Abstract:
A method and apparatus for mitigating dynamic overvoltage (DOV) in an AC network. A distribution and power transformer has its primary connected to the high voltage bus of the network and its secondary connected to a switching device. Upon the occurrence of a condition known to cause a DOV and the DOV a control system cause the switching device to change from a nonconductive mode to a conductive mode in less than the time for one cycle of the operating frequency of the AC network. This change in switching device conduction places a short circuit across the transformer secondary.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority of U.S. provisional patent application Ser. No. 60/903,751 filed on Feb. 27, 2007, entitled “Method And Apparatus For Mitigation Of Dynamic Overvoltage” the contents of which are relied upon and incorporated herein by reference in their entirety, and the benefit of priority under 35 U.S.C. 119(e) is hereby claimed. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the occurrence of temporary overvoltages on AC networks and more particularly to the mitigation of those voltages. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    Undesirable temporary overvoltages can occur for a variety of reasons on AC networks and have the potential to adversely affect connected equipment, possibly resulting in equipment damage. For example, the temporary overvoltages can occur in conjunction with conventional (line commutated) high voltage DC (HVDC) terminals in cases where the DC system is blocked (conduction halted) when a large amount of capacitive shunt compensation and harmonic filters are connected because of the operation of some kind of protection in the DC system. The HVDC control system is normally designed to trip the shunt compensation at the instant of converter blocking, thereby limiting the duration of the overvoltage to a few cycles, corresponding to the breaker opening times. 
         [0004]    The magnitude of the dynamic overvoltage is directly related to the short circuit capacity of the AC system and the size of the shunt compensation. Dynamic overvoltage (DOV) can be an issue in those systems that have a low short circuit capacity compared to the rating of the HVDC transmission. Systems having this characteristic are known as weak systems. 
         [0005]    Dynamic overvoltage can be particularly problematic in systems with generation connected at or near the bus experiencing the DOV event. In such a configuration, avoidance of generator self-excitation and/or generator overflux can be a decisive design consideration. 
         [0006]    Static Var Compensators (SVC) where that term includes SVCs, Voltage Source Converter SVC (VSC-SVC) sold by various manufacturers such as ABB and others by using trademarks or tradenames such as SVC Light or STATCOM, thyristor switched or thyristor controlled reactors (TSR, TCR) and synchronous condensers are used to provide reactive power compensation to a power system. Part of the function of these systems may include suppression of dynamic overvoltages but their cost can be prohibitive since these SVC systems are normally designed for longer-term and more frequent operation than is needed to correct DOV. Therefore they would include additional capability that is not needed for mitigation of DOV. 
         [0007]    Referring now to  FIG. 1  there is shown a dynamic overvoltage event with and without SVC or STATCOM mitigation. In  FIG. 1 , the solid line shows the DOV without such mitigation and the dotted line shows the DOV with such mitigation. 
       SUMMARY OF THE INVENTION 
       [0008]    An apparatus for mitigating in less than the time for one cycle of a predetermined operating frequency of an AC network a short term dynamic overvoltage on the AC network. The apparatus has: 
         [0009]    a distribution and power transformer having a primary connected to a high voltage bus and a secondary connected to a switching device that can switch from a nonconductive mode to a conductive mode within a time that is less than the time for one cycle of the network predetermined operating frequency; 
         [0010]    a system connected to the switching device, the system capable of detecting the occurrence of a condition known to cause a dynamic overvoltage on the AC network and the occurrence of a dynamic overvoltage on the AC network and causing the switching device to change in less than the time for one cycle of the predetermined operating frequency from the nonconductive mode to the conductive mode to thereby short circuit the transformer secondary when both the occurrence of a condition known to cause a dynamic overvoltage on the AC network and the occurrence of a dynamic overvoltage on the AC network are detected at the same time. 
         [0011]    A method for mitigating in less than the time for one cycle of a predetermined operating frequency of an AC network a short term dynamic overvoltage on the AC network. The method: 
         [0012]    detects the occurrence of a condition known to cause a dynamic overvoltage on the AC network and the occurrence of a dynamic overvoltage on the AC network; and 
         [0013]    causes a switching device connected to the secondary of a transformer whose primary is connected to the AC network to change in less than the time for one cycle of the predetermined operating frequency from a nonconductive mode to a conductive mode to thereby short circuit the transformer secondary when both the occurrence of a condition known to cause a dynamic overvoltage on the AC network and the occurrence of a dynamic overvoltage on the AC network are detected at the same time. 
     
    
     
       DESCRIPTION OF THE DRAWING 
         [0014]      FIG. 1  shows a dynamic overvoltage event with and without prior art mitigation of the event. 
           [0015]      FIG. 2  shows a block diagram of a DOV mitigation apparatus embodied in accordance with the present invention. 
           [0016]      FIG. 3   a  graphs show the high side bus voltage and the generator flux magnitude without the Subcycle DOV Limiting Device (SDLD) of the present invention. 
           [0017]      FIG. 3   b  graphs show the high side bus voltage and the generator flux magnitude with the Subcycle DOV Limiting Device (SDLD) of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring now to  FIG. 2 , there is shown a block diagram of an apparatus  10  embodied in accordance with the present invention that is designed only to mitigate DOV and especially short term DOV where short term means dynamic overvoltages that are less than two seconds. The apparatus  10  includes a SDLD that includes a transformer  12 , a high voltage breaker  14 , a high speed switching device  20 , an overvoltage detector  24 , a control system  26  and a DOV condition detector  28  each of which are described in more detail below. 
         [0019]    The transformer  12  may be embodied by any well known distribution and power transformer, such as for example and without limitation, a two winding or auto transformer, with secondary voltage in the medium voltage range. The transformer  12  is designed with MVA rating and impedance values selected to give the needed reactive power absorption at the primary for conditions with the secondary short circuited. For example, a transformer  12  with 500 MVAR absorption capability at the primary (at nominal primary voltage) can be made using a 50 MVA base rated transformer with 10% leakage reactance. 
         [0020]    The SDLD and thus apparatus  10  also includes the high voltage breaker  14  that is connected between the HV primary side of transformer  12  and the HV bus  16 . The secondary or MV side of transformer  12  is connected to MV bus  18  and to high speed switching device  20 . 
         [0021]    As is shown in  FIG. 2 , a typical embodiment for apparatus  10  and thus the SDLD further includes the voltage measuring device  22  connected between the HV bus  16  and the overvoltage detector  24 . Detector  24  detects the occurrence of an actual overvoltage condition on HV bus  16  by measuring the AC voltage on the bus and upon the occurrence of such a condition provides an input signal to control system  26 . 
         [0022]    The DOV condition detector  28  provides an input signal to control system  26  upon the occurrence of a condition that is known to cause a DOV. As is well known the DOV condition detector  28  senses certain preset conditions that are known to cause a DOV in a particular power system. While DOV condition detector  28  is shown in  FIG. 2  as a block that is separate from control system  28  it should be appreciated that detector  28  may be implemented in software in system  26  or may be implemented in hardware as a logic circuit. 
         [0023]    The control system  26  can either be an existing system such as the Mach 2 system presently used by ABB for its HVDC technology, with additional programming for the SDLD of the present invention which is well within the skill of those of ordinary skill in this art, or a special-purpose control system built for use in apparatus  10 . 
         [0024]    For normal operating conditions, the transformer  12  is energized from the high voltage side by its connection to the HV bus  16 , and its secondary circuit is open (no load). During dynamic overvoltage, the transformer secondary is short circuited by the high speed (subcycle) switching device  20 . The signal from the DOV condition detection  28  causes the control system  26  to connect all three phases on the secondary side of the transformer  12  to a common point without delay if, for example, device  26  detects that the HVDC converters have blocked during an AC network configuration and power level known to cause DOV. It should be appreciated that the standards used by transformer manufacturers such as ABB for designing the power transformers that can be used to embody the SDLD of the present invention require that the transformer withstand such a secondary side connection for a limited time which is typically less than two (2) seconds. The input signal from the overvoltage detection  24  to control system  26  causes the control system  26  to short circuit the secondary of the tranformer  12  if an overvoltage is detected. 
         [0025]    As is well known to those of ordinary skill in this art, the close timing of the individual poles (phases) of the high speed switching device  20  may optionally be selected by control system  26  in such a way as to minimize DC offsets in the secondary side currents of transformer  12 . DC offsets may cause more stress on transformer  12  as the mechanical force on the transformer is proportional to the peak I in the transformer. Minimizing DC offsets also minimizes the current that has to be carried by switching device  20 . 
         [0026]    After the capacitive MVARs (such as AC filters at a HVDC station, shunt capacitor banks, or lightly loaded high voltage transmission lines) that have caused the DOV are disconnected from the system, the transformer reactive power absorption is removed from the system, either by opening the switching device  20  on the transformer secondary circuit or by opening the high voltage breaker  14  on the primary side. 
         [0027]    A fast switching device  20  capable of conducting high currents is used on the secondary side of transformer  12  in order to mitigate the DOV in the subcyle time frame, i.e. less than eight (8) milliseconds for a 60 Hertz system. The device  20  may be, but is not limited to, one of the following: 
         [0028]    Fast mechanical switch 
         [0029]    Triggered spark gap 
         [0030]    Thyristors, or other power electronic switches 
         [0000]    The device  20  chosen for a given application depends on specific conditions and detailed design studies of the power system. 
         [0031]    While DOV detector  24 , control system  26  and DOV condition detector  28  are shown in  FIG. 2  as separate physical elements it should be appreciated that all of these elements could reside in a single physical system. 
         [0032]    An example showing the performance of the SDLD of the present invention in mitigating generator overflux and DOV, that is high side bus voltage, is shown by the graphs in  FIGS. 3   a  and  3   b.  The graphs in  FIG. 3   a  show the high side bus voltage and the generator flux magnitude without SDLD of the present invention and the bottom set shown those two same parameters with the SDLD of the present invention. 
         [0033]    In the system that was tested to obtain the graphs shown in  FIGS. 3   a  and  3   b,  the maximum desired high side bus voltage was 1.4 per unit and the maximum desired generator flux magnitude was 1.3 per unit. As can be seen from the left hand graph shown in  FIG. 3   b,  the present invention easily meets the maximum desired high side bus voltage requirement when the SDLD of the present invention is used and as can be seen from the right hand graph in  FIG. 3   b  the present invention also meets the maximum generator flux magnitude requirement when the SDLD is used. 
         [0034]    It is to be understood that the description of the preferred embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.