Abstract:
The invention features a system and approach for minimizing the step voltage change as seen by the utility customer as well minimizing transients imposed on the fundamental waveform of a normal voltage carried on a utility power network when a reactive power source (e.g., capacitor bank) is instantaneously connected to the utility power. The reactive power source is adapted to transfer reactive power of a first polarity (e.g., capacitive reactive power) to the utility power network. The system includes a reactive power compensation device configured to transfer a variable quantity of reactive power of a second, opposite polarity to the utility power network, and a controller which, in response to the need to connect the shunt reactive power source to the utility power network, activates the reactive power compensation device and, substantially simultaneously, causes the shunt reactive power source to be connected to the utility power.

Description:
INCORPORATION BY REFERENCE  
       [0001]    This application herein incorporates by reference the following applications: U.S. application Ser. No. 09/240,751, which was filed on Jan. 29, 1999, U.S. application Serial No. 60/117,784, filed Jan. 29, 1999, U.S. application Ser. No. ______, entitled “Discharging a Superconducting Magnet”, filed Nov. 24, 1999; U.S. application Ser. No. ______, entitled “Method and Apparatus for Controlling a Phase Angle”, filed Nov. 24, 1999;; U.S. application Ser. No. ______, entitled “Voltage Regulation of a Utility Power Network”, filed Nov. 24, 1999; U.S. application Ser. No. ______, entitled “Method and Apparatus for Providing Power to a Utility Network, filed Nov. 24, 1999; and U.S. application Ser. No. ______, entitled “Electric Utility System with Superconducting Magnetic Energy Storage”, filed Nov. 24, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to electric power utility networks including generating systems, transmission systems, and distribution systems serving loads.  
           [0003]    Utility power systems, particularly at the transmission level, are primarily inductive, due to the impedance of transmission lines and the presence of numerous transformers. Further, many of the largest loads connected to the utility power system are typically inductive. Large motors used, for example, in lumber mills, rock crushing plants, steel mills, and to drive pumps, shift the power factor of the system away from the desired unity level, thereby decreasing the efficiency of the power system. Because of the daily and hourly load variations, it is necessary to change the amount of compensation applied to counteract the effects of these changing inductive loads  
           [0004]    One approach for providing compensation to the system is to connect one or more large shunt capacitor banks to provide a capacitive reactance (e.g., as much as 36 MVARs) to the system in the event of a contingency (i.e., a nonscheduled event or interruption of service) or sag in the nominal voltage detected on the utility power system. By selecting the proper amount of capacitance and connection location, these capacitor banks provide a level of control of the line voltage or power factor. Mechanical contactors are typically employed to connect and switch the capacitor banks to compensate for the changing inductive loads.  
         SUMMARY OF THE INVENTION  
         [0005]    The invention features a system and approach for minimizing the step voltage change experienced by the utility customer as well minimizing transients imposed on the fundamental waveform of a normal voltage carried on a utility power network when a reactive power source (e.g., capacitor bank) is instantaneously connected to the utility power. The reactive power source is adapted to transfer reactive power of a first polarity (e.g., capacitive reactive power) to the utility power network.  
           [0006]    In one aspect of the invention, the system includes a reactive power compensation device configured to transfer a variable quantity of reactive power of a second, opposite polarity to the utility power network, and a controller which, in response to the need to connect the shunt reactive power source to the utility power network, activates the reactive power compensation device and, substantially simultaneously, causes the shunt reactive power source to be connected to the utility power network.  
           [0007]    In another aspect of the invention, a method of providing reactive power compensation from a reactive power source to a utility power network carrying a nominal voltage includes the following steps. A change in magnitude in the desired nominal voltage on the utility power network is detected, and such change results in voltage deviating outside of a utility specified acceptable range. In response to detecting the change in the desired nominal voltage, the reactive power source is connected to the utility power network to provide reactive power compensation of a first polarity. For a predetermined first duration, reactive power compensation of a second opposite polarity is provided to the utility power network in a period substantially coincident with connecting the reactive power source to the utility power network.  
           [0008]    By transferring reactive power of a second, opposite polarity to the network when the switch is closed, the magnitude of a potentially large step-like change in reactive power introduced from the reactive power source is offset for a period of time, thereby minimizing potential transients which would normally be imposed over the fundamental utility waveform carried on the utility power network. These transients are caused by the generally step-like change in voltage when the reactive power source is connected to the utility power network. Although there are many forms of transients, which can be imposed on the utility waveform, such transients are typically in the form of oscillatory “ringing” imposed over the fundamental waveform. Such ringing can cause among other problems, false switching of power devices and overvoltage failures. In addition, the sudden step voltage change induced by switching the utility reactive device can disrupt sensitive industrial control systems and processes. An overvoltage failure can be catastrophic to customers. In essence, the system “softens” the sharp, step-like introduction of reactive energy from the reactive power source.  
           [0009]    Embodiments of these aspects of the invention may include one or more of the following features.  
           [0010]    In a preferred embodiment, the controller is configured to activate the reactive power compensation device to transfer reactive power compensation of the first polarity to the utility power network prior to connecting the shunt reactive power source to the utility power network. As stated above, providing reactive power compensation of the second, opposite polarity to the utility power network opposes the abrupt step like introduction to the utility power network of reactive power of the first polarity delivered by the shunt reactive power source. Providing reactive power compensation of the first polarity prior to connecting the shunt reactive power source to the utility power network, allows a significantly greater magnitude of change in reactance when the reactive power compensation of the second polarity is introduced. Furthermore, the reactive power compensation device provides additional voltage support to the system prior to the shunt reactive power source being connected to the utility power network.  
           [0011]    The reactive power compensation of the first polarity is generally provided for a duration between 1 and 2 seconds.  
           [0012]    The impedance of a utility power network is primarily inductive, due to the long line lengths and presence of transformers. Thus, in a preferred embodiment, the reactive power source is a capacitor bank and during particular time periods the reactive power compensation device provides inductive power compensation.  
           [0013]    The system and method are used with a utility power network that includes a transmission network and a distribution network electrically connected to the transmission network. The distribution network has distribution lines, with the reactive power source normally connected to the transmission network and the reactive power compensation device connected to the distribution network of the utility power network and proximally to each other.  
           [0014]    Typically, reactive power compensation is switched on when the nominal voltage drops below 98% and switched off when voltage exceeds 102% of the nominal voltage. Moreover, the allowable step change in the voltage due to switching of the reactive compensation device is typically limited to about 2% at the transmission voltage level  
           [0015]    In certain applications, after providing reactive power compensation of the second opposite polarity, a second stage of reactive power compensation of the first polarity is provided in conjunction with the reactive power source providing reactive power compensation. In other words, the reactive power compensation device supplements the reactive power provided by the reactive power source.  
           [0016]    For example, in an emergency mode operation, the voltage on the utility power network may have dropped significantly. In this case, the inverter will operate continuously to provide reactive power in conjunction with the capacitor bank. If the inverter is only operated for a relatively short, emergency mode, the inverter may be operated in overload fashion to provide a maximum amount of reactance. Alternatively, the inverter can be operated in a steady state mode, to provide a lower reactance level over a longer, indefinite duration.  
           [0017]    These and other features and advantages of the invention will be apparent from the following description of a presently preferred embodiment and the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram representation of a voltage recovery device and switched capacitor bank connected to a utility power network.  
         [0019]    [0019]FIG. 2 is a block diagram of a portion of reactive power compensation device of FIG. 1 connected to a distribution line.  
         [0020]    [0020]FIG. 3 is a flow diagram illustrating the general steps for operating the voltage recovery device.  
         [0021]    [0021]FIG. 4 is a graph showing the output of the reactive compensation device.  
         [0022]    [0022]FIG. 5 is a graph showing the utility voltage characteristic as a function of time using the reactive compensation device. 
     
    
     DETAILED DESCRIPTION  
       [0023]    Referring to FIG. 1, a reactive power compensation system  30  is shown connected in shunt with a distribution line  20  of utility power network. Distribution line  20  is shown connected to a transmission line  18  of the transmission line network through a first transformer  22   a,  which steps down the higher voltage (e.g., greater than 25 kV carried on transmission line  18  to a lower voltage, here 6 kV. A second transformer  22   b  steps down the 6 kV to a voltage suitable for a load  24 , here 480 V.  
         [0024]    Reactive power compensation system  30  includes an energy storage unit  32 , an inverter system  44 , and a controller  60 , which is used in conjunction with a transmission capacitor bank  31 . Energy storage unit  32  may be in a part of a D-SMES module, which is capable, together with inverter system  44 , of delivering both real and reactive power, separately or in combination, to distribution line  20 . In this embodiment, D-SMES module could be sized at 3.0 MVA and with inverter  44  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 indefinitely. Further details relating to the operation and construction of the D-SMES module can be found in co-pending application, Ser. No. ______, filed on Nov. 24, 1999, by ______, and entitled “Electric Utility System with Superconducting Magnetic Energy Storage.  
         [0025]    As will be described in greater detail below, inverter  44 , under the intelligent control of controller  60 , serves to transfer reactive power to and from the utility power network. In particular, during the initial period in which capacitor bank  31  begins delivering reactive power to the utility power network, inverter  44  provides an inductive reactance to counteract the abrupt, step-like introduction of capacitive reactive power from capacitor bank  31  on the utility power network. Furthermore, inverter  44  can be controlled to provide additional voltage support to the system prior to capacitive bank  31  being connected to the utility power.  
         [0026]    Capacitor bank  31  provides a capacitive reactance (e.g., as much as 36 MVARs) to the system in the event of a contingency (i.e., a nonscheduled event or interruption of service) or sag in the nominal voltage detected on the utility power system. Capacitive banks suitable for use with reactive power compensation system  30  are commercially available from ABB, Zurich Switzerland. Further details relating to capacitor banks used in conjunction with superconducting energy storage systems can be found in U.S. Pat. No. 4,962,354, U.S. Pat. No. 5,194,803, and U.S. Pat. No. 5,376,828, all of which are incorporated herein by reference.  
         [0027]    Capacitor bank  31  is coupled to transmission line  18  through a relay switch  35  and a switchgear unit  39 , which provide over-current protection and to facilitate maintenance and troubleshooting of capacitor bank  31 . A protective fuse  41  is connected between switchgear  39  and transmission line  18 .  
         [0028]    Referring to FIG. 2, inverter system  44  converts DC voltage from energy storage unit  32  to AC voltage and, in this embodiment, includes four inverter units  46 . In general, inverter  44  can act as a source for leading and lagging reactive power. In general, inverter can only source real power from energy storage unit  32  for as long as real power is available from the energy storage unit. However, inverter  44  can source reactive power indefinitely assuming the inverter is operating at its nominally rated capacity. Thus, inverter  44  can provide reactive power without utilizing power from energy storage unit  32 . Further details regarding the arrangement and operation of inverter  44  can be found in co-pending application, Ser. No. ______, filed on Nov. 24, 1999, by ______, and entitled “Electric Utility System with Superconducting Magnetic Energy Storage.” 
         [0029]    Each inverter unit  46  is capable of providing 750 KVA continuously and 1.875 MVA in overload for one second. The outputs of each inverter unit  46  are combined on the medium-voltage side of the power transformers to yield the system ratings in accordance with the following table.  
                                       Power Flow   Value   Duration                   MVA delivered, leading or   3.0   Continuously       lagging       MVA delivered, leading or   7.5   1-2 seconds in event of       lagging, overload condition       transmission or distribution fault               detection       Average MW delivered to utility   2.0   0.4 seconds in event of       (for an exemplary D-SMES       transmission or distribution fault       module).       detection                  
 
         [0030]    Each inverter unit  46  includes three 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 is equipped with a local Slave Controller that manages local functions such as device protection, current regulation, thermal protection, power balance among modules, and diagnostics, among others. Inverter units and modules are mounted in racks with integral power distribution and cooling systems.  
         [0031]    Inverter system  44  is coupled to distribution line  20  through step-down transformers  50  and switchgear units  52 . Each power transformer  50  is a 6 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 generally mounted outdoors adjacent to the system enclosure with power cabling protected within an enclosed conduit (not shown). As is shown in FIG. 1, a fuse  53  is connected between step-down transformer  50  and distribution line  20 .  
         [0032]    Each switchgear unit  52  provides over-current protection between power transformers  50  and inverter units  46 . Each of the four main inverter outputs feeds a circuit breaker rated at 480 V, 900 A RMS continuous per phase with 45 kA interruption capacity. Switchgear units  52  also serve as the primary disconnect means for safety and maintenance purposes. The switchgear units are generally mounted adjacent to the inverter unit enclosures.  
         [0033]    Referring again to FIG. 1, system control unit  60  has a response time sufficient to ensure that the transfer of power to or from energy storage unit  30  occurs at a speed to address a fault or contingency on the utility system. In general, it is desirable that the fault is detected within 1 line cycle (i.e., {fraction (1/60)} second for 60 Hz, {fraction (1/50)} second for 50 Hz). In one embodiment, the response time is less than 500 microseconds.  
         [0034]    With reference to FIGS.  3 - 5 , the operation of controller  60  and inverter  44  is described in conjunction with an exemplary contingency occurring on the utility power network. At the outset, the nominal voltage of the utility power system is monitored (step  200 ). For example, the nominal voltage on transmission line  18  is sensed either directly or from a remote device. FIG. 5 shows that in this particular example the voltage is detected as being 98% of nominal value at t=0. When the nominal voltage has dropped below a predetermined threshold value (e.g., here 98%), an input control signal is transmitted to controller  60  which, in turn, transmits a trigger signal  73  (at point  75  of FIG. 5) to activate inverter  44  (step  202 ) and begin ramping inverter reactive output from zero to full overload rating in 0 to 2 seconds. When full leading output of the inverter has been achieved, a signal is sent to close mechanical contactor  35  (step  204 ).  
         [0035]    Referring to FIG. 4, prior to enabling switch  35  to operate, inverter system  44  is activated to ramp upward to provide the maximum amount of capacitive reactance available, for example, +7.5 MVARs). Because inverter is not intended to provide this maximum reactive power for more than a few seconds, inverter system  44  is operated in an overload mode. Simultaneous with the closing of contactor  35  (at point  77  of FIGS. 4 and 5), inverter  44  is controlled to now provide the maximum available inductive reactance, for example, −7.5 MVARs (step  206 ). The time period between step  202  and setup  206  is set based on the known characteristics of mechanical contactor  35  or can be learned by controller  60  which monitors the change in voltage. As shown in FIG. 5, inverter  44  alone has increased the voltage by 1.45% prior to energizing capacitor bank  31 .  
         [0036]    In a second step, when contactor  35  closes, capacitor bank  31  injects capacitive reactance, here 36 MVARs, onto the utility power system. During this period in which capacitive bank  31  is switched into the circuit, the voltage increases an additional 0.98%. The inductive reactance provided by inverter  44  cancels in part the capacitive reactance from capacitor bank  31 . This mitigates possible “ringing” caused by the rapid introduction of reactance onto the sagging utility power signal were capacitor bank  31  be allowed to unleash its full 36 MVARs onto the utility power network.  
         [0037]    In a third step—immediately after contactor  35  is closed—the inductive reactance provided by inverter  44  ramps down (at point  79 ) until the inverter no longer generates reactive power (at point  81 ). During this third step the voltage increases an additional 1.14%. At this point, the sole reactance being introduced to the utility power network is from capacitor bank  31 .  
         [0038]    As can be seen from FIG. 5, this approach softens the otherwise step-like injection of capacitive reactance from capacitor bank  31  (represented by dashed line  83 ). Moreover, the full 3.6% voltage increase provided by capacitor bank  31  has been accomplished without an abrupt step-like injection of reactive power. Furthermore, the full 3.6% voltage increase is provided in three steps, none of which exceeds the 2% limit that utilities generally require.  
         [0039]    However, in circumstances in which additional capacitive reactance, beyond that provided by capacitive bank  31 , would be desirable, inverter  44  can be controlled to provide supplemental capacitive reactance.  
         [0040]    Referring to FIG. 4, inverter  44  is controlled to provide additional capacitive reactance in an “emergency overload mode.” It is important to note that during this second capacitive reactance period  87 , capacitor bank  31  is also providing capacitive reactance to the utility power network. In this overload mode, inverter  44  provides the maximum reactance available. In an alternative application, where capacitive reactance is desired over longer periods (perhaps, indefinitely), inverter  44  may be controlled to provide a lower level (e.g., 2-3 MVARs) in a steady state mode of operation. In applications where real power does not need to be supplied to the utility power network, the invention would be implemented without energy storage unit  32 .  
         [0041]    Further details relating to the control of inverter  44  to adjust the phase angle of the reactance, can be found in co-pending applications, Ser. No. ______, filed on Nov. 23, 1999, by ______, entitled “Phase Angle Control,” and in Ser. No. ______, filed on Nov. 23, 1999, by ______, entitled “Voltage Regulation Control.  
         [0042]    It is also important to appreciate that the invention is equally applicable in situations when capacitor bank  31  is removed from the utility power network. That is, a similar step voltage would decrease occur when capacitor bank is switched off. In this case, the process described above in conjunction with FIGS.  3 - 5  is reversed.  
         [0043]    Other embodiments are within the scope of the claims. For example, in the embodiment described above in conjunction with FIGS. 1 and 2, a D-SMES unit was discussed as being used to provide the real and reactive power needed to recover the voltage on the transmission network. However, it is important to appreciate that other voltage recovery devices capable of providing both real and reactive power, including flywheels, batteries, an energy storage capacitive systems bank, compressed gas energy sources, and fuel cell systems (e.g., those that convert carbon based fuels into electricity) are also within the scope of the invention.  
         [0044]    Still other embodiments are within the scope of the claims. For example, the invention can also be used in conjunction with other approaches for minimizing transient effects. For example, the invention can complement those approaches using zero-switching techniques, such as that described in U.S. Pat. No. 5,134,356, which is incorporated herein by reference. The utility power network described above in conjunction with FIG. 1 included distribution lines connected to a load  24 .