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I am studying the RTS 96 test system for a power system simulation in Matlab. I have noticed that under the Branch Data file, there are some buses which have two lines connecting them. For example (from Table-12.txt) between buses 115 and 121

ID#       =  Branch identifier.
             Inter area branches are indicated by double letter ID.
             Circuits on a common tower have hyphenated ID#.
Lam-p     =  Permanent Outage Rate (outages/year).
Dur       =  Permanent Outage Duration (Hours).
Lam-t     =  Transient Outage Rate (outages/year).
Con       =  Continuous rating.
LTE       =  Long-time emergency rating (24 hour).
STE       =  Short-time emergency rating (15 minute).
Tr        =  Transformer off-nominal ratio.
             Transformer branches are indicated by Tr <> 0.

 ID  From  To  L -  Perm.     Tran.   R     X     B   Con LTE STE Tr
 #    Bus Bus miles Lam-p Dur Lam-t   pu    pu    pu  MVA MVA MVA pu
A25-1 115 121  34   .41    11  0.8  0.006 0.049 0.103 500 600 625 0
A25-2 115 121  34   .41    11  0.8  0.006 0.049 0.103 500 600 625 0

Is the reason purely physical, meaning, introducing redundancy to increase system security? When it comes to modeling, can I just combine these into one transmission line (with appropriately modified parameters)?

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Placing two or more transmission lines (transformers, cables, ...) in parallel is a common way to obtain increased capacity and redundancy against single failures. It also lets you take one line out of service for maintenance, while the second line carries the load.

Mathematically, you would obtain the same results by combining the parallel transmission lines, but you would lose the ability to simulate a scenario where only one of the two transmission lines is in use. Simulating contingency scenarios ("what happens if one line has a fault?") is one of the main reasons we do power systems analysis, so I would tend to model the two transmission lines as two separate elements.

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