in a
prior-art system. Particular end users are able to enjoy greater
reliability (e.g. up-time) with respect to the entirety of their
energy needs, as compared with prior-art systems. The benefits that
flow from generation of power locally to where it is needed, as
compared with the legacy approach of generation power at great
distances from where it is needed, may be fully enjoyed. The
embodiment 41 may, under some circumstances, also permit an
end-user to save money as compared with purchasing most of its
power from the utility.
[0027] It will be appreciated that while many embodiments of a
system such as that discussed in connection with FIG. 1 will be
embodiments where the distribution grid 12 is operated and powered
by a commercial electrical power utility, the teachings of the
invention and its benefits are not limited thereto. For example,
the grid 12 might be a grid operated by a military unit, perhaps up
to the size of a division, in an isolated area such as a desert
area.
[0028] The system 41 of FIG. 1 will now be described from a
different perspective, namely that of an expanded power
distribution system with several independent grids 14.
[0029] The architecture and control features of the independent
grid 14 allow for an almost unlimited ability to expand and
integrate with existing grids 12 as future growth desires.
[0030] The figure shows a small-town distribution system 41 that
has been added to with small on-site power 21, 20 and larger
independent power units 22, 23. The existing distribution system 12
could not support interconnection of these generators onto the
distribution side of the transformer 24.
[0031] By adding the power converters and control scheme, the power
can be integrated into the existing grid and supplied to the end
users without the need for upgrades to the existing grid 12.
[0032] Further expansion could include larger independent grids or
new ones connected through more power converters to the existing
grid or to the other independent grids or both.
[0033] FIG. 2 shows an end-user system 40. The end-user system may
represent an entire building, or may represent a load-panel area
within a large building. In some cases the end-user system 40 may
represent a plurality of buildings nearby to each other.
[0034] The system 40 comprises a plurality of inverters 29, 32, 33,
37 powering a local AC bus 26. Each inverter has a line 28
connecting to the bus 26. The bus 26 has local loads 27.
[0035] The system 40 receives utility power from line 12, which may
be stepped down locally by means of transformer 30. For example the
utility may provide 13 kV on the line 12, stepped down to
three-phase 480 V AC by means of the transformer.
[0036] In day-to-day operation, the 480 VAC power from the utility
is coupled by means of coupler 29 to local AC bus 26, and thence to
local loads 27.
[0037] Importantly, however, there may be one or more local power
generation or storage devices forming part of system 40. As one
example there may be an engine and generator or alternator 34,
generating AC power which is coupled by coupler 33 to the local bus
26. Importantly this power source 34 can be very large, much larger
than what would be permitted in a prior-art system. A portion of
the power (the portion permitted by the utility) can be fed back
into the grid 12 by means of coupler 29.
[0038] What will also be appreciated is that spare power in the
local bus 26 can also be fed into feeder 14 to be made available to
other end users. This takes place through coupler 32 and optional
transformer 31, which may for example step up locally generated
power at 480VC to a 13 kV level which is more suitable to
distribution to the other end users, who might be a kilometer
distant. (The higher voltage reduces resistive losses in the
feeder.) The amount of power that can be shared from one end-user
location to the next (using feeder 14) is far, far greater than the
amount of power that could be shared if the only sharing mechanism
were that of the utility grid 12. It is difficult to overstate the
benefit of what has just been said about the feeder 14 when
compared with a prior-art system where the only sharing opportunity
is the legacy grid 12 operated by a legacy utility that may be
suspicious of end users who wish to generate large amounts of power
locally.
[0039] In FIG. 2 we see examples of other equipment that might be
interconnected with the local bus 26. For example a DC storage
system 38, 39 permits storing DC energy received from the local bus
26, or storing DC energy received from generation facility 35.
Generation facility 35 may be a cogeneration facility providing
waste heat to some other process while generating power to be
delivered to the local bus 26 (and/or to storage 38, 39).
[0040] Stated differently, FIG. 2 shows a proposed building power
distribution system 40, which has been made into an independent
grid 26, unsynchronized with the main existing grid 12. There is
power coming from two different, unsynchronized external feeds 12,
14, as well as two onsite generators 34, 35 and incorporating
energy storage 38, 39 in one of them.
[0041] The inverters 29, 32, 33, 37 are controlled in such as way
as to feed the 480 VAC main (local) bus 26. In this way isolated,
independent DC sources are made to behave like a single, larger
voltage source feeding the bus 26.
[0042] Separate system controls allow for the net energy supply to
be varied between the isolated units 29, 32, 33, 37 to allow for
more economic control of energy sources and to improve
redundancy.
[0043] In the above arrangement, the DC link voltages of the
various inverters 29, 32, 33, 37 are not connected together. This
increases redundancy since the failure of a single DC link will not
cause the failure of any of the others.
[0044] It will be appreciated that while many embodiments of a
system such as that discussed in connection with FIG. 2 will be
embodiments where the local AC bus is a three-phase "wye" grid, the
teachings of the invention and its benefits are not limited
thereto. For example, the local C bus grid 12 might be a
single-phase AC bus, or might be a "delta" three-phase system.
[0045] In the event of failure of the system 40, it is possible to
restore service to the local bus 26 by opening breakers 28 and
closing a bypass or transfer switch 25. In an exemplary embodiment,
these steps are carried out manually, and it is anticipated that
these steps would be required only very infrequently.
[0046] In a system of which FIG. 2 is an example, there is no limit
to the number of different power sources. While FIG. 2 shows four
possible power sources, there could be more. Typically there would
be N+1 or N+2 redundancy. The internal building loads 27 are the
same as they would be for a standard utility power distribution
layout.
[0047] The key to controlling the layout 41 of FIG. 2 is that all
the inverters 29, 32, 33, 37 that are tied to the main AC bus 26
must share the load 27.
[0048] One embodiment being described herein is a power system
which includes a utility grid providing AC electric power
connectivity and extending geographically to a plurality of first
end-user locations and to a plurality of second end-user locations.
The first end-user locations comprise more than one and less than
all of the end-user locations of the utility grid. Each of the
first end-user locations comprises a local bus providing AC
electric power connectivity. Each local bus has associated with it
at least one AC load metallically connected thereto and drawing
power therefrom. Each local bus is coupled by means of a respective
first at least one coupler to the utility grid.
[0049] The at least one coupler comprises a first
power-supply-inverter and a second power-supply-inverter, each
power-supply-inverter having an AC interface and a DC interface and
a controller. Each power-supply-inverter is disposed in a first
mode to receive DC power received at the DC interface and to
generate AC power delivered at the AC interface, and is disposed in
a second mode to receive AC power received at the AC interface and
to generate DC power delivered at the DC interface. The DC
interface of the first power-supply-inverter is electrically
connected to the DC interface of the second
power-supply-inverter.
[0050] The at least one coupler has a first AC interface defined as
the AC interface of the first power-supply-inverter and a second AC
interface defined as the AC interface of the second
power-supply-inverter. The controller of the first
power-supply-inverter and the controller of the second
power-supply-inverter coupled so as to prevent the first
power-supply-inverter and the second power-supply-inverter from
being in the first mode simultaneously for extended intervals, and
so as to prevent the first power-supply-inverter and the second
power-supply-inverter from being in the second mode simultaneously
for extended intervals. By "extended intervals" we may mean in
excess of a few milliseconds or in excess of half a second.
[0051] Each power-supply-inverter is disposed when in its first
mode to generate its AC power delivered at its AC interface
consistent in voltage and phase and frequency with any AC power
present external to said AC interface.
[0052] FIG. 3 shows a detail of a bank of inverters 44, 46, 48, 50,
with particular attention to out-of-band control links 52, 53.
[0053] FIG. 3 is intended to portray a basic power layout, ignoring
for the moment the source of DC power to the inverters. In this
embodiment the AC grid (local bus) 26 is a four-wire system with
Neutral line 57, fed directly by the inverters 44, 46, 48, 50
without a transformer downstream of them (between the inverter and
the local bus 26).
[0054] It will be appreciated that a control scheme is required
such that all of the inverters that are sourcing both kW and kVAR
current into the same voltage node (here, the local bus 26) will
work together and not fight each other for control. The control
scheme used in this system to solve this problem works by having
one unit, called the master, maintain the voltage of the four-wire
480/277 VAC bus. Its individual phase currents are measured and the
values passed along a high speed communication link 52 to the slave
units to be copied. For example at a particular moment, the master
might be inverter 46 having respective controller 47, while the
other controllers 45, 49, 51 are acting as slaves, each controlling
its respective inverter 44, 48, 50.
[0055] Each individual inverter 44, 46, 48, 50 still maintains
over-current and other protection features, and a main breaker 56
can be remotely tripped by the system or inverter controller under
certain conditions.
[0056] A standard prior-art way of paralleling a group of
synchronous rotating generators gives oscillations because they are
trying to share power and speed but can only be controlled by a
mechanical fuel throttle that has a slow reaction time between
throttle change and output power change. There are no power
electronics in such a system. The power electronic converters of
the present embodiments can change power at least 500 to 1000 times
faster.
[0057] The method for providing maximum fault current as fast as
possible involves the use of measured voltage limits by each
individual inverter, such that if a voltage is out of range due to
a fault, the maximum amount of current is sourced by each unit for
as long as possible or until the fault is cleared.
[0058] The fault current must be sourced to open a distribution
breaker. This is a difficult inverter challenge while being
controlled in a current-sharing manner. See FIG. 5 which shows an
internal power circuit 73 for a building. Within the building is a
480 VAC main bus 71. Branch feeders 72, 74, 76, 78 deliver AC power
to circuits throughout the building. Independent 800 VDC supplies
81, 82, 83, and 84 are shown, each able to deliver AC power to the
main bus 71.
[0059] Consider what happens if there is a fault 85 giving rise to
a fault current 86. The inverters 81, 82, 83, 84 must be able
collectively to source enough fault current to open the protection
on the faulted feeder without taking down the whole independent
grid.
[0060] One of the biggest limitations of inverter based microgrids
is their inability to be retro-fitted to existing buildings without
completely re-doing the breaker scheme because the inverters cannot
source enough fault current to trip a breaker. Thus the inverters
will trip off (instead of a breaker being tripped) and the whole
building goes dark, because the inverters have tripped off. This
contrasts with a simpler prior-art system in which a fault current
would have blasted open the breaker of one of the branch feeders
and the rest of the building would have stayed on.
[0061] With a single, low impedance voltage source it is a simple
calculation to determine how much fault current can be sourced
versus how much is needed for certain breakers. If the breaker is
too big for the inverter (that is, if the tripping current for the
breaker exceeds the current-sourcing capability of the inverter),
then the breaker needs to have intelligent fault sensing controls
added, or else the problem of the whole building going dark comes
back.
[0062] With parallel inverters, what is needed is for the inverters
to act more quickly than they could in a simple current-following
mode. In an exemplary embodiment, the inverters are configured to
sense a fault through voltage sensing, and then each inverter is
released to feed maximum current until the voltage comes back,
which happens after the breaker feeding the fault opens.
[0063] FIG. 5 also shows optional power sources 90, 91 which may
provide AC power to the AC bus 71. For example an inverter 91 may
receive power from (say) a photovoltaic array. Induction generator
90 may receive power from a source of rotary energy such as a
turbine. As will be discussed further below, each of these sources
is necessarily synchronous to whatever voltage source defines the
AC voltage on the AC bus 71. Here, the voltage source is the one or
more inverters 81, 82, 83, 84. What may happen, and what is in fact
not uncommon, is that the power from one or more of these sources
may be of poor quality. The source may only deliver power to one
phase. Even if the source delivers power to all three phases, it
may not be in perfect phase relationship. Loads that are
assymmetric (as between the three phases) or that introduce
power-factor loads may be present on the AC bus 71, and if they do,
this may be beyond the ability of the sources 90, 91 to correct or
compensate for. As will be discussed below, however, with suitable
configuration the inverters 81, 82, 83, 84 can sample the voltage
waveforms present on the three phases, and can nearly
instantaneously deliver voltage to the phases in such a way as to
overcome nearly all such problems. In this way, an end user of a
system 73 can make use of commercial, off-the-shelf power sources
even if they produce power of poor non-utility-grade quality. In an
exemplary embodiment, the combined power generation capacity of the
non-utility-grade sources 90, 91 might be up to fifty percent of
the combined power generation capacity of the inverters 81, 82, 83,
84. In another embodiment the percentage might be sixty-five
percent.
[0064] To recap, in the layout of FIG. 4 above, the DC link
voltages of the various inverters are not connected together. This
helps reduce fault current levels and increases redundancy. The
output AC inverters are still able to work together as a single
unit voltage source by having the transistor PWM (pulse-width
modulation) pattern sent from a master controller (one of 45, 47,
49, 51) and passed through to the slave controller (the others of
45, 47, 49, 51) of each individual inverter control.
[0065] The Master PWM Controller acts as it would act if it were
driving a single inverter, adjusting the PWM pattern to maintain a
fixed voltage and frequency no matter what the load. This can
include adjusting the PWM pattern to compensate for non-linear load
characteristics that would otherwise cause voltage distortion, thus
actively filtering harmonic currents.
[0066] To take advantage of price differentials at different times
for different sources of energy, it is necessary to control how
much of each energy source supplies the load. In the exemplary
control scheme the power flow can be controlled accordingly.
[0067] Corrected elsewhere, the DC voltage will now stay stable and
the power flow change will be done by each slave inverter varying
its PWM pattern based on the current slave signal multiplied by (x)
a proportioning signal coming from the CanBUS. Note that this may
include a proportion greater than 100%, which will cause the slave
to supply more power than the master.
[0068] It is instructive to return briefly to FIG. 2 for a
description of the distributed generation control strategies.
[0069] The energy sources for the on-site power 34, 35 can come
from many different sources, but the most common are the burning of
natural gas or syngas, the use of variable speed engine generators
or turbines, the use of fuel cells, and the use of solar power.
Many of these sources require DC/AC conversion.
[0070] In addition, energy storage technologies 38, 39 can help
balance power in the grid, smooth the peak generation requirements,
and provide short-term power during loss of other generation
sources.
[0071] The power electronics and control scheme can take advantage
of the ability to run rotating machinery at variable speeds and
convert the electric power. Importantly, in a typical prior-art
system, any rotating-machinery power source such as a generator or
alternator is required to run at some fixed fraction or multiple of
the frequency of the local bus, and is required to maintain a fixed
phase relationship with the local bus. But in the approach of FIG.
2, a power source drawing upon rotating machinery is able to serve
its purpose even if the rotation is at some frequency and phase
that is not linked to anything about the local bus. Saying this in
a different way, there is no requirement that the rotating
machinery be rotating at any particular frequency, to be able to
generate power to be supplied to the local bus 26.
[0072] The approach according to the invention is thus a much more
efficient approach than the standard prior-art way of generating
fixed-frequency electric power from rotating generators, especially
over wide power ranges which are more necessary with smaller
grids.
[0073] Similarly if a gas or steam turbine generator is used with a
system according to the invention, it becomes possible to eliminate
a gearbox. This decreases size and allows variable speeds, which
increases efficiency. Size and weight issues can be particularly
important in dense areas where construction space is limited.
[0074] Returning to FIG. 3, it is noted that the DC/AC or DC/DC
inverters shown each have a CanBUS connection 53 and a dedicated
high-speed link 52 as shown.
[0075] The main redundant feature of the inverter controls is to be
able to keep running if one unit fails. If the master unit fails,
the next slave unit down the line becomes the master and continues
on. So for example, if controller 47 is the master and if the other
controllers are slaves, a provision must be made for the
possibility that the controller 47 (or its inverter 46) may fail.
In that case, an arbitration mechanism is employed to promote one
of the other controllers 45, 49, 51 to "master" status.
[0076] A series of alarm and warning messages can be sent via
ETHERNET 55 over the internet to remote monitoring facilities. This
enables remote diagnostic capabilities and the ability to more
quickly dispatch necessary maintenance support.
[0077] Under this arrangement according to the invention, since any
single inverter or energy source can fail without causing complete
grid failure, then the faster the failure can be fixed, the less
the chance of a grid failure due to a second equipment failure.
[0078] Depending upon the particular grid and design and cost
considerations, it may be required that the grid run at a reduced
load until repairs can be completed. This may be carried out using
demand response signals from the system level controller 54 to an
intelligent load shedding control, but done in such a way that the
AC voltage on the grid always stays within specification.
[0079] In the event of a failure of the system level controller 54,
the inverters 44, 46, 48, 50 are programmed to go into a default
mode and keep supplying the grid with balanced or pre-set
proportions from the various energy sources. An alarm will be sent
via ETHERNET 55 or simply the absence of the required signal will
trigger an alarm upstream. Again, this is done seamlessly.
[0080] These features cover all the significant faults that could
stop the independent grid 26 from providing voltage within the
specified limits under any single point of failure.
[0081] The final redundant feature is the ability to switch the
whole system off and go to a bypass switch (25 in FIG. 2) to feed
the load just as a traditional distribution system does. This would
normally be done manually for safety reasons.
[0082] The bypass feature is easy to integrate because the
independent grid is designed to integrate into existing grids with
very few changes required.
[0083] A system control scheme for a single independent grid will
now be described.
[0084] To both balance thermal energy requirements and take
advantage of price differentials at different times for different
sources of energy, it is necessary to control how much of each
energy source supplies the load at a given moment.
[0085] In the control scheme according to an embodiment of the
invention, this is done by sending a mathematical multiplier to
each inverter 44, 46, 48, 50 via CanBUS 53, such that it can be
multiplied inside the slave units to the proportioning signals
coming from whichever inverter is serving as the master inverter at
that time.
[0086] Two-way communication via CanBUS 53 is used to adjust the
amount of energy from different sources in the event of an inverter
failure. The inverters will automatically re-assign master control
to the next slave unit, but the total amount of energy fed to the
independent grid must be maintained.
[0087] The system may also include intelligent switchgear on
certain loads to enable fast load shedding of less critical loads
in the event of disruptions from an energy source causing total
load capacity problems.
[0088] The communication to a central station for optimizing energy
use and monitoring system conditions is done via ETHERNET link 55
to the internet.
[0089] As may be appreciated from the above discussion, what is
described includes the application of a control scheme to control
multiple inverters in order to drive an independent electrical grid
fed by multiple energy sources, controlling the energy flow from
the various sources, along with redundant back-up capabilities.
[0090] A main purpose behind creating such a control scheme is to
facilitate the efficient integration of distributed energy
resources (DER) into the existing power grid, without being limited
to existing penetration level limits imposed by operators of the
existing grid. This ability to integrate unlimited amounts of
various sources of electrical energy, without regard to the present
state of the existing grid, is at the heart of the need for this
technology application.
[0091] The main way in which this task is accomplished is by having
the Independent Grid (feeder 14) be seen by the main grid 12 as a
load-reduction type device, as opposed to a parallel interconnected
generator.
[0092] For projects requiring capacity larger than a single grid
can support, the independent grids 14 are capable of being
interconnected with other independent grids, for unlimited
expandability.
[0093] The system provides for the independent grid and the
efficient use of distributed generation assets, while overcoming
the main challenges of integration with respect to control and
stability of the existing grid.
[0094] Additional control features to improve redundant capability.
The Multiple Inverter Control Scheme can include a redundant,
always on-line, back up PWM generator controller. If the Master PWM
healthy signal is lost, the voltage source inverters can switch to
the backup without interruption or delay.
[0095] There are other software features to command the voltage
source inverters and energy source converters to behave in certain
ways under certain conditions to prevent complete system trips. The
system may also include intelligent switchgear on certain loads to
enable fast load shedding of less critical loads in the event of
disruptions from an energy source causing total load capacity
problems.
[0096] Upstream grid fault current limiting. In a Distributed
Generation application with the inverter tied in parallel to a
larger grid, the path of power flow during an upstream grid fault
is difficult to predict.
[0097] The main grid operator will want to shut down all connected
distributed generation sources as quickly as possible so that the
existing protection breakers do not see an increase in the amount
of fault current that they must interrupt.
[0098] By simply measuring current, the inverter controller cannot
tell that such an upstream fault has occurred due to the various
loads (such as induction motors) and other generation sources which
may create resonant circulating paths for the current. The only way
to sense an upstream fault is through voltage measurement and
comparison between the phases.
[0099] The inverter controller can be programmed to measure such
anomalies and shut down on the next 200-microsecond transistor
switch. The controller must have the capability to filter noise
from the measurements such that it does not trip on nuisance
events.
[0100] High Speed current control/power flow control. As noted
above, with the inverter switching 40+times per half cycle, the
ability to control current is much faster than any other means of
standard electrical controls.
[0101] The inverter controller monitors the AC voltage and switches
ON and OFF the transistors to create current flow. The controller
monitors the feedback of the current sensor and can adjust the
switching of the transistors for both amplitude and phase angle of
the current relative to the voltage.
[0102] The control bandwidth is a function of the transistor
switching frequency, with the feedback sampling frequency at a
higher rate for improved accuracy.
[0103] When the inverter is tied to an infinite grid, it is always
in current control mode. If the grid is unstable, the inverter can
help stabilize it by monitoring the frequency and voltage and
adjusting real or reactive current, or both, as required.
[0104] The inverter will have some inherent harmonic compensation
for the main grid due to the low source impedance of the current
source. Active harmonic filtering through current control of AC
voltages measured at multiple frequencies of the fundamental can be
incorporated, but this is a significant extra feature and will
require switching frequencies higher than 5 kHz for harmonics
beyond 5th and 7th.
[0105] Output AC voltage source operating mode. If the inverter is
NOT connected to an existing infinite grid, it can act as the
voltage source. In this mode it will turn the transistors ON and
OFF and measure the output voltage, adjusting to keep an accurate
60 Hz 480V sinusoidal waveform. The current draw will be dependent
upon the load, with the current sensors acting as protection
devices. The inverter may be programmed to lower the output voltage
under heavy current draw if desired.
[0106] The inverters can be placed in parallel to increase the
capacity of the overall output. With special controls, multiple
inverters can be grouped together to act as one, even though the DC
link sources are not connected together.
[0107] FIG. 4 shows detail of an inverter.
[0108] The diagram shows a simple circuit of 6 transistors
including 61, 62 connected in 3 series pairs between a DC supply 63
through an inductor to an AC line 58. The output AC current and
voltage are measured on lines 59 and fed back to the inverter
controller 60. Thus there is current and voltage feedback as to all
3 phases of the AC line.
[0109] The basic concept of the power flow control is that either
the top transistor 61 or bottom one 62 is switched ON (depending
upon the AC waveform polarity at the time) to create a path from
the AC voltage through the inductor to the + or - DC link, causing
current to flow and energy to be stored in the magnetic field of
the inductor.
[0110] When this transistor turns OFF, the energy stored will be
dumped through the freewheel diode of the opposite transistor into
the DC link capacitors.
[0111] The timing of the transistor firing can be manipulated such
that the amplitude of the current and phase angle of the voltage
and current can be controlled. This allows for separate real and
reactive current control, current limits and with 180 degree phase
shifts, the direction of the net power flow.
[0112] Fast Response. Due to the switching frequency of the
transistors (5 kHz), changes can be made every 200 uSeconds, or
41.5 times in each half cycle (at 60 Hz).
[0113] FIG. 6 shows a prior-art standard interconnection. Power is
generated by a utility company at generators 101, 102, each linked
by a transformer 103, 104 to a 230 kilovolt (for example)
transmission line 106. This is stepped down by transformers 107,
108 to a 115 kilovolt transmission line 109. This power may be
stepped down by a transformer 110 to a 13.8 kilovolt distribution
line 111. The power may also be stepped down by a transformer 114
to a 13.8 kilovolt distribution line 115. The transmission lines
may be any of several voltages including 230 kV or 345 kV or 500
kV.
[0114] Distribution line 115 may serve buildings 116, 117, and 118,
each having a respective transformer 119, 120, 121. Within each
building is a respective AC power bus 139, 140, 141.
[0115] A single building 118 may have a generator 123 and an
inverter-based local source of generated power 124. AC power from
these sources is carried to the building 118 by means of a bus 125.
As will be appreciated from the above discussion, the regulatory
environment will typically place a strict and low limit on the
amount of power that may be passed from bus 125 back into the
utility grid at 115, 109, and so on.
[0116] Importantly, the transformer connections are each, by
definition, synchronous as between the primary and secondary
windings thereof. This means that the generator 123 is strictly
required to be synchronous with the utility grid and with the
utility generators 101, 102. Likewise the inverter 125 is strictly
required to be synchronous with the utility grid and with the
utility generators 101, 102.
[0117] FIG. 7 shows an installation with an independent grid for a
single building 126. The 115 kilovolt transmission line 109 is
seen, just as in FIG. 6. Distribution line 115 is seen, as in FIG.
6. Building 129 functions as in FIG. 6. Importantly, however, in
FIG. 7 it may be seen that building 126 is quite different. Utility
power is passed through transformer 121 to line 127, where it is
rectified at 128 and provided to DC bus 129. In this example a
power source 130 provides power to a DC/DC converter 131 to the DC
bus 129. Also in this example a bidirectional DC/DC converter 133
connects a DC energy storage device 132 with the DC bus 129. The
energy storage device 132 might, for example, be a battery.
[0118] What is important here is that the direction of the power
flow is FROM the grid not TO the grid, thus the independent grid is
not seen by the existing grid as a generator but as a load, and in
this case, a reduced load, due to the on-site generators providing
much of the energy required for the building.
[0119] A generator 134 provides power to rectifier 135, and from
there to the DC bus 129. The DC power at the DC bus 129 is inverted
at 126 and is provided via line 137 to internal AC bus 138 to power
loads in the building 126.
[0120] Importantly, it will be appreciated that there is no
requirement that the generator 134 be synchronous with anything
else.
[0121] FIG. 8 shows an installation 160 with multiple independent
grids, as well as an unsychronized feeder 153. FIG. 8 shows many of
the same functional blocks as FIG. 7. New functional blocks in FIG.
8 include coupler 155 which couples the DC bus 129 with the feeder
153 (perhaps through a transformer 154 as shown). Building 144,
line building 126, has an internal bus 146 that is not tied
metallically to the building feed at 142. In this way building 144
is similar to building 126, which likewise has an internal bus 138
that is not tied metallically to the building feed at 121.
[0122] Note that in an exemplary embodiment, the feeder is a
distribution level voltage, 13.8 kV. The power conversions take
place at 480V, but any distance will be required to have 13.8 kV
lines. Note that this is also the highest voltage that can be used
with the generator, in this case a 10 MW turbine.
[0123] Associated with building 144 is a local DC bus 150, much as
building 126 has associated with it a local DC bus 129. This
permits building 144 to draw power from the feeder 153 as desired,
thereby perhaps making use of the power from sources 130, 132, or
134.
[0124] In this example there is a waste treatment plant 162, with a
gas turbine 161. The turbine 161 turns generator 163, which
supplies AC power (in this example, at 13.8 kilovolts) to the
feeder 153. In this way either or both of buildings 144, 126 is
able to draw upon the power from the generator 163.
[0125] To the extent that coupler 155 serves as an inverter
(delivering power from DC bus 129 to feeder 153), the coupler 155
is required to be synchronous with generator 163. But neither
coupler 155 nor generator 163 is required to be synchronous with
the utility grid at 113 and 115.
[0126] Note that Inverter 155 is a three-wire current source, and
it synchronizes to the voltage source of the 10 MW generator. It
can cause current to flow, and control the phase angle of this
current to produce +/- kW or VAR depending on the phase angle. A
zero degree shift is kW, a 90 degree shift is VAR, and in between
is a vector sum of the two. Inverter 128 is simply programmed not
to allow -kW phase angles of current to flow.
[0127] Any feeder such as feeder 153 necessarily has exactly one
power source that defines the voltages on the feeder (thus called a
"voltage source"). Any other power sources that also deliver power
to the feeder 153 are necessarily not voltage sources but are mere
current sources. Thus in a typical arrangement the inverter 155
(e.g. in FIG. 8) serves as a current source and not a voltage
source.
[0128] Alternatively, however, it might develop that the generator
163 might go out of service. If this were to happen, then some
other power source such as inverter 155 could be reconfigured to
serve as the voltage source for the bus 153. In this way, power
could be delivered (see FIG. 8) from the microsite containing power
sources 121, 130, 134 to a different microsite such as that of
building 144 even if generator 163 were out of service.
[0129] Inverter 136 is a four-wire voltage source. It outputs three
single phase voltages between phases A, B, C and N. It does this no
matter what the load, up to the silicon limit of the transistors so
long as the DC can be maintained within limits by whatever energy
sources are feeding or taking from it.
[0130] With further reference to FIG. 8, many advantages and
benefits of the invention may be appreciated.
[0131] Consider, for example, the waste treatment plant 162. In a
prior-art system, such a waste treatment plant 162 might be nearly
unusable because of regulatory and engineering constraints imposed
by the utility company providing power at 113. In the arrangement
shown in FIG. 8, however, power from the waste treatment plant 162
can pass through transformer 152 and electronics 151 to a local DC
bus 150 associated with building 144. Similarly, power from the
waste treatment plant 162 can pass through transformer 154 and
electronics 155 to a local DC bus 129 associated with building 126.
Nothing about this system requires that the power from the waste
treatment plant 162 be "utility grade". It could be inaccurate in
its frequency or phasing, for example, as compared with the utility
grid at 113. If the power from the plant 162 is three-phase power
(as it is very likely to be in most embodiments), it is no problem
at all if there are unbalanced loads somewhere on the feeder 153.
Even if there are unbalanced loads, the rectified power passed to
DC bus 150 or 129 is capable of being put fully to use.
[0132] In FIG. 8, the electronics 128 may be configured so that the
power flow is solely from left to right in FIG. 8. If so, then on a
practical level the system that is to the right of electronics 128
will appear to the existing utility grid as a load reduction and
not as an interconnected generator.
[0133] As mentioned above, in an exemplary embodiment the plant 162
with its generator 163 might well not be generating utility grade
power. For example the power generated by the generator 163 may
sometimes be of slightly higher frequency relative to
specifications, and may at other times be of slightly lower
frequency. There might also be unbalanced loads somewhere on the
feeder 153 as between the three phases. There could also be
poor-power-factor loads somewhere on the feeder 153, giving rise to
a condition of the voltage and current on the feeder 153 being
pulled out of phase with each other. In the face of all of these
possible degradations in the quality of the power supplied on the
feeder 153, it is possible to configure inverter 155 (together with
local DC bus 129 and energy storage 133, 132) to bring about
substantial improvements in the power quality on the feeder 153, as
will now be discussed.
[0134] In an exemplary power-quality-enhancement approach, inverter
155 draws power from feeder 153 from time to time, some of which is
stored in energy storage 133, 132. The power drawn need not be
power of high quality since it will be rectified anyway on its way
to storage 133, 132 (passing over DC bus 129). Inverter 155,
however, also monitors the instantaneous voltages present on the
three (typical) phases of the feeder 153. If inverter 155 notices
an imperfect voltage waveform on one of the phases, including
timing issues of the waveform that diverge from the desired
frequency regulation, then it nearly instantaneously pumps some
power into that phase (or draws power from that phase) so as to
bring the waveform closer to the ideal. The availability of power
on the desired time scale--far faster than the reaction time of the
mechanical throttle on generator 134--is achieved through the
energy storage device 132. This activity by the inverter 155 is
carried out upon each of the three phases and can result in the
effective waveforms on the phases of the feeder 153 being
utility-grade quality.
[0135] In this arrangement, the generator 163 serves as a voltage
source, while inverter 155 serves as a three-wire power source (or
sink) for the purpose of dynamic frequency stabilization.
[0136] The power quality issues that are voltage related can be
solved by various forms of VAR control or harmonic filtering, but
that will not fix a frequency problem, which can only be fixed by
changing net kW flow from an energy source. It is not correct to
say that the inverter 155 sinks or sources current (which a D-VAR
does as well); it sinks or sources power (kW), which it gets or
sends from the energy storage source.
[0137] On a practical level this could mean, among other things,
that the power provided to other end users on the feeder 153 can be
sold at a price that is appropriate for utility-grade power
(typically a retail price). In contrast, had the generator 163 been
connected in a prior-art fashion directly to a utility grid such as
at 115, the power would only be able to be sold at much lower
wholesale prices. Said differently, the availability of the
hardware 155, 129, 133, 132 together with appropriate configuration
permits providing frequency stability so that non-utility-grade
power is rendered utility-grade, offering economic benefits.
[0138] Returning now to FIG. 8, another benefit may be seen.
Consider the effect upon building 144 or 126 if the utility grid
113, 114, 115 is lost to the end users at buildings 144 and 126. In
a prior-art arrangement, any local backup power generation is
permitted to be connected to the building only by means of a
"transfer switch". The transfer switch connects either the utility
grid 115 to the building bus 146, or the local backup power such as
from generator 134 or generator 163. With such an arrangement, any
switching of the transfer switch will result in a disruption of
power.
[0139] Some transfer switches require tens or hundreds of
milliseconds to switch. But another issue is that some utilities
require that the circuit "go dark" for at least hundreds of
milliseconds, before the local backup power comes on line. As such,
this is often a regulatory, not technical, limitation. But as
mentioned above, the systems according to the invention do not
suffer from such interruptions.
[0140] In contrast, however, with an arrangement 160 as shown in
FIG. 8, a loss of the utility grid 115 need not result in any
disruption at all of the power to the end user AC buses 146, 138.
The loss of utility power merely means that electronics 148, 128
are then unable to provide DC power to the local DC buses 150, 129
respectively. But other sources of DC power permit continued and
uninterrupted AC power through inverters 149, 136 to local AC buses
146, 138.
[0141] Returning again to FIG. 8, it will be appreciated that
electronics 155, 128, 151, 148 can be configured to respond to
out-of-band signaling that permits taking action based upon price
changes among the various sources of electrical power. If a
particular power source becomes more expensive, the electronics can
draw less power from that source. If a particular power source
becomes less expensive, then the electronics can draw more power
from that source.
[0142] Any electrical power distribution system will have fault
protection devices which are intended to open in the event of a
fault, so as to isolate a failed portion of the system from other
non-failed portions of the system. What is undesirable, however, is
if in the event of a fault, the fault currents fail to rise to a
level that suffices to trigger the relevant fault protection
device. With appropriate configuration, however, the inverters
feeding a given feeder or bus or grid can respond to internal
control logic to sense voltage anomalies on the feeder or bus or
grid; when this happens the inverters can supply extra current so
as to help clear the relevant fault protection devices as quickly
as possible.
CONCLUSION
[0143] The advantages of this control scheme, over other ways to
perform the sharing of multiple power sources, include: [0144] A
lower output source impedance requirement for the inverters since
impedance is not required to be added to aid active power sharing.
This reduces cost and efficiency loss, and increases the amount of
available fault current that can be sourced from the inverters.
[0145] A simple control scheme may be employed, with no need for
active Micro-grid type controls. [0146] It is possible to retain
redundant operation with no single-component failure able to cause
a complete shutdown.
[0144] While the invention has been described with respect to
particular embodiments, the invention is not limited thereto. Those
skilled in the relevant arts will have no difficulty devising
myriad obvious improvements and variations, all of which are
intended to be within the scope of the claims which follow, when
properly construed.
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