Abstract:
A system and method for synchronizing power system data gathered in response to unscheduled power system events such as faults. A plurality of protective devices communicate over a peer-to-peer communication network, and when a first device detects the occurrence of a power system event, the device records cycles of power system data before, after, or surrounding the power system event. The first device informs remote devices via the network of the event and a time tag associated with the event, and the remote devices record synchronized power system data.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to relays that provide protective control of power distribution systems. More particularly, the present invention relates to a technique for synchronizing the gathering of fault data from multiple protective relays. 
     Power system faults can start and finish rapidly. Protective relays are designed to sense faults and provide protective control by operating a circuit breaker to interrupt the fault. A modem protective relay incorporates digital signal processing to analyze fault data by capturing sampled waveforms before and after the protective relay interrupts the power system. To fully analyze the cause and extent of a fault, analysis of fault data at multiple locations on the power system is necessary. 
     Conventional systems for monitoring protective relay fault data are primarily master-slave communication systems in which a central control station (master) initiates communications to the protective relays (slaves). In master-slave systems, the master initiates all communications. In a common approach, the master control station cyclically and sequentially monitors each slave device for a change of state. This monitoring process introduces significant delays between the time that the relay reacts to a fault and the control station learns of the fault. The control station can signal other relays to record a waveform, but the ensuing delay is unacceptable in master-slave systems for capturing information at the time of the fault across the system. 
     U.S. Pat. Nos. 5,224,054 and 5,233,538 to Wallis disclose the capturing of synchronized sampled data in master-slave systems. These patents disclose a technique which employs a control station which sends a command signal to multiple “circuit monitors” to synchronously sample the system at the present or a future time. While this approach is adequate for characterizing a power system on demand or at a scheduled time, this approach does not adequately address the synchronization of fault data because faults do not occur according to a schedule. 
     Current advanced trip units and protective relays capture waveforms at the time of the fault. In a typical arrangement, a protective relay will continuously measure and discard up to 72 cycles of power system data. When the (unexpected) fault occurs, the protective relay continues to sample the waveform (and other calculated parameters and status flags) for a preset number of cycles. After this period, the full number of stored samples, comprising cycles of power system information before and after the fault are place in storage for future communication to a local or remote computer. While this process effectively provides fault data information, it does so at only one point in the system. 
     SUMMARY OF THE INVENTION 
     In view of the above discussion, it would be desirable to provide a method for synchronously recording sampled power system data at multiple points in the power system when a power system event occurs. It would further be desirable for such data to be post-processed to synchronize the information from multiple sources. 
     In accordance with the exemplary embodiments described below, the present invention can be implemented by sensing a power system event, such as a fault, in a first protective relay associated with a first point on the power system; recording a time tag of the power system event and a first set of power system data; and transmitting a data message from the first protective relay to remote protective relays over a peer-to-peer communications network. The first protective relays and remote protective relays are preferably synchronized (e.g., by a common time standard), The data message preferably includes the time tag of the power system event. Each remote relay can then use the time tag to save its own set of power system data which is synchronized to the first set of power system data. 
     The technique of the present invention advantageously provides system-wide data surrounding a fault, power surge, or other power system event. This information permits a user to analyze more accurately the cause and effects of the power system event. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention can be understood more clearly by reading the following Detailed Description, in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a power system under the protective control of a power protection scheme according to one embodiment of the present invention; and 
     FIG. 2 is a flow chart describing a method according to an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a power system under the protective control of a protection scheme according to one embodiment of the present invention is shown. In FIG. 1, a plurality of protective relays  10  is operatively connected to provide protective control to a power system  12 . The protective relays  10  are further operatively connected, via appropriate communications ports, to a peer-to-peer communications network  14 . The protective relays  10  each include a microprocessor  16 , an associated memory  18 , and a time clock (not shown). The time clocks of the relays  10  are synchronized by a time standard  20 , which can be operatively connected to the peer-to-peer communications network  14 , or can be operatively connected to each of the relays by other appropriate means. Each protective relay  10  samples power system data at an associated point on the power system, and temporarily holds (e.g., in a buffer associated with the microprocessor  16 ) some number of cycles of power system data. The temporarily stored (sampled and held) cycles of power system data are replaced by new cycles of power system data as the relay  10  continues to sample and hold new power system data. 
     In operation, the system of FIG. 1 achieves synchronized sampled data capture at multiple locations in the power system for unplanned events, such as faults. The peer-to-peer communications network  14 , as contrasted with a conventional master/slave communications network, uses a protocol which allows each device on the communications network  14  to initiate communications if the network is available (that is, if the communications bus is not already in use). In a conventional master/slave system, only the master can initiate communications, and when a slave device senses a condition in the system, the slave cannot communicate this fact until the master requests information from (polls) the slave device. Examples of peer-to-peer communications systems include a so-called Field Messaging System under the Profibus or Ethernet protocol. 
     Time synchronization is practiced in conventional protective relay systems. Protective relays are conventionally provided with an internal real time clock which continuously keeps time within the relay. Each event within the relay can be time tagged to this internal clock. Time synchronization can be achieved by connecting each device to a time standard, such as the IRIG-B time standard, which not only synchronizes all the devices to each other, but also to universal coordinated time. The time standard includes a radio receiver, which allows the standard to receive accurate time updates from one of a number of international time standard transmitters. Thus, a connection to a time standard  20  can provide an exact time pulse to the protective relay and a digital time command by which the protective relay can update its internal time clock. 
     Referring now to FIG. 2, a flow chart describing a method according to one embodiment of the present invention is shown. The method of FIG. 2 begins in step  100 , where a first protective relay  10  (FIG. 1) associated with a first point on the power system senses a power system event, such as a fault. In step  102 , the first protective relay records a time tag of the power system event, and the processor associated with the relay saves a first set of power system data (e.g., 72 cycles) relating to the detected event. The first set of power system data preferably includes data cycles leading up to the power system event, as well as some power system data cycles following the event. Step  102  can be performed by the processor collecting storing the first set of power system data in memory  18 , or in other suitable data storage means for storing data more permanently than the relay&#39;s data sampling system. For example, the microprocessor  16  can select 72 cycles of buffered data, which might include a distribution of 60 cycles of power system data before a fault and 12 cycles of power system data following the fault. In step  104 , the first protective relay initiates the transmission of a data to other protective relays over a peer-to-peer communications network, indicating that a power system event has occurred. The data message preferably includes the time tag of the event, and can also include an indication of the number and/or distribution of cycles of power system data recorded by the first protective relay. Alternatively, each relay can be programmed (via its associated microprocessor) to record a predetermined number and distribution of cycles of power system data surrounding a power system event. In step  106 , the other devices associated with the peer-to-peer network receive the data message, and each receiving device uses the received time tag to determine which cycles of power system data should be saved in its associated memory. In step  108 , each relay records one or more cycles of power system data corresponding to the first set based on the time difference determined in step  106 , along with the time tag of the event. Preferably, each protective relay sets its sampled data system, under the control of the microprocessor associated with the relay, to save exact same number and distribution of data cycles relating to the power system event, indicated by the time tag, as the first protective relay (the device originating the communication message). 
     In the manner of the above example, the technique of the present invention results in synchronized power system data for unexpected events, and the synchronized data can be readily retrieved at a later time for analysis (e.g., in an analysis program) of the causes and effects of the power system event. The synchronized data can show the data sampled by different protective relays at the same time just prior to, or just following, a power system event. 
     The present invention achieves numerous advantages over known systems, especially those that employ master/slave communication systems. Because in master/slave communication systems, a master polls slave devices to determine the occurrence of a fault, the master may not be aware of the fault until it is too late (e.g., because the slave device&#39;s sample and hold system has already discarded relevant data) to instruct the slave devices to save desired data. 
     While the foregoing description includes numerous details and specificities, these are provided for purposes of explanation only, and are not to be construed as limitations of the present invention. Many modifications to the above examples will be readily apparent to those of ordinary skill in the art which are within the spirit and scope of the invention, as defined by the following claims and their legal equivalents.