Patent Number: 
Section: claims

1. A plant operation data monitoring apparatus comprising:plant-data inputting means for inputting plant data from a plant facility;a plant-data most-recent-value table for temporarily storing a most recent value of said plant data;plant-data recording means for retrieving said most recent value from said plant-data most-recent-value table and recording long-term time-series plant data in a plant data history table;monitoring point editing means for editing to register data regarding one or more monitoring points for monitoring an error of said plant facility in response to an operator request;a monitoring-point registering table for storing a result registered by said monitoring-point editing means;data-correlation determining means for selecting one or more main variable points having a strong correlation to the corresponding monitoring point from said plant-data history table using said monitoring point as a key;a main-variable-point registering table for registering a main variable point selected by said data-correlation determining means;limit calculating means for calculating to define a statistical upper/lower limit value function for said plant-data history table on the basis of time-series plant data of said monitoring point and said main variable point;a limit-value function table for storing a limit-value function defined by said limit-value calculating means;limit-value determining means for referring to table information in said limit-value function table and for periodically matching content of said plant-data most-recent-value table with the table information, and if the most recent value is outside the limit value range, deviation information due to a plant error is determined and stored in a determination result table;plant-error monitoring/outputting means for monitoring and outputting an error signal and content of said determination result table if a new plant error occurs; anda display device for displaying an output from said plant-error monitoring/outputting means. 2. The plant operation data monitoring apparatus according to claim 1, further comprising:a data-count/weighting-factor conversion function, whereinwhen statistical upper/lower limit values are calculated with monitoring points and main variable points by said limit-value calculating means, an optimum upper/lower limit range is obtained by performing a limit calculation considering a data distribution density in units of divided clusters. 3. The plant operation data monitoring apparatus according to claim 1, wherein:a condition indicating the plant is stable is provided from outside; when statistical upper/lower limit values are calculated with monitoring points and main variable points, history data is extracted on the basis of said condition to calculate a less varying limit value; andthe limit value is used to perform monitoring checking. 4. The plant operation data monitoring apparatus according to claim 1, wherein as a result of statistical processing, if a distribution in a correlation between said monitoring points and said main variable points is divided into two or more groups, a condition for distinguishing these groups is provided from outside. 5. The plant operation data monitoring apparatus according to claim 4, wherein as a condition for distinguishing a plurality of groups, a data mining method is applied to history data to automatically extract the most appropriate plant condition for distinguishing groups. 6. The plant operation data monitoring apparatus according to claim 1, wherein:a plurality of plant states are defined;one or more main variable points having a strong correlation to said monitoring point in each plant state are automatically calculated;the statistical upper/lower limit values of said monitoring point are provided as a plurality of functions of said main variable points for monitoring whether the function is in a defined plant state; andthe upper/lower limit values of the matched function are used for monitoring. 7. The plant operation data monitoring apparatus according to claim 6, wherein a priority statistical process is performed so that said main variable points may be reduced to a same point even in a plurality of plant states. 8. The plant operation data monitoring apparatus according to claim 6, wherein said main variable point is provided from outside to calculate the statistical upper/lower limit values in a plurality of plant states. 9. The plant operation data monitoring apparatus according to claim 6, wherein the statistical upper/lower limit values of a plurality of said monitoring points in each of the plurality of plant states are calculated. 10. The plant operation data monitoring apparatus according to claim 9, wherein:when a common plant state is defined for a plurality of said monitoring points to perform monitoring, an independent plant condition is provided for each of the said monitoring points; andthe statistical upper/lower limit values are calculated to perform monitoring. 11. The plant operation data monitoring apparatus according to claim 10, wherein:the calculated statistical upper/lower limit values of said monitoring points and said plant data used for calculation are displayed on a same graph;at the same time, upper/lower limit matched data count and outlier count are displayed; andvalidation is performed on the calculated values. 12. The plant operation data monitoring apparatus according to claim 11, wherein:a functional expression indicating a statistical upper/lower limit value or a factor thereof is displayed;the factor is manually modified; andthe modified factor can be confirmed on a graph display. 13. The plant operation data monitoring apparatus according to claim 12, wherein the statistical upper/lower limit values and said plant data used for calculation in a plurality of common plant states and independent plant conditions can be displayed on a same graph by changing colors and marker shapes. 14. The plant operation data monitoring apparatus according to claim 13, further comprising:means for not displaying information in a certain state as needed when a plurality of states are overlay-displayed.