Note: Descriptions are shown in the official language in which they were submitted.
DYNAMIC DIRECT POWER CONTROL METHOD AND SYSTEM FOR A
GRID CONNECTED CONVERTER
TECHNICAL FIELD
[0001] The
disclosure herein relates to methods and systems for
controlling power converters operatively coupled to an electrical grid.
BACKGROUND
[0002]
[000] Grid connected converters became the most critical power electronics
application for interfacing Renewable Energy Resources (RES) to existing
grids, transferring power through DC links (HVDC), and motor drive
applications. The demand for fast and accurate power control became a
dominant factor for designing the power converters. Moreover, the power
converters should be controlled to perform efficiently under abnormal
condition of the grid. Direct Power Control (DPC) method for controlling such
converters has gained much attention due to its superior feature of fast
dynamic response. However, conventional DPC method depends on using a
static switching table to produce the required switching signals to the
converter regardless of the grid and the dc link variations. The conventional
DPC with a static switching table cannot perform efficiently under abnormal
grid conditions such as voltage dips, frequency change, phase jump etc or
dc link variation. With a sever condition of voltage dip or dc link voltage
variation, the conventional DPC could fail to achieve the power demands
hence, it will be interrupted or disconnected from the grid which is not
acceptable for many applications. A method for adapting the conventional
DPC under the abnormal grid operation became a serious topic which needs
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to be resolved for the DPC method to gain trust for those critical
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] FIG. 1 illustrates an example embodiment of a grid connected
power converter controlled by the proposed Dynamic Direct Power Control
(DDPC) method.
[0004] FIG. 2A illustrates, in an example embodiment, Three-Level
Converter Space Vectors.
[0005] FIG 2B illustrates, in an example embodiment, 12-Sectors
classification of the Conventional Three-Level Converter Space Vectors which
is used also for regulating the positive power demand only as well as the
reactive power.
[0006] FIG 2C illustrates an example embodiment of a proposed Six-
Sectors with a dynamic 4-subsectors classification of the Three-Level
Converter Space Vectors designed for the proposed Dynamic Direct Power
Control (DDPC) Method.
[0007] FIG. 3A illustrates example embodiment representations of
sector associations with the selected space vectors for conventional DPC
method.
[0008] FIG 3B illustrates an example of embodiment representation of
the proposed sector associations with the new selected space vectors based
on a crossover angle method as defined herein.
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[0009] FIG. 4A and FIG. 4B illustrate, in example embodiments, the
new generic switching table representations for positive and negative power
demands for various sectors of the power converter space vectors.
[0010] FIG. 4C and FIG. 4D illustrate, in example embodiments, the
new generic switching table implementation for sector number 1.
[0011] FIG. 5 illustrates, in an example embodiment, a method of
operation in implementing dynamic direct power control.
[0012] FIG. 6A illustrates an example application case of the invention
herein to a grid-connected converter for a wind turbine system.
[0013] FIG. 6B illustrates a further example application case of the
invention herein to a grid-connected converter for a high voltage direct
current (HVDC) system.
[0014] FIG. 6C illustrates a further example application case of the
invention herein to a grid-connected converter for a medium voltage motors
drive application.
[0015] FIG 7A illustrates an example of embodiment representation for
the affinity of the power converter sector no. 1 Nearest Three Vectors (N3V)
to alter the active power and related crossover angles with respect to grid
voltage vector location.
[0016] FIG 7B illustrates an example of embodiment representation for
the affinity of the power converter sector no. 1 Nearest Three Vectors to
alter the reactive power with respect to grid voltage vector location.
DETAILED DESCRIPTION
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[0017] Among other benefits and advantages of the proposed dynamic
direct power control (DDPC) technique and system of the disclosure herein,
in recognizing that conventional static switching tables are typically
designed
presuming nominal or constant voltages of the grid and the dc link, the
disclosure herein provides a novel dynamic switching table for the DPC. The
dynamic switching table is based at least in part on actual variation in grid
and dc-link voltages for operational control of a grid-connected power
converter, such as due to a sudden or unexpected grid faults or heavy
loading, where the grid voltage and the dc-link voltage may be subjected to
considerable changes, especially in case of weak grids. The proposed
dynamic switching table incorporates a new definition of power influence
crossover angles of each space vector of the power converter hence, an
advantageous and novel classification for space vectors sectors is
introduced. The proposed scheme dynamically adapts the switching table to
select the optimum space vector by feeding forward the actual grid and dc-
link voltages, advantageously resulting in more precise control of power
converter output. The proposed technique and system may be applied in
various grid connected converter applications, including but not limited to
grid-connected converters in back-to-back configuration for motor drive
systems, grid-connected converters for wind turbine applications, and grid-
connected sending and receiving converters for HVAC application, among
others.
[0018] Provided is a method of controlling a power converter operatively
coupled with an electric grid. The method comprises operating the power
converter in a direct power control mode under substantially constant
voltage conditions of a grid voltage of the electric grid and a dc link
voltage
of the power converter based at least in part on a switching table, the
switching table including a set of space vector parameters associated with
the dc link and grid voltages, detecting a grid fault voltage event in
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accordance with a measured change of the grid and dc link voltages relative
to nominal operating voltage conditions, determining a set of crossover
angles for the Nearest Three Vectors related to voltage vector location inside
space vector hexagonal diagram (N3V) based on measured grid and the dc
link voltages, dynamically adapting the switching table based on algorithmic
feedforward of the set of crossover angles to the set of space vector
parameters of the switching table, and regulating an output power of the
power converter in accordance with the dynamically adapted switching table.
[0019] Also provided is a power converter control module coupled to an
electric grid. The power converter control module includes a processor, and
a memory storing instructions. The instructions are executable in the
processor to operate the power converter in a direct power control mode
under substantially constant voltage conditions of a grid voltage of the
electric grid and a dc link voltage of the power converter based at least in
part on a switching table, the switching table including a set of space vector
parameters associated with the dc link and grid voltages, detect a grid fault
voltage event in accordance with a measured change of the grid and dc link
voltages relative to nominal operating voltage conditions, determine a set of
crossover angles for the Nearest Three Vectors related to voltage vector
location inside space vector hexagonal diagram (N3V) based on measured
grid and the dc link voltages, dynamically adapt the switching table based
on algorithmic feedforward of the set of crossover angles to the set of space
vector parameters of the switching table, and regulate an output power of
the power converter in accordance with the dynamically adapted switching
table.
[0020] Further provided is a non-transitory computer readable memory
storing instructions. The instructions are executable in a processor to
operate a power converter that is operatively coupled in a direct power
control mode under nominal operating conditions of a grid voltage of the
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electric grid and a dc link voltage of the power converter based at least in
part on a switching table, the switching table including a set of space vector
parameters associated with the dc link and grid voltages, detect a grid fault
voltage event in accordance with a measured change of the grid and dc link
voltages relative to the substantially constant voltage conditions, determine
a set of crossover angles for the Nearest Three Vectors related to voltage
vector location inside space vector hexagonal diagram (N3V) based on
measured grid and the dc link voltages, dynamically adapt the switching
table based on algorithmic feedforward of the set of crossover angles to the
set of space vector parameters of the switching table, and regulate output
power of the power converter in accordance with the dynamically adapted
switching table.
[0021] One or more embodiments described herein can be implemented
using programmatic modules, engines, or components. A programmatic
module, engine, or component can include a program, a sub-routine, a
portion of a program, or a software component or a hardware component
capable of performing one or more stated tasks or functions. As used herein,
a module or component can exist on a hardware component independently
of other modules or components. Alternatively, a module or component
can be a shared element or process of other modules, programs or
machines.
[0022] Furthermore, one or more embodiments described herein may be
implemented through the use of logic instructions that are executable by one
or more processors. These instructions may be carried on a computer-
readable medium. In particular, machines shown with embodiments herein
include one or more processors and various forms of memory for storing
data and processor-executable instructions. Embodiments described herein
may be implemented in the form of computer processor- executable logic
instructions or programs stored on computer memory mediums.
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[0023] FIG.
1 illustrates, in an example embodiment configured as test
and simulation system 100, a grid-connected power converter electrically
connected within an electrical grid. The DC link may be a capacitor device in
one embodiment, electrically interposed between the power converter and
other power electronic application. The power converter may be electrically
controlled via dynamic direct power control (DDPC) module 105. Dynamic
direct power control module 105 may include any combination of hardware
elements, electrical circuitry and encoded software instructions, in one
example embodiment, a processor and a non-transient memory storing
instructions executable in the processor to control functioning and response
of the power converter within the electrical grid. In one embodiment,
dynamic direct power control module 105 may include capability for
operating the power converter based on a dynamic adapted switching table
in response to variations in grid and dc link voltages.
[0024] FIG.
2A illustrates, in an example embodiment 201, Three-Level
Converter Space Vectors.
[0025] FIG
2B illustrates, in an example embodiment, 12-Sectors
classification of the Conventional Three-Level Converter Space Vectors of
FIG. 2A, hexagonal diagram 202 divided into 12 sectors representing space
vectors which increase the active power output.
[0026] FIG
2C illustrates an example embodiment of a proposed Six-
Sectors with a dynamic 4-subsectors classification of the Three-Level
Converter Space Vectors designed for the proposed Dynamic Direct Power
Control (DDPC) method (DDPC), with the active power decrease influence
vectors divided into 6 sectors, each sector of hexagon 203 being sub-divided
into 4 regions
[0027] The
conventional DPC method directly selects the optimum
converter vectors that increase or decrease the active and reactive power
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from a static switching table. The optimum converter vectors are selected by
well-defined rules triggered from hysteresis comparators, which use the
instantaneous measured values of active and reactive power without
introducing any filtering stage. With reference to FIG. 2A, the conventional
DPC controller selects the appropriate vector uk at each sampling frequency
to regulate the converter power as well as the dc-link voltage. The
relationship between the converter vector, grid vector and the rate of
converter's power change may be expressed as:
dP 1.5 E. .2
= 1 _
dt L NO
9
dQ 1.5
dt = L9 UgdUk COS( 09 ¨ 0k)]
Ugdlik Sin(Og -- 0k)
where Lg is the equivalent upstream inductance, Ugd is the d-axis
component of grid voltage vector, uk is the converter voltage vector, and
'Og ¨ Ok' is the relative angle between grid vector angle Eig and converter
voltage vector angle Ok which can be defined in relation to the space vector
diagram shown in FIG. 2A.
[0028] As referred to herein, the crossover angles in one embodiment
may be derived assuming that the grid and the dc-link voltages are
dynamically changing. The converter space vectors can be presented by dc-
link voltage as follows:
uk = Akudc.actLek
where Udc.act is the actual dc-link voltage, Ok is the converter space vector
'uk' angle ,and Ak is the converter space vector amplitude coefficient which
1 1
equals to ¨ for small vectors (u13, U14, U15, u ), 16, u17 and
u18 ¨ for
3 v-3-
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2
medium vectors (U2, U4, U6, Ug, 1210 and u12) and ¨3 for big vectors (u1, u3,
U5, U7, U9 and u11). The actual dc-link voltage can be expressed using the
nominal value as:
Udc.act = kdcUdc.nom.
The setup minimum value of the dc-link voltage boost is commonly selected
more than times the converter's line-to-line voltage. Moreover, the dc-
link setup voltage should be further boosted by a kboost factor to
compensate for the voltage drop across the upstream equivalent inductance.
Thus, the nominal setup of the dc-link voltage can be expressed as:
Udc.nominal = kboost = kdc =Vline.RMS.
The converter terminal voltage vector amplitude in this case can be
expressed as:
Iuk I =
2Akk
-boostkdcVline.RMS.
The converter terminal voltage may be expressed by the positive sequence
component of the grid voltage, ugd under balanced three phase system as:
A/2 l Tr Ugdr= v line.RMS.
Accordingly, the converter terminal vector may be expressed as:
Uk = 0-AkkboostkdcUgd.nom. Lek
The actual grid voltage may be represented using a variation factor k as:
= kv=Ugd.nom.
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The converter voltage magnitude may be represented based on the latter
relations as:
kdc 3, I
114 \1
1 = .¨ Akk boost ¨kv I"-g di
[0029] The converter delivered power rate of change in the dynamic case
under dc link and grid voltage variation may be expressed in terms of the
dc-link and the grid voltages as:
dP 3u
1 2
_ gd [
2 ArjAkkboostkdc
= dt 2Lg 1 k
cos(Og ¨ Ok)]
v
dQ 3ugd AljAkkboostkac
¨= cos(Og ¨19k)1
dt 2L9 kv
[0030] The crossover angles of any vector uk can be obtained by setting
the power rate of change of above equations to zero as referred to herein
dQ
may be defined by setting ¨dP = 0 and ¨ = 0 as:
dt dt
P.cross
0 k
= Ok + COS-1 ________________________ kv
IjAkkboostkdc
Q.cross =
The latter relation indicates that the crossover angle of the converters space
vectors for reactive power producing is exactly equal to the converter space
vector angle itself which means that there is no any crossover angles within
each hexagonal 12 sectors. Accordingly, the hexagonal diagram in
accordance with FIG. 2A may be divided similar to the conventional DPC
scheme into 12 sectors so that the reactive power influence curves cross the
x-axis exactly at sectors' borders. On the contrary, the crossover angle of
the active power influence vectors takes place inside each sector. As
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illustrated before through equations, the crossover angles of each vector
change dynamically with the actual value of the dc-link and grid voltages.
[0031] The influence of each vector on active and reactive power may be
represented using the power rate of change as expressed above based on dc
link and grid voltages. The converter space vectors' influence on active and
reactive power in relation with the actual grid voltage vector location and
amplitude can be classified into two groups: a first group of converters space
vectors which increase the active power, and a second group of converters
space vectors which decrease the reactive power. Usually, the converter
space vectors are graphically presented in the hexagonal diagram which is
divided into 12 sectors 201 as shown in FIG. 2A. The influence of each
converter space vector is calculated using the power rate of change
expression above, assuming nominal grid and dc-link voltages with 1.3 boost
factor, in one embodiment.
[0032]
FIGS. 3A illustrate example embodiment representations of
sectors 1 and 2 in a 12 sector space vector hexagonal presentation
associations with related in space vectors transition, It is clear from FIG 3A
that the conventional method alter the converter space vectors based on the
power hysterics comparators (static switching table) once the grid vector
cross the sector boarder regardless of the value of dc link voltage or grid
vector voltage amplitude.
[0033] FIG.
3B illustrates the subdivided Sectors A, B, C, and D in the
proposed 6 sector space vectors hexagonal presentation associations with
related in space vectors transition, in relation to a crossover angle as
defined
herein 302. in relation to a crossover angle as defined herein 301. The
crossover angles ak for each converter space vector (uk) can be calculated
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dP
using the actual measured grid and dc-link voltages by setting ¨ = 0, as
dQ
follows:
Ugcj
ak = Ok + COS-1(
uk
where 0 k is the converter space vector angle.
FIGs 3AB can be discussed in details as follows: the active power decreasing
demand (Hp= -1) and reactive power high increasing demand (HQ=2), the
standard sectors shown in FIG. 3A utilizes only vector 2/3 for sectors 1 and
sector 2, while the proposed subsectors shown in FIG. 3B utilizes vector U2
in subsector A and once the grid vector cross the calculated cross angle of
vector u3 (9cros53), the converter space vector U3 changes its affinity from
decreasing the active power (subsector A) to increasing the active power
(subsector B) . Similarly, as shown for converter space vectors u1 and u2
when the power demands is presented by (Hp= -1, 142=-2). Based on the
proposed crossover angles concept as defined herein, the DDPC controller
takes action to change the converter applied space vector from U2 to U3
immediately when the grid vector cross sub-sector A.
[0034] FIG.
4A and FIG. 4B illustrate, in example embodiments 401a,
401b respectively, the new generic switching table representations for
positive and negative power demands for various sectors of the power
converter space vector. In details, the converter space vector are classified
into three categories, Big Vectors ub, Medium Vectors Um and Small Vectors
Us. To construct the switching table in any sector, knowing the position of
the grid voltage vector in that sector, the table in 401a and 401b can be
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used. For example referring to 401a, if it is required to slightly increase
the
active power P(+) and slightly increase the reactive power Q(+), the "NEXT
Small Vector" us+1 should be used based on the actual converter space
vectors located in that specific sector. In this example referring to FIG. 2A
and Sector 1, if the grid voltage vector is located in sector 1, the next
converter space vector that will achieve the above power demands is U14.
Similarly, the switching table for sector no. 5 selects the next Small Vector
2/16, and so,.
[0035] FIG. 4C and FIG. 4D illustrate, in example embodiments 401c,
401d respectively, the new generic switching table implementation for sector
number 1. By completing all sectors, the entire switching tables can be
constructed. However, the generic method is a smart programmatical way to
reduce the effort of constructing the entire table manually and also it saves
the processor memory space. In addition to, the proposed method is
dynamically created the switching table as the crossover angles always
varies.
[0036] The crossover angles may be dynamically calculated and
algorithmically feedforwarded to the switching table. The online crossover
angles can be calculated, in an embodiment, using the relation as described
above:
u d
Ocross = Ok cos( ______________ g )
AkUdc.act
By the dynamic calculation of the crossover angles, the static switching table
is adapted or transformed into a dynamic switching table which increases
accuracy of the DDPC and prevents the cyclic pulsation of active and reactive
power outputs of the power converter during the transients as well as the
steady-state operation.
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[0037] FIGS. 4C and 4D represent the static switching table 401c, 401d
based upon which the dynamically switched tables 401a, 401b of FIG. 4A
and FIG. 4B may be generated based on algorithmic feedforward of
crossover angles. The influence magnitude of converter space vectors which
increase the active power may be denoted by `+' for small increments, µ-F +'
for medium increments and µ-F + +' for big increments. Similarly, µ-' for
small decrements, - -' for medium decrements and `- - -' for big decrements.
[0038] FIG. 5 illustrates, in an example embodiment, method 500 of
operation 500 of dynamic direct power control module 105. In describing the
example of FIG. 5, reference is made to the examples of FIGS. 1- 4
collectively for purposes of illustrating suitable components or elements for
performing a step or sub-step being described.
[0039] At step 510, operating the power converter 101 in a direct power
control mode under substantially constant voltage conditions of a grid
voltage of the electric grid 102 and a dc link voltage of the power converter
based at least in part on a switching table, the switching table including a
set of vector space parameters associated with the dc link and grid voltages.
[0040] At
step 520, detecting a grid fault voltage event in accordance
with a measured change of the grid and dc link voltages relative to the
substantially constant voltage conditions.
[0041] In one embodiment, a grid fault voltage event, or an unexpected
heavy loading fluctuation, may be detected by evaluating the positive or the
negative sequence component of the grid voltage vector in the synchronous
reference frame (dq-axis) to determining whether that dq-axis value of the
grid voltage exceeds a threshold limit.
[0042] At step 530, determining a set of crossover angles for the Nearest
Three Vectors related to voltage vector location inside space vector
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hexagonal diagram (N3V) based on measured grid and the dc link voltages.
The set of crossover angles is determined for a set of power converter space
vectors to construct the dynamic switching table. The set of crossover
angles, in one embodiment, is determined for a set of power converter space
vectors based on dynamically constructing the switching table and selecting
the optimum converter space vector based at least on the present location of
the grid vector inside the hexagonal space vector diagram.
[0043] At step 540, dynamically adapting the switching table based on
algorithmic feedforward of the set of crossover angles to the set of converter
space vectors.
[0044] In one embodiment, the method further comprises a hybrid
Hexagonal Space Vectors including a first Hexagonal Space Vector diagram
classified into 6 sectors and 4 subsectors, and a second Hexagonal Space
Vector diagram classified into 12 sectors based on the calculated set of the
crossover angles, wherein the first Hexagonal Space Vector diagram is used
to generate the optimum converter space vector for producing the negative
active power (13), and the second Hexagonal Space Vector diagram is used
to generate the optimum converter space vector for producing the positive
active power (Pt) as well as producing both the positive and negative
reactive power (Q+) and (Q-).
[0045] At
step 550, regulating output power of the power converter in
accordance with the dynamically adapted switching table.
[0046] In
one embodiment, regulating the output power includes
regulating one or both of the active power output and the reactive power
output. The converter is controlled by the dynamic switching table to
eliminate, or at least minimize, any undesirable cyclic pulsation in the
active
and reactive power components.
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[0047] In
other embodiments, the method steps described may be
embodied as processor-executable instructions stored in a non-transitory
storage medium or memory and incorporated into, or made communicatively
accessible to, any one or more of an electrical grid controller device, a
power
converter controller device, a terminal computing device or a server
computing device.
[0048] It is contemplated for embodiments described herein to extend to
individual elements and concepts described herein, independently of other
concepts, ideas or system, and also for the embodiments to include
combinations of elements recited throughout this application. Although
embodiments are described in detail herein with reference to the
accompanying drawings, it is contemplated that the invention is not limited
to such embodiments. As such, many modifications and variations will be
apparent to practitioners skilled in this art.
[0049] For
instance, and by way of illustration of additional examples of
applying the dynamic direct power control methods and systems disclosed
herein, FIG. 6A illustrates an example application case of the invention
herein that implements dynamic direct power control module 605 to grid-
connected converter for wind turbine system 601. FIG. 6B illustrates a
further example application case of the invention herein that implements
dynamic direct power control module 610 to a grid-connected converter for
high voltage direct current (HVDC) system 602. FIG. 6C illustrates a further
example application case of the invention herein that implements dynamic
direct power control module 615 to a grid-connected converter for a medium
voltage motors drive application 603.
[0050] FIG
7A illustrates an example of embodiment representation for
the affinity of the power converter sector no. 1 Nearest Three Vectors to
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alter the active power and related crossover angles with respect to grid
voltage vector location.
[0051] FIG 7B illustrates an example of embodiment representation for
the affinity of the power converter sector no. 1 Nearest Three Vectors to
alter the reactive power with respect to grid voltage vector location.
[0052] Accordingly, it is intended that the scope of the invention be
defined by the following claims and their equivalents. Furthermore, it is
contemplated that a particular feature described either individually or as
part
of an embodiment can be combined with other individually described
features, or parts of other embodiments, even if the other features and
embodiments make no specific mention of the particular combination of
features. Thus, any absence of describing combinations should not preclude
the inventors from claiming rights to such combinations.
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