Patent Publication Number: US-8111492-B2

Title: Apparatus, system, and method for creating one or more slow-speed communications channels utilizing a real-time communication channel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/540,253, filed 29 Sep. 2006 now U.S. Pat. No. 7,701,836, which is a continuation-in-part of U.S. patent application Ser. No. 11/211,816, filed Aug. 25, 2005, and now U.S. Pat. No. 7,463,467, which is itself a continuation-in-part of U.S. patent application Ser. No. 09/900,098, filed Jul. 6, 2001, and now U.S. Pat. No. 6,947,269. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to apparatus, systems, and methods for maximizing the use of communications resources within a power protection device, and more particularly to the creation of one or more virtual communications channels from a small amount of data within a real time data stream. 
     DESCRIPTION OF THE PRIOR ART 
     In U.S. Pat. No. 5,793,750, the contents of which are hereby incorporated by reference, a communication system between two microprocessor-based protective relays for an electric power system is disclosed. Each of the two relays in that system has both transmit and receive modules, for directly transmitting indication status bits indicative of the result of selected protective functions of one relay from that one relay to the other, and vice versa. 
     The output status indication bits are sometimes used to identify the existence and location of a fault on the power line portion served by the two relays. One or both of the relays might initiate a circuit breaker trip action on the basis of the exchange of such information. The output status indication bits may be the result of processing functions in one of the relays involving the voltages and/or currents on the power line monitored by that relay. The output status indication bits may be used for various control, status, indication and protection functions. Examples of protection functions include permissive overreaching transfer trip (POTT) actions, permissive under-reaching transfer trip (PUTT) actions, directional comparison unblocking (DCUB) and direct transfer trip (DTT) actions. Other relay-to-relay operations are possible using particular output status indication bits. 
     The advantage of the communication system described in the &#39;750 patent is that it is fast and secure. Protective relays typically accomplish their monitoring functions several times each power system cycle. The communication system described in the &#39;750 patent provides the results of these monitoring functions of one relay, to the other relay. The information is transmitted directly over a communications link from an originating relay which may or may not trip its associated circuit breaker based on its operational results, to another relay. The receiving relay then uses the transmitted information, in the form of digital bits, to perform its own on-going calculations, producing various protection actions such as tripping and closing a circuit breaker when appropriate. The communication between the two relays may be bidirectional, allowing the two relays to exchange information concerning the results of their own calculations both quickly and securely. 
     Communication systems in power protection devices often rely on low-bandwidth connections, such as 56 kbps or 64 kbps serial connections. However, these connections are often used to communicate real time data, such as, for example, direct transfer tripping, i.e., tripping a local circuit breaker after receiving a command from a remote protection device, permission-to-trip communications a permissive direction comparison protection scheme, and blocking of tripping in a blocking directional comparison scheme. As these communications are time-critical, such schemes generally demand the entire bandwidth of a low-bandwidth connection so that the data can be communicated as needed, and with a sufficient guarantee of integrity. 
     Another communication application utilized by power protection devices is known as “Line Current Differential,” or 87L function. An 87L function communicates local currents as samples or phasors, restraining terms derived from local currents, on/off bits associated with various functions such as, for example, indication of time synchronization to an external device, detection of external faults, test in progress, charging current compensation in progress, and other quantities. In addition, user configurable bits and timing information, such as, for example, time stamps, sequence numbers, and message hold up time, may be communicated as well to facilitate synchronization of data between two devices utilizing the communications channel. As an 87L application is time critical, it also often utilizes the entire communications bandwidth. 
     While the above functions are critical to the operation of power protection devices, it would be beneficial to also support and utilize other communications between the said devices without the installation of a separate physical communications channel. 
     SUMMARY OF THE INVENTION 
     The disclosed invention achieves its objectives through the use of an intelligent electronic device providing a real-time communications system as well as one or more virtual communications channels. 
     In one embodiment of the invention, an intelligent electronic device includes a processor and a transmit module coupled to a slow-speed serial communications link. The processor causes the transmit module to transmit a real-time data channel consuming nearly all of the bandwidth available on the communications link. However, the processor further causes the transmit module to inject a background session into one or more unused bits of the real-time data channel, thereby creating one or more virtual communications channels. 
     Further refinements of the disclosed invention describe different uses of the communications channel. For example, a multi-ended fault location service can be realized by sharing voltage data and/or calculated system impedances. Further, fault records can be transmitted between the devices without interrupting normal operation of the intelligent electronic device. Similarly, device diagnostics data can be transmitted as well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Although the characteristic features of this invention will be particularly pointed out in the claims, the invention itself, and the manner in which it can be made and used, can be better understood by referring to the following description taken in connection with the accompanying drawings forming a part hereof, wherein like reference numerals refer to like parts throughout the several views and in which: 
         FIG. 1  is a simplified single line schematic diagram of a typical wide area power system. 
         FIG. 2  is a simplified block diagram of a relay-to-relay direct communication system within the power system of  FIG. 1  constructed in accordance with an embodiment of the invention. 
         FIG. 3  is an exemplary-received frame of the relay-to-relay direct communication system of  FIG. 2 . 
         FIG. 4  is a simplified functional block diagram of a system constructed in accordance with an embodiment of the invention wherein a primary power protection device controls the functioning of a backup power protection device. 
         FIG. 5  is a simplified block diagram of a relay-to-relay direct communication system for use in the power system of  FIG. 1 , constructed in accordance with an embodiment of the invention. 
         FIG. 6  is an exemplary-received frame of the relay-to-relay direct communication system of  FIG. 4 . 
         FIG. 7  is a simplified block diagram of a relay-to-relay direct communication system within the power system of  FIG. 1  constructed in accordance with an alternate embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     As indicated above, the present invention is based on and is an improvement of the communication system of U.S. Pat. No. 5,793,750, which includes a direct communication link between two protective relays serving an electric power apparatus, the system supporting a communication arrangement or protocol involving eight data channels for exchange of output status indication bits between the two relays both quickly and securely. The channel data bits TMB 1 -TMB 8  identify eight transmit bits, on eight data channels. 
     Those bits, when received by the other relay, are identified as received channel data bits RMB 1 -RMB 8 , wherein RMB 1 -RMB 8  are the “mirror” or replica of the transmit channel data bits. The eight data channels can accommodate at least eight output status indication bits. As indicated above, however, in many two-relay arrangements, only two or perhaps three channels are necessary to communicate the output status indication bits. Utilizing the present invention, the otherwise vacant channel space can now be used by selected additional data (discussed below) and an associated synchronization channel to synchronize the additional data. 
     The additional data can be digitized analog quantities, such as metering data, or can be “virtual terminal” data. In a virtual terminal implementation, a human user or another application utilizes the direct communication link to communicate with the other relay. For example, the human user could utilize the direct communications link to control or query the other relay. An application such as, for example, an integration protocol like as DNP 3 , could also utilize the communications link in the virtual terminal implementation. 
       FIG. 1  is a simplified single line schematic diagram of a typical wide area power system  10 . As illustrated in  FIG. 1 , the power system  10  includes, among other things, two generators  12  each configured to generate three-phase sinusoidal waveforms, for example, three-phase 12 kV sinusoidal waveforms, two step-up power transformers  14  configured to increase the 12 kV sinusoidal waveforms to a higher voltage, such as 138 kV, and a number of circuit breakers  18 . The step-up power transformers  14  provide the higher voltage sinusoidal waveforms to a number of long distance transmission lines such as the transmission lines  20 . In one embodiment, a first substation  16  may be defined to include the generators  12 , the step-up transformers  14  and the circuit breakers  18 , all interconnected via a first bus  19 . At the end of the long distance transmission lines  20 , a second substation  22  may include step-down power transformers  24  to transform the higher voltage sinusoidal waveforms to lower voltage sinusoidal waveforms (e.g., 15 kV) suitable for distribution via a distribution line to various end users  26  and loads  30 . 
     As previously mentioned, the power system  10  includes protective devices and procedures to protect the power system elements from faults or other abnormal conditions The protective devices and procedures utilize a variety of protective logic schemes to determine whether a fault or other problem exists in the power system. For example, some types of protective relays utilize a current differential comparison to determine whether a fault exists in the protection zone. Other types of protective relays compare the magnitudes of calculated phasors, representative of the power system sinusoidal waveforms, to determine whether a fault exists in the protection zone. Frequency sensing techniques and harmonic content detection is also incorporated in protective relays to detect fault conditions. Similarly, thermal model schemes are utilized by protective relays to determine whether a thermal problem exists in the protection zone. 
     Referring again to  FIG. 1 , also included are a first and a second protective relay  100  and  102  adapted to provide for example, overcurrent protection for the transmission line  21 . As described below, the first and second protective relays  100 ,  102  are also adapted to communicate via a communication link  34  that can be configured using one of a number of suitable media. Additional protective relays such as a protective relay  104 , adapted to communicate with the first protective relay  100  and/or the second protective relay  102 , may also be included in the power system  10 . 
       FIG. 2  is a simplified block diagram of a relay-to-relay direct communication system  40  incorporated in the power system  10 . Although illustrated using the first and second protective relays  100 ,  102 , it should be understood that the communication system  40  can include additional protective relays operatively coupled to the first and/or second relay  100 ,  102  and adapted to operate as described below. Further, although illustrated using the first and second protective relays  100 ,  102 , it should be understood that the apparatus and method described herein is applicable to communication between any intelligent electronic device (IED) of the power system  10 . 
     For ease of discussion, the first protective relay  100  is shown as the transmitting relay and includes, inter alia, a “transmit” module  41 , having a microcontroller  42  operatively coupled to a receive and transmit interface means; in this example, a universal asynchronous receiver/transmitter (UART)  43 . The (transmitting) UART  43  is configured to convert bytes of channel data bits (corresponding to the channel data) resulting from operation of the first protective relay  100  into a single serial message stream for outbound transmission via the communication link  34  to the second protective relay  102 , and to convert an inbound serial message stream (from the second protective relay  102 ) into bytes of channel data suitable for use by the first protective relay  100 . 
     Similarly, the second protective relay  102  is shown as the receiving relay and includes, inter alia, a “receive” module  44  having a second microcontroller  45  operatively coupled to another UART  46 , operational and configured as described above. Although not separately illustrated, each of the first and second protective relays  100 ,  102  include both transmit and receive capability to enable communication. While illustrated as transmit and receive modules  41 ,  44 , in a simplified functional block diagram format, the relay-to-relay direct communication system and method described herein may be implemented by means of a microprocessor or field programmable gate array (FPGA) executing a computer program, protection algorithm or relay logic scheme. Further, although illustrated as a UART  43  operatively coupled to the first microcontroller  42 , and a UART  46  operatively coupled to the second microcontroller  45 , one of any suitable transmit and receive interface means may be utilized to convert bytes of channel data bits into a serial message stream for transmission via the communication link  34 . 
     The transmit module  41  and the receive module  44  are operatively connected via the communication link  34 . As noted above, the communication link  34  may be implemented as an RF link, a microwave link, an audio link, a fiber optic link, or another other type of suitable media adapted to carry serial data. As illustrated, in addition to output status indication bits, each of the transmit and receive modules  41 ,  44  is capable of transmitting/receiving other types of channel data in the form of serial messages. For example, the channel data may include digitized analog values, derived from analog quantities, that require more than a single bit such as metering information, breaker failure system security enhancement information, reclose enable information, instrument transformer checking and multi-terminal fault location information, to name a few. 
     Referring to the transmit module  41 , an eight data channel arrangement is configured such that two data channels, a data channel  47  and a data channel  48 , correspond to the conventional output status indication bits  57  transmitted as channel data bit TMB 1  and TMB 2 , respectively, from the transmit module  41  of the first protective relay  100  to the receive module  44  of the second protective relay  102 . Three data channels, a data channel  49 , a data channel  50  and a data channel  51 , are dedicated to digitized analog values  59 ,  60  and  61  transmitted as channel data bits TMB 3 , TMB 4  and TMB 5 , respectively, from the transmit module  41  of the first protective relay  100  to the receive module  44  of the second protective relay  102 . Each of the digitized analog values  59 ,  60 ,  61  are formed by, for example, converting a 32-bit floating point number representing an analog quantity (e.g., system impedances, currents, voltages) into an 18-bit floating point number. The 18-bit floating point number is then serialized such that one bit from each of the digitized analog values  59 ,  60 ,  61  is included as channel data bits TMB 3 , TMB 4  and TMB 5 , respectively, in sequential transmitted messages until all of the bits associated with the digitized analog values  59 ,  60 ,  61  are transmitted. For example, if each of the digitized analog values  59 ,  60 ,  61  is expressed in 18 bits, eighteen sequential serial messages are transmitted where the first serial message includes the first bit of the digitized analog value  59  transmitted as channel data bit TMB 3 , the first bit of the digitized analog value  60  transmitted as channel data bit TMB 4 , and the first bit of the digitized analog value  61  transmitted as channel data bit TMB 5 . Similarly, the second serial message includes the second bit of the digitized analog value  59  transmitted as channel data bit TMB 3 , the second bit of the digitized analog value  60  transmitted as channel data bit TMB 4 , and the second bit of the digitized analog value  61  transmitted as channel data bit TMB 5 , and so on. 
     It should be noted that while compromising some precision, the conversion scheme that converts a 32-bit floating point number (representing the analog quantity) into a corresponding 18-bit floating point number, enables quicker transmission to the second protective relay  102 . It should also be noted that other conversion schemes may be utilized depending on the analog quantity measured, the precision required, and the speed of transmission desired. 
     Two additional data channels, a data channel  52  and a data channel  53  facilitate virtual terminal data transmitted as channel data bits TMB 6  and TMB 7 , respectively, from the transmit module  41  of the first protective relay  100  to the receive module  44  of the second protective relay  102 . As noted above, virtual terminal data refers to data provided by a user located at a local relay (e.g., the first relay  100 ), to a remote relay (e.g., the second relay  102 ) via the communication link  34 . In such a configuration, the local relay operates as a virtual terminal to allow the user to query and/or control the remote relay with the familiar serial port user interface passing data on otherwise unused channels. The virtual terminal scheme also adds fast meter/operate capability. Like the digitized analog values described above, the virtual terminal data is serialized bit-by-bit such that, for example, 18-bit virtual terminal data is transmitted bit-by-bit in 18 sequential serial messages where the first two bits are payload flags and the last sixteen bits are two 8-bit data bytes. For example, the 18-bit virtual terminal data may be expressed as: p 1 p 2 d 16 d 15 d 14 d 13 d 12 d 11 d 10 d 9 d 8 d 7 d 6 d 5 d 4 d 3 d 1  where p 1 =1 indicates that d 1 -d 8  is a payload byte, and p 2 =1 indicates that d 9 -d 16  is a payload byte (see,  FIG. 3 ). 
     The eighth data channel  54  is dedicated to synchronization information transmitted as channel data bit TMB 8  from the transmit module  41  of the first protective relay  100  to the receive module  44  of the second protective relay  102 . The synchronization information enables synchronization of the data channels associated with the analog values  59 ,  60 ,  61  and the virtual terminal data  62 . Thus, when any of the data channels  47 - 53  are used for anything other than the output status indication bits, a dedicated synchronous channel is allocated for synchronization information transmitted as channel data bit TMB 8 . 
     Although illustrated utilizing an eight data channel arrangement, it should be understood that a different number or arrangement and/or assignment of data channels can be used by the first and second protective relays  100 ,  102  of the communication system  40 . Accordingly, the two data channels of output status indication bits in combination with the three data channels of analog values and the two data channels of virtual terminal data illustrated in  FIG. 2  is arbitrary. The output status indication bits could occupy more or less or no data channels, the analog values could occupy more or less or no data channels, and the virtual terminal data could occupy more or less or no data channels. In addition, one analog value can occupy more than one data channel for speedier transmission. Similarly, virtual terminal data can occupy more than one data channel for speedier transmission. 
     Further, in one embodiment of the invention, the arrangement and/or assignment of the data channels may be fixed, while in another embodiment, the arrangement and/or assignment of the data channels may be dynamically changed during relay operation, depending on the desired configuration of the protective relay(s)  100 ,  102 . As a result, speed of receipt of the channel data by the receive module  44  is adjustable based on the assignment of the channel data to the number of data channels. 
     For example, if 18-bit virtual terminal data is dynamically assigned to one data channel during a high activity period of relay operation, it is transmitted bit-by-bit in 18 sequential serial messages, and then reassembled for use by the receiving relay. If one message is transmitted every 1 millisecond via the communication link  34 , 18 milliseconds are required for receipt of the entire 18-bit virtual terminal data. In contrast, if the same 18-bit virtual terminal data is dynamically assigned to three data channels during a lower activity period of relay operation, it is transmitted in groups of 3 bits in 6 sequential serial messages, requiring six milliseconds. 
     Prior to transmission, each of the eight channel data bits TMB 1 -TMB 8  is encoded by an encoder  65  to form an encoded message  66  using one of any number of suitable techniques. The encoded message  66  may therefore have one of any number of suitable formats, depending on the encoding scheme selected. For example, in one encoding scheme, the encoded message  66  may include 36 or 40 bits, divided into four 9-bit (for 36 bit length) or 10-bit (for 40 bit length) characters plus a number of idle bits. The number of idle bits may vary depending upon the selected transmission speed. 
     Continuing with the example, the bits may be assembled such that the first 9-10 bit character includes a single start bit followed by the six channel data bits TMB 1 -TMB 6 , followed by an odd parity bit and one or two stop bits, as selected by the user. The second character may include a second single start bit, followed by the six channel data bits TMB 5 , TMB 6 , TMB 7 , TMB 8 , TMB 1  and TMB 2 , followed by an odd parity bit and one or two stop bits. The third character may include a start bit followed by the six channel data bits TMB 7 , TMB 8 , TMB 1 , TMB 2 , TMB 3  and TMB 4 , followed by an odd parity bit and one or two stop bits. The fourth and final character in the message may include a single start bit followed by the six channel data bits TMB 3 -TMB 8 , followed by an odd parity bit and one or two stop bits. The remaining bits, if any, are a variable number of idle bits, depending upon transmission speed of the data. 
     Using such an encoding scheme, each of the channel data bits TMB 1 -TMB 8  are repeated three times in the four character portions of one encoded message  66  with single stop and parity bits and one or two stop bits inserted between each character portion of the encoded message  66 . This encoding scheme allows the receiving, or second protective relay  102 , to check for errors that may have occurred during transmission. 
     In addition to assembling the bits into messages, each of the first and second protective relays  100 ,  102  may be adapted to further encode and decode using an identifier pattern selected during system configuration. For example, if preprogrammed to include one particular identifier pattern, the transmit encoder  65  logically inverts one of the four characters in each of the messages as a means of encoding the identifier pattern into the message. As described below, the receiving, or second, relay  102  then ensures that the received message has been encoded with the correct identifier pattern. Although described as assembling messages where one character is logically inverted, it should be understood that other suitable formats and encoding schemes may be utilized by the encoder  65  to generate the encoded message  66 . 
     The encoded message  66  is then applied to the UART  43 , adapted to satisfy several operating parameters for the system. In general, the UART  43  converts the encoded message  66  into a serial message  67  for transmission as part of a serial message stream via the communication link  34 . Accordingly, the receiving UART  46  must also be capable of checking the received serial message  67  for proper framing (the presence of one stop bit per byte) and proper parity, and detecting overrun errors. 
     The UART  43  can be programmed for various baud rates. For example, it can be programmed for baud rates ranging from 300 through 115,000. The UART  43  is additionally adapted to synchronize both transmit and receive serial messages using transmit and receive clocks externally supplied. As will be appreciated by one skilled in the art, the method of bit synchronization, using start and stop bits or using synchronizing clocks, is one of any number of suitable methods for synchronization. 
     Subsequent to being prepared for transmission by the UART  43 , the serial message  67  is transmitted over the communication link  34  to the receive-module  44 . The sampling and transmission rates can be varied depending on the desired operation of the transmitting relay. 
     Referring now to the receive module  44 , the receiving UART  46  provides the counterpart functions of the transmitting UART  43 . When the serial message  67  is received by the receive module  44 , the UART  46  performs several data checks on each character of the serial message  67 . It also checks each character of the serial messages  67  for proper framing, parity and overrun errors. 
     From UART  46 , the characters of the serial message  67  are passed to a decoder  68 . In general, the decoder  68  reassembles groups of four characters in order to reconstruct the four character message. Next, the decoder  68  checks each message for errors, and also examines the results of the UART checks described above. If any of the checks fail, the decoder  68  discards the message and de-asserts a DOK (data OK) flag  94  for that message in a register  95  (see,  FIG. 3 ). 
     More specifically, in the illustrated example, the decoder  68  ensures that there are the three copies of the eight channel data bits TMB 1 -TMB 8  included in the transmitted four-character encoded message  66 . If an identifier pattern was used to encoder the encoded message  66 , the decoder  68  also checks to ensure that the encoded message  66  includes the identifier pattern. It should be noted that the encoding/decoding scheme described above is one of any number of suitable encoding/decoding schemes to enable error detection that may be utilized in the method and apparatus of the invention. 
     As a result of operation of the decoder  68 , the DOK flag  94  and the channel data bits RMB 1 -RMB 8  are provided. The received channel data bits RMB 1 -RMB 8  are the mirror or replica of transmitted channel data bits TMB 1 -TMB 8 . The data OK (DOK) flag  94  provides an indication of whether errors were detected in the received message. 
     Like the transmit module  41  of the first relay  102 , the receive module  44  of the second relay  102  includes an eight data channel arrangement where two data channels are dedicated to the output status indication bits, three data channels are dedicated to three digitized analog values, two data channels are dedicated to virtual terminal data and one data channel is dedicated to synchronization information. Accordingly, the output status indication bits  57  are received as channel data bits RMB 1  and RMB 2  via data channels  70  and  71 , respectively, and are applied to one or more security counters  69 . The security counters  69  operate to ensure that the state of the received channel data bits RMB 1  and RMB 2  remain constant for a pre-selected number of received serial messages  67  before the output status indication bits are utilized by downstream processes. Ensuring that the state of the output status indication bits remain constant increases the reliability and security associated with the output status indication bits  57 . 
     Because the two channel data bits RMB 1  and RMB 2  are transmitted bit by bit, no synchronization of those bits is required. The channel data bits RMB 1  and RMB 2  are used by the second relay  102  to make determinations concerning operation of the power system  10  (as detected by the first protective relay  100 ) including possible circuit breaker trip action when appropriate. In the illustrated example, the digitized analog values  59 ,  60  and  61  are received as channel data bits RMB 3 , RMB 4 , and RMB 5  via a data channel  72 , a channel  73  and a channel  74 , respectively. Each of the three digitized analog values  59 ,  60 ,  61  are received serially one bit per message per data channel, and are then parallelized in a parallelize element  78 . The parallelize element  78  re-assembles each of the three digitized analog values from received successive decoded messages  58 . As noted above, in the illustrated example, each of the digitized analog values  59 ,  60 ,  61  includes eighteen bits. In an embodiment, sixteen bits are used for information while the remaining two bits are unused. Therefore, for every 18 messages, a complete original analog value is received on each corresponding data channel. 
     Similarly, the virtual terminal data  62  is received as channel data bits RMB 6  and RMB 7  via data channels  75  and  76 , respectively. Like the analog values  59 ,  60 ,  61 , the virtual terminal data  62  is received serially one bit per message per data channel, and is also parallelized in the parallelize element  78 . In the illustrated embodiment, the virtual terminal data  62  includes eighteen bits. Sixteen bits of the eighteen bits are utilized for virtual terminal data, where the sixteen bits are divided into two eight-bit bytes. The two remaining bits are used to indicate which of the two eight-bit byte fields actually contain virtual terminal data, and which, if any, are idle, (e.g., waiting for user input). Thus, for every 18 decoded messages  58 , two virtual terminal bytes are received on each corresponding data channel  75 ,  76 . After parallelization via the parallelize element  78 , the analog values and the virtual terminal data are provided to the second protective relay  102 . 
     Again, the particular arrangement of the eight data channel bits TMB 1 -TMB 8  is established in accordance with the user&#39;s communication requirements. Different numbers of output status indication bits, analog values and virtual terminal data can be utilized to form seven bits of the eight channel data bits TMB 1 -TMB 8 . 
     A data channel  77 , or synchronization channel, is dedicated to the remaining channel data bit, RMB 8 . The channel data bits RMB 8  of the synchronization channel enable the receiving decoder  68  and parallelize element  78  to find the start and stop boundaries serial messages that include the digitized analog values and virtual terminal data. The synchronization channel is necessary when any of the other channel data bits include the digitized analog values or the virtual terminal data. If all of the channel data bits are used for output status indication bits only, no synchronization is necessary and the data channel  77  may be used for output status indication bits. 
     In order to determine that a complete (four character) bit message has been received, the second relay  102  identifies the first byte of each of the bit messages via message synchronization. In one embodiment, message synchronization is maintained by counting modulo  4  from the first received byte after byte synchronization is achieved. Accordingly, each time the counter rolls over, the first byte is received. 
       FIG. 3  illustrates an exemplary received frame  80  of the relay-to-relay direct communication system  40 , according to an embodiment of the invention. As illustrated, the received frame  80  includes 18 messages where a series of the “bottom” channel data bit (TMB 8 ) provides the 18-bit synchronization information after encoding, transmission and decoding. In addition, the analog values and virtual terminal data are received as channel data bits RMB 3 -RMB 7  via data channels  72 - 76 . 
     Referring to the data channel  77 , or the synchronization channel, a special frame synchronization pattern, for example 000001, is utilized to indicate that all other data channels (e.g., data channels  70 - 76 ) are at the beginning of a frame. In the illustrated example, when the last six bits received on the synchronization channel are 000001 (the 1 being most recent), then the other data channels are determined to be at a frame boundary. For example, the synchronization channel may be expressed as d 8 d 7 d 6 d 5 vd 4 d 3 d 2 d 1 1pt000001 where, d x =virtual terminal data, 1=binary one, 0=binary zero, p=1 indicates that the virtual terminal data is valid, v is a virtual terminal flag byte; it is normally 1, but is set to 0 to indicate a special flag byte is in the virtual terminal data, and t=time sync bit. 
     A comparator  91  in  FIG. 3  is adapted to enable detection of the special frame synchronization pattern in the six most recently received channel data bits (from the six most recently received messages). Upon detecting the special frame synchronization pattern via operation of the comparator  91 , a modulo  18  counter  92  is interrogated. If the modulo  18  counter  92  is not zero, it is reset to zero and the data on the synchronization, virtual terminal data and analog value channels (i.e., channels  72 - 77 ) since the last valid frame sync (FS) signal  97  is discarded. Therefore, if the modulo  18  counter  92  is at zero, if all of the 18 most recent data OK (DOK) flags in register  95  are valid (e.g., a binary 1 value) and if the comparator  91  is asserted indicating detection of the special frame synchronization pattern, then an AND-gate  96  asserts the FS signal  97 , resulting in the analog values and virtual terminal data being utilized by the receiving, or second relay  102 . 
     The synchronization channel, dedicated to the channel data bit RMB 8 , includes an additional virtual terminal character separated into two four-bit segments  80  and  82 . Further, a bit  84  has a binary 1 value if the additional virtual terminal character contains valid data, and has binary 0 value if the additional virtual terminal character is idle (such as might be the case if the virtual terminal session is waiting for input from the user). A bit  85  of the synchronization channel  77  has a binary 1 value, and a bit  86  typically has a binary 1 value, except under special conditions described below. When both of the bits  84  and  85  have a binary 1 value, five consecutive zeros in the synchronization channel are not possible. This ensures that the frame synchronization pattern 000001 detected by comparator  91  can only occur at frame boundaries. 
     The additional terminal character contained in half-bytes  80  and  82  can also include control characters, intended to indicate from one relay (transmitting) to the other (receiving) when virtual terminal communication should be established, terminated, paused, etc. When one of these control characters is included in the additional virtual terminal character, bit  86  is forced to a binary 0 value. The special control characters are chosen carefully by the system designer such that, even with bit  86  at the binary 0 value, the frame synchronization pattern 000001 can only occur at a frame boundary. 
     In addition, a bit T  98  in the synchronization channel comprises a separate serial data stream, transmitted at the rate of one bit per 18 messages (frame). This separate serial data stream contains date and time information. Each time the FS signal  97  asserts, a time synchronization device  88  accepts the bit T  98 . An additional frame synchronization system, similar to the frame synchronization system described above, allows the time synchronization device  88  to recognize the boundaries between successive time synchronization messages. Namely, a specific frame synchronization pattern is placed in the serial data stream formed by the bit t  98  (i.e., a bit t serial data stream). A comparator detects the specific frame synchronization pattern, and signals that the time-of-day and calendar day information, contained in the bit T serial data stream may be used. The data included in the bit T serial data stream is formatted such that the frame synchronization pattern can only occur at frame boundaries. The time synchronization device  88  then updates the time-of-day clock and the calendar day with the time-of-day and calendar day information contained in the bit T serial data stream. 
     Unlike typical (analog) control inputs of protective relays, the relay-to-relay direct communication system disclosed herein includes communication link monitoring capability via detection of corrupted serial messages when they occur. That is, when a corrupted serial message is received by the receive module  44 , it may be concluded by the receive module that the corrupted serial message is the result of faulty operation or degradation of the communication link  34  and/or associated transmission equipment. Suitable alarming may be utilized to notify the user of the condition where the communication link  34  and/or associated equipment remains faulty for a predetermined duration. 
     The relay-to-relay direct communication system disclosed herein also includes communication link monitoring via detection of missing serial messages. Because, the serial messages  67  are transmitted via the communication link  34  at pre-determined periodic intervals, or at a predictable rate, it can be concluded by the receive module that the missing serial message(s)  67  is/(are) the result of faulty operation or degradation of the communication link  34  and/or associated transmission equipment. For example, if the transmit module  41  is transmitting 250 serial messages every second (a rate of one message every 4 milliseconds), and the receive module  44  does not receive a serial message in an 8 millisecond period (4 ms normal rate plus a 4 ms margin), a problem with the communication link and/or associated equipment may be concluded. In both-instances, the DOK flag  94  indicates the problem with the communication link  34  and/or associated equipment, and the received analog values and/or virtual terminal data is not utilized by the receiving relay (see,  FIG. 3 ). 
     The relay-to-relay direct communication system disclosed herein further includes an ability to determine communication link availability, or channel availability, defined as that portion of time the communication link  34  and/or associated equipment is capable of properly delivering uncorrupted serial messages  67 . Communication link availability may be calculated by dividing the aggregate number of all of the received uncorrupted serial messages by the total expected serial messages in a recording period. For example, for a recording period of 24 hours, at 250 serial messages per second the transmitting module  41  transmits 21,600,000 messages and the receive module  44  receives 21,590,000 serial messages  67  because 9000 of the serial messages were corrupted and 1000 of the serial messages were missing. The channel availability would therefore be 21,590,000/21,600,000=99.9537%. Suitable alarming may be utilized to notify the user when the channel availability falls below a predetermined threshold. 
     As will be appreciated by one skilled in the art, variations of availability calculations are possible such as, for example, counting received frames  80  to determine availability of the digitized analog values and/or virtual terminal data. For example, because 18 received frames are needed to reconstruct an 18-bit digitized analog value, receipt of only 17 of the 18 frames would indicate an analog value availability of 94.44%. 
     Accordingly, the relay-to-relay direct-communication system disclosed herein is adapted to (1) directly communicate output status indication bits which represent the result of protection functions by one of the relays, (2) directly communicate selected analog values representing one or more functions of the relay, (3) directly communicate virtual terminal data provided by a user to one of the relays via the other relay, (4) monitor the communication link between the two relays, (5) determine communication link availability and (6) provide time synchronization. The analog values and the virtual terminal data are processed in serial fashion in successive messages on channels not used by the output status indication bits. The time synchronization data is processed in serial fashion in successive frames (18 messages) of data. 
     As noted above, the number of and assignment of data channels for the output status indication bits and the additional data (analog values and virtual terminal data) can be pre-selected by an operator or can be dynamically selected during relay operation. The additional data may include analog values only, virtual terminal data only or a combination of analog values and virtual terminal data. The synchronization channel is dedicated for purposes of synchronizing the additional data, to transmit/receive additional virtual terminal data, time information and calendar (date) information. This results in the channel capability of the basic transmission arrangement disclosed in the &#39;750 patent being used to its maximum extent, while providing the benefits of the existing fast and highly secure transmission of output status indication bits. 
       FIG. 4  illustrates a power protection system  400  utilizing a primary  410  and a backup  420  protection device to oversee the operation of a circuit breaker  430 . As illustrated, both the primary device  410  and the backup device  420  are identical. The circuit breaker provides a breaker status signal  434 , 436  to both the primary  410  and backup  420  devices. The breaker status signal  434 , 436  indicates whether the breaker is open or closed, and is used by both the primary device  410  and the backup device  420  to determine whether or not to open the circuit breaker on detection of a fault. 
     The primary device  410  and the backup device  420  also provide overcurrent indications  444 , 446  to an alarm grid  440 . Further, the breaker provides a breaker gas pressure alarm signal  438  to the backup device  420 . As explained later, the backup device transmits this alarm condition over a link  415  to the primary device  410 . The primary device  410  processes the transmission and outputs a breaker gas pressure alarm  448  to the alarm grid  440 . 
     The primary device  410  is also connected to a communications network  450 . An operator may use a computer  460  connected to the same network  450  to send commands to the primary device  410 . The operator may also direct commands to the backup device  420  through the primary device  410 , which is coupled to the backup device by link  415 . 
       FIG. 5  is a block diagram of a device-to-device direct communication system  500  within the power system  400 , constructed according to an embodiment of the invention.  FIG. 5  is largely analogous to  FIG. 2 , discussed earlier. However,  FIG. 5  utilizes sixteen data channels, and their specific use is described below. The first eight data channels  531 - 538  are used as “virtual output bits” for the primary device. The virtual output bits  511 - 518  denoted VOB 1 -VOB 8  are transferred into the first eight data channels  531 - 538 . The primary protective device  504  transmits the status of the virtual output channels to the backup protective device  556  which operates output contacts in accordance with the virtual output channels, as explained later. 
     The next six data channels  519 - 524 , denoted IB 1 -IB 6  (input bit) are virtual input channels. The primary protective device  504  collects the status of its input contacts  508  and places the collected status into the input bits  519 - 524 . Those bits are then transmitted to the backup protective device  556 , which maintains corresponding virtual input bits, and may use the virtual input bits in its internal calculations. 
     The final two data channels  525 - 526 , denoted CMD 1 -CMD 2  are command channels. Using these channels, the primary protective device  504  may issue commands to the backup device  556 . The commands may be relayed through the primary protective device  504  from an operator as illustrated in  FIG. 4 . The commands may also be sourced from the operation of the primary device  504 . For example, an external operator could alter the cold load pickup setting both the primary protective device  504  and backup protective device  556  using the CMD data channels  525 - 526 . 
     When a frame is ready to transmit, the processor encodes at  550  the data using any one of a number of suitable techniques. The data is then passed to a UART  552 , where it is transmitted by a link  555  to the backup protection device  556 . The backup protection device  556  then retrieves the data from a UART  579  and decodes at  580  the data into sixteen parallel received bits  561 - 576 . The received bits  561 - 576  are separated into OB 1-8  (output bits  1 - 8 )  581 - 588 , VIB 1-6  (virtual input bits  1 - 6 )  589 - 594 , and CMD 1-2  (command bits  1 - 2 )  595 - 596 . The backup protection device adjusts its output contacts (not shown) to conform to the received output bits. It also updates its internal operations with the virtual input bits, and executes any commands required by the command bits. 
       FIG. 6  is an exemplary received frame  600  of the device-to-device direct communication system  500 , according to an embodiment of the invention. As illustrated, the received frame includes sixteen bits. Of these, eight are output bits  620 . The processor  660  reconfigures the output contacts  651 - 658  to match the state of the output bits  620 . The received frame also includes six virtual input bits  616 , which correspond to inputs of the primary protection device  504 . The processor  660  adjusts its internal memory and operating state based on the virtual input bits  630 . Finally, the received frame includes two command channels  640 . These channels may encompass messages that are many bits long, and will need to be assembled frame by frame before they can be executed. Once the command messages are assembled, they are executed by the command processor  640 , which adjusts the internal state of processor  660  and output contacts  650 . 
     The virtual input bits  616  can be used to convey the status of an input contact from one device to the other device. For instance, one device may monitor signals, such as a circuit breaker-in service signal, a circuit breaker test mode signal, or a circuit breaker manual close indication signal. The monitored signals may then be transformed into digital bits and transferred to the other device, where they are used internally in the second devices calculations. Other signals which may be monitored using virtual input bits are cold load pickup on/off, or the status of a second circuit breaker. 
     Each virtual output bit may be based on a single setting within the primary protection device, or a combination of settings within the primary protection device. The table below illustrates some common power protection settings: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Setting 
                 Description 
               
               
                   
                   
               
             
            
               
                   
                 50PH 
                 Phase Instantaneous Definite-Time 
               
               
                   
                   
                 Overcurrent Elements 
               
               
                   
                 50N 
                 Residual Ground Instantaneous Definite- 
               
               
                   
                   
                 Time Overcurrent Elements 
               
               
                   
                 E50Q 
                 Negative-Sequence Instantaneous 
               
               
                   
                   
                 Definite-Time Overcurrent Elements 
               
               
                   
                 E51S 
                 Selectable Operating Quantity Inverse 
               
               
                   
                   
                 Time Overcurrent Element 
               
               
                   
                 EV/D1 
                 Manual Close Command for Circuit 
               
               
                   
                   
                 Breaker D1 
               
               
                   
                 EV/D2 
                 Manual Close Command for Circuit 
               
               
                   
                   
                 Breaker D2 
               
               
                   
                 43OP 
                 Reclosing Control Activation Parameter 
               
               
                   
                 43PR 
                 51PR Detector Activation Parameter 
               
               
                   
                   
               
            
           
         
       
     
     Virtual output bits may also be used so that one device; i.e.; the primary device; 
     may control output contacts on another device; i.e.; the backup device. Some virtual output bit functions can be ground overcurrent on/off indication, remote on/off indication, and auto-reclosing of the second circuit breaker. 
     So, for instance, using the disclosed invention, a virtual output bit could be set in the primary based on a negative-sequence instantaneous definite-time overcurrent element (E50Q), or it could be based on a negative sequence instantaneous definite-time overcurrent element and a phase instantaneous definite-time overcurrent element (50PH). 
       FIG. 7  depicts a further refinement of the disclosed communications system. As with the previously discussed communications system, this communications system makes use of unused bits within the payload of a real-time communications stream. However, instead of communicating virtual terminal data, the refined communications system creates one or more general purpose virtual communications channels. These virtual communications channels can be transmitted along with real-time bandwidth intensive data over the same communications lines used by the real-time data. This aspect of the disclosed communications system is especially useful where a real-time quantity is communicated over a relatively slow speed connection, such as, for example, a 56 kbps or 64 kbps serial connection. In such circumstances, a single real-time signal may require the entire bandwidth of the communications medium, and any additional quantities that were desired to be communicated would require additional physical communications lines. However, by using the disclosed communications system, a number of slow-speed virtual channels can be created using spare bits from the primary real-time communications session. 
     As an example, a “Line Current Differential” or “87L” function transmits local currents as samples or phasors, as well as restraining terms derived from local currents, and various one bit quantities related to the core 87L function, such as, for example, an indication of synchronization of the device to an external time source, an indication of the detection of an external fault, a test-in-progress indication, and a charging current compensation in progress indication. As the 87L function is time-critical, using a relatively slow speed connection, an 87L function may transmit a packet approximately every 1-8 ms. Accordingly, assuming 1 unused bit within an 87L packet is used to create a single virtual channel, a communications rate of 250 bps can be achieved, assuming that an 87L packet is sent every 4 ms. 
     There are numerous functions that the low bandwidth virtual communications channels disclosed herein can utilize. Such functions include, for example, multi-ended fault location, i.e., the sharing of voltages, calculated system impedances, or other special quantities to facilitate better fault location as compared with a single-ended fault location method that uses just local data, executing commands on a remote relay, such as putting it in test mode, extracting fault records and/or metering and event logs from a remote device, extracting device diagnostics data from a remote device, changing settings on a remote device, crude synchronizing of clocks and calendars in devices that are not independently synchronized to another external time source, the creation of a virtual terminal by which commands can be entered without interrupting the functioning of the device, and periodically sending metering data to facilitate automatic control and/or protection applications. Other applications where a human operator needs to interface with a device located remotely from the operator, or automatic functions that utilize low bandwidth/slow data can also be accommodated using the virtual channels disclosed herein. 
     Generally, a serial communications channel requires a data line, a clock, framing, and a data integrity verification method. The disclosed virtual communications channels provide data bits using one or a few bits of an underlying real-time session, such as an 87L session. As packets of real-time quantities are usually sent at regular intervals, the packet itself can serve as a clock. Similarly, sequence numbers within the packets of the underlying session can serve as a framing mechanism. Similarly, the underlying packets will contain integrity checks, such as CRC or BCH checks, and accordingly, errors introduced by noise in the communications channel can be assumed to be detected and eliminated through a retry mechanism. Alternatively, an additional integrity check can be appended onto the packets transmitted via the virtual channel, decreasing available bandwidth, but adding an additional guarantee of data integrity. 
       FIG. 7  is similar to  FIG. 2 , except that certain elements have been modified and/or re-labeled. Transmit module  741  includes several re-labeled elements such as element  57 , labeled “output bits” has been replaced with element  757 , labeled “Real-time protection data,” elements  59 - 62  have been removed and replaced with element  762  labeled “background session data,” elements  47 - 54  have been removed and replaced with element  747  labeled “data frame,” and element  743  labeled serial communications channel has been substituted for element  43  labeled “UART.” The function of element  757  is similar to that of removed element  57 , except that element  757  may contain additional real-time protection data, such as, for example, “Line Current Differential Protection Data.” Elements  59 - 62  have been replaced with a general purpose communications session over which analog values, virtual terminal data, multi-ended fault location data, meter data, and time synchronization data may be transmitted. The eight bit data channel represented by elements  47 - 54  of  FIG. 2  have been replaced with element  747  depicting a communications channel of any size, such as, for example, 256 bits, of which a few bits are dedicated to a background session. The bits used for the background session may be background session bits. Element  43  of  FIG. 2  has been replaced with a synchronous serial communications channel interface  743  that may be configured to be compatible with a number of standard communications links, such as, for example, ITU-T G.703, EIA-422, and IEEE C37.94 Element  743  may also be configured to support one or more direct fiber links. 
     The changes to the receiver track the changes to the transmitter. That is, the receive module  744  includes a serial communications channel interface  746  that may be compatible with a number of standard communications links. Data frame  777  replaces the elements  70 - 77  and may be a communications channel of any size, such as, for example 256 bits, of which a few bits are dedicated to a background session. Real-time protection data module  787  may contain additional real-time protection data received from the transmit module  741 . Further, background session data module  792  receives the parallelized background session data. 
     Note that the invention described herein utilizes a digital processor. As the algorithms described do not require any particular processing characteristics, any type of processor will suffice. For instance, microprocessors, microcontrollers, digital signal processors, field programmable gate arrays, application specific integrated circuits (ASIC) and other devices capable of digital computations are acceptable where the term processor is used. 
     In addition, the term intelligent electronic device is used. An intelligent electronic device is defined, for terms of this application, as a power protection device (i.e.; non-power protection devices such as general computers are not intended) including a processor for decision making. Examples of intelligent electronic devices are relays of various types and recloser controls. 
     The foregoing description of the invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The description was selected to best explain the principles of the invention and practical application of these principles to enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention not be limited by the specification, but be defined by the claims set forth below.