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IEE 2nd International Conference on Advances in Power System Control Operation and Management December 1993 Hong Kong Power System Monitoring and Control Facilities on Protective Relays S M Haden and R Squires GEC ALSTHOM Protection Value Cells and Setting Control Cells Heading Cells contain a simple piece of text These are used throughout the database as place markers to split the database into different areas The most common Heading Cells are the database column heading cells Value Cells contain a piece of text to describe their contents and a value which may be read Intrinsic in this value is a data type which instructs the master station how the data is to be converted Typical Value Cells are measured values such as phase currents device information such as model number waveform records etc Setting Control Cells are similar to Value Cells but their contents can be changed These cells additionally contain information about the minimum and maximum values for the cell and the valid steps Typical SettingKontrol Cells are relay protection settings such as current thresholds system control cells such as circuit breaker control etc Individual cells are grouped together into columns of related information The cell in the first row of each column is a heading cell which describes the contents of the column This organization is invariant across all relays Thus the contents of any relay can be read in the same way First the column headings are extracted and presented to the user as a menu From this menu the user selects a particular column The text and values for each cell in the selected column are then extracted and again presented to the user as a menu Individual cells may then be selected for further operation Typically this could be change of setting assignment to a measurement value on a mimic display log to disc or real time graphing In practice it is found that all relay types contain a certain amount of common information This includes the relay type niodel number and serial number its location communications address etc This information is generally required by the master station when the relay is first connected A special command could be provided to extract this data however a better solution is to group all the data together in a single resewed column The format of the column is fixed but the data can now be extracted in the same way as all other data The method of access described can be implemented in a way that is simple intuitive and requires no reference manual for the user to access a particular piece of data Moreover the method is consistent across any number of relays and need not be updated if further relays are added to the system From a master station point of view the commands required to access the database are few The most common are 95 Get Column Headings Get Column Text Get Column Values Get Cell Text Get Cell Data Get Cell Limits Preload New Setting Execute Setting Abort Setting From these simple commands more complex sequences may be built up as required 2 5 Time Alignment and Sequence Of Event Recording One of the important functions of existing SCADA systems is sequence of event recording This gives the system engineer valuable insight into the order in which events on the system occur The typical accuracy of older systems is lOms This function is currently carried out by the RTU s of the SCADA system which monitor system events using digital inputs These events are generally time tagged using a system synchronizing pulse distributed around the substation Often of more importance than the absolute time of an event is its time relative to other events across the system The new generations of protective relays now include their own sequence of event recording facilities Moreover these facilities can be extended using spare relay input channels and separate I O modules to cover events not specifically related to the protection In order to achieve this a new method of time synchronization has been devized which removes the need or separate clock synchronization wiring Rather than trying to synchronize the clocks within each individual relay the relay clocks are allowed to free run Events within each relay are time tagged relative to the internal free running clock This is derived from the microprocessor s clock and resolutions of flms are obtainable Clearly when these event records are transmitted to the substation computer events from different relays will be out of step This problem is solved by also transmitting the current value of the relay s clock This is compared with the substation computer s clock and the difference used to calculate the actual time of the event 3 Conclusions recently as digital technology has advanced protection performance and the operator interface have been improved The latest generation of microprocessor based relays will in future offer facilities far in excess of their predecessors In particular it will be possible to integrate the protection systems with both the local and remote control systems The result of this integration will be a reduction in costs and an increase in the data available from the power system If the full advantages of this integration are to be achieved a structured approach is required so that a system wide solution may be derived A hierarchical system has been presented with the remote control system connected to the substation network via a substation computer The substation computer is additionally capable of replacing much of the local control system For such a system to be widely adopted at distribution as well as transmission levels it is important that the set up costs associated with the system are reduced This is achieved by using a low cost communications network and a language which s