Patent Publication Number: US-11646573-B2

Title: Closed loop restoration

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from the U.S. Provisional Application No. 63/086,219, filed on Oct. 1, 2020, the disclosure of which is hereby expressly incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     Field 
     The present disclosure relates generally to a system and method for restoring power to a power distribution network and, more particularly, to a system and method for restoring power to a closed loop power distribution network, where the method includes performing a power restoration process for a radial network and then estimating power flow and an available power capacity from switching devices in the network for providing power to sections that were left unpowered by the radial restoration process. 
     Discussion of the Related Art 
     An electrical power distribution network, often referred to as an electrical grid, typically includes a number of power generation plants each having a number of power generators, such as gas turbines, nuclear reactors, coal-fired generators, hydro-electric dams, etc. The power plants provide power at a variety of medium voltages that are then stepped up by transformers to a high voltage AC signal to be connected to high voltage transmission lines that deliver electrical power to a number of substations typically located within a community, where the voltage is stepped down to a medium voltage for distribution. The substations provide the medium voltage power to a number of three-phase feeders including three single-phase feeder lines that carry the same current, but are 120° apart in phase. A number of three-phase and single-phase lateral lines are tapped off of the feeder that provide the medium voltage to various distribution transformers, where the voltage is stepped down to a low voltage and is provided to a number of loads, such as homes, businesses, etc. 
     Periodically, faults occur in the distribution network as a result of various things, such as animals touching the lines, lightning strikes, tree branches falling on the lines, vehicle collisions with utility poles, etc. Faults may create a short-circuit that increases the load on the network, which may cause the current flow from the substation to significantly increase, for example, many times above the normal current, along the fault path. This amount of current causes the electrical lines to significantly heat up and possibly melt, and also could cause mechanical damage to various components in the substation and in the network. Many times the fault will be a temporary or intermittent fault as opposed to a permanent or bolted fault, where the thing that caused the fault is removed a short time after the fault occurs, for example, a lightning strike, and where the distribution network will almost immediately begin operating normally. 
     Fault interrupters, for example, reclosers that employ vacuum interrupters, are provided on utility poles and in underground circuits along a power line and have a switch to allow or prevent power flow downstream of the recloser. These reclosers detect the current and voltage on the line to monitor current flow and look for problems with the network circuit, such as detecting a fault. If fault current is detected the recloser is opened in response thereto, and then after a short delay closed to determine whether the fault is a temporary fault. If fault current flows when the recloser is closed, it is immediately opened. If the fault current is detected again or two more times during subsequent opening and closing operations indicating a permanent fault, then the recloser remains open, where the time between detection tests may increase after each test. For a typical reclosing operation for fault detection tests, about 3-6 cycles of fault current pass through the recloser before it is opened. 
     A sectionalizer is a self-contained, circuit-opening device used in combination with source-side protective devices, such as reclosers or circuit breakers, to automatically isolate faulted sections of an electrical distribution network. A faulted circuit indicator is a device that automatically detects and identifies faults in an electrical distribution network, but does not have switching capabilities to open a power line. These devices are typically distributed between and among the reclosers to provide a system for isolating smaller sections of the network in response to a fault. Faulted circuit indicators and sectionalizers rely on observing a sequence of fault currents and the presence and absence of voltage either to indicate the presence of a fault or count the number of reclosing attempts, and then perform circuit isolation sectionalizing when the maximum number of reclosing attempts has been reached. Existing power distribution circuit sectionalizers detect the passage of fault currents, including both the initial fault event and subsequent recloser-initiated events, as part of more elaborate fault isolation and restoration processes. These processes may include counting discrete intervals of fault current passage, or counting discrete intervals of voltage presence and absence. 
     Most power distribution networks typically have a radial configuration where a power source at one end of a feeder provides power to loads at the other end the feeder. A fault, downed conductor or loss of source in a distribution feeder will cause load sections between reclosers or other switching devices downstream of the power loss location to be unpowered. When this occurs, it is desirable to reconfigure the network by opening and closing some of the switching devices to redirect power to as many load sections as possible from other power sources while still meeting source capacity, line ampacity and voltage constraints. For example, the first normally closed switching device downstream of the power loss location is opened and a normally open switching device at the end of the feeder from the power source is closed to allow power from another power source to provide power to the loads downstream of the now open switching device. In some designs there are multiple normally open switching devices allowing power to be provided from multiple power sources. 
     The decision to reconfigure the switching devices to power the unpowered downstream loads without over-burdening the network should be made quickly, and with limited computational resources. However, opening and closing switching devices in a desirable manner is often a complex process requiring significant coordination between the switching devices. More specifically, when reconfiguring a distribution network to provide power to unpowered sections as described, it is desirable to coordinate all of the operations to ensure that no sources become overloaded, no unnecessary sections lose power and no unnecessary switching actions are performed. In a distributed restoration scheme, information messages are passed between the switching devices or between the switching devices and a central control center using a suitable communications scheme, such as a wireless radio or fiber optics. It is desirable to pass this information in as little time and number of messages as possible. 
     SUMMARY 
     The following discussion discloses and describes a system and method for restoring power in a closed-loop power distribution network. The network includes at least two power sources, at least one feeder and a plurality of switching devices positioned along the at least one feeder and being in communication with each other, where the network is represented by a plurality of teams where each team includes one side of adjacent switching devices and loads and power line sections therebetween. The method determines that there is a power loss in the network and that one or more of the sections is not receiving power. The method performs a radial restoration process for restoring power to the network where any section only receives power from one power source, and then determines that at least one of the sections is not receiving power after the radial restoration process has been performed. The method estimates power flow through each switching device and determines an available power capacity from each switching device, where estimating the power flow and additional capacity includes using a breadth first search (BFS). The method then determines if the unpowered sections can be powered by any of their neighbor and non-neighbor devices. The method virtually closes the switching devices to power the unpowered sections and updates the estimation of power flow through each switching device and determination of available power capacity from each switching device. 
     Additional features of the present disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a simplified schematic illustration of an electrical power distribution network illustrating a radial network restoration process; 
         FIG.  2    is a simplified block diagram of a switching device; 
         FIG.  3    is a simplified schematic type diagram of an electrical power distribution network illustrating a closed-loop network restoration process; 
         FIG.  4    is a flow chart diagram illustrating the closed-loop restoration process; 
         FIGS.  5 - 7    show variations of a simplified schematic type diagram of an electrical power distribution network illustrating a process for selecting a leader switching device; 
         FIG.  8    is a flow chart diagram illustrating a process for identifying leader devices in the network shown in  FIGS.  5 - 7   ; and 
         FIG.  9    is a simplified schematic type diagram of an electrical power distribution network illustrating how the process responds to a capacity request. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the disclosure directed to a system and method for restoring power to a closed loop power distribution network, where the method includes performing a power restoration process for a radial network and then estimating power flow and an available power capacity from switching devices in the network for providing power to sections that were left unpowered by the radial restoration process is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. 
     This disclosure proposes a power restoration algorithm for restoring power to as many sections of a radial power distribution network as possible in response to loss of power, for example, from a fault, that will check all allowable electrical configurations of the network to find the most desired configuration, whereas heuristic approaches may lead to less ideal scenarios. The algorithm performs an exhaustive search that checks all possible solutions for reconfiguring the network to optimally deliver power to as many sections as possible, which becomes prohibitive when the number of solutions becomes very large, where a section is an electrically connected area bounded by switching devices. The algorithm builds the solutions in a way that only feasible configurations will be considered, thus substantially reducing the computation time. If the number of solutions becomes larger than the available memory or a defined time limit is reached before an optimal restoration solution is reached, a solution will still be produced and used, however, it may not be optimal. 
     The algorithm begins with a set of data knowledge of the network that includes network topology, capacity of sources, loads and other information and data that is used to characterize the validity and optimality of a restoration solution. From this data set, a depth first search (DFS) is performed from section to section in the network, where the DFS is started at each section defined as an independent source. As is well understood by those skilled in the art, a DFS is an algorithm for traversing or searching tree or graph data structures that starts at a selected root node, here a power source, and explores as far as possible along each branch of the structure before backtracking. The search performed from each source is independent from the searches performed by the other sources and can be run on a different processor if a parallel computing architecture is available. As the search traverses out from the source section, the capacity of the source is decremented by the amount of load in each section as well as the constraints of any devices that power has passed through, which limits the number of sections a source will try to connect to. The search will also stop if another source is reached because a radial solution cannot have multiple sources connected to the sections. 
     When a branch of the DFS cannot continue, it will begin building a possible solution by tracing its path back towards the source. While building the possible solutions on the reverse traversal of the sections, the search will wait at each section until all downstream branches complete their traversal back to the current section. This allows the accumulation of all load and configurations downstream of the current section. At this step the restrictions of the devices and other constraints can be checked on a subsection of the solution, which allows solutions to be eliminated that are not feasible. When the search ends back at the starting source, a full list of all of the sections and configurations that the source can support is obtained. Once a list of restoration solutions for each source is created, a solution from each source will be exhaustively combined. This full solution will be scored based on predetermined criteria, such as total load restored, smallest number of switching operations, etc. The highest scoring solution is then returned as the optimal configuration. It is noted that this approach can handle distributed generation (DG) on the network, where sections with DG sources are treated as negative load. It is further noted that in the discussion below, a team is defined as a section of powerlines bounded by switching devices and a division is defined as a group of teams that is powered by a single power source and could be electrically isolated from other divisions by normally open switching devices. 
       FIG.  1    is a simplified schematic type diagram of an electrical power distribution network  10  that employs distributed control consistent with the discussion herein, and can be used as an example to illustrate the restoration algorithm outlined above. The network  10  includes AC power sources  12 ,  14  and  16 , such as electrical substations that step down high voltage power from a high voltage power line (not shown) to medium voltage power. The network includes a feeder  18  serviced by the source  12 , a feeder section  20  serviced by the source  16  and a feeder section  22  serviced by the source  14 . The network  10  also includes switching devices  40 ,  42 ,  44 ,  46 ,  52  and  54  positioned along the feeder  18 , a switching device  50  positioned at the end of the feeder section  20 , a switching device  48  positioned at the end of the feeder section  22 , where the switching devices  40 - 54  can be any suitable device for the purposes discussed herein, such as reclosers, breakers, sectionalizers, etc. A section  60  of the feeder  18  is provided between the source  12  and the device  40 , a section  62  of the feeder  18  is provided between the devices  40 ,  42  and  50 , a section  64  of the feeder  18  is provided between the devices  42  and  44 , a section  66  of the feeder  18  is provided between the devices  44  and  46 , a section  68  of the feeder  18  is provided between the devices  46 ,  48 ,  52  and  54 , a section  72  of the feeder  18  is provided off of the device  52  and a section  74  of the feeder  18  is provided off of the device  54 . 
     The devices  40 ,  42 ,  44 ,  46 ,  52  and  54  are normally closed devices and the devices  48  and  50  are normally open devices so that the source  12  provides power to the sections  60 ,  62 ,  64 ,  66 ,  68 ,  72  and  74  and the sources  14  and  16  are prevented from providing power to those sections. For some network configurations referred to herein as closed loop, one or both of the switching devices  48  and  50  may be closed at the same time when the device  40  is closed. In the network  10 , the combination of one side of the devices  40 ,  42  and  50  and the section  62  define Team A, the combination of the other side of the device  50 , the feeder section  20  and the source  16  define Team B, the combination of one side of the devices  42  and  44  and the section  64  define Team C, the combination of one side of the devices  44  and  46  and the section  66  define Team D, the combination of one side of the devices  46 ,  48 ,  52  and  54  and the section  68  define Team E, the combination of the other side of the device  48 , the source  14  and the section  22  define Team F, the combination of the other side of the device  52  and the section  72  define Team G, and the combination of the other side of the device  54  and the section  74  define Team H. As will be discussed in detail below, the topology, i.e., network configuration, will be known by all of the devices  40 - 54  by communications therebetween, where the topology of the network  10  will be automatically revised and updated as devices are removed and added to the network  10 . A control center  76  may be in, for example, radio communication with the switching devices  40 - 54  that provides switching control consistent with the discussion herein. 
       FIG.  2    is a simplified schematic block diagram of a switching device  80  intended to be a non-limiting representation of any one of the switching devices  40 - 54  in the network  10 , where the device  80  may include a switch  82 , voltage/current sensors  84 , a controller  86 , a memory/buffer  88 , a transceiver  90  and a timer  92 . 
     The radial network restoration system and method described herein that provides an optimal configuration of the switches  40 - 54  to power all of the loads in the network  10  when the switch  40  is opened when, for example, there is a fault in the section  60  and the source  12  is unable to power the sections that are downstream of the switch  40  from the source  12 . The restoration algorithm is performed in one of the switching devices  40 - 54 , or the control center  76 , who has been designated the leader device, discussed below, where the leader device is in communication with all of the other devices through any suitable communications protocol, such as radio transmissions, fiber optics, etc. First, the algorithm identifies how much power those teams with sources can provide and how much power those teams with loads need, where the switching devices  40 - 54  have stored data about how much load is on and power can be provided by each team. In this non-limiting example, the source  14  can provide 4 MW of power (Team F) and the source  16  can provide 3 MW of power (Team B). The loads in the Teams A, C, D, E, G and H each require 1 MW of power. 
     The restoration algorithm identifies all of the configurations or solutions where the switches  40 - 54  are selectively opened and closed so that power is provided to all of the Teams A, C, D, E, G and H without overloading the capacity of the sources  14  and  16 , and where the algorithm allocates enough memory for 2 N  restoration solutions, where N is the number of the switching devices being considered. The algorithm selects a source team, such as Team B, and performs the DFS, where the algorithm determines which combination of sections it is able to power in a radial manner. For example, the algorithm determines that Team B can power Team A with 2 MW of power left, and then Team A can power Team C with one MW of power to spare, and then Team C can power Team D with no power capacity left. The algorithm then traverses back from Team D to the source  16  identifying all of the possible restoration solutions, and from this it now knows that the source  16  can provide power to Team B; Teams B and A; Teams B, A and C; and Teams B, A, C and D as possible solutions. 
     The algorithm then selects Team F and performs the DFS, where the algorithm again determines which sections the source  14  is able to power in a radial manner. In this example, the algorithm determines that it can power Team E with 2 MW of power to spare, and then Team E can power Team D with 1 MW of power to spare and then Team D can power Team C with no capacity left. The algorithm then traverses back from Team C to the source  14  identifying the possible solutions, and it now knows that the source  14  can provide power to Team F; Teams F and E; Teams F, E and D; and Teams F, E, D and C as possible solutions. In the same manner, the algorithm also determines the other possible solutions for power from the source  14  as: 
     Teams F, E, H and D; 
     Teams F, E and G; 
     Teams F, E, H and G; and 
     Teams F, E, D and G. 
     The possible solutions for power from both of the sources  14  and  16  are then identified as: 
     Teams B and F; 
     Teams B, F and E; 
     Teams B, F, E and D; 
     Teams B, F, E, D and C; 
     Teams B, F, E, H and D; 
     Teams B, F, E and G; 
     Teams B, F, E, H and G; 
     Teams B, F, E, D and G; 
     Teams B, A and F; 
     Teams B, A, F and E; 
     Teams B, A, F, E and D; 
     Teams B, A, F, E, D and C; 
     Teams B, A, F, E, H and D; 
     Teams B, A, F, E and G; 
     Teams B, A, F, E, H and G; 
     Teams B, A, F, E, D and G; 
     Teams B, A, C and F; 
     Teams B, A, C, F and E; 
     Teams B, A, C, F, E and D; 
     Teams B, A, C, F, E, D and C; 
     Teams B, A, C, F, E, H and D; 
     Teams B, A, C, F, E and G; 
     Teams B, A, C, F, E, H and G; 
     Teams B, A, C, F, E, D and G; 
     Teams B, A, C, D and F; 
     Teams B, A, C, D, F and E; 
     Teams B, A, C, D, F, E and D; 
     Teams B, A, C, D, F, E, D and C; 
     Teams B, A, C, D, F, E, H and D; 
     Teams B, A, C, D, F, E and G; 
     Teams B, A, C, D, F, E, H and G; and 
     Teams B, A, C, D, F, E, D and G. 
     Those possible solutions that include two of the teams, specifically Team D and Team C are not allowed and are excluded because they would create a closed loop. The final possible list of solutions is then provided as: 
     Teams B and F; 
     Teams B, F and E; 
     Teams B, F, E and D; 
     Teams B, F, E, D and C; 
     Teams B, F, E, H and D; 
     Teams B, F, E and G; 
     Teams B, F, E, H and G; 
     Teams B, F, E, D and G; 
     Teams B, A and F; 
     Teams B, A, F and E; 
     Teams B, A, F, E and D; 
     Teams B, A, F, E, D and C; 
     Teams B, A, F, E, H and D; 
     Teams B, A, F, E and G; 
     Teams B, A, F, E, H and G; 
     Teams B, A, F, E, D and G; 
     Teams B, A, C and F; 
     Teams B, A, C, F and E; 
     Teams B, A, C, F, E and D; 
     Teams B, A, C, F, E, H and D; 
     Teams B, A, C, F, E and G; 
     Teams B, A, C, F, E, H and G; 
     Teams B, A, C, F, E, D and G; 
     Teams B, A, C, D and F; 
     Teams B, A, C, D, F and E; 
     Teams B, A, C, D, F, E and G; and 
     Teams B, A, C, D, F, E, H and G. 
     Those solutions are then analyzed to determine which one will be selected as the optimal solution based on predetermined selection criteria. The first and most important criteria is connecting load by predetermined priority, which includes the amount of load and any other customer specified priority, such as hospitals and other types of designated high priority loads. The analysis wants to make sure that those high priority loads are provided power. If there is more than one of the solutions having the same priority, then the next most important criteria can be, for example, the number of loads that are powered. Then, after removing those solutions that don&#39;t pick up the highest number of loads, and assuming there are still one or more solutions left, the next criteria can be, for example, the number of switching operations that need to be performed to provide the solution, where fewer switching operations is better. If more than one possible solution is still available after applying that criteria, the last criteria can be, for example, providing the best power source balancing. If there is more than one solution left after all of the criteria have been applied, then one of those solutions is selected at random. The solution selected in this example is Teams B, A, C, D, F, E, H and G and another algorithm is then initiated that provides the switching operation that includes opening the switching device  46  and closing all of the switching devices  40 ,  42 ,  44 ,  48 ,  50 ,  52  and  54  so that all of the sections  62 - 74  are powered. 
     As mentioned, the discussion above concerning the power restoration algorithm and process is for restoring power in radial networks. That algorithm can be extended to restoring power to as many sections as possible in closed-loop networks, where more than one source is connected to a load. As the radial restoration searches for possible restoration solutions from a source, the distance into a line that the search traverses is limited by the capacity of the source, i.e., once the loads in the teams use up the available power from the source, the search is stopped. However, the depth of a closed-loop restoration search does not have such a limit because the search must continue after the source has been depleted in case another source is reached at the end of the line. The closed-loop restoration algorithm virtually determines using a section-by-section basis the feasibility of powering load sections. This results in a new desired network configuration for the network restoration. The algorithm connects unpowered loads to one or more neighbor sections that were already connected to a source in previous steps. When connecting these load sections, the affected sections upstream are taken into consideration to ensure that capacity, ampacity, voltage and other constraints aren&#39;t violated. 
     In a typical distribution feeder, two types of sources are common, namely, constant voltage sources and controllable constant power sources. A constant power source that has a known or controllable setpoint does not require a power flow and can be used to pick up load sections with minimal computational effort. If there are multiple voltage sources, many factors play a role in the amount of output power from a given source, such as admittance and voltage angle, requiring a power flow to be run. 
     For the closed-loop restoration process, the full information about the network being connected is not necessary, where with equivalent admittance and voltage measurements, an equivalent of the network can be generated and used to greatly simplify the power flow calculations. Further, only network configurations with a high chance of success are calculated, thus avoiding the need to attempt a power flow for each possible way to power a section. 
       FIG.  3    is a simplified schematic type diagram of an electrical power distribution network  100  that is used below as a non-limiting example to describe the closed-loop restoration process referred to above. The network  100  includes a first main AC source  102  coupled to a primary feeder  104  and having a capacity of 45 MW, a second main AC source  106  coupled to a secondary feeder  108  that is coupled to the feeder  104 , a first alternate AC source  110  coupled to a secondary feeder  112  that is coupled to the feeder  104  and having a capacity of 15 MW, a second alternate AC source  114  coupled to a secondary feeder  116  that is coupled to the feeder  104  and having a capacity of 25 MW, and a third alternate AC source  118  coupled to a secondary feeder  120  that is coupled to the feeder  104  and having a capacity of 2 MW. Secondary feeders  126 ,  128  and  130  are also coupled to the feeder  104 , and switching devices  132 ,  134 ,  136  and  138  are provided along the feeder  104 , a switching device  140  is provided in the feeder  126 , a switching device  142  is provided in the feeder  128 , a switching device  144  is provided in the feeder  130 , a switching device  146  is provided in the feeder  112 , a switching device  148  is provided in the feeder  116 , a switching device  150  is provided in the feeder  120 , and a switching device  152  is provided in the feeder  108 . A section  156  having a load of 10 MW is defined between the devices  132 ,  134  and  142 , a section  158  having a load of 10 MW is defined between the devices  134 ,  136 ,  144  and  152 , a section  160  having a load of 10 MW is defined between the devices  136 ,  138  and  146 , a section  162  having a load of 10 MW is defined between the devices  138 ,  148  and  150 , a section  164  having a load of 10 MW is defined downstream of the device  140 , a section  166  having a load of 10 MW is defined downstream of the device  142 , and a section  168  having a load of 10 MW is defined downstream of the device  144 . The switching devices  132 ,  140  and  152  are normally closed and the switching devices  146 ,  148  and  150  are normally open so that power is provided to the network  100  by both of the main sources  102  and  106  in a closed-loop manner and not by the auxiliary sources  110 ,  114  and  118  during normal operation. Further, in this non-limiting example, the device  132  has a restriction that a maximum of 27 MW of power, i.e., current, can pass through it. 
     If a fault occurs at or near the main source  106 , the switching device  152  will perform a reclosing operation and eventually remain open if the fault remains so that the source  106  does not provide power to the network  100 . The source  102  is still able to provide power to the network  100 , but it does not have enough capacity to satisfy all of the loads in all of the sections  156 ,  158 ,  160 ,  162 ,  164 ,  166  and  168 , i.e., 70 MW, and further only 27 MW of power can pass through the switching device  132 . A radial restoration solution is first obtained before the closed-loop restoration algorithm is initiated, which can be obtained in the manner described above, or by brute force techniques, or by some other process that is known by those skilled in the art. For this example, the selected radial solution that is the most optimal closes the devices  146  and  148  and opens the devices  134 ,  136 ,  138  and  144  so that the sections  156 ,  166 ,  160  and  162  receive power from the sources  102 ,  110  and  114 , but the section  158  does not receive power. It is noted that the source  102  has enough capacity to power the section  158 , but it won&#39;t because of the restriction of the device  132 . In other words, if the device  134  was closed to pick up the section  158 , 30 MW of power would flow through the device  132 , thus violating the restriction, so the radial restoration algorithm would not allow that as a solution. 
     Since there is additional capacity in the sources  102 ,  110 ,  114  and  118 , the closed-loop restoration algorithm will be initiated in an attempt to power all of the sections. Generally, as will be described in detail below, the closed-loop restoration algorithm estimates the flow through each device in the current configuration for the radial solution, determines the available capacity from each device, determines if the unpowered teams or sections can be powered by any of their neighbor devices, determines if the unpowered teams or sections can be powered from distant devices, virtually closes the devices to power the unpowered teams, and then updates the flow estimates and available capacities. 
     For the selected radial solution, 20 MW of power flows through the device  132  to the sections  156  and  166 , 10 MW of power flows through the device  140  to the section  164 , 10 MW of power flows through the device  142  to the section  166 , 10 MW of power flows through the device  146  to the section  160 , 10 MW of power flows through the device  148  to the section  162  and 0 MW of power flows through the devices  134 ,  136 ,  138 ,  144  and  150 . The algorithm finds the capacity that each device will be able to provide to a team or section next to it. Since the source  114  can provide 25 MW and 10 MW is used by the section  162 , 15 MW is available to be provided to either the downstream side of the devices  138  and  150 , but practically just to the section  160 . Likewise, the source  110  could provide 5 MW of power that is not used by the section  160  to the section  158  through the device  136  or to the section  162  through the device  138 , and the source  118  could provide 2 MW of power to the section  162 . Further, the source  102  has 30 MW available that is not used by the sections  164 ,  166  and  156 , but can only provide 7 MW to the section  158  because of the restriction on the device  132 . The algorithm then identifies the secondary capacities of the devices, i.e., if an open device was to close, how much additional capacity would another device obtain, were identifying secondary capacities does not occur across unpowered sections. For example, if the device  150  were to close, then the section  162  would obtain 2 MW of additional capacity, and if the device  138  were to close, then the section  160  would obtain 5 MW of additional capacity and the device  136  would obtain 15 MW of additional capacity. 
     Since 7 MW can be provided by the device  134  and 5 MW can be provided by the device  136 , those devices are virtually closed and power is virtually provided to the section  158 , which uses 10 MW of that available power, and the flow estimates and available capacities are updated in each of the devices in the manner as discussed above. Now, the available capacity on the device  144  is 2 MW of power when the devices  134  and  136  are closed, which is not enough to power the section  168 , which needs 10 MW of power. However, the algorithm knows that an additional 15 MW of power is available from the source  114  if the device  138  is closed. Thus, this solution is implemented and the devices  134 ,  136 ,  138  and  144  are actually and simultaneously closed, and power is delivered to the entire network  100 , and the capacities and flow estimates are updated. It is noted that the closed-loop restoration algorithm may not identify the most optimal solution, but likely will identify a sufficient solution without checking every possible solution. 
       FIG.  4    is a flow chart diagram  180  illustrating the closed-loop restoration process as discussed above. The process begins with the network  100  in a radial topology where no voltage source is connected to another voltage source. Using a simple breadth first search (BFS), the algorithm estimates the load flow and additional capacity at each of the devices already connected to a power source at box  182 . As is well understood by those skilled in the art, a BFS is an algorithm for traversing or searching a tree or graph data structure that starts at an arbitrary node of the graph and explores all of the neighbor nodes at the present depth prior to moving on to the nodes at the next depth level. Upon the completion of the BFS, the next step selects one of the unpowered sections to try and reconnect at box  184 , which can be based on any suitable factors or user defined priority, such as load size, total capacity available to the section, etc. Once the algorithm selects an unpowered section, the various available combinations or solutions for closing the devices to power the section are identified and examined at box  186 . With additional data determined during the BFS step, distant open devices can also be considered if closing them would increase the capacity of the open device connected to the section in question. 
     The best combination or solution that has enough capacity to power the selected section is selected at box  188 , where, for example, the combinations or solutions are ordered by the number of required switching operations. Decision diamond  190  determines if this combination or solution connects only one additional voltage source, possibly to other constant power sources, and if so, that combination or solution is accepted as the new desired configuration at box  192 . Estimates of the load flow and additional capacity at each of the devices now connected to a source are updated at box  194 , and the process returns to the box  184  to select the next unpowered section. In this scenario, only an additional BFS is required to update the flows and capacities at the box  194 . 
     If the accepted solution requires connecting more than one power sources at the decision diamond  190 , that combination or solution is only considered as a candidate combination or solution. This is because the power flow resulting from connecting multiple voltage sources together cannot be predetermined with the information gathered, where the flows depend on phase angle, network impedance, etc. and require a power flow calculation. To perform a computationally light power flow calculation, a simplified virtual network is constructed at box  196  using equivalent section admittances, source impedances, voltage and current measurements, and the equivalent admittance of adjacent feeders. Sections connected to multiple devices are modeled as multi-port edges. The operating voltage of known voltage sources is computed using the voltage and current measurements from the devices connected to the section in which the source resides, and the admittance of that section. Adjacent powered feeders, for which detailed information is not available, are modeled as an equivalent voltage source behind impedance using the minimal information available from the open device. The power flows are then computed on this simplified virtual network and the results are mapped back to the real devices in the network  100  at box  198 . The algorithm determines whether the candidate combination or solution meets all of the requirements of source capacity, line ampacity and voltage constraints and is valid at decision diamond  200  and, if so, the solution is accepted and is implemented at box  202 , and the process returns to the box  184  to select the next unpowered section. If the solution is not valid at the decision diamond  200 , the solution is removed at box  204 , and the algorithm returns to the box  188  to obtain the next best solution. Once a section is virtually powered, or all combinations to power that section have been exhausted, the algorithm stops. If any section is successfully connected, that triggers all of the previously checked sections to be processed once again. 
     As mentioned above, to ensure the coordination of the restoration efforts and processes, a leader device is selected for all of the switching devices in each division for steady state operation. During transitional periods a device can take on or relinquish leadership to other devices, through messaging, but each division will only have one leader device at any given time, except for open devices, which may have two leader devices. These open devices need to coordinate with both of the leader devices on either side of the device that are in different divisions. All requests for reconfiguration of a network will be calculated and executed by the leader device of that division. Anytime two divisions must interact with each other, such as closing an open tie device, both leader devices must agree to the plan before continuing. Upon reaching steady state, leadership will be passed between the devices as necessary to ensure only one leader device resides over a division. 
     In a distributed restoration scheme, each leader device will only have knowledge of a portion of the entire network. When a fault occurs, where two downstream sections need to be restored, these sections should be restored together to ensure a more optimal solution. When receiving a restoration request from a device, the leader device can analyze the division to see if there are additional sections that are expected to need restoring. If there are multiple sections expected to be restored, the leader device can wait a finite amount of time until all requests are processed and solve the restoration problem. When multiple devices need to operate to restore a section of the network, the operations need to be performed in certain steps so as to not violate network constraints during the transition. Devices are grouped into categories that can be operated together without violating constraints. These groups are then operated once the devices in the previous group confirm a successful operation. When an open device is commanded to close to repower a section, voltage must be present on the other side of the device to restore the power. However, closing devices may not have a voltage sensor on both sides. Upon receiving a permission request an open device will ensure voltage is on its other side. If the device is not capable of making the measurement, it will ask a neighbor device that has a sensor if voltage is present. If no voltage is detected at either time, the open device reports this back to the leader device, and the leader device removes the open device from consideration for closing. 
     During steady state, the capacity available from each open device changes and must be recalculated. As data propagates across the division, the available capacity can be calculated at each device according to its own constraints, and the capacity of the neighbor devices. Once the message reaches an open device, that device can send the equivalent capacity of itself to the neighbor division. When an open device receives a permission request to close, it must confirm with its leader device that it is clear to close on the other side. When a leader device receives a request to allow a device to close, it must check that there are no other activities going on in the division, and that the device has the required capacity to power the load on the other side. With the most up-to-date, data, the leader device can decide that the network is in a steady state. Additionally, it can perform a capacity calculation and determine the device&#39;s capacity. This can be relayed back to the requesting leader device if there is not enough capacity to make a new calculation, so the requesting leader device can re-calculate its restoration effort. 
     Once the network is reconfigured, the leader device may no longer be connected, or it may not be the best candidate for leadership. A new leader device needs be elected that has the latest data to be ready for the next event. Whenever a leader device finishes a reconfiguration, it will calculate a new leader device for each new section created. It will send a leadership command to the new leader devices, as well as the latest relevant data. 
     When a device is closed or opened by a process outside of restoration, the leadership will have to be re-designated to match the new configuration of the network. When an open device is closed, it will contact the leader devices on both sides of it with information on the other leader device. Whichever leader device determines it is not the best fit it will transfer all of its data about the division to the other leader device. Conversely, when a device opens, it will contact its only leader device. The leader device will then elect a new leader device for the division on the side of the newly opened device that it does not reside. Multiple open devices may be shared by two divisions. When restoring one of these divisions, multiple of these open devices cannot be closed independently. The configuration of both sides of the open devices must be taken into consideration when multiple devices are closed. During steady-state operation, open devices broadcast to both leader devices the information about the sources to which they are connected on the other side. The broadcast messages also contain the information about power limitations for devices via which they are connected to those sources. This data is used to resolve the source inter-dependence among multiple open devices and allow for correct computation of available source capacity. When a section needs more power to reenergize than a neighbor division can provide and more capacity is requested than a neighbor division can provide, the leader device of the powered division can run a capacity calculation. This consists of adding a virtual load at the device that is connected to the unpowered section and running the normal restoration logic. If no section is dropped, and the virtual load is picked up, the division can be reconfigured to provide the needed capacity. If the virtual load cannot be picked up, the responding leader device can request more power from its powered neighbor divisions, following the same approach in a recursive manner up to a user configurable depth. 
     The improvements mentioned above allow for additional scenarios to be handled gracefully, when compared to the known power restoration techniques, where some of these scenarios are listed here as examples. When using multiple tie devices, if a single open device does not have enough capacity to power the whole division, the division will be broken down into multiple segments, each either being powered from a different open device, or left unpowered. In the event of multiple simultaneous faults, since all information is routed to a single leader device, all of the faults can be considered when solving the power restoration problem. With sequential fault events, since each leader device is always up to date on the state of the division it controls, each fault can be acted upon immediately and regardless of the initial state of the division. For load shifting, if a neighbor division does not have enough capacity to restore a division that is out of power, load sections can be shifted to adjacent divisions to allow the source to gain additional capacity to power the unpowered division. 
     During startup and periods of no activity, leader devices are elected for each division through a predetermined hierarchy of devices based on information periodically passed around. The responsibilities of the leader devices start when an event triggers a restoration process. An event trigger such as a restoration request from a device isolating a fault will cause a leader device to enter the process. Upon entering the process, the leader device will trigger a restoration calculation, where typically it will be decided which of the devices to open and close. Before any switching operations take place, the leader device must gain permission from the second leader device of any closing devices. By performing this first, no unnecessary opening operations are performed in the case a tie cannot close. For this, messaging will occur from the restoring leader device to the open device, then to the opposite leader device of the open device. Before forwarding the message to the opposite leader device, the open device will ensure voltage is present on the powered side of the device. When this is confirmed, the permission is forwarded. For the opposite leader device to confirm that an open device can close, it must ensure that closing the device will not cause any issues in its own division. If there are no other open devices set to close, and no events are taking place within the division, the leader device can issue a confirmation. The response of the opposite leader device is then transmitted back to the open device, then the restoring leader device. This ensures that both divisions that are involved are coordinated in the restoration effort. 
     In the case where multiple devices need to be closed, which connect to the same source on the powered side, the leader device on the restoring side can construct a virtual network to assess whether the source may become overloaded. This network is referred to as a virtual network because it is not a part of the connected division being restored. 
     The power restoration algorithm running in the leader device has the entire topology of its own division, but has very limited information about the neighbor division(s). The virtual network mechanism is a distributed algorithm that runs on both the bounding open devices, and on the restoring leader devices. The algorithm sends to the leader device of the neighbor division the information about the available power/capacity, sends to the leader device of the neighbor division the interconnections and power restrictions imposed for each open device considered in the restoration plan, sends the necessary information in a compact form to avoid broadcasting of the entire division&#39;s topology, and assembles the virtual network in the leader device that is used in assessing possible restoration solutions. 
     Once all of the permissions have been obtained by the restoring leader device, the devices can then be operated. These devices are grouped based on the network and their position to not violate any constraints during the transition. The grouping will depend on whether the restoration is due to a fault event or a capacity request. For a fault event, all of the opening devices are in a first group and all of the closing devices are in a second group. For a capacity request, all of the closing devices are in the first group and all of the opening devices are in the second group. To begin, all of the devices in the first occupied group are commanded to operate. Once all of the devices respond, or timeout, the leader device will advance the restoration process. If all of the devices operated successfully, the next group will be commanded to operate. If any device did not operate successfully, or a new event occurred within the restoring section, the restoration calculation will be re-run and the process started again. Upon completion of all of the devices which should operate, the leader device will designate the most appropriate new leader device for each division. 
     Periodically, open devices will send the capacity they have available. The immediate capacity, i.e., the amount which can be used from the source(s) in the neighbor division, is considered the first level of capacity. Additionally, the open device can calculate a hypothetical scenario where load in its own division can be shifted to adjacent powered divisions to find out what the most capacity available could be after executing the load shifting. This is considered the second level of capacity. The third level is like the second level, but it is assumed that the neighbor divisions are using their second level of capacity. In the case where a leader device calculates that no additional power is available in the surrounding divisions that can be used to pick up more load, but identifies that enough distant capacity may be available, a capacity request is sent. The mechanism will be like obtaining permission to close an open device, but the leader device on the other side of the open device will trigger its own restoration effort that can obtain the power needed for the requesting leader device. This process can be repeated at each leader device, allowing a division to retrieve power from several divisions away. 
       FIGS.  5 ,  6  and  7    are simplified schematic type diagrams of an electrical power distribution network  210  that is used below to describe a non-limiting example to illustrate how the process selects a leader device in the manner as discussed above. The network  210  includes an AC source  212  coupled to one end of a primary feeder  214  and an AC source  216  coupled to an opposite end of the feeder  214 . A series of switching devices  218 ,  220 ,  222 ,  224 ,  226  and  228  are provided along the feeder  214 , where the devices  218 - 224  and  228  are normally closed devices and the device  226  is a normally open device to electrically separate the sources  212  and  216 . The network  210  also includes a secondary feeder  232  coupled to the feeder  214  and having a normally open switching device  234 , and a secondary feeder  236  coupled to the feeder  214  and having a normally open switching device  238 . The network  210  also includes a secondary feeder  242  coupled to the feeder  236  at one end and to an AC source  244  at an opposite end. A series of switching devices  246 ,  248 ,  250  and  252  are provided along the feeder  242 , where the devices  246 ,  250  and  252  are normally closed devices and the device  248  is a normally open device to electrically separate the sources  212 ,  216  and  244 . A secondary feeder  254  is coupled to the feeder  242  and includes a normally open switching device  256 . 
     The normally open devices  226 ,  234 ,  238 ,  248  and  256  define electrically separated divisions  260 ,  262 ,  264 ,  266 ,  268  and  270  in the network  210 . One of the devices in each division is initially designated as the leader device and all of the devices in the division know which one is the leader device. In this example, the device  238  is the leader device of the division  260 , the device  226  is the leader device of the division  262 , and the device  256  is the leader device of the divisions  264  and  270 , where the leader devices of the divisions  266  and  268  are not shown. 
     If a fault occurs on the feeder  214  in, for example, the section between the devices  218  and  220 , the device  218  will perform a reclosing operation for protection and the device  218  will eventually open if the fault remains, where power is cut off to the downstream sections being serviced by the source  212 . The device  218  then messages the leader device  238  for the division  260  to request restoration by any suitable technique, such as those described above. The algorithm operating in the leader device  238  wants to isolate the fault between the devices  218  and  220  by also opening the device  220  and restore power to the sections in the division  260  downstream of the device  220 . The algorithm operating in the leader device  238  determines which of the devices to open and close, in what order they are to be opened and closed, obtains permission from the other leader devices to open and close the devices, sends the commands to open and close the devices and transfers the leadership responsibility, if necessary. In this example, the device  238  obtains permission from the device  226  for it to close and from the device  256  for the device  248  to close and, if granted, commands the device  222  to open and then the devices  226  and  248  to close. 
     As shown in  FIG.  7   , the network  210  now has the configuration where the source  212  and the devices  218 ,  220  and  234  are in new division  400 , the source  244  and the devices  220 ,  222 ,  238 ,  246 ,  248 ,  250 ,  252  and  256  are in new division  402 , and the source  216  and the devices  222 ,  224 ,  226  and  228  are in new division  404 . At this point, the device  238  is still the leader device of the division  400  because the device  218  is still going through the reclosing operation. Eventually the device  218  will lock open, and will message the device  238  requesting restoration. The device  238  will know that the restoration is already complete, and will separate section  406  from the division  400 , as shown in  FIG.  7   . The device  238  will then pass leadership data to the device  234  for the division  400 , the device  220  for the section  406 , the device  256  for the division  402  and the device  222  for the division  404 . 
       FIG.  8    is a flow chart diagram  280  illustrating the algorithm, which is operating in a leader device of a division, discussed above for selecting leader devices. The algorithm starts at box  282  where an event is triggered, such as a fault, where all current events occurring in the network  210  are cancelled at box  284 . At box  286 , the algorithm waits for pending devices to send appropriate messages, which does not happen when the restoration event initially occurs. The algorithm then proceeds to perform the restoration process at box  288 , where for a new event or if a device did not operate properly, the restoration calculations are performed at box  290  to determine what switching devices are operated and in what order, which in this example is open the device  222  and close the devices  226  and  248 . The algorithm then determines if the restoration process is complete at decision diamond  292 , which initially is no because the actual switching operation of the devices  222 ,  226  and  248  has not been performed yet. The algorithm determines if all of the devices that are to be closed have permission to close at decision diamond  294 , which is initially no because the algorithm has not performed those steps yet either. The algorithm obtains permission to close the tie devices  226  and  248  between the divisions at box  296 , which requires a voltage check on an opposite side of the tie devices from the fault at box  298 , where the tie devices  226  and  248  contact the leader device in the adjacent division to obtain permission to close at box  300 , and where the other leader devices may or may not allow the device to close. For example, if another fault is occurring in those divisions and the source was not available, the permission would be denied by the leader device. 
     Once the leader device  238  has permission or not the algorithm returns to the box  286  and the advance restoration box  288 , where it is not a new event so no additional calculations are necessary, assuming that permission was given to open all of the closed devices. The restoration is still not complete at the decision diamond  292 , but the devices to be closed do have permission at the decision diamond  294 , and those devices are opened at box  302 . The leader device  238  waits at the box  286  for messages that the device  222  has opened and the devices  226  and  248  have closed, and when that happens the restoration is complete at the decision diamond  292 . When the restoration is complete at the decision diamond  292 , the algorithm determines whether the event was triggered for more capacity at decision diamond  304 , where in this case it is no because the event was triggered by a fault, so the algorithm proceeds to box  306  to select new leader devices for the new divisions  400 ,  402  and  404 . 
     If the event was triggered for more capacity at the decision diamond  304 , the algorithm moves to box  308  to respond to the capacity request.  FIG.  9    is a simplified schematic type diagram of an electrical power distribution network  410  that is used below as a non-limiting example to illustrate how the process responds to this request. The network  410  includes an AC source  412  coupled to a feeder  414  at one end and an AC source  416  coupled to the feeder  414  at an opposite end, where the both of the sources  412  and  416  have a 10 MW capacity. Switching devices  418 ,  420  and  422  are provided along the feeder  414  and define sections  424  and  426 , where the switching device  418  is normally open, the switching devices  420  and  422  are normally closed, the section  424  has a 3 MW load and the section  426  has a 2 MW load. A secondary feeder  428  is coupled to the section  426  and includes switching devices  430  and  432  defining sections  434  and  436  that are unpowered in this example, where the section  434  has a 6 MW load and the section  436  has a 1 MW load. The source  412  and the device  418  are in one division, the source  416  and the devices  420 ,  422  and  430  are in another division and the devices  430  and  432  are in another division, where the devices  418  and  430  are tie devices. 
     The source  416  only has 5 MW that is available after powering the sections  424  and  426 , and therefore cannot power the sections  434  and  436  by closing the device  430 . Restoration algorithms of the type discussed above are operated virtually in the leader devices to see if the source  412  division and the source  416  division can coordinate to provide the requested capacity. This restoration process identifies that if the device  420  is closed and the device  418  is opened, then the section  424  can be powered by the source  416  and 8 MW will be available at the device  430  for the sections  434  and  436 , which is enough if the device  430  is opened. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.