Patent Publication Number: US-2023138879-A1

Title: Network protector with a communications interface

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 63/272,932, filed on Oct. 28, 2021 and titled NETWORK PROTECTOR WITH A COMMUNICATIONS INTERFACE, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a network protector with a communications interface. The communications interface allows peer-to-peer communications with other network protectors. 
     BACKGROUND 
     A network protector includes a resettable switching apparatus and may be electrically connected to a feeder in a distribution system to control an electrical connection between a load and the feeder. 
     SUMMARY 
     In one aspect, a network protector includes: a first resettable switching apparatus configured to control an electrical connection between a distribution transformer and a first electrical feeder of a secondary electrical distribution network; a first communications interface; and a first controller configured to: determine a direction of power flow in the first electrical feeder; cause the first communications interface to provide a first indication of the direction of power flow in the first electrical feeder to a second network protector; and receive a second indication from the second network protector. The second indication includes an indication of the direction of power flow in a second electrical feeder of the secondary electrical distribution network. 
     Implementations may include one or more of the following features. 
     The first electrical feeder and the second electrical feeder may be electrically connected in parallel to an alternating current (AC) power source, and the first controller may be further configured to: determine whether the direction of power flow in the first electrical feeder is forward or reverse. Forward power flow is away from the power source and reverse power flow is toward the power source. The first controller may be further configured to: not open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is reverse; and open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is forward. The first controller may be configured to not open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is reverse. 
     The first resettable switching apparatus may be a circuit breaker or a vacuum interrupter. 
     In another aspect, a system includes: a first network protector configured to control an electrical connection between a first distribution transformer and a first electrical feeder of a secondary electrical distribution network, the first network protector including: a first communications interface; and a first resettable switching apparatus. The system also includes: a second network protector configured to control an electrical connection between second distribution transformer and a second electrical feeder of the secondary electrical distribution network, the second network protector including: a second communications interface; and a second resettable switching apparatus. The first communications interface is configured to: provide information related to a direction of power flow in the first electrical feeder to the second communications interface, and to receive information related to a direction of power flow in the second electrical feeder from the second communications interface. The second communications interface is configured to: provide information related to a direction of power flow in the second electrical feeder to the first communications interface, and to receive information related to a direction of power flow in the first electrical feeder from the first communications interface. 
     Implementations may include one or more of the following features. 
     The system also may include a vault, and the first network protector and the second network protector may be enclosed in the vault. 
     The first electrical feeder and the second electrical feeder may be electrically connected in parallel to a power source, and the first controller may be further configured to: determine whether the direction of power flow in the first electrical feeder is forward or reverse. Forward power flow is away from the power source and reverse power flow is toward the power source. The first controller may be further configured to: not open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is reverse; and open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is forward. The first controller may be configured to not open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is reverse at the same time. 
     The first controller may be further configured to: not open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is reverse; and open the first resettable switching apparatus if the power flow in the first feeder is reverse and the power flow in the second feeder is forward. The second controller may be further configured to: not open the second resettable switching apparatus if the power flow in the second feeder is reverse and the power flow in the first feeder is reverse; and open the second resettable switching apparatus if the power flow in the second feeder is reverse and the power flow in the first feeder is forward. The first electrical feeder and the second electrical feeder may be configured to electrically connect to one or more distributed energy resources. 
     In another aspect, a method includes: controlling a first network protector to provide a first indication, the first indication being an indication of the direction of power flow in a first electrical feeder of a secondary electrical distribution network; comparing the first indication and a second indication, the second indication being an indication of the direction of power flow in a second electrical feeder of the secondary electrical distribution network; and determining whether to control the first network protector or a second network protector based on the comparison. 
     Implementations may include one or more of the following features. 
     Controlling the first network protector may include controlling the first network protector to provide the first indication to the second network protector. Comparing the first indication and the second indication may include comparing the first indication to the second indication at the second network protector. Comparing the first indication and the second indication may include determining whether the direction of power flow is away from a load in the first electrical feeder and away from the load in the second electrical feeder; and, if the direction of power flow is away from the load in the first electrical feeder and away from the load in the second electrical feeder, the method further includes maintaining a resettable switching apparatus in the first network protector in a closed state and maintaining a resettable switching apparatus in the second network protector in a closed state such that the power flow continues in the secondary electrical distribution network. The load may include one or more distributed energy resources (DER), and at least some of the power flow away from the load in the first electrical feeder and in the second electrical feeder is based on electrical power generated by the one or more DERs. 
     Implementations of any of the techniques described herein may include 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 a network protector. 
         FIG.  4    is a block diagram of another example of an electrical power system. 
         FIG.  5    is a side block diagram of a network protector system. 
         FIG.  6    is a flow chart of an example of a process of operating a network protector. 
         FIGS.  7 A- 7 D  show simulated data related to a fault condition. 
         FIGS.  8 A- 8 D  show simulated data related to ordinary operating conditions in which excess energy was produced by one or more solar arrays. 
     
    
    
     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 distribution network  101  that includes network protectors  130 _ 1  and  130 _ 2  coupled to respective feeders  104 _ 1  and  104 _ 2 . The feeders  104 _ 1  and  104 _ 2  are connected to a load or loads  103 . The network protectors  130 _ 1  and  130 _ 2  are configured to communicate with each other via a communications path  165 . 
     The secondary distribution network  101  is connected to an alternating (AC) power source  102  through 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 the voltage on the respective feeders  106 _ 1  and  106 _ 2  (which is determined by the source  102 ) 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 the feeder  104 _ 1 ,  104 _ 2 , respectively, of the secondary distribution network  101 . A medium-voltage circuit breaker  105 _ 1  is coupled to the feeder  106 _ 1 . A medium-voltage circuit breaker  105 _ 2  is coupled to the feeder  106 _ 2 . 
     The AC power source  102  operates at a fundamental frequency of, for example, 50 or 60 Hertz (Hz). The power source  102  may be, for example, 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, 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 network protectors  130 _ 1  and  130 _ 2  control the flow of electrical power to and from the secondary distribution network  101 . As discussed in greater detail below, the network protectors  130 _ 1  and  130 _ 2  are configured to communicate information related to the direction of power flow on respective feeders  104 _ 1  and  104 _ 2  via a communications path  165  (shown as a dotted line in  FIG.  1   ). The information related to the direction of power flow is used to determine whether an error condition exists in the system  100  or the distribution network  101 . 
     For example, in some implementations, the network protector  130 _ 1  receives an indication of the direction of power flow on the feeder  104 _ 2  from the network protector  130 _ 2  and compares the received indication with an indication of the direction of power flow on the feeder  104 _ 1 . If the direction of power flow is the same on both feeders  104 _ 1 ,  104 _ 2 , no error condition exists, the network protector  130 _ 1  remains closed, and power continues to flow in the feeder  104 _ 1 . If the power flows in the feeder  104 _ 1  in a different direction than in the feeder  104 _ 2 , an error condition exists. When an error condition is determined to exist, the network protector  130 _ 1   and/or the network protector  130 _ 2  opens such that power no longer flows in the feeder  104 _ 1  and/or  104 _ 2 . 
     The configuration the network protectors  130 _ 1  and  130 _ 2  allows the network protectors  130 _ 1  and  130 _ 2  to accept bi-directional power flow (power flow away from or toward the source  102 ) while also allowing the network protectors  130 _ 1  and  130 _ 2  to protect the load  103  from abnormal conditions. Reverse power flow is power that flows toward the source  102  and forward power flow is power that flows away from the source  102 . Bi-directional power flow includes reverse power flow and forward power flow. 
     Forward power flow is typically present during normal and expected operation of the system  100 . Reverse power flow may arise from error conditions or during ordinary and error-free operation. Error conditions include, for example, maintenance conditions and fault conditions. A maintenance condition is a condition that is intentionally caused due to scheduled maintenance or other intentional action that involves opening the medium-voltage circuit breaker  105 _ 1  or the medium-voltage circuit breaker  105 _ 2 . A fault condition is an unintentional event that changes the flow of power in the secondary distribution network  101  and/or the system  100 . Examples of unintentional events include, for example, phase-to-ground faults, overcurrent conditions, and over-voltage conditions. Unintentional events may be caused by falling objects, ingress of moisture, storms, equipment malfunction, and other unplanned events. The medium-voltage circuit breaker  105 _ 1  and/or the medium-voltage circuit breaker  105 _ 2  open in the presence of a fault condition. 
     Reverse power flow due to a fault condition does not flow on all of the feeders in the secondary distribution network  101 . For example, when the medium-voltage circuit breaker  105 _ 2  is opened (due to a fault or for maintenance), the feeder  104 _ 1  has forward power flow and the feeder  104 _ 2  has reverse power flow. 
     Reverse power flow that arises from ordinary operation flows on all of the feeders in the network  101 . For example, reverse power flow may arise from excess power that is generated by a distributed energy resource (DER) connected to the distribution network  101 . A DER is an electricity-producing resource and/or a controllable load. Examples of DERs 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  include one or more DERs and also may include devices and systems that are not DERs. For example, the loads  103  also may include motors, lighting systems, and/or machines. 
     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 network  101 . This returned electrical power is reverse power that flows from the loads  103  toward the source  102 . Reverse power flow that is caused by excess DER power generation appears on all of the feeders in the secondary distribution network  101  (the feeder  104 _ 1  and the feeder  104 _ 1  in the example of  FIG.  1   ). 
     Although reverse power flow from error conditions is undesirable, reverse power flow that arises from DER power generation is generally desirable and may be used by other systems within the power system  100 . Traditional network protectors are configured with logic that assumes that reverse power flow is an indication of a fault condition, and these traditional network protectors open and disconnect their load based on a detection of reverse power flow in the feeder associated with the network protector. Thus, such traditional network protectors are unable to return excess power generated by a DER to the grid because the traditional network protectors always open or trip in the presence of reverse power flow. 
     On the other hand, the network protector  130 _ 1  and the network protector  130 _ 2  distinguish between normal operation (including reverse power flow caused by excess DER power generation) and abnormal conditions using information from one or more other network protectors. The network protectors  130 _ 1  and  130 _ 2  are not configured to assume that reverse power flow is always caused by an error condition. Instead, the network protectors  130 _ 1  and  130 _ 2  allow reverse power flow so long as no error condition exists. 
     The network protectors  130 _ 1  and  130 _ 2  have fewer tripping (or opening events) than a traditionally configured network protector and, as a result, may have a longer lifetime and may cause fewer service interruptions than a traditionally configured network protector. Additionally, the network protectors  130 _ 1  and  130 _ 2  encourage efficient use of generated energy. Moreover, the network protectors  130 _ 1  and  130 _ 2  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. 
     Before discussing the network protectors  130 _ 1  and  130 _ 2  in greater detail, an overview of the secondary distribution network  101  is provided. 
     The secondary 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 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, building, 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 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 hospital, or an apartment building. The spot network  201  includes 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 overcurrent 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, 600 V, or another voltage below 1 kV. 
     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 . Each switch device  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  may be, for example, a network protector. Each switch device  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  is configured to communicate with at least one other of the switch devices  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4 . Each switch devices  230 _ 1 ,  230 _ 2 ,  230 _ 3 ,  230 _ 4  is configured to communicate information related to the direction of power flow on the respective low-voltage feeder  204 _ 1 ,  204 _ 2 ,  204 _ 3 ,  204 _ 4 . 
       FIG.  3    is a block diagram of a network protector  330 . The network protector  330  may be used as the network protector  130 _ 1 ,  130 _ 2 ,  230 _ 1 ,  230 _ 2 ,  230 _ 3 , or  230 _ 4 . The network protector  330  includes a resettable switching apparatus  332 , a sensing apparatus  334 , and a switch control mechanism  335 . The network protector  330  also includes a communications interface  360  that sends and receives data, information, and/or commands over a communications path  365 . 
     The sensing apparatus  334  monitors the electrical power on a low-voltage feeder  304  and the switch control mechanism  335  operates the resettable switching apparatus  332 . The switch control mechanism  335  may be, for example, a relay. The network protector  330  also includes a controller  350 . The controller  350  may be an electronic controller, such as, for example, a microcontroller. The controller  350  analyzes data collected by the sensing apparatus  334  and provides commands to the switch control mechanism  335  such that the controller  350  controls the state of the resettable switching apparatus  332 . The switch control mechanism  335  may be coupled to the controller  350  or implemented as part of the controller  350 . 
     The resettable switching apparatus  332  is any type of switch that is capable of opening and closing the feeder  304 . For example, the resettable switching apparatus  332  may be an air circuit breaker. 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  304 . When the electrical contacts are separated, current cannot flow in the feeder  304 . The resettable switching apparatus  332  is configured for repeated operation. For example, after the resettable switching apparatus  332  opens the feeder  304  to stop or prevent current flow, the resettable switching apparatus  332  is able to close the feeder  304  such that current flow in the feeder  304  resumes. The resettable switching apparatus  332  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  332 . 
     The communications interface  360  is any type of interface that is capable of receiving and sending data, information, and/or commands over the communications path  365 . For example, the communications interface  360  may be a network interface (such as an Ethernet interface), a Bluetooth interface, a serial interface (for example, RS-232 or RS-485), or an International Electrotechnical Commission’s (IEC) 61850 interface. The communications path  365  may be wired or wireless. The communications path  365  may be configured to transmit data, information, and commands using an industrial protocol such as, for example, the common industrial protocol (CIP), Modbus, HART protocol, FOUNDATION fieldbus, or Ethernet Powerlink. 
     The communications interface  360  is coupled to the controller  350  such that the controller  350  can cause data, information, and/or commands to be sent from the network controller  330  and received by the network controller  330 . 
       FIGS.  1 ,  2 , and  3    are provided as examples, and other configurations are possible. For example, the secondary distribution network  201  may have fewer or more than four parallel low-voltage feeders. 
       FIG.  4    is a block diagram of a system  400  that includes a secondary distribution network  401 . The distribution network  401  is a low-voltage secondary distribution network and includes a plurality of feeders. The distribution network  401  may be a spot network. In the example of  FIG.  4   , two feeders  404 _ 1  and  404 _ 2  are shown. A network protector  430 _ 1  is coupled to the feeder  404 _ 1  and a network protector  430 _ 2  is coupled to the feeder  404 _ 2 . A single phase is shown in  FIG.  4   . However, the network protectors  430 _ 1  and  430 _ 2  may be multi-phase (for example, three-phase) network protectors. 
     The network protector  430 _ 1  includes a resettable switching apparatus  432 _ 1 , a sensing apparatus  434 _ 1 , a controller  450 _ 1 , and a communications interface  460 _ 1 . The network protector  430 _ 2  includes a resettable switching apparatus  432 _ 2 , a sensing apparatus  434 _ 2 , a controller  450 _ 2 , and a communications interface  460 _ 2 . The sensing apparatus  434 _ 2  monitors the feeder  404 _ 2 . The sensing apparatus  434 _ 2  provides data related to the direction of power flow on the feeder  404 _ 2  to the controller  450 _ 2 . Data, information, and/or commands are sent through the communications interface  460 _ 1  to the communications interface  460 _ 2 . Data, information, and/or commands are sent through the communications interface  460 _ 2  to the communications interface  460 _ 1 . Thus, the network protectors  430 _ 1  and  430 _ 2  communicate with each other through the communications interfaces  460 _ 1  and  460 _ 2 . 
     The network protector  430 _ 1  is discussed in more detail. The resettable switching apparatus  432 _ 1  is any type of switch that is capable of opening and closing the feeder  404 _ 1 . For example, the resettable switching apparatus  432 _ 1  may be an air circuit breaker. The resettable switching apparatus  432 _ 1  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 _ 2 . 
     The sensing apparatus  434 _ 1  includes one or more detectors or sensors, each of which is configured to sense one or more properties of the power that flows in the feeder  404 _ 1 . The sensing apparatus  434 _ 1  may include any type of current sensor, such as, for example, a current transformer (CT) or a Rogowski coil. In some implementations, a conductor-mounted power flow sensor with a high sampling rate, such as the GridAdvisor Series II smart sensor, available from the Eaton Corporation of Cleveland, Ohio, may be used. Alternately or additionally, the sensing apparatus  434 _ 1  may include one or more voltage sensors and/or one or more power sensors. The sensing apparatus  434 _ 1  may include other related devices, such as timers or other devices that measure the passage of time. 
     The sensing apparatus  434 _ 1  produces data related to the direction of power flow on the feeder  404 _ 1  and/or data from which the direction of power flow may be derived. For example, in some implementations, the sensing apparatus  434 _ 1  produces a binary indicator that has a first value when current flows toward the loads  103  and second value when current flows toward the source  102 . In another example, the sensing apparatus  434 _ 1  produces data related to a measured quantity, such as a numerical value of measured real power on the feeder  404 _ 1 , and the direction of power flow is derived from the measured quantity. In another example, when a CT is part of the sensing apparatus  434 _ 1 , the direction of current is determined based on the polarity. 
     The controller  450 _ 1  is an electronic controller that includes an electronic processing module  452 _ 1 , an electronic storage  454 _ 1 , and an input/output (I/O) interface  456 _ 1 . The electronic processing module  452 _ 1  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 _ 1  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 _ 1  may include volatile and/or non-volatile components. The electronic storage  454 _ 1  and the processing module  452 _ 1  are coupled such that the processing module  452 _ 1  can access or read data from and write data to the electronic storage  454 _ 1 . 
     The electronic storage  454 _ 1  stores executable instructions, for example, as a computer program, logic, or software, that cause the processing module  452 _ 1  to perform various operations. The electronic storage  454 _ 1  stores instructions that cause the processing module  452 _ 1  to send information, data, and/or commands to an external device through the communications interface  460 _ 1  and instructions that cause the processing module  452 _ 1  to process information, data, and/or commands that are received through the communications interface  460 _ 1  from an external device. For example, the electronic storage  454 _ 1  stores executable instructions that cause the processing module  452 _ 1  to perform the process  600  of  FIG.  6   . To provide another example, the electronic storage  454 _ 1  may store instructions that cause readings from the sensing apparatus  434 _ 1  to be stored on the electronic storage  454 _ 1 . The instructions also may include instructions that compare the readings obtained by the sensing apparatus  434 _ 1  to one or more threshold values or specifications stored on the electronic storage  454 _ 1 . 
     Furthermore, the electronic storage  454 _ 1  may store instructions that, when executed, cause the electronic processing module  452 _ 1  to generate a command signal that causes the resettable switching apparatus  432 _ 1  to change state. For example, the electronic processing module  452 _ 1  send a switch control mechanism (such as the relay  135 _ 1  shown in  FIG.  1 B ) a command signal that causes the resettable switching apparatus  432 _ 1  to open or close, or the electronic processing module  452 _ 1  may send a command signal directly to the resettable switching apparatus  432 _ 1 . 
     Furthermore, the electronic storage  454 _ 1  may include instructions that implement techniques for filtering and/or preparing the data produced by the sensing apparatus  434 _ 1 . For example, the electronic storage  454 _ 1  may include instructions that implement an analog-to-digital (A/D) converter that digitizes analog data from the sensing apparatus  434 _ 1 . 
     The I/O interface  456 _ 1  may be any interface that allows a human operator and/or an autonomous process to interact with the controller  450 _ 1 . The I/O interface  456 _ 1  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, an Ethernet interface. The I/O interface  456 _ 1  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 _ 1  may be, for example, operated, configured, modified, or updated through the I/O interface  456 _ 1 . 
     The I/O interface  456 _ 1  also may allow the controller  450 _ 1  to communicate with systems external to and remote from the network protector  430 _ 1 . For example, the I/O interface  456 _ 1  may include a communications interface that allows communication between the controller  450 _ 1  and a remote station (not shown), or between the controller  450 _ 1  and a separate electrical apparatus in the power system  100  ( FIG.  1 A ) 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 _ 1   without making physical contact with the network protector  430 _ 1  or the controller  450 _ 1 . 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 _ 1  via a services protocol, or a remote control that connects to the controller  450 _ 1  via a radio-frequency signal. The controller  450 _ 1  may communicate information such as whether or not an error condition exist to the remote station through the I/O interface  456 _ 1 . 
     The controller  450 _ 1  is coupled to the communications interface  460 _ 1 . The communications interface  460 _ 1  may be any type of interface sends and receives data, commands, and/or information through a communications path  465 . The communications path  465  may be wired or wireless and may send data, information, and commands using a specific communications protocol such as, for example, an industrial protocol, a services protocol, or an Internet protocol. 
     The communications interface  460 _ 1  may be a type of network interface, such as, for example, an Ethernet interface. In another example, the communications interface  460 _ 1  is an IEC 61850 interface. The communications interface  460 _ 1  also may allow communication without physical contact through, for example, an IEEE 802.11, Bluetooth, or a near-field communication (NFC) connection. In some implementations, the communication interface  460 _ 1  is implemented as part of the I/O interface  456 _ 1 . 
     The network protector  430 _ 2  is configured in the same manner. In the example of  FIG.  4   , the network protector  430 _ 2  monitors the feeder  404 _ 2 . The network protector  430 _ 2  and the controller  450 _ 2  provide an indication of the direction of power flow on the feeder  404 _ 2  through the communications interface  460 _ 2  and may receive an indication of the direction of power flow on the feeder  404 _ 1  from the network protector  430 _ 1 . 
     Other implementations are possible. For example, the low-voltage distribution network  401  may include more than two low-voltage feeders and more than two network protectors. In these examples, each network protector  430 _ 1  and  430 _ 2  is configured to communicate with more than one other network protector. 
     Moreover, in the example shown in  FIG.  4   , each network protector  430 _ 1  and  430 _ 2  includes a respective controller  450 _ 1  and  450 _ 2 . However, in other implementations, one controller may be used by the network protectors  430 _ 1  and  430 _ 2 . In these implementations, the controller may be implemented separately from the network protector  430 _ 1  and/or the network protector  430 _ 2 . For example, the controller  450 _ 1  may be configured to control the components of the network protector  430 _ 1  and the components of the network protector  430 _ 2  and to access data from the sensing apparatus  434 _ 1  and the sensing apparatus  434 _ 2 . 
     In some implementations, the network protectors  430 _ 1  and  430 _ 2  do not include an integrated controller and instead communicate with a controller that is separate from the network protectors  430 _ 1  and  430 _ 2 . The separate controller controls the components of both network protectors  430 _ 1 ,  430 _ 2  and accesses the data from the sensing apparatuses  434 _ 1 ,  434 _ 2 .In these implementations, the separate controller may be configured in the same manner as the controller  450 _ 1  or  450 _ 2 . 
     Furthermore, in some implementations, the network protectors  430 _ 1  and  430 _ 2  are enclosed within or otherwise held in a common unit, such as shown in  FIG.  5   . 
       FIG.  5    is a side block diagram of a network protector system  570 . The network protector system  570  includes a housing  572  that encloses the network protectors  430 _ 1  and  430 _ 2 . The housing  572  is a three-dimensional body that defines an enclosed or partially enclosed interior space  573 . The housing  572  may be, for example, a cabinet or a vault. The housing  572  may be configured for above-ground or underground use. The housing  572  may be made of any solid, durable material. For example, the housing  572  may be made of aluminum, steel, or a ruggedized polymer material. The housing  572  is sized to accommodate the network protectors  430 _ 1  and  430 _ 2 , and the communications path  465 . The length of the communications path  465  may be, for example, about 1 to 2 meters. 
     The housing  572  also may include other components that are not shown in  FIG.  5   . For example, the housing  572  may include bushings to accommodate the feeders  404 _ 1  and  404 _ 2 . 
       FIG.  6    is a flow chart of a process  600 . The process  600  is an example of a process of operating a network protector. The process  600  may be performed by the controller  450 _ 1 , the controller  450 _ 2 , or by a separate controller that is coupled to the network protector  430 _ 1  and the network protector  430 _ 2 . The process  600  is discussed with respect to the controller  450 _ 1 . 
     The controller  450 _ 1  obtains an indication of the direction of power flow on the feeder  404 _ 2  ( 610 ). The controller  450 _ 1  may obtain the indication of the direction of power flow on the feeder  404 _ 2  by producing a command signal and transmitting the command signal over the communications path  465  to the network protector  430 _ 2 . The controller  450 _ 2  receives the command signal and provides an indication of the direction of power flow on the feeder  404 _ 2  to the network protector  430 _ 1  via the communication path  465  in response. In some implementations, the network protector  430 _ 2  provides the indication of direction of power flow on the feeder  404 _ 2  to the network protector  430 _ 1  on a regular basis and without being commanded to do so. 
     The indication of the direction of power flow is based on data from the sensing apparatus  434 _ 2 . The indication of the direction of power flow may be, for example, a binary value, a symbol, or a numerical value that represents a measured electrical quantity. For example, in implementations that use a binary value to indicate the direction, a value of 1 may indicate forward power flow and a value of 0 may indicate reverse power flow. In another example, in implementations in which the sensing apparatus  434 _ 2  produces a numerical value that represents a measured electrical quantity (such as real power), the indication of the direction of power flow is derived from the measured data. For example, a value of real power that is less than zero indicates reverse power flow and a value of real power that is greater than zero indicates forward power flow. In this example, the indication of the direction of power flow on the feeder  404 _ 2  is referred to as the first indication of power flow. 
     The controller  450 _ 1  compares the first indication from the network protector  430 _ 2  to an indication of the direction of power flow on the feeder  404 _ 1  ( 620 ). The indication of the direction of power flow on the feeder  404 _ 1  is obtained from the sensing apparatus  434 _ 1  and/or derived from data produced by the sensing apparatus  434 _ 1 . In this example, the indication of the direction of power flow on the feeder  404 _ 1  is referred to as the second indication. 
     The first and second indications may be compared by, for example, subtraction, or in any other suitable manner. The comparison produces an output that indicates whether the first and second indications are the same. If the first and second indications are the same, then the direction of power flow is the same on the feeder  404 _ 1  and the feeder  404 _ 2 , and no error condition exists. If the first and second indications are not the same, the direction of power flow on the feeder  404 _ 1  is different than the direction of power flow on the feeder  404 _ 2 , and an error condition may exist. In some implementations, the first and second indications are formatted prior to being compared. For example, if the first and second indications are numerical values that represent measured real power, the indications may be converted into binary numbers that indicate the direction of power flow prior to being compared. 
     If no error condition exists ( 630 ), the controller  450 _ 1  does not act on the network protector  430 _ 1  or the network protector  430 _ 2 , and power continues to flow in the feeders  404 _ 1  and  404 _ 2 . The process  600  returns to ( 610 ) or may end. 
     If an error condition exists ( 630 ), the controller  450 _ 1  opens the switching apparatus  432 _ 1  or the switching apparatus  432 _ 2  ( 640 ). The controller  450 _ 1  also determines which feeder  404 _ 1  or  404 _ 2  has reverse power flow based on the first and second indications. The controller  450 _ 1  commands the switching apparatus of the network protector that is coupled to the feeder with reverse power flow to open because the reverse power flow exists due to an error condition. For example, if the reverse power is on the feeder  404 _ 1 , the controller  450 _ 1  commands the resettable switching apparatus  432 _ 1  to open. The resettable switching apparatus  432 _ 2  remains closed. If the reverse power is on the feeder  404 _ 2 , the controller  450 _ 1  commands the resettable switching apparatus  432 _ 2  to open by issuing a command to the network protector  430 _ 2  via the communications path  465 . The resettable switching apparatus  432 _ 1  remains closed. 
       FIGS.  7 A- 7 D and  8 A- 8 D  show simulated data for a spot network. The spot network included two low-voltage feeders: feeder 1 and feeder 2, a network protector 36 on feeder 1, and a network protector 37 on feeder 2. The network protectors 36 and 37 were configured for peer-to-peer communication with each other, similar to the network protectors  130 _ 1 ,  130 _ 2 ,  330 ,  430 _ 1 , and  430 _ 2  discussed above. Each feeder 1 and 2 was connected to an AC power source through a medium-voltage circuit breaker (MVCB). The spot network was connected to DERs (solar arrays in the simulation).  FIGS.  7 A- 7 D  relate to a simulation in which the MVCP on feeder 1 was opened due to a fault condition. In  FIGS.  7 A- 7 D , the spot network had a 20% photovoltaic (PV) penetration.  FIGS.  8 A- 8 D  relate to a simulation in which excess energy was produced by the solar arrays. In  FIGS.  8 A- 8 D , the spot network had an 80% photovoltaic (PV) penetration. 
       FIG.  7 A  shows power drawn by the load ( 781 ) and power generated by the solar arrays ( 782 ) as a function of time in seconds.  FIG.  7 B  shows power through the network protector 36 as a function of time in seconds. In  FIG.  7 B , real power is labeled  783  and reactive power is labeled  784 .  FIG.  7 C  shows power through the network protector 37 as a function of time in seconds. In  FIG.  7 C , real power is labeled  785  and reactive power is labeled  786 .  FIG.  7 D  shows the state of the network protector 36 ( 787 ) and the state of the network protector 37 ( 788 ) as a function of time. In  FIG.  7 D , a value of 1 on the y-axis indicated an open state and a value of 0 on the y-axis indicated a closed state. The time scale (x-axis) is the same in  FIGS.  7 A- 7 D . 
     As shown in  FIG.  7 A , the power generated by the solar arrays ( 782 ) did not exceed the power demand of the load ( 781 ) during the simulation. At about time t = 15 seconds (s), the MVCB on feeder 1 opens due to a fault. The MVCB on feeder 2 remains closed. Reverse power flow occurs on the feeder 1, and the network protector 36 senses reverse power flow, as shown in  FIG.  7 B . As shown in  FIG.  7 C , forward power flow continues on feeder 2, and the network protector 37 does not sense reverse power flow. The network protector 36 and/or the network protector 37 perform the process  600 . For example, the network protector 37 shares the forward power flow status with the network protector 36. Because the power flow is in different directions on the feeders 1 and 2, the network protector 36 determines that a fault condition exists and the reverse power flow is due to the fault condition. At about time t= 17.6 s, the network protector 36 opens because the reverse power flow is on feeder 1. 
       FIGS.  8 A- 8 D  show a simulation of the same spot network in a situation in which the power generated by the solar arrays exceeds the power demand of the load.  FIG.  8 A  shows power drawn by the load ( 881 ) and power generated by the solar arrays ( 882 ) as a function of time in seconds.  FIG.  8 B  shows power through the network protector 36 as a function of time in seconds. In  FIG.  8 B , real power is labeled  883  and reactive power is labeled  884 .  FIG.  8 C  shows power through the network protector 37 as a function of time in seconds. In  FIG.  8 C , real power is labeled  885  and reactive power is labeled  886 .  FIG.  8 D  shows the state of the network protector 36 ( 887 ) and the state of the network protector 37 ( 888 ) as a function of time. In  FIG.  8 D , a value of 1 on the y-axis indicated an open state and a value of 0 on the y-axis indicated a closed state. The time scale (x-axis) is the same in  FIGS.  8 A- 8 D . 
     As shown in  FIG.  8 A , from time t = 0 s to around time t =  100  s, the power demand of the load ( 881 ) exceeds the power generated by the solar arrays ( 882 ). Around time = 100 s, the power generated by the solar arrays ( 882 ) exceeds the power demand of the load ( 881 ) intermittently. For example, the power generated by the solar arrays ( 882 ) exceeds the power demand of the load ( 881 ) at around time t = 120 s. The excess power generated by the solar arrays flows as reverse power on the feeder 1 and the feeder 2. As shown in  FIG.  8 B  and  FIG.  8 C , reverse power flows in the network protector 36 and the network protector 37 and the real power detected by the network protector 36 and the network protector 37 is negative (for example around time t = 120 s). The network protector 36 and/or the network protector 37 performs the process  600  and determines that the reverse flow is on feeders 1 and 2. Thus, and as shown in  FIG.  8 D , no error condition is detected, the network protectors 36 and 37 remain closed, and the excess power from the solar arrays is returned to the grid. 
     These and other implementations are within the scope of the claims.