U.S. patent application number 09/449436 was filed with the patent office on 2002-01-03 for method and apparatus for controlling a phase angle of ac power to keep dc voltage from an energy source constant.
Invention is credited to BUCKLES, WARREN ELLIOTT, FOLTS, DOUGLAS C..
Application Number | 20020002115 09/449436 |
Document ID | / |
Family ID | 23784157 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020002115 |
Kind Code |
A1 |
BUCKLES, WARREN ELLIOTT ; et
al. |
January 3, 2002 |
METHOD AND APPARATUS FOR CONTROLLING A PHASE ANGLE OF AC POWER TO
KEEP DC VOLTAGE FROM AN ENERGY SOURCE CONSTANT
Abstract
A transfer of power between an energy storage device and a load
is controlled by obtaining a DC voltage from the energy storage
device, and controlling a phase angle of AC power delivered to the
load to keep the DC voltage substantially constant. The phase angle
of the AC power is controlled by controlling a current component of
the AC power.
Inventors: |
BUCKLES, WARREN ELLIOTT;
(Madison, WI) ; FOLTS, DOUGLAS C.; (Baraboo,
WI) |
Correspondence
Address: |
GARY A WALPERT
FISH & RICHARDSON PC
225 FRANKLIN STREET
BOSTON
MA
021102804
|
Family ID: |
23784157 |
Appl. No.: |
09/449436 |
Filed: |
November 24, 1999 |
Current U.S.
Class: |
363/14 |
Current CPC
Class: |
H02J 15/007 20200101;
H02J 3/28 20130101; H02J 15/00 20130101 |
Class at
Publication: |
505/867 |
International
Class: |
H02M 007/537; H02M
005/00 |
Claims
What is claimed is:
1. A method of controlling a transfer of power between an energy
storage device and a load, comprising: obtaining a DC voltage from
the energy storage device; and controlling a phase angle of AC
power delivered to the load to keep the DC voltage substantially
constant.
2. The method of claim 1, wherein the energy storage device
comprises a current-mode energy storage device.
3. The method of claim 2, wherein the current-mode energy storage
device comprises a superconducting magnet.
4. The method of claim 1, further comprising maintaining the DC
voltage substantially constant by controlling a current component
of the AC power.
5. The method of claim 1, further comprising controlling a current
component of the AC power to keep an output voltage component of
the AC power substantially constant.
6. The method of claim 1, wherein controlling the phase angle is
performed using an inverter.
7. The method of claim 6, wherein controlling the phase angle of
the AC power includes compensating for power losses in the
inverter.
8. The method of claim 7, wherein compensating for power losses in
the inverter comprises: determining a difference between voltage at
input terminals of the inverter and a preset voltage value; and
changing the phase angle of the AC power in accordance with the
difference.
9. The method of claim 1, wherein the transfer of power comprises
discharging power from the energy storage device to the load.
10. A system for controlling a transfer of power between an energy
storage device and a load, comprising: an inverter arranged between
the energy storage device and the load, the inverter having input
terminals which exchange DC voltage with the energy storage device
and output terminals which exchange AC power with the load; and a
controller which (i) obtains the DC voltage at the input terminals,
and (ii) controls a phase angle of AC power delivered to the load
to keep the DC voltage substantially constant.
11. The system of claim 10, wherein the energy storage device
comprises a current-mode energy storage device.
12. The system of claim 11, wherein the currentmode energy storage
device comprises a superconducting magnet.
13. The system of claim 10, wherein the controller maintains the DC
voltage substantially constant by controlling a current component
of the AC power.
14. The system of claim 10, wherein the controller controls a
current component of the AC power to keep an output voltage
component of the AC power substantially constant.
15. The system of claim 10, wherein controlling the phase angle of
the AC power comprises compensating for power losses in the
inverter.
16. The system of claim 15, wherein compensating for power losses
in the inverter comprises: determining a difference between voltage
at input terminals of the inverter and a preset voltage value; and
changing the phase angle of the AC power in accordance with the
difference.
17. The system of claim 10, wherein the transfer of power comprises
discharging power from the energy storage device to the load.
18. A computer program stored on a computer-readable medium for
controlling a transfer of power between an energy storage device
and a load, the computer program comprising instructions that cause
a computer to: obtain a DC voltage from the energy storage device;
and control a phase angle of AC power delivered to the load to keep
the DC voltage substantially constant.
19. The computer program of claim 18, wherein the energy storage
device comprises a current-mode energy storage device.
20. The computer program of claim 19, wherein the current-mode
energy storage device comprises a superconducting magnet.
21. The computer program of claim 18, further comprising
instructions that cause the computer to maintain the DC voltage
substantially constant by controlling a current component of the AC
power.
22. The computer program of claim 18, further comprising
instructions that cause the computer to control a current component
of the AC power to keep an output voltage component of the AC power
substantially constant.
23. The computer program of claim 18, wherein the AC power is
output from an inverter and controlling the phase angle of the AC
power comprises compensating for power losses in the inverter.
24. The computer program of claim 23, wherein compensating for
power losses in the inverter comprises: determining a difference
between voltage at input terminals of the inverter and a preset
voltage value; and changing the phase angle of the AC power in
accordance with the difference.
25. The computer program of claim 18, wherein the transfer of power
comprises discharging power from the energy storage device to the
load.
Description
INCORPORATION BY REFERENCE
[0001] The following applications are hereby incorporated by
reference into the subject application as if set forth herein in
full: (1) U.S. Patent Application No. 09/240,751, entitled
"Electric Utility Network With Superconducting Magnetic Energy
Storage", filed Jan. 29, 1999; (2) U.S. Provisional Application No.
60/117,784, entitled "Electric Utility Network With Superconducting
Magnetic Energy Storage", filed Jan. 29, 1999; (3) U.S. patent
application No.______, entitled "Method And Apparatus For Providing
Power To A Utility Network", filed Nov. 24, 1999 (Atty. Docket No.
05770/091001); (4) U.S. patent application No.______ , entitled
"Method And Apparatus For Discharging A Superconducting Magnet",
filed Nov. 24, 1999 (Atty. Docket No. 05770/087001); (5) U.S.
patent application No.______ , entitled "Capacitor Bank Switching",
filed Nov. 24, 1999 (Atty. Docket No. 05770/089001); (6) U.S.
Provisional Application No.______ , entitled "Voltage Regulation Of
A Utility Power Network", filed Nov. 24, 1999 (Atty. Docket No.
05770/090001); and (7) U.S. patent application No.______ , entitled
"Electric Utility System With Superconducting Magnetic Energy
Storage", filed Nov. 24, 1999 (Atty. Docket No. 05770/092001).
BACKGROUND OF THE INVENTION
[0002] This invention relates to controlling the transfer of energy
between a current-mode storage device and a load by controlling a
phase angle of power delivered to the load.
[0003] Energy storage devices, such as a superconducting magnet,
have been contemplated to provide power to a utility network to
compensate for power shortfalls in the network. For example, large
superconducting magnetic energy storage ("SMES") systems have been
proposed for diurnal energy storage. In such devices, energy is
transferred from a utility network to the storage device when the
utility demand is low (e.g., at night) and energy is transferred
from the storage device to the utility network when the demand is
high (e.g., summer afternoons).
SUMMARY OF THE INVENTION
[0004] Energy transfers between a current-mode storage device and a
load (such as a utility network) are more efficient when a voltage
across output terminals of the current-mode storage device is
constant. Such energy transfers are referred to as "constant
voltagey" transfers.
[0005] The invention features a method of controlling a transfer of
power between an energy storage device and a load. In general, in
one aspect, the method includes obtaining a DC voltage from the
energy storage device, and controlling a phase angle of AC power
delivered to the load to keep the DC voltage substantially
constant. By controlling the phase angle to keep the DC voltage
substantially constant, constant voltage energy transfers can be
performed more efficiently without complex switching mechanisms or
additional hardware, such as hysteretic controllers or storage
cells.
[0006] This aspect of the invention may include one or more of the
following features. The energy storage device includes a
current-mode energy storage device, such as a superconducting
magnet. The transfer of power includes discharging power from the
energy storage device to the load. The method maintains the DC
voltage substantially constant by controlling a current component
of the AC power. The method controls the current component of the
AC power to keep an output voltage component of the AC power
substantially constant. Thus, the invention can be used to meet the
constant voltage requirements of many utilities.
[0007] Discharging the magnet at a substantially constant voltage
decreases discharging time (relative to constant power or constant
resistance discharge). As a result, the likelihood (and/or amount)
of damage to the magnet can be reduced. Also, if the magnet is
connected to a utility network, for example through an inverter,
discharging the magnet at a substantially constant voltage
increases the rate at which power can be supplied to the utility
network. As a result, the utility network can be stabilized more
quickly following a fault.
[0008] The method may be performed using an inverter. Power losses
in the inverter may be compensated by controlling the phase angle
of the AC power. Compensating for power losses in the inverter
includes determining a difference between voltage at input
terminals of the inverter and a preset voltage value, and changing
the phase angle of the AC power delivered by the inverter in
accordance with the difference.
[0009] Other advantages and features of the invention will become
apparent from the following description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is view of a superconducting magnet.
[0011] FIG. 2 is block diagram of circuitry for discharging the
superconducting magnet into a load.
[0012] FIG. 3 is a block diagram of a control board used in the
circuitry of FIG. 2.
[0013] FIG. 4 is a flow diagram showing a process for discharging
the superconducting magnet.
[0014] FIG. 5 is a flow diagram showing a process for controlling a
phase angle of output current/power to discharge the
superconducting magnet at constant voltage.
[0015] FIG. 6 is block diagram of alternative circuitry for
discharging the superconducting magnet into a load.
[0016] FIG. 7 is block diagram of alternative circuitry for
discharging the superconducting magnet into a load.
[0017] FIG. 8 is block diagram of alternative circuitry for
discharging the superconducting magnet into a load.
[0018] FIG. 9 is a vector diagram showing real and reactive
components of output AC power.
[0019] FIG. 10 is a flow diagram showing a process for discharging
the superconducting magnet at constant voltage.
[0020] FIG. 11 is a block diagram of circuitry for compensating for
inverter power loss during discharging of the superconducting
magnet.
[0021] FIG. 12 is a flow diagram showing a process for controlling
the phase angle of AC output current to keep the AC output voltage
substantially constant.
[0022] FIG. 13 is a graph showing a mode of magnet discharge.
[0023] FIG. 14 is a graph showing an alternative mode of magnet
discharge.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring to FIG. 1, a superconducting magnet 10,
representing a superconducting magnetic energy storage ("SMES")
device and cryogenic refrigeration unit, includes an energy storage
magnetic coil 11 positioned within a containment vessel 12 of the
cryogenic refrigeration unit. In one embodiment, containment vessel
12 encloses magnetic coil 11 in liquid helium 14, and is fabricated
from two austenitic stainless steel vessels 13 separated by a
vacuum insulated space 15. In alternative embodiments, vessels 13
may be formed of stainless steel, aluminum, or epoxy fiberglass
composite.
[0025] The cryogenic refrigeration unit includes one or more
Gifford-McMahon type coolers (not shown) operating in concert to
maintain cryogenic temperatures within vessel 12 and to re-liquify
helium vapor building up within the vessel. Under normal operating
conditions, helium (liquid or gaseous) does not circulate outside
vessel 12. External, room temperature, refrigeration system gasses
are not interchanged with the internal helium supply. Preferably,
the system design permits continuous operation, with one or both
coolers inoperable, for a minimum of 48 hours.
[0026] Superconducting magnetic coil 11 is wound with a
low-temperature superconducting cable formed from niobiumtitanium
copper-based matrix wire that has been cabled into a mechanically
stable form and insulated prior to winding. An alternative
embodiment uses a coil fabricated of high-temperature
superconductor. Superconducting coil 11 also can be embodied with a
high temperature superconductor cooled with an appropriate cryogen,
such as helium or nitrogen. A suitable superconducting magnetic
coil is available from American Superconductor Corporation of
Westborough, Mass.
[0027] Discharging the Superconducting Magnet FIG. 2 shows
superconducting magnet 10 coupled to an AC ("Alternating Current")
load 15. Load 15 may represent, but is not limited to, a utility
network, such as that described in U.S. patent application No.
09/240,751 and U.S. Provisional Application No. 60/117,784. Thus,
load 15 may include reactive as well as resistive elements (not
shown).
[0028] Arranged between load 15 and magnet 10 is circuitry 16 which
couples magnet 10 to load 15. Circuitry 16 includes a magnet
charger 19 for charging magnet 10. Charger 19 may be an inverter or
other device which receives power from the utility network or any
other external source. Circuitry 16 also includes a DC-AC (with
"DC" referring to "Direct Current") power inverter 17; however,
other types of circuitry may be used instead of, or in addition to,
such an inverter. Inverter 17 can be any type of DC-AC power
converter, such as single level H-bridge, single level six-switch,
multi-level, and stacked H-bridge inverters.
[0029] Inverter 17 operates in a "discharge" mode, during which
energy is provided from magnet 10 to load 15. In the "discharge"
mode, inverter 17 converts DC power from superconducting magnet 10
into AC power, and provides that AC power to load 15. In this mode,
thyristor 20, which serves as a switch, is biased to produce an
open circuit and diode 22, which also serves as a switch, is biased
to produce a short circuit. This allows current to flow from magnet
10 to load 15.
[0030] Control circuitry 24 detects increases in resistance (i.e.,
quenches) in superconducting magnet 10 and, in response, controls
inverter 17 and switch 20 so that superconducting magnet 10
discharges its energy into load 15 at a substantially constant
voltage (V.sub.DC across terminals 25). Control circuitry 24 may
include, for example, a quench comparator circuit. The quench
comparator circuit detects increases in voltage across one or more
portions of coil 11, which are indicative of resistance in coil 11.
Control circuitry 24, including the quench comparator circuit, may
be implemented as hardware or as software (computer instructions)
executing on one or more controllers.
[0031] For example, as shown in FIG. 3, control circuitry 24 may
comprise three controllers 27, 29 and 30 (e.g., microprocessors) on
a single circuit board. In addition, this circuitry includes
appropriate driver circuits and analog-to-digital ("A/D ")
converters (not shown).
[0032] Controller 27 executes computer instructions to control
current and voltage loops in inverter 17 (for use in discharging
magnet 10 and maintaining a constant output AC voltage from
inverter 17, as described below). Controller 29 functions as the
system controller, meaning that it executes computer instructions
to detect an amount of power at load 15, to detect the quench
status of superconducting magnet 10, to open or close switch 20, to
control magnet charger 19, and to provide information to controller
27. Controller 30 records information obtained by or generated by
controllers 27 and 29, formats that information, and displays it to
a user. Although this embodiment shows three controllers, any
number of controllers (e.g., one controller) can be used to perform
the functions attributed to controllers 27, 29 and 30. The computer
instructions executed in each controller may be stored in one or
more memories (not shown) in circuitry 24 or an internal memory of
each controller.
[0033] FIG. 4 shows a process 32 for discharging magnet 10. Process
32 begins by detecting 401 a quench in magnet 10. As noted, the
quench is detected when a change, in particular an increase, is
detected in the voltage across superconducting coil 11 in magnet
10. Detecting 401 may be performed by a cryostat controller (not
shown) or by controller 29. The controller monitors the voltage
across two or more symmetric winding segments of magnet coil 11. If
the voltage difference between the symmetric segments exceeds a
predetermined threshold, a quench is detected. In response to the
increase in voltage, controller 29 instructs controller 27 to
discharge energy from magnet 10. Controller 27 controls discharging
(402) so that V.sub.DC remains substantially constant throughout
the discharging time.
[0034] To maintain V.sub.DC substantially constant, controller 27
controls a phase relationship between the voltage and the current
at the output terminals 34 (FIG. 2) of inverter 17. Specifically,
controller 27 perturbates the nominal 90 degree phase angle between
output AC voltage and output AC current to maintain the voltage
level at DC terminals 25 at a substantially constant value.
Normally, to keep V.sub.DC constant, as the output current from
magnet 10 (namely, I.sub.DEVICE) decreases, the output current
magnitude from inverter 17 would be decreased. Since the output
power is the product of the output current and utility voltage,
this effectively decreases the inverter's output power to match the
power available from magnet 10, which is the product of
I.sub.DEVICE and V.sub.DC. For utility grid stabilization, however,
it is desirable to maximize both real and reactive power. Because
of this, it is preferable to operate the inverter at its maximum
output current rating. Hence, controlling the phase angle of the
inverter output current with respect to the utility voltage is the
way by which this embodiment regulates power flow and, thus,
V.sub.DC.
[0035] One process for controlling the phase relationship between
voltage and current is shown in FIG. 5 below; however, other
processes may be used. During discharge, inverter 17 may be
operated in overload mode (e.g., substantially above its continuous
power rating). The period during which the inverter can operate in
overload mode depends upon the thermal capacity of the
inverter.
[0036] Returning to FIG. 4, once magnet 10 is discharged to a
predetermined level, process 32 ends. During normal discharge of
magnet 10, for utility grid voltage stabilization purposes, magnet
10 may be discharged to a non-zero value. However, when magnet 10
is discharged in response to a quench, magnet 10 may discharge to
zero.
[0037] Other circuit configurations may also be used to implement
magnet discharging process 32. For example, FIG. 6 shows magnet 10
coupled to a DC load 36. In this configuration, DC load 36 includes
two gate turn-off devices, such as IGCT ("Integrated Gate
Commutated Thyristor") power switching devices 37 and 39 which are
connected to resistive load 39a. Additional capacitive
inductive-resistive circuitry (e.g., a snubber circuit) (not shown)
may be connected in parallel to the gate turn-off devices. IGCT
devices 37 and 39 are alternatively opened and closed (by control
circuitry 24) to discharge magnet 10 in such a way that a
substantially constant voltage V.sub.DC is maintained across
terminals 25. The resistive load 39a and IGCT devices modulate the
discharged power in accordance with control signals from control
circuitry 24. Output diodes 38a and 38b control power flow between
IGCT devices 37 and 39. IGCT devices 37 and 39 may also be other
gate-controlled semiconductor switches, such as, but not limited
to, GTO's ("Gate Turn-off Thyristors") or IGBT's ("Insulated Gate
Bipolar Transistors") used either singly or in series/parallel.
[0038] FIG. 7 shows still another circuit configuration that may be
used to implement magnet discharging process 32. This configuration
is similar to that of FIG. 2, except that it includes a profiling
circuit 40 in shunt across terminals 41. Profiling circuit 40
includes a resistor 42 and an SCR ("Silicon Controlled Rectifier")
device 44 which operates as either an open or closed switch. If the
current of magnet 10 drops below a predetermined level, such as
below 25% of its maximum value (which usually occurs near the end
of a discharging cycle), controller 27 outputs a signal to close
the switch of SCR device 44. Triggering SCR device 44 causes
current from magnet 10 to flow through profiling circuit 40, thus
changing the discharging process from a constant voltage
discharging process to a constant resistance discharging
process.
[0039] A relatively small resistance for resistor 42 causes magnet
10 to do a resistive discharge, in which the voltage of magnet 10
is low and discharge time is long (see FIG. 13). A relatively
higher value for resistor 42 results in a constant voltage
discharge for times t.sub.0-t.sub.1 and in a constant resistance
discharge after time t.sub.1 (see FIG. 14).
[0040] Profiling circuit 40 can be "switched in" at other points
during the discharging process. For example, if inverter 17
malfunctions before or during magnet discharging, profiling circuit
40 can be "switched in" to discharge magnet 10.
[0041] Controlling Power Phase Angle to Maintain Constant
Voltaqe
[0042] Referring to FIG. 8, the real power output from inverter 17
is a product of I.sub.DEVICE and V.sub.DC where I.sub.DEVICE is the
DC current from magnet 10 and V.sub.DC is the input DC voltage to
inverter 17. As magnet 10 drains, the value of I.sub.DEVICE
decreases. To keep V.sub.DC substantially constant in spite of this
decrease in I.sub.DEVICE inverter 17 varies the phase angle of its
output AC current (and thus the phase angle of the output AC
power). Inverter 17 changes the phase angle to increase power flow
towards the DC side or into the inverter when V.sub.DC is below a
nominal value and to increase power flow out of the inverter when
V.sub.DC is above the nominal value.
[0043] Referring to FIG. 9, a vector diagram 50 illustrates the
(sinusoidal) AC power output by inverter 17 as a function of real
power (axis 52) and reactive power (axis 54). At real power axis
52,I.sub.DEVICE is at its maximum value. This corresponds to a time
when superconducting magnet 10 is fully, or almost fully, charged.
Inverter 17 outputs its maximum amount of real power ("kW") along
real power axis 52. This is because real power is a function of the
product of V.sub.DC and I.sub.DEVICE and I.sub.DEVICE is at its
maximum. The vector output would lie on the real power axis 52 only
for the special case of magnet power (V.sub.DC I.sub.DEVICE) equal
to the maximum inverter rating. Typically, the inverter power
rating is greater than magnet power.
[0044] Along the reactive power axis 54, I.sub.DEVICE is at its
minimum (in this case, zero). Inverter 17 outputs its maximum
amount of reactive power ("kVARI") along reactive power axis 54 and
uses the energy from this reactive power to maintain V.sub.DC
substantially constant (since reactive power does not result in
aggregate energy dissipation to the load). Reactive power may be
sourced from the utility network and then output back to the
utility network, or it may be obtained from other sources.
[0045] Between axes 52 and 54, the power output of inverter 17 is a
combination of both real and reactive components. Power vector 51
represents the amount of power that is output by inverter 17. The
degree to which power vector 51 is real or reactive is determined
on the basis of phase angle (".theta.") 55 between vector 51 and
real power axis 52. For example, to determine the amount of real
power provided by vector 51 at .theta., a line 56 is drawn from
vector 51 to real power axis 52. Intersection point 59 corresponds
to the amount of real power provided by vector 51. Reactive power
is determined similarly (by drawing a line from vector 51 to
reactive power axis 54).
[0046] As energy is discharged from magnet 10, I.sub.DEVICE drops .
The amount of current (and thus real power) output from inverter 17
can be reduced correspondingly by varying the phase angle. For
example, in FIG. 8, the phase angle of the output current can be
varied so that less real power is provided by inverter 17 (thus
increasing .theta.).
[0047] FIG. 5 shows a process 62 for varying the phase angle of the
AC output current during magnet discharging to keep V.sub.DC
constant. Process 62 can be implemented as a computer program
(instructions) executed by controller 27. Controller 27 first
obtains a value of V.sub.DC (501). This value may be obtained,
e.g., from controller 29 (which, as described above, monitors
V.sub.DC). Alternatively, controller 27 may obtain the value of
V.sub.DC itself (by monitoring terminals 41) or from another
source.
[0048] The phase angle of the AC power output from inverter 17 is
controlled to keep V.sub.DC substantially constant (502).
Controlling is performed by comparing (503 to 506) the value for
V.sub.DC obtained in step 501 to a target (predetermined) value.
This value may be preset in controller 27, or it may be set, for
example, based on an initial value of V.sub.DC. Alternatively, a
controller within inverter 17 could perform this function.
[0049] The results of the comparisons in steps 503 to 506 determine
how much, and in what direction, to vary the phase angle of the
inverter's output AC current. For example, if process 62 determines
(503) that the measured value of V.sub.DC is considerably larger
than the target value (for example, a voltage nearing the
inverter's maximum voltage rating), then process 62 decreases (507)
the phase angle at a fairly rapid rate, such as the inverter's slew
rate limit. This rapidly increases the amount of real power being
supplied to utility network 61, and correspondingly decreases the
value of V.sub.DC. If process 62 determines (504) that the measured
value of V.sub.DC is larger than the target value, but not
inordinately so, then process 62 decreases (508) the phase angle at
a less rapid rate, such as half of the inverter's slew rate limit.
This increases the amount of real power being supplied to utility
network 61, and correspondingly decreases the value of
V.sub.DC.
[0050] On the other hand, if process 62 determines (505) that the
measured value of V.sub.DC is considerably less than the target
value, then process 62 increases (509) the phase angle at a fairly
rapid rate, such as the inverter's slew rate limit. This rapidly
decreases the amount of real power being supplied to utility
network 61, increases the amount of reactive power being supplied,
and correspondingly increases the value of V.sub.DC. If process 62
determines (506) that the measured value of V.sub.DC is less than
the target value, but not inordinately so, then process 62
increases (510) the phase angle at a less rapid rate, such as half
of the inverter's slew rate limit. This decreases the amount of
real power being supplied to utility network 61, increases the
amount of reactive power being supplied, and correspondingly
increases the value of V.sub.DC. Process 502 depicts a proportional
control system which can be implemented in hardware, software, or a
combination of the two. In simple terms, an error signal (V.sub.DC
-V.sub.TARGET) is used to control a system variable (phase angle).
The rate-of-change of the system variable is proportional to the
error, hence the name: proportional controller.
[0051] The foregoing comparisons 503 to 506 operate to maintain
V.sub.DC substantially constant during discharging (or charging) of
magnet 10. While four such comparisons are shown, the invention is
not limited as such. For example, there may only be two
comparisons--one for determining if V.sub.DC is greater than the
nominal value and one for determining if V.sub.DC is less than the
nominal value. In this case, only one rate is used for increasing
the phase angle of the output AC current and one rate is for
decreasing that phase angle. Alternatively, the invention may
include more than the four comparisons of process 62, each with its
own corresponding rate for varying the phase angle.
[0052] Process 62 may be incorporated into other processes. For
example, FIG. 10 shows a process 64 that uses process 62 to
discharge superconducting magnet 10 (FIG. 8) at a substantially
constant V.sub.DC voltage. Process 64 may be implemented by a
computer program (instructions) executing on one or more of
controllers 27, 29 and 30 (FIG. 3).
[0053] To begin, process 64 monitors (1001) magnet 10 to detect a
quench. This is described above. Once a quench has been detected,
process 64 sets (1002) the phase angle of the inverter's output AC
current to 900 (full leading). Process 64 initiates (1003) the
overload mode of inverter 17, and opens (1004) switch 60 (FIG. 8)
thereby allowing all current from magnet 10 to flow to inverter 17.
In addition, it commands magnet charger 19 to turn off, thereby
preventing additional power from flowing to the magnet. A minimum
overload value in this case is 100% overload for one second (or two
PU for one second, where "PU" is "Per Unit", the ideal continuous
output current of inverter 17). Process 64 moves (1005) the phase
angle of the output current in accordance with detected conditions
in the magnet. For example, if a large resistive zone is detected
in magnet 10, process 64 may decrease the phase angle considerably
to discharge the magnet quickly. Then, process 64 calls (1006)
process 62 for discharging the magnet while maintaining V.sub.DC
substantially constant. Process 62 is performed until (1007) magnet
10 reaches a preset energy level, which is generally not zero.
[0054] Process 64 closes (1008) switch 60 and determines (1009) if
the overload mode of the inverter has ended. Once the overload mode
ends, process 64 ends, whereafter magnet 10 may be repaired and/or
replaced, as required.
[0055] Power losses in inverter 17 may result in minor deviations
in the phase angle of the output power. Such losses can cause minor
fluctuations in V.sub.DC during charging and/or discharging. FIG.
11 is a block diagram of circuitry 70 that can be used to
compensate for the power losses in inverter 17, and thus reduce
fluctuations in V.sub.DC. This circuitry 70 can be implemented
using discrete hardware components and/or instructions executing on
a controller. Master controller may be, for example, the control
circuitry 24 of FIG. 3. PLL 74 may be implemented in software
within one of controllers 27, 29 or 30. Phase compensator 71 may
reside in software within a controller in inverter 17.
[0056] Circuitry 70 includes phase angle compensator 71, master
controller 72, and phase-locked loop ("PLL") 74. Also included in
FIG. 11 are monitor 75 and DC bus capacitor 76. DC bus capacitor 76
stores an actual value of V.sub.DC across the input terminals of
inverter 17. Monitor 75 monitors this value, compares it to a
preset target voltage value, and determines a difference between
the actual value of V.sub.DC and the preset nominal value. This
information is provided to phase angle compensator 71.
[0057] PLL 74 and master controller 72 determine a phase angle of
power on utility system 61 and output current sampled from inverter
17. This determination is based on voltage sampled from utility
system 61. The phase angle is provided to phase angle compensator
71 (via master controller 72). Phase angle compensator 71 uses this
phase angle and the difference in voltage provided from monitor 75
to determine a phase angle offset. The phase angle offset is the
amount by which a phase angle set by inverter 17 must be offset to
compensate for power losses in inverter 17. Generally, the phase
angle offset has a negative value, which means that inverter 17
receives power from utility system 61 in order to compensate for
the lost power.
[0058] Maintaining Constant Output Voltage
[0059] Inverter 17 is also controlled to maintain a substantially
constant output AC voltage. The value of this output voltage may be
dictated by the utility network or by whatever system is receiving
power from inverter 17.
[0060] The output AC voltage is controlled by regulating the output
AC current and phase from inverter 17. Specifically, controller 27
senses (FIG. 3) the output AC voltage at terminals 77 of inverter
17 (FIG. 8) and regulates the magnitude of reactive current output
from inverter 17 in order to keep the output AC voltage
substantially constant. FIG. 12 shows a process 80, which is
implemented by computer instructions executing on controller 27,
for maintaining a substantially constant output AC voltage from
inverter 17.
[0061] Process 80 begins by integrating (1101) the root-mean-square
("RMS") of the AC voltage output from inverter 17 over a period of
time. This integration is performed several times on all three
phases of voltage, and the results from all three phases are summed
and are averaged (1102) to determine the average output voltage
V.sub.avg of inverter 17. Process 80 compares (1103) V.sub.avg to a
target voltage value, typically 1 PU which, in this example, is 480
V. If V.sub.avg is greater than this nominal value, process 80
decreases (1104) the reactive current out of inverter 17, which
causes a corresponding decrease in output AC voltage. If V.sub.avg
is less than this nominal value (1105), process 80 increases (1106)
the reactive current out of inverter 17, which causes a
corresponding increase in output AC voltage. Process 80 may be
implemented independently on each phase or line of the utility
system using a 3-phase inverter of a suitable type or three
single-phase inverters.
[0062] It is noted that the reactive current can be leading or
lagging. To boost or increase the voltage on a utility system, one
can either decrease the amount of lagging reactive current or
increase the amount of leading reactive current being injected into
the utility. Likewise, to buck or decrease the utility voltage ,
one can decrease the leading current or increase the lagging
injected current.
[0063] Other embodiments not described herein are also within the
scope of the following claims. For example, the invention can be
used in connection with any current-mode energy storage device,
such as a synchronous flywheel, and not just superconducting
magnets. Also, combinations of hardware and/or software not
described herein may be used. For example, GTO ("Gate Turn-off
Thyristor") or IGBT ("Insulated Gate Bipolar Transistor") switches
may be used in the embodiments of the foregoing figures. The
invention may be used in a DSMES ("Distributed Superconducting
Magnetic Energy Storage System"), such as that described in U.S.
patent application No. 09/240,751 and U.S. Provisional Application
No. 60/117,784.
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