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  What is the application of 2-W Transformer I-ST Protection?  
  The 2-Winding Transformer component in PTW v6.5 has a new feature for fast and easy modeling of the transformer differential protection for Arc Flash Evaluation. The following provides a brief description of its use and application in the Arc Flash Evaluation study module.  
  1. The one-line diagram, TX3 2-Winding Transformer subview, and Arc Flash study results for the line-side of the B-SWBD1 breaker are displayed in Figures 1, 2 and 3, respectively.  
 
Figure 1 Figure 2
 
   
  Figure 3  
  2. As shown in Figure 2, the check box for the INST Protection of 2-Winding Transformer represents the transformer differential protection in Arc Flash. For this example, the     box is left unchecked before running the Arc Flash Evaluation with the Line Side option selected in the Study Options menu (Figure 4).  
   
  Figure 4  
  3. The Arc Flash study result shows that the B-SWBD1 and LVP1 breakers interrupted the arcing fault current – leading to a 5.3 cal/cm2 incident energy.  
  4. If the INST Protection checkbox is checked the in 2-Winding Transformer Subview (Figure 5) and the arc flash study is completed for the line side of the same breaker,     then the Trip Delay Time data field color will turn white. This means that this field can be edited by the user (Figure 5). The protective device name from the transformer     contribution branch will include “INST Protection” in parenthesis.  
  5. If the total trip time of differential relay plus the opening time of respective breakers is entered into the Trip Delay Time data field and the Arc Flash study is re-ran, then     the entered time will be used for the calculation of the incident energy (figure 6). The (*N6) – Special Instantaneous Protection note indicates that this protection     function has been used instead of the original device for interrupting of the arcing fault current.  
   
  Figure 5  
   
  Figure 6