U.S. patent application number 09/755993 was filed with the patent office on 2001-10-04 for independent load sharing between parallel inverter units in an ac power system.
Invention is credited to Jensen, Uffe Borup.
Application Number | 20010026461 09/755993 |
Document ID | / |
Family ID | 8158876 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010026461 |
Kind Code |
A1 |
Jensen, Uffe Borup |
October 4, 2001 |
Independent load sharing between parallel inverter units in an AC
power system
Abstract
For independent load sharing of current harmonics between
parallel inverter units (1a, 1b, 1c) in an AC power system without
intra-unit control signal communication, where each inverter unit
is connected to a common load (8) through a separate impedance (7a,
7b, 7c), amplitude and phase of a selected current harmonic is
determined for each unit by processing of a current value M
measured in a power output line (51a, 51b, 51c) from the unit
before the separate impedance and is compared with amplitude and
phase of a feed-back voltage or current (V) from the power output
line to produce an error signal, which is used for determination of
a control signal component for the selected harmonic for
incorporation into a control signal input to control means for
controlling the content of said harmonic in the voltage and/or
current supplied by the inverter unit (1a, 1b, 1c) to the common
load (8).
Inventors: |
Jensen, Uffe Borup; (Odense,
DK) |
Correspondence
Address: |
THOMPSON HINE L.L.P.
2000 COURTHOUSE PLAZA , N.E.
10 WEST SECOND STREET
DAYTON
OH
45402
US
|
Family ID: |
8158876 |
Appl. No.: |
09/755993 |
Filed: |
January 5, 2001 |
Current U.S.
Class: |
363/71 |
Current CPC
Class: |
H02M 7/493 20130101;
H02M 3/1584 20130101 |
Class at
Publication: |
363/71 |
International
Class: |
H02M 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 6, 2000 |
DK |
PA 2000 00007 |
Claims
1. A method for independent load sharing of current harmonics
between parallel inverter units (1a, 1b, 1c; 22a, 22b, 22c) in an
AC power system without intra-unit control signal communication,
each of said inverter units comprising a DC to AC converter means
(2; 23) with associated control means (5; 26) for controlling
switching times of said converter means and each being connected to
a common load (8; 29) through a separate impedance (7a, 7b, 7c;
28a, 28b, 28c), said method comprising for each of said units the
steps of measuring at least a current in a power output line (51a,
51b, 51c) from the unit before said separate impedance, processing
the measured current value for determination of amplitude and phase
of a selected current harmonic (I), comparing the amplitude and
phase of the selected current harmonic with amplitude and phase of
a feed-back voltage or current (V) from said power output line to
produce an error signal and using said error signal for
determination of a control signal component for the selected
harmonic for incorporation into a control signal input to said
control means (5; 26) for controlling the content of said harmonic
in the voltage and/or current supplied by the inverter unit (1a,
1b, 1c; 22a, 22b, 22c) to the common load (8; 29) via said separate
impedance (7a, 7b, 7c; 28a, 28b, 28c).
2. A method as claimed in claim 1, wherein said control means (5)
is controlled to supply a PWM control signal for said converter
means.
3. A method as claimed in claim 2, wherein said determination of
amplitude and phase of a selected current harmonic is made by a
discrete Fourier transformation.
4. A method as claimed in claim 2 or 3, wherein said control signal
component is determined to include a phase and amplitude
relationship with respect to the selected current harmonic.
5. A method as claimed in claim 4, wherein said control signal
component is determined to include a phase difference of 90.degree.
and a linear amplitude relationship with respect to the selected
current harmonic.
6. A method as claimed in any of the preceding claims, wherein the
determination of said control signal component comprises
integration of said error signal and back-transformation of the
integrated signal to a time domain.
7. A method as claimed in any of the preceding claims, wherein said
feed-back signal is obtained by measuring a voltage in said power
output line (51a, 51b, 51c) from said inverter unit (1a, 1b,
1c)
8. A method as claimed in any of claims 1 to 6, wherein said
inverter units (22a, 22b, 22c) are controlled to provide active
compensation of load current harmonics by injection of compensating
current components into the load current lines (54) and said
feed-back signal is obtained by measuring said load current.
9. An AC power system comprising a number of inverter units (1a,
1b, 1c; 22a, 22b, 22c) connected in parallel with a common load (8;
29) without intra-unit control signal communication, each of said
inverter units (1a, 1b, 1c; 22a, 22b, 22c) comprising a DC to AC
converter means (2;23) with associated control means (5;26) for
controlling switching times of said converter means and each being
connected to the common load through a separate impedance (7a, 7b,
7c; 28a, 28b, 28c), each of said inverter units further comprising
means for measuring at least a current in a power output line (51a,
51b, 51c) from the unit before said separate impedance, means (15,
16) for processing the measured current value for determination of
amplitude and phase of a selected current harmonic, means for
comparing the amplitude and phase of the selected current harmonic
with amplitude and phase of a feed-back voltage or current from
said power output line to produce an error signal and means (18,
19) for determination from said error signal of a control signal
component for the selected harmonic for incorporation into a
control signal input to said control means (5; 26) for controlling
the content of said harmonic in the voltage and/or current supplied
by the inverter unit (1a, 1b, 1c; 22a, 22b, 22c) to the common load
(8; 29) via said separate impedance.
10. An AC power system as claimed in claim 9, said control means
(5;26) comprises PWM control means.
11. An AC power system as claimed in claim 10, wherein said
processing means (15, 16) comprises discrete Fourier transformation
means.
12. An AC power system as claimed in claim 9, 10 or 11, wherein
said determination means comprises integrating means (18) for
integration of said error signal and inverse discrete Fourier
transformation means (19) for back-transformation of the integrated
signal to a time domain.
13. An AC power system as claimed in any of claims 9 to 12, wherein
said measuring means comprises means for measuring a voltage in
said power output line (51a, 51b, 51c) from said inverter unit (1a,
1b, 1c) to provide said feed-back error signal.
14. An AC power system as claimed in any of claims 9 to 12, further
comprising means for controlling said control means to provide
active compensation of load current harmonics by injection of
compensating current components into the load current lines (54),
said measuring means comprising means for measuring said load
current to provide said feed-back error signal.
Description
[0001] The present invention relates to a method for independent
load sharing of current harmonics between parallel inverter units
in an AC power system and an AC power system comprising a number of
inverter units connected in parallel with a common load without
intra-unit control signal communication.
[0002] In AC power systems of this kind use is typically made of
inverter units that controlled by pulse width modulation can supply
a filtered output voltage to the load, in which both fundamental
frequency and harmonics can be controlled.
[0003] AC power systems comprising a number of such inverter units
connected in parallel are known in the art and may be used as
redundant power supply systems in applications, where maintenance
of electrical power supply is critical, such as electrical power
supply in computer systems, aircraft, radar installations, hospital
systems and similar applications. Typically, the redundancy of such
systems will be provided by the use of two or more units for
continued supply of the common load in situations of failure or
fall-out of a unit, whereby power supply to the load is maintained
by the remaining unit or units.
[0004] In other applications of AC power systems parallel
connection of inverter units may be necessary to maintain a desired
power supply capacity.
[0005] In U.S. Pat. No. 5,745,356 a parallel redundant power supply
system without use of intra-unit signaling is disclosed, in which
the information necessary for load sharing is derived solely from
the output power level of each unit, whereby the output of a unit
is phase-locked to the output of other units connected in parallel
therewith. Power level samples are generated either from a DC input
voltage from the DC input power source or the AC output voltage
from AC output voltage generating means and used for determination
of the AC output power by control of the AC output phase and
frequency by a control signal determined by integration of a
derivative of the output power level of the AC output voltage
generating means as obtained from the power level samples.
[0006] In such a system distribution of load current harmonics
between the individual inverter units is not possible.
[0007] In installations, however, requiring electrical power supply
of non-linear loads, typically a load consisting entirely or partly
of rectifiers, e.g. diode or thyristor rectifiers, such as computer
aircraft and radar installations sharing of load current harmonics
between inverter units is critical. A rectifier load causes
distortion of the current supplied by an inverter and, in result,
the load current will have a content of harmonics in addition to
the fundamental frequency. If such current harmonics are not
distributed evenly between the inverters there will be a risk of
overloading of the individual inverter and, in result a reduction
of its life time.
[0008] In U.S. Pat. No. 5,436,512 a method for active sharing of a
harmonic load between inverters connected in parallel is disclosed.
This method requires electrical connection between the inverter
units in addition to their common connection to the load. Beyond
adding complication and expense such a solution may create problems
with respect to reliability and will not be suitable for use in a
distributed system with individual inverters placed geographically
apart from each other.
[0009] For single inverter units supplying an output voltage which
becomes distorted to an unacceptable extent by load current
harmonics various methods for active limitation or reduction of the
distortion, e.g. by use of the dynamics of the inverter to
compensate for the distortion, have been disclosed in the art.
Thus, U.S. Pat. No. 5,001,619 and U.S. Pat. No. 5,377,092 discloses
methods, by which amplitude and phase relationship for a single
harmonic is determined by discrete Fourier transformation. A vector
representing the amplitude and phase shift of the harmonic is used
as input to an integrating control unit to produce an integrated
reference quantity, which is transformed back to the time domain by
inverse discrete Fourier transformation and added to the control
reference for the inverter. Thereby, the inverter is controlled to
compensate for a load current harmonic such that the output voltage
is not significantly distorted, even by a non-linear load.
[0010] In U.S. Pat. No. 5,383,107 an inverter control unit is
disclosed in which a complex objective function based upon harmonic
content in a minimum number of the phase-to-phase AC voltages
produced by the inverter, e.g. two out of the three phase-to-phase
voltages, is determined to control the switches of the inverter to
minimize the objective function and thus control the harmonic
content in the inverter output voltages.
[0011] In general, these prior art methods for reduction of
distortion cannot be applied to a system of several inverters
connected in parallel. Thus, in compensation methods a fatal
situation may arise, when two or more inverter units counteract
each other, e.g. by an inverter unit generating a compensating
voltage harmonic, which another inverter unit will try to remove.
In result circulating current harmonics may flow from one inverter
unit to the other and, not only from an inverter unit to the load
as desired. Such circulating currents will cause increasing loss
and a reduction of the overall capacity made available to the
load.
[0012] On this background it is the object of the invention to
provide a method and an AC power system of the kind set forth,
which will provide active load sharing of current harmonics between
inverters connected in parallel.
[0013] According to the invention this object is achieved by a
method for independent load sharing of current harmonics between
parallel inverter units in an AC power system without intra-unit
control signal communication, each of said inverter units
comprising a DC to AC converter means with associated control means
for controlling switching times of said converter means and each
being connected to a common load through a separate impedance, said
method comprising for each of said units the steps of measuring at
least a current in a power output line from the unit before said
separate impedance, processing the measured current value for
determination of amplitude and phase of a selected current
harmonic, comparing the amplitude and phase of the selected current
harmonic with amplitude and phase of a feed-back voltage or current
from said power output line to produce an error signal and using
said error signal for determination of a control signal component
for the selected harmonic for incorporation into a control signal
input to said control means for controlling the content of said
harmonic in the voltage and/or current supplied by the inverter
unit to the common load via said separate impedance.
[0014] The method according to the invention may be applied to
control of one or more harmonics in the inverter output voltage by
determination of a control signal component for each such harmonic,
the control signal component being variable from zero to a value
determined by the measured current harmonic.
[0015] For each inverter unit the control signal component may be
determined by methods as disclosed in the art, e.g. in U.S. Pat.
No. 5,001,619 and U.S. Pat. No. 5,377,092 mentioned above.
[0016] Thus, by the method according to the invention the reference
signal for the inverter control unit is modified to adapt the
selected output voltage harmonic to the current in a way so as to
avoid circulating currents between individual inverter units and,
provide equal sharing of load current harmonics between the
individual inverter units.
[0017] Preferred implementations of the method are stated in
dependent claims 2 to 8.
[0018] Further in accordance with the invention an AC power system
is provided, comprising a number of inverter units connected in
parallel with a common load without intra-unit control signal
communication, each of said inverter units comprising a DC to AC
converter means with associated control means for controlling
switching times of said converter means and each being connected to
the common load through a separate impedance, each of said inverter
units further comprising means for measuring at least a current in
a power output line from the unit before said separate impedance,
means for processing the measured current value for determination
of amplitude and phase of a selected current harmonic, means for
comparing the amplitude and phase of the selected current harmonic
with amplitude and phase of a feed-back voltage or current from
said power output line to produce an error signal and means for
determination from said error signal of a control signal component
for the selected harmonic for incorporation into a control signal
input to said control means for controlling the content of said
harmonic in the voltage and/or current supplied by the inverter
unit to the common load via said separate impedance.
[0019] Preferred and advantageous embodiments of such a power
system are stated in dependent claim 10 to 14.
[0020] Application of the method and AC power system of the
invention is not limited to any particular type or number of
inverter units, nor to the number of output voltage phases or any
particular method for determination and control of current and/or
voltage harmonics.
[0021] In the following the invention will be described in more
detail with reference to the accompanying schematical drawings, in
which
[0022] FIG. 1 is a block diagram of an AC power system comprising
three inverter units in a parallel arrangement connected to a
common load;
[0023] FIG. 2 is a block diagram of an inverter unit in the system
shown in FIG. 1 with control means embodying the invention;
[0024] FIG. 3 is a graphic vector representation of a current
harmonic and a corresponding reference voltage component;
[0025] FIG. 4 is an illustration of the relationship between real
and imaginary components of the vectors represented in FIG. 3;
[0026] FIG. 5 is an example of a graphic transient signal waveform
representation by use of the method according to the invention;
[0027] FIG. 6 is a block diagram of a further embodiment of a load
sharing AC power system according to the invention with means for
compensation of current harmonics; and
[0028] FIG. 7 is a block diagram of an inverter unit, on the system
shown in FIG. 6 with control means embodying the invention.
[0029] In the AC power system shown in FIG. 1 each of three
inverter units 1a, 1b and 1c connected in a parallel coupling
comprises an inverter bridge 2, which is supplied with a DC
voltage. The inverter bridge 2 feeds a three-phase transformer 3,
the output voltages of which are supplied to an output filter 4,
e.g. of LC type, attenuating the high frequency content in the
voltages from the inverter bridge 2. The output lines 51a to 51c
from the filters 4 of the individual inverters 1a to 1c are
connected via separate impedances 7a, 7b and 7c and a common
junction 52 to the common load 8. The separate impedances, each of
which may as shown comprise a coil or could alternatively consist
of a piece of cable, are necessary for the individual control of
the output voltage from each of inverter units 1a to 1c.
[0030] As shown for inverter unit 1a only, the voltage and current
in the output line 51a after the filter 4 are measured and the
measured values of the voltage V and the current I are supplied via
lines 9 and 10 to a processing unit 6 for determination of a
control signal component, which is supplied to a pulse width
modulator 5 acting as a control unit for controlling switching
instants of the inverter bridge 2.
[0031] In the following the processing unit 6 and the determination
of the control signal component for a single phase and a single
harmonic will be explained in more detail with reference to FIGS. 2
to 4.
[0032] As shown in FIG. 2 the measured value of the current I
supplied via line 10 is subjected in a block 16 to a discrete
Fourier transformation DFT, as a result of which calculation is
made of the real and imaginary components Re({overscore (I)}.sub.h)
and Im({overscore (I)}.sub.h) of a selected current harmonic
{overscore (I)}.sub.h as shown by the vector representation 11 in
FIG. 3.
[0033] As functions of the real and imaginary components
Re({overscore (I)}.sub.h) and Im({overscore (I)}.sub.h) of the
current harmonic I.sub.h real and imaginary components
Re(V.sub.h,ref) and Im(V.sub.h,ref) of a reference signal component
which is calculated in a processing block 17, as illustrated by the
graphic representation in FIG. 4, in accordance with the equations
1 Re ( V h , ref ) = K h Im ( I _ h ) ; and Im ( V h , ref ) = - K
h Re ( I _ h )
[0034] where K.sub.h is a factor representing a proportional
relationship between the calculated current harmonic Ih obtained
from DFT block 16 and the corresponding reference signal component
V.sub.h for the selected harmonic.
[0035] As result of these calculation a reference signal component
V.sub.h,ref is calculated as shown by the vector representation 12
in FIG. 3. In the example illustrated by the equations above and
the graphic representations in FIGS. 3 and 4 a linear or
proportional amplitude relationship and a phase relationship
corresponding to a phase difference of 90.degree. has been chosen
as a preferred relationship. However, within the scope of the
invention arbitrary phase and amplitude relationship may be
used.
[0036] As further shown in FIG. 2 also the measured value V
supplied via line 9 is subjected in the processing unit 6 to a
discrete Fourier transformation DFT in a block 15, as a result of
which a voltage harmonic vector representation 21 is calculated.
The vector 21 for the voltage harmonic forms part of a feed-back
signal to the processing unit 6. The vector 21 representing the
feed-back voltage harmonic is now in a regulator block 18
subtracted from the vector 12 representing the reference signal
component to form an error signal vector which is supplied to an I
or PI integrator, from which the integrated error signal is
transformed back to the time domain by an inverse discrete Fourier
transformation IDFT in a block 19 to form a control voltage
harmonic, which is added to a feed-back signal supplied from an RMS
control block for control of the fundamental frequency of the
output signal to form the overall control signal to be supplied to
the PWM modulator 5 in FIG. 1.
[0037] In accordance with the example described above the graphic
representation in FIG. 5 illustrates transient phenomena for the
5th harmonic from two inverter units connected in parallel such as
inverter units 1a and 1b in FIG. 1 represented by current harmonic
vectors 11a and 11b and output or feedback voltage harmonic vectors
21a and 21b. As will appear the current harmonics from the two
inverters are shared during the transient situation and shared with
high accuracy in the stationary situation, whereby a significant
reduction of distortion in the output is obtained. This results
from the described phase and amplitude relationship, by which the
voltage harmonic vectors 11a and 11b are controlled to be
phase-shifted 90.degree. in front of the current harmonic vectors,
as represented by vectors 21a and 21b with an amplitude
proportional to the amplitude of the current harmonic vectors.
[0038] In FIG. 6 an alternative embodiment is shown to illustrate
application of the invention to active compensation of harmonics in
a load current by means of a parallel arrangement of inverter
units. In case of a non-linear load 29 the supply voltage from an
AC voltage source 53 is distorted. This distortion is avoided by
injection of current harmonics in counterphase to the load current
harmonics. Without the measures of the invention parallel coupling
of inverter units for such a purpose without intra-unit
communication would lead to a fatal situation, in which circulating
currents between the units cannot be avoided. By application of the
invention the load current is shared between the inverter units
without any need of communication between the units.
[0039] In the arrangement in FIG. 6 three inverter units 22a, 22b
and 22c are connected in parallel, each of which comprises an
inverter bridge 23 supplying a three-phase transformer 24, the
output currents of which are supplied to an output filter 25, which
may be of L-type for damping of the current ripple. In line 54
connecting the load 29 with the AC source 53 the load current is
measured and the measured current value is supplied via line 30 to
the control unit 27 together with the measured current from output
filter 25 supplied via line 31 for determination of the reference
signal component for the PWM modulator 26, which calculates the
switching instants of the inverter bridge.
[0040] As in the arrangement in FIG. 1 each of inverter units 22a
to 22c are connected with the load through a separate impedance,
e.g. in the form of a coil 28a, 28b and 28c, respectively, or
alternatively a piece of cable, and a common junction 40.
[0041] In FIG. 7 the control unit 27 is shown in greater detail
with respect to its operation for a single phase and a single
harmonic. As explained above with reference to FIG. 2 a current
harmonic vector 32 is calculated by discrete Fourier
transformation, DFT in a block 35 and is converted in the block 36
to a reference signal component for the selected harmonic, which is
supplied to the regulator block 37.
[0042] From the measured load current value supplied via line 30
and representing a feed-back signal for regulator unit 37 a current
harmonic vector 39 is determined by discrete Fourier
transformation, DFT in a block 34.
[0043] In the regulator 37 an error vector is determined as the
vector difference between the measured and feed-back current
harmonic vectors 33 and 39 and is inputted to an I or PI integrator
supplying an integrated error signal, which is transformed back to
the time domain by inverse discrete Fourier transformation, IDFT in
a block 38, from which the control voltage harmonic is supplied to
the PWM modulator 26.
[0044] Whereas the method and AC power system of the invention have
been explained with reference to specific embodiments in hardware
implementation it will be appreciated by persons skilled in the art
that within the scope of the invention many deviations will be
possible including, in particular, software implementation of the
processing steps conducted in the inverter units for determination
of a control signal harmonic to influence the PWM control of an
inverter bridge.
* * * * *