|Publication number||USRE41170 E1|
|Application number||US 11/655,817|
|Publication date||Mar 30, 2010|
|Priority date||Nov 24, 1999|
|Also published as||CA2392409A1, CA2392409C, CA2676240A1, DE60031158D1, DE60031158T2, EP1236261A1, EP1236261B1, EP1701425A2, EP1701425A3, US6577108, US20020075701, US20030026114, WO2001039349A1|
|Publication number||11655817, 655817, US RE41170 E1, US RE41170E1, US-E1-RE41170, USRE41170 E1, USRE41170E1|
|Inventors||Thomas Gregory Hubert, Douglas C. Folts, Warren Elliott Buckles|
|Original Assignee||American Superconductor Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (34), Non-Patent Citations (4), Referenced by (8), Classifications (10), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
ThisMore than one reissue application has been filed for the reissue of U.S. Pat. No. 6,577,108. The present reissue application Ser. No. 11/655,817 is a divisional of reissue application 11/150,603 (now abandoned), which is a reissue of U.S. application Ser. No. 10/196,707, which application is a continuation (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 10/002,847, filed Nov. 14, 2001, now abandoned, of U.S. application Ser. No. 09/718,672, filed Nov. 22, 2000 now abandoned, and of U.S. Provisional Application Serial No. 60/167,377, filed Nov. 24, 1999. The disclosures of the prior applications are considered part of (and are incorporated by reference in) the disclosure of this application.
The following applications are hereby incorporated by reference into this application as if set forth herein in full: (1) U.S. patent application Ser. No. 09/240,751, entitled “Electric Utility Network With Superconducting Magnetic Energy Storage” and filed on Jan. 29, 1999; (2) U.S. Provisional Application No. 60/117,784, entitled “Electric Utility Network With Superconducting Magnetic Energy Storage” and filed on Jan. 29, 1999; (3) U.S. patent application Ser. No. 09/449,505, entitled “Method and Apparatus for Discharging a Superconducting Magnet” and filed on Nov. 24, 1999; (4) U.S. patent application Ser. No. 09/449,436, entitled “Method and Apparatus for Controlling a Phase Angle” and filed on Nov. 24, 1999; (5) U.S. patent application Ser. No. 09/449,378, entitled “Capacitor Switching” and filed on Nov. 24, 1999; (6) U.S. patent application Ser. No. 09/449,375, entitled “Method and Apparatus for Providing Power to a Utility Network” and filed on Nov. 24, 1999; (7) U.S. patent application Ser. No. 09/449,435, entitled “Electric Utility System with Superconducting Magnetic Energy Storage” and filed on Nov. 24, 1999; and (8) U.S. Provisional Application No. 60/167,377, entitled “Voltage Regulation of a Utility Power Network” and filed on Nov. 24, 1999.
This invention relates to electric power utility networks including generating systems, transmission systems, and distribution systems serving loads. In particular, the invention relates to controlling the transfer of energy to and from a utility power network. Energy storage devices, including capacitor banks and superconducting magnetic energy storage devices (SMES), are used to provide power to a utility power network in order to compensate for power shortfalls or voltage instability problems on the network. For example, in the event of a fault or outage on the network, power may be transferred from an energy storage device to the network to ensure that the amount of power on the network remains within acceptable limits.
The invention features a system for controlling a power compensation device, such as an inverter connected to a utility power network, to operate in an “overload” mode. Operating in an overload mode means operating the power compensation device in excess of its maximum steady-state power delivery characteristic (e.g., power delivery rating). This reduces the cost of heat dissipation elements in the compensating device and reduces the number of solid state switching devices required therein.
In one aspect, the invention is a system that includes a controller which controls a reactive power compensation device to deliver, for a first period of time and in response to a detected change in a nominal voltage, reactive power to the utility power network. In a second period of time following the first period of time, the controller controls the reactive power compensation device to provide reactive power to the utility power network at a level that is a factor N(N>1) greater than a maximum power capability characteristic of the reactive power compensation device.
In another aspect, the invention is directed to providing power compensation from a power compensation device to a utility power network carrying a nominal voltage, the power compensation device having a steady-state power delivery characteristic. This aspect features detecting a change of a predetermined magnitude in the nominal voltage on the utility power network, and controlling the power compensation device to deliver, for a first period of time and in response to the detected change in the nominal voltage, reactive power to the utility power network. The power compensation device is controlled to deliver, for a second period of time following the first period of time, reactive power to the utility power network at a level that is a factor N(N>1) greater than the steady-state power delivery characteristic of the power compensation device.
Having detected and reacted to a change of a predetermined magnitude in the nominal voltage on the utility power network by increasing injected power to a level that is as much as N times higher than the maximum steady-state power delivery characteristic of the compensation device, power injection of the compensating device can be purposefully and gradually reduced to the maximum steady-state value so as not to include a transient response by the network that could result in voltage instability and/or other undesirable events.
Among other advantages, these aspects of the invention provide an approach for operating a reactive power compensation device in an overload mode for a maximum period of time without incurring an abrupt, step-like change in inverter current at the time the overload capability of the compensating device has been expended, thereby forcing the compensating device's current to be at or below a specified level. Thus, as noted, the invention reduces the possibility of undesirable transients (e.g., ringing oscillations) in the utility power network. Furthermore, a substantially optimum ramp down profile can be determined on the basis of the characteristic impedance of the network.
Embodiments of the foregoing aspects of the invention may include one or more of the following features. During the first period of time, the compensation device provides real power and reactive power to the utility power network. After the second period of time, the reactive power from the compensation device is non-discontinuously decreased to the steady-state power delivery characteristic. The factor N is generally determined on the basis of a transient thermal capacity characteristic (e.g., a 1% rating) of the compensation device. The second period of time is determined on the basis of the ability of the compensation device to absorb thermal energy. The ramp down profile may be determined on the basis of the characteristic impedance of the network. The characteristic impedance of the network may be determined using known characteristics of the network. Alternatively, the reactive power compensation device can apply a stimulus to the network and a response measured.
These and other features and advantages of the invention will be apparent from the following description, drawings and claims.
Power compensation system 30 includes an energy storage unit 32, an inverter system 44, and a controller 60. Energy storage unit 32 may be a part of a D-SMES (Distributed SMES) module which, together with inverter system 44, is capable of delivering both real and reactive power, separately or in combination, to distribution line 20. In this embodiment, the DSMES module is sized a 3.0 MVA and is capable of delivering an average of 2 MWatts for periods as long as 400 milliseconds, 7.5 MVA for a full second, and 3.0 MVAR of reactive power for an indefinite period of time. As described below, inverter 44, under the intelligent control of controller 60, transfers reactive power to and from the utility power network.
Each of the four inverter units 46 is capable of providing 750 KVA continuously and 1.875 MVA in overload mode for one second. The outputs of each inverter unit 46 are combined on the medium-voltage side of the power transformers to yield system ratings in accordance with the following table.
MVA delivered, leading or
MVA delivered, leading or
1-2 seconds in event of
lagging, overload condition
transmission or distribution fault
Average MW delivered to utility
0.4 seconds in event of
(for an exemplary D-SMES
transmission or distribution fault
Each inverter unit 46 includes three parallel inverter modules (not shown). Because inverter units 46 are modular in form, a degree of versatility is provided to accommodate other system ratings with standard, field-proven inverter modules. A level of fault tolerance is also possible with this modular approach, although system capability may be reduced. Each inverter module 46 is equipped with a local slave controller (not shown) that manages local functions, such as device protection, current regulation, thermal protection, power balance among modules, and diagnostics, among others. The inverter units and modules are mounted in racks with integral power distribution and cooling systems.
Inverter 44 is coupled to distribution line 20 through one or more step-down power transformers 50 and one or more switchgear units 52 (see also FIG. 1). Each power transformer 50 is a 24.9 kV/480 V three-phase oil-filled pad mount transformer having a nominal impedance of 5.75% on its own base rating. The power transformers are mounted outdoors adjacent to the system enclosure with power cabling protected within an enclosed conduit (not shown). As is shown in
Referring back to
Referring again to
With reference to
As can be seen from
Referring again to
where 1/f,=tS, the sample period and In is the sampled instantaneous inverter current.
To obtain a value that is proportional to the energy that is dissipated above the rated, steady-state dissipation capability of the inverter (i.e., a value related to the transient thermal capacity limit), a ratio of the instantaneous inverter current (In) to the steady state limit (Imax=InvtrIRefMax) is obtained as follows:
This expression represents the accumulated thermal energy of the inverter, a static variable that is updated every AC line cycle. Calculation of the accumulation of energy continues, as shown by the dotted line of FIG. 3.
Referring again to
The ramp-down profile is typically a function of the characteristic impedance of the utility network to which it is connected. However, the characteristic impedance of a network changes unpredictably over time. In one approach, a suitable characteristic impedance value of the network can be derived from knowledge of the types of loads, conductors, reactive devices and transformers connected to the network. Alternatively, the characteristic impedance of the network can be determined by periodically applying a stimulus (e.g., a step function load) to the network and measuring the response of the network. In particular, inverter 44 can be used to apply the step function load, while controller 60 measures the response. Of course, the step function load would be of sufficiently low magnitude to prevent stimulation of undesirable oscillations. The characteristic impedance is then used to determine the ramp-down profile.
The summation of each cycle of inverter heat energy being accumulated must be calculated for each AC line cycle beginning with the initiation of the overload current above the steady-state maximum value. This summation is mathematically simple. But, the accumulation must also be dynamically estimated for each remaining cycle of the ramp-down process in order to be able to determine when to initiate the processes, as well as to ensure that ramp-down is proceeding such that the inverter's heat capacity limit will not be exceeded. Because the value of inverter current is controlled and predictable for each cycle of the process, a conceptually straightforward summation of each of the heat contributions during each of these cycles can be performed, but not without significant mathematical overhead, in practice. However, this mathematically intensive calculation can be simplified dramatically using the closed form approach described below. Simplifying this calculation permits the use of a less costly controller and/or significantly conserves the controller's bandwidth for other tasks.
where I is the inverter current, InvtrIRefMax(=Imax) is as defined above, and AmpsPerCycle is the slope of the ramp-down of the current. The foregoing expression represents the thermal capacity predictor for determining when the inverter must begin or continue the ramp-down of overcurrent toward the maximum steady-state value.
The final expression for limiting the overcurrent period of inverter 44 is the sum of equations (1) and (2), as follows:
Note that the slope of the thermal energy content (heat content) of the inverter gradually declines during the ramp-down period in which the capacitive reactance from inverter 44 is reduced, and the slope becomes negative only after the inverter current reaches its maximum steady-state rating.
At this point, the process has computed the accumulation of energy being dissipated in the inverter through regions 74 and 82. Region 74 refers to that part that has actually accumulated, while region 82 represents the estimated accumulation that will occur from the current sample until the inverter current reaches the steady-state level. Samples are accumulated once per cycle for both regions 74 and 82, although the accumulation in region 82 is for estimation purposes. Moreover, inverter 44 generally cannot dissipate its heat at the same rate that the power delivered to the utility network is reduced. Thus, controller 60 must have sufficient intelligence to recognize that, in the event of a subsequent contingency, the thermal energy content of the inverter may not have decreased back to a level corresponding to the steady-state current level.
When the inverter current declines to the InvtrIRefMax level (212) (FIG. 3), the inverter will begin to cool. To reflect the cooling process, the accumulation procedure must be modified. In particular, although accumulation of heat energy is still computed, what is accumulated is a recovered capacity rather than an extended capacity. To do this, controller 60 begins the process by selecting (214) an incrementally higher value of estimated inverter current than the level of InvtrIRefMax (the maximum steady-state value) and using this value as if it were the actual inverter current. By using this value in the heat accumulation estimation process described above, controller 60 can verify whether the estimated current can be successfully reduced to InvtrIRefMax quickly enough so as not to exceed the thermal capacity limit of the inverter (in the event that a subsequent request for an over-current is required). In particular, controller 60 determines whether the inverter thermal capacity limit will be exceeded if the ramp-down process were to be initiated at the incrementally-larger estimated current level previously mentioned. If it is not exceeded, a constant value is subtracted from the accumulation of heat energy (216) and the value of the current is incremented by the value depicting the slope of the ramp-down process, called AmpsPerCycle. The estimate is again performed at the next sample period. The constant value represents the inverter's thermal recovery increment, a value that essentially gauges the state of recovery of the inverter from the overload. If the estimated current results in a prediction that exceeds the inverter's heat capacity limit, the thermal recovery increment is still decremented by the constant value, provided that the inverter current is actually at or below InvtrIRefMax, but the inverter current estimate remains unchanged, as it is used to constrain the peak current if a new overload current is requested. The process continues and, eventually, the full overload thermal capability of the inverter is restored and the overload current reaches its limit of N times the steady-state rating.
Thus, controller 60 controls inverter 44 to provide a maximum amount of inverter current should another contingency occur. Controller 60 does so without exceeding the capability the inverter and by providing a ramping-down to the steady state InvtrIRefMax level, while ensuring that the thermal capacity of the inverter is not exceeded by the time that the current declines to the InvtrIRefMax level.
For example, as shown in
Other embodiments not explicitly described herein are also within the scope of the claims. For example, in the embodiment described above in conjunction with
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|U.S. Classification||323/207, 363/37, 323/205|
|International Classification||H02J3/18, G05F3/30, H02M5/45, G05F1/70|
|Cooperative Classification||Y02E40/22, H02J3/1842|
|Oct 26, 2007||AS||Assignment|
Owner name: AMERICAN SUPERCONDUCTOR CORPORATION,MASSACHUSETTS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:HUBERT, THOMAS GREGORY;FOLTS, DOUGLAS C.;BUCKLES, WARRENT ELLIOTT;SIGNING DATES FROM 20010509 TO 20010514;REEL/FRAME:020022/0373
|Jun 8, 2010||CC||Certificate of correction|
|Dec 10, 2010||FPAY||Fee payment|
Year of fee payment: 8
|Dec 10, 2014||FPAY||Fee payment|
Year of fee payment: 12