U.S. patent number RE44,901 [Application Number 13/212,811] was granted by the patent office on 2014-05-20 for method for converting direct voltage into three phase alternating voltage.
This patent grant is currently assigned to SMA Solar Technology AG. The grantee listed for this patent is Holger Becker, Sven Bremicker, Guenther Cramer, Thorsten Dingel, Bernd Engel, Frank Greizer, Wilfried Groote, Ralf Joachim Laschinski, Matthias Victor, Torben Westphal. Invention is credited to Holger Becker, Sven Bremicker, Guenther Cramer, Thorsten Dingel, Bernd Engel, Frank Greizer, Wilfried Groote, Ralf Joachim Laschinski, Matthias Victor, Torben Westphal.
United States Patent |
RE44,901 |
Becker , et al. |
May 20, 2014 |
Method for converting direct voltage into three phase alternating
voltage
Abstract
A method of converting a direct voltage generated by a
decentralized power supply system into three-phase alternating
voltage by means of a plurality of single-phase inverters
(WR1-WR3), said alternating voltage being provided for supplying an
electric mains, is intended to avoid inadmissible load unbalances
using single-phase inverters. This is achieved in that, upon
failure of one inverter (WR1-WR3), an asymmetrical power supply
distribution is reduced by limiting the output of the other
inverters. The method makes it possible to simplify three-phase
voltage monitoring.
Inventors: |
Becker; Holger (Elmshorn,
DE), Cramer; Guenther (Kassel, DE),
Bremicker; Sven (Alheim, DE), Dingel; Thorsten
(Niestetal, DE), Engel; Bernd (Wolfenbuettel,
DE), Groote; Wilfried (Vellmar, DE),
Greizer; Frank (Kaufungen, DE), Laschinski; Ralf
Joachim (Kassel, DE), Victor; Matthias
(Niestetal, DE), Westphal; Torben (Aurich,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Becker; Holger
Cramer; Guenther
Bremicker; Sven
Dingel; Thorsten
Engel; Bernd
Groote; Wilfried
Greizer; Frank
Laschinski; Ralf Joachim
Victor; Matthias
Westphal; Torben |
Elmshorn
Kassel
Alheim
Niestetal
Wolfenbuettel
Vellmar
Kaufungen
Kassel
Niestetal
Aurich |
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A |
DE
DE
DE
DE
DE
DE
DE
DE
DE
DE |
|
|
Assignee: |
SMA Solar Technology AG
(Niestetal, DE)
|
Family
ID: |
38281957 |
Appl.
No.: |
13/212,811 |
Filed: |
August 18, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
11650781 |
Jan 8, 2007 |
7576449 |
Aug 18, 2009 |
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Foreign Application Priority Data
|
|
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|
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Jan 27, 2006 [DE] |
|
|
10 2006 003 904 |
|
Current U.S.
Class: |
307/45; 700/293;
700/292; 307/44; 700/286; 307/82 |
Current CPC
Class: |
H02J
3/26 (20130101); H02J 3/383 (20130101); H02J
3/46 (20130101); H02J 3/381 (20130101); Y02E
40/50 (20130101); Y02E 10/56 (20130101); H02J
2300/24 (20200101) |
Current International
Class: |
H02J
1/12 (20060101); H02J 1/00 (20060101); H02J
7/34 (20060101); H02J 3/00 (20060101); H02J
5/00 (20060101); G05D 17/00 (20060101); H02J
3/38 (20060101); H03K 19/00 (20060101); G05D
9/00 (20060101); G05D 11/00 (20060101); G05D
3/12 (20060101); G05D 5/00 (20060101) |
Field of
Search: |
;361/45 ;307/45-46 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 817 350 |
|
Jan 1998 |
|
EP |
|
05 308780 |
|
Nov 1993 |
|
JP |
|
2005/117136 |
|
Dec 2005 |
|
WO |
|
2006/084294 |
|
Aug 2006 |
|
WO |
|
Primary Examiner: Patel; Dharti
Attorney, Agent or Firm: Slater & Matsil, L.L.P.
Claims
We claim:
1. A method of converting direct voltage generated by a mains
connected system for decentralized power supply into a three-phase
alternating voltage by means of a plurality of single-phase
inverters .[.(WR1-WR3).]. connected to a different phase (L1, L2,
L3), said alternating voltage being provided for supplying .[.an
electric.]. .Iadd.the .Iaddend.mains and for decentralized power
supply, .[.whereby.]. .Iadd.wherein .Iaddend.upon failure of one
inverter .[.(WR1-WR3).]. .Iadd.of the plurality of
inverters.Iaddend., an asymmetrical power supply distribution is
reduced by limiting .[.the.]. output of the other inverters
.Iadd.of the plurality of inverters.Iaddend., .[.whereby.].
.Iadd.wherein .Iaddend.the failure of the .Iadd.one
.Iaddend.inverter .[.(WR1-WR3).]. is registered by a measurement
signal of an additional circuit of .Iadd.each of .Iaddend.the
.[.inverter.]. .Iadd.other inverters.Iaddend., a communication
signal resulting from the measurement signal being provided for
limiting the output or for disconnecting the .Iadd.other
.Iaddend.inverters from the mains, .[.whereby.]. .Iadd.wherein
.Iaddend.the measurement signal is generated by a current of a
constant current source, said current of said constant current
source being in .[.the.]. .Iadd.a .Iaddend.milliampere range.
2. The method as set forth in claim 1, .[.whereby that.].
.Iadd.wherein .Iaddend.the output of each .[.inverter (WR1-WR3).].
.Iadd.of the other inverters .Iaddend.is durably limited to 2-20
kVA.[., more specifically to about 4.6 kVA.]..
3. The method as set forth in claim 1, .[.whereby.]. .Iadd.wherein
.Iaddend.the output of each .[.inverter (WR1-WR3).]. .Iadd.of the
other inverters .Iaddend.is limited temporarily.[., more
specifically for about 10 minutes,.]. to about 5 kVA.
4. The method as set forth in claim 1, .[.whereby.]. .Iadd.wherein
.Iaddend.the communication signal is communicated to .[.the.].
processors of .Iadd.each of .Iaddend.the .Iadd.other
.Iaddend.inverters .[.(WR1-WR3).]. for limiting the output of
.Iadd.each of .Iaddend.the .Iadd.other .Iaddend.inverters by means
of processor control.
5. The method as set forth in claim 1, .[.whereby the.].
.Iadd.wherein a .Iaddend.communication takes place between the
.Iadd.plurality of .Iaddend.inverters .[.(WR1-WR3).]., through
which each inverter communicates through one or a plurality of
external cables .[.that it is ready for supply.]..
6. The method as set forth in claim 1, .[.whereby the.].
.Iadd.wherein a .Iaddend.communication takes place between the
.Iadd.plurality of .Iaddend.inverters .[.(WR1-WR3).]., each
inverter communicating by radio .[.that it is ready for
supply.]..
7. A circuit arrangement comprising means for implementing the
method as set forth in claim 1, with a plurality of single-phase
inverters .[.(WR1-WR3).]. of a decentralized power supply system,
wherein each inverter has a failure measuring circuit .[.(1).]. for
reducing the power of the .[.other inverters.]. .Iadd.inverter
.Iaddend.upon .Iadd.an .Iaddend.asymmetrical power supply resulting
from the failure of .Iadd.one of .Iaddend.the .[.inverter.].
.Iadd.inverters.Iaddend..
8. The circuit arrangement as set forth in claim 7, .[.whereby.].
.Iadd.wherein .Iaddend.each measuring circuit includes a direct
current source .[.(3), more specifically a constant current source,
that may be additionally.]. .Iadd.but only one constant current
source is .Iaddend.connected through an electrically conductive
bridge .[.(4), so that only one of the current sources (3) delivers
the.]. .Iadd.in order to deliver a .Iaddend.measurement current,
each measuring circuit comprising a connection for a measurement
and/or output signal in such a manner that a failure or an OK
signal can be communicated .[.to the other two inverters.]..
9. The circuit arrangement as set forth in claim 7, .[.whereby.].
.Iadd.wherein .Iaddend.each inverter is implemented with an
external communication line .[.(2).]..
10. The circuit arrangement as set forth in claim 7, .[.whereby.].
.Iadd.wherein .Iaddend.a plurality of groups .[.(GR1, GR2).]. of
three single-phase inverters .[.(11-16).]. is connected to
.[.the.]. .Iadd.an .Iaddend.output side of a three-phase protection
switchgear, said switchgear being adapted to be switched off
through .[.the.]. .Iadd.a .Iaddend.manual, external clearing means
.[.(18).]..
11. The circuit arrangement as set forth in claim 10, .[.whereby.].
.Iadd.wherein .Iaddend.the switchgear is a contactor
.[.(19).]..
12. The circuit arrangement as set forth in claim 7, .[.whereby an
implementation such that.]. .Iadd.wherein.Iaddend., upon failure of
.Iadd.the .Iaddend.one .[.inverter.]. .Iadd.of the
inverters.Iaddend., all the inverters are disconnected from the
mains when the voltage of one phase exceeds, or falls short of,
imposed limit values.
13. The circuit arrangement as set forth in claim .[.12 whereby an
implementation such that.]. .Iadd.10 wherein all .Iaddend.the
inverters are disconnected from the mains by directly switching
them off, a manual, external clearing means .[.(18).]. being
directly connected to .[.all.]. the .[.inverter.]. .Iadd.plurality
of .Iaddend.groups.
.Iadd.14. A method comprising: applying a measurement current in a
milliampere range from a constant current source to an inverter
system comprising a first single phase inverter, a second single
phase inverter and a third single phase inverter; monitoring the
inverter system through a first circuit in the first single phase
inverter; monitoring the inverter system through a second circuit
in the second single phase inverter; monitoring the inverter system
through a third circuit in the third single phase inverter; upon a
failure of the third single phase inverter, measuring the failure
in the first single phase inverter and the second single phase
inverter; reducing a first output power of the first single phase
inverter based on the measured failure in the first single phase
inverter; and reducing a second output power of the second single
phase inverter based on the measured failure in the second single
phase inverter thereby reducing an asymmetrical power
supply..Iaddend.
.Iadd.15. The method according to claim 14, wherein measuring the
failure comprises communicating a first communication signal to a
first processor of the first single phase inverter and
communicating a second communication signal to a second processor
of the second single phase inverter..Iaddend.
.Iadd.16. The method according to claim 14, wherein reducing the
first output power and the second output power comprises limiting
the first output power and the second output power to 2 kVA-20 kVA,
respectively..Iaddend.
.Iadd.17. A method of converting a direct voltage into a three
phase alternating voltage for supplying into a mains, each single
phase inverter of a plurality of single phase inverters having a
different phase, the method comprising: monitoring a plurality of
single phase inverters by measuring a constant current in a
milliampere range at a circuit of each single phase inverter; upon
measuring a failure of one of the single phase inverters,
communicating the failure to a processor of each of the other
single phase inverters; and reducing an asymmetrical power supply
by limiting an output power of each of the other single phase
inverters..Iaddend.
.Iadd.18. The method according to claim 17, wherein reducing the
output power comprises limiting the output power to 2 kVA-20 kVA,
respectively..Iaddend.
.Iadd.19. A method comprising: monitoring an operating condition of
a first single phase inverter, a second single phase inverter and a
third single phase inverter by measuring a constant current in a
milliampere range; upon measuring a failure of the first single
phase inverter in circuits of the second single phase inverter and
the third single phase inverter; and reducing an asymmetrical power
supply by reducing output powers of the second single phase
inverter and the third single phase inverter by communicating to
processors of the second single phase inverter and the third single
phase inverter the failure of the first single phase
inverter..Iaddend.
.Iadd.20. The method according to claim 19, wherein reducing the
output power comprises limiting the output power to 2 kVA-20
kVA..Iaddend.
.Iadd.21. A system comprising: a plurality of single-phase
inverters, each single phase inverter configured to provide a
different single phase and each single-phase inverter comprising a
monitoring circuit configured to measure a constant current in a
milliampere range; and a controller configured to control a power
output of the respective single phase inverter; and a connection
connecting the monitoring circuits of the plurality of the single
phase inverters, wherein upon a failure of one of the single phase
inverters, the monitoring circuits of the other single phase
inverters send a communication signal to their respective
controllers, and wherein the controllers of the other single-phase
inverters reduce the power outputs of the other single-phase
inverters to reduce an asymmetrical power supply..Iaddend.
.Iadd.22. The system according to claim 21, wherein the power
outputs of the other single-phase inverters are limited to 2 kVA-20
kVA..Iaddend.
.Iadd.23. The system according to claim 21, further comprising a
communication circuit having one or a plurality of external
cables..Iaddend.
.Iadd.24. The system according to claim 21, further comprising a
wireless communication circuit..Iaddend.
.Iadd.25. A system comprising: a first single phase inverter
configured to produce a first single phase, the first single phase
inverter comprising a first monitoring circuit; a first constant
current source configured to provide a current in a milliampere
range; and a first controller configured to control a first power
output of the first single phase inverter; a second single phase
inverter configured to produce a second single phase, the second
single phase inverter comprising a second monitoring circuit; and a
second controller configured to control a second power output of
the second single phase inverter; a third single phase inverter
configured to produce a third single phase, wherein the first,
second and third single phase are different; and a connection
connecting the first single-phase inverter, the second single-phase
inverter and the third single phase inverter, wherein the
monitoring circuit of the first single phase inverter is configured
to send, upon detection of a failure of the third single phase
inverter, a first communication signal to the controller of the
first single phase inverter, wherein the monitoring circuit of the
second single phase inverter is configured to send, upon detection
of the failure of the third single phase inverter, a second
communication signal to the controller of the second single phase
inverter, wherein the first single phase inverter reduces a first
output power, wherein the second single phase inverter reduces a
second output power, and wherein an asymmetrical power supply is
reduced..Iaddend.
.Iadd.26. The system according to claim 25, wherein the first
output power comprises 2 kVA-20 kVA, and wherein the second output
power comprises 2 kVA-20 kVA..Iaddend.
.Iadd.27. The system according to claim 25, wherein the first
output power comprises about 4.6 kVA, and wherein the second output
power comprises about 4.6 kVA..Iaddend.
.Iadd.28. The system according to claim 25, further comprising a
communication circuit comprises one or a plurality of external
cables..Iaddend.
.Iadd.29. The system according to claim 25, further comprising a
wireless communication circuit..Iaddend.
.Iadd.30. The system according to claim 25, wherein the first
single phase inverter comprises a conductive bridge between the
constant current source and the monitoring circuit..Iaddend.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims Priority from German Application No. DE 10
2006 003 904.1-32 filed on 27 Jan. 2006
FIELD OF THE INVENTION
The invention relates to a method having the features recited in
the preamble of claim 1.
DESCRIPTION OF THE PRIOR ART
Mains connected systems for decentralized energy supply, e.g.,
photovoltaic systems with solar modules and inverters or systems
with fuel cells and inverters are known. For low output, a
single-phase inverter is usually used for single-phase power
supply; for high output, the power supply is three-phased. However,
the use of a three-phase inverter is expensive, since it has less
efficiency and is produced in smaller quantities.
This is the reason why, for high output, one usually uses three
single-phase inverters. However, it suffices a failure of but one
of the inverters, as a result of a short for example, to often
cause asymmetrical power supply exceeding the admissible limit or a
load unbalance to occur as a result of the independent supply.
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to indicate a method that avoids
inadmissible load unbalances using single-phase inverters.
This object is solved with the characterizing features of claim 1
in connection with the features recited in the preamble thereof as
well as with the characterizing features of claim 9 in connection
with the features of the preamble thereof.
The invention relies on the idea consisting in limiting
asymmetrical power supply to an admissible limit and not to switch
off the entire system so that the intact inverters are capable of
proceeding with feeding in the associated phases. In not shutting
down the entire system, the quality of current supply or the
current availability increases. This means that, in accordance with
the invention, the power of the other two inverters is initiated to
an imposed value when only one of the inverter fails, for what
reason so ever, e.g., as a result of a short circuit. This value
may be greater than or equal to zero.
The invention allows for improving the quality of the electric
mains while tolerating an admissible load unbalance, with
single-phase inverters permitting to achieve high efficiency,
modularity and low-cost manufacturing.
In an advantageous developed implementation of the method of the
invention, there is provided that the power of every inverter be
limited durably to 2-20 kVA, more specifically to about 4.6 kVA.
There is preferably provided that the performance of every inverter
be limited temporarily, more specifically for about 10 minutes, to
about 5 kVA. This measure is particularly suited for mains
connected systems for the decentralized energy supply with
inverters having a higher rated output, more specifically having an
output greater than 4.6 kVA.
It is advantageous if the failure of the inverter is detected by a
measurement signal of an additional circuit of the inverter, a
power limiting communication signal resulting from the measurement
signal being provided. The measurement signal simply detects the
failure of one inverter, with such an additional circuit being
housed in an additional mounting plate on each of the inverters.
This allows for utilizing commercially available single-phase
inverters that only need little conversion to implement the method
of the invention.
In order to limit the power of the inverters, there is practically
provided that the communication signal is communicated to the
processors of the inverters in order to limit the power of the
inverters by controlling the processors. The processor or
microprocessor in each of the intact phases receives a command to
limit the power and the inverter may produce the power needed
through a corresponding PWM control for example.
Another advantageous measure is characterized in that the
measurement signal is produced by a current from a constant current
source, said current of said constant current source being in the
milliampere range. Through the low measurement current that may
preferably be produced in each additional mounting plate, it is
possible to put into practice the detection of the failure, using
little additional power or rather with high efficiency. It is
particularly advantageous if, upon failure of the inverter, the
fault is communicated through one or a plurality of external cables
between the inverters. As a result, the independent single-phase
inverters may practically communicate together so that the status
of an inverter is recognized. In case of failure of one or a
plurality of inverters, the inventive power limitation of the other
inverters is realized.
Other advantageous developed implementations of the invention will
become apparent from the dependent claims.
The invention will be better understood upon reading the more
detailed description of the Figures, which describes other
advantages thereof.
In said Figures:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an exemplary embodiment of a circuit arrangement of
the invention for limiting a load unbalance,
FIG. 2 shows a circuit with two groups of single-phase inverters
with a mains contactor and a three-phase voltage monitoring
module,
FIG. 3 shows a circuit with two groups of single-phase inverters
with a mains contactor without three-phase voltage monitoring
module and
FIG. 4 shows a circuit with two groups of single-phase inverters,
the mains being disconnected directly by the inverters.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an example with three single-phase inverters WR1, WR2,
WR3, more specifically with photovoltaic inverters, which supply
three-phase alternating voltage to a low-voltage main system. As
will be shown later, the inverters convert a decentralized direct
voltage, produced by solar modules in particular, into a
standardized low voltage of 230V/400V/50 Hz for example.
Every inverter WR1-WR3 is provided with a circuit that forms what
is referred to as an intertripping or a failure measuring circuit
1. The failure measuring circuit 1 is an additional electronic
circuit that connects the three single-phase inverters WR1-WR3
through an external connection 2. Upon failure of one inverter,
this failure is communicated between the inverters WR1-WR3 through
one or a plurality of external cables 5.
The task of the circuit shown is to detect the failure of an
inverter so that, upon asymmetrical power supply resulting from the
failure of one inverter, the power of the other inverters may be
lowered in order to reduce load unbalance to an admissible limit.
In permanent operation, the individual power of the inverters
WR1-WR3 is to be limited to 4.6 kVA for example.
Each measuring circuit 1 includes a direct current source 3, more
specifically a constant current source. Said source can be
connected additionally through an electrically conductive bridge 4
so that only one of the current sources 3 delivers a measurement
current, as can be seen in FIG. 1. Further, each measuring circuit
includes a connection for a measurement and/or an output signal,
which has not been illustrated in greater detail herein. As a
result, a failure signal or an OK-signal can be communicated to the
other two operative inverters.
It is preferred that every inverter WR1-WR3 be provided with an
additional circuit, each inverter having the same additional
circuit. The circuit has connections for the direct current source
and for the communication lines. One of the current sources
delivers the measurement current for measuring the failure, said
measurement current being a few mA. In FIG. 1, it is the inverter
WR1 that delivers the measurement current. Every inverter however
has the possibility of communicating a failure or an OK signal to
the other two inverters through the output signal. A clearing
signal is only generated if the inverters are ready for supply in
all of the three phases. Then, all the inverters communicate an OK
signal. The communication may be by radio.
In a circuit arrangement that has not been illustrated in greater
detail herein and that is provided in the additional circuit, the
failure of the inverter is registered by a measurement signal. A
communication signal resulting from said measurement signal
regulates the power limitation to a value of 4.6 kVA for example.
For the communication signal is communicated to the processors of
the inverters in order to limit the power of the inverters by means
of processor control or of PWM control for example.
FIG. 2 shows a circuit with two groups GR1, GR2 of single-phase
inverters 11-16 that are connected in parallel for increasing the
power. The solution in groups is lower in cost since single-phase
inverters are manufactured in large quantities and have greater
efficiency.
Each inverter 11-16 is supplied, on its input side, with direct
voltage generated by solar modules 9. One inverter practically
generates alternating current of one phase L1, L2 or L3. Each
inverter is moreover provided with an external communication line
2.
For safety reasons, the system shown usually includes a switching
center with disconnecting function or clearing means 18 that is
accessible to the personnel of the mains operator.
Switching stations may be overground connecting points of a service
tap to the supply mains such as a cable connection box, a cable
distribution cabinet, a transformer station or a service box, as
long as it is unrestrictedly accessible to the personnel of the
mains operator.
The clearing means 18 is connected to a contactor 19 in such a
manner that said contactor 19 is capable of causing the mains to
disconnect through actuation of the clearing means 18. Moreover,
the contactor 19 is coupled to a three-phase voltage monitoring
module 20 so that an additional network limiter is provided. As
shown in FIG. 2, the clearing means 18 is connected to the
three-phase voltage monitoring module 20. This module 20 allows for
disconnecting all the inverters from the mains when the voltage of
one phase exceeds, or falls short of, imposed limit values. Network
limitation is further improved as a result thereof.
Preferably, a plurality of groups of three single-phased inverters
are connected to the output side of a three-phase protection
switchgear (contactor 19), said switchgear being adapted to be
switched off through the manual, external clearing means 18 or
through a voltage monitoring module 20.
In the solution shown in FIG. 3, one has made the economy of the
module 20 so that the installation expense is reduced. The
contactor 19 however is still connected to the clearing means
18.
As shown in FIG. 4, the failure circuit may further be implemented
in such a manner that a signal occurs from outside, for example by
actuation of the clearing means 18, said signal causing the
inverters 11-16 to become disconnected from the mains. This can
obviate the need for the contactor 19, an air break switch
disconnector or another switchgear.
Accordingly, the devices shown for monitoring the mains are
provided with a respective associated switching member or with a
clearing station. With single-phase power supply through one of the
inverters 11-16 to the mains supply system, such a device is
utilized up to a rated output 4.6 kVA, in a three-phased
implementation, up to an output of 30 kVA for PV inverters
(PV=photovoltaic). It may be integrated into the PV inverter or
implemented as an independent protective device. In addition
thereto, the device may include a voltage and frequency monitoring
device and may evaluate as an additional criterion located mains
impedance leaps of a certain order of magnitude. Thanks to these
criteria, namely the redundant implementation of the switching
members and the self-monitoring of the measurement system, the
device is capable of meeting safety demands so that the required
switching center with disconnection function, which has to be
always accessible to the personnel of the mains operator, and the
separate voltage and frequency monitoring device otherwise provided
for preventing decoupling can be simplified.
For PV-systems with rated outputs >30 kVA, both an always
accessible clearing station and a three-phase voltage monitoring
device are needed. On systems with inverters, this is preferably
realized in the following manner:
The system may be disconnected by the contactor 19 in the main
line, which connects the system to the mains connecting point. As
shown, groups of inverters are thereby formed in larger systems.
Each of these groups can be disconnected from the mains by its own
contactor. Savings can thus be made since a high-capacity contactor
is more expensive than a plurality of contactors having a lower
switching capacity. Motor-driven switches are also possible as
switchgears for protecting the mains.
A separate voltage monitoring relay (module 20 and contactor 19)
measuring the voltages in the three phases of the mains connection
may act onto the protector as shown in FIG. 2. If one voltage
leaves the adjusted range, the voltage monitoring relay opens the
contactor 19.
In order to implement the always accessible clearing station a turn
knob snap switch, which also acts onto the mains disconnecting
contactor 19, can be housed on the outside of a house, for example
in a lockable box.
If an inverter for single-phase power supply has a three-phase
mains monitoring device, the voltage monitoring module 20 can be
obviated. If three inverters for single-phase power supply are
distributed over three mains phases and are connected together
using an intertripping or a failure circuit 1 for the other two
inverters to also switch off upon failure of one inverter, this
corresponds to a three-phase voltage monitoring. In this case, the
voltage monitoring relay according to FIG. 2 can be obviated.
The possibility of disconnecting the inverters 11-16 from the mains
through a contact from the outside is, in terms of construction,
easy to combine with the intertripping illustrated in FIG. 2;
therefore the current source 3 of the intertripping (see I.sub.mess
1 in FIG. 2) can be practically switched on and off through an
external switch. The costs incurred by this additional function are
minimal. In this case, the main disconnecting contactor 19 can be
obviated.
In the Figures, there has been described a method of converting a
direct voltage generated by a photovoltaic system into a
three-phase alternating current by means of a plurality of
single-phase inverters, the alternating current being provided for
being supplied to an electric mains supply. In accordance with the
invention, upon failure of one inverter, an asymmetrical power
distribution of the mains electricity supply is reduced by limiting
the output of the other inverters.
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