Abstract:
A plurality of preprogrammed switches is disposed in a looped distribution line downstream from a source of power to respond to a short circuit and to reconfigure the line to isolate the short circuit. Some of the preprogrammed switches are each provided with a unique open time interval, such as t1, t2 and t3. Others of the plurality of preprogrammed switches can then determine which switch is opening in response to the short circuit and can identify the portion of the line that is shorted. Certain of the preprogrammed switches can then reconfigure to protect the identified portion of the line.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to electric power distribution systems. More particularly, the invention relates to such electric power distribution systems that utilize loop sectionalizing for improvement in responding to short circuits on distribution lines.  
       BACKGROUND OF THE INVENTION  
       [0000]     A. General  
         [0002]     Reference is frequently made herein to circuit breakers, sectionalizers, and reclosers. All of these devices are designed to switch distribution circuits on and off by opening or closing switches therein. Typically modern breakers, sectionalizers, and reclosers do not contain any means to determine when or if they should open or close. Instead these devices are attached to control devices which measure power system currents and/or voltages, and send signals to the reclosers, sectionalizers, and circuit breakers to open and close. The methods described below, which decide when to open and close the circuit breakers, sectionalizers, and reclosers, are typically implemented in these control devices. The usual practice in the art is to refer to a device that controls a circuit breaker as a protective relay. Similarly, a device that controls a recloser is typically referred to as a recloser control and a device that controls a sectionalizer is typically referred to as a sectionalizer control.  
         [0003]     Thus, even though reclosers, sectionalizers, and breakers are frequently referred to herein as measuring current, sensing short circuit current, measuring voltage, and sensing when a line is energized or deenergized, it will be readily apparent to those skilled in the art that the control device associated with each circuit breaker, recloser, or sectionalizer is actually performing the measurements, detecting short circuits or line energization states, and making the decision to open and close the connected recloser, sectionalizer, or circuit breaker. Thus, when a circuit breaker, recloser, and/or sectionalizer is referred to herein, it means the combination of recloser and recloser control, sectionalizer and sectionalizer control, and circuit breaker and protective relay.  
         [0000]     B. Power Distribution Systems with Radial Distribution Lines  
         [0004]     Electric power distribution typically occurs at voltages in the range 4 kV to 35 kV. Historically, distribution lines were connected radially from distribution substations to loads. The prior art example of  FIG. 1  illustrates a system  21  with a single distribution line  20  feeding several loads, Load  1  through Load  4 , from a distribution substation  22 . Usually several distribution lines radiate from the substation  22 , but for simplicity, only one distribution line  20  is shown in  FIG. 1 .  
         [0005]     A short circuit on such a radial line typically causes a power outage for all connected loads in a radial distribution system. Inside the substation  22  is a circuit breaker  24  that helps protect the distribution line  20  from short circuits. If a short circuit occurs anywhere on the line, then large currents will begin to flow from the substation  22  to the short circuit on the distribution line  20 . Over-current detecting equipment, called protective relays, will detect the large current and will signal the circuit breaker  24  to open. When the circuit breaker  24  opens, the distribution line  20  is disconnected from the power source within the substation  22 . This interrupts power to all of the loads, Load  1  through Load  4 , connected to the radial distribution line  20 . The loads then remain without power until a line crew travels to the site of the short circuit, repairs the short-circuited conductors, and then closes the circuit breaker  24 . The duration of such a power outage is typically several hours.  
         [0006]     Power can be restored to the loads faster after temporary short circuits by reclosing the circuit breaker. Still referring to  FIG. 1 , many short circuits are transient in nature. If the short-circuited line is disconnected from the power source by the circuit breaker  24 , temporary short circuits can be self-healed. When the circuit breaker  24  recloses, the short circuit will be gone, and power will be restored to the loads immediately without the need to dispatch a line crew to repair the short circuit. This returns power to the loads faster, in a matter of seconds instead of hours.  
         [0007]     Of course, there is no guarantee that the short circuit is temporary, and it may still be present when the circuit breaker closes. In that case, the circuit breaker  24  is again opened, and may be reclosed one or more additional times testing to see if the short circuit has self-healed. If the short circuit goes away, then the breaker remains closed. If the short circuit is permanent, then the breaker  24  opens a predetermined number of times, waiting a predetermined time between each closing, and then opens a final time and remains open. Again all of the connected loads are without power until a line crew locates and repairs the short circuit, and the circuit breaker is closed. This can take several hours. Since some portion of faults are temporary, reclosing of the circuit breaker  24  decreases the chances that a load will be without power for several hours, and increases the chance that the load will be without power for only a few seconds.  
         [0008]     Sectionalizers and reclosers can be used to limit the size of the power outages caused by permanent short circuits. Some distribution lines are constructed with switches along the length of the line, as shown in  FIG. 2 . In particular, in  FIG. 2 , a system  23  with a sectionalizer switch  26  allows power to be restored to some loads faster. First, the operation of sectionalizer switches will be discussed, and then the operation of line reclosers will be discussed. Both reclosers and sectionalizers historically have the ability to measure the current flowing through the switch, and the voltage at least on one side of the switch. When the voltage is measured on only one side of the switch (the usual case), it is measured on the side of the switch closer to the sub-station  22  or to the power source.  
         [0009]     The switch  26 , also marked S, in  FIG. 2  is a sectionalizer switch. It measures current flowing through the switch, and the voltage on the left (substation or power source) side of the switch. If a short circuit occurs between the substation  22  and the sectionalizer switch  26 , the system of  FIG. 2  behaves the same as the system of  FIG. 1 . If a short circuit  28  occurs between the sectionalizer switch  26  and the end of the distribution line  20  then the system of  FIG. 2  acts differently than the system of  FIG. 1 . This short circuit  28  is said to be “down stream” of the sectionalizer  26 .  
         [0010]     The short circuit causes a large current to flow from the power source in the substation  22 , through the substation circuit breaker  24 , along the distribution line  20 , through the sectionalizer switch  26 , and to the short circuit  28 . The sectionalizer  26  senses the large current flow, and then “knows” that the short circuit  28  is down stream from the sectionalizer location. If the short circuit location were upstream of the sectionalizer location, then the sectionalizer switch  26  would not have detected the large current.  
         [0011]     As with the system  21  of  FIG. 1 , the circuit breaker  24  in the substation  22  opens and removes power from the distribution line  20 , all of the connected loads, Load  1  through Load  4 , and from the short circuit  28 . This causes the voltage along the distribution line  20  to drop from several thousand volts to substantially zero volts. The sectionalizer  26  senses the absence of voltage, and “knows” that the substation breaker  24  has opened. The substation breaker  24  may be programmed to test several times for a temporary short circuit before finally opening and staying open if the short circuit  28  is permanent. The sectionalizer switch  26  counts each time the substation breaker  24  closes and opens. The sectionalizer  26  does this by sensing the short circuit current flowing through the sectionalizer switch when the substation breaker  24  closes, and senses the absence of voltage when the substation breaker  24  opens.  
         [0012]     The sectionalizer switch S is programmed to open during one of the “open-intervals” of the substation breaker  24 . This open-interval is the time when the substation breaker  24 , or any device, is open. During some pre-determined open interval, the sectionalizer opens.  
         [0013]     When the substation breaker  24  recloses with the sectionalizer switch open, there will be no large current flow because the short circuit  28  has been isolated from the power source by the open condition of the sectionalizer switch  26 . The circuit breaker  24  will then remain closed. In other words, the sectionalizer switch  26  made the permanent short circuit  28  shown in  FIG. 2  appear to be a temporary short circuit. The result is that after the substation breaker  24  closes, power is restored to Load  1  and Load  2  after just a few seconds, even though the short circuit  28  is permanent. Only the customers at Load  3  and at Load  4  will experience an extended power outage while the line crew searches for and repairs the short circuit  28 . After the short circuit is repaired, the sectionalizer switch  26  is closed, and power returns to Load  3  and Load  4 . This is an improvement over the system of  FIG. 1  where a permanent short circuit caused an extended power outage for all of the loads.  
         [0014]      FIG. 3  illustrates a system  27  with a recloser  30 , also marked R, in place of the sectionalizer  26  in the system  23  of  FIG. 2 . This line recloser  30  further reduces the size and length of a power outage. For short circuits between the substation circuit breaker  24  and the recloser  30 , the system  27  of  FIG. 3  operates the same as the system  21  of  FIG. 1 . For short circuits downstream of the recloser  30 , the systems operate differently. The short circuit  28  shown in  FIG. 3  will cause a large current to flow from the substation  22  through the recloser  30  and to the short circuit  28 . The recloser  30  senses the large current and opens quickly. The substation circuit breaker  24  is programmed to open with a longer time delay than the recloser  30 , so the recloser opens and the substation breaker remains closed. This removes power from loads Load  3  and Load  4 , but power to Load  1  and Load  2  is only degraded while the large current is flowing, and is completely restored when the recloser  30  opens. Typically the recloser will open in less than one second, so the short circuit  28  in  FIG. 3  will only cause a power degradation lasting less than one second for Load  1  and for Load  2 . The recloser  30  may be programmed to reclose after some time to test if the short circuit  28  is temporary. If the short circuit is temporary, then Load  3  and Load  4  will be without power for only a few seconds.  
         [0015]     If the short circuit is permanent, then the recloser  30  will open and close a predetermined number of times, then open one final time and remain opened. Load  3  and Load  4  will remain un-powered for several hours while the line crew searches for and repairs the short circuit. Load  1  and Load  2  will experience a power degradation lasting less than one second each time the recloser  30  closes to test if the short circuit  28  still exists. This is an improvement over the system  23  of  FIG. 2  where Load  1  and Load  2  experienced a power outage lasting several seconds.  
         [0000]     C. Power Distribution Systems with Looped Distribution Lines  
         [0016]     A system  31  in  FIG. 4  shows a further improvement over the radial distribution systems of  FIGS. 1-3  with looped distribution circuits providing rapid power restoration following permanent short circuits. The previously considered systems  21 ,  23  and  27  in  FIGS. 1-3  involved radial distribution lines. The system  31  of  FIG. 4  involves a looped distribution line  32 . The term “looped” means that power can be fed to any of the loads, Load  1  through Load  6 , from either direction. This system  31  is normally operated in the state shown in  FIG. 4 . In the normal operational state of the system shown in  FIG. 4 , reclosers R 1 , R 2 , R 4  and R 5  are closed, and recloser R 3  is open. Recloser R 3  is normally open because if all of the reclosers were closed, it would be difficult to control the amount of power flowing through the looped line, especially if the two circuit breakers  24  and  25  are in different substations.  
         [0017]     Reclosers R 1 , R 2 , R 4  and R 5  can sense current flowing through them, and can also measure or at least sense the presence or absence of voltage on at least one side; typically the side closer to the substation  22  or the power source. Recloser R 3  can measure current and can measure voltage on both sides of itself. Methods of measuring current and measuring or sensing voltage are, of course, well known in the art.  
         [0018]     Recall that the system  27  shown in  FIG. 3  performed very well for the short circuit  28  shown in  FIG. 3 . Load  1  and Load  2  experienced power degradation lasting less than one second even for a permanent fault. However, if the permanent short circuit  28  were upstream of the recloser  30  in  FIG. 3 , then all four loads would experience an extended power outage. So, the performance of system  27  depends on the location of the short circuit  28 . The system  31  shown in  FIG. 4  removes the dependence on short circuit location. This system performs substantially equally well regardless of where the short circuit exists or occurs.  
         [0019]     For example,  FIG. 5  shows a permanent short circuit  28  upstream of recloser R 1 . If the system  31  were not looped, then this short circuit would result in an extended power outage for loads Load  1 , Load  2 , and Load  3 , because the substation breaker  24  would open and would de-energize the line serving all of those loads.  
         [0020]      FIG. 6  and  FIG. 7  show how the system  31  limits the size and duration of the outage for the short circuit  28  shown in  FIG. 5 . The short circuit  28  causes a large current to flow from the power source inside the substation  22  to the short circuit  28 . The substation circuit breaker  24  opens for a pre-determined time, causing a temporary power outage for loads Load  1 , Load  2  and Load  3 . The substation breaker  24  typically tests the line  32  several times to see if the short circuit  28  is temporary. If the short circuit is temporary, then when the circuit breaker  24  closes the short circuit  28  will no longer exist, and the system  31  will return to the state shown in  FIG. 4 . If the short circuit  28  is permanent, the substation breaker  24  opens and closes a predetermined number of times, i.e., the breaker tests the line for a temporary short circuit, and then breaker  24  remains open. Reclosers R 1  and R 2  sense that the line connected to them is de-energized for an extended time. Reclosers R 1  and R 2  are both programmed to take specific action when they sense that the line  32  is de-energized for an extended time (e.g. longer than some predetermined time). Recloser R 1  is programmed to open following a predetermined time delay after the line becomes de-energized. Recloser R 2  is programmed to reconfigure to protect the section of line  32  between reclosers R 2  and R 1 . Recloser R 2  was previously configured to protect the section of line between reclosers R 2  and R 3  because the power source or substation  22  was to the left of recloser R 2 . Recloser R 3  is programmed to close after a predetermined time when it senses that the line  32  connecting to either side of recloser R 3  is de-energized. After the substation breaker  24  has finished testing the line for a temporary short circuit and breaker  24  is open, and all of the reclosers perform their programmed tasks, the system  31  is as shown in  FIG. 7 .  
         [0021]     Notice that only Load  1  connected between recloser R 1  and the substation breaker  24  experiences a prolonged power outage. Load  2  and Load  3  would have also experienced a prolonged power outage if the system were radial. However since the system  31  is looped, Load  2  and Load  3  are without power only for a few tens of seconds while circuit breaker  24  is open and while the predetermined time delays elapse before recloser R 1  opens, recloser R 2  reconfigures, and recloser R 3  closes. A timeline for the entire process is shown in the bottom portion of  FIG. 7 .  
         [0022]     Now assume a permanent short circuit  28  occurs between reclosers R 1  and R 2 , as shown in  FIG. 8 . The permanent short circuit causes a large current to flow from the substation power source through the circuit breaker  24  and through recloser R 1  to the short circuit  28 . Recloser R 1  is programmed to open with less delay than the substation breaker  24 , so recloser R 1  opens and circuit breaker  24  remains closed. I.e., when recloser R 1  opens, the short circuit current ceases, so the substation breaker  24  does not open. Recloser R 1  closes several times to test the line  32  for a temporary short circuit. In this example, the short circuit  28  is permanent, so recloser R 1  eventually opens permanently. It opens before the substation breaker  24  is programmed to open, so the substation breaker remains closed.  
         [0023]     The system  31  is now in the state shown in  FIG. 9 . Load  2  and Load  3  are de-energized. Recloser R 2  performs exactly as in the previous example. If it senses that the line connected to it is de-energized for an extended time (e.g. longer than some predetermined time), it reconfigures to protect the section of line  32  between reclosers R 1  and R 2 . Recloser R 3  also acts exactly the same as it did in the previous example. It reconfigures to protect the section of line between reclosers R 2  and R 3 , and closes, which brings the system  31  to the state shown in  FIG. 10 .  
         [0024]     The permanent short circuit now causes a large current to flow from the substation, through reclosers R 5 , R 4 , R 3 , and R 2 , causing a temporary power degradation to Load  2 , Load  3 , Load  4 , Load  5 , and Load  6 . When recloser R 2  senses a large current after reconfiguring to protect the line between reclosers R 2  and R 1 , it opens very rapidly, and remains open. It does not attempt to reclose. Because recloser R 2  opens very rapidly, reclosers R 3 , R 4  and R 5  and circuit breaker  25  all remain closed.  
         [0025]     The system  31  now resides in the state shown in  FIG. 11 . Notice that the permanent short circuit  28  between reclosers R 1  and R 2  only caused an extended power outage for the Load  2  connected between reclosers R 1  and R 2 . Load  1  experienced one to several temporary power degradations as recloser R 1  was testing the line for a temporary short circuit. Load  3  experienced one to several temporary power outages lasting several seconds as recloser R 1  was testing for a temporary short circuit, and then experienced a longer power outage as reclosers R 2  and R 3  reconfigured. Finally Load  3  experienced a temporary power degradation when recloser R 3  closed, causing short circuit current to flow through reclosers R 2 , R 3 , R 4  and R 5 . Load  4 , Load  5  and Load  6  all experienced a temporary power degradation lasting less than one second when recloser R 3  closed. If the substation breakers  24  and  25  are located in the same substation  22 , or in substations electrically close to each other, then Load  4 , Load  5  and Load  6  may also experience temporary degradations in power due to the initial short circuit, and when recloser R 1  tests the line for a temporary short circuit. A timeline for the entire process is shown in the bottom portion of  FIG. 11 .  
         [0026]     In our final example of this configuration, assume a permanent fault  28  between reclosers R 2  and R 3 , as shown in  FIG. 12 . This short circuit causes large short circuit current to flow through the substation breaker  24  and reclosers R 1  and R 2 . Recloser R 2  is programmed to open with less delay than the substation breaker  24  and recloser R 1 , so recloser R 2  opens. The short circuit current ceases, so the substation breaker  24  and recloser R 1  remain closed. Recloser R 2  tests the line  32  several times for a temporary short circuit, and finally opens permanently. Each time recloser R 2  opens, it does so before recloser R 1  and the substation breaker  24  react, so each time recloser R 1  and the substation breaker  24  remain closed.  
         [0027]     The system  31  is now in the state shown in  FIG. 13 . As with the previous examples, recloser R 3  is programmed to close after it senses either line connected to it is de-energized. When recloser R 3  closes, a large short circuit current flows through reclosers R 3 , R 4  and R 5  and circuit breaker  25 . Recloser R 3  is programmed to open and not reclose in response to short circuit current. After recloser R 3  opens, the system reverts to the state shown in  FIG. 13 . Notice that Load  1  and Load  2  only experienced one to several temporary power degradations while recloser R 2  tested the line for a permanent fault. Load  4 , Load  5  and Load  6  experienced one temporary power degradation when recloser R 3  closed. Load  3  is the only load that experiences an extended power outage. A timeline for the entire process is shown in the bottom portion of  FIG. 13 .  
         [0028]     All three of the previous examples could have included short circuits on the lower half of the distribution loop, and the results would have been similar except that breaker  25 , reclosers R 3 , R 4 , and R 5 , and loads Load  4 , Load  5 , and Load  6  would have been involved.  
         [0029]     Each of the reclosers in looped distribution lines has a set of rules for operation. From the preceding discussion, it might seem that each recloser performs different functions depending on the location of the short circuit. However, each recloser follows a certain preprogrammed sequence of actions regardless of the location of the short circuit.  FIG. 14  shows a flow chart of the preprogrammed sequence of actions that are taken by recloser R 3 ,  FIG. 15  shows a flow chart of the preprogrammed sequence of actions that are taken by reclosers R 2  and R 4  and  FIG. 16  shows a flow chart of the preprogrammed sequence of actions that are taken by reclosers R 1  and R 5 .  
         [0030]     Each flowchart,  FIGS. 14 through 19 , has a start and end bubble. The methods move from the end bubble or from anywhere in the flowchart to the start bubble when the scheme is reset or restarted. The reset or restart occurs after the permanent short circuit is repaired or when an operator determines the method should be reset. The reset or restart can be manual, such as when a person issues a reset signal or command to the recloser, or automatic, such as when the reclosers reset themselves when they detect some sufficient condition.  
         [0031]     The above-described operation of recloser R 3  is summarized in  FIG. 14 . After starting at bubble  40 , recloser R 3  determines if line  32  is de-energized for a predetermined time on one side only at decision block  41 . If the line is energized on both sides of recloser R 3 , or de-energized on both sides of recloser R 3 , it continues to measure voltage on both side of recloser R 3  until line  32  is de-energized on one side only for some predetermined time. If the line is de-energized on one side only for a predetermined time, the process proceeds to decision block  42  to determine if line  32  is de-energized to the right of recloser R 3 . If so, recloser R 3  configures to protect line  32  to the right of recloser R 3  at block  43 . If not, recloser R 3  configures to protect line  32  to the left of recloser R 3  at block  44 . In either situation, recloser R 3  closes at block  45 . It then continues to sense for short circuit current at decision block  46 . If recloser R 3  detects a short circuit current lasting longer than a predetermined time, recloser R 3  opens at block  47  and the process terminates at end bubble  48 .  
         [0032]     The above-described operation of reclosers R 2  and R 4  is summarized in  FIG. 15 . After starting at bubble  50 , reclosers R 2  and R 4  determine if a short circuit current is present on line  32  at decision block  51 . If a short circuit is present, reclosers R 2  and R 4  determine at decision block  52  if the short circuit current lasts longer than a first predetermined time. If not, the process begins again at start bubble  50 . If the short circuit current lasts longer than a predetermined time, then recloser R 2  or R 4  opens at block  58 . Decision block  59  determines if the line has been tested for temporary short circuits more than a predetermined number of times. If it has, then the process ends at block  57 . If the line has been tested for a temporary short circuit less than a predetermined number of times, then the recloser R 2  or R 4  is closed at block  60  and the process continues from the start bubble. If no short circuit current is detected at decision block  51 , then a check is performed to determine if the line is energized at decision block  53 . If the line is not de-energized, i.e., if the line is still energized, then the process continues from the start bubble. If the line is de-energized, then recloser R 2  or R 4  is reconfigured to protect the upstream line, i.e., recloser R 2  or R 4  is reconfigured to protect the line between reclosers R 1  and R 2  or between reclosers R 5  and R 4 . Reclosers R 2  and R 4  again monitor line  32  for a short circuit current that lasts longer than a predetermined amount of time at block  55 . If a short circuit current is detected and lasts longer than the predetermined amount of time, recloser R 2  and/or recloser R 4  open at block  56  and end the process at bubble  57 .  
         [0033]     The above-described operation of reclosers R 1  and R 5  is summarized in  FIG. 16 . After starting at bubble  61 , reclosers R 1  and R 5  determine if a short circuit current is present on line  32  at decision block  62 . If a short circuit is present, reclosers R 1  and R 5  determine at decision block  63  if the short current lasts longer than a first predetermined time. If the short circuit current does not last longer than a predetermined time, then the process reverts to the start bubble  61 . If the short circuit lasts longer than a predetermined time, then recloser R 1  or R 5  opens at block  67 . Decision block  68  determines if the line has been tested for temporary short circuits more than a predetermined number of times. If it has, then the process ends at bubble  66 . If the line has been tested for a temporary short circuit less than a predetermined number of times, then recloser R 1  or R 5  is closed at block  69  and the process continues from the start bubble. If short circuit current is not detected at decision block  62 , then recloser R 1  or R 5  determine if the line has been de-energized for longer than a predetermined time at decision block  64 . If the line has not been de-energized for longer than a predetermined time, then the process reverts to start bubble  61 . If the line has been de-energized for a predetermined time, then recloser R 1  or R 5  opens at block  65 , and the process ends at bubble  66 .  
         [0034]     These processes in  FIGS. 14-16  are sufficient to reduce the extended power outage to only the section of looped distribution line  32  containing the permanent short circuit  28 . Notice that these processes cover only the function of the reclosers related to controlling the looped distribution line  32 . Each recloser may perform various other functions, such as metering, reporting, etc. These other functions are not shown in  FIGS. 14, 15  and  16 .  
         [0035]     A number of problems exist with respect to the present method of controlling looped distribution lines, as represented by  FIGS. 14-16 . The method implemented by the processes discussed above has several non-idealities: 
        A. Permanent short circuits  28  between reclosers R 1  and R 2 , or between reclosers R 2  and R 3  cause a temporary power degradation to Load  4 , Load  5  and Load  6  when recloser R 3  closes. If the system were not configured as a loop, i.e. if recloser R 3  did not connect the top distribution line to the bottom distribution line, then this temporary power degradation would not occur (assuming the looped distribution line terminated in electrically separated substations), or this temporary power degradation would be less severe. In other words, this arrangement decreases the quality of power supplied to the distribution line that does not have the short circuit  28  while it increases the quality of power supplied to the distribution line that does have a short circuit.     B. Permanent short circuits  28  between reclosers R 1  and R 2 , or between reclosers R 2  and R 3 , cause added stress on the power system when recloser R 3  closes. This stress includes large short circuit currents that stress transformers, generators, conductors, etc. The added stress also includes decreased voltages that stress many types of connected electrical loads such as motors and electronics. The added stress also includes added wear on recloser R 3  when recloser R 3  must open after closing with a permanent fault between reclosers R 2  and R 3 , and added wear on recloser R 2  when R 2  must open after recloser R 3  closes with a permanent fault between reclosers R 1  and R 2 .     C. If the power source in the substation is de-energized, then the distribution line is de-energized. Recloser R 1  responds to this by opening (Blocks  64  and  65  in  FIG. 16 ). This is a nuisance because after power is restored to the substation, the loads downstream of recloser R 1  remain de-energized until recloser R 1  is closed, possibly by a manual operation after several hours.        
 
         [0039]     The shortcomings described are also applicable to the other side of the loop in  FIGS. 14-16 , i.e., the side of the loop containing breaker  25  and reclosers R 4  and R 5 .  
         [0040]     There has been a long-felt need for methods or systems that efficiently and effectively reconfigure an electrical power distribution system to provide power to most of the loads upon the occurrence of a short circuit on the distribution line.  
         [0041]     Accordingly, it is a general object of the present invention to provide improved methods and systems that reconfigure a looped distribution line in a manner that continues to supply power to most of the loads when a short circuit occurs.  
         [0042]     Another general object of the present invention is to provide improved methods and systems for sectionalizing a looped distribution line to reduce the stress on the power system and on the power system components when a short circuit occurs.  
         [0043]     Another general object of the present invention is to provide improved methods and systems for sectionalizing a looped distribution line to reduce the unnecessary outages in the distribution power system when no short circuit exists in the distribution power system.  
         [0044]     Yet another object of the present invention is to provide a plurality of preprogrammed switches, with at least some of the preprogrammed switches having at least one unique open interval when responding to a short circuit, such that other preprogrammed switches can determine which preprogrammed switch opened in response to the short circuit.  
         [0045]     A further object of the present invention is to provide a power distribution system and methods in which a normally open preprogrammed switch does not close until an adjacent preprogrammed switch opens when the short circuit is downstream from the adjacent preprogrammed switch.  
         [0046]     A still further object of the present invention is to provide methods for determining which preprogrammed switch opened in response to the occurrence of a short circuit.  
       BRIEF SUMMARY OF THE INVENTION  
       [0047]     This invention is directed to methods and systems for sectionalizing a looped distribution line, such as from a substation in an electric power distribution system, that supplies electrical power to a plurality of loads and that also reduces the stress on the power system and on the power system components when a short circuit occurs and that also reduces unnecessary temporary outages. The electric power distribution system includes a plurality of preprogrammed switches disposed between at least some of the plurality of loads. The plurality of preprogrammed switches includes a first preprogrammed switch and a fifth preprogrammed switch each disposed in the looped distribution line downstream from said substation, a second preprogrammed switch and a fourth preprogrammed switch each disposed in the looped distribution line downstream from said first and fifth preprogrammed switches and a third preprogrammed switch disposed in the looped distribution line between said second and fourth preprogrammed switches. The third preprogrammed switch is in a normally open condition. All of said preprogrammed switches are programmed to respond to the occurrence of a short circuit in the looped distribution line and to reconfigure the looped distribution line to isolate the short circuit.  
         [0048]     Methods of the present invention include the steps of providing some of the plurality of preprogrammed switches with a unique open interval time for at least one of its open intervals when responding to a short circuit condition on the looped distribution line, determining the length of the unique open interval by at least one of the preprogrammed switches in response to the occurrence of a short circuit, identifying that the short circuit is in a portion of the looped distribution line that is downstream from the preprogrammed switch associated with the determined unique open time interval, and configuring the preprogrammed switch downstream to protect the identified portion of the looped distribution line.  
         [0049]     The step of configuring the next downstream preprogrammed switch may include the steps of immediately opening the second preprogrammed switch upon determining that the short circuit is between the first and second preprogrammed switches, configuring the third preprogrammed switch to protect the line between the second and third preprogrammed switches and closing the third preprogrammed switch. The step of configuring the next downstream preprogrammed switch may also include the steps of immediately opening the fourth preprogrammed switch upon determining that the short circuit is between the fifth and fourth preprogrammed switches, configuring the third preprogrammed switch to protect the line between the fourth and third preprogrammed switches and closing the third preprogrammed switch.  
         [0050]     The step of providing each preprogrammed switch with a unique open time interval may include the steps of providing the preprogrammed switches in the substation with a unique open time interval of time t1, providing the first and fifth preprogrammed switches with a unique open time interval of time t2, with t2 greater than t1, and providing the second and fourth preprogrammed switches with a unique open time interval of time t3, with t3 greater than t2. The step of determining at each preprogrammed switch the length of said open time interval may include the additional steps of determining if the open interval used to test the line for a temporary short circuit is greater than time t1 but less than time t2 and determining if the open interval used to test the line for a temporary short circuit is greater than time t2 but less than time t3.  
         [0051]     Systems in accordance with the present invention may include a plurality of preprogrammed switches disposed between at least some of the plurality of loads, one of the preprogrammed switches being in a normally open condition, some of said plurality of preprogrammed switches programmed to respond to the occurrence of a short circuit in the looped distribution line and to reconfigure the looped distribution line to isolate the short circuit, some of said plurality of preprogrammed switches provided with a unique open time interval for at least one of its open intervals when responding to a short circuit condition on the looped distribution line and at least one of said plurality of preprogrammed switches capable of determining the length of said unique open time interval in response to the occurrence of a short circuit to identify that the short circuit is in a portion of the looped distribution line that is downstream from the preprogrammed switch associated with the determined unique open time interval; whereby the next downstream preprogrammed switch is configured to protect the identified portion of the looped distribution line that is downstream from the preprogrammed switch with the determined unique open interval. The normally open switch may typically determine the amount of time of the unique open time interval.  
         [0052]     Systems may further include a preprogrammed switch disposed adjacently to said normally open preprogrammed switch that immediately opens upon determining that the short circuit is between the adjacent, the normally open preprogrammed switches and the normally open preprogrammed switch configures to protect the line between the adjacent and the normally open preprogrammed switches and the normally open switch closes after the adjacent switch has opened. Systems may further include a first pair of preprogrammed switches disposed at each end of the looped distribution line in the substation, said first pair of preprogrammed switches provided with a unique open time interval of time t1, a second pair of preprogrammed switches disposed next downstream in the looped distribution system from the first pair of preprogrammed switches in the substation, said second pair of preprogrammed switches provided with a unique open time interval of time t2, with t2 greater than t1 and a third pair of preprogrammed switches disposed next downstream in the looped distribution system from the second pair of preprogrammed switches, said third pair of preprogrammed switches provided with a unique open time interval of time t3, with t3 greater than t2. At least one of the preprogrammed switches may determine if the open interval used to test the line for a temporary short circuit is greater than time t1 but less than time t2 and/or determine if the open interval used to test the line for a temporary short circuit is greater than time t2 but less than time t3. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0053]     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the figures in which like reference numerals identify like elements, and in which:  
         [0054]      FIG. 1  illustrates a single radial distribution line feeding several loads from a distribution substation;  
         [0055]      FIG. 2  illustrates a sectionalizer switch interposed between some of the loads in the radial distribution line of  FIG. 1 ;  
         [0056]      FIG. 3  illustrates a recloser interposed between some of the loads in the radial distribution line in place of the sectionalizer switch of  FIG. 2 ;  
         [0057]      FIG. 4  illustrates a plurality of reclosers, with each recloser interposed between respective loads in a looped distribution system;  
         [0058]      FIG. 5  illustrates a short circuit in the looped distribution line of  FIG. 4 ;  
         [0059]      FIGS. 6 and 7  illustrate the response of the plurality of reclosers to reconfigure the distribution system in response to sensing conditions on the looped distribution line caused by the short circuit in  FIG. 5 ;  
         [0060]      FIG. 8  illustrates a short circuit at a different location in the looped distribution line from that shown in  FIG. 5 ;  
         [0061]      FIGS. 9 through 11  illustrate the response of the plurality of reclosers to reconfigure the distribution system in response to sensing conditions on the looped distribution line caused by the short circuit in  FIG. 8 ;  
         [0062]      FIG. 12  illustrates a short circuit at a still different location in the looped distribution line from that shown in  FIG. 5  or  8 ;  
         [0063]      FIG. 13  illustrates the response of the plurality of reclosers to reconfigure the distribution system in response to sensing conditions on the looped distribution line caused by the short circuit in  FIG. 12 ;  
         [0064]      FIG. 14  is a flow chart of a preprogrammed sequence of actions that are taken by a recloser in a looped distribution system;  
         [0065]      FIG. 15  is a flow chart of a preprogrammed sequence of actions that are taken by certain other reclosers in a looped distribution system;  
         [0066]      FIG. 16  is a flow chart of a preprogrammed sequence of actions that are taken by still certain other reclosers in a looped distribution system;  
         [0067]      FIG. 17  is a flow chart of a preprogrammed sequence of actions that are taken by a recloser in a looped distribution system in accordance with the present invention;  
         [0068]      FIG. 18  is a flow chart of a preprogrammed sequence of actions that are taken by certain other reclosers in a looped distribution system also in accordance with the present invention;  
         [0069]      FIG. 19  is a flow chart of a preprogrammed sequence of actions that are taken by still certain other reclosers in a looped distribution system also in accordance with the present invention; and  
         [0070]      FIG. 20  illustrates a three recloser embodiment of a looped distribution system with each recloser interposed between respective loads. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0071]     The shortcomings of prior art looped distribution systems described above in Paragraphs 0035-0036 can be overcome by facilitating the reclosers to communicate with each other. For example, for a permanent short circuit  28  between reclosers R 1  and R 2 , after recloser R 1  opens permanently, recloser R 1  could send a signal to recloser R 2  telling recloser R 2  to also open. Then, when recloser R 3  closes, there would be no large short circuit current which would stress the system as described above, and there would be no temporary power degradation for Load  3 , Load  4 , Load  5  and Load  6 . Similar benefits are possible for other short circuit locations when the reclosers communicate with each other. In another example, if the power source inside the substation is de-energized, then the protective relay associated with circuit breaker  24  could send a signal to recloser R 1  not to open. Then, when the power source inside the power station is reenergized, recloser R 1  would still be closed and the downstream loads would be reenergized immediately without manual intervention.  
         [0072]     However, communications infrastructure is rarely available to allow reclosers to communicate with each other in a typical looped distribution line  32 . What is needed is a way to prevent the shortcomings described in A:, B: and C: above, without requiring dedicated or traditional communications between the reclosers.  
         [0073]     Note that while a sectionalizer switch  26  is used in  FIG. 2 , reclosers R 1 -R 5  are used in  FIGS. 4-13  and circuit breakers  24 - 25  are used at the substation  22  in  FIGS. 1-13 , that these devices, including their respective controllers, may be more generally characterized as preprogrammed switches. Of interest with respect to the present invention is that these preprogrammed switches are programmed or set to have at least one open interval in response to a short circuit that is of a known length or duration. Preferably, the preprogrammed switches in the upper segment of the looped distribution line (such as circuit breaker  24  and reclosers R 1  and R 2  in  FIGS. 4-13 ), each have a unique open interval of different times such that the other preprogrammed switches in that segment and the normally open recloser R 3  can determine which preprogrammed switch opened in response to the presence of a short circuit on the looped distribution line  32 .  
         [0074]     In accordance with one aspect of the present invention, the methods described herein prevent added stress to the power system and to the power system components when recloser R 3  closes in a looped distribution system  31  similar to that shown in  FIG. 4 . To prevent the problematic short circuit current from flowing when recloser R 3  closes, it is necessary to:  
         [0075]     1: Open recloser R 2  before recloser R 3  closes when a permanent short circuit exists between reclosers R 1  and R 2 .  
         [0076]     2: Prevent recloser R 3  from closing when a permanent short circuit exists between reclosers R 2  and R 3 . Again, similar discussions and results exist for the other half of the looped distribution line.  
         [0077]     Notice that in both required actions above, both reclosers R 2  and R 3  apparently need to know the location of the short circuit. The present invention allows recloser R 2  and R 3  to operate as required in 1: and 2: above without requiring detailed knowledge of the short circuit location and without the need for communications circuits between the reclosers. The reclosers still operate from a fixed set of processes, but the processes are different than for the system previously described.  
         [0078]     In accordance with another aspect of the present invention, the methods described herein prevent unnecessary extended outages when the power source inside the substation de-energizes. Such an outage is caused when recloser R 1  or R 5  opens during the source de-energization. When recloser R 1  or R 5  opens during such a source de-energization, manual intervention may be necessary to close recloser R 1  or R 5  after the source is reenergized. To prevent such an extended outage, it is required to prevent recloser R 1  or R 5  from opening during source de-energizations.  
         [0079]      FIG. 17  shows the rules of operation for recloser R 3 . The flow charts refer to conditions C 1 _L, C 1 _R, C 2  and C 3 . These names have no significant meanings, and the conditions are defined later. Bold lines in  FIG. 17  indicate changes to the prior art, and non-bold lines are the prior art.  
         [0080]      FIG. 18  shows a flow chart that includes the rules of operation for reclosers R 2  and R 4 . Again, the bold lines are changes to the prior art, and non-bold lines are the prior art.  
         [0081]      FIG. 19  shows a flow chart that includes the rules of operation for reclosers R 1  and R 5 . Again, the bold lines are changes to the prior art, and non-bold lines are the prior art.  
         [0082]     Suitable selections for conditions C 1 _L, C 1 _R, C 2  and C 3  are now presented. Conditions C 1 _L, C 1 _R, C 2  and C 3  are checks against the duration of the final open interval (or possibly some other open interval, or some combination of open intervals) of some other recloser or circuit breaker as sensed by the recloser performing the condition checks. An open interval is the time when a recloser or circuit breaker is open between the closings that test the line for a temporary short circuit. One of the aspects of the present invention is to detect which recloser or substation breaker is opening and closing by setting each device with a unique open interval duration for at least one of the open intervals. All other reclosers on the same circuit can then measure the open interval and know which recloser or substation breaker is operating. By knowing which device is operating, the recloser sensing the duration of the open interval can know between which two devices the short circuit exists.  
         [0083]     In the examples which follow, we concentrate on the final open interval before a recloser or substation breaker finally opens and remains open due to a permanent short circuit. Other modifications to the present invention could be to choose some other open interval, such as the first, second, shortest, longest, etc.  
         [0084]     As an example of the selections, assume that the substation breaker has a final open interval of about 30 seconds, recloser R 1  has a final open interval of about 45 seconds, and recloser R 2  has a final open interval of about 60 seconds. Condition C 2  could be chosen as “shorter than about 35 seconds”. In other words, in  FIG. 4  when recloser R 2  detects a final open interval shorter than about 35 seconds, it “knows” that the substation breaker was performing the line tests, because the substation breaker has a final open interval of about 30 seconds. Recloser R 2  “knows” that it must not have been recloser R 1  performing the line tests because recloser R 1  has a final open interval of about 45 seconds. Since the substation breaker was performing the line tests, and recloser R 1  was not performing the line tests, the short circuit must lie between recloser R 1  and the substation breaker. In that case, per  FIG. 18 , recloser R 2  would configure to protect the line between reclosers R 1  and R 2  in preparation for recloser R 3  closing and recloser R 1  opening.  
         [0085]     If on the other hand, recloser R 2  sensed a last open interval longer than about 35 seconds, then it “knows” recloser R 1  was testing the line because it has a final open interval of about 45 seconds. Recloser R 2  then “knows” that the short circuit lies between reclosers R 1  and R 2 . Per  FIG. 18 , recloser R 2  opens to prevent stressing the rest of the system with a large short circuit current when recloser R 3  closes.  
         [0086]     A suitable selection for condition C 1 _R is “shorter than about 50 seconds”. In other words when recloser R 3  in  FIG. 4  detects a final open interval shorter than about 50 seconds, it “knows” that recloser R 2  was not testing the line for a temporary short circuit, because recloser R 2  has an open interval of about 60 seconds. Since recloser R 2  was not testing the line, the permanent short circuit does not lie between recloser R 3  and R 2 , and it is safe for recloser R 3  to close. Accordingly, as shown in  FIG. 17 , recloser R 3  will configure to protect the “right” line, or the line between reclosers R 3  and R 2 , and will close.  
         [0087]     On the other hand, if recloser R 3  detects an open interval longer than about 50 seconds, it “knows” that recloser R 2  was testing the line, and the permanent short circuit lies between reclosers R 3  and R 2 . Accordingly, as shown in  FIG. 17 , recloser R 3  would not reconfigure to protect either right or left line, and more importantly recloser R 3  would not close. This prevents stress to the system, because if recloser R 3  closes, a large short circuit will flow from the substation through reclosers R 4 , R 5  and R 3  to the short circuit between reclosers R 3  and R 2 .  
         [0088]     Note that condition C 1 _L could be the same as or different than condition C 1 _R depending on the open intervals selected for reclosers R 4  and R 5  and the substation breaker attached to recloser R 5 .  
         [0089]     A suitable selection for condition C 3  in  FIG. 19  might be “longer than 25 seconds”. In other words, when recloser R 1  detects an open interval longer than 25 seconds, it knows that the substation breaker was testing the line for temporary short circuits because the substation breaker has a final open interval of 30 seconds. If the line is de-energized and there are either no attempts to test the line for short circuits, or the last open interval does not match the required last open interval of the breaker, then the power source within the substation must have been de-energized. Accordingly, recloser R 1  will not open, so that when the power source inside the substation is reenergized, the loads downstream of recloser R 1  will be reenergized immediately without delay or need for manual intervention.  
         [0090]     The above representative time durations are for illustrative purposes only. Actual times could be significantly longer or shorter. The conditions C 1 _R, C 1 _L, C 2  and C 3  used herein are only one possible set of conditions that create the desired result. Other condition sets are possible.  
         [0091]     The operation of recloser R 3  in accordance with the present invention is summarized in  FIG. 17 . After starting at bubble  70 , recloser R 3  determines if line  32  is de-energized on only one side of recloser R 3  at decision block  71 . If the line is energized on both sides, or de-energized on both sides, recloser R 3  continues to check the voltage on line  32  until line  32  is de-energized on either side of recloser R 3 , but not on both sides of recloser R 3 . When recloser R 3  detects that the line is de-energized on exactly one side of recloser R 3 , the process proceeds to decision block  72  to determine if line  32  is de-energized to the right of recloser R 3 . If so, recloser R 3  determines if the last open interval satisfied the condition C 1 _R at block  73 . If not, the process ends at end bubble  78 . However, if the condition C 1 _R is satisfied at block  73 , recloser R 3  configures to protect line  32  to the right at block  74 . Recloser R 3  then closes, as indicated at block  75 . It then continues to sense for short circuit current at decision block  76 . If a short circuit current is detected, recloser R 3  opens at block  77  and the process terminates at end bubble  78 .  
         [0092]     Returning to decision block  72  in  FIG. 17 , if recloser R 3  determines that the line to the right is not de-energized, recloser R 3  determines if the last open interval satisfied the condition C 1 _L at block  79 . If not, the process ends at end bubble  78 . However, if the condition C 1 _L is satisfied at block  79 , recloser R 3  configures to protect line  32  to the left at block  80 . The process then continues to block  75  where recloser R 3  closes. It then continues to sense for short circuit current at decision block  76 . If a short circuit current is detected, recloser R 3  opens at block  77  and the process terminates at end bubble  78 .  
         [0093]     The operation of reclosers R 2  and R 4  in accordance with the present invention is summarized in  FIG. 18 . After starting at bubble  80 , reclosers R 2  and R 4  determine if a short circuit current is present on line  32  at decision block  81 . If a short circuit is present, reclosers R 2  and R 4  determine at decision block  82  if the short circuit current lasts longer than a first predetermined time. If not, the process begins again at start bubble  80 . If the short circuit current lasts longer than a predetermined time, then recloser R 2  or R 4  opens at block  89 . Decision block  90  determines if the line has been tested for temporary short circuits more than a predetermined number of times. If it has, then the process ends at block  88 . If the line has been tested for a temporary short circuit less than a predetermined number of times then the recloser R 2  or R 4  is closed at block  91  and the process continues from the start bubble. If no short circuit current is detected at decision block  81 , then a check is performed to determine if the line is energized for longer than a predetermined time at decision block  83 . If the line is not de-energized for longer than a predetermined time, then the process continues from the start bubble. If the line is de-energized for longer than a predetermined time, then a check is made at decision block  84  to determine if the last open interval satisfies condition C 2 . If the last open interval satisfies condition C 2 , then recloser R 2  or R 4  is reconfigured to protect the upstream line, i.e. recloser R 2  or R 4  is reconfigured to protect the line between R 1  and R 2  or between R 5  and R 4 . Reclosers R 2  and R 4  again monitor line  32  for a short circuit current at decision block  86 . If a short circuit current is detected, recloser R 2  and/or recloser R 4  open at block  87  and the process ends at bubble  88 .  
         [0094]     However, if the condition C 2  was not satisfied at decision block  84 , reclosers R 2  and/or R 4  skip blocks  85  and  86 , proceeding to block  87  where reclosers R 2  and/or R 4  are opened. Thus, in this instance, reclosers R 2  and/or R 4  immediately open and skip the steps of first configuring to protect the upstream line (block  85 ) and to first detect a short circuit current (block  86 ). The process then ends at bubble  88 .  
         [0095]     The operation of reclosers R 1  and R 5  in accordance with the present invention is summarized in  FIG. 19 . After starting at bubble  93 , reclosers R 1  and R 5  determine if a short circuit current is present on line  32  at decision block  94 . If a short circuit is present, reclosers R 1  and R 5  determine at decision block  95  if the short current lasts longer than a first predetermined time. If the short circuit current does not last longer than a predetermined time, then the process reverts to start bubble  93 . If the short circuit current lasts longer than a predetermined time, then recloser R 1  or R 5  opens at block  99 . Decision block  100  determines if the line has been tested for temporary short circuits more than a predetermined number of times. If it has, then the process ends at bubble  98 . If the line has been tested for a temporary short circuit less than a predetermined number of times then the recloser R 1  or R 5  is closed at block  101  and the process continues from the start bubble. If short circuit current is not detected at decision block  94 , then recloser R 1  or R 5  determines if the line has been de-energized for longer than a predetermined time at decision block  96 . If the line has not been de-energized for longer than a predetermined time, then the process reverts to start bubble  93 . If the line has been de-energized for a predetermined number of times, then recloser R 1  or R 5  determines if the last open interval satisfies condition C 3  at decision block  102 . If the last open interval does not satisfy condition C 3 , then the process reverts to the start bubble  93 . If the last open interval satisfies condition C 3  at block  102 , then recloser R 1  or R 5  opens at block  97 , and the process ends at bubble  98 .  
         [0096]     It should be noted that while all of the examples shown above included five reclosers and two substation breakers, the invention is also effective at reducing stress to power system components and reducing the number of unnecessary temporary outages if there are fewer than five reclosers in the scheme. As an example,  FIG. 20  shows a connection of two substation circuit breakers and three reclosers. In  FIG. 20 , recloser R 1  and R 5  operate according to the flow diagram of  FIG. 19 , and recloser R 3  operates according to  FIG. 17 . The system of three reclosers shown in  FIG. 20  is as effective at reducing stress on power system components and reducing the number of unnecessary temporary outages as the system of five reclosers described previously.  
         [0097]     Moreover, many of the drawing figures illustrate a load disposed between each adjacent pair of reclosers. It will be appreciated by those skilled in the art that loads may not always be disposed in the distribution system between each adjacent pair of reclosers. Likewise, certain reclosers have been illustrated in various drawing figures as being located within a substation. Again, it will be appreciated that such reclosers are not limited to a specific location, but may be disposed at other locations in the distribution system.  
         [0098]     It will be further appreciated that, while the looped distribution systems shown in  FIGS. 4-13  and  20 , and the flow charts shown in  FIGS. 14-16 , have been indicated as prior art, the systems shown in  FIGS. 4-13  and  20  and the flow charts in  FIGS. 14-16  are not prior art when the circuit breakers and reclosers illustrated therein are programmed to operate in accordance with the teachings of  FIGS. 17-19 . Instead, such systems and flow charts then become part of the present invention.  
         [0099]     While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made therein without departing from the invention in its broader aspects.