U.S. patent application number 14/970513 was filed with the patent office on 2016-04-07 for power flow controller with a fractionally rated back-to-back converter.
This patent application is currently assigned to Varentec, Inc.. The applicant listed for this patent is Varentec, Inc.. Invention is credited to DEEPAKRAJ M. DIVAN, RAJENDRA PRASAD KANDULA, ANISH PRASAI.
Application Number | 20160099653 14/970513 |
Document ID | / |
Family ID | 49878402 |
Filed Date | 2016-04-07 |
United States Patent
Application |
20160099653 |
Kind Code |
A1 |
DIVAN; DEEPAKRAJ M. ; et
al. |
April 7, 2016 |
POWER FLOW CONTROLLER WITH A FRACTIONALLY RATED BACK-TO-BACK
CONVERTER
Abstract
A power flow controller with a fractionally rated back-to-back
(BTB) converter is provided. The power flow controller provide
dynamic control of both active and reactive power of a power
system. The power flow controller inserts a voltage with
controllable magnitude and phase between two AC sources at the same
frequency; thereby effecting control of active and reactive power
flows between the two AC sources. A transformer may be augmented
with a fractionally rated bi-directional Back to Back (B TB)
converter. The fractionally rated BTB converter comprises a
transformer side converter (TSC), a direct-current (DC) link, and a
line side converter (LSC). By controlling the switches of the BTB
converter, the effective phase angle between the two AC source
voltages may be regulated, and the amplitude of the voltage
inserted by the power flow controller may be adjusted with respect
to the AC source voltages.
Inventors: |
DIVAN; DEEPAKRAJ M.; (Santa
Clara, CA) ; KANDULA; RAJENDRA PRASAD; (Santa Clara,
CA) ; PRASAI; ANISH; (Santa Clara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Varentec, Inc. |
|
|
|
|
|
Assignee: |
Varentec, Inc.
Santa Clara
CA
|
Family ID: |
49878402 |
Appl. No.: |
14/970513 |
Filed: |
December 15, 2015 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13673966 |
Nov 9, 2012 |
|
|
|
14970513 |
|
|
|
|
61558706 |
Nov 11, 2011 |
|
|
|
Current U.S.
Class: |
363/35 |
Current CPC
Class: |
H02M 5/458 20130101;
H02M 5/4585 20130101; H02M 2001/0093 20130101; H02J 3/06
20130101 |
International
Class: |
H02M 5/458 20060101
H02M005/458 |
Goverment Interests
STATEMENT OF RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH
[0002] This invention was made with Government support under
DE-AR0000108 awarded by the United States Department of Energy. The
Government has certain rights in the invention.
Claims
1. An apparatus for controlling real and reactive power flow
between a first AC source and a second AC source, comprising: a
back-to-back converter comprising an AC-DC converter coupled
between a first tap and a second tap of a transformer, the AC-DC
converter comprising a first set of switches, a DC-AC converter
coupled to the second AC source, the DC-AC converter comprising a
second set of switches, and a DC link coupled to the AC-DC
converter and the DC-AC converter, wherein a third tap of the
transformer is coupled to the first AC source, and an input voltage
to the back-to-back converter is less than a voltage at the third
tap of the transformer.
2. The apparatus of claim 1, wherein an input voltage to the
back-to-back converter is a fraction of the voltage at the third
tap of the transformer.
3. The apparatus of claim 1, wherein the transformer further
comprises a set of taps floating around the voltage at the third
tap of the transformer.
4. The apparatus of claim 1, wherein the transformer is an auto
transformer or an isolated step-down transformer.
5. The apparatus of claim 1, further comprising the
transformer.
6. The apparatus of claim 1, wherein the third tap is located
between the first tap and the second tap.
7. The apparatus of claim 1, wherein the AC-DC converter comprises
a first leg and a second leg, the first leg coupled to the first
tap of the transformer and the second leg coupled to the second tap
of the transformer.
8. The apparatus of claim 1 having an output, wherein the DC-AC
converter comprises a first leg and a second leg, the first leg and
the second leg coupled to the output of the apparatus.
9. The apparatus of claim 1, further comprising a control module,
wherein the control module generates a common mode control signal
such that the common mode current is within a first predetermined
value and generates a differential mode control signal such that a
differential current is within a second predetermined value.
10. The apparatus of claim 9, wherein the control module generates
a first set of switching pulses to the first set of switches
according to the differential mode control signal, and a second set
of switching pulse to the second set of switches according to the
common mode control signal.
11. The apparatus of claim 1, wherein the first and the second set
of switches are two-quadrant switches.
12. The apparatus of claim 1, wherein the DC-AC inverter is a
multiple level inverter.
13. The apparatus of claim 1 having an output, further comprising:
an input filter coupled to the transformer and the AC-DC converter;
and an output filter coupled to the DC-AC converter and the output
of the apparatus.
14. The apparatus of claim 1, wherein the DC link comprises a
capacitor.
15. The apparatus of claim 1, further comprising a fail-normal
switch, wherein the fail-normal switch is coupled across the
back-to-back converter.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/673,966, filed on Nov. 9, 2012, entitled
"Power Flow Controller with a Fractionally Rated Back-to-Back
Converter," which claims the benefit of U.S. Provisional Patent
Application No. 61/558,706, filed on Nov. 11, 2011, entitled "Power
Flow Controller with a Fractionally Rated Back-to-Back Converter,"
which are incorporated by reference herein in their entireties.
TECHNICAL FIELD
[0003] The present invention(s) relate generally to controlling
power flow in an electric power system. More particularly, the
invention(s) relate to power flow controllers with back-to-back
converters.
DESCRIPTION OF THE RELATED ART
[0004] An electric power system is a network of interconnected
electrical equipment that generates, transmits, and consumes
electric power. Electric power is delivered to consumers through a
transmission network and a distribution network from generators to
consumers. The transmission network and the distribution network
are often known as the transmission grid and the distribution grid,
respectively. Operation of the transmission grid and the
distribution grid was once straightforward before the deregulation
of the electric power market, but became extremely complex as a
result of the competition among various utility companies.
Increased amount of electric power is flowing in the electric power
system and causing congestion and overflow in certain parts of the
electric power system, which may limit the capacity and also impact
the reliability of the electric power system. As the electric power
system is highly dynamic, real-time power flow control ensures the
electric power system's reliability and increases its capacity and
efficiency. As a result, the increasing load demand, increasing
level of penetration of renewable energy and limited transmission
infrastructure investments have significantly increased the need
for a smart dynamically controllable grid.
[0005] Traditionally, power flow control has been achieved by
generator control, shunt VAR compensation and LTC tap settings.
However, the range of control achieved is not very significant and
the dynamic response is very slow. Various devices can be installed
on the electric power system to perform electric power flow
controls such as a Phase Angle Regulator (PAR), also known as a
Phase Shifting Transformer (PST), a Unified Power Flow Controller
(UPFC), and a Back-to-Back (BTB) HVDC link.
[0006] PARs or PSTs correct the phase angle difference between two
parallel connected electrical transmission systems and thereby
control the power flow between the two systems so that each can be
loaded to its maximum capacity. Conventional PARs and PSTs insert a
series voltage to a phase that is in quadrature with the
line-to-neutral voltage. However, conventional PARs or PSTs cannot
control the reactive power flow independently from the active power
flow. Their dynamic capabilities, if they exist, are also very
limited. UPFCs consist of two inverters with an intermediate DC bus
with energy storage. One inverter is connected in shunt through
transformer, while the second inserts a series voltage in the line,
again through transformer coupling. UPFCs typically can insert a
desired series voltage, balancing average power flow using the
shunt inverter. This allows a UPFC to source or sink active and
reactive power. UPFCs are typically used at very high power and
voltage levels (100 MW @ 345 KV). The need for UPFCs to survive
faults and abnormal events on the grid makes their design complex
and expensive because the series transformers and inverters for
operation under system faults are large and expensive. Moreover,
the shunt transformer and inverters for operation under transient
voltages also add cost. As a result, although UPFCs have been
commercially available for decades, few have been deployed.
[0007] BTB HVDC links consist of two inverters with an intermediate
DC bus with energy storage. BTB HVDC links provide a wide control
range (+/-1 p.u.) for both active and reactive power. However, for
a 1 p.u. control range, the converter has to be rated for at least
2 p.u. (two converters of 1 p.u. each). Building such high power
controllers for transmission or sub-transmission systems is
extremely complex and expensive. Also, the size and complexity may
affect their reliability. As the two inverters are connected in
series, effectively a single point of failure in the system is
created.
BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION
[0008] According to various embodiments of the invention, a power
flow controller with a fractionally rated back-to-back (BTB)
converter is provided. Various embodiments provide dynamic control
of both active and reactive power of a power system. The power flow
controller inserts a voltage with controllable magnitude and phase
between two AC sources; thereby effecting control of active and
reactive power flows between the two AC sources. In one embodiment,
a transformer is augmented with a fractionally rated bi-directional
Back to Back (BTB) converter. In various embodiments, low-rating
insulated gate bipolar transistors (IGBTs) are used as switches in
the fractionally rated converters. Further, a power flow controller
may be isolated from a system fault. In some embodiments, a
fail-normal switch bypasses the power flow controller in case of a
contingency.
[0009] A power flow controller with a fractionally rated BTB
converter provides control of both the active and reactive power
flow between two AC sources at the same frequency. In various
embodiments, the fractionally rated BTB converter comprises a
transformer side converter (TSC), a direct-current (DC) link, and a
line side converter (LSC). By controlling the switches of the BTB
converter, the effective phase angle between the two AC source
voltages may be regulated, and the amplitude of the voltage
inserted by the power flow controller may be adjusted with respect
to the AC source voltages. Various embodiments may be implemented
at various voltage levels such as 13 kV, 69 kV, and 139 kV.
Further, in some embodiments, a power flow controller comprises a
fail-normal switch. As the fault current is diverted by the bypass
switch until line breakers trip, the transformer and the BTB
converter of the power flow controller are isolated from fault
currents or high transient voltages during any fault.
[0010] Other features and aspects of the invention will become
apparent from the following detailed description, taken in
conjunction with the accompanying drawings, which illustrate, by
way of example, the features in accordance with embodiments of the
invention. The summary is not intended to limit the scope of the
invention, which is defined solely by the claims attached
hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The present invention, in accordance with one or more
various embodiments, is described in detail with reference to the
following figures. The drawings are provided for purposes of
illustration only and merely depict typical or example embodiments
of the invention. These drawings are provided to facilitate the
reader's understanding of the invention and shall not be considered
limiting of the breadth, scope, or applicability of the invention.
It should be noted that for clarity and ease of illustration these
drawings are not necessarily made to scale.
[0012] FIG. 1 illustrates an exemplary system diagram of an
electric power system where various embodiments of the invention
can be implemented.
[0013] FIG. 2 illustrates an exemplary schematic diagram of a
single-phase power flow controller in accordance with an embodiment
of the invention.
[0014] FIG. 3 illustrates an exemplary schematic diagram of a
single-phase 3-level power flow controller in accordance with an
embodiment of the invention.
[0015] FIG. 4A is a diagram illustrating a system with an
installation of a power flow controller in accordance with an
embodiment of the invention.
[0016] FIG. 4B is a vector diagram illustrating principles of
operation of a power flow controller in accordance with an
embodiment of the invention.
[0017] FIGS. 5A-C illustrate simulation waveforms of an embodiment
of the invention as described herein.
[0018] FIG. 6A depicts common mode and differential mode controls
implemented in various embodiments of the invention as described
herein.
[0019] FIG. 6B-D illustrate control block diagrams of various
embodiments of the invention as described herein.
[0020] FIG. 7 illustrates an example computing module that may be
used in implementing various features of embodiments of the
invention.
[0021] The figures are not intended to be exhaustive or to limit
the invention to the precise form disclosed. It should be
understood that the invention can be practiced with modification
and alteration, and that the invention be limited only by the
claims and the equivalents thereof.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
[0022] According to various embodiments of the invention, a power
flow controller with a fractionally rated back to back (BTB)
converter is provided. Various embodiments provide dynamic control
of both active and reactive power of a power system. The power flow
controller inserts a voltage with controllable magnitude and phase
between two AC sources; thereby effecting control of active and
reactive power flows between the two sources. In one embodiment, a
transformer is augmented with a fractionally rated bi-directional
BTB converter. In various embodiments, switches of the BTB
converter may be implemented by low-rating insulated gate bipolar
transistors (IGBTs). Further, the power flow controller may
comprise a fail-normal switch that bypasses the power flow
controller in case of a contingency. As the fault current is
diverted by the bypass switch until line breakers trip, the
transformer and the BTB converter of the power flow controller are
isolated from fault currents or high transient voltages during any
fault.
[0023] A power flow controller with a BTB converter controls both
the active and the reactive power flow between two AC sources at
the same frequency. In various embodiments, the fractionally rated
BTB converter comprises a transformer side converter (TSC), a
direct-current link, and a line side converter (LSC). By
controlling the switches in the fractionally rated converter, the
effective phase angle between the two voltages may be regulated and
the amplitude of the voltage inserted by the power flow controller
may be adjusted with respect to the AC source voltages.
[0024] Before describing the invention in detail, it is useful to
describe a few example environments with which the invention can be
implemented. One such example is that of illustrated in FIG. 1.
[0025] FIG. 1 illustrates an exemplary system diagram of an
electric power system 100 where various embodiments of the
invention can be implemented. The electric power system 100
comprises generators 101 and 102; loads 110 and 111; and
transmission lines 103-107, which may have different ratings and
are loaded differently. Various power flow controllers may be
deployed to the power system 100. In the illustrated example, two
power flow controllers 108 and 109 are installed. As a result of
this installation, power flows of the power system 100 may be
controlled. In other words, both the active and reactive power
along each transmission line of the power system 100 may be
redirected.
[0026] From time-to-time, the present invention is described herein
in terms of this example environment. Description in terms of these
environments is provided to allow the various features and
embodiments of the invention to be portrayed in the context of an
exemplary application. After reading this description, it will
become apparent to one of ordinary skill in the art how the
invention can be implemented in different and alternative
environments.
[0027] FIG. 2 illustrates an exemplary schematic diagram of a
single-phase power flow controller 200 in accordance with an
embodiment of the invention. The example power flow controller 200
comprises a transformer 201 and a BTB converter 230. In one
embodiment, the transformer 201 may be an autotransformer. In one
embodiment, the transformer 201 may be an isolated step-down
transformer. The BTB converter 230 comprises a transformer side
converter (TSC) 202 which is an AC-DC converter, a direct-current
(DC) link 203, a line side converter (LSC) 204 which is a DC-AC
converter, input filter inductors 215 and 216, and output filter
inductors 217 and 218. The LSC 204 may be a full-bridge or a
half-bridge DC-AC converter. The TSC 202 and the LSC 204 are
connected through the common DC link 203. In the illustrated
example, the common DC link 203 comprises a capacitor 205 and is a
capacitor link. The TSC 202 comprises switches 206-209, and the LSC
comprises switches 210-213. The TSC 202 comprises legs 220 and 221:
the leg 220 comprises switches 206 and 208, and the leg 221
comprises switches 207 and 209. The legs 220 and 221 of the TSC 202
may be coupled between two taps 241 and 242 of the transformer 201.
In one embodiment, the legs 220 and 221 are coupled between taps
+/-n of the transformer 201, respectively. Further, the LSC 204
comprises legs 222 and 223: the leg 222 comprises switches 210 and
212 and the leg 223 comprises switches 211 and 213. The legs of the
LSC 204 may be both coupled to the output of the BTB converter 230,
either directly or indirectly.
[0028] The power flow controller 200 may be installed between two
AC two sources. In one embodiment, the power flow controller 200
may be installed in series with a transmission line between the two
AC sources. In various embodiments, taps of the transformer 201 may
be floating around the voltage level of the transmission line where
the power flow controller is installed. The input of the power flow
controller 200 is connected to one node of a transmission line and
the output of the power flow controller 200 is connected to another
node of the transmission line. In various embodiments, the
mid-point of the BTB converter 230 may serve as the input of the
power flow controller 200. In further embodiments, the tap 240 of
the transformer 201 may serve as the input of the power flow
controller 200. Taps of the transformer 201 may be floating around
the voltage level at the tap 240 of the transformer 201, which may
be the line voltage. The legs 220 and 221 of the TSC 202 may be
coupled between taps 241 and 242 of the transformer 201,
respectively, either directly or indirectly. The tap 240 may be a
tap that is located between the taps 241 and 242. In one
embodiment, the taps 241 and 242 are +/-n of the transformer 201,
and the tap 240 is the center tap of the tap winding (i.e., +/-n)
of the transformer 201. In various embodiments, the output of the
power flow controller 200 is the output of the BTB converter 230.
The output of the power flow controller 200 is coupled to another
node of the transmission line.
[0029] The control module 220 regulates the switches 206-213. In
various embodiments, the control module 220 may generate switching
pulses to regulate the turn-on and turn-off of each switch. In some
embodiments, the control module 220 may interact with the gate
drivers for switches 206-213. In various embodiments, the switches
206-213 are two-quadrant switches that conduct currents in both
directions but may block voltages in one direction. In some
embodiments, insulated-gate bipolar transistors (IGBTs) with an
antiparallel diode or metal-oxide-semiconductor field-effect
transistor (MOSFETs) with an antiparallel diode may be used as
switches 206-213. An ordinary skill in the art should appreciate
that switches 206-213 may be implemented by other devices.
[0030] Further, the power flow controller 200 may comprise a
fail-normal switch 214. The fail-normal switch 214 may be connected
across the BTB converter 230. In one embodiment, the fail-normal
switch 214 may be coupled to the tap 240 of the transformer 201. In
various embodiments, the fail-normal switch 214 may be realized by
two anti-parallel thyristors. In further embodiments, the
fail-normal switch 214 may be realized by electromechanical or
vacuum switches in parallel with the thyristors. The fail-normal
switch 214 provides fast response. In cases of a converter failure
or a line side fault, the fail-normal switch 214 turns on and
bypasses the transformer 201 and the BTB converter 230 of the power
flow controller 200, which avoids single-point failures and
increases the system reliability. As system faults may result in
currents of the order of 10-20 kA for duration of 5-10 cycles
before being interrupted by protective mechanism. When a fault is
detected, the BTB converter 230 is switched off and the fail-normal
switch 214 is turned on such that the fault current flows through
the fail-normal switch 214. The DC capacitor 205 of the BTB
converter 230 only needs to handle the fault current for a short
delay (10-20 micro seconds) between the fault detection and the
fail-normal switch turn on time. In some embodiments where the
transformer 201 is a load tap changing (LTC) transformer, the
fail-normal switch 214 retains the passive transformer
functionality.
[0031] The BTB converter 230 is a fractionally rated BTB converter
as the input voltage V.sub.S of the BTB converter 230 is a fraction
of the line voltage V.sub.1. For example, the input voltage V.sub.S
of the BTB converter 230 may be less than 10% of the rated voltage
of the transformer 201. In various embodiments, the BTB converter
230 may be coupled between +/-10% taps of the transformer 201. In
turn, the rating of the BTB converter 230 is only a fraction (e.g.,
equal or less than 10%) of the total controlled power rating as the
switches 206-213 handle only a fraction of the transformer 201
rated voltage. Since the BTB converter 230 achieves the fractional
rating because of the fractional voltage rather than the current,
multi-level converter may be implemented in various embodiments
where series operation of the switches cannot be avoided.
[0032] FIG. 3 illustrates an exemplary schematic diagram of a
single-phase 3-level power flow controller 300 in accordance with
an embodiment of the invention. The example power flow controller
300 comprises a transformer 301 and a 3-level BTB converter 340. In
one embodiment, the transformer 301 may be an autotransformer. In
one embodiment, the transformer 301 may be an isolated step-down
transformer. The BTB converter 340 comprises a transformer side
converter (TSC) 302 which is an AC-DC converter, a direct-current
(DC) link 303, a line side converter (LSC) 304 which is a DC-AC
converter, input filter inductors 315-316, and output filter
inductors 317-318 and capacitor 319. In various embodiments, the
LSC 304 may be a full-bridge or a half-bridge DC-AC inverter. The
TSC 302 and the LSC 304 are coupled through the common DC link 303.
In the illustrated example, the common DC link 303 is a capacitor
link comprising two capacitors 305 and 306. The TSC 302 comprises
switches 320-327 and the LSC comprises switches 328-335. The TSC
302 comprises legs 341 and 342: the leg 341 comprises switches
320-323 and the leg 342 comprises switches 324-327. The legs 341
and 342 of the TSC 302 may be coupled between two taps of the
transformer 301. The legs 341 and 342 of the TSC 302 may be coupled
between the taps +/-n of the transformer 301, respectively.
Further, the LSC 304 comprises legs 343 and 344: the leg 343
comprises switches 328-331 and the leg 344 comprises switches
332-335. The legs of the LSC 304 may be coupled to the output of
the BTB converter 340, either directly or indirectly.
[0033] The power flow controller 300 may be installed between two
AC two sources. The power flow controller 300 may be connected in
series with a transmission line. In various embodiments, taps of
the transformer 301 may be floating around the voltage level of the
transmission line where the power flow controller is installed. The
input of the power flow controller 300 is connected to one node of
a transmission line and the output of the power flow controller 300
is connected to another node of the transmission line. In various
embodiments, the mid-point of the BTB converter 340 may serve as
the input of the power flow controller 300. In further embodiments,
the tap 360 of the transformer 301 may serve as the input of the
power flow controller 300. Taps of the transformer 301 may be
floating around the voltage level at the tap 360 of the transformer
301, which may be the line voltage. The legs 341 and 342 of the TSC
302 may be coupled between taps 361 and 362 of the transformer 301,
respectively, either directly or indirectly. The tap 360 may be a
tap that is located between the taps 361 and 362. In one
embodiment, the taps 361 and 362 are +/-n of the transformer 301,
and the tap 360 is the center tap of the tap winding (i.e., +/-n)
of the transformer 301. As illustrated, the tap 360 of the
transformer 301 may be connected to the mid-point of the LSC 304.
In one embodiment, the center tap of the tap winding of the
transformer 301 may be connected to mid-point of the LSC 304 to
reduce the loss of the multiple-level BTB converter 340 including
the switching/conduction and passive losses and to reduce
overheating of the capacitors comprised in the DC link as this
neutral line allows current to flow directly from the input of the
BTB converter to the output of the BTB converter. In various
embodiments, the output of the power flow controller 300 is the
output of the BTB converter 340. The output of the power flow
controller 300 is coupled to another node of the transmission
line.
[0034] The BTB converter 340 is a fractionally rated BTB converter
as the input voltage V.sub.S of the BTB converter 340 is a fraction
of the line voltage V.sub.1. For example, the input voltage V.sub.S
of the BTB converter 340 may be less than 10% of the rated voltage
of the transformer 301. In various embodiments, the BTB converter
340 is connected between +/-10% taps of the transformer 301. In
turn, the rating of the BTB converter 340 is only a fraction (e.g.,
equal or less than 10%) of the total controlled power rating as the
switches 320-335 handle only a fraction of the transformer 301
rated voltage.
[0035] Still referring to FIG. 3, the control module 350 regulates
the switches 320-335. In various embodiments, the control module
350 may generate switching pulses to regulate the turn-on and
turn-off of each switch. In some embodiments, the control module
350 may interact with the gate drivers for switches 320-335. In the
illustrated example, the switches 320-335 are two-quadrant switches
that are implemented by an IGBT and an anti-parallel diode. In some
embodiments, metal-oxide-semiconductor field-effect transistor
(MOSFETs) with an antiparallel diode may be used as switches
320-335. An ordinary skill in the art would appreciate that
switches 320-335 may be implemented by other devices.
[0036] Further, the power flow controller 300 may comprise a
fail-normal switch 315. The fail-normal switch 315 may be connected
across the BTB converter 340. In one embodiment, the fail-normal
switch 315 may be coupled to the tap 360 of the transformer 301. In
various embodiments, the fail-normal switch 315 is realized by two
anti-parallel thyristors. In further embodiments, the fail-normal
switch 315 may be realized by electromechanical or vacuum switches
in parallel with the thyristors. The fail-normal switch 315
provides fast response. In cases of a converter failure or a line
side fault, the fail-normal switch 315 turns on and bypasses the
transformer 301 and the BTB converter 340 of the power flow
controller 300, which avoids single-point failures and increases
the system reliability. As system faults may result in currents of
the order of 10-20 kA for duration of 5-10 cycles before being
interrupted by protective mechanism. When a fault is detected, the
BTB converter 340 is switched off and the fail-normal switch 315 is
turned on such that the fault current flows through the fail-normal
switch 315. The DC capacitors 305-306 of the BTB converter 340 only
need to handle the fault current for a short delay (10-20 micro
seconds) between the fault detection and the fail-normal switch
turn on time. In some embodiments where the transformer 301 is a
load tap changing (LTC) transformer, the fail-normal switch 315
retains the passive transformer functionality.
[0037] FIGS. 4A and 4B illustrate principles of operation of
various embodiments of the power flow controllers with BTB
converters as described herein. FIG. 4A is a diagram illustrating a
system with an installation of a power flow controller 406 in
accordance with an embodiment of the power flow controllers with
BTB converters as described herein. FIG. 4B is a vector diagram
illustrating principles of operation of a power flow controller in
accordance with an embodiment of the invention. The exemplary
system 400 comprises two generators 401 and 403, two buses 402 and
404, and a transmission line 405. V.sub.1 is the voltage at Bus
402, and V.sub.2 is the voltage at Bus 404. In the illustrate
example, the power flow controller 406 is installed in series with
the transmission line 405. The power flow controller 406 performs
dynamic power flow control of both active and reactive power of the
power system 400. Such dynamic power flow control is achieved by
actively controlling the phase and magnitude of the transfer
voltage in a certain range. The BTB converter 407 synthesizes the
converter input voltage V.sub.S to generate a voltage V.sub.CONV
that may be of different magnitude and phase compared to V.sub.S.
As a result, the phase and magnitude of the output voltage
V.sub.out, resultant of Bus 1 Voltage V.sub.1 and V.sub.CONV, may
be controlled to achieve both active and reactive power
control.
[0038] Referring to FIG. 4B, as illustrated, the initial phase
difference between bus 402 voltage V.sub.1 and bus 404 voltage
V.sub.2 is .delta.. The power flow controller 406 inserts a voltage
V.sub.CONV to V.sub.1, which creates the output voltage V.sub.out.
The output voltage V.sub.out and the Bus 402 voltage V.sub.1 may
have different phases and amplitudes. The amplitude of the output
voltage V.sub.out may be adjusted by adjusting the amplitude and
phase angle of the inserted voltage V.sub.CONV. Further, the phase
difference between the output voltage V.sub.out and the Bus 404
voltage V.sub.2 is (.delta.+.phi.), which may be adjusted by
adjusting the amplitude and phase angle of the inserted voltage
V.sub.CONV. As such, control of both active power and reactive
power is achieved as the active power transferred between buses 402
and 404
( P = V out V 2 X Line sin ( .delta. + .phi. ) , ##EQU00001##
where X.sub.Line is the line impedance) is a function of
(.delta.+.phi.), and the reactive power transferred between buses
402 and 404
( Q = V out V 2 X Line ( cos ( .delta. + .phi. ) - V out V 1 ) ,
##EQU00002##
where X.sub.Line is the line impedance) is a function of the
voltage amplitude V.sub.out and V.sub.2.
[0039] The series voltage V.sub.CONV that the power flow controller
can generate is a function of the input voltage V.sub.S, which in
turn depends on the transformer taps across which the BTB converter
is connected. As shown, the range of the voltage V.sub.CONV is a
circle 410 with a radius of
V S 2 . ##EQU00003##
The power flow control range of the power flow controller is a
function of the input voltage V.sub.S, line impedance X.sub.Line,
and the phase difference .delta. between the sending end voltage
V.sub.1 and the receiving end voltage V.sub.2. The active power P,
the sending end reactive power at Bus 402 Q.sub.1, and the
receiving end reactive power at Bus 404 Q.sub.2 may be expressed in
Equations (1), (2), and (3) respectively:
P = V out V 2 X sin ( .delta. + .phi. ) ( 1 ) Q 1 = V out X ( V out
- V 2 cos ( .delta. + .phi. ) ) ( 2 ) Q 2 = V out X ( V 2 - V out
cos ( .delta. + .phi. ) ) wher e V out = V 1 2 + V CONV 2 .phi. =
tan - 1 V CONV sin .theta. V 1 + V CONV cos .theta. ( 3 )
##EQU00004##
[0040] Further, referring back to FIG. 4A, the power flow
controller 406 also has shunt VAR capability. The shunt VAR range
is the same as the BTB converter 407 rating. The range of the shunt
VAR is given in Equation (4):
Q.sub.SHUNT=2n* {square root over
(3)}*V.sub.Bus*I.sub.CONV.sub._.sub.rating (4)
where V.sub.Bus is the line-to-line voltage, n is the transformer
tap ratio, and I.sub.CONV.sub._.sub.rating is the current rating of
the BTB converter.
[0041] As illustrated, various embodiments have circular range of
operation. For a converter with input voltage V.sub.S and the DC
link voltage V.sub.DC, the fundamental voltage that the converter
may synthesize is given by Equation (5):
V CONV = V s , PEAK V DC 2 ( k q sin .theta. + k p cos .theta. )
such that ( k q - 0.5 ) 2 + k p 2 < 0.5 ( 5 ) ##EQU00005##
where V.sub.S,PEAK is the peak voltage of the converter input
voltage, k.sub.q is the reactive power coefficient, and k.sub.p is
the active power coefficient.
[0042] The in-phase component k.sub.q sin .theta. of the converter
voltage controls the reactive power flowing through the line where
the power flow controller is deployed while the out of phase
component k.sub.p cos .theta. controls the active power.
[0043] FIGS. 5A-C illustrate simulation waveforms of an embodiment
of the invention as described herein. One embodiment of a power
flow controller is simulated in a 2-bus 138 kV system with
parameters shown in Table 1. The converter parameters are given in
Table 2. The control parameters are designed using standard bode
plot techniques and are given in Table 3.
TABLE-US-00001 TABLE 1 138 kV Test System Parameters Parameter
Value Voltage 138 kV L-L, 80 kV L-G Line 30 miles, 0.168 + j0.79
ohms/mile Taps (n) +/-5%
TABLE-US-00002 TABLE 2 Converter Parameters Parameter Value V.sub.S
8.0 kV V.sub.DC 12.5 kV C.sub.DC 1 mF L.sub.diff 4 mH L.sub.f 1 mH
C.sub.f 50 microF .DELTA. 2 degrees
TABLE-US-00003 TABLE 3 Converter Control Parameters Parameter Value
Differential Mode V.sub.DC (base) 12.5 kV I.sub.diff (base) 500 A
K.sub.p k.sub.i (voltage loop) 4, 10 respectively K.sub.p k.sub.i
(current loop) 0.05, 0.5 respectively Common Mode V.sub.CONV (base)
4 kV I.sub.line (base) 800 A K.sub.p k.sub.i 0.1, 0.5
respectively
[0044] Referring to FIG. 5A, when .delta. is 2 degrees, the power
flow in the line without the power flow controller is 28 MW. The
power flow controller may vary power flow from 66 MW to -10 MW,
thereby providing a control range of 38 MW while maintaining the
reactive power constant. Referring to FIG. 5B, the converter input
voltage V.sub.S, line current and converter voltage V.sub.CONV at
maximum active power range are shown. The maximum V.sub.CONV that
the converter can generate
V S 2 . ##EQU00006##
Referring to FIG. 5C, the differential current being controlled to
regulate the DC bus voltage is shown. A controllability range of
+/-38 MW/MVAR is achieved with a converter rating of 10% of the
power being controlled. As the converter is connected between +/-5%
taps, the converter only handles peak voltage of 12 kV.
[0045] FIGS. 6A-C depict the control block diagrams in various
embodiments of the invention, which may be implemented by a
computing module as illustrated in FIG. 7. FIG. 6A depicts the
common mode and differential mode control in various embodiments of
the invention as described herein. In the illustrated example, the
LSC 604 is a half-bridge DC-AC converter. In various embodiments,
the power flow in the power flow controller comprises two
components: a common mode and a differential mode. The common mode
is the primary component controlling the line current while the
differential component is used for auxiliary purposes such as
controlling the DC link 603 voltage.
[0046] The common mode power is controlled by the LSC 604 to
generate requisite power flow in the line. The common mode
converter voltage V.sub.comm is regulated controlled to regulate
the common mode current I.sub.Comm, which is the same as the line
current. The LSC 604 may generate either
V S + V DC 2 or V S - V DC 2 ##EQU00007##
depending on the status of the switches 610 and 611 of the LSC 604.
Any voltage waveform that remains within the range of
V S + V DC 2 and V S - V DC 2 ##EQU00008##
may be synthesized by regulating the duty cycle of the switches 610
and 611 of the LSC 604. Accordingly, the maximum fundamental
voltage that may be generated by the converter with respect to the
center tap of the transformer 601 has a peak voltage of
V DC 2 . ##EQU00009##
In some embodiments, V.sub.DC is controlled to be the peak of
V.sub.S.
[0047] The differential power component shuffles energy between the
transformer 601 and the DC bus through the TSC 602. The active
component of the differential mode current I.sub.Diff is controlled
to regulate the mean DC capacitor 609 voltage and the reactive
component is controlled to regulate the shunt VAR of the power flow
controller.
[0048] FIGS. 6B-D illustrate the control block diagrams in various
embodiments of the invention as described herein. The control
schemes comprise a common mode control scheme and a differential
mode control scheme, which may be implemented by various control
modules of different embodiments, for example, the control module
220 as illustrated in FIG. 2 and the control module 350 as
illustrated in FIG. 3.
[0049] FIG. 6B illustrates the common mode control block diagram in
various embodiments of the invention as described herein. In one
embodiment, the common mode control is realized in a current loop.
For desired active power and series VAR control, in various
embodiments, the control module sets the current references for the
common mode control. The control is achieved in d-q synchronous
reference frame. The desired line currents I.sub.comm.sup.ref are
compared with the actual current I.sub.comm, and the error is
propagated through the PI controller, which in turn generates
converter output voltage V.sub.comm.sup.ref. The reference voltage
is then compared with the carrier wave to generate switching pulses
for the LSC of a power flow controller.
[0050] FIG. 6C illustrates a differential mode control block
diagram in various embodiments of the invention as described
herein. In one embodiment, the common mode control is realized in a
voltage loop as the objective of the differential mode control is
to maintain the average DC link voltage V.sub.DC,0 at a desired
value and also control the shunt VAR. The average value of the DC
link voltage is extracted with a low pass filter and then compared
with the reference value. The voltage error is fed to the PI
regulator which in turn generates the TSC reference current
I.sub.diff,d.sup.ref. The control generates the reference for the
reactive component of the differential current
I.sub.diff,q.sup.ref, as per the shunt VAR requirements. The
differential currents are regulated by generating appropriate
switching pulses for the TSC of a power flow controller.
[0051] FIG. 6D illustrates a differential mode control block
diagram in various embodiments of the invention as described
herein. In various embodiments, the reactive power flow across the
capacitor of the DC link may introduce low frequency ripples such
as a 2.sup.nd harmonic voltage ripple across the capacitor. By
achieving power balance between input and output of the converter,
the low frequency ripples and the capacitor size may be minimized.
Instantaneous power balance between the input and the output of the
BTB converter of a power flow controller may be achieved by
introducing controlled harmonics, for example, 3.sup.rd and
5.sup.th harmonic in the input side of the BTB converter, thereby
minimizing instantaneous power imbalance on either side of the
converter. The input power available at the harmonic frequency
together with the available power at the fundamental frequency will
try to balance the fundamental power required on the output of the
converter at each instant. In various embodiments, the DC link
capacitor may be reduced by this control scheme. As the induced
harmonics on the input are controlled, the peak currents through
the switches of the BTB converter are limited. Further, no
harmonics are induced in the line current.
[0052] As illustrated, the in-phase component
V.sub.dc.sub._.sub.LSC,2d and the out-of-phase component
V.sub.dc.sub._.sub.LSC,2q of the 2.sup.nd harmonic ripple flowing
through the DC link of a BTB converter caused by the common mode
current may be determined from the current I.sub.LSC and the
voltage V.sub.LSC of the LSC of a BTB converter. The in-phase
component V.sub.dc.sub._.sub.LSC,2d may be compensated by the
differential mode reactive current component which in turn is
controlled by the q-axis TSC voltage V.sub.TSC,1q. The out-of-phase
component V.sub.dc.sub._.sub.LSC,1q may be compensated by the
3.sup.rd harmonic current in the differential mode, which is
controlled by the 3.sup.rd harmonic voltage V.sub.TSC,3q of the
TSC. The fundamental and the 3.sup.rd harmonic reference voltages
are summed up and compared with the carrier wave to generate
switching pulses for the TSC of a BTB converter. In various
embodiments, the carrier wave is a pulse-width modulation (PWM)
carrier wave.
[0053] As used herein, the term set may refer to any collection of
elements, whether finite or infinite. The term subset may refer to
any collection of elements, wherein the elements are taken from a
parent set; a subset may be the entire parent set. The term proper
subset refers to a subset containing fewer elements than the parent
set. The term sequence may refer to an ordered set or subset. The
terms less than, less than or equal to, greater than, and greater
than or equal to, may be used herein to describe the relations
between various objects or members of ordered sets or sequences;
these terms will be understood to refer to any appropriate ordering
relation applicable to the objects being ordered.
[0054] As used herein, the term module might describe a given unit
of functionality that can be performed in accordance with one or
more embodiments of the present invention. As used herein, a module
might be implemented utilizing any form of hardware, software, or a
combination thereof. For example, one or more processors,
controllers, ASICs, PLAs, PALs, CPLDs, FPGAs, logical components,
software routines or other mechanisms might be implemented to make
up a module. In implementation, the various modules described
herein might be implemented as discrete modules or the functions
and features described can be shared in part or in total among one
or more modules. In other words, as would be apparent to one of
ordinary skill in the art after reading this description, the
various features and functionality described herein may be
implemented in any given application and can be implemented in one
or more separate or shared modules in various combinations and
permutations. Even though various features or elements of
functionality may be individually described or claimed as separate
modules, one of ordinary skill in the art will understand that
these features and functionality can be shared among one or more
common software and hardware elements, and such description shall
not require or imply that separate hardware or software components
are used to implement such features or functionality.
[0055] Where components or modules of the invention are implemented
in whole or in part using software, in one embodiment, these
software elements can be implemented to operate with a computing or
processing module capable of carrying out the functionality
described with respect thereto. One such example computing module
is shown in FIG. 8. Various embodiments are described in terms of
this example-computing module 800. After reading this description,
it will become apparent to a person skilled in the relevant art how
to implement the invention using other computing modules or
architectures.
[0056] Referring now to FIG. 7, computing module 700 may represent,
for example, computing or processing capabilities found within
desktop, laptop and notebook computers; hand-held computing devices
(PDA's, smart phones, cell phones, palmtops, etc.); mainframes,
supercomputers, workstations or servers; or any other type of
special-purpose or general-purpose computing devices as may be
desirable or appropriate for a given application or environment.
Computing module 700 might also represent computing capabilities
embedded within or otherwise available to a given device. For
example, a computing module might be found in other electronic
devices such as, for example, digital cameras, navigation systems,
cellular telephones, portable computing devices, modems, routers,
WAPs, terminals and other electronic devices that might include
some form of processing capability.
[0057] Computing module 700 might include, for example, one or more
processors, controllers, control modules, or other processing
devices, such as a processor 704. Processor 704 might be
implemented using a general-purpose or special-purpose processing
engine such as, for example, a microprocessor, controller, or other
control logic. In the illustrated example, processor 704 is
connected to a bus 702, although any communication medium can be
used to facilitate interaction with other components of computing
module 700 or to communicate externally.
[0058] Computing module 700 might also include one or more memory
modules, simply referred to herein as main memory 708. For example,
preferably random access memory (RAM) or other dynamic memory,
might be used for storing information and instructions to be
executed by processor 704. Main memory 708 might also be used for
storing temporary variables or other intermediate information
during execution of instructions to be executed by processor 704.
Computing module 700 might likewise include a read only memory
("ROM") or other static storage device coupled to bus 702 for
storing static information and instructions for processor 704.
[0059] The computing module 700 might also include one or more
various forms of information storage mechanism 710, which might
include, for example, a media drive 712 and a storage unit
interface 720. The media drive 712 might include a drive or other
mechanism to support fixed or removable storage media 714. For
example, a hard disk drive, a floppy disk drive, a magnetic tape
drive, an optical disk drive, a CD or DVD drive (R or RW), or other
removable or fixed media drive might be provided. Accordingly,
storage media 814 might include, for example, a hard disk, a floppy
disk, magnetic tape, cartridge, optical disk, a CD or DVD, or other
fixed or removable medium that is read by, written to or accessed
by media drive 712. As these examples illustrate, the storage media
714 can include a computer usable storage medium having stored
therein computer software or data.
[0060] In alternative embodiments, information storage mechanism
710 might include other similar instrumentalities for allowing
computer programs or other instructions or data to be loaded into
computing module 700. Such instrumentalities might include, for
example, a fixed or removable storage unit 722 and an interface
720. Examples of such storage units 722 and interfaces 720 can
include a program cartridge and cartridge interface, a removable
memory (for example, a flash memory or other removable memory
module) and memory slot, a PCMCIA slot and card, and other fixed or
removable storage units 722 and interfaces 720 that allow software
and data to be transferred from the storage unit 722 to computing
module 700.
[0061] Computing module 700 might also include a communications
interface 724. Communications interface 724 might be used to allow
software and data to be transferred between computing module 700
and external devices. Examples of communications interface 724
might include a modem or softmodem, a network interface (such as an
Ethernet, network interface card, WiMedia, IEEE 802.XX or other
interface), a communications port (such as for example, a USB port,
IR port, RS232 port Bluetooth.RTM. interface, or other port), or
other communications interface. Software and data transferred via
communications interface 724 might typically be carried on signals,
which can be electronic, electromagnetic (which includes optical)
or other signals capable of being exchanged by a given
communications interface 724. These signals might be provided to
communications interface 724 via a channel 728. This channel 728
might carry signals and might be implemented using a wired or
wireless communication medium. Some examples of a channel might
include a phone line, a cellular link, an RF link, an optical link,
a network interface, a local or wide area network, and other wired
or wireless communications channels.
[0062] In this document, the terms "computer program medium" and
"computer usable medium" are used to generally refer to media such
as, for example, memory 708, storage unit 720, media 714, and
channel 728. These and other various forms of computer program
media or computer usable media may be involved in carrying one or
more sequences of one or more instructions to a processing device
for execution. Such instructions embodied on the medium, are
generally referred to as "computer program code" or a "computer
program product" (which may be grouped in the form of computer
programs or other groupings). When executed, such instructions
might enable the computing module 700 to perform features or
functions of the present invention as discussed herein.
[0063] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example only, and not of limitation. Likewise,
the various diagrams may depict an example architectural or other
configuration for the invention, which is done to aid in
understanding the features and functionality that can be included
in the invention. The invention is not restricted to the
illustrated example architectures or configurations, but the
desired features can be implemented using a variety of alternative
architectures and configurations. Indeed, it will be apparent to
one of skill in the art how alternative functional, logical or
physical partitioning and configurations can be implemented to
implement the desired features of the present invention. Also, a
multitude of different constituent module names other than those
depicted herein can be applied to the various partitions.
Additionally, with regard to flow diagrams, operational
descriptions and method claims, the order in which the steps are
presented herein shall not mandate that various embodiments be
implemented to perform the recited functionality in the same order
unless the context dictates otherwise.
[0064] Although the invention is described above in terms of
various exemplary embodiments and implementations, it should be
understood that the various features, aspects and functionality
described in one or more of the individual embodiments are not
limited in their applicability to the particular embodiment with
which they are described, but instead can be applied, alone or in
various combinations, to one or more of the other embodiments of
the invention, whether or not such embodiments are described and
whether or not such features are presented as being a part of a
described embodiment. Thus, the breadth and scope of the present
invention should not be limited by any of the above-described
exemplary embodiments.
[0065] Terms and phrases used in this document, and variations
thereof, unless otherwise expressly stated, should be construed as
open ended as opposed to limiting. As examples of the foregoing:
the term "including" should be read as meaning "including, without
limitation" or the like; the term "example" is used to provide
exemplary instances of the item in discussion, not an exhaustive or
limiting list thereof; the terms "a" or "an" should be read as
meaning "at least one," "one or more" or the like; and adjectives
such as "conventional," "traditional," "normal," "standard,"
"known" and terms of similar meaning should not be construed as
limiting the item described to a given time period or to an item
available as of a given time, but instead should be read to
encompass conventional, traditional, normal, or standard
technologies that may be available or known now or at any time in
the future. Likewise, where this document refers to technologies
that would be apparent or known to one of ordinary skill in the
art, such technologies encompass those apparent or known to the
skilled artisan now or at any time in the future.
[0066] The presence of broadening words and phrases such as "one or
more," "at least," "but not limited to" or other like phrases in
some instances shall not be read to mean that the narrower case is
intended or required in instances where such broadening phrases may
be absent. The use of the term "module" does not imply that the
components or functionality described or claimed as part of the
module are all configured in a common package. Indeed, any or all
of the various components of a module, whether control logic or
other components, can be combined in a single package or separately
maintained and can further be distributed in multiple groupings or
packages or across multiple locations.
[0067] Additionally, the various embodiments set forth herein are
described in terms of exemplary block diagrams, flow charts and
other illustrations. As will become apparent to one of ordinary
skill in the art after reading this document, the illustrated
embodiments and their various alternatives can be implemented
without confinement to the illustrated examples. For example, block
diagrams and their accompanying description should not be construed
as mandating a particular architecture or configuration.
* * * * *