Electric power conversion systems – Current conversion – Having plural converters for single conversion
Utility Patent
2000-03-21
2001-01-02
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
Utility Patent
active
06169679
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to methods and system for synchronizing the phase angles of parallel connected inverters and, more particularly, to a method and system for transmitting phase angle information between inverters in a parallel connected inverter system to synchronize the phase angles of the inverters.
BACKGROUND ART
An inverter is a device which converts direct current (DC) electricity to alternating current (AC) electricity. Inverters are often used to power AC devices in automobiles. In practice, multiple AC voltage sources are needed to be connected in parallel to meet the higher power output requirements of vehicle devices or loads.
FIG. 1
illustrates an electric system
10
having a plurality of inverters
12
a
,
12
b
, and
12
n
and a load
14
. Inverters
12
are connected in parallel between one end
18
of load
14
and ground
16
. The other end
20
of load
18
is connected to ground
16
. Each of inverters
12
a
,
12
b
, and
12
n
includes a respective AC voltage source
22
a
,
22
b
, and
22
n
connected to a respective inductance source
24
a
,
24
b
, and
24
n
. Each AC voltage source
22
provides an output voltage vector v
i
and an output inductance L
i
, where i=a, b, . . ., n. The voltage vector v
i
has an associated voltage magnitude and phase angle. The phase angle varies from 0° to 360°. AC voltage sources
22
are configured from electronic voltage source inverters (VSIs).
The phase angle difference among parallel voltage sources can cause circulating real power among these voltage sources. The circulating real power reduces the power capability and efficiency of these voltage sources. The circulating real power may also cause malfunction of the inverters. To eliminate and minimize the circulating real power it is necessary to eliminate and minimize the phase angle difference between the inverters.
FIG. 2
illustrates a parallel system
30
having two inverters
32
a
and
32
b
. Each inverter
32
has a respective AC voltage source
34
a
and
34
b
and an inductance source
36
a
and
36
b
. AC voltage sources
34
a
and
34
b
provide respective output voltage vectors v
1
, and v
2
.
FIG. 3
illustrates the phase angle difference &dgr; between the two voltage vectors v
1
and v
2
. To reduce the circulating real power it is desired to reduce the phase angle difference &dgr; to the smallest value possible.
The circulating real power between output voltage vectors v
1
and v
2
is determined by:
circulating real power =V
1
*V
2
*sin(&dgr;)/(2&pgr;
f
)/(L
1
+L
2
)
where f is the fundamental operation frequency of inverters
32
, V
1
and V
2
are the root-mean-square (rms) value of the AC output voltage of v
1
and v
2
, respectively.
There is no circulating real power if the phase angles are the same, i.e., &dgr;=zero. Therefore, it is desired to eliminate and minimize the phase angle differences between inverters in a parallel connected voltage source inverter system.
DISCLOSURE OF INVENTION
Accordingly, it is an object of the present invention to provide a method and system for transmitting phase angle information between inverters in a parallel connected inverter system to synchronize the phase angles of the inverters.
In accordance with the object and other objects, the present invention provides a method for transmitting phase angle information between inverters in a parallel connected inverter system to synchronize the phase angles of the inverters. The inverter system has a master inverter and a plurality of server inverters connected in parallel. Each inverter has a voltage source. Each of the voltage sources generate a voltage having an associated phase angle which varies as a function of time in a phase cycle.
The method includes transmitting a common phase angle signal to each of the server inverters. The common phase angle signal is a function of the phase angle of the master inverter. The common phase angle signal is indicative of the absolute phase angle of the master inverter when the phase angle of the master inverter is equal to a phase angle value associated with a new phase cycle. The common phase angle signal is indicative of an incremental phase angle change of the phase angle of the master inverter in a given time period while the phase angle of the master inverter is different from the value associated with a new phase cycle.
The method preferably further includes controlling the phase angle of each of the server inverters in accordance with the common phase angle signal such that the phase angles of the master inverter and the server inverters are synchronized. The phase angle of each of the server inverters is adjusted to be equal to the absolute phase angle of the master inverter when the common phase angle signal is indicative of the absolute phase angle of the master inverter. The phase angle of each of the server inverters is adjusted to be equal to the summed value of 1) the incremental phase angle change of the phase angle of the master inverter in the given time period and 2) the phase angle of the respective server inverter prior to the given time period when the common phase angle signal is indicative of the incremental phase angle change of the master inverter.
Preferably, the common phase angle signal includes an absolute phase angle data format indicative of the absolute phase angle of the master inverter and an incremental phase angle data format indicative of an incremental phase angle change of the phase angle of the master inverter in a given time period. The absolute phase angle data format includes a flag set high and the incremental phase angle data format includes a flag set low. The absolute angle data format and the incremental phase angle data format are data bit formats having a fixed amount of data bits.
Further, in accordance with the object and other objects, the present invention provides an inverter system for carrying out the method of the present invention. The inverter system includes a master inverter having a voltage source. The voltage source of the master inverter provides a voltage having an associated phase angle which varies as a function of time in a phase cycle. The inverter system further includes a plurality of server inverters each having a voltage source. Each of the voltage sources of the server inverters provide a voltage having an associated phase angle which varies as a function of time in a phase cycle. The master inverter and the server inverters are connected in parallel.
The inverter system further includes a controller associated with the master inverter. The controller is operable for transmitting a common phase angle signal to each of the server inverters. The common phase angle signal is a function of the phase angle of the master inverter. The common phase angle signal is indicative of the absolute phase angle of the master inverter when the phase angle of the master inverter is equal to a phase angle value associated with a new phase cycle. The common phase angle signal is indicative of an incremental phase angle change of the phase angle of the master inverter in a given time period while the phase angle of the master inverter is different from the value associated with a new phase cycle.
REFERENCES:
patent: 3675037 (1972-07-01), Hamilton
patent: 4328429 (1982-05-01), Kublick
patent: 4565953 (1986-01-01), Espelage et al.
patent: 5001620 (1991-03-01), Smith
patent: 5016158 (1991-05-01), Matsui et al.
patent: 5225973 (1993-07-01), Patel et al.
patent: 5257180 (1993-10-01), Sashida et al.
Deng Doug D.
Wang Kon-King Mike
Ford Motor Company
May Roger L.
Riley Shawn
Sparschu Mark S.
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