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Detection of Inrush Current and Overexcitation Condition (contd..) |
| (2) |
Voltage Based Restraint (contd..) |
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Expressions 5, 6 and 7 are self explanatory.
The last restraining function  is known as the transient monitor function. The transient monitor function is used to qualify the purity of data. Typically, in a numerical relaying set up, with moving window algorithm, there are a certain number of windows in which both pre fault and post fault data points are present. Any phasor computation done with them is meaningless because the window does not fit with either prefault or post fault signal. Under such condition, the residual error  (  sample) between estimated (reconstructed) and measured signal is quite high. For a half cycle window with 6 samples in it,  . When is greater than threshold value
 , then tripping decision should be inhibited. |
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Fig 39.9 shows the flux current plane associated with no fault and internal fault regions. It is difficult to evaluate the actual flux in the core, because the initial condition is unknown. It depends also upon the remanant flux. Fortunately, to distinguish no fault (or external fault) from the internal fault, we are interested in the slope of  curve rather than the actual values. |
