Patent Publication Number: US-2023136795-A1

Title: Network protector that detects an error condition

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 63/272,941, filed on Oct. 28, 2021 and titled NETWORK PROTECTOR THAT DETECTS AN ERROR CONDITION, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a network protector that detects an error condition. 
     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 resettable switching apparatus configured to control an electrical connection between a first distribution transformer and a first electrical feeder of a secondary electrical distribution network; a sensor apparatus configured to sense one or more properties of electrical power in the first electrical feeder; and a controller configured to: analyze one or more of the sensed properties of the electrical power in the first electrical feeder to determine whether an error condition exists in the secondary electrical distribution network; and open the resettable switching apparatus if an error condition exists. 
     Implementations may include one or more of the following features. 
     The error condition may be one or more of a fault condition or a maintenance condition. 
     The one or more sensed properties may include one or more properties of electrical current that flows in the first electrical feeder. The one or more properties of the electrical current may include a magnitude of one or more frequency harmonics of the electrical current, and the controller may be configured to determine that the reverse power flow is from an error condition if the magnitude of one or more frequency harmonics exceeds a threshold value. The electrical current includes reverse current flow generated by a distributed energy resource, and the controller may be configured to open the resettable switching apparatus only if the one or more properties of the reverse current indicate that the error condition exists. The one or more properties of the electrical current may include a characteristic of a transient of the electrical current. 
     The controller also may be configured to determine whether reverse power flow exists in the first electrical feeder, and the controller may be configured to analyze the one or more sensed properties only if reverse power flow exists in the first electrical feeder. 
     The first electrical feeder may be configured to be electrically connected to one or more distributed energy resources. 
     The first electrical feeder may be configured to be electrically connected to an energy source that is also electrically connected to a second electrical feeder of the secondary electrical distribution network, and the first electrical feeder and the second electrical feeder may be electrically connected to the same load. 
     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 distribution network, the first network protector including: a first resettable switching apparatus; a first sensor apparatus configured to sense one or more properties of electrical power in the first electrical feeder; and a first controller configured to analyze one or more of the sensed properties of the electrical power in the first electrical feeder to determine whether an error condition exists in the first electrical feeder. The system also includes a second network protector configured to control an electrical connection between a second distribution transformer and a second electrical feeder of the secondary distribution network, the second network protector including: a second resettable switching apparatus; a second sensor apparatus configured to sense one or more properties of electrical power in the second electrical feeder; and a second controller configured to analyze one or more of the sensed properties of the electrical power in the first electrical feeder to determine whether an error condition exists in the second electrical feeder. 
     Implementations may include one or more of the following features. 
     The error condition may include one or more of a fault condition or a maintenance condition. 
     The one or more sensed properties may include one or more properties of electrical current that flows in the first electrical feeder. The one or more sensed properties of the electrical power in the first electrical feeder may include a frequency characteristic of the electrical current in the first electrical feeder, and the one or more sensed properties of the electrical power in the second electrical feeder may include a frequency characteristic of the electrical current in the second electrical feeder. The frequency characteristic of the electrical current in the first electrical feeder may include an amplitude of one or more harmonic components of the electrical current in the first electrical feeder, and the frequency characteristic of the electrical current in the second electrical feeder may include an amplitude of one or more harmonic components of the electrical current in the second electrical feeder. 
     The system also may include: a first circuit breaker configured to control an electrical connection between the first distribution transformer and an AC power source, and a second circuit breaker configured to control an electrical connection between the second distribution transformer and the same AC power source. 
     The system also may include: a first circuit breaker configured to control an electrical connection between the first distribution transformer and a first AC power source, and a second circuit breaker configured to control an electrical connection between a second AC power source that is distinct from the first AC source. 
     In another aspect, a method includes: accessing, at a network protector, one or more values related to one or more properties of electrical power in an electrical feeder of a secondary electrical distribution network; analyzing at least one value to determine whether an error condition exists in the secondary electrical distribution network; and if an error condition exists, opening a resettable switching apparatus in the network protector to disconnect the electrical feeder from a distribution transformer. 
     Implementations may include one or more of the following features. 
     A plurality of values may be accessed, each of the plurality of values being an amplitude of a harmonic component of current flowing in the secondary electrical distribution network, and the method also may include: comparing the amplitude of each harmonic component to a corresponding threshold value for that harmonic component. An error condition may exist when one or more of the amplitudes exceeds the corresponding threshold value. 
     A plurality of values may be accessed, each of the plurality of values being an amplitude of a harmonic component of current flowing in the secondary electrical distribution network, and the method also may include: comparing the amplitude of each harmonic component to a corresponding prior value for that harmonic component. An error condition may exist when a difference between one or more of the amplitudes exceeds a threshold value. 
     The method also may include: determining whether reverse current flow exists in the electrical feeder based on the one or more values; and, in some implementations, the at least one value is analyzed to determine whether an error condition exists in the secondary electrical distribution network only if reverse current flow exists in the electrical feeder. 
     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 A  is a block diagram of an example of an electrical power system. 
         FIG.  1 B  is a block diagram of an example of a network protector that may be used in the power system of  FIG.  1 A . 
         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 an example of a secondary network that is fed by a single alternating current (AC) source. 
         FIG.  5    is a block diagram of an example of a secondary network that is fed by two independent (AC) sources. 
         FIG.  6    is a flow chart of an example of a process of operating a network protector to determine whether an error condition exists in a secondary distribution network. 
         FIGS.  7 A and  7 B  are examples of simulated data of current flow in a secondary network when there is no fault condition. 
         FIGS.  8 A and  8 B  are examples of simulated data of current flow in a secondary network when there is a fault condition. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1 A  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 A  for simplicity. The electrical power system  100  includes a secondary electrical power distribution network  101  that includes network protectors  130 _ 1  and  130 _ 2  coupled to respective feeders  104 _ 1  and  104 _ 2 . The secondary electrical power distribution network  101  is electrically connected to an alternating current (AC) source  102  and to a load or loads  103 . The network protectors  130 _ 1  and  130 _ 2  control the electrical connection between the respective feeder  104 _ 1  and  104 _ 2  and loads  103 . 
     The system  100  also includes a controller  150 . As discussed in more detail below, the controller  150  analyzes one or more properties of the power that flows on the feeders  104 _ 1  and  104 _ 2  to determine whether an error condition exists. The configuration of the controller  150  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 or error conditions. 
     In the discussion below, 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. However, reverse power flow also may arise from excess power that is generated by a distributed energy resource (DER) and/or from circulating current that may arise when the source  102  is implemented as more than one independent AC power source. 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 . Moreover, reverse power flow that arises from a circulating current caused by a differences in the phase of voltage generated by independent AC power sources is not necessarily undesirable. 
     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. Thus, such traditional network protectors are unable to return excess power generated by a DER to the grid and are also unable to be used in a secondary network that is fed by more than one independent AC power source. 
     On the other hand, the controller  150  is configured to analyze properties and characteristics of the power that flows in the feeders  104 _ 1  and/or  104 _ 2  in order to determine whether an error condition exists. The properties and characteristics analyzed by the controller  150  include, for example, harmonic content of the current and/or voltage, overshoot characteristics, and/or transient characteristics. The controller  150  does not assume that reverse power flow is caused by an error condition. Instead, the controller  150  is configured to allow reverse power flow so long as no error condition exists. The error condition may be, for example, a maintenance condition or a fault condition. A maintenance condition is a condition that is intentionally caused due to scheduled maintenance or other intentional maintenance that halts or changes the flow of power in the secondary distribution network  101 . A fault condition is an unintentional event that halts or changes the flow of power in the secondary distribution network  101 . Examples of unintentional events include, for example, a phase-to-ground fault, an overcurrent condition caused by a short, or an over-voltage condition. Unintentional events may be caused by falling objects, ingress of moisture, storms, equipment malfunction, and other unplanned events. 
     The controller  150  and 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 controller  150  and the network protectors  130 _ 1  and  130 _ 2  encourage efficient use of generated energy and may be used in a secondary distribution network that is fed by more than one AC source. 
     Before discussing the controller  150  in greater detail, an overview of the system  100  and its various components is provided. 
     Referring also to  FIG.  1 B , the network protector  130 _ 1  includes a resettable switching apparatus  132 _ 1 , a sensor apparatus  134 _ 1 , and a switch control mechanism  135 _ 1 . The sensor apparatus  134 _ 1  monitors the electrical power on the feeder  104 _ 1  and the switch control mechanism  135 _ 1  operates the resettable switching apparatus  132 _ 1 . The switch control mechanism  135 _ 1  may be, for example, a relay. The switch control mechanism  135 _ 1  may be coupled to the controller  150  or implemented as part of the controller  150 . 
     The resettable switching apparatus  132 _ 1  is any type of switch that is capable of opening and closing the feeder  104 _ 1 . For example, the resettable switching apparatus  132 _ 1  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  104 _ 1 . When the electrical contacts are separated, current cannot flow in the feeder  104 _ 1 . The resettable switching apparatus  132 _ 1  is configured for repeated operation. For example, after the resettable switching apparatus  132 _ 1  opens the feeder  104 _ 1  to stop or prevent current flow, the resettable switching apparatus  132 _ 1  is able to close the feeder  104 _ 1  such that current flow in the feeder  104 _ 2  resumes. The resettable switching apparatus  132 _ 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  132 _ 1 . 
     The network protector  130 _ 2  is configured in a similar manner. Moreover, other implementations are possible. For example, although only one instance of the controller  150  is shown in  FIGS.  1 A and  1 B , in some implementations each network protector  130 _ 1  and  130 _ 2  includes a separate instance of the controller  150 . 
     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 A . 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 power source  102  is connected to the feeder  106 _ 1  through a medium-voltage circuit breaker  105 _ 1  and to the feeder  106 _ 2  through a medium-voltage circuit breaker  105 _ 2 . The medium-voltage circuit breakers  105 _ 1  and  105 _ 2  operate in the event of a fault to disconnect the source  102  from the system  100 . The medium-voltage circuit breakers  105 _ 1  and  105 _ 2  also may be opened intentionally and in a planned manner to perform maintenance on the system  100 . In some implementations, multiple independent AC power sources feed the secondary distribution network  101 . An example of such an implementation is shown in  FIG.  5   . 
     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 A , 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 a respective feeder  104 _ 1 ,  104 _ 2  of the secondary distribution network  101 . 
     The loads  103  may 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. 
     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 . As noted above, network protectors 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. 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 analyzing one or more sensed properties or characteristics of the power that flows in the secondary distribution network  101 . Thus, the network protectors  130 _ 1  and  130 _ 2  are configured to provide the excess energy generated by the DERs to the system  100 . Moreover, the network protectors  130 _ 1  and  130 _ 2  and the control  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. 
     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 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, 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 . 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), network protectors  130 , and transformers  342  that provide power to the loads  103 . The area network  301  may include tens of redundant feeders  304  and network protectors  130 , and the loads  103  may include tens, hundreds, or thousands of loads. The network protectors  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 A,  2 , and  3    are provided as examples, and the controller  150  may be used with distribution networks having other configurations. For example, although one AC power source  102  is shown in  FIGS.  2  and  3   , the spot network  201  and the area network  301  may be fed by more than one independent AC power source. In another example, the distribution network  201  may have fewer or more than four parallel low-voltage feeders. Moreover, the examples discussed above relate to network protectors. However, other resettable switching apparatuses, for example, reclosers and/or switchgear, may be used in the secondary distribution network  101  and may be implemented with the controller  150 . 
       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. 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 , and a controller  450 _ 1 . In the example of  FIG.  4   , the controller  450 _ 1  is part of the network protector  430 _ 1 . For example, the controller  450 _ 1  and the resettable switching apparatus  432 _ 1  may be contained together in the same housing. In some implementations, the controller  450 _ 1  is separate from the network protector  430 _ 1  but is in communication with the resettable switching apparatus  432 _ 1  and the sensor apparatus  434 _ 1 . 
     The network protector  430 _ 1  also includes a sensor apparatus  434 _ 1 . The sensor 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 sensor 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 sensor apparatus  434 _ 1  may include one or more voltage sensors and/or one or more power sensors. The sensor apparatus  434 _ 1  may include other related devices, such as timers or other devices that measure the passage of time, and/or a spectrum analyzer or other device configured to measure the frequency content of the power that flows in the secondary network  401 . 
     The sensor apparatus  434 _ 1  produces data related to the properties and/or characteristics of the power that flows on the feeder  404 _ 1 . The data may be frequency domain data produced by a device such as a spectrum analyzer. For example, the sensor apparatus  434 _ 1  may produce values or other indicators of the amplitude of one or more frequency harmonics of the electrical current that flows on the feeder  404 _ 1 . The amplitude may be expressed as an absolute value in amperes (A) or as a percentage of the amplitude of the fundamental current harmonic. In some implementations, the data from the sensor apparatus  434 _ 1  is time domain data. 
     For example, the data from the sensor apparatus  434 _ 1  may be time-domain data, such as a collection of current amplitude values, power values, and/or voltage values that are each associated with a time stamp. In some implementations, the sensor apparatus  434 _ 1  produces an indication of a transient and/or an overshoot of the electrical current that flows on the feeder  404 _ 1  and/or the voltage on the feeder  401 _ 1 . A transient is a relatively short-lived burst of current or voltage that has a greater than typical amplitude. The indication of the transient may be a time duration during which the current on the feeder  404 _ 1  is above a threshold value or the rise time of the transient. The rise time of the transient may be the time that the current and/or voltage increases from the typical or expected amplitude to a threshold amount or to the maximum amplitude. Another example of time-domain data that the sensor apparatus  434 _ 1  may produce is an indication of an overshoot of the current on the feeder  404 _ 1 . The overshoot is the difference between the expected current amplitude and a measured current. In some implementations, the sensor apparatus  434 _ 1  produces time-domain data and frequency-domain data. 
     The sensor apparatus  434 _ 1  may produce additional data about the power that flows in the feeder  404 _ 1 . For example, the sensor apparatus  434 _ 1  may be configured to produce an indication of the direction of current flow on the feeder  404 _ 1 . In these implementations, the sensor apparatus  434 _ 1  may include a directional device, such as a diode, that is capable of providing an indication of the direction of current flow on the feeder  404 _ 1 . In these implementations, the sensor apparatus  434 _ 1  produces an indication of the direction of current flow. For example, the sensor apparatus  434 _ 1  may produce 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 . 
     The network protector  430 _ 1  also includes the controller  450 _ 1 . The controller  450 _ 1  analyzes data from the sensor apparatus  434 _ 1  to determine whether or not an error condition exists in the secondary network  401 . If an error condition exists in the secondary network  401 , the controller  450 _ 1  causes the resettable switching apparatus  432 _ 1  to open. 
     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. 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 may store instructions that cause readings from the sensor 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 sensor apparatus  434 _ 1  to one or more threshold values or specifications stored on the electronic storage  454 _ 1 . 
     The electronic storage  454 _ 1  also may store information about the network protector  430 _ 1  and/or the feeder  404 _ 1 , such as one or more threshold values and/or a specification used for determining whether a change has occurred in one or more measured properties. For example, the electronic storage  454 _ 1  may store instructions that determine whether the amplitude of one or more current harmonics exceeds a respective threshold. Additionally, the electronic storage  454 _ 1  may store instructions that implement various mathematical approaches, such as instructions that implement a Wavelet transform. 
     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 mechanism  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 mechanism  432 _ 1  to open or close, or the electronic processing module  452 _ 1  may send a command signal directly to the resettable switching mechanism  432 _ 1 . Moreover, the electronic storage  454 _ 1  may store information related to the conditions under which the network protector  430 _ 1  is to be opened or closed. For example, the electronic storage  454 _ 1  may store a threshold value that represents a maximum acceptable difference between network-side and source-side voltage vectors to allow the network protector  430 _ 1  to transition from the open state to the closed state. 
     Furthermore, the electronic storage  454 _ 1  may include instructions that implement techniques for filtering and/or preparing the data produced by the sensor 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 sensor apparatus  434 _ 1 . In another example, the instructions may include instructions that convert time-domain data from the sensor  434 _ 1  into frequency-domain data and vice versa. 
     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, Ethernet. 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 the determined tap position through the I/O interface  456 _ 1  to the remote station or to a separate device in the power system  400 . 
     The network protector  430 _ 2  is configured in the same manner. The network protector  430 _ 2  includes a resettable switching apparatus  432 _ 2 , a sensor apparatus  434 _ 2 , and a controller  450 _ 2 . The network protector  430 _ 2  monitors and controls the power flow in the feeder  404 _ 2 . 
     Other implementations are possible. For example, in  FIG.  4   , the source  102  is a single AC source that is connected to the secondary distribution network  401 . However, the network protectors  430 _ 1  and  430 _ 1  may be used to determine whether an error condition exists in implementations in which the secondary distribution network  401  is fed by multiple independent and distinct AC power sources.  FIG.  5    is a block diagram of a system  500  that includes two distinct AC sources: a first AC source  502 _ 1  and a second AC source  502 _ 2 . The first AC source  502 _ 1  is connected to the circuit breaker  105 _ 1 , and the second AC source  502 _ 2  is connected to the circuit breaker  105 _ 2 . The first AC source  105 _ 1  and the second AC source  502 _ 2  are separate and independent AC sources and may produce voltage having different amplitudes and/or phases. 
     Differences between the amplitude and/or phase of the voltage produced by the AC sources  502 _ 1  and  502 _ 2  may produce a circulating current that flows in the secondary network  401  toward the sources  502 _ 1  and/or  502 _ 2 . Thus, the circulating current is a reverse current. However, the circulating current is not necessarily a fault condition and the system  500  may operate as expected in the presence of the circulating current. 
     As discussed above, a conventional or traditional network protector opens in the presence of a reverse current. However, the controllers  450 _ 1  and  450 _ 2  are configured to analyze the current that flows in the feeders  404 _ 1  and  404 _ 2  to determine whether an error condition exists. Thus, the network protectors  530 _ 1  and  530 _ 2  also reduce unnecessary operation of the network protectors  530 _ 1  and  530 _ 2  and reduce service outages in implementations in which the secondary distribution system  401  is fed by more than one distinct AC power source. 
       FIG.  6    is a flow chart of a process  600 . The process  600  is an example of a process of operating a network protector to determine whether an error condition exists in a secondary distribution network. The process  600  may be performed by the controller  150 , the controller  450 _ 1 , or the controller  450 _ 2 . The process  600  is discussed with respect to the controller  450 _ 1 . 
     Data from the sensor apparatus  434 _ 1  is accessed ( 610 ). The data may include frequency domain and/or time domain data. The data from the sensor apparatus  434 _ 1  may be an indication of the current, power, and/or voltage on the feeder  404 _ 1  over time. For example, the data from the sensor apparatus  434 _ 1  may be time-domain data, such as a collection of current amplitude values, power values, and/or voltage values that are each associated with a time stamp. The data may include frequency domain data, such as an amplitude of one or more harmonics of the AC current that flows on the feeder  404 _ 1 . The data also may include data that indicates a direction of current flow on the feeder  404 _ 1 . 
     The data from the sensor apparatus  434 _ 1  may be accessed by retrieving the data from the sensor apparatus  434 _ 1 . In some implementations, the sensor apparatus  434 _ 1  provides the data to the controller  450 _ 1  continuously or on a regular basis via a communications path. 
     The data is analyzed to determine whether an error condition exists in the secondary network  401  ( 620 ). The existence of an error condition causes the properties and characteristics of the power on the feeder  404 _ 1  to change compared to when there is no error condition. Reverse power generated by DERs and circulating current that arises from the secondary network  401  being fed by more than one independent AC power source have characteristics that are the same as forward current flow that occurs in ordinary and normal conditions, so long as no error condition is present. In other words, the properties of the reverse current flow generated by excess power generated by DERs and the properties of reverse current that is circulating current have the same features and characteristics as normal, non-fault forward current. Thus, by analyzing the properties of the power flow on the feeder  404 _ 1 , the controller  450 _ 1  determines whether or not an error condition exists. 
     For example, a high-impedance fault in the secondary distribution network may be caused by a phase-to-ground fault. Such a fault changes the amplitude of the harmonics of the current that flows on the feeder  404 _ 1 . The current that flows on the feeder  404 _ 1  has a fundamental frequency that is determined by the AC source or sources that feed the secondary distribution network  401 . The fundamental frequency may be, for example, 60 Hz. The current also includes components that have frequencies that are integer multiples of the fundamental frequency. For example, if the fundamental frequency is 60 Hz, the AC current on the feeder  404 _ 1  also may include components at 180 Hz (the third harmonic in this example). Each component or current harmonic has a corresponding amplitude that is expressed in amperes. Under ordinary conditions when there is no error condition, the component at the fundamental frequency has the largest amplitude and the amplitude of each of the other harmonics is much smaller. In the presence of a high-impedance fault, the amplitude of one or more odd-numbered harmonics (for example, the 3 rd , 5 th , 7 th , 13 th , 15 th , and/or 17 th  harmonics) increases substantially and the current also may include even-numbered components. 
     In some implementations, the data from the sensor apparatus  434 _ 1  includes information about the current harmonics, and this data is analyzed to determine whether or not an error condition exists. For example, the magnitude of one or more harmonics may be compared to a threshold value that represents the magnitude of that harmonic under ordinary conditions. In another example, the measured amplitude of a harmonic is compared to the amplitude of that same harmonic measured at an earlier time when no error condition existed. In another example, a shape or profile of the amplitude of the current and/or voltage harmonics over time are analyzed. 
     In some implementations, a Wavelet transform is used to provide additional information about the harmonics. The Wavelet transform is a signal processing technique that decomposes the data from the sensor apparatus  434 _ 1  into different ranges of frequencies using a series of low-pass and high-pass filters. The Wavelet transform provides a time-frequency multi-resolution analysis, the results of which provide an indication of short-lived, sudden, and/or abrupt variations in the electrical parameters represented in the data from the sensor apparatus  434 _ 1 . For example, the Wavelet transform may provide indications of short-lived, sudden, and/or abrupt variations in voltage, phase, current, and/or frequency of the power flow on the feeder  404 _ 1 . The Wavelet transform helps to detect and/or identify variation in harmonics of current and/or voltage, and these variations indicate whether or not a fault condition is present. 
     Faults and planned opening of the circuit breaker  105 _ 1  also may cause changes to the properties and characteristics of the power that flows in the feeder  404 _ 1 . For example, a planned opening of the circuit breaker  105 _ 1  causes transient currents on the feeder  404 _ 1 . Data from the sensor  434 _ 1  may be analyzed to identify such transients. For example, time series data from the sensor  434 _ 1  (current or voltage on the feeder  404 _ 1  as a function of time) may be analyzed to identify a transient by, for example, comparing the rate of change of the current or voltage to a threshold, comparing the maximum amplitude of the current and/or voltage to a threshold, and/or determining a duration of time during which the current and/or voltage exceeds a threshold value and then comparing that determined duration to a specification. In this way, an error condition, whether caused by a fault condition or by planned maintenance or other intentional action, may be detected based on the data from the sensor apparatus  434 _ 1 . 
     In some implementations, the controller  450 _ 1  determines a direction of current flow before analyzing the data from the sensor apparatus  434 _ 1  further. In these implementations, the controller  450 _ 1  only analyzes the data from the sensor apparatus  434 _ 1  if there is reverse current flow is detected on the feeder  404 _ 1 . 
     If an error condition does not exist ( 630 ), the process  600  returns to ( 610 ) to continue monitoring the feeder  404 _ 1  or the process  600  ends. If an error condition exists ( 630 ), then the controller  450 _ 1  produces a command that causes the network protector  430 _ 1  to open such that the distribution transformer  142 _ 1  is disconnected from the feeder  404 _ 1 . 
       FIGS.  7 A and  7 B and  8 A and  8 B  show characteristics of current flow as a function of time at a primary side of a distribution transformer that is connected to a spot network.  FIGS.  7 A and  7 B  show the current as a function of time during ordinary operating conditions with reverse power flow due to excess PV generation.  FIGS.  8 A and  8 B  show simulated data for the same spot network during a fault condition.  FIG.  7 A  shows the magnitude of the fundamental harmonic of current that flows on the primary side of the distribution transformer as a function of time in seconds (s) when there is no fault condition but reverse power flow from excess PV generation exists.  FIG.  7 B  shows the magnitude of the higher-order current harmonics through the 7 th  harmonic as a function of time in seconds when there is no fault condition but reverse power flow from excess PV generation exists.  FIGS.  7 A and  7 B  have the same time scale.  FIG.  8 A  shows the magnitude of the fundamental current harmonic as a function of time in seconds during a fault condition.  FIG.  8 B  shows the magnitude of the higher-order current harmonics through the 7 th  harmonic as a function of time during the fault condition.  FIGS.  8 A and  8 B  have the same time scale. 
     As shown by comparing  FIG.  7 A  to  FIG.  8 A  and by comparing  FIG.  7 B  to  FIG.  8 B , the current harmonics have different characteristics during ordinary operation (even with reverse power flow) than in the presence of the fault. For example, the profile of the amplitude of the current harmonics over time has a different shape during normal operation than when a fault is present. In another example, the amplitude of the current harmonics are different during normal operation than when a fault is present. The amplitude of the higher-order current harmonics ( FIG.  8 B ) during the fault condition is generally greater than the amplitude of the higher-order current harmonics under ordinary conditions ( FIG.  7 B ). 
     The process  600  uses such characteristics to distinguish between normal operation and a fault condition instead of assuming that reverse power flow always indicates that a fault is present. In this way, the network protectors  430 _ 1  and  430 _ 2  (and other network protectors implemented with a controller that performs the process  600 ) allow reverse power flow in a secondary network so long as a fault condition is not present. This capability reduces service outages, and allows the network protectors  430 _ 1  and  430 _ 2  to be used in secondary networks that are fed by more than one independent AC power source and/or secondary networks that include DERs. 
     These and other implementations are within the scope of the claims.