Patent Publication Number: US-2023135520-A1

Title: Network protector for secondary distribution network that includes distributed energy resources

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 63/272,949, filed on Oct. 28, 2021 and titled NETWORK PROTECTOR FOR SECONDARY DISTRIBUTION NETWORK THAT INCLUDES DISTRIBUTED ENERGY RESOURCES, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a network protector for a secondary distribution network that includes distributed energy resources (DER). 
     BACKGROUND 
     A network protector permits two or more electrical feeders to be connected to a common low-voltage bus. The network protector may include a sensor system the monitors electrical conditions on one of the electrical feeders and a resettable switching apparatus that controls current flow in the one of the electrical feeders based on the monitored electrical conditions. 
     SUMMARY 
     In one aspect, a system includes a network protector. The network protector includes a resettable switching apparatus configured to control an electrical connection between a distribution transformer and a first electrical feeder of a secondary electrical distribution network, the secondary distribution network electrically connected to one or more distributed energy resources (DERs). The network protector also includes a controller configured to: determine, after a test signal is provided to the secondary electrical distribution network, whether the secondary electrical distribution network has a radial structure or a mesh structure; if the secondary electrical distribution network has a radial structure, control the resettable switching apparatus to open and disconnect the distribution transformer from the first electrical feeder of the secondary electrical distribution network; and if the secondary electrical distribution network has a mesh structure, control the resettable switching apparatus such that the distribution transformer and the first electrical feeder are connected to thereby allow one or more of the DERs to provide electrical power to the secondary electrical distribution network. 
     Implementations may include one or more of the following features. The network protector also may include a sensor configured to measure one or more electrical properties of electrical power in the first electrical feeder, and the controller also may be configured to: determine whether there is a change in at least one measured electrical property; and if there is a change in at least one electrical property, cause the test signal to be provided to the secondary electrical distribution network. The one or more electrical properties may include any of current flow direction, voltage magnitude, voltage phase angle, active power, reactive power, and impedance. 
     The controller also may be configured to cause the test signal to be provided the secondary network by controlling the distribution transformer to perform a tap change operation, and, to determine whether the secondary electrical distribution network has a radial structure, the controller may be configured to compare a voltage on a source side of the distribution transformer after the tap change operation to the voltage on the source side of the distribution transformer before the tap change operation. In some implementations, the tap change operation is associated with a tap step voltage. In these implementations, if the voltage on the source side changes by the tap step voltage, the secondary distribution network is determined to have a radial structure, and if the voltage on the source side does not change by the tap step voltage, the secondary distribution network is determined to have a mesh structure. The system also may include the distribution transformer. 
     In some implementations, the controller is further configured to cause the test signal to be provided to the first electrical feeder by controlling a reactive power generation apparatus to inject the test signal into the secondary electrical distribution network, the test signal having a known amount of reactive power; and, to determine whether the secondary electrical distribution network has a radial structure, the controller is configured to: compare an amount of reactive power on a side of the reactive power generation apparatus after the test signal is provided to the first electrical feeder to an amount of reactive power on the side of the reactive power generation apparatus before the test signal was provided. If the reactive power on the side of the reactive power generation apparatus changes by the known amount of reactive power, the secondary electrical distribution network may be determined to have a radial structure; and if the reactive power on the side of the reactive power generation apparatus does not change by the known amount of reactive power, the secondary electrical distribution may be determined to have a mesh structure. If the reactive power on the side of the reactive power generation apparatus does not change, the secondary electrical distribution network may be determined to have a radial structure; and if the reactive power on the side of the reactive power generation apparatus changes, the secondary electrical distribution may be determined to have a mesh structure. The system also may include the reactive power generation apparatus. The reactive power generation apparatus may be a capacitor bank or an inverter. 
     The controller may be further configured to: determine whether the resettable switching apparatus is open; if the resettable switching apparatus is open, compare a magnitude of a voltage vector on a source side of the distribution transformer to a magnitude of a voltage vector on a secondary network side of the distribution transformer; and determine whether to close the resettable switching apparatus based on the comparison. 
     In another aspect, a network protector includes a resettable switching apparatus configured to electrically connect to a low-voltage feeder of a secondary distribution network; a switch control configured to control a state of the resettable switching apparatus to thereby determine whether electrical current flows through the switching apparatus; and a controller configured to: determine whether a fault condition exists; and if a fault condition does not exist, allow electrical power to flow through the resettable switching apparatus in any direction. 
     Implementations may include one or more of the following features. 
     The resettable switching apparatus may be an air circuit breaker. 
     The controller may be configured to determine whether a fault condition exists by determining a structure of the low-voltage feeder. The controller may be further configured to: cause a test generation device to generate a test signal; and determine the structure of the low-voltage feeder based on a response to the test signal. 
     The switch control may be a relay. 
     The network protector also may include a sensor system configured to measure one or more electrical properties of electrical power on the low-voltage feeder. 
     The controller also may be configured to cause the switch control to open the resettable switching apparatus if a fault condition exists. 
     In another aspect, a method of operating a network protector includes: detecting reverse power flow from a load toward a source on a low-voltage electrical feeder of a secondary electrical distribution network; causing a test signal to be provided to the low-voltage electrical feeder of the secondary electrical distribution network; analyzing a response of the secondary electrical distribution system to the test signal; and determining whether to allow the reverse power to flow based on the analysis. 
     Implementations of any of the techniques described herein may a system, a network protector, a controller, a method, a process, or executable instructions stored on a machine-readable medium. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       DRAWING DESCRIPTION 
         FIG.  1    is a block diagram of an example of an electrical power system. 
         FIG.  2    is a block diagram of an example of a spot network. 
         FIG.  3    is a block diagram of an example of an area network. 
         FIG.  4    is a block diagram of another example of an electrical power system. 
         FIG.  5 A  is a block diagram of another example of an electrical power system. 
         FIG.  5 B  is a block diagram of an example of a distribution transformer. 
         FIG.  5 C  is a block diagram of an example of a network protector. 
         FIGS.  6 A and  6 B  are block diagrams of another example of an electrical power system. 
         FIG.  7    is a flow chart of an example of a process for determining the structure of a distribution network. 
         FIG.  8    is a flow chart of an example of a process for reclosing or closing an open switch device. 
         FIG.  9    is a plot of reclose zones. 
         FIGS.  10 A- 10 D  show examples of simulated data. 
         FIGS.  11 A- 11 D  show examples of simulated data. 
         FIGS.  12 A- 12 D  show examples of simulated data. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a block diagram of an example of an electrical power system  100 . The power system  100  may be a single-phase power system or a multi-phase (for example, three-phase) power system. A single phase is shown in  FIG.  1    for simplicity. The electrical power system  100  includes a secondary electrical power distribution network  101  that includes switch devices  130 _ 1  and  130 _ 2 . The switch devices  130 _ 1  and  130 _ 2  may be any type of switch that is capable of repeatedly controlling the electrical connection between the respective feeder  104 _ 1  and  104 _ 2  and the loads  103 . For example, the switch devices  130 _ 1  and  130 _ 2  may be network protectors, reclosers, or switchgears. The secondary electrical power distribution network  101  is electrically connected to an alternating current (AC) source  102  and to load or loads  103 . As discussed below, the switch devices  130 _ 1  and  130 _ 2  are configured to allow bi-directional power flow (power flow toward and away from the source  102 ) while also providing protection from abnormal conditions (such as fault conditions). The configuration allows the secondary distribution network  101  to provide electrical power generated by a distributed energy resource (DER) to the power system  100 . 
     The AC power source  102  operates at a fundamental frequency of, for example, 50 or 60 Hertz (Hz). The power source  102  may be a generator, a power plant, an electrical substation, or a renewable energy source. The power source  102  may be medium-voltage or distribution voltage (for example, between 1 kilovolts (kV) and 35 kV) or high-voltage (for example, 35 kV and greater). Moreover, the power source  102  may receive power from other electrical power sources that are not shown in  FIG.  1   . For example, each of the power source  102  may be a medium-voltage substation that receives and transforms high-voltage AC power into medium-voltage AC power that is provided to feeders  106 _ 1  and  106 _ 2 . 
     The feeders  106 _ 1  and  106 _ 2  transfer AC electrical power from the power source  102  to a primary or source side of respective distribution transformers  142 _ 1  and  142 _ 2 . A distribution transformer is a transformer performs a voltage transformation at an end point or node of a distribution grid. In the example of  FIG.  1   , the distribution transformers  142 _ 1  and  142 _ 2  convert voltage on the respective feeders  106 _ 1  and  106 _ 2  to lower voltages that are suitable for general household, industrial, and/or commercial use. For example, the distribution transformers  142 _ 1  and  142 _ 2  may transform the voltage on the respective feeders  106 _ 1  and  106 _ 2  to a voltage of 1 kV or less. The secondary side of each distribution transformer  142 _ 1 ,  142 _ 2  is connected to a respective feeder  104 _ 1 ,  104 _ 2  of the secondary distribution network  101 . 
     The network protector  130   1  controls an electrical connection between the distribution transformer  142 _ 1  and loads  103 . The network protector  130 _ 2  controls an electrical connection between the distribution transformer  142   2  and the loads  103 . The loads  103  include one or more distributed energy resources (DER). A DER is an electricity-producing resource and/or a controllable load. Examples of DER include, for example, solar-based energy sources such as, for example, solar panels and solar arrays; wind-based energy sources, such as, for example wind turbines and windmills; combined heat and power plants; rechargeable sources (such as batteries); natural gas-fueled generators; electric vehicles; and controllable loads, such as, for example, some heating, ventilation, air conditioning (HVAC) systems and electric water heaters. The loads  103  also may include devices and systems that are not DERs. For example, the loads  103  also may include motors, lighting systems, and/or machines. The secondary electrical distribution network  101  includes switch devices  130 _ 1  and  130 _ 2 . The system  100  also includes a controller  150 . 
     Under some conditions, the power generated by the DERs exceeds the power demand of the loads  103 , and the DERs return electrical power to the secondary distribution system  101 . This returned electrical power is reverse power that flows from the loads  103  toward the source  102 . In a traditional configuration of an electrical distribution system, it is assumed that power flows from the source to the loads under expected or ordinary conditions, and that reverse power flow (for example, a current that flows from the loads to the source) is an indication of an abnormal condition, such as the presence of a fault. Switch devices that are configured in the traditional manner open based on detection of power flowing from the load to the source, even if the reverse flow does not arise from an abnormal operating condition. Moreover, switch devices configured in the traditional manner only reclose when conditions in the system indicate that power flow from the sources to the load is guaranteed. 
     On the other hand, in the system  100 , the controller  150  is configured to determine whether or not the distribution network  101  is in an abnormal condition by determining the structure of the distribution network  101 . Under normal or expected operating conditions, source-side circuit breakers  105   1  and  105   2  are closed, and the distribution network  101  has a mesh or loop structure. During an abnormal operating condition, such as an overcurrent or overvoltage condition, at least one of the breakers  105 _ 1 ,  105 _ 2  is open, and the distribution network  101  has a radial structure. Thus, by determining whether or not the distribution network  101  has a radial structure, the controller  150  also determines whether the electrical distribution network  101  has an abnormal condition directly and without relying on an assumption that reverse power flow indicates a fault condition. 
     Various techniques for determining the structure of the distribution network  101  are discussed below. Regardless of the specific approach used to determine the structure of the distribution network  101 , the configuration of the controller  150  results in improved performance of the system  100  and greater utility for the switch devices  130 _ 1  and  130 _ 2  as compared to the traditional approach of assuming that reverse power flow indicates an abnormal condition. For example, the structure-determination approach provided by the controller  150  allows power generated by DERs to flow toward the source  102  when the network  101  is operating under normal or expected operating conditions. In contrast, a traditional approach does not allow power to flow toward the source  102  under any conditions, even if the network  101  is operating under normal conditions and the reverse power flow is generated by DERs. On the other hand, by allowing reverse power to flow under some conditions, the structure-determination approach results in fewer outages for customers and fewer unnecessary operations of the switch devices  130 _ 1  and  130 _ 2 . Accordingly, the structure-determination approach improves the performance and efficiency of the system  100  and the switch devices  130 _ 1  and  130 _ 2  while also improving customer satisfaction. Moreover, allowing electrical power generated by the DERs to flow in the network  101  reduces waste and results in a more environmentally sound approach. 
     Additionally, the switch devices  130 _ 1  and  130 _ 2  and the controller  150  may be used in implementations in which the secondary distribution network  101  has a relatively high penetration of DER power generation, for example, a 90% or greater penetration. DER penetration is the ratio of nominal capacity of DER power generation to the nominal load of the feeder to which the DERs are connected. The likelihood of reverse power arising from DER power generation occurring increases with DER penetration. 
     Prior to discussing the controller  150  in more detail, an overview of the distribution network  101  is provided. 
     The secondary electrical power distribution network  101  is a low-voltage network (for example, a network that distributes electricity having a voltage of 1 kV or less). The secondary electrical power distribution network  101  may be a spot network or an area network. In a spot network, two or more feeders are connected in parallel to a common bus to provide power to a specific location or spot. A grid or area network includes redundant feeders. Regardless of the configuration of the low-voltage network, the network protectors  130 _ 1  and  130 _ 2  improve the overall performance of the low-voltage network. For example, reverse power caused by DER generation exceeding the demand causes in a network protector with a traditional configuration to open, even if there is no fault condition. In a spot network that employs traditional network protectors, any reverse power causes the network protectors to open, which results in a service outage for the load. In an area or grid network that employs only traditional network protectors, the presence of reverse power may cause fewer than all network protectors to open, however, reliability is reduced when even some of the network protectors open. Thus, the network protectors  130 _ 1  and  130 _ 2 , which do not assume that reverse power flow is caused by a fault condition, improve the performance of spot and area networks. 
       FIG.  2    is a block diagram of an electrical power system  200  that includes a spot network  201 . The spot network  201  includes four parallel low-voltage feeders  204 _ 1 ,  204 _ 2 ,  204 _ 3 ,  204 _ 4  that are all connected to a spot, which is the loads  103  in the example of  FIG.  2   . The loads  103  may be, for example, a variety of electrical loads that are all within one large building or location, such as an airport terminal, a data center, a hospital, or an apartment building. The spot network  201  includes one or more DERs. The loads  103  include one or more DERs. 
     The spot network  201  receives electrical power from four medium-voltage feeders  206 _ 1 ,  206 _ 2 ,  206 _ 3 ,  206 _ 4  that are fed by the AC power source  102 . The feeders  206 _ 1 ,  206 _ 2 ,  206 _ 3 ,  206 _ 4  include respective circuit breakers  105 _ 1 ,  105 _ 2 ,  105 _ 3 , and  105 _ 4  that open in the presence of an abnormal condition, such as a fault (for example, an over-voltage or over-current condition) or scheduled maintenance. 
     Each medium-voltage feeder  206 _ 1 ,  206 _ 2 ,  203 _ 3 ,  206 _ 4  is electrically connected to a primary side of a respective distribution transformer  242 _ 1 ,  242 _ 3 ,  242 _ 3 ,  242 _ 4 . The voltage at on each feeder  206 _ 1 ,  206 _ 2 ,  206 _ 3 ,  206 _ 4  and at the primary side of each respective distribution transformer  242 _ 1 ,  242 _ 3 ,  242 _ 3 ,  242 _ 4  is determined by the voltage of the source  102 . The distribution transformers  242 _ 1 ,  242 _ 3 ,  242 _ 3 ,  242 _ 4  step down (reduce) the voltage from the source  102  such that the voltage at a secondary side of each transformer is lower than the voltage at the primary side. The voltage at the primary side of the distribution transformers may be, for example, between 1 kV and 35 kV, and the voltage at the secondary side of the distribution transformers may be, for example, 240 V, 480 V, or 600 V. 
     The secondary side of each distribution transformer  242 _ 1 ,  242 _ 3 ,  242 _ 3 ,  242 _ 4  is electrically connected to a respective low-voltage feeder  204 _ 1 ,  204 _ 2 ,  204 _ 3 ,  204 _ 4 . Respective switch devices  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  control the electrical connection between the loads  103  and each low-voltage feeder  204 _ 1 ,  204 _ 2 ,  204 _ 3 ,  204 _ 4 . The operation of the switch devices  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  is controlled by the controller  150 . Although the controller  150  is shown as a single element, in some implementations, each switch device  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  has a dedicated local controller that controls the operations of that switch device. In these implementations, the spot network  201  includes four instances of the controller  150 , and each instance of the controller is associated with one switch device. Each switch device  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  may be, for example, a network protector. In implementations in which the switch devices  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  are network protectors, each network protector may have a dedicated local controller. 
       FIG.  3    is a block diagram of a power system  300  that includes an area network  301 . The area network  301  includes redundant feeders  304  (only one of which is labeled), switch devices  130  (each of which may be a network protector), and transformers  342  that provide power to the loads  103 . The area network  301  may include tens of redundant feeders  304  and switch devices  130 , and the loads  103  may include tens, hundreds, or thousands of loads. The switch devices  130  are controlled by the controller  150 . Although the controller  150  is shown as a single element, in some implementations, each switch device  130  has a dedicated local controller. 
       FIGS.  1 ,  2 , and  3    are provided as examples, and the controller  150  may be used with distribution networks having other configurations. For example, the distribution network  201  may have fewer or more than four parallel low-voltage feeders. 
       FIG.  4    is a block diagram of a controller  450  in a power system  400  that includes a distribution network  401 . The controller  450  is an example of an implementation of the controller  150 . The distribution network  401  may be a medium-voltage distribution network or a low-voltage secondary distribution network. The distribution network  401  includes a plurality of feeders. In the example of  FIG.  4   , two feeders are shown and one is labeled as feeder  404 . A switch device  430  is coupled to the feeder  404 . The distribution network  401  includes additional switch devices  430  coupled to the other feeders. In the example of  FIG.  4   , an additional switch device is labeled as  430 _ 1 . 
     The switch device  430  may be a single-phase or multi-phase switch device. The switch device  430  includes a resettable switching apparatus  432  and a sensing system  434 . The switch device  430  is coupled to the controller  450 . The controller  450  is shown as being separate from the switch device  430 . However, in some implementations, the controller  450  is included in the switch device  430 . For example, the controller  450  and the switch device  430  may be contained together in the same housing. The switch device  430  is electrically connected to the feeder  404 . 
     The resettable switching apparatus  432  is any type of switch that is capable of opening and closing the feeder  404 . The resettable switching apparatus  432  is configured for repeated operation. For example, after the resettable switching apparatus  432  opens the feeder  404  to stop or prevent current flow, the resettable switching apparatus  432  is able to close the feeder  404  such that current flow in the feeder  404  resumes. The resettable switching apparatus  432  also may include additional components and systems such as actuators, motors, springs, levers, and/or driving electronics that facilitate the operation of the switching apparatus  432 . 
     In some implementations, the feeder  404  is a high-voltage or medium-voltage feeder, and the switch device  430  is configured to open and close such a feeder. For example, the switch device  430  may be a switchgear or a recloser. In these implementations, the resettable switching apparatus  432  may be a plurality of electrically conductive contacts that are joined to close the feeder  404  and separated to open the feeder  404 . For example, the resettable switching apparatus  432  may include a first electrical contact and a second electrical contact configured to move relative to the first electrical contact to open and close the feeder  404 . The resettable switching apparatus  432  may be a vacuum interrupter or a high-voltage or medium voltage circuit breaker. 
     In some implementations, the feeder  404  is a low-voltage feeder that is in a secondary electrical distribution network. In these implementations, the switch device  430  may be, for example, a network protector or other switch device configured for low-voltage use. In implementations in which the switch device  430  is a network protector, the resettable switching apparatus  432  may be an air circuit breaker operated by relay that monitors the voltage across the open contacts and the current through the closed contacts. An air circuit breaker includes two electrical contacts that operate in air at atmospheric pressure. When the electrical contacts are joined, current can flow in the feeder  404 . When the electrical contacts are separated, current cannot flow in the feeder  404 . 
     The switch device  430  also includes a sensing system  434 . The sensing system  434  includes one or more detectors or sensors, each of which is configured to sense one or more properties of the electrical current in the feeder  404 . The sensing system  434  may include any type of current sensor, such as, for example, a current transformer (CT) or a Rogowski coil. Alternately or additionally, the sensing system  434  may include one or more voltage sensors and/or one or more power sensors. The sensing system  434  may include a relay. 
     The switch device  430  also includes a controller  450 . The controller  450  is coupled to the switch device  430  and to a test generation device  440  via a communication path  455 . The communication path  455  may be any type of wired and/or wireless path capable of transporting signals, information, and/or data. For example, the communication path  455  may be a control cable, a wire, and/or an Ethernet or other network cable. 
     The test generation device  440  may be, for example, a device capable of injecting reactive power, such as the reactive power generation apparatus  644  of  FIGS.  6 A and  6 B , or a device capable of changing the amount of voltage on the feeder  404 , such as the distribution transformer  542 _ 1  of  FIG.  5 A , a voltage regulator, or any other device or apparatus that includes a tap changer. The power generation apparatus  644  may be, for example, a capacitor bank, power electronic switches, a power converter, and/or an inverter. The test generation device  440  is electrically connected to the feeder  404 , however, the test generation device  440  is not necessarily connected between the switch device  430  and the loads  103  as shown in  FIG.  4   . Moreover, although the test generation device  440  is shown as being distinct from the switch device  430 , in some implementations, the test generation device  440  is integrated with the switch device  430 . For example, the test generation device  440  may be housed within a unit or container that also houses the resettable switching apparatus  432 . 
     The controller  450  is an electronic controller that includes an electronic processing module  452 , an electronic storage  454 , and an input/output (I/O) interface  456 . The electronic processing module  452  includes one or more electronic processors, each of which may be any type of electronic processor and may or may not include a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a field-programmable gate array (FPGA), Complex Programmable Logic Device (CPLD), and/or an application-specific integrated circuit (ASIC). 
     The electronic storage  454  may be any type of electronic memory that is capable of storing data and instructions in the form of computer programs or software, and the electronic storage  454  may include volatile and/or non-volatile components. The electronic storage  454  and the processing module  452  are coupled such that the processing module  452  can access or read data from and write data to the electronic storage  454 . 
     The electronic storage  454  stores executable instructions, for example, as a computer program, logic, or software, that cause the processing module  452  to perform various operations. For example, the electronic storage  454  stores executable instructions that cause the processing module  452  to determine the structure of the distribution network  401  using the process  700  of  FIG.  7   . To provide another example, the electronic storage may store instructions that cause readings from the sensing system  434  to be stored on the electronic storage  454 . The instructions also may include instructions that compare the readings obtained by the sensing system  434  at different times to determine whether one or more properties of the electrical power in the feeder  404  change over time. The properties include, for example, the direction of current flow on the feeder  404 , magnitude of voltage on the feeder  404 , phase angle of voltage on the feeder  404 , magnitude of current on the feeder  404 , phase angle of current on the feeder  404 , active power on the feeder  404 , reactive power on the feeder  404 , or impedance at a known point on the feeder  404 . 
     The electronic storage  454  also may store information about the switch device  430  and/or the feeder  404 , such as one or more threshold values used for determining whether a change has occurred in one or more measured properties. For example, as discussed above, the electronic storage  454  may store instructions that determine whether one or more measured properties on the feeder  404  changes over time. In this example, if the magnitude of the measured voltage on the feeder  404  changes by at least the voltage change threshold over the associated time, the controller  450  produces an indication that the voltage magnitude has changed. Thresholds for other properties also may be stored on the electronic storage  454 . Moreover, the threshold may be expressed as a percentage change that is stored on the electronic storage  454 . The same percentage change may be used as a threshold for all measured properties, or each measured property may have a different threshold. In some implementations, threshold values or threshold percentage changes may be entered into the controller  450  via the I/O interface  456 . 
     Furthermore, the electronic storage  454  may store instructions that, when executed, cause the electronic processing module  452  to send the test generation device  440  a command signal that causes the test generation device  440  to generate a test signal  445  and provide the test signal  445  to the feeder  404 . The electronic storage  454  also may store instructions that cause the electronic processing module  452  to analyze values of one or more properties of electrical power on the feeder  404  and/or in a power system that includes the feeder  404  after controlling the test generation device  440 . 
     Furthermore, the electronic storage  454  may include instructions that implement techniques for filtering and/or preparing the data produced by the sensing system  434 . For example, the electronic storage  454  may include instructions that implement an analog-to-digital (A/D) converter that digitizes analog data from the sensing system  434 . 
     Additionally, the electronic storage  454  may store instructions related to the operation of the switch device  430 . For example, the electronic storage  454  may store instructions, that when executed by the processing module  452 , cause the controller  450  to issue a command to the switch device  430  such that the switch device  430  opens or closes. Moreover, the electronic storage  454  may store information related to the conditions under which the switch device  430  is to be opened or closed. For example, the electronic storage  454  may store a threshold value that represents a maximum acceptable difference between the network-side and source-side voltage vectors to allow the switch device  430  to transition from the open state to the closed state, as discussed with respect to  FIG.  8   . 
     The I/O interface  456  may be any interface that allows a human operator and/or an autonomous process to interact with the controller  450 . The I/O interface  456  may include, for example, a display (such as a liquid crystal display (LCD)), a keyboard, audio input and/or output (such as speakers and/or a microphone), visual output (such as lights, light emitting diodes (LED)) that are in addition to or instead of the display, serial or parallel port, a Universal Serial Bus (USB) connection, and/or any type of network interface, such as, for example, Ethernet. The I/O interface  456  also may allow communication without physical contact through, for example, an IEEE 802.11, Bluetooth, or a near-field communication (NFC) connection. The controller  450  may be, for example, operated, configured, modified, or updated through the I/O interface  456 . 
     The I/O interface  456  also may allow the controller  450  to communicate with systems external to and remote from the switch device  430 . For example, the I/O interface  456  may include a communications interface that allows communication between the controller  450  and a remote station (not shown), or between the controller  450  and a separate electrical apparatus in the power system  100  ( FIG.  1   ) using, for example, the Supervisory Control and Data Acquisition (SCADA) protocol or another services protocol, such as Secure Shell (SSH) or the Hypertext Transfer Protocol (HTTP). The remote station may be any type of station through which an operator is able to communicate with the controller  450  without making physical contact with the switch device  430  or the controller  450 . For example, the remote station may be a computer-based work station, a smart phone, tablet, or a laptop computer that connects to the controller  450  via a services protocol, or a remote control that connects to the controller  450  via a radio-frequency signal. The controller  450  may communicate information such as the determined tap position through the I/O interface  456  to the remote station or to a separate device in the power system  400 . 
     As discussed above, the test generation device  440  may be a reactive power generation apparatus or a distribution transformer.  FIG.  5 A  shows an example that includes distribution transformers  542 _ 1  and  542 _ 2 .  FIGS.  6 A and  6 B  shows an example that includes a reactive power generation apparatus  644 . 
       FIG.  5 A  is a block diagram of a power system  500 . The power system  500  includes distribution network  501 . In the example of  FIG.  5 A , the distribution network  501  is a low-voltage secondary distribution network that includes low-voltage feeders  504 _ 1  and  504 _ 2 . The distribution network  501  includes a first medium-voltage feeder  506 _ 1  and a second medium-voltage feeder  506   2  that are feed by the source  102 . The first and second medium-voltage feeders  506 _ 1  and  506 _ 2  include respective circuit breakers  105 _ 1  and  105 _ 2 . The system  500  also includes medium-voltage loads  518 _ 1  and  518 _ 2 , which are respectively connected to the medium-voltage feeders  506 _ 1  and  506 _ 2 . 
     The power system  500  also includes network protectors  530 _ 1  and  530 _ 2 . The network protector  530   1  controls an electrical connection between the low-voltage feeder  504 _ 1  and the loads  103 . The network protector  530 _ 2  controls an electrical connection between the low-voltage feeder  504 _ 2  and the loads  103 . Each network protector  530 _ 1  and  530 _ 2  includes an instance of the controller  450 . 
       FIG.  5 C  is a block diagram of the network protector  530 _ 1 . The network protector  530 _ 1  includes a resettable switching apparatus  532 , a sensor system  534 , and a relay or switch control  535 . The resettable switching apparatus  532  may be, for example, an air circuit breaker. 
     The sensing system  534  is configured to measure voltage and/or current in the respective low-voltage feeder  504 _ 1 . The sensing system  534  may include, for example, a current sensor that measures current in the low-voltage feeder  504 _ 1 , a first voltage sensor that measures voltage between the source side of the network protector  530 _ 1  and the load side of the network protector  530 _ 1 , and a second voltage sensor that measures voltage between the low-voltage feeder  504 _ 1  and ground or other reference potential. The relay  535  controls the state of the switching apparatus  532  based on measurements from the sensing system  534 . The network protector  530 _ 2  is configured in the same manner. Any type of sensor capable of measuring voltage and/or current may be used in the sensing system  534 . The current sensor may be, for example, a current transformer (CT) or Rogowski coil. The voltage sensor may be, for example, a potential transformer (PT). 
     The system  500  also includes distribution transformers  542 _ 1  and  542 _ 2 , which are substantially identical to each other.  FIG.  5 B  shows the distribution transformer  542 _ 1  in greater detail. The transformer  542 _ 1  includes a primary winding  543   a  and a secondary winding  543 b that are magnetically coupled by a core  541 . The primary winding  543   a  and the secondary winding  543 b are made of an electrically conductive material such as, a metal or a metal alloy, for example, copper or a copper alloy. The primary winding  543   a  is on a primary or source side of the transformer  542 _ 1 , and the secondary winding  543 b is on a secondary side of the transformer  542 _ 1 . 
     The core  541  is made of a ferromagnetic material, such as, for example, iron or steel. The core  541  may be a gapped core or an un-gapped core. In implementations in which the core  541  is an un-gapped core, the core  541  is a contiguous segment of ferromagnetic material. A gapped core includes a gap that is not ferromagnetic material. The gap may be, for example, air, nylon, or any other material that is not ferromagnetic. Thus, in implementations in which the core  541  is a gapped core, the core includes at least one segment of a ferromagnetic material and at least one segment of a material that is not a ferromagnetic material. 
     Referring also to  FIG.  5 B , the primary winding  543   a  includes T taps  546 , where T is an integer number that is greater than one. The taps  546  are made of an electrically conductive material (such as, for example, metal), and the taps  546  are electrically connected to the primary winding  543   a.  Each tap is separated from the nearest other tap, with at least one turn being between any two adjacent taps. During operational use of the distribution transformer  542 _ 1 , there is a potential difference V_T between any two adjacent taps  546 . 
     The medium-voltage feeder  506 _ 1  is electrically connected to a tap selector  547 . The tap selector  547  is made from an electrically conductive material and is configured to electrically connect the medium-voltage feeder  506 _ 1  to one of the taps  546 . The amount of voltage provided to the low-voltage feeder  504 _ 1  depends on which of the taps  546  the tap selector  547  is connected to, thus, the controller  450  is able to change the voltage on the low-voltage distribution feeder  504 _ 1  by a known amount by moving the tap selector  547  from one of the taps  546  to another one of the taps  546 . 
     The controller  450  in each network protector  530 _ 1  and  530 _ 2  receives data from respective source-side monitors  534 _ 1  and  534 _ 2 . The source-side monitors  534 _ 1  and  534 _ 2  may be sensors that are configured to measure, for example, the voltage and/or other electrical properties on the primary side of the transformers  542 _ 1  and  542 _ 2 . In some implementations, the source-side monitors  534 _ 1  and  534 _ 2  are part of the respective transformers  542 _ 1  and  542   2 . 
     During normal operation of the power system  500 , the medium-voltage circuit breakers  105 _ 1  and  105 _ 2  are closed, and the feeders  504 _ 1  and  504 _ 2  of the secondary distribution network  501  form a mesh or loop structure. The voltage at the primary side of the transformers  542 _ 1  and  542 _ 2  is dictated by the sources  102 _ 1  and  102 _ 2 , respectively. Performing a tap change operation by moving the tap selector  547  to another tap does not change the voltage at the primary side of the transformers  542 _ 1  and  542 _ 2 . 
     During abnormal operating conditions, the circuit breaker  105 _ 1  or  105 _ 2  opens, for example, due to a fault or scheduled maintenance, and the secondary distribution network  501  has a radial structure. When the secondary distribution network  501  has a radial structure, the voltage at the primary side of the transformers  542 _ 1  and  542 _ 2  depends on the voltage drop due to the loading on the secondary side. Thus, under an abnormal condition, a tap change operation changes the voltage on the primary side of the transformers  542 _ 1  and  542 _ 2 . In the implementation shown in  FIG.  5 A , the source-side monitors  534 _ 1  and  534 _ 2  are used to monitor the voltage on the primary side of the respective transformers  542 _ 1  and  542 _ 2 . 
     By performing a tap change operation and then monitoring the response to that tap change operation, the controller  450  is able to determine whether or not an abnormal condition exists in the distribution network  501 . Moreover, the controller  450  may determine that the distribution network  501  is operating in an expected or normal condition even if there is reverse power flow on the feeder  504 _ 1  and/or the feeder  504 _ 2 . 
     In some implementations, the controller  450  monitors one or more properties or parameters of electrical power in the feeder  504 _ 1 . A significant change in one or more of the monitored properties or parameters triggers the controller  450  to issue a tap change test command to the distribution transformer  542 _ 1  to perform a tap change operation to generate a test signal  545 . The voltage at the primary side of the distribution transformer  542 _ 1  is analyzed based on data from the source-side monitor  534 _ 1 . If the voltage at the primary side of the distribution transformer  542 _ 1  changes, the controller  450  determines that the feeder  504 _ 1  has a radial structure and thus is in an abnormal condition. The controller  450  issues a command that causes the network protector  530 _ 1  to open such that the feeder  504 _ 1  is disconnected from the loads  103 . If the voltage at the primary side of the distribution transformer  542 _ 2  does not change, the feeder  504 _ 1  has a mesh structure and is in a normal or expected operating condition. The network protector  530 _ 1  remains closed. 
     The feeder  504 _ 2  (and any other feeders in the distribution network  501 ) may be monitored and analyzed in the same manner. Moreover, in some implementations, the controllers  450  also control the reclosing, or closing after opening, of the network protector  530 _ 1  and  530 _ 2  using, for example, the process  800  discussed with respect to  FIG.  8   . 
       FIG.  6 A  is a block diagram of a power system  600 . The power system  600  is similar to the power system  500  ( FIG.  5 A ), except the power system  600  also includes a reactive power generation apparatus  644 . The reactive power generation apparatus  644  may be any device or system that is capable of producing a test signal  645  that has a known amount of reactive power (Qc). Reactive power is the product of the voltage and current that is out-of-phase with each other. For example, the reactive power generation apparatus  644  may be a capacitor bank, an inductor bank, a bank of devices that includes inductors and capacitors, a power converter, or an inverter. The reactive power generation apparatus  644  is coupled to the controller  450  and is configured to be controlled by the controller  450 . For example, the controller  450  may be configured to generate command signals that cause the reactive power generation apparatus  644  to inject the test signal  645  into the distribution network  601 . 
     In the example shown in  FIG.  6 A , the reactive power generation apparatus  644  is electrically connected to the low-voltage feeder  504 _ 2  between the loads  103  and the network protector  530 _ 2 . However, other implementations are possible. For example, the reactive power generation apparatus  644  may be between the distribution transformer  542 _ 1  and the network protector  530 _ 1 . In another example, the reactive power generation apparatus  644  may be integrated with the network protector  530 _ 2 . Furthermore, the system  600  also may include an additional reactive power generation apparatus connected to the low-voltage feeder  504 _ 1 . 
     As discussed below, the test signal  645  is observed on a first side  644   a  and/or a second side  644   b  of the reactive power generation apparatus  644  to determine the structure of the distribution network  601 . 
       FIG.  6 A  shows the power system  600  during normal or expected operating conditions. During normal and expected operation of the power system  600 , the circuit breakers  105 _ 1  and  105 _ 2  are closed, the feeders  504 _ 1  and  504 _ 2  of the secondary distribution network  601  form a mesh or loop structure and, based on Kirchoff s Current Law (KCL), the reactive power (Qc) in the test signal  645  is divided into a first reactive power  648  that flows toward the loads  103  and a second reactive power  649  that flows toward the transformer  542 _ 2 . Under normal or expected operating conditions, the amount of reactive power in  648  and  649  depends on the impedance of the path toward the loads  103  and the path toward the transformer  542 _ 2 , respectively. 
       FIG.  6 B  shows the power system  600  during an abnormal condition after the circuit breaker  105 _ 2  has opened due to, for example, a fault or scheduled maintenance. In  FIG.  6 B , the circuit breaker  105 _ 2  is in grey shading to indicate that it is open. The circuit breaker  105 _ 1  remains closed, and the source  102  remains connected to the feeder  506 _ 1 . During abnormal conditions, the network  601  has a radial structure. According to Kirchoff s Current Law (KCL), in a radial structure, the injected reactive power Qc becomes the source of reactive power on the second side  644   b  of the reactive power generation apparatus  644 . Therefore, during abnormal conditions, the reactive power flow  649  on the second side  644   b  of the reactive power generation apparatus  644  remains constant before and after the injection of the test signal  645 , while the reactive power flow  648  on the source side of the reactive power generation apparatus  644  (the first side  644   a  in the example of  FIG.  6 B ) changes by Qc, which is the amount of reactive power in the test signal  645 . 
     Thus, to determine the structure of the distribution network  601 , the reactive power is observed on the first side  644   a  and/or the second side  644   b  of the reactive power generation apparatus  644  before and after injection of the test signal  645 . For example, if the reactive power  648  on the source side of the reactive power generation apparatus  644  (the first side  644   a  in the example of  FIG.  6 B ) changes by Qc after injection of the test signal  645 , the distribution network  601  has a radial structure. If the reactive power  648  on the source side of the reactive power generation apparatus  644  does not change by Qc after the injection of the test signal  645 , the distribution network  601  has a mesh or loop structure. Alternatively or additionally, the reactive power  649  on the second side  644   b  of the reactive power generation apparatus  644  may be observed before and after the injection of the test signal  645 . If the reactive power  649  does not change, the distribution network  601  has a radial structure. If the reactive power  649  changes, the distribution network  601  has a mesh or loop structure. The reactive power on either side  644   a,    644   b  of the reactive power generation apparatus  644  may be measured by the sensing system  534  or by a separate sensor (not shown). 
     In some implementations, the controller  450  monitors one or more properties or parameters of the electrical power in the feeder  504 _ 1 . A significant change in at least one of the monitored properties or parameters triggers the controller  450  to issue a command to the reactive power generation apparatus  644 , and the test signal  645  is generated and injected into the feeder  504 _ 2 . Furthermore, if the controller  450  determines that the network  601  has a radial structure, the controller  450  opens the network protector  530 _ 2 . Such an implementation is discussed below with respect to  FIG.  7   . Moreover, in some implementations, the controllers  450  also control the reclosing, or closing after opening, of the network protectors  530 _ 1  and  530 _ 2  using a process such as the process  800  discussed with respect to  FIG.  8   . 
       FIG.  7    is a flow chart of a process  700 . The process  700  is an example of a process for determining the structure of a distribution network. The process  700  may be used to determine the structure of any of the distribution networks  101 ,  201 ,  301 ,  401 ,  501 , and  601  discussed above. Moreover, the process  700  may be used to determine the structure of other distribution networks. The process  700  may be performed by the controller  150  or the controller  450 . The process  700  is discussed with respect to  FIG.  4    for the purposes of providing an example. 
     The distribution network  401  is monitored ( 710 ). For example, and referring to  FIG.  4   , the sensing system  434  continuously monitors properties and parameters of the voltage and/or current on the feeder  404  and the measured data is stored on the electronic storage  454 . 
     Whether or not a change in one or more monitored properties of the electricity in the network  401  is determined ( 715 ). The electronic storage  454  includes executable instructions that compare the measured data over time to determine whether one or more properties of the power flow in the feeder  404  has changed over a particular time period. The time period may be, for example, 1 millisecond (ms) to 10 seconds (s). The properties may be any measured or derived property and include, without limitation, amplitude and/or phase angle of voltage and/or current, reactive power, impedance, active power, impedance, and/or current flow direction. 
     To provide a specific example, the sensing system  434  may be configured to measure the magnitude of current flow in the feeder  404  every  5  ms, and the controller  450  may be configured to determine the amount of change between each measurement or between every other measurement. In this way, the controller  450  determines the amount of change in the measured current magnitude over a specified time period. The controller  450  compares the determined amount of change to a threshold related to that property. Continuing the example of current magnitude, the controller  450  is configured to compare the change in the current magnitude to a pre-determined current magnitude threshold. 
     To provide another example, the property may be the direction of current flow on the feeder  404 . A change in direction is considered a change in a property, but is not assumed by the controller  450  to necessarily indicate the presence of an abnormal condition. Moreover, the electronic storage  454  may store a threshold or a specification related to the current direction property. For example, the threshold may specify a period of time over which the direction is reverse (toward the source) before the direction of current flow is determined to have changed. 
     If none of the changes exceed a threshold, there has not been a change in one or more properties at ( 715 ), the process  700  returns to ( 710 ) and continues to monitor the feeder  404 . 
     If the change exceeds the threshold, then the controller  450  determines that there has been a change in an electrical property, and the process  700  moves to ( 720 ). In some implementations, the controller  450  only detects that a change in an electrical property has occurred if the amount of change of more than one property exceeds the appropriate threshold for that property. 
     The controller  450  causes the test generation device  440  to generate the test signal  445 , which is provided to the feeder  404  ( 720 ). In implementations in which the test generation device  440  is a distribution transformer (such as shown in  FIG.  5 A ), the controller  450  causes the tap selector  547  to move to a different one of the taps  546 . In implementations in which the test generation device  440  is a reactive power generation apparatus (such as shown in  FIGS.  6 A and  6 B ), the controller  450  causes the reactive power generation apparatus  644  to generate the test signal  645  by providing a known amount of reactive power to the feeder  404 . 
     In some implementations, the controller  450  causes the test signal to be produced without performing ( 710 ) and ( 715 ). For example, an operator of the system  400  may manually trigger the generation of the test signal  445  by communicating with the controller  450  through the I/O interface  456 . In other words, the process  700  may be performed without performing ( 710 ) and ( 720 ), and the generation of the test signal  445  may be triggered by events other than a change in a monitored electrical property. 
     After providing the test signal  445  to the feeder  404 , the controller  450  analyzes the response to the test signal  445  ( 730 ). For example, if the test signal is the reactive power test signal  645  ( FIGS.  6 A and  6 B ), the controller  450  determines whether the reactive power on a side of the reactive power generation apparatus  644  changes after the reactive power test signal  645  is injected. In another example, if the test signal is the tap change voltage  545 , the controller  450  determines whether the voltage on the primary side of the transformer  542 _ 1  changes after the tap change occurs. 
     The controller  450  determines the structure of the electrical distribution network  401  ( 740 ). The analysis for determining the structure of the distribution networks  501  and  601  discussed above with respect to  FIGS.  5 A,  6 A, and  6 B  are additional examples of ( 740 ). 
     If the distribution network  401  has a radial structure ( 750 ), the controller  450  declares a fault ( 760 ). The controller  450  may declare a fault by producing a perceivable indicator at the I/O interface  256 . The controller  450  also issues a command signal to the switch device  430  that causes the switch device  430  to open to thereby disconnect the feeder  404  from the loads  103 . For example, in the abnormal condition shown in  FIG.  6 B , the controller  450  would control the network protector  530 _ 2  to open after determining that a radial structure existed. 
     If the distribution network  401  does not have a radial structure ( 750 ), then the switch device  430  remains closed and the feeder  404 . The process  700  may end or return to ( 710 ) to continue monitoring the feeder  404 . 
       FIG.  8    is a flow chart of a process  800 . The process  800  is an example of a process for reclosing or closing an open switch device. The process  800  may be performed by the controller  150  or the controller  450  and with any of the switch devices discussed above. To provide an example, the process  800  is discussed with respect to reclosing the network protector  530 _ 2  of  FIG.  6 B . As discussed above,  FIG.  6 B  illustrates an abnormal condition. The network protector  530 _ 2  is opened after determining that the network  601  has a radial structure. 
     As noted above, in traditional network protectors, forward power flow (from source to load) is typically a required condition to reclosing a switch device. However, the configuration of the controller  450  allows reclosing even in the presence of reverse flow. In other words, forward power flow is not required for reclosing the network protector  530 _ 2 . The controller  450  first checks the distribution transformer  542 _ 2  to determine whether or not it is energized and the loads  103  to determine whether or not the loads  103  have voltage. If the distribution transformer  542 _ 2  is energized and the loads  103  have voltage, the controller  450  determines a magnitude of a difference (Vtn) between a voltage vector (Vt) on the primary side of the transformer  542 _ 2  and a voltage vector (Vn) on the network side of the transformer  542 _ 2  ( 810 ), and determines if the magnitude of the difference is within an acceptable limit ( 820 ) according to Equation 1: 
       |Vtn|&lt;threshold   Equation (1),
 
     where Vtn=Vt−Vn, and threshold is the maximum allowable difference between the voltage vectors Vt and Vn. The threshold may be stored on the electronic storage  454  or provided through the I/O interface  456 . The threshold value is a numerical value and may be, for example, 135 V. 
     If the magnitude of the voltage difference is greater than the threshold ( 825 ), the network protector  530 _ 2  is not closed, and the process  800  returns to ( 810 ). 
     If the magnitude of the voltage difference is less than the threshold ( 825 ), the controller  450  issues a close command to the network protector  530 _ 2  ( 830 ), and the reclose action is performed. The reclose action may take, for example, 1 minute, 2 minutes, or 5 minutes. 
     After the reclose action is complete, the controller  450  may issue a command to the reactive power generation apparatus  644  to issue the test signal  645  ( 840 ). The controller  450  may then analyze the response to the test signal  645  as discussed above to confirm that the feeder  504 _ 2  and the distribution network  601  are in the loop or mesh configuration. If the distribution network  601  has the loop or mesh structure, the process  800  ends or enters ( 710 ) of the process  700  to begin monitoring for an abnormal condition. If the distribution network  601  has a radial structure, the controller  450  declares a fault condition ( 760 ). Other network protectors in the distribution network  601  may be reclosed in the same manner. Furthermore, the process  800  may be used to reclose switch devices in any of the distribution networks  101 ,  201 ,  301 ,  401 , and  501 . 
       FIG.  9    shows reclose zones  951  and  952 . The reclose zone  951  is outlined in a dashed line style and is for a traditional network protector, which is controlled by a traditional controller that assumes an abnormal condition is present if there is reverse power flow. In other words, the reclose zone  951  is for a system that does not include the controller  450 . The reclose zone  952  is outlined in a solid line style and is for the network protector  530 _ 2 , which is controlled by the controller  450 . In  FIG.  9   , the x-axis (horizontal axis) represents the voltage vector (Vt) measured at the primary side of the distribution transformer  542 _ 2 , and the y-axis (vertical axis) represents the voltage vector (Vn) on the secondary or network side of the distribution transformer  542 _ 2 . The reclose zone  952  is defined by all of the differences between Vt and Vn that are less than a threshold. The reclose zone  951  is determined by traditional criteria. 
     If the measured voltage vector Vn at a particular time and the measured voltage vector Vn measured at that time fall inside the reclose zone  952 , then the controller  450  issues a command to the network protector  530 _ 2  to close. As shown in  FIG.  9   , the reclose zone  952  is larger than the reclose zone  951  and the reclose zone  952  also substantially overlaps the reclose zone  951 . Accordingly, as compared to the traditional configuration, the configuration of the controller  450  allows more opportunities to reclose the network protector  530 _ 2  and thus improves overall performance. The network protector  530 _ 1  has a reclose zone that is similar to the reclose zone  952 . 
       FIGS.  10 A- 10 D ,  FIGS.  11 A- 11 D , and  FIGS.  12 A- 12 D  show examples of data from simulations of a spot network with 90% photovoltaic (PV) penetration. PV penetration of 90% indicates that the nominal PV power generation capacity is 90% of the nominal load.  FIGS.  10 A- 10 D and  11 A- 11 D  show data from a simulation in which a secondary electrical spot distribution network included two network protectors (NWP_ 36  and NWP_ 47 ) configured in a traditional manner and configured to trip in the presence of reverse power flow. The simulated system was fed by an AC power source and also included circuit breakers BRK GA 05 , which was between the AC power source and NWP_ 36 , and BRK GA 02 , which was between the AC power source and NWP_ 47 .  FIGS.  12 A- 12 D  show data from a simulation in which the same secondary electrical distribution network included two network protectors configured in a manner similar to the network protectors  130 _ 1  and  130 _ 2 ,  430 ,  530 _ 1  or  530 _ 2 . In other words,  FIGS.  12 A- 12 D  show data from a simulation in which the network protectors allow reverse power flow if there is no fault or other abnormal condition. 
       FIGS.  10 A- 10 D  show unintentional tripping of the traditionally configured network protectors due to reverse power flow.  FIG.  10 A  shows generated PV power ( 1090 ) and the power demand of the load ( 1091 ) as a function of time.  FIG.  10 B  shows real ( 1092 ) and reactive power ( 1093 ) at the network protector NWP_ 36  as a function of time.  FIG.  10 C  shows real ( 1094 ) and reactive power ( 1095 ) at the network protector NWP_ 47  as a function of time.  FIG.  10 D  shows the status of NWP_ 36 , NWP_ 47 , BRK_GA 05 , and BRK_GA 02  as a function of time, with a value of one (1) indicating open and a value of zero (0) indicating open. In  FIG.  10 D , NWP_ 36  and NWP_ 47  are represented in the plot labeled  1096 , and BRK_GA 05  and BRk_GA 02  are represented by the plot labeled  1098 .  FIGS.  10 A- 10 D  have the same time axis. 
     Referring to  FIG.  10 A , around time t=55s, the amount of generated solar power ( 1090 ) exceeds the power demand of the load ( 1091 ). The excess power is returned to the distribution network as reverse power, and the network protectors NWP_ 36  and NWP_ 47  trip open (as shown in  FIG.  10 D ) even though there is no fault or other abnormal condition. Referring to  FIGS.  10 B and  10 C , after the network protectors NWP_ 36  and NWP_ 47  trip open, and no power flows. Thus, the excess solar power is not utilized and the network protectors NWP_ 36  and NWP_ 47  open unnecessarily. 
       FIGS.  11 A- 11 D  show the operation of the secondary spot network over a different period of time. In the simulation that produced the data in  FIGS.  11 A- 11 D , the medium-voltage circuit breaker (MVCB) BRK_GA 05  associated with the network protector NWP_ 36  was opened at time t= 5  s and reclosed at time t= 8  s.  FIG.  11 A  shows generated PV power ( 1190 ) and the power demand of the load ( 1191 ) as a function of time.  FIG.  11 B  shows real ( 1192 ) and reactive power ( 1193 ) at a first network protector NWP_ 36  as a function of time.  FIG.  11 C  shows real ( 1194 ) and reactive power ( 1195 ) at a second network protector NWP_ 47  as a function of time.  FIG.  11 D  shows the status of status of NWP_ 36  ( 1196 ), NWP_ 47 , BRK_GA 05  ( 1098 ), and BRK_GA 02  as a function of time, with a value of one (1) indicating open and a value of zero (0) indicating open. 
     Referring to  FIG.  11 B , the NWP_ 36  responds to the opening of the MVCB BRK_GA 05  after about  3  seconds (at time t=8 s). However, due to the presence reverse power flow (the generated solar power shown as  1191  in  FIG.  11 A ), the NWP_ 36  fails to reclose even after the MVCB recloses at time t=8 s. As shown in  FIG.  11 D , the status of the NWP_ 36  remains open even after the MVCB BRK_GA 05  Thus, the traditionally configured NWP_ 36  is unable to distinguish between reverse power flow that occurs due to a fault or other abnormality and reverse power flow due to DER power generation. 
       FIGS.  12 A- 12 D  show data from a simulation of the same spot network (90% PV penetration) in which the network protectors NWP_ 36  and NWP_ 47  were configured in a manner similar to the network protectors  130 _ 1  and  130 _ 2 ,  430 ,  530 _ 1  or  530 _ 2 .  FIG.  11 A  shows generated PV power ( 1290 ) and the power demand of the load ( 1291 ) as a function of time.  FIG.  12 B  shows real ( 1292 ) and reactive power ( 1293 ) at a first network protector NWP_ 36  as a function of time.  FIG.  12 C  shows real ( 1294 ) and reactive power ( 1295 ) at a second network protector NWP_ 47  as a function of time.  FIG.  12 D  shows the status of status of NWP_ 36  and NWP_ 47  as a function of time, with a value of one (1) indicating open and a value of zero (0) indicating open. 
     At the time t=55s generated solar power ( 1290 ) exceeded the power drawn by the load ( 1291 ) and the network protectors NWP_ 36  and NWP_ 47  observe reverse power flow. The trip function is activated, and a capacitor bank is controlled to inject 25 kVAR reactive power into the secondary spot distribution network. The reactive power is measured on either side of the capacitor bank (using, for example, the technique discussed with respect to  FIGS.  6 A and  6 B ) to determine the structure of the secondary spot distribution network. Because the reverse power flow is from excess PV energy generation, the network protectors NWP_ 36  and NWP_ 47  are not tripped (as shown in  FIG.  12 D ), and the excess PV energy is utilized. 
     These and other implementations are within the scope of the claims.