Abstract:
Disclosed is a method for reducing the variation in voltage, due to Ferranti effect, using the impedance injection capability of distributed impedance injection modules. The Ferranti effect is an increase in voltage occurring at the receiving end of a long transmission line in comparison to the voltage at the sending end. This effect is more pronounced on longer lies and underground lines when the high-voltage power lines are energized with a very low load, when there is a change from a high load to a very light load, or the load is disconnected from the high-voltage power lines of the power grid. This effect creates a problem for voltage control at the distribution end of the power grid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 15/055,422 filed Feb. 26, 2016, which claims the benefit of U.S. Provisional Patent Application No. 62/264,739 filed Dec. 8, 2015, and is a continuation-in-part of U.S. patent application Ser. No. 15/069,785 filed Mar. 14, 2016, which claims the benefit of U.S. Provisional Application No. 62/264,744 filed Dec. 8, 2015, and this application also claims the benefit of U.S. Provisional Patent Application No. 62/359,885 filed Jul. 8, 2016. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to systems and methods for distributed control of the inductive and capacitive loading of high voltage power transmission lines by using online injection modules that hang on or are connected to the power lines and are enabled for line balancing and distributed control. 
         [0004]    2. Prior Art 
         [0005]    Long transmission lines  102  of the power transmission system  100  shown in  FIG. 1  are hung from transmission towers  101  and are used to transfer power, schematically shown as transferring power from generator  103  to load  104 . These transmission lines, if long, can have a considerable shunt capacitance and series inductance, together with the transmission line resistance, which are distributed along the entire length of the line. Under these conditions, when the receiving-end power load is very small, or the circuit is open, the voltage at the receiving-end of the transmission line can rise to a level substantially higher than the voltage at the supply-end of the transmission line. 
         [0006]    The foregoing effect is referred to as the Ferranti effect, and is caused by the combined effects of the distributed shunt capacitance and series inductance giving rise to a charging current in the transmission line. In particular, the transmission line  102  of  FIG. 1  can be represented by the distributed π-model  200  shown in  FIG. 2 .  FIG. 2  illustrates the power line with distributed resistance R  204  and inductance L  205  per unit length segment  201 . Capacitance C  203  represents a shunt capacitance between the neighboring lines and ground. 
         [0007]    Consider the lumped model of  FIG. 3  which illustrates a lumped RLC circuit  300  representation of the distributed model  200  of the high voltage transmission line.  FIG. 3  is a simplified representation of the high voltage transmission line drawing power from the supply side to the receiving side. Supply voltage {right arrow over (V R )}  206  has a shunt capacitance C S    302  and capacitive supply current {right arrow over (I CS )}  308  with ground. Receiving voltage {right arrow over (V R )}  207  has a shunt capacitance {right arrow over (C R )}  303  and capacitive receiving current {right arrow over (I CR )}  309  with ground, in parallel with the Load RL    312  having a load current {right arrow over (I Load )}  311 . 
         [0008]    Now consider  FIG. 4A , the phase diagram  400 , and to most simply illustrate the Ferranti effect, consider the Load RL    312  to be zero, that is the Load RL    312  is open circuited (disconnected) with the load current {right arrow over (I Load )}  311  equal to zero, then Vector {right arrow over (V R )}  207  represents the voltage at the receiving end of the transmission line, and appears across the capacitance C R    303 . Because the AC current into a capacitor leads the AC voltage on the capacitor by  90  degrees, the AC current {right arrow over (I CR )}  309  in the transmission line leads the voltage on the capacitor C R , namely {right arrow over (V R )}  207 , as shown in  FIG. 4A . Thus the voltage drop in the line caused by the charging current for capacitor C R    303 , being supplied through the line resistance R  304 , is the voltage and {right arrow over (ΔV R )}  403  across resistor  304 , is inphase with the charging current {right arrow over (I CR )}  309  as shown. That charging current also passes through the transmission line inductance L  305 . Since the AC voltage in an inductor L leads the AC current on an inductor by 90 degrees, the voltage Δ{right arrow over (V L )}  404  across the inductor L is phase shifted another 90 degrees, as shown in the phase diagram  400 . The net result is the voltage {right arrow over (V S )}  206 , the voltage at the source end of the transmission line, which as can be seen in  FIG. 4A , has a magnitude that is less than the voltage {right arrow over (V R )}  207  at the receiving end of the transmission line. 
         [0009]    When the Load RL    312  is not zero, the Ferranti effect does not disappear, but at least for substantial loads, the effects of the capacitive shunt current becomes masked by the dominance of the effects of the load current {right arrow over (I Load )}  311 , that is the resultant series inductive and resistive drops due the load current {right arrow over (I Load )}  311  which is normally much larger than the drops due to the capacitive charging current at no-load conditions. The effects of the shunt capacitor charging current, under these conditions, tends to be lumped with the overall characteristics of the power distribution system for any corrections that may be attempted to maintain voltages, power factors, etc. within desirable limits. 
         [0010]    It will be good to have an adaptive solution that can prevent or control the Ferranti effect from increasing the voltage at the receiving end of transmission lines when the load is reduced. What is proposed by the present invention is such a solution implemented using the distributed injection modules, which are also sometimes referred to as active impedance injection modules or distributed voltage/impedance injection modules that have been proposed by the parent applications of the current invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The drawings are made to point out and distinguish the invention from the prior art. The objects, features, and advantages of the invention are detailed in the description taken together with the drawings. 
           [0012]      FIG. 1  is a block diagram of the transmission line as per the prior art. 
           [0013]      FIG. 2  is a schematic representation of the (distributed it-model) equivalent circuit of the transmission line of  FIG. 1  showing the distributed nature of the resistance, inductance, and capacitance associated with the transmission line. 
           [0014]      FIG. 3  is a schematic representation of the lumped constant (π-model) equivalent circuit of the transmission line of  FIGS. 1 and 2 . 
           [0015]      FIG. 4A  is a phase diagram of the current-voltage relationships illustrating the Ferranti effect and its cause. 
           [0016]      FIG. 4B  is a phase diagram  410  of the current-voltage relationship after correction voltage has been applied to mitigate Ferranti effect. 
           [0017]      FIG. 5  is a block diagram of a first embodiment of a distributed Voltage/Impedance injection module with a multi-turn primary winding. 
           [0018]      FIG. 6  is a block diagram of a second embodiment of a distributed voltage/impedance injection module having a plurality of single turn primary windings. 
           [0019]      FIG. 7  is a block diagram of the transmission line with distributed voltage/impedance injection modules attached directly to the HV-transmission-lines. 
           [0020]      FIG. 8  shows a block diagram representation of a long HV transmission line showing the distributed voltage/impedance injection modules covering each segment of the HV transmission line. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The invention disclosed herein is generally directed at providing a system and method for controlling or limiting the increase in voltage at the load end of a long transmission line by using impedance/voltage injection modules, such as, but not limited to, those disclosed in U.S. Pat. No. 7,105,952. 
         [0022]    A plurality of implementations of injection modules has been proposed by the present inventors to allow balancing of the power transferred over a transmission line by changing the line impedance locally. By making these injection modules intelligent, self-aware, and with local intercommunication capability, it is possible to monitor locally and control the power flow over the high-voltage transmission (HV transmission) lines of the power grid. The distributed injection modules are enabled to recognize changes in the power flow characteristics and inject impedances to compensate for the unwanted changes in the high-voltage (HV) transmission lines. 
         [0023]    While these injection modules could be directly connected in series with the transmission line with ground connection, this is not normally done because of the need to provide insulation that can sustain high line voltage differences to ground at each module, which will become prohibitively expensive and also make the modules heavy. In practice, the impedance modules are can be electrically and mechanically connected to the transmission line, and are at line potential, or alternatively only magnetically coupled to the transmission line, but in either case, they are enabled to respond to transmission line current variations. These impedance injection modules are ideal candidates for distributed implementation, as existing high voltage power transmission systems can be retrofitted without disturbing the transmission line itself, though alternate implementation of impedance/voltage injection module installations using movable and other support structures are also usable for correcting the Ferranti effect. (The distributed impedance/voltage injection is described in detail as it is the preferred mode for this application). Thus in substantially all cases, the impedance/voltage injection modules respond to transmission line current and not transmission line voltage. Any transmission line corrections that require line voltage information such as corrections in voltage and power factor by each injection module are made by instructions received by wireless, wireline or power line communication to the module based on current measurements made by that module, other communicably linked modules or other line current measurement capability coupled to the high voltage power lines and again communicably linked to the impedance/voltage injection modules, and voltage measurements made at the next line voltage measurement facility, typically at the next substation. 
         [0024]    As previously described, phase diagram  FIG. 4A  illustrates the Ferranti effect. As can be seen from that  FIG. 4A , the Ferranti effect arises from the AC-charging current in the line inductance L ( FIG. 3 ). Accordingly, injection of a correcting impedance/voltage onto the line can reduce or eliminate this component, leaving the Δ{right arrow over (V R )}  403  component, which can be considered a load characteristic and handled as part of a power factor correction. 
         [0025]      FIG. 4B  shows the voltage-current vector relationship  410  with the correction voltage vector {right arrow over (V Cor )}  402  applied to correct the increase in voltage due to Ferranti effect. The resultant voltage vector at the receiver {right arrow over (V R )}, new  407  is the compensated voltage across the capacitor C R    303  when the Load RL    312  is not present or is very low. 
         [0026]      FIG. 5  shows an injection module  500  previously disclosed (U.S. patent application Ser. No. 15/055,422) having a transformer  501  with a multi-turn primary winding which is attached to the transmission line by cutting the line and splicing the module in series with the transmission line. The injection module  500  has a controller  504  that senses the changes in current transmitted over the transmission line  102  and voltage information transmitted to the module, and uses them to control the output of a converter  503 . Alternatively, the injection module may wirelessly communicate current measurements to the next substation, and the substation, or a separate control station (not shown) wirelessly send specific instructions to the injection module  500  and similar injection modules on the transmission line. The converter  503  generates an impedance (or voltage) that when impressed in a distributed fashion on the transmission line via the transformer  501 , can compensate for the changes on the transmission line  102 . In this configuration, the module  500  can be electrically connected directly to the power transmission line  102 , if desired, and insulated from ground, or allowed to float with respect to the transmission line. The power for the controller is also extracted from the transmission line by a second transformer  502  connected to a power supply module. Any transmission line voltages needed for transmission line control are measured at one or more substations and wirelessly transmitted to the injection modules, or the current measured by the injection modules transmitted to a control location also having the transmission line voltage measurements and the control location transmitting instructions to the injection modules for the desired injection module response. 
         [0027]      FIG. 6  shows a second embodiment of intelligent and self-aware injection module  600  previously disclosed (U.S. patent application Ser. No. 15/069,785) that uses a plurality of single turn primary transformers  601  to couple the output produced by the converters  603  to couple the control voltage to the power line  102 . A sensor and power supply module  602  that couples to the power transmission line is used to sense the changes in current in the transmission line  102  and feed it to the controller  604 . The controller  604  then provides the necessary instructions to the converters  603  coupled to it. The sensor and power supply module  602  also extracts power from the transmission line to supply to the controller  604  and the other electronic control circuits of the module. This magnetic coupling requires no direct connection to the high voltage transmission line and/or cutting and splicing thereof, and therefore is better suited for a retrofit of existing transmission systems, and even for new installations, should be less costly to implement. The only consideration is the support and insulation for the injection module. In that regard, modules that are only magnetically coupled to the high voltage transmission line are not grounded, but allowed to electrically float with the respect to the transmission line so must be supported using the required insulative characteristics, though usually adequate support and insulation are already available at the support towers, provided the size and weight of the injection modules is appropriately chosen by design. 
         [0028]      FIG. 7  is a schematic illustration  700  of a long distribution line where the intelligent and self-aware injection modules  701  are distributed over the transmission line  102  at the towers  101  to monitor and control the impedance of the line and provide line balancing and power transfer control capability from the generators  103  to the load  104 . These injection modules  701  can also be effectively used to reduce or eliminate the Ferranti effect by injecting a corrective inductance in the high voltage line to reduce or eliminate the Δ{right arrow over (V L )}  404  voltage component in  FIG. 4A  which gives rise to the Ferranti effect. Instead or in addition, a corrective capacitance could be injected by the injection modules  701  which would reduce the transmission line current {right arrow over (I RL )}  310  charging capacitor C R    303  and thus reduce the current through and hence the voltage Δ{right arrow over (V L )}  404  across the inductance L  305  ( FIG. 3 ). 
         [0029]    As discussed earlier, concerning the phase diagram  400  in  FIG. 4A , the Ferranti effect is produced when the load current on the transmission lines is very low, the load is removed, or the line is open-circuited. These scenarios result in the current of the high voltage transmission line comprising mainly the current in the distributed shunt capacitor of the high voltage transmission line. This capacitive current produces voltage drops in the distributed series inductors L  205  and the distributed resistors R  204  of the high voltage transmission line. The phasor diagram of  FIG. 4A  illustrates that due to the phase differences introduced by the capacitive current {right arrow over (I CR )}  309  flowing in the line inductance L  305 , an additional voltage drop is produced, shown by the phasor Δ{right arrow over (V L )}  404  . This additional voltage drop is 180 degrees out of phase with the received voltage V R    207  at the receiving end of the transmission line. This combined with the inphase resistive drop {right arrow over (ΔV R )}  403 , in the line resistance R  304 , produces the resultant Ferranti effect voltage vector {right arrow over (V S )}-{right arrow over (V R )}  401  that makes the magnitude of sending end voltage {right arrow over (V S )}  206  illustrated in  FIG. 4A  smaller than the magnitude of the voltage at the receiving end {right arrow over (V R )}  207  of  FIG. 4A . 
         [0030]    Since the voltage rise is a distributed function that is cumulative over the length of the HV transmission line, it is possible to introduce corrective changes by the preferred method of injecting the correct voltage (impedance) values by the voltage/impedance injection modules distributed over the HV transmission line. Alternately the correction can be applied, as injected voltages to correct the cumulative effect over sections of the high voltage transmission line by using groups of impedance/voltage injection modules, at different points in the line. Both such corrections will be very effective in overcoming the Ferranti effect. 
         [0031]      FIG. 8  shows each of the distributed segments  801 - 1  and  801 - 2  of the HV transmission line  102 . At least one injection module  701  is attached to or suspended from and used to provide a transmission line balancing capability. The distributed segments  801  of the HV transmission line are represented by the distributed impedance diagram  201  of  FIG. 2 . The injection modules  701  receive wireless control signals from the receiving substation, or from separate localized intelligent centers which themselves receive the needed information from the receiving substation (not shown). Since a long transmission line is involved, the transmission line length may exceed the capabilities of the wireless communication, though the needed control information may easily be sequentially relayed by the injection modules themselves, or by the localized intelligent centers previously referred to. In that regard, note that it is not a disturbance that is being corrected, but a characteristic of the transmission line itself. Consequently, the injection modules themselves, having a transmission line current sensing capability, can be programmed to use a default Ferranti effect correction when they sense the low transmission line current characteristic of an unloaded transmission line, which default correction can then be adjusted for any changes in transmission line characteristics due to weather, etc., based on the actual voltage measurements at the receiving end of the transmission line. 
         [0032]    Note also that the Ferranti effect is not a “strong” effect in comparison to other conditions sought to be corrected by the injection modules (line balancing, power factor correction, etc.), but is a cumulative effect of a long transmission line  102 . Consequently, the number of injection modules used for overall transmission line control may easily exceed the number needed simply for correction of the Ferranti effect, in which case one might use an equally spaced subset of the available modules for this purpose. 
         [0033]    The injection modules are sometimes referred to as impedance/voltage injection modules, as the function of the modules for the present invention is the injection of an effect onto the transmission line with the proper phasing to achieve the desired result, specifically the diminishing or cancellation of the voltage {right arrow over (ΔV L )}  404  as shown in  FIG. 4B . The injection might be considered an injection of an opposing voltage {right arrow over (V Cor )}  402 , or alternatively, the injection of a cancelling inductance, that counteract the line inductance L  305 . Any power needed for the injection itself is also taken from the transmission line prior to the injection through the magnetic coupling from and to the transmission line. 
         [0034]    Even though the invention disclosed is described using specific implementations, circuits, and components, it is intended only to be exemplary and non-limiting. The practitioners of the art will be able to understand and modify the same based on new innovations and concepts, as they are made available. The invention is intended to encompass these modifications.