Abstract:
Provided is an intelligent electronic device for protection, monitoring, controlling, metering or automation of electrical power system. The system, method and device of the present invention preserves current differential protection active during a breaker bypass or similar operation. A current differential protection system is coordinated by one relay (transfer), which simultaneously establishes and handles multiple two-terminal  87 L protection zones with several relays. This “enhanced multiple-terminal system” requires no change to protection settings on any local or remote relays during a bypass process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    None. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to electric power systems including intelligent electronic devices (IEDs) for protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines. More specifically, the present invention relates to a system, method and device for preserving current differential protection communication active during the process involved in a breaker bypass or similar operation. 
         [0003]    Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include IEDs and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like. 
         [0004]    Generally, IEDs are also used for protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines. For example, certain IEDs and procedures may act to isolate some power system element(s) from the remainder of the power system upon detection of an abnormal condition or a fault in, or related to, the power system element(s). IEDs may include protective devices such as protective relays or otherwise, remote terminal units (RTUs), power line communication devices (PLCs), bay controllers, supervisory control and data acquisition (SCADA) systems, general computer systems, meters, and any other comparable devices used for protecting, monitoring, controlling, metering and/or automating electric power systems and their associated power lines. 
         [0005]    In one example, a particular type of IED generally known as a current differential protective relay protects an associated power line by analyzing the current at different terminals of the line. The general implementation of a current differential protective relay is illustrated in  FIG. 1A . A current differential protective relay R 1  measures the current I 1  situated at one bus  102  via current transformer CT 1  on an associated power line  108 . Another protective relay R 2  measures the current I 2  situated at another bus  104  via current transformer CT 2  on the same power line  108 . The current vector quantity I 2  (magnitude and angle) measured by protective relay R 2  is transmitted to the current differential protective relay R 1  via communication link  130 . 
         [0006]    During operation, the current differential protective relay R 1  then calculates a vector sum of the currents [Σ(I 1 , I 2 )]. Under no-fault conditions, the resulting vector sum equals about zero amperes. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal about zero amperes. Upon detection of a fault or abnormal condition, the current differential protective relay R 1  sends a trip signal or command to an associated circuit breaker  110  to isolate the condition. 
         [0007]    In order to fully isolate the fault condition, it is to be noted that the other protective relay R 2  is also a current differential protective relay. In this arrangement, the other current differential protective relay R 2  may be adapted to concurrently receive the current measurement I 1  from current differential protective relay R 1  via communication link  130  and calculate a vector sum therefrom. 
         [0008]    When protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines, it is often beneficial to reroute data streams such as communication signals therein in order to calculate maintenance on protective devices or on power system elements associated thereto. For example, a power system element may require maintenance wherein the power system element and its associated protective device must be isolated from its associated power line. In order to maintain power distribution through the power line, power may be rerouted around the element that requires maintenance. In order to maintain protection, control, monitoring etc. of the power line, data streams such as communication signals must also be rerouted. 
         [0009]    U.S. Pat. No. 6,639,330 for a “Transfer Relay for Computer Base Equipment” describes a power switching transfer relay to automatically switch an electrical load, such as that drawn by a computer or other sensitive electrical or electronic equipment, from a primary power source to a secondary, or backup, power source upon interruption or loss of the primary source. The transfer relay includes a power relay and two control relays that are arranged to switch the electrical power input from the primary source to the backup source upon failure of the primary power source in the space of less than one cycle, and to actuate an alarm upon loss of the primary power source, loss of the backup power source, or the occurrence of a relay fault. 
         [0010]    U.S. Pat. No. 5,347,417 for a “Power Supply Protection System Applied to Optical Subscriber Network” describes a system for protecting a remote power supply for supplying power to an optical subscriber network, via a pair of power supply lines, from a remote power supply apparatus, with the power supply branch apparatuses inserted into the power supply lines in correspondence with each power receiving circuit respectively mounted in subscriber transmission nodes. Each of the power supply branch apparatuses comprises relay contacts inserted into its own power supply branch lines connected between the power supply lines and its own power receiving circuit, and a relay energized by an overcurrent detector or first and second communication units to change over the relay contacts. The relay contacts are opened and closed subscriber by subscriber sequentially to detect a faulty portion, and thereafter, the power is fed again selectively to the subscribers which have not experienced the fault. 
         [0011]    U.S. Pat. No. 5,132,867, for a “Method and Apparatus for Transfer Bus Protection of Plural Feeder Lines” describes a microprocessor based tie relay for controlling a tie circuit breaker between a main bus and a transfer bus to which any one of a number of feeder lines may be connected through a disconnect switch when the feeder circuit breaker associated with that feeder line is out of service. Settings for the protection characteristics of each of the feeder relays controlling the feeder circuit breakers are stored in non-volatile memory together with a default protection characteristic suitable for protecting any of the feeder lines. The appropriate protection characteristic for the feeder line connected to the transfer bus is selected for use by the tie relay in controlling the tie circuit breaker. This selection may be made manually by an operator, or preferably automatically by the microprocessor of the tie relay which monitors the states of the feeder circuit breakers and of the disconnect switches and selects the settings associated with the feeder line whose feeder circuit breaker is open and disconnect switch is closed. If the microprocessor does not recognize only one feeder line connected to the transfer bus, the default protection characteristic is selected and an alarm is generated. 
         [0012]    U.S. Pat. No. 5,041,737 for a “Programmable Bus-Tie Relay having a Plurality of Selectable Setting Groups” describes a bus-tie relay apparatus which includes a multi-position mechanical switch and a logic circuit responsive to the position of the mechanical switch for producing digital signals on five digital line, wherein a valid digital signal comprises the presence of high conditions on two, and two only, of said digital lines. A sensor senses the condition of the digital lines and retrieves the values of a relay element setting group from memory associated with that digital signal. A plurality of such relay element setting groups are stored in the apparatus, each one of which comprises values corresponding to the characteristics of an in-place relay associated with a particular one power line in a group thereof. 
         [0013]      FIG. 1B  generally provides an illustration of a traditional system for applying IEDs, such as protective devices, in order to maintain protection, monitoring, controlling, metering and/or automating of an associated power line. It should be clear that while  FIG. 1B  and other figures (including those illustrating the embodiments of the present invention) show two power lines emanating from a single substation, the methods and systems described herein may be generally extended to more or less than two lines, delivered to one or more substations. In the described systems, local protective relays R 1 , R n-1  are associated with respective circuit breakers  110 ,  111  for primary protection. For primary protection, local protective relays R 1 , R n-1  are current differential protective relays similar to those described with respect to  FIG. 1A . 
         [0014]    In the arrangement of  FIG. 1  B, local protective relays R 1 , R, n-1  receive current measurements I 2 , I n  from remote protective relays R 2 , R n  via communication link  130   b ,  130   a  in order to preserve current differential protection on power lines  108 ,  109  as discussed with respect to  FIG. 1A . Upon detection of a fault or abnormal condition on power lines  108 ,  109 , the local protective relay R 1 , R n-1  associated with that particular power line  108 ,  109  signals a corresponding circuit breaker  110 ,  111  to isolate the condition. In order to fully isolate the fault condition, it is to be noted that remote protective relays R 2 , R n  are also current differential protective relays. 
         [0015]    Circuit breakers (e.g.,  110  and  111 ) are high maintenance devices that experience some wear each time they interrupt a fault condition. Accordingly, a substation is typically constructed such that each primary circuit breaker  110 ,  111  may be taken out of service for maintenance purposes or replacement while leaving its associated power line  108 ,  109  associated therewith energized. In these instances, prior art arrangements have isolated the primary circuit breaker  110 ,  111  along with its associated local protective relay R 1 , R n-1  in order to provide for secondary protection on the energized power line  108 ,  109 . The local protective relay R 1 , R n-1  associated with the primary circuit breaker  110 ,  111  is commonly referred to as the primary relay. 
         [0016]    A method for isolating a primary circuit breaker such as  110  or  111  while providing secondary protection in such instances is commonly referred to as a breaker bypass operation. As shown in  FIG. 1B , one traditional arrangement for providing secondary protection in such instances includes having a transfer bus  106  associated with a main or primary bus  102 . In this arrangement, to isolate or take primary circuit breakers  110 ,  111  out of service, all other lines are typically connected to the main bus  102  by proper configuration of switches S 2 , S 5 , S 6 , and S 7 . 
         [0017]    For example, all other power lines are connected to the main bus  102  by closing switches S 2 , S 6  and opening switches S 5 , S 7 . During a breaker bypass operation, switches S 5 , S 7  are closed, whereas switches S 1 , S 2 , S 6 , S 8  are opened such that power lines  108 ,  109  are now connected to transfer bus  106 . Accordingly, current differential protection of either power line  108 ,  109  is now maintained through protective relay R x  and circuit breaker  114 . The circuit breaker  114  which provides secondary protection is commonly referred to as a transfer breaker, tie breaker, or coupler breaker, whereas its associated relay R x  is commonly referred to as a transfer breaker, tie breaker, or coupler relay. Communication (e.g., communication of current vector quantities as discussed above) between remote relays R 2 , R n  and transfer relay R x  may be optionally routed through communications switch  200 . 
         [0018]    Nevertheless, the arrangement of  FIG. 1B  poses a number of challenges for current differential protection of power lines. For example, current differential protection generally cannot be maintained during the entire bypass process due to the resulting parallel lines that feed a protected power line through both the main and transfer buses during the switching process of a breaker bypass operation. The hypothetical condition of keeping line current differential protection active on the local and remote relays during the switching process, would inaccurately cause these relays to detect a fault or abnormal condition on the power line. This is because the switching process of a bypass operation on the aforementioned bus arrangement, creates a parallel feed path onto the bus, changing the local measured quantity, which causes the vector sums of the currents to be unequal to zero on each relay. 
         [0019]    In order to overcome this shortcoming, during a breaker bypass or similar operation, current differential protection is often disconnected and replaced by backup protection such as step-distance. This, however, compromises the quality of the power line protection as step-distance protection is generally known to be slower and less reliable than current differential protection. Most faults associated with a breaker bypass operation generally occur due to human error. For example, operators may inadvertently cause a bus-to-ground fault while they intend to create a parallel current path that will allow for isolation of the circuit breaker. Therefore, during manual modifications to the bus configurations during a bypass operation, the risk of causing a fault is the highest. 
         [0020]    Accordingly, it is an object of the invention to provide a system and method for maintaining current differential protection of a power line even during a breaker bypass operation. 
         [0021]    This and other desired benefits of the preferred embodiments, including combinations of features thereof, of the invention will become apparent from the following description. It will be understood, however, that a process or arrangement could still appropriate the claimed invention without accomplishing each and every one of these desired benefits, including those gleaned from the following description. The appended claims, not these desired benefits, define the subject matter of the invention. Any and all benefits are derived from the multiple embodiments of the invention, not necessarily the invention in general. 
       SUMMARY OF THE INVENTION 
       [0022]    In accordance with the invention, an intelligent electronic device for protection, monitoring, controlling, metering or automation of power lines in an electrical power system is provided. The system, method, and devices of the present invention are adapted to provide protection of a power system. In other embodiments, a system, method, and device are provided which preserve line current differential protection during a breaker bypass or a similar operation. 
         [0023]    In one embodiment, a system is provided for maintaining current differential protection of a power line using a plurality of IEDs. The system generally includes a local IED associated with a location of the power line. The local IED is adapted to measure and transmit the current vector quantity associated with the location of the local IED. A remote IED associated with a location of the power line is further provided, wherein the remote IED is adapted to measure and transmit the current vector quantity associated with the location of the remote IED. 
         [0024]    A transfer IED in communication with the local and remote IEDs is adapted to receive the currents vector quantities transmitted by the local and remote IEDs. 
         [0025]    The transfer IED is further associated with a second location on the same bus arrangement as the local IED interconnected with the protected power line. This second location may be on a power line which is parallel to the power line of the local and remote relays. The transfer IED calculates the sum of the currents associated with its own location in the bus and the currents received from the local and remote IEDs. When the sum of the currents is not equal to about zero amperes, the transfer IED transmits a signal to cause tripping of a circuit breaker associated therewith, thereby isolating the protected power line. 
         [0026]    In accordance with yet another embodiment of the invention, the transfer IED is further adapted to transmit the current measured by the transfer IED and the current measured by the remote IED to the local IED. In turn, the local IED is adapted to receive the current quantity sent by the transfer IED, which is the vector sum of the currents measured by the transfer and the remote IED. The local IED will utilize the received current quantity and its own current measurement to evaluate a whether to assert a tripping signal to the associated local circuit breaker, in case these quantities do not add up to zero amperes. 
         [0027]    In accordance with yet another embodiment of the invention, the transfer current differential IED is further adapted to transmit the current measured by the transfer IED and the current measured by the local IED to the remote IED. In turn, the remote IED is adapted to receive the current quantity sent by the transfer IED, which is the vector sum of the currents measured by the transfer and the local IED. The remote IED will utilize the received current quantity and its own current measurement to evaluate whether to assert a tripping signal to the associated remote circuit breaker, in case these quantities do not add up to zero amperes. 
         [0028]    In accordance with yet another embodiment of the present invention, each of the transfer, local and remote IEDs are current differential IEDs which preserve line current differential protection during a breaker bypass or a similar operation. 
         [0029]    In yet another embodiment of the present invention, a method for maintaining current differential protection of a power line in a power system is provided including the steps of measuring the current associated with a location of the power line; measuring the current associated with another location of the power line; measuring the current associated with a location of second power line interconnected with first power line; calculating the sum of the currents associated the locations of the first and second power lines; and transmitting a signal to a circuit breaker associated with the second power line when the sum of the currents is not equal to about zero amperes. 
         [0030]    In yet another embodiment of the present invention a method for maintaining current differential protection of a power line in a power system is provided including the steps of measuring the current associated with a location of the power line; measuring the current associated with another location of the power line; measuring the current associated with a location of second power line interconnected with first power line; calculating the sum of the current associated with the first location of the first power line and current associated with the location of the second transmission associated the locations of the first and second power lines; and transmitting a signal to a circuit breaker associated with the second location of the second power line when the sum of the currents is not equal to about the current associated with the second location of the second power line. 
         [0031]    It should be understood that the present invention includes a number of different aspects or features which may have utility alone and/or in combination with other aspects or features. Accordingly, this summary is not exhaustive identification of each such aspect or feature that is now or may hereafter be claimed, but represents an overview of certain aspects of the present invention to assist in understanding the more detailed description that follows. The scope of the invention is not limited to the specific embodiments described below, but is set forth in the claims now or hereafter filed. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0032]      FIG. 1A  is a single line schematic diagram of a prior art system for current differential protection of a power line. 
           [0033]      FIG. 1B  is a single line schematic diagram of a prior art system for providing a bypass or similar operation for a power line. 
           [0034]      FIG. 2A  is a single line schematic diagram of a system for providing protection during a bypass or similar operation for a power line having two IEDs associated therewith in accordance with an embodiment of the present invention. 
           [0035]      FIG. 2B  is a single line schematic diagram of a system for maintaining current differential protection during a bypass or similar operation for a power line having a plurality of IEDs associated therewith in accordance with an embodiment of the present invention. 
           [0036]      FIG. 3A  is a single line schematic diagram of the system for maintaining current differential protection during a bypass or similar operation of  FIG. 2B , wherein the breaker bypass operation is consummated. 
           [0037]      FIG. 3B  is a single line schematic diagram of the system for maintaining current differential protection during a bypass or similar operation of  FIG. 3A , wherein the communication switch is adapted to include coordinate the communication with other associated IEDs in accordance with an embodiment of the present invention. 
           [0038]      FIG. 3C  is a single line schematic diagram of the system for maintaining current differential protection during a bypass or similar operation of  FIG. 3A , wherein the transfer relay includes a communication switch and a MUX in a single device in accordance with an embodiment of the present invention. 
           [0039]      FIG. 3D  is a diagram of the system for maintaining current differential protection during a bypass or similar operation for a power line having a plurality of IEDs associated therewith in accordance with an embodiment of the present invention. 
           [0040]      FIG. 3E  is a block diagram of an IED for a system for maintaining current differential protection during a bypass or similar operation for a power line having a plurality of IEDs associated therewith in accordance with an embodiment of the present invention. 
           [0041]      FIG. 3F  is a block diagram of an IED for a system for maintaining current differential protection during a bypass or similar operation for a power line having a plurality of IEDs associated therewith in accordance with an embodiment of the present invention. 
           [0042]      FIG. 4A  is a flow chart depicting a method for maintaining current differential protection by a transfer relay during a bypass or similar operation for a power line having a plurality of IEDs associated therewith in accordance with an embodiment of the present invention. 
           [0043]      FIG. 4B  is a flow chart depicting a method for maintaining current differential protection by an IED associated with the transfer relay of  FIG. 4A  during a bypass or similar operation for a power line having a plurality of IEDs associated therewith in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0044]    The present invention generally relates to a method and apparatus for customization of an IED. Generally, IEDs are used for protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines. IEDs may include protective devices such as protective relays, or otherwise, RTUs, PLCs, bay controllers, SCADA systems, general computer systems, meters, and any other comparable devices used for protecting, monitoring, controlling, metering and/or automating electric power systems and their associated power lines. 
         [0045]    Although the embodiments described herein are preferably associated with protective devices, such as protective relays including transfer relays, local relays and remote relays, it is contemplated that the embodiments may also be associated with any suitable power system control or protective devices such as those mentioned or described above or below. 
         [0046]      FIG. 2A  illustrates an embodiment of the invention for providing protection during a circuit breaker  110  bypass or similar operation using a transfer relay R c  to simultaneously establish and coordinate a three-terminal protection with a local relay R a  and a remote relay R b , which communicate with the transfer relay R c  on a two-terminal protection mode. 
         [0047]    During primary protection of power line  108 , local current differential protective relay R a  measures the current I 1  situated at one bus  102  via current transformer CT 1  on an associated power line  108 . Another remote protective relay R b  measures the current I 2  situated at another bus  104  via current transformer CT 2  on an associated power line  108 . Current I 2  measured by remote relay R b  is transmitted to local relay R a  via communication link  130 . 
         [0048]    The communication link  130  may be a wired link such as a fiber optic, regular metallic, Ethernet copper wired or a wireless link such as digital radio, RF or microwave communication. Current I 2  may be further communicated on the communication link  130  as time-aligned vector (magnitude and phase angle) quantities. A secured communication may further be achieved by using known encryption technologies such as data encryption standard (DES), triple DES (3DES), advanced encryption standard (AES), Rivest Cipher (RC4). The current I 2  may be further communicated on the communication link  130  as time-aligned vector quantities. 
         [0049]    For purposes of this embodiment, communication among protective devices may be generally achieved by a bidirectional communications means. For example, data streams or communication signals maybe transferred as described in U.S. Pat. No. 5,793,750 for “System for Communicating Output Function Status Indications Between Two or More Power System Protective Relays” and U.S. Pat. No. 6,947,269 for “Relay-to-Relay Direct Communication System in an Electric Power System.” 
         [0050]    During protection of a two-terminal line, the local relay R a  combines the current I 1  that it measures with current I 2  measured and communicated by the remote relay R b . The local relay R a  calculates a vector sum of the currents (Σ(I 1 , I 2 )). Under normal conditions, the resulting vector sum equals about zero amperes. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal about zero amperes. Upon detection of a fault or abnormal condition, local relay R a  sends a trip signal to an associated circuit breaker  110  to isolate the condition. 
         [0051]    The bus arrangement containing circuit breaker  110  is designed such that it may take circuit breaker  110  out of service for maintenance purposes or replacement while leaving its associated power line  108  therewith energized and protected by a transfer relay R c . For example, during a bypass or similar operation, circuit breaker  110  may be isolated. However, unlike traditional bypass arrangements (e.g., as described with respect to  FIG. 1B ), communication to and from local relay R a  is not terminated, but rather rerouted to transfer relay R c . More specifically, protection is established by communication between the transfer relay R c  and each of the local relay R a  and the remote relay R b . 
         [0052]    In an embodiment, in order to initiate a bypass or similar operation, an operator may signal to communication switch  200  to reroute communications. For example, the operator may initiate such via control inputs  202 . More specifically, an operator may signal to communication switch  200  to cease communication between the local relay R a  and the remote relay R b  and, instead, commence communication between the transfer relay R c  with each of the local relay R a  and the remote relay R b . An example of a communications switch that may be used for this application is that described in U.S. Patent Application No. 60/718,365 for a Method and Apparatus for Routing Data Streams Among Intelligent Electronic Devices or the SEL 2126 Fiber Optic Transfer Switch manufactured by Schweitzer Engineering Laboratories, Inc., both of which are incorporated herein in their entirety and for all purposes. 
         [0053]    In this new configuration, transfer relay R c  is configured to receive currents quantities I 1 , I 2  respectively measured by local and remote relays R a , R b . The transfer relay R c  is further adapted to measure transfer current I x . With these values, transfer relay R c  calculates a vector sum of the transfer current I x  and the currents I 1 , I 2  respectively transmitted by local and remote relays R a , R b  [Σ(I 1 , I 2 , I x )]. Under normal conditions, the resulting vector sum equals approximately zero amperes. It shall be noted that this is the case because under normal conditions, I I 2  I generally equals to I Σ(I 1 , I x ) I. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal zero amperes. Upon detection of a fault or abnormal condition, the transfer relay R c  sends a trip signal to an associated circuit breaker  114  to isolate the condition. 
         [0054]      FIG. 2B  illustrates an embodiment of the invention for preserving current differential protection active during a circuit breaker bypass or similar operation using a transfer relay R x  to establish a multiple feed line terminal. The embodiment of  FIG. 2B  differs from the embodiment of  FIG. 2A  in that all of transfer relay R x , local relay R 1 , and remote relay R 2  are current differential relays. 
         [0055]    Like the arrangement of  FIG. 2B , the bus arrangement containing circuit breaker  110  is designed such that it may take circuit breaker  110  out of service for maintenance purposes or replacement while leaving its associated power line  108  therewith energized and protected by a transfer current differential relay R x . For example, during a bypass or similar operation, circuit breaker  110  may be isolated. However, unlike traditional bypass arrangements, communication to and from local current differential relay R 1  is not isolated, but rather rerouted to transfer current differential relay R x  in accordance with an aspect of the present invention. More specifically, current differential protection is maintained by establishing communication between the transfer current differential relay R x  and each of the local current differential relay R 1  and the remote current differential relay R 2 . 
         [0056]    In one embodiment, in order to initiate a bypass or similar operation, an operator may close switch S 5 . The operator further signals to communication switch  200  to reroute communications via control inputs  202 . More specifically, an operator may signal to communication switch  200  to cease communication between the local current differential relay R 1  and the remote current differential relay R 2  and, instead, commence communication between the transfer current differential relay R x  with each of the local current differential relay R 1  and the remote current differential relay R 2 . 
         [0057]    In one embodiment, the control inputs  202  may be optionally controlled by a multiplexer (or MUX)  204 . It is to be noted that the communications switch  200  and the MUX  204  are included to reduce the number of communications channels involved and for automation purposes. In another embodiment (not shown), the communication between the transfer current differential relay R x  with each of the local current differential relay R 1  and the remote current differential relay R 2  may be initiated by directly linking each of the local current differential relay R 1  and the remote current differential relay R 2  to transfer current differential relay R x  without a communications switch or a MUX. 
         [0058]    Referring back to  FIG. 2B , in the depicted configuration, communication between the transfer current differential relay R x  with each of the local current differential relay R 1  and the remote current differential relay R 2  establishes a bypass or similar operation. Transfer current differential relay R x  is configured to receive currents I 1 , I 2  respectively measured by local and remote current differential relays R 1 , R 2 . The transfer current differential relay R x  is further adapted to measure transfer current I x . Transfer current differential relay R x  calculates a vector sum of the transfer current and the currents received from local current differential relay R 1  and remote current differential relay R 1  [Σ(I 1 , I 2 , I x )]. Under normal conditions, the resulting vector sum equals about zero amperes. It shall be noted that this is the case because under normal conditions, I I 2  I generally equals to I Σ(I 1 , I x ) I. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal about zero amperes. 
         [0059]    Simultaneously, transfer current differential relay R x  further calculates a vector sum of the currents Σ(I 2 , I x ) and Σ(I 1 , I x ) and communicates these vector sums back to local current differential relay R 1  and remote current differential relay R 2 , respectively through corresponding communications links  130   b ,  140   b  and  130   a ,  140   a . Local current differential relay R 1  calculates a vector sum of the current measured I 1  and the vector sum Σ(I 2 , I x ) received from transfer current differential relay R x  (Σ(I 1 , I 2 , I x ). Under normal conditions, the resulting vector sum equals about zero amperes. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal about zero amperes. 
         [0060]    Remote current differential relay R 2  calculates a vector sum of the current measured I 2  and the vector sum Σ(I 1 , I x ) received from transfer current differential relay R x  (Σ(I 2 , I 1 , I x ). Under normal conditions, the resulting vector sum equals about zero amperes. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal about zero amperes. Upon detection of a fault or abnormal condition, the associated relay R 1 , R 2 , or R x  sends a trip signal to an associated circuit breaker  110 , or  114  to isolate the condition. In this way, the tripping of circuit breaker  110  and  114  fully isolates a fault associated power line  108  and the parallel power line  106 . It is to be noted that additional relays R n-1 , R n , breakers, and communications links (not shown) may further be added and provided protection in accordance with the teachings above. 
         [0061]    The main advantage of the invention, which is built into the transfer current differential relay (e.g., R x ), is the high system reliability achieved by preserving current differential protection during the entire process of a bypass operation. In addition, when setting up a multiple-terminal line system, the transfer current differential relay (e.g., R x ) in accordance with the teachings of the present invention will not require any connected IEDs to adjust its settings to communicate using any special mode other than the standard two-terminal current differential mode. This is important because the actual implementation requires less communications channels and less commissioning time, because no IED settings are required to be controlled remotely on the local or remote relays (e.g., R 1 , R 2 ). 
         [0062]    In yet another embodiment, communication links  140   a  and  140   b  may be combined into a single communication link. In such an embodiment, a multiplexer (MUX) may replace the communications switch  200  in order to simplify communication traffic from the plurality of communication links  140   a ,  140   b  into a single channel communication link. Examples of MUXs known in the art that may be used herein include the Focus MUX manufactured by Pulsar Technologies, Inc., the Jungle MUX manufactured by General Electric Company, and the IMUX manufactured by RFL Electronics Inc. 
         [0063]      FIG. 3A  illustrates an embodiment of the invention for consummating circuit breaker bypass operation after preserving current differential protection active during a circuit breaker bypass or similar operation using a transfer current differential relay R x  to establish a multiple feed line terminal as illustrated in  FIG. 2B . 
         [0064]    After rerouting the communications among the current differential relays R x , R 1 , R 2  in accordance with the bypass operation as illustrated in  FIG. 2B , the local circuit breaker  110  may be safely isolated by opening switch S 1  and S 2 . Because of the rerouting of communications as discussed with respect to  FIG. 2B , current differential protection is maintained for power line  108  and power line  109 . Also, during normal conditions, the current values I x , I 2  respectively measured by transfer current differential relay R x  and remote current differential relay R 2  are approximately equal upon a successful bypass or similar operation. 
         [0065]    In order to restore local breaker  110  or place local feed line  152  back to service, the process reverses by closing local breaker  110  and the switches S 1 , S 2  associated therewith. 
         [0066]    In order to ensure proper restoration of the local breaker, transfer current differential relay R x  calculates a vector sum of the transfer current and the currents received from local current differential relay R 1  and remote current differential relay R 1  [Σ(I 1 , I 2 , I x )]. Under normal conditions, the resulting vector sum equals about zero amperes. In contrast, if the local breaker is improperly restored or if there is an abnormal condition thereof, the resulting vector sum does not equal about zero 
         [0067]    Simultaneously, transfer current differential relay R x  further calculates a vector sum of the currents Σ(I 2 , I x ) and Σ(I 1 , I x ) and communicates these vector sums back to local current differential relay R 1  and remote current differential relay R 2 , respectively through corresponding communications links  130   b ,  140   b  and  130   a ,  140   a . Local current differential relay R 1  calculates a vector sum of the current measured I 1  and the vector sum Σ(I 2 , I x ) received from transfer current differential relay R x  (Σ(I 1 , I 2 , I x ). Under normal conditions, the resulting vector sum equals about zero amperes. In contrast, if the local breaker is improperly restored or if there is an abnormal condition thereof, the resulting vector sum does not equal about zero Remote current differential relay R 2  calculates a vector sum of the current measured I 2  and the vector sum Σ(I 1 , I x ) received from transfer current differential relay R x  (Σ(I 2 , I 1 , I x ). Under normal conditions, the resulting vector sum equals about zero amperes. In contrast, if the local breaker is improperly restored or if there is an abnormal condition thereof, the resulting vector sum does not equal about zero 
         [0068]    If a fault or abnormal condition is not detected, a restoration operation is initiated to open transfer switch S 5  while closing switches S 1  and S 2  associated with the previously bypassed circuit breaker  110 . In contrast, if a fault or abnormal condition is detected, the associated current differential relay R 1 , R 2 , or R x  will communicate a trip signal to open its associated circuit breakers  110 ,  111 , or  114 . 
         [0069]    The transfer current differential relay R x  may further be adapted to coordinate with communication switch  200  to disconnect communication links between  130   a  and  140   a , and between  130   b  and  140   b . Moreover, transfer current differential relay R x  may further be adapted to re-establish links between communication link  130   a  and  130   b . As such, the transfer bus  106  is freed up to service another local feed line such as local feed line  154  in the system through the communication switch  200  using the system and method of the present invention or any other bypass means. 
         [0070]    As illustrated in  FIG. 3B , in accordance with yet another aspect of the present invention, the transfer current differential relay R x  may further be adapted to communicate with local current differential relay R n-1  and remote current differential relay R n  in order to provide current differential protection during a bypass or similar operation using similar principles as discussed in greater detail with respect to  FIG. 2B and 3A . In yet another embodiment of  FIG. 3B , communication links  140   a ,  140   b ,  140   c  and  140   d  may be combined into a single communication link. In such an embodiment, a multiplexer (MUX) may replace the communications switch  200  in order to simplify communication traffic from the plurality of communication links  140   a ,  140   b ,  140   c ,  140   d  into a single channel communication link. 
         [0071]    As illustrated in  FIG. 3C , in accordance with yet another aspect of the present invention, the transfer current differential relay R x  may further be adapted to include both transfer relay capability with that of a communication switch  200  in a single device  300 . The single device  300  may further optionally include a MUX  204  therein. 
         [0072]    As illustrated in  FIG. 3D , transfer current differential relay R x  may be adapted to preserve current differential protection active during a bypass or similar operation for a plurality of relays R 1 , R 2 , R 3 , R 4 , R n  associated with a power line. The equations of this figure represent the vector sum of measured current at which each current differential relay operates under a normal condition wherein no fault or other abnormal condition exists on the power line. For example, current differential relay R 4  detects a normal condition when I R4 =Σ(I x , I 1 , I 2 , I 3 , . . . I n ), whereas a fault condition or an abnormal condition is detected when I R4 ≠Σ(I x , I 1 , I 2 , I 3 , . . . I n ). Upon detection of a fault, current differential relay R 4  may be adapted to send a trip signal to an associated circuit breaker to isolate the condition. 
         [0073]    In the embodiments of the present invention as illustrated in  FIGS. 2A-3D , the transfer current differential relay R x  is the only affected relay which requires modification of settings contained therein; therefore, this present invention system and method is flexible and may be readily implemented throughout the power system. 
         [0074]    In accordance with an aspect of the present invention,  FIG. 3E  illustrates a block diagram of an IED  300  for preserving current differential protection active for a plurality of current differential relays R 1  to R n  during a bypass or similar operation. This IED  300  may be utilized for transfer current differential relay R x  functionality in the embodiments of the invention as described above. 
         [0075]    In one embodiment, IED  300  measures the transfer current I x  including any or all three phases of the current I xA , I xB , I xC . Simultaneously, IED  300  is adapted to receive input data  352  from a plurality of serial inputs carrying digitized vector current quantities I 1  to I n  (and any or all three phases thereof) measured and communicated by respective current differential relays R 1  to R n  (not shown in this figure). This input data may be transmitted over a plurality of communications links (e.g., if connected directly to the relays or a communications switch) or a single communication link (e.g., if connected to a MUX). 
         [0076]    The measured analog transfer current vector quantities I xA , I xB , I xC  may be filtered using low pass filters  312 ,  314   316 ; optionally multiplexed through MUX  322 ; and digitized through an analog to digital (A/D) converter  324 . The resulting digitized current values may further be respectively filtered through digital band pass filters  326 ,  328 ,  330  to further reduce noise. 
         [0077]    A micro-controller  336  is provided to calculate a vector sum of the transfer current quantities I xA , I xB , I xC  and the measured currents I 1 , I 2 , I 3  . . . I n  received from current differential relays [Σ(I 1 , I 2 , I 3  . . . I x )]. Under normal conditions, the resulting vector sum equals about zero; therefore, an optional no trip signal is communicated at communication port  346  which may be connected to an associated circuit breaker. In contrast, if a fault or abnormal condition is detected, the resulting vector sum does not equal about zero; therefore, a trip signal is communicated at communication port  344  which may be connected to an associated circuit breaker. 
         [0078]    Simultaneously, transfer current differential relay R x  further calculates a vector sum of the currents for each current differential relay, wherein I R1 =Σ(I x , I 2 , I 3 , . . . I n ); I R2 =Σ(I x , I 1 , I 3 , . . . I n ) and I R3 =Σ(I x , I 1 , I 2 , . . . I n ). The transfer current differential relay R x  is adapted to transmit these values to corresponding current differential relays via communication ports  338 ,  340 ,  342 . Each current differential relay determines whether a normal or an abnormal condition exists on the power line. For example, a normal condition is detected when I R3 =Σ(I x , I 1 , I 2 , . . . I n ), whereas a fault condition or an abnormal condition is detected when I R3 ≠Σ(I x , I 1 , I 2 , . . . I n ). As discussed above, upon detection of a fault, current differential relay R 3  may be adapted to send a trip signal to an associated circuit breaker to isolate the condition. 
         [0079]    In an alternate embodiment of IED  300 , the input data  352  may interface with a field programmable gate array (FPGA)  350  or an equivalent programmable logic device. The FPGA may be adapted to provide a data interface which includes DBPF  326 ,  328 ,  330  and micro-controller  336 . As the system becomes more complex, one or more FPGAs with multiple microcontrollers may be included to perform other specific protection, monitoring, controlling, metering and/or automating functions. 
         [0080]    As illustrated in  FIG. 3F , in yet another embodiment of IED  300 , the communication ports  338 ,  340 ,  342 ,  344 , and  346  of  FIG. 3E  may be replaced with a single communications link  358 . An example of a suitable communication link is a network communication link  358  such as an Ethernet wide area network (not shown). The communication link  358  may be adapted such that multiple data frames may be sent and received through the same link  358 . The IEC 61850 standard communication protocol is an example of a suitable protocol for fast communications between IEDS. In the embodiment of  FIG. 3F , the communication link  358  may further be adapted to communicate the digitized current vector quantities I 1 , I 2 , I 3  . . . I n  measured and transmitted by their respective relays. 
         [0081]    In accordance with an aspect of the present invention,  FIG. 4A  illustrates a method wherein a transfer current differential relay R x  communicates with each of the plurality of current differential relays R 1 , R 2  . . . R n  in order to preserve current differential protection of an associated power line. In step  450 , a transfer current differential relay R x  receives current vector quantities I 1 , I 2 . . . I n  from associated relays through a suitable communication link(s). Preferably, the current vector quantities transmitted to the transfer current differential relay R x  are time-aligned in order to maintain power system synchronization. Concurrently, the transfer current differential relay R x  measures its local current vector quantity I x  through its current transformer. It is to be noted that the transfer current differential relay R x  may be adapted to measure any or all three phases of the current I x . 
         [0082]    In step  452 , the transfer current differential relay R x  is adapted to calculate the vector sum of calculate a vector sum of the transfer current value I x  and the measured currents I 1 , I 2 , . . . I n  received from current differential relays [Σ(I 1 , I 2 , . . . I x )]. Simultaneously, transfer current differential relay R x  further calculates a vector sum of the currents for each current differential relay, wherein I R1 =Σ(I x , I 2 , . . . I n ); I R2 =Σ(I x , I 1 . . . I n ) and I Rn =Σ(I x , I 1 , I 2 , . . . I n-1 ). The transfer current differential relay R x  is adapted to transmit these values to corresponding relays via a suitable communication port Com  1 , Com  2  . . . Com n, respectively. 
         [0083]    In step  454 , the transfer current differential relay R x  determines whether the vector sum of transfer current value I x  and the measured currents I 1 , I 2 , . . . I n  received from relays [Σ(I 1 , I 2  . . . I x )] equals about zero amperes. Under normal conditions, the resulting vector sum equals about zero amperes. Therefore, in such cases, the method is reestablished in order to monitor fault conditions on the associated line. In contrast, if a fault or abnormal condition is detected, the resulting vector sum does not equal about zero; therefore, a trip command is communicated as shown at  456  which may be communicated to an associated circuit breaker. 
         [0084]    In accordance with yet another aspect of the present invention,  FIG. 4B  illustrates a method wherein a current differential relay R 1  associated with the transfer current differential relay R x  of  FIG. 4A  communicates with such in order to preserve current differential protection of an associated power line. In step  460 , current differential relay R 1  measures its local current vector quantity I 1  through its current transformer. Concurrently, this current vector quantity I 1  is transmitted to transfer current differential relay R x . 
         [0085]    In step  462 , current differential relay R 1  receives the vector sum of currents I R1  transmitted from transfer current differential relay R x  (I R1 =Σ(I x , I 2 , . . . I n )), the calculation of which is explained in detail above with respect to  FIG. 4A . 
         [0086]    In step  464 , the transfer current differential relay R x  determines whether the vector sum of the measured current value I 1  and vector sum of currents I R1  equals about zero amperes. Under normal conditions, the resulting vector sum equals about zero amperes. Therefore, in such cases, the method is reestablished in order to monitor fault conditions on the associated line. In contrast, if a fault or abnormal condition is detected, the resulting vector sum does not equal about zero; therefore, a trip command is communicated as shown at  466  which may be communicated to an associated circuit breaker. 
         [0087]    While this invention has been described with reference to certain illustrative aspects, it will be understood that this description shall not be construed in a limiting sense. Rather, various changes and modifications can be made to the illustrative embodiments without departing from the true spirit, central characteristics and scope of the invention, including those combinations of features that are individually disclosed or claimed herein. Furthermore, it will be appreciated that any such changes and modifications will be recognized by those skilled in the art as an equivalent to one or more elements of the following claims, and shall be covered by such claims to the fullest extent permitted by law.