U.S. patent application number 15/449243 was filed with the patent office on 2017-11-09 for inductive power transfer system.
The applicant listed for this patent is Auckland Uniservices Limited. Invention is credited to Udaya Kumara MADAWALA, Duleepa Jayanath THRIMAWITHANA.
Application Number | 20170323721 15/449243 |
Document ID | / |
Family ID | 45441695 |
Filed Date | 2017-11-09 |
United States Patent
Application |
20170323721 |
Kind Code |
A1 |
MADAWALA; Udaya Kumara ; et
al. |
November 9, 2017 |
INDUCTIVE POWER TRANSFER SYSTEM
Abstract
An inductive power transfer OPT system) includes an AC-AC
full-bridge converter (T.sub.p1-T.sub.p4) provided between the
primary conductive path (L.sub.pt) and an alternating current power
supply (V.sub.in). The system may include a controller for
controlling the pick-up device to shape the input current drawn
from the alternating current power supply (V.sub.in).
Inventors: |
MADAWALA; Udaya Kumara;
(Manukau, NZ) ; THRIMAWITHANA; Duleepa Jayanath;
(Auckland, NZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Auckland Uniservices Limited |
Auckland |
|
NZ |
|
|
Family ID: |
45441695 |
Appl. No.: |
15/449243 |
Filed: |
March 3, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13807436 |
Apr 22, 2013 |
9653207 |
|
|
PCT/NZ2011/000124 |
Jun 30, 2011 |
|
|
|
15449243 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M 3/33561 20130101;
H02J 2310/48 20200101; H02J 50/12 20160201; H01F 38/14 20130101;
H02J 5/005 20130101; H02M 7/797 20130101; H02J 7/00712 20200101;
H02M 2001/007 20130101; H02J 50/10 20160201; H02J 7/025
20130101 |
International
Class: |
H01F 38/14 20060101
H01F038/14; H02M 7/797 20060101 H02M007/797; H02M 3/335 20060101
H02M003/335; H02J 5/00 20060101 H02J005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2010 |
NZ |
586526 |
Claims
1. An inductive power transfer (IPT system) comprising a primary
conductive path adapted to provide a magnetic field for reception
by a pick-up device, and; an AC-AC full-bridge converter provided
between the primary conductive path and an alternating current
power supply to provide a controlled current to the primary
conductive path for provision of the magnetic field.
2. An IPT system as claimed in claim 1, further comprising a
controller for controlling the pick-up device to shape an input
current drawn from the alternating current power supply.
3. An IPT system as claimed in claim 2 wherein the controller
modulates the pick-up device to shape the input current drawn from
the alternating current power supply.
4. An IPT system as claimed in claim 2 wherein the pick-up includes
a full-bridge converter having two pairs of complementary switches,
and the controller controls the pick-up device by controlling a
phase angle between the pairs of complementary switches.
5. An IPT system as claimed in claim 1 wherein the alternating
current power supply comprises a mains utility power supply.
6. An IPT system as claimed in claim 1 wherein the IPT system
comprises a bi-directional IPT system.
7. An IPT system as claimed in claim 1, wherein the AC-AC
full-bridge converter connects the alternating current power supply
to the primary conductive path to provide a current in the primary
conductive path having a frequency which is greater than a
frequency of the alternating current power supply when power is
being transferred to the pick-up device.
8. An IPT system as claimed in claim 1, wherein the AC-AC
full-bridge converter connects the primary conductive path to the
alternating current power supply to provide a current to the
alternating current power supply having a frequency which is less
than a frequency of the current in the primary conductive path when
power is being transferred to a alternating current power
supply.
9. A primary circuit for an IPT system, the primary circuit
comprising: a primary conductive path adapted to provide a magnetic
field for reception by a pick-up device; and an AC-AC converter
provided between the primary conductive path and an alternating
current power supply.
10. A primary circuit for an IPT system as claimed in claim 9
wherein the alternating current power supply comprises a mains
utility power supply.
11. A primary circuit for an IPT system as claimed in claim 9
wherein the primary circuit comprises part of a bi-directional IPT
system.
12. A primary circuit for an IPT system as claimed in claim 9
wherein the AC-AC converter connects the alternating current power
supply to the primary conductive path to provide a current in the
primary conductive path having a frequency which is greater than a
frequency of the alternating current power supply when power is
being transferred to an IPT pick-up device.
13. A primary circuit for an IPT system as claimed in claim 9
wherein the AC-AC converter connects the primary conductive path to
the alternating current power supply to provide a current to the
alternating current power supply having a frequency which is less
than a frequency of the a current in the primary conductive path
when power is being transferred from an IPT pick-up device to the
alternating current power supply.
14. A method for controlling an inductive power transfer (IPT)
system having an AC-AC full-bridge converter provided between an AC
power supply and a primary conductive path, the method comprising:
controlling a pick-up device of the IPT system to shape an input
current drawn from an alternating current power supply.
15. A method as claimed in claim 14, further comprising modulating
operation of the pick-up device to shape an input current drawn
from the alternating current power supply.
16. A method as claimed in claim 14, wherein the pick-up device
includes a full-bridge converter having two pairs of complementary
switches, and the method comprises controlling the pick-up device
by controlling a phase angle between the pairs of complementary
switches.
17. A method as claimed in claim 14 wherein the alternating current
power supply comprises a mains utility power supply.
18. A method as claimed in claim 14 wherein the IPT system
comprises a bi-directional IPT system.
19. A method as claimed in claim 14, further comprising controlling
the AC-AC full-bridge converter to connect the alternating current
power supply to the primary conductive path to provide a current in
the primary conductive path having a frequency which is greater
than a frequency of the alternating current power supply when power
is being transferred to the pick-up device.
20. A method as claimed in claim 14, further comprising controlling
the AC-AC full-bridge converter to connect the primary conductive
path to the alternating current power supply to provide a current
to the alternating current power supply having a frequency which is
less than a frequency of a current in the primary conductive path
when power is being transferred to the alternating current power
supply.
21. (canceled)
22. (canceled)
23. (canceled)
Description
REFERENCE TO RELATED APPLICATIONS
[0001] The present disclosure is based on and claims benefit from
U.S. patent application Ser. No. 13/807,436 filed on Dec. 28, 2012
which is based on PCT Publication Number WO 2012/005607, which
corresponds to International Application Number PCT/NZ2011/000124
filed on Jun. 30, 2011 which claims benefit from New Zealand
application 586526 filed Jun. 30, 2010, the entire contents of each
of which are herein incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to Inductive Power Transfer (IPT)
systems, and has particular, but not necessarily exclusive,
application to bi-directional IPT systems.
BACKGROUND
[0003] Sustainable generation, transmission, distribution and
utilization of energy have all become a priority for addressing
global concerns in relation to both depletion and irresponsible use
of fossil fuel reserves. Encouragements with intensives for wider
exploitation of renewable resources can be considered as an
integral part of this mission. As a result, over the past several
years many large renewable energy plants have been built and
incorporated into the main power network. This trend soon changed
in favour of decentralized energy generation or sometimes referred
to as distributed generation (DG). More recently, DG systems became
Green Energy (GE) systems being solely based on renewable or Green
energy sources through which more economic, environmental and
sustainability benefits can be achieved. A GE system, which
typically derives power from wind, solar or bio-gas, is operated at
either medium or low power levels and allows the energy to be
consumed or grid-connected at or near the point of generation. A
medium power GE system is usually capable of supplying power for
industry, large offices and community complexes, whilst a low power
GE unit would be of a power level that is adequate to power either
grid-connected or stand-alone houses, farms, lighthouses and
telecommunications facilities.
[0004] Power generation through GE system is unpredictable in
nature due mainly to the dependence of renewable energy sources on
climate conditions. Some form of energy storage is therefore an
essential and integral part of most, if not all, GE systems as it
allows both storage and retrieval of energy when necessary.
Electric Vehicles (EVs) have recently emerged as one way forward
for clean or green transport, and also means for addressing energy
fluctuations in the power network. The latter became popular as
vehicle-to-grid (V2G) power. Although EVs are primarily considered
as a method of clean transport, they can also be used in GE systems
to supplement the energy storage, and such systems have been
referred to as `Living & Mobility`. Irrespective of the
application, an EV essentially requires some form of a power
interface to the grid or power supply to charge its battery
storage. In situations, where the battery storage of an EV is used
for both V2G and G2V applications, or to supplement an existing
battery storage as in the case of `Living & Mobility`, the
power interface should necessarily be bi-directional to allow for
both charging and discharging of the vehicle. A hard-wired power
interface between the EV and the grid is simple and can be used to
either charge or discharge batteries but such wired interfaces are
now considered to be inconvenient and inflexible, and pose safety
concerns. Wireless or contactless power interfaces have thus become
an attractive alternative for charging and/or discharging EVs.
Amongst the existing wireless power transfer technologies,
Inductive power Transfer (IPT) is a key technology that has widely
been accepted as suitable for charging/discharging EVs or V2G and
G2V applications
[0005] IPT systems produce voltages and currents at a much higher
frequency in contrast to low grid frequency. Therefore existing IPT
systems essentially require an additional low-frequency DC-AC
converter stage for grid integration with bi-directional power
flow.
[0006] The additional converter stage with a DC link capacitor
significantly increases the system cost and complexity, and reduces
the efficiency and reliability.
OBJECT
[0007] It is an object of the present invention to at least
ameliorate one or more of the disadvantages of the prior art, or to
at least provide the public with a useful alternative.
SUMMARY
[0008] Accordingly in one aspect the invention provides an
inductive power transfer (IPT system) comprising: [0009] a primary
conductive path adapted to provide a magnetic field for reception
by a pick-up device, and; [0010] an AC-AC full-bridge converter
provided between the primary conductive path and an alternating
current power supply to provide a controlled current to the primary
conductive path for provision of the magnetic field.
[0011] Preferably the system includes a controller for controlling
the pick-up device to shape the input current drawn from the
alternating current power supply.
[0012] Preferably the controller modulates the pick-up device to
shape the input current drawn from the alternating current power
supply.
[0013] Preferably the pick-up includes a full-bridge converter
having two pairs of complementary switches, and the controller
controls the pick-up by controlling the phase angle between the
pairs of complementary switches.
[0014] Preferably the alternating current power supply comprises a
mains utility power supply.
[0015] Preferably the system comprises a bi-directional IPT
system.
[0016] Preferably the AC-AC converter connects the alternating
supply to the primary inductive path to provide a current in the
primary conductive path having a frequency which is greater than
the frequency of the alternating current supply when power is being
transferred to the pick-up device.
[0017] Preferably the AC to AC converter connects the primary
conductive path to the alternating current supply to provide a
current to the alternating current supply having a frequency which
is less than the frequency of the current in the primary conductive
path when power is being transferred to the alternating current
supply.
[0018] In a further aspect the invention provides a primary circuit
for an IPT system, the primary circuit including a primary
conductive path adapted to provide a magnetic field for reception
by a pick-up device, and an AC-AC converter provided between the
primary conductive path and an alternating current power
supply.
[0019] Preferably the alternating current power supply comprises a
mains utility power supply.
[0020] Preferably the primary circuit comprises part of a
bi-directional IPT system.
[0021] Preferably the AC-AC converter connects the alternating
supply to the primary inductive path to provide a current in the
primary conductive path having a frequency which is greater than
the frequency of the alternating current supply when power is being
transferred to an IPT pick-up device.
[0022] Preferably the AC to AC converter connects the primary
conductive path to the alternating current supply to provide a
current to the alternating current supply having a frequency which
is less than the frequency of the current in the primary conductive
path when power is being transferred from an IPT pick-up device to
the alternating current supply.
[0023] In a further aspect the invention provides a method for
controlling an inductive power transfer (IPT) system having an AC
to AC full-bridge converter provided between an AC power supply and
a primary conductive path, the method comprising:
[0024] controlling a pick-up device of the IPT system to shape the
input current drawn from the alternating current power supply.
[0025] Preferably the method includes modulating the operation of
the pick-up device to shape the input current drawn from the
alternating current power supply.
[0026] Preferably the pick-up includes a full-bridge converter
having two pairs of complementary switches, and the method includes
controlling the pick-up by controlling the phase angle between the
pairs of complementary switches.
[0027] Preferably the alternating current power supply comprises a
mains utility power supply.
[0028] Preferably the system comprises a bi-directional IPT
system.
[0029] Preferably the method includes controlling the AC-AC
converter to connect the alternating supply to the primary
inductive path to provide a current in the primary conductive path
having a frequency which is greater than the frequency of the
alternating current supply when power is being transferred to the
pick-up device.
[0030] Preferably the method includes controlling the AC-AC
converter to connect the primary conductive path to the alternating
current supply to provide a current to the alternating current
supply having a frequency which is less than the frequency of the
current in the primary conductive path when power is being
transferred to the alternating current supply.
[0031] In another aspect, the invention broadly consists in a
primary circuit for an IPT system, the primary circuit including a
primary conductive path adapted to provide a magnetic field for
reception by one or more pick-up devices, a matrix bridge converter
provided between the primary conductive path and an alternating
current power supply, and a control means adapted to control
switches of the converter to connect the alternating current supply
to the primary conductive path.
[0032] Preferably the control means controls complimentary switches
of the bridge-matrix converter to provide two voltages, one voltage
being applied to one end of the primary conductive path and the
other voltage being applied to the other end of the primary
conductive path, the control means providing a controlled phase
delay between the two voltages so as to control the voltage applied
to the primary conductive path.
[0033] In one embodiment the primary conductive path includes one
or more reactive elements.
[0034] Preferably the primary conductive path comprises an LCL
network, or an appropriate var or reactive energy compensation
network.
[0035] In a further aspect the invention broadly provides a method
for controlling an IPT system including a bridge-matrix converter
provided between a AC power supply of the and a magnetic field
producing or receiving circuit whereby complementary switches of
the bridge-matrix converter are controlled to provide first and
second voltages, the first and second voltage being provided to the
field producing or receiving circuit, and the control means
controlling the phase difference between the voltages so as to
control the current in the field producing or receiving
circuit.
[0036] Preferably the field producing or receiving circuit
comprises an LCL circuit.
[0037] Preferably, the IPT system as set forth in any one of the
preceding statements may comprise a multiphase IPT system.
[0038] Preferably, the matrix converter as set forth in any one of
the preceding statements may comprise a multiphase matrix
converter.
[0039] Advantageously, use of a multiphase IPT system results in
lower power losses and lower current ripple.
[0040] Preferably, the IPT system as set forth in any one of the
preceding statements may include multiple primary supplies and/or
or primary conductive paths, and/or multiple pick-ups and/or
multiple pick-up windings.
[0041] Preferably, the IPT system according to any one of the
preceding statements includes a primary and/or pick-up which may be
an active load or a passive load. Therefore, for example, the
primary maybe connected to an AC load.
[0042] Preferably, the IPT system according to any one of the
preceding statements can be used in both stand-alone and
grid-connected modes.
[0043] Further aspects of the invention will become apparent from
the following description.
[0044] For convenience the term "matrix converter" is used in this
document. This term is intended to refer to any type of single
phase (or where appropriate polyphase) full-bridge AC-AC
converter.
[0045] The invention thus provides a technique that allows for
direct integration of an IPT system to the grid without an
additional converter stage. This is attractive and more appropriate
than existing systems which use a low frequency DC-AC converter
stage. This document therefore proposes a novel single-stage IPT
power interface that is suitable for direct grid integration. The
proposed IPT grid interface utilizes a matrix converter to
eliminate an additional low frequency power conversion stage. Such
a matrix converter based IPT topology or a control strategy has not
been previously disclosed or suggested. Mathematical analysis and
simulation results are presented for a single-phase bi-directional
IPT system for example, to show that the proposed technique is
viable and requires a simple control strategy to effectively
control both direction and amount of power flow. Without an
additional power conversion stage, the IPT power interface is low
in cost, low in power losses and ideal for wireless charging and
discharging of single or multiple EVs or V2G applications. Although
the invention is described by way of example with reference to a
bi-directional IPT system, those skilled in the art will appreciate
that the invention is also applicable to uni-directional systems.
The invention may also be implemented in polyphase systems.
DRAWING DESCRIPTION
[0046] One or more embodiments of the invention will be described
further below by way of example with reference to the accompanying
drawings, in which:
[0047] FIG. 1 is a conventional grid connected bi-directional IPT
system
[0048] FIG. 2 is a bi-directional IPT system according to one
embodiment of the present invention
[0049] FIG. 3 is an equivalent circuit model of the system of FIG.
2
[0050] FIG. 4 is a diagram of normalised magnitude spectrum of
current in the primary of the system of
[0051] FIG. 2 for two different phase angles implemented in the
matrix converter.
[0052] FIGS. 5 and 6 are diagrams of possible bi-directional
switches for use in a matrix converter.
[0053] FIG. 7 is a diagram of a generalised matrix converter based
IPT system which facilitates direct AC to AC conversion.
[0054] FIG. 8 is a switching pattern diagram.
[0055] FIG. 9 is a diagram of harmonic currents at the mains input
for a converter as shown in the previous drawings.
[0056] FIG. 10 is one example of a pick-up controller.
[0057] FIG. 11 shows one example of a primary controller.
[0058] FIGS. 12 and 13 are plots of primary and pick-up line
voltages and track currents.
[0059] FIG. 14 is a plot of power and ripple currents.
[0060] FIGS. 15 and 16 are plots of primary and pick-up line
voltages and track currents
[0061] FIG. 17 is a plot of power and ripple currents.
[0062] FIGS. 18 and 19 are plots of primary and pick-up line
voltages and track currents
[0063] FIG. 20 is a plot of power and ripple currents.
[0064] FIG. 21 is a table illustrating Switching Patterns according
to embodiments of the present disclosure.
[0065] FIG. 22 is a table illustrating Design Parameters according
to embodiments of the present disclosure.
DESCRIPTION OF ONE OR MORE EMBODIMENTS
[0066] A Typical Grid-Connected IPT System
[0067] A typical grid connected bi-directional IPT system is
schematically shown in FIG. 1. As illustrated in the diagram, the
primary IPT circuit, which comprises a full-bridge converter
(commonly known as an active front end, reversible rectifier or a
controlled rectifier) and a tuned LCL circuit, derives power from
the DC bus and generates a track current in the primary conductive
path or track L.sub.pt, which is loosely coupled to the pick-up
winding (L.sub.st) or the secondary pick-up circuit. The output of
the pick-up circuit can be connected to an active load that is
capable of consuming or generating power, which is represented as a
DC supply in FIG. 1. The primary and pick-up circuits are
implemented with virtually identical electronics, which include a
full-bridge converter and a tuned LCL circuit, to facilitate
bidirectional power flow between the primary supply and the pick-up
load. Each LCL circuit is tuned to the track frequency, which is
generated by the primary full-bridge converter and is typically
around 10-50 kHz. Both full-bridge converters are operated at the
same frequency either in the inverting or rectifying mode,
depending on the direction of the power flow. Voltages and phase
angle between the full-bridge converter will determine the amount
and direction of power flow. Although the word "track" is used in
this document to refer to the primary conductive path which is
primarily represented by L.sub.pt, this may take a variety of
physical forms, for example, it may consist of a winding or an
elongate loop, or multiple windings. The primary conductive path
may also include additional elements such as L.sub.pi and C.sub.pt.
i.e. it may comprise the LCL circuit.
[0068] As evident from FIG. 1, an additional full-bridge converter
stage, indicated as "grid inverter", is used to interface the IPT
converter to the utility grid. The grid inverter is controlled to
maintain a constant DC bus voltage either by extracting power from
the grid or delivering power to the grid. When the IPT supply is
delivering power to the load, the grid inverter functions as an
active rectifier, whereas when the power flow is reversed it works
as an inverter generating power at grid frequency. The introduction
of a separate grid inverter creates switching losses and requires a
sophisticated control subsystem. Furthermore, the grid inverter
requires a large inductor (L.sub.f1) to regulate the ripple current
drawn or supplied to the grid and a significantly large DC bus
capacitance to minimize voltage ripples. Consequently, conventional
grid connected IPT system are significantly more expensive, have
higher losses and tend to be bulky.
[0069] Proposed Matrix Converter Based IPT System
[0070] The shortcomings of a conventional grid connected IPT power
interface can be alleviated by employing a matrix converter, which
replaces both the grid and primary side full-bridge converter of
the IPT system in FIG. 1. Both the primary and pick-up can either
be an active source or a passive load. Note that in this situation
the EV or the pick-up output is represented by a battery as an
active source. A schematic of this proposed IPT topology is
depicted in FIG. 2. Since the primary side full-bridge converter is
directly connected to the utility grid, bi-directional switches
T.sub.p1-T.sub.p4 are used to drive the primary LCL circuit of the
proposed system at a suitable track frequency. Moreover, the
proposed topology eliminates the need for large and expensive
DC-bus capacitors. An inductor between the grid and the primary
converter is not required to control the power flow between the
grid and load of this topology, due to inherent current sourced
nature of the proposed IPT system. However, a smaller n-filter
network may be desirable at the input to attenuate the high
frequency switching noise, generated by the matrix converter to an
acceptable level.
[0071] Steady State Analysis
[0072] According to FIG. 2, the matrix converter produces a
symmetrical bipolar square wave voltage V.sub.pi to drive the LCL
resonant circuit of the primary supply at a suitable track
frequency, f.sub.T. A square wave voltage V.sub.pa that has an
approximate magnitude of |V.sub.insin(.omega..sub.Lt| at 50% duty
cycle is generated by switching bi-directional switches T.sub.p1
and T.sub.p3 at frequency f.sub.T. Similarly, a voltage V.sub.pb
that is delayed in phase by .phi..sub.1 radians with respect to
V.sub.pa is generated by using switches T.sub.p2 and T.sub.p4. The
phase delay between V.sub.pa and V.sub.pb is controlled to regulate
the average voltage that appears across the LCL resonant circuit. A
phase delay of 0 degrees corresponds to a short-circuit across
V.sub.pi whereas a phase delay of 180 degrees corresponds to
maximum V.sub.pi. As such fundamental and harmonics of V.sub.pi are
a function of .phi..sub.1 and the input voltage
|V.sub.insin(.omega.t)| as given by,
V pi = - V in sin ( .omega. L t ) 4 .pi. n = 1 , 3 .infin. 1 n cos
( n .omega. T t + n .PHI. 1 2 ) sin ( n .PHI. 1 2 ) ( 1 )
##EQU00001##
where .omega..sub.L is the mains angular frequency and
.omega..sub.T=2.pi.f.sub.T.
[0073] An equivalent circuit model that can be used to analyze the
steady state operation of this converter is illustrated in FIG. 3.
The LCL circuits in the primary and the pick-up are both tuned to
the track frequency (f.sub.t) and therefore,
.omega. T 2 = ( 2 .pi. f T ) 2 = 1 L p t C p t = 1 L pi C p t = 1 L
st C s t = 1 L si C s t ( 2 ) ##EQU00002##
[0074] Therefore ignoring the induced voltage V.sub.pr due to
I.sub.st in the receiving coil L.sub.st, the track current I.sub.pt
can be given by,
I ^ pt = - j V ^ pi .omega. L pt ( 2 - .omega. 2 L p t C p t ) ( 3
) ##EQU00003##
[0075] An expression for I.sub.pt in terms of V.sub.in can be
obtained by substituting (1) in (3) as given below,
I p t = - V in sin ( .omega. L t ) 4 .pi. n = 1 , 3 .infin. { 1 n 2
.omega. T L p t ( 2 - n 2 ) .times. sin ( n .omega. T t + n .PHI. 1
2 ) sin ( n .PHI. 1 2 ) } ( 4 ) ##EQU00004##
[0076] The normalized magnitude spectrum of I.sub.pt for two
different .phi. values is shown in FIG. 4. It is evident from FIG.
4 that harmonics of I.sub.pt are significantly attenuated by the
LCL tuned circuit. Furthermore the 3.sup.rd harmonic of I.sub.pt is
nullified when the phase delay between V.sub.pa and V.sub.pb is 120
degrees. For example, when the phase delay is 120 degrees the
largest harmonic current generated is approximately 55 dB smaller
than the fundamental of the track current. Similarly it can be
shown that the harmonic currents of I.sub.st are significantly
smaller than the fundamental. Therefore to simplify the analysis,
I.sub.pt and I.sub.st are assumed to be ideal sinusoidal currents.
If I.sub.pt and I.sub.st are considered to be ideal sinusoidal
currents, then the voltages induced in the track and pick-up
inductor, denoted by V.sub.pr and V.sub.sr, respectively, are
sinusoidal voltages with a frequency of f.sub.T. Thus I.sub.pt and
I.sub.st are independent of the induced voltages and can be given
by,
I p t = - V in sin ( .omega. L t ) 4 .pi. .omega. T L p t sin (
.omega. T t + .PHI. 1 2 ) sin ( .PHI. 1 2 ) ( 5 ) I st = - V out 4
.pi. .omega. T L st sin ( .omega. T t + .theta. + .PHI. 2 2 ) sin (
.PHI. 2 2 ) ( 6 ) ##EQU00005##
where .theta. is the relative phase difference between V.sub.pi and
V.sub.si, which is used to control the direction and the magnitude
of power flow.
[0077] The induced voltages on the primary and the pick-up are
given by,
V pr = - V out 4 M .pi. L st cos ( .omega. T t + .theta. + .PHI. 2
2 ) sin ( .PHI. 2 2 ) ( 7 ) V sr = - V in sin ( .omega. L t ) 4
.pi. L p t cos ( .omega. T t + .PHI. 1 2 ) sin ( .PHI. 1 2 ) ( 8 )
##EQU00006##
[0078] The input current drawn by the primary is affected by both
V.sub.pi and V.sub.pr, and is given by,
I ^ pi = 1 j .omega. L p t ( 1 - .omega. 2 L p t C 2 - .omega. 2 L
p t C V ^ pi - V ^ pr ) ( 9 ) ##EQU00007##
[0079] Substituting (1) and (7) in (9) results in,
I pi = - V out 4 M .pi. .omega. T L p t L st sin ( .omega. T t +
.theta. + .PHI. 2 2 ) sin ( .PHI. 2 2 ) - V in sin ( .omega. L t )
.times. 4 .pi. .omega. T L p t n = 1 , 3 .infin. 1 - n 2 n 2 ( 2 -
n 2 ) sin ( n .omega. T t + n .PHI. 1 2 ) sin ( n .PHI. 1 2 ) ( 10
) ##EQU00008##
[0080] From (1) and (10) it can be seen that only the terms with
the fundamental track frequency contribute to real power flow from
V.sub.pi. The output power averaged over a single switching cycle
of the track frequency can be given by,
P o = M .omega. T L p t L st 8 .pi. 2 V in sin ( .omega. L t ) V
out sin ( .theta. ) sin ( .PHI. 1 2 ) sin ( .PHI. 2 2 ) ( 11 )
##EQU00009##
[0081] The average power flow into the IPT system over one cycle at
grid frequency can therefore be given by,
P o , avg = M .omega. T L p t L st 16 .pi. 3 V in V out sin (
.theta. ) sin ( .PHI. 1 2 ) sin ( .PHI. 2 2 ) ( 12 )
##EQU00010##
[0082] From (12) it is evident that maximum power transfer takes
place when the phase delay .theta. between the primary and pick-up
full-bridge converter is .+-.90.degree.. A leading phase angle
constitutes power transfer from the pick-up to the grid while a
lagging phase angle enables power transfer from the grid to the
pick-up. Furthermore, the magnitude of the power transferred
between the grid and the load can be regulated by controlling
.phi..sub.1 and .phi..sub.2, the phase shift in switches of the
primary and pick-up full-bridge converters respectively. Therefore,
for a given input and output voltage, both the amount and direction
of power flow between the track and the pick-up can be regulated by
controlling either the magnitude or phase angle of the voltage
generated by the primary and pick-up full-bridge converters.
[0083] Implementation of the Converter
[0084] As depicted in FIG. 1, both the primary and the pick-up of
the IPT system consist of a full-bridge converter which drives a
tuned LCL circuit. Since the pick-up is supplying a DC load, a
standard full-bridge converter is utilised in the pick-up to
regulate the power transfer. However, the switches of the primary
full-bridge converter T.sub.p1-T.sub.p4 are bi-directional
switches, which can be realised using standard IGBTs/MOSFETs as
indicated in FIG. 5. Bi-directional IGBT switch modules are
available from a few manufacturers. The control algorithm discussed
below is based on an IPT system that uses a Matrix converter with
bi-directional switches as indicated in FIG. 6. For this
discussion; the top switch of the AC switch is named T.sub.pxa and
the bottom switch is named T.sub.pxb where x is the switch number.
Therefore for example T.sub.p1 would be made up of two switches
T.sub.p1a and T.sub.p1b. However this can be extended to cater for
any alternative implementations of AC-AC converters. Although this
example illustrates the use of this concept for a single-phase
single-pick-up bi-directional IPT system, it will be apparent to
those skilled in the art that this can be extended to cater for
three-phase or/and multi-pick-up unidirectional/bi-directional
systems. A generalised diagram of a matrix converter based IPT
system is illustrated in FIG. 7. As can be seen from FIG. 7, the
single primary circuit may be loosely coupled to a single pick-up,
or to multiple pick-ups.
[0085] Control Algorithm
[0086] The primary inverter/rectifier, which is a matrix converter,
is operated to generate a suitable track current at the tuned
frequency f.sub.T. In case of a single pick-up system the track
current can be variable allowing it to optimize the track current
with load to minimize losses. However in multi-pick-up systems a
constant track current may be preferred to supply all the pick-up
loads optimally. The output voltage V.sub.pi produced by the matrix
converter to drive the LCL resonant tank is controlled either
through a PWM or a phase modulation strategy to regulate the track
current I.sub.pt accordingly. Although PWM techniques help reduce
the harmonic contents in V.sub.pi the switching losses may be
elevated due to high switching frequencies. Phase-modulation allows
the converter switches to be operated at f.sub.T thereby reducing
switching losses but harmonic content in V.sub.pi is significantly
higher. The discussion presented here is based on phase-modulation
but can be easily adopted to suit PWM switching schemes.
[0087] As alluded to above, in phase-modulated control,
complimentary switches of the matrix converter T.sub.p1 and
T.sub.p3 are operated as a pair to produce a voltage V.sub.pa
whereas complimentary switches T.sub.p2 and T.sub.p4 are operated
as a pair to produce a voltage V.sub.pb. Both V.sub.pa and V.sub.pb
are square-wave signals with a frequency of f.sub.T and a duty
cycle of 50%. The output voltage V.sub.pi is the difference between
V.sub.pa and V.sub.pb and thus can be regulated by changing the
relative phase between V.sub.pa and V.sub.pb. If the phase
difference between V.sub.pa and V.sub.pb is .phi..sub.1 then the
output voltage produced by the matrix converter can be given by
equation (1).
[0088] Therefore it can be seen that a phase difference of 180
degrees corresponds to maximum V.sub.pi whereas a phase delay of 0
degrees corresponds to 0 V across V.sub.pi. The track current
I.sub.pt is related to V.sub.pi and thus I.sub.pt can be regulated
to a desired value by controlling the phase difference between
V.sub.pa and V.sub.pb, .phi..sub.1. In case of an LCL compensated
primary as illustrated in FIG. 1 the track current can be given by
equation (5) above.
[0089] The matrix converter does not provide inherent current
freewheeling paths. Therefore in addition to phase-modulated
control of V.sub.pi, during commutation of the bi-directional
switches, the control algorithm should be capable of providing
forced freewheeling paths for the current to flow. The proposed
control scheme monitors the full-bridge converter current I.sub.pi
and input voltage V.sub.in and decides the switching pattern as
summarised in Table 1 (see FIG. 21). As evident from the table the
switching pattern of the converter during the positive half cycle
of the input voltage is the inverse of the pattern used during the
negative half cycle of the input voltage thereby avoiding the 180
degree phase transition that could occur in the track current.
[0090] FIG. 8 below illustrates the switching pattern of the
switches Tp.sub.1 and Tp.sub.3 for both positive and negative
currents in the matrix converter. As evident from FIG. 8 the
proposed switching pattern provides two freewheeling paths for the
current during each switch commutation.
[0091] The pick-up full-bridge converter, which will be supplying a
DC load, will be controlled using the same phase-modulation
technique to regulate the pick-up inductor current I.sub.st. If the
phase delay between the two switch pairs in the pick-up is
.phi..sub.2 then the current I.sub.st produced by the pick-up
full-bridge converter is given by equation (6) above. In (6) the
phase-shift .theta. is the phase difference between the primary and
pick-up converters voltages V.sub.pi and V.sub.si.
[0092] Under the above conditions the input current supplied by the
primary full-bridge converter can be given by equation (10)
above.
[0093] Thus the power transferred between the grid and the pick-up
load can be calculated and is given in equation (11). As evident
from (11), the direction and magnitude of power flow can be
regulated by controlling the phase-shift .theta.. A leading
phase-shift constitutes power transfer from the pick-up to the grid
while a lagging phase angle enables power transfer from the grid to
the pick-up. Furthermore, maximum power transfer between the grid
and the pick-up load takes place when the phase-shift .theta.
between the primary and pick-up full-bridge converter is .+-.90
degrees and under this condition the reactive power
supplied/received by the grid is ideally zero. Thus in some
situations it is advantageous to operate the IPT system at a fixed
phase-shift of .+-.90 degrees that is determined by the direction
of power transfer, and control the magnitude of power flow by
regulating either/both .phi..sub.1 or/and .phi..sub.2.
[0094] This system will produce a significant amount of mains
harmonic currents at the input if both the primary and the pick-up
are operated with fixed steady state values of .phi..sub.1 and
.phi..sub.2 as the input current under such conditions is nearly a
square-wave. This can be resolved by operating either/both primary
or/and pick-up full-bridge converters with variable .phi..sub.1 and
.phi..sub.2 to shape the input current drawn by the system. In
particular, an unexpected benefit of the use of an AC-AC converter
between the AC supply and the primary conductive path L.sub.pt is
that the pick-up full-bridge converter (T.sub.s1-T.sub.s4) can be
used to shape the input current drawn from the AC supply to which
the AC-AC converter is connected. This cannot be achieved with the
prior art converter topologies used in IPT systems since they
require the presence of a DC capacitor between the alternating
current power supply and the primary conductive path. FIG. 9
illustrates the harmonic contents of the input current drawn by
this proposed IPT system when the pick-up phase delay .phi..sub.2
is modulated to shape the input current to follow the grid voltage.
As evident from the diagram the THD produced by this converter is
below the limits set by IEEE standards for grid connected
full-bridge converters. Most of the harmonic energy is contained in
frequencies higher than the track frequency which can easily be
filtered to improve the THD further.
[0095] There are many possible control algorithms that can be
implemented to achieve above mentioned control tasks. FIG. 10
illustrates one such algorithm which is used by the pick-up of the
IPT system considered in this example. EMF induced across a sense
winding is used to obtain the phase of the track current I.sub.pt
and a PLL is used to produce a reference angle that is synchronized
with this EMF. This signal produced by the PLL is delayed in phase
by 0 or 180 degrees, which corresponds to a phase-shift .theta. of
-90 or +90 degrees respectively, is used to drive the switches
T.sub.s1-T.sub.s4. The phase delay .phi..sub.2 of the pick-up
full-bridge converter is modulated using a reference angle that is
in-phase with the mains frequency. The power throughput of the
converter is regulated by controlling the track current by varying
.phi..sub.1 of the matrix converter. An example controller diagram
for the primary is given in FIG. 11. As an alternative the power
controller can be integrated to the pick-up controller and the
input current shaping can be achieved through the primary
controller.
[0096] Simulation Results
[0097] A 2.8 kW matrix converter based grid-connected IPT system
capable of transferring bi-directional power has been designed and
simulated in MATLAB Simulink.TM., and results are presented to
verify the viability of the proposed concept. The primary of the
system is powered by a 230 V.sub.ac source and the pick-up is
connected to a 250 V battery, representing an EV or an active load.
A complete set of design parameters of the simulated system is
given in Table 2 (see FIG. 22).
[0098] The simulated voltages and currents of both the primary and
the pick-up of the proposed IPT system over a 20 ms period are
shown in FIG. 12. As predicted from (1) and (5), the voltage
V.sub.pi and the track current I.sub.pt produced by the matrix
converter exhibit an envelope of 50 Hz modulation due to time
varying input voltage. Since the pick-up is supplied by a DC
source, the current in the pick-up inductor I.sub.st has a constant
amplitude as given by (6). FIG. 13 illustrates a few cycles of
these waveforms, and it can be seen that both the matrix converter
and the pick-up full-bridge converter are operated at a 50% duty
cycle and with a 180 degrees phase shift. The corresponding
currents produced in L.sub.pt and L.sub.st are at 20 kHz and lags
the full-bridge converter voltages by 90 degrees as given by (5)
and (6). Furthermore, it is evident from the figure that currents
I.sub.pt and I.sub.st are sinusoidal and therefore validate the
assumptions made in (5)-(8). The primary track current given by
(5), and similarly the pick-up track current, are both independent
of the loading and fixed by circuit parameters. However in
practice, the track currents will reduce as the load increases due
to losses and component tolerances. Therefore the full-bridge
converter voltages V.sub.pi and V.sub.si need to be regulated in
order to maintain a constant track current.
[0099] The input and output power of the system along with the
input and output currents are shown in FIG. 14. Under the above
conditions and according to (12), the IPT system delivers an
average output power of 2.8 kW to the load. It can be noted that
both the output and the input power exhibit a 100 Hz ripple. This
is caused by the 50 Hz modulation that exists in the track current
as illustrated in FIG. 12. Since the voltage produced by the
pick-up full-bridge converter lags the voltage that produced by the
primary matrix converter, a positive power is delivered to the load
in accordance with (11). The peak load power is approximately 4 kW.
The input current, produced by the control scheme employed in the
simulation, appears to generate considerable amounts of 3rd and 5th
harmonic currents. However by controlling the full-bridge converter
phase angles .phi..sub.1 and .phi..sub.2 in (10), the harmonics
currents can be significantly reduced.
[0100] The direction of the power flow between primary and the
pick-up can be reversed by driving the pick-up full-bridge
converter with a leading phase angle .theta.. The pick-up of the
simulated IPT system is driven at a 90 degrees leading phase angle
with respect to the primary full-bridge converter, and the
simulations results are shown in FIG. 15 and FIG. 16. As evident
from FIG. 16 the pick-up full-bridge converter voltage V.sub.si is
leading V.sub.pi by 90 degrees. However, since the input voltages
are unchanged, the primary and pick-up track currents remain
constant, regardless of the phase difference between the
full-bridge converter.
[0101] As illustrated in FIG. 17, the pick-up is now delivering
about 2.8 kW to the grid through the mutual coupling that exists
between the track and the pick-up inductors. As a result, in
comparison to FIG. 14, the direction of the current flow has now
reversed. According to (12), the power throughput of the converter
can be regulated either by changing the relative phase angle
.theta. between the primary and the pick-up full-bridge converters
or by varying .phi..sub.1 and .phi..sub.2. However, the input power
factor of this system can be maintained close to unity by
maintaining .theta. at .+-.90 degrees. Thus the output power of the
simulated IPT system is regulated by controlling .phi..sub.2 ,
which in turn reduces or increases the pick-up inductor current
I.sub.st. The power throughput of the pick-up can be reduced by
reducing the phase difference .phi..sub.2 between the switches of
the pick-up full-bridge converter. The voltages and currents of the
system when .phi..sub.2 is reduced to 90 degrees while maintaining
.theta. at +90 degrees are shown in FIG. 16 and FIG. 17. As evident
from FIG. 16, the pick-up current I.sub.st was reduced to about 70
A in accordance with (6). Consequently the power throughput of the
converter was been reduced to about 2 kW as illustrated in FIG.
20.
[0102] The IPT system disclosed herein can be used in standby
applications where there is a requirement to supply an AC supply,
for example 230 V AC, to a load in the event of a grid failure for
example. Therefore, the invention provides an IPT system which can
be used in both standby and grid connected modes.
[0103] Furthermore, the invention is applicable to multiphase
systems. Therefore, the invention provides a multiphase matrix
converter based IPT system. This has advantages of both lower
losses and low current ripples.
[0104] The invention provides an IPT system which can also be
extended to multiple primary and/or multiple pick-up systems.
Furthermore, multiple primary conductive paths i.e. tracks and/or
multiple pick-up windings may be used.
[0105] In another aspect, the invention also allows both the
primary, or primaries and the pick-up, or pick-ups to be either
active loads or passive loads. For example, rather than the primary
being connected to an AC source, it can be connected to an AC load
when the pick-up is connected to a battery (an EV).
[0106] From the foregoing it will be seen that the invention
provides a novel matrix converter based IPT system that requires
only a single stage power conversion process to facilitate
contactless and bidirectional power flow. The proposed system
wirelessly transfers power through loose magnetic coupling, and a
mathematical analysis together with simulation results have been
presented to show that the proposed technique is viable and
requires a simple control strategy to effectively control both
direction and amount of power flow. The proposed IPT power
interface is reliable, efficient and low in cost without an
additional power conversion stage, and is attractive for
applications which require wireless power.
* * * * *