U.S. patent application number 10/694650 was filed with the patent office on 2005-04-28 for harmonic neutralized frequency changer.
Invention is credited to Baumgart, Gary E..
Application Number | 20050088861 10/694650 |
Document ID | / |
Family ID | 34435470 |
Filed Date | 2005-04-28 |
United States Patent
Application |
20050088861 |
Kind Code |
A1 |
Baumgart, Gary E. |
April 28, 2005 |
HARMONIC NEUTRALIZED FREQUENCY CHANGER
Abstract
A power converter that produces variable frequency multiphase AC
power from fixed or variable frequency AC power. The converter can
be used to drive an AC motor for propulsion applications or other
motors and loads that require variable frequency AC power. The
power converter system is based on a combination of several power
conversion technologies used in a power circuit topology and a
regulator control system that allows for higher electrical
efficiency, higher power density and lower power distortion to be
achieved than is possible from any of the individual technologies.
Specifically, the input and output power distortion of a frequency
changer is monitored, and a group of high performance inverters are
used to inject harmonic currents into a specially designed
transformer to neutralize the power distortion to a specified
acceptable level. By this neutralization, the power density of a
solid-state electric power converter is increased (e.g., by a
factor of 5-6) and power quality distortion is reduced (e.g., below
0.1%). These features are especially useful in the electric power
conversion markets particularly for surface ship and submarine
propulsion drive applications.
Inventors: |
Baumgart, Gary E.;
(Pittsburgh, PA) |
Correspondence
Address: |
Robert D. Kucler, Esq.
REED SMITH LLP
P.O. Box 488
Pittsburgh
PA
15230-0488
US
|
Family ID: |
34435470 |
Appl. No.: |
10/694650 |
Filed: |
October 27, 2003 |
Current U.S.
Class: |
363/40 |
Current CPC
Class: |
H02M 1/12 20130101; Y02E
40/40 20130101; H02J 3/01 20130101; H02M 5/271 20130101 |
Class at
Publication: |
363/040 |
International
Class: |
H02M 001/12 |
Claims
What is claimed is:
1. A power converter system, comprising: a direct conversion
frequency changer, implemented as an Unrestricted Frequency Changer
or a Matrix Converter, including an input and an output, the
frequency changer adapted to accept an input voltage at an input
frequency at the input and deliver an output voltage at an output
frequency at the output; an input high bandwidth inverter; an
output high bandwidth inverter; an inverter controller adapted to
calculate harmonics at the input and output of the frequency
changer and control the input and output high bandwidth inverters
to generate input and output harmonic cancellation signals; an
input harmonic injection transformer connected to the input
inverter and the input of the frequency changer to inject said
input harmonic cancellation signals; and an output harmonic
injection transformer connected to the output inverter and the
output of the frequency changer to inject said output harmonic
cancellation signals.
2. The power converter system of claim 1, wherein the frequency of
the output voltage is approximately at or above the frequency of
the input voltage.
3. The power converter system of claim 1, wherein the inverter
controller calculates harmonics at the input and output of the
frequency changer from a control algorithm selected from the group
consisting of: fundamental differential harmonic neutralization by
series voltage injection; discrete harmonic neutralization by
series voltage injection; and discrete harmonic neutralization by
current injection.
4. The power converter system of claim 1, wherein said frequency
changer is implemented as a three-phase to three-phase matrix
converter.
5. The power converter system of claim 1, wherein said high
bandwidth inverters have a multilevel cascade H-bridge
topology.
6. The power converter system of claim 1, wherein said high
bandwidth inverters have a multilevel diode-clamped inverter
topology.
7. A power converter system, comprising: a frequency changer with
an input and an output, the frequency changer adapted to accept an
input voltage at an input frequency at the input and deliver an
output voltage at an output frequency at the output; an input high
bandwidth inverter; an output high bandwidth inverter; an inverter
controller adapted to calculate harmonics at the input and output
of the frequency changer and control the input and output high
bandwidth inverters to generate input and output harmonic
cancellation signals; an input harmonic injection transformer
connected to the input inverter and the input of the frequency
changer to inject said input harmonic cancellation signals; and an
output harmonic injection transformer connected to the output
inverter and the output of the frequency changer to inject said
output harmonic cancellation signals, wherein said harmonics
calculated by the inverter controller are input and output voltage
harmonics.
8. The power converter of claim 7, wherein said input harmonic
injection transformer is in series with said frequency changer.
9. The power converter system of claim 7, wherein said frequency
changer is implemented as an Unrestricted Frequency Changer or a
Matrix Converter.
10. The power converter system of claim 7, wherein the inverter
controller calculates harmonics at the input and output of the
frequency changer from a control algorithm selected from the group
consisting of: fundamental differential harmonic neutralization by
series voltage injection; and discrete harmonic neutralization by
series voltage injection.
11. The power converter system of claim 7, wherein said frequency
changer is implemented as a three-phase to three-phase matrix
converter.
12. The power converter system of claim 7, wherein said input and
output high bandwidth inverters comprise a circuit topology
selected from the group consisting of: multilevel cascade H-bridge
and multilevel diode-clamped inverter topologies.
13. The power converter system of claim 12, wherein said input and
output high bandwidth inverters include switching frequencies in
the range of about 40 kHz to about 50 kHz.
14. A power converter system, comprising: a frequency changer with
an input and an output, the frequency changer adapted to accept an
input voltage at an input frequency at the input and deliver an
output voltage at an output frequency at the output; an input high
bandwidth inverter; an output high bandwidth inverter; an inverter
controller adapted to calculate harmonics at the input and output
of the frequency changer and control the input and output high
bandwidth inverters to generate input and output harmonic
cancellation signals; an input harmonic injection transformer
connected to the input inverter and the input of the frequency
changer to inject said input harmonic cancellation signals; and an
output harmonic injection transformer connected to the output
inverter and the output of the frequency changer to inject said
output harmonic cancellation signals, wherein said generated input
and output harmonic cancellation signals are narrow band discrete
harmonic cancellation signals.
15. The power converter system of claim 14, wherein said calculated
harmonics are voltage harmonics.
16. The power converter system of claim 14, wherein said calculated
harmonics are current harmonics.
17. The power converter system of claim 14, wherein said input
harmonic injection transformer is in series with said frequency
changer.
18. The power converter system of claim 14, wherein said input
harmonic injection transformer is in parallel with said frequency
changer.
19. The power converter system of claim 14, wherein said frequency
changer is implemented as an Unrestricted Frequency Changer or a
Matrix Converter.
20. The power converter system of claim 15, wherein said frequency
changer is implemented as a three-phase to three-phase matrix
converter.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to frequency changer
circuits for electric motor systems, and more specifically, the
present invention relates to the calculation, generation,
application and control of currents to neutralize unwanted harmonic
frequencies in conventional frequency changer circuits.
[0003] 2. DESCRIPTION OF THE BACKGROUND
[0004] In many diverse environments, electric motors are used as
drive or propulsion systems. The requirement for low noise and low
distortion power conversion systems has made the multilevel PWM
(pulse width modulation) technology a strong contender for many
different electric motor drive applications. However, the
efficiency and power density of this technology is limited by the
need for two to three stages of power conversion. Specifically,
power must first be converted from AC to DC and then from DC back
to AC by accommodating the PWM technology.
[0005] These multistage conversions hinder system efficiency and
lower power density. For example, the losses in the conversion
processes typically range from 2-3% of applied power. Moreover, the
power density for the PWM multilevel inverter has traditionally
hovered in the range of from 0.5 to just over 1.0 MW/m.sup.3. In
fact, when all of the equipment required for the total conversion
process, (e.g., transformers, rectifiers, inverters and filters)
are included in the power density calculation, the system power
density is typically in the range of 0.5-0.75 MW/m.sup.3.
[0006] In many applications, these losses are not acceptable. For
example, any wasted power in "onboard" or self-sustained systems
such as ships and submarines significantly decreases propulsion
system performance. Where power is at a minimum, the present
invention finds its most effective applications.
[0007] In addition to these specific propulsion applications,
improvements in the power density via a reduction in losses is
continually sought in all electric motor arts. As such, the present
invention preferably provides a system architecture capable of
improving the power density of electric motors over traditional PWM
systems by a factor of at least 3-6 times and reducing system
losses to about 1%.
SUMMARY OF THE INVENTION
[0008] In accordance with at least one preferred embodiment, the
present invention provides a frequency changer circuit that
compensates and corrects for unwanted harmonic frequencies that are
characteristic of convention frequency changers. Specifically, the
present invention includes the calculation and application of
signals that cancel the unwanted harmonic frequencies that
traditionally exist at both the input and output ends of a
conventional frequency changer.
[0009] Specifically, the present invention includes a conventional
frequency changer adapted to change AC power at an input frequency
to output power at a different frequency (single phase or
multiphase). Because unwanted harmonic frequencies appear at both
the input and output of this frequency changer, the present
invention injects signals via injection transformers at the input
and output of the frequency changer in order to cancel these
unwanted harmonics.
[0010] A multilevel DC link controller and DC link are connected to
an input and output high bandwidth PWM inverter to produce the
compensating signals. The outputs of the inverters are filtered and
then applied to the input and output of the frequency changer with
specially-designed transform and controller circuits. These
components may all optionally be built right into the existing
motor circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For the present invention to be clearly understood and
readily practiced, the present invention will be described in
conjunction with the following figures, wherein like reference
characters designate the same or similar elements, which figures
are incorporated into and constitute a part of the specification,
wherein:
[0012] FIG. 1 shows a system block diagram for the advanced
harmonic neutralized frequency changer of the present
invention;
[0013] FIG. 2 depicts a general three-phase to three-phase
frequency changer;
[0014] FIG. 3 depicts a three-phase to three-phase matrix
converter;
[0015] FIG. 4 details three exemplary configurations (4A-4C) of
bi-directional switches;
[0016] FIG. 5 details an exemplary clamped-diode inverter;
[0017] FIG. 6 shows an exemplary DC link controller;
[0018] FIG. 7 shows an exemplary frequency changer controller;
[0019] FIG. 8 shows an exemplary input inverter controller; and
[0020] FIG. 9 depicts various harmonic injection transformers
including in series with the frequency changer (FIG. 9A) and in
parallel with the frequency changer (FIG. 9B).
DETAILED DESCRIPTION OF THE INVENTION
[0021] It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements
that are relevant for a clear understanding of the invention, while
eliminating, for purposes of clarity, other elements that may be
well known. Those of ordinary skill in the art will recognize that
other elements are desirable and/or required in order to implement
the present invention. However, because such elements are well
known in the art, and because they do not facilitate a better
understanding of the present invention, a discussion of such
elements is not provided herein. The detailed description will be
provided hereinbelow with reference to the attached drawings.
[0022] In at least one preferred embodiment, the present invention
comprise a novel system architecture that will reduce system losses
in electric motors to less than 1% and increase the power density
by a factor of 3 to 6 times better than basic multilevel PWM power
converters. The architecture is described as a "harmonic
neutralized frequency changer." FIG. 1 depicts a general block
diagram of the system architecture according to the present
invention, and each part of the system will be described in turn
below.
[0023] The power converter design (100) of FIG. 1 is based on a
unique topology that employs a direct conversion frequency changer
to convert the source AC power to variable frequency output power.
In a practical application, the source AC power can be a generator
and the output power can be applied to a motor load, but this is
only an exemplary component selection for purposes of describing
the present invention.
[0024] These types of power converters are very efficient and have
high power densities compared to DC link power converters. However,
direct power converters generate a rich spectrum of unwanted
harmonics in the input and output current. These generated
harmonics are a function of the generator frequency and the
variable output frequency. The power converter topology 100 of the
present invention shown in FIG. 1 neutralizes the dominant harmonic
currents in both the input and output by injecting neutralizing
harmonic currents generated by high bandwidth inverters 105, 106
while only demanding modest power capability. By use of the present
invention, the total harmonic current distortion is preferably less
than 0.1% of rated current, and the worst-case single harmonic
current is preferably less than 0.05% of the rated current.
[0025] As seen in FIG. 1, the traditional path from an input
voltage (Vi) to the output voltage (Vo) is accomplished through a
conventional frequency changer 110. The present invention, however,
adds harmonic injection transformers at both the input 115 and
output 116 of the frequency changer. These harmonic injection
transformers 115, 116 are used to inject signals that neutralize
the unwanted harmonics that exist at the input and output of the
frequency changer 110. The neutralizing harmonics are generated by
high bandwidth PWM inverters 105, 106 controlled by a multilevel DC
link controller 130 as described below.
[0026] In order to facilitate a better understanding of the present
invention, each of the major components of the harmonic neutralized
frequency changer system 100 will now be presented. Although many
component alternatives are presented, each of these components are
provided by way of example only, and one skilled in the art can
easily envision additional alternatives within the scope of the
present disclosure.
[0027] Frequency Changer
[0028] Most electronic power converters can be characterized as
some sort of frequency changers. For example, a simple rectifier
circuit converts multiphase input AC power to one-phase output
power with a frequency of zero. Similarly, a simple inverter
converts one-phase input power with a frequency of zero to
multiphase AC output power with a specified frequency. Extending
these concepts, the general frequency changer 110 converts AC input
power with a specified non-zero frequency to AC output power with a
specified non-zero frequency.
[0029] There are two basic circuit topologies for a three-phase AC
to three-phase AC frequency change. The general three-phase to
three-phase frequency changer, shown in FIG. 2, uses a combination
of bi-directional switch modules 200 in order to accomplish the
frequency change. Likewise, the three-phase to three-phase matrix
converter, shown in FIG. 3, uses a simplified combination of
bi-directional switch modules 210 to accomplish the frequency
change.
[0030] There are a variety of direct power converter technologies
that utilize these general circuit topologies that can be
functionally applied in this invention. Some of these circuit
topologies, all of which utilize bi-directional switches and are
common in the art, include: (1) the Naturally Commutated
Cycloconverter (NCC); (2) the Unrestricted Frequency Changer (UFC);
(3) the Unity Displacement Factor Frequency Changer (UDFFC); (4)
the Controllable Displacement Factor Frequency Changer (CDFFC); and
(5) the Matrix Converter (MC).
[0031] In each of these technologies, the output frequency and
voltage can be controlled continuously and adjusted independently
of each other. Further, the output power equals the input power
allowing for minor internal losses. Any of these frequency changer
topologies can be applied in this invention. However, the NCC, UFC
and MC are, for a number of reasons, most appropriate for motor
drive applications. The NCC can be effectively applied when the
output frequency is less than the input frequency. The UFC and MC
are good choices when the output frequency approaches or exceeds
the input frequency.
[0032] The controlled bi-directional switch 200, 210 used in these
topologies must be able to carry current in both the forward and
reverse direction, and the switch must be able to be turned on at
specific phase angles with respect to the input voltage source.
Partially controlled switches are adapted to be turned on at any
angle but are naturally commutated off by source voltage. Fully
controlled switches, on the other hand, are adapted to be turned on
or off at any angle. There are a variety of implementations for
bi-directional switches. Some of the most practical implementations
of bi-directional switches are shown in FIG. 4. Specifically, FIG.
4A shows a fully controlled IGBT bi-directional switch module, FIG.
4B shows a fully controllable GTO bi-directional switch module, and
FIG. 4C shows a partially controlled thyristor bi-directional
switch module.
[0033] Some key exemplary characteristics and control limitations
of each of these power converter technologies are given in Table I
(below).
1TABLE I Characteristics of Direct AC Power Converters Number of
Switches for Power Output Output Type of Three Converter Frequency
Voltage Source Power Modulation Power Phase Type Range Range Factor
Function Switch System NCC 0 .ltoreq. fo .ltoreq. fi 0 .ltoreq. Vo
.ltoreq. Vmax .theta..sub.i = .vertline. f (Vo, .theta..sub.o)
.vertline. Periodic Bi- 18 directional Partially Controlled UFC 0
.ltoreq. fo .ltoreq. .infin. 0 .ltoreq. Vo .ltoreq. Vmax
.theta..sub.i = .+-. .theta..sub.o Linear Bi- 18 (PWM) directional
Fully Controlled UDFFC 0 .ltoreq. fo .ltoreq. fi 0 .ltoreq. Vo
.ltoreq. Vmax .theta..sub.i = 0 Periodic Bi- 18 directional Fully
Controlled CDFFC 0 .ltoreq. fo .ltoreq. fi 0 .ltoreq. Vo .ltoreq.
Vmax - .theta. max .ltoreq. .theta..sub.i .ltoreq. .theta. max
Periodic Bi- 18 .theta. max = f (Vo, .theta..sub.o) directional
Fully Controlled MC 0 .ltoreq. fo .ltoreq. .infin. 0 .ltoreq. Vo
.ltoreq. Vmax/2 .theta.i = .+-. .theta..sub.o Linear Bi- 9 (PWM)
directional Fully Controlled
[0034] All frequency changers consist of an array switches that
allow a multiplicity of input AC voltages to be switched to a
multiplicity of output phases to construct a controlled AC output
voltage where output voltage and output frequency can be
controlled. In some cases the source power factor can also be
controlled. The main difference between the various types of
frequency changers is in the method of synthesizing the output
waveform.
[0035] The unique characteristics of the various types of frequency
changers are determined by the sequence of switching and a
modulation function applied to the array of switches. Two
modulation functions are typically used in frequency changers to
generate sinusoidal output voltage. These two functions are the
linear (triangular) function given by: M(t)=arc
sin(sin(.omega..sub..smallcircle.t+.PSI.)), and the periodic
function given by: M(t)=arc
sin(r*sin(.omega..sub..smallcircle.t+.PSI.)), where r is the ratio
of the output voltage amplitude to the input voltage amplitude.
[0036] At the center of the problem addressed by the present
invention, a by-product of the output voltage waveform synthesis is
a broad spectrum of unwanted frequencies. These unwanted harmonic
components will appear as currents in both the input and output of
the frequency changer. The frequency spectrum is complex and varies
for each type of frequency changer. In addition, the magnitude and
frequency of the unwanted harmonics change with the output load,
voltage and frequency. Therefore, simple filtering techniques are
not effective in attenuating these unwanted frequencies.
[0037] In general, the output voltage for the frequency changer (of
whatever type) will include a fundamental voltage component plus:
(1) harmonics of the output frequency; (2) third order harmonic
components of the input frequency; and (3) sideband harmonics. The
output voltage for a three-phase output frequency changer operating
into a balanced symmetrical load can thus be generalized by the
following expression: 1 V o = 3 3 V i [ sin ( o t + o ) + m = 2
.infin. a m sin ( m o t + m o ) + n = 1 .infin. b n sin ( 3 n i t )
+ n = 1 .infin. m = 1 .infin. { c mn sin ( 3 n i t 2 m o t 2 m o )
+ d mn sin ( 3 n i t m o t m o ) } ]
[0038] where .psi. is the phase voltage displacement angle with
respect to the input voltage reference.
[0039] In addition to the output harmonics, the input current for
the frequency changer will also contain a spectrum of unwanted
frequencies. The input current will contain the fundamental current
component for each of the output phases and the sideband harmonic
frequencies for each of the output phases. We can generalize the
input current for a three phase output frequency changer with a
balanced symmetrical load by the following expression: 2 I i = 1 3
I o [ sin ( o t + o - 2 3 ) + sin ( o t + o - 4 3 ) ] + 1 I o [ m =
1 .infin. a m { sin ( m i t + ( m + 1 ) o t + o ) - sin ( m i t + (
m - 1 ) o t + o ) } ] + 1 I o [ m = 0 .infin. a m { sin ( m i t + (
m + 1 ) o t + o - ( m + 1 ) 2 3 ) - sin ( m i t + ( m - 1 ) o t + o
- ( m - 1 ) 2 3 ) } ] + 1 I o [ m = 1 .infin. a m { sin ( m i t + (
m + 1 ) o t + o - ( m - 1 ) 4 3 ) - sin ( m i t + ( m - 1 ) o t + o
- ( m - 1 ) 4 3 ) } ]
[0040] In high performance power converter applications where good
input and output power quality is a requirement, the unwanted
harmonics must be removed or neutralized. The generation of these
neutralizing signals is the function of the input and output high
bandwidth inverters 105, 106 shown in FIG. 1.
[0041] High Bandwidth PWM Inverter
[0042] There are a variety of different inverter circuit topologies
that may be applied to the concepts of the present invention and
that are understood by those skilled in the art. For example, the
multilevel cascade H-bridge and the multilevel diode-clamped
inverter are two exemplary topologies. The necessary requirement
for the PWM inverter circuit 105, 106 is that the inverter must
have a high bandwidth in order to neutralize currents in the range
of the harmonics generated by the frequency changer 110. This will
require effective switching frequencies in the range of about 40 to
50 kHz--well above the range obtainable by basic IGBT
inverters.
[0043] Typical IGBTs can be switched in the range of about 5 to 10
kHz and in some case as high as 20 kHz. To obtain the required
switching frequency for the present invention, IGBT switching
information must be multiplexed between multiple devices.
Multiplexing can be accomplished in the distributed voltage domain
or in the time domain. The aforementioned multilevel inverters are
used to accomplish the former. The latter time domain multiplexing
is not discussed herein.
[0044] The inverter function does not generate or consume real
power except for losses within the inverter itself. However, these
losses may be significant because of the high switching frequency.
Moreover, switching losses may contribute as much as 80% of the
total inverter losses.
[0045] The rating of the inverter will be based on the reactive
power required to neutralize the harmonics in the input and output
current. Fortunately, this will be a small fraction of the total
power rating of the system. This fact weighs heavily on the
improved performance of the present invention. For example, the
reactive current that must be generated by the inverter to
neutralize the input harmonics of an NCC frequency changer is given
by: 3 I iq = 3 3 4 Io n = 0 .infin. a n cos ( 2 n o ) 4 n 2 - 1 cos
( i t ) where n = 0 .infin. a n 4 n 2 - 1 converges rapidly .
[0046] As an exemplary (but not limiting) embodiment, a multilevel
clamped-diode inverter 105, 106 is shown in FIG. 5 for clarity.
FIG. 5 details one inverter phase for the input inverter and the
output inverter connected back-to-back. The advantage of this
topology is the input and output neutralization inverters can be
connected back-to-back and share a common DC link (see also FIG.
1). Therefore, fewer controlled DC link power sources are required
when compared with the cascade H-bridge topology. The number of
controlled supplies will equal (n-1) where n is the level number
for the inverter. The level number is selected so that the basic
switching frequency for the inverter IGBT module multiplied by
2(n-1) exceeds 40 kHz (i.e., f.sub.s*2(n-1)>40,000).
[0047] As briefly described above, the inverter output will
preferably be controlled by pulse width modulation (PWM) of the
inverter power switches. This form of modulation produces sideband
harmonics centered on the effective switching frequency. The
inverter filters 125, 126, shown in FIG. 1, attached to the
inverter output 105, 106 must be applied to attenuate these
harmonics. The reactive power of the harmonic spectrum is
proportional to the product of the IGBT switching frequency and the
square of the total DC link voltage-divided by 4(n-1).sup.2. The
size of the filter 125, 126 is therefore increased linearly with
the switching frequency and reduced by the square of the inverter
level number. For example, a five-level inverter 105, 106 will
require a filter 125, 126 only {fraction (1/16)} the size of the
filter for a two-level inverter switching at the same effective
frequency.
[0048] An ideal filter will not absorb any of the harmonic energy
but will effectively circulate the harmonic currents between the
filter and the power source where it is dissipated. Minimizing the
energy of the switching frequency harmonics will reduce these
losses and improve the efficiency of the system. The filter 125,
126 is thus designed to reduce the switching harmonics to the
required distortion level (typically .about.0.1%). The active
control bandwidth will be approximately 0.1 to 0.2 times the
effective switching frequency of the inverter 105, 106. Therefore,
the filter attenuation factor must be the total DC link voltage
times 1/(n-1).multidot.10.sup.3 per decade. This requires a 2 to 4
pole filter to achieve the attenuation, and the response should be
flat up to the break point set above the active control
bandwidth.
[0049] DC Link Controller
[0050] Multiple controlled DC power supplies are required to
operate the inverters 105, 106; however, the exact number is
dependent on the type of inverter selected for the system. The DC
link controller 130 must provide power to balance the losses of the
inverters 105, 106. Except for these losses, the net real power
requirement for the inverters 105, 106 is zero. However, certain
components in the rectifier circuit must be rated to carry current
between adjacent rectifier bridges to transfer energy between
adjacent levels to maintain a constant voltage at each level under
changing conditions.
[0051] A controlled regenerative rectifier is provided for each
inverter level as specified above. Each rectifier will consist of a
dual full-wave bridge and an inductor to control the DC ripple
current. Each rectifier will be independently self-controlled with
an inner current loop and outer voltage loop. The bandwidth of the
current loop is preferably high (>1000 rad/s) to maintain good
voltage regulation. FIG. 6 shows a typical implementation of the DC
link controller 130, with an exploded view of the regenerative
rectifier.
[0052] Power Converter Regulator Control System
[0053] The power converter regulator control system can be divided
into three independent sections: (1) the Frequency Changer
Controller; (2) the Input Inverter Controller; and (3) the Output
Inverter Controller.
[0054] The function of the frequency changer controller is to
control the fundamental current and the bulk power transfer of the
power converter system. FIG. 7 details an exemplary embodiment of
the frequency changer controller. The controller is structured as a
current controller that can be applied with outer control loops for
frequency, voltage, flux, torque and speed. The outer loops feed
into the independent direct and quadrature current references shown
in FIG. 7. The flexibility of this controller allows it to be
applied to motor drives as well as power conditioning systems.
[0055] In this exemplary embodiment, the frequency changer gate
controller is slaved to the source voltage reference by a phase
lock loop. Signal sampling and processing are also synchronized to
the source voltage. The controller applies reference
transformations to demodulate the converter fundamental current to
a DC level in a rotating coordinate system synchronous with the
output fundamental frequency. The control facilitates independent
d-q current control of the fundamental current only. The harmonics
are removed from the signal by the demodulation process and the
natural roll-off in gain for the P-I current controller. Whereas
the frequency changer 110 controls the bulk power of the system,
the input 105 and output 106 inverters control the injected
harmonic currents to neutralize the harmonically corrupted bulk
power. The inverters 105, 106 need only be sized to provide
reactive power that allows the inverters to be rated at a fraction
of the system rating.
[0056] There are several control strategies that can be applied to
the present invention. Briefly, some main control strategies can be
summarized as: (1) fundamental differential harmonic neutralization
by series voltage injection; (2) fundamental differential harmonic
neutralization by current injection; (3) discrete harmonic
neutralization by series voltage injection; and (4) discrete
harmonic neutralization by current injection. Each of these control
strategies can be applied to both the input and output inverter
functions. For the purpose of this exemplary embodiment, the
fundamental differential harmonic neutralization by series voltage
injection method will be described. The controller is shown in FIG.
8 for the input inverter. Only small differences in nomenclature
differentiate it from the output inverter.
[0057] The input inverter PWM controller is slaved to the source
voltage reference by a phase lock loop. Signal sampling and
processing are also synchronized to the source voltage. The
controller applies reference transformations to demodulate the
converter fundamental current to a DC level in a rotating
coordinate system synchronous with the power converter output
fundamental frequency. The harmonic components are removed from the
d-q fundamental currents by filtering. The resulting signal is then
transformed into the .alpha.-.beta. coordinate plane. The
fundamental .alpha.-.beta. currents are subtracted from the total
current .alpha.-.beta. components. The resulting signal contains
the harmonic current information. This is then transformed back
into the d-q plane and regulated to zero.
[0058] This control strategy will not be effective for higher
frequency harmonic components. For those cases, the discrete
harmonic neutralization strategy can be used. Individual harmonics
can be independently controlled. The individual harmonic
frequencies are calculated based on the equations presented above.
The control is only limited by the effective switching frequency of
the inverter and the number of harmonics to be included. In most
situations, 90% of the harmonic current THD is from the 24 most
prominent harmonics.
[0059] Harmonic Injection Network
[0060] As shown at a high level in FIG. 1, the harmonic currents or
voltage generated by the input 105 and output 106 inverters must be
injected (at 115 and 116) into the frequency changer 110 input and
output circuits to neutralize the unwanted harmonics. There are two
main methods used to accomplish the injection. The first method is
to inject a voltage in series with the frequency changer generated
voltage, and the second method is to inject a current in parallel
with the frequency changer generated current.
[0061] The first of these solutions is depicted in FIG. 9A and the
second solution is shown in FIG. 9B. The injection network
components 115, 116 can be separate transformers for power
conditioner applications, or in the case of motor drives and ship
propulsion applications, the injection network components can be
incorporated as compensation windings in the power source generator
and the motor. Existing systems have never combined the injection
network components into the generator and motor.
[0062] System Performance and Applications
[0063] According to computer simulation, the total harmonic
distortion of the frequency changer system may be reduced
dramatically. This invention provides high power conversion
efficiency by reducing the number of electric conversions to just
one. Other benefits include the reduction in the volume and weight
of the power converter, and a highly responsive means to control
the current harmonics in the generator and propulsion motor to very
low levels as compared to current systems.
[0064] Alternative Discrete Harmonic Neutralization
[0065] The above-described inverter controller methodology was
based on wideband harmonic neutralization based on fundamental
discrimination. An alternative method of discrete harmonic
neutralization based on pre-calculated harmonic spectrum for the
power converter will now be described.
[0066] The control concept applies equally to input-side and
output-side inverters. However, the description provided in this
disclosure will apply to the output-side inverter. The harmonic
injection can be implemented as a shunt current into the output to
cancel the resident harmonic currents. (Similar to the concept
shown in FIG. 8).
[0067] The inverter controller is synchronized to the frequency
changer reference by a phase-lock-loop. Signal sampling and
processing are also synchronized to the source voltage. For this
configuration, frequency changer current must be the observed
control signal. Therefore, the frequency changer current is sampled
in the natural multiphase stationary reference system. These
signals are then applied to a series of reference transformations
to demodulate the converter fundamental current to a DC level in a
rotating coordinate system synchronous with the output fundamental
frequency.
[0068] An appropriate transformation for converting multi-phase
stationary reference signals to two-phase signals is used. The
stationary reference signal two-phase signal is subsequently
transformed to two-phase rotating reference signals without any
loss of system state information. However, the transformation is
rotated at the frequency of a specified harmonic signal. The result
of this transformation is a signal equal to the specified harmonic
as observed in the rotating reference.
[0069] The harmonic signals are applied to a pair of proportional
plus integral controllers that independently regulate inverter the
direct and quadrature current components by generating voltage
references for the inverter. The voltage references are two-phase
rotating reference signals that must be transformed back to the
multiphase stationary reference format using the same
transformations from above. The individual harmonic signals for
each phase in the stationary reference are subsequently summed to
generate a reference signal for each phase.
[0070] The multiphase, stationary reference signals are sent to the
PWM Inverter controller that generates the appropriate level of
harmonic voltage or harmonic current for injection into the load
circuit. In this process the inverter in association with a reactor
becomes a harmonic current source. At the point of injection the
harmonic signals are inverted and cancel the resident harmonic
components.
[0071] This process is applied to a specified number of the
dominant harmonics produced by the frequency changer. The value of
"n" for the specified harmonics can be calculated for any specified
type of frequency changer for a specified operating condition. If
the most dominant (e.g., six to twelve) harmonics are neutralized
by this process the output distortion of the frequency changer can
be reduced to any specified level by adding additional harmonic
controller components.
[0072] Although described by exemplary embodiments, the present
invention may be applied to virtually any power conditioning or
electric motor drive application. However, the greatest advantages
will be realized in applications where power density and power
quality must be maximized. Propulsion and pump drives for ships and
submarines are examples of applications that greatly benefit from
high power density and high power quality.
[0073] Nothing in the above description is meant to limit the
present invention to any specific materials, geometry, or
orientation of elements. Many part/orientation substitutions are
contemplated within the scope of the present invention and will be
apparent to those skilled in the art. The embodiments described
herein were presented by way of example only and should not be used
to limit the scope of the invention.
[0074] Although the invention has been described in terms of
particular embodiments in an application, one of ordinary skill in
the art, in light of the teachings herein, can generate additional
embodiments and modifications without departing from the spirit of,
or exceeding the scope of, the claimed invention. Accordingly, it
is understood that the drawings and the descriptions herein are
proffered only to facilitate comprehension of the invention and
should not be construed to limit the scope thereof.
* * * * *