Abstract:
A method of controlling a static VAR compensator includes providing a static VAR compensator having a capacitive component and a thyristor for switching the capacitive component into and out of a power distribution network; monitoring an electrical characteristic associated with the capacitive component; and controlling operation of the thyristor at least in part on the basis of the electrical characteristic associated with the capacitive component.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to electrical power distribution, and in particular, to controlling the complex power vectors in a power transmission system. 
       BACKGROUND 
       [0002]    The waveform that exists at all points in a power transmission system is ideally a sinusoid of constant frequency. In the US, that frequency is 60 Hz. However, in many parts of the world, the frequency is 50 Hz. 
         [0003]    The power transmitted by a power transmission and distribution system is represented by a power vector in the complex plane. The imaginary component of the power vector represents reactive power, while the real component represents the power that actually carries out useful work. The ratio between the real component of the power vector and the magnitude of the power vector is referred to as a “power factor.” 
         [0004]    The relative magnitudes of the real and reactive power depend on system loads. These system loads, which can change from time to time, affect a phase relationship between the voltage and current waveform on the transmission line. This phase relationship depends on the imaginary part of the impedance (i.e. the reactance) seen by the transmission line. As these loads increase, the transmission and/or distribution line voltages can change from their desired levels. 
         [0005]    In response to changes in the load, one can provide or withdraw reactive VARs to control the voltage level on the network. One known device for carrying out this function is a static VAR compensator. These static VAR compensators use high-voltage thyristors to connect and disconnect a reactive capacitor from the line. The control of these thyristors is critical to avoid damaging them. 
         [0006]    The control of the thyristors depends in part on the voltage waveform present on the line. This voltage waveform is subject to electrical disturbances. In some cases, these electrical disturbances are dominated by harmonic content that is, to some extent, predictable. When this is the case, techniques such as that described in U.S. patent application Ser. No. 12/749,390, filed on Mar. 30, 2010 and incorporated herein by reference, can be used to determine when gate current can safely be turned off, and when it should be applied. However, when the electrical disturbances occur at random, the techniques disclosed in the foregoing patent application are less effective. 
       SUMMARY 
       [0007]    In one aspect, the invention features a method of controlling a static VAR compensator. Such a method includes providing a static VAR compensator having a capacitive component and a thyristor for switching the capacitive component into and out of a power distribution network; monitoring an electrical characteristic associated with the capacitive component; and controlling operation of the thyristor at least in part on the basis of the electrical characteristic associated with the capacitive component. 
         [0008]    In some practices of the invention, the electrical characteristic is a voltage across the capacitive component. Among these practices are those that also include obtaining an estimate of a time rate of change, the time rate of change being a rate of change of the voltage waveform with respect to time. In such cases, controlling operation of the thyristor includes applying a current to a gate of the thyristor based at least in part on a relationship between the estimate and a safety threshold. In yet others of these practices, controlling operation of the thyristor includes controlling operation at least in part based on the estimate. 
         [0009]    Some additional practices of the invention also include obtaining an estimate of a derivative of the time rate of change. In these practices, controlling operation of the thyristor includes controlling operation of the thyristor at least in part on the basis of the estimate of the second derivative. 
         [0010]    Alternative practices include those in which controlling operation of the thyristor includes applying a gate current to the thyristor when the estimate of the time rate of change reaches a first designated value, as well as those in which controlling operation of the thyristor includes ceasing application of a gate current to the thyristor when the estimate of the time rate of change reaches a second designated value. 
         [0011]    In yet other practices, controlling operation of the thyristor includes applying a gate current to the thyristor when the estimate of the time rate of change reaches a first designated value, and ceasing application of the gate current when the estimate of the time rate of change next reaches the designated value. 
         [0012]    In another aspect, the invention features an apparatus for static VAR compensation in a power distribution network. Such an apparatus includes a capacitive load; a thyristor for causing the reactive load to be switched into and out the power distribution network; a current source for applying a gate current to the thyristor; and a controller for causing gate current to be applied and removed on the basis of an estimate of a time rate of change, the time rate of change being a rate of change of a voltage across the capacitive load. 
         [0013]    Some embodiments also include a digital signal processing circuit for providing the estimate of the time rate of change based on a signal representative of a voltage across the capacitive load. In other embodiments, an analog circuit provides the estimate of the time rate of change based on a signal representative of a voltage across the capacitive load. 
         [0014]    In another aspect, the invention features an apparatus for static VAR compensator in a power distribution network. Such an apparatus includes a capacitive load; a thyristor for causing the reactive load to be switched into and out the power distribution network; means for applying a gate current to the thyristor; and means for causing gate current to be applied and removed at least in part on the basis of an estimate of a time rate of change, the time rate of change being a rate of change of a voltage across the capacitive load. 
         [0015]    Among the embodiments are those that also include means for estimating the time rate of change, and those in which the means for causing gate current to be applied and removed further includes means for measuring a second derivative, the second derivative being a rate of change of the time rate of change. 
         [0016]    These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0017]      FIG. 1  shows a static VAR compensator with a thyristor-switched capacitor; and 
           [0018]      FIG. 2  compares a thyristor-current waveform and a voltage across a capacitor. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A typical static VAR compensator  10 , shown in  FIG. 1 , brings the power factor to unity or brings the voltage between first and second lines  11   a ,  11   b  to a desired level by controlling the reactance presented to a power transmission system. Alternatively, a typical static VAR compensator can correct for fast voltage dips and/or flicker. The second line  11   b  is typically a transmission line. The first line  11   a  is either a neutral line or another transmission line at another voltage. 
         [0020]    The static VAR compensator  10  shown in  FIG. 1  includes a valve  12  in series with a reactive element  14 . The valve  12  includes a thyristor  16  in parallel with a diode  18  of opposite polarity. In operation, the valve  12  selectively switches the reactive element  14  in and out. Within the reactive element  14 , a capacitor  20  provides a primary impedance and a detuning inductive reactor  22 , which provides a secondary impedance. The polarity of the static VAR compensator  10  shown in  FIG. 1  can be reversed without changing the principles of its operation. 
         [0021]    The thyristor  16  has three terminals: a gate  23   a , a cathode  23   b , and an anode  23   c . Applying a gate current to the thyristor&#39;s gate terminal  23   a  causes a conducting path for thyristor-current between the thyristor&#39;s cathode  23   b  and the anode  23   c . This conducting path continues to exist even when the gate current is turned off. As a result, the thyristor  16  latches into a conducting state. Once latched in the conductive state, the thyristor provides a conductive path for the thyristor-current without the need to continuously provide gate current. 
         [0022]    As a result of the conducting path, a thyristor-current begins to flow between cathode  23   b  and anode  23   c  of the thyristor  16 . This connects the reactive element  14  to the power transmission system and thereby alters the impedance presented to the system. When the correct reactance is switched into the circuit at the correct time, the power vector rotates toward the positive real axis, thus bringing the power factor closer to unity or the line voltage to a desired level. Alternatively, switching the correct reactance into the circuit at the correct time reduces the voltage dip and/or flicker. 
         [0023]    The thyristor  16  maintains the conducting path between anode  23   c  and cathode  23   b  for as long as the thyristor-current remains above a quenching threshold. This quenching threshold is slightly above zero amps, but for most practical purposes, is treated as zero amps. If the thyristor-current falls below this quenching threshold, the conducting path between anode  23   c  and cathode  23   b  disappears. The disappearance of this conducting path in turn disconnects the reactive element  14  from the circuit. 
         [0024]    As the conducting path disappears, charge carriers (i.e. holes and electrons) still present within the thyristor  16  recombine. The recombination process takes some time to complete. The period during which this recombination takes place is called the “refractory period.” After the refractory period, it becomes safe to turn the thyristor  16  on again. 
         [0025]    During the refractory period, the thyristor  16  is particularly vulnerable to damage. If, during the refractory period, the thyristor-current were to somehow rise back above the quenching threshold, even momentarily, the thyristor could be damaged when no gate current has been applied. This spontaneous and uncontrolled re-opening of the conducting path between the thyristor&#39;s anode  23   c  and cathode  23   b  can result in serious physical damage to the thyristor  16 . 
         [0026]    In operation, one would apply a gate current whenever there is a risk that the thyristor-current will fall below the quenching threshold. Doing so reduces the likelihood of damaging the thyristor during a refractory period. 
         [0027]    A difficulty that arises, however, is that it is sometimes difficult to know exactly when the thyristor current will fall below the quenching threshold. In general, the current waveform is not a perfect sinusoid. Instead, the current waveform often includes various disturbances superimposed on it. These disturbances can include harmonic content, which lends itself to some predictably. However, the disturbances can also include randomly occurring current drops that cannot readily be predicted. 
         [0028]    As noted above, if thyristor-current momentarily rises above the quenching threshold during a refractory period, the thyristor  16  may sustain serious damage. Consequently, it is preferable to make sure a gate current is present whenever the thyristor-current is near the quenching threshold. This reduces the risk of the thyristor-current momentarily rising above the quenching threshold in the absence of any gate current. 
         [0029]    Known methods of controlling the timing of the gate current rely, to a great extent, on an educated guess based on what disturbances can be expected in the thyristor-current. Thus, using these known methods, it is possible that an unanticipated disturbance will unexpectedly drive the thyristor-current below the quenching threshold while no gate current is present. This can result in the conducting path spontaneously re-opening during the refractory period. 
         [0030]    An alternative approach, which avoids the foregoing difficulties, is to observe a time rate of change of voltage across the capacitor  20 . When this time rate of change falls below a predefined safety threshold, one applies a gate pulse since the capacitor and thyristor currents would then be near zero. 
         [0031]    In  FIG. 1 , a voltage measuring element  24  provides a measurement of capacitor voltage to a digital signal processor  26 , which then calculates a time rate of change of the capacitor voltage and provides the result to a controller  28 . The controller  28  uses that result to determine when to drive a current source  30  that is connected to the gate terminal  23   a  of the thyristor  16 . 
         [0032]    In some embodiments, the controller  28  determines that that the time rate of change of the capacitor voltage is less than some predefined safety threshold and adaptively controls when the gate pulse should occur. In other embodiments, the digital signal processor  39  also calculates a second derivative. This second derivative is then provided to the controller  28 , which then uses it to determine whether the capacitor voltage, even if close to zero, is moving away or towards zero. If the former is the case, the controller  28  does not apply a gate pulse, whereas if the latter is the case, the controller  28  applies a gate pulse. In either case, the controller  28  maintains the gate pulse until the time rate of change of the capacitor voltage rises beyond some safety threshold. 
         [0033]    The value of the safety threshold will vary from one installation to another, and will depend to some extent on how quickly a gate pulse can be applied. For example, if the safety threshold is too low, by the time the controller  28  can apply a gate pulse, it may be too late and damage to the thyristor  16  may have occurred. If the safety threshold is too high, the controller  28  will apply a gate pulse prematurely, thus wasting energy. 
         [0034]      FIG. 2  shows a typical thyristor-current  32  and the accompanying a capacitor voltage  34  measured across the capacitor  20 . It is apparent from the figure that the thyristor-current  32  goes to zero when the time rate of change of capacitor voltage  34  goes to zero. It is also apparent that the thyristor current  32  approaches zero whenever the second derivative of the capacitor voltage  34  is negative. 
         [0035]    While the foregoing discussion has referred to “current waveform,” it will be apparent that current waveform is related to other electrical waveforms, such as voltage and power waveforms, and that the methods described herein can readily be adapted to such waveforms by minor modifications. 
         [0036]    In other embodiments, the digital signal processor  26  is replaced by an analog circuit. For example, an analog circuit could have a differentiator circuit connected to the voltage across the capacitor  20 . The output of the differentiator circuit is then provided to a comparator circuit for comparing the resulting time rate of change of capacitor voltage with a safety threshold. Such an analog circuit could be extended to measure a second derivative by placing another differentiator circuit in series with the first differentiator circuit and using the output of that other differentiator circuit as a basis for estimating a second derivative.