Transmission line taps can be a more economical way of interconnecting load and generation than construction of new switching stations or new transmission lines to existing switching stations. Recognizing that virtually any tapped connection to a transmission line will degrade the reliability of the line to some extent, the question arises as to how to identify whether or not the degree of degradation will be acceptable. This report investigates the factors that are used to evaluate the acceptability of tapped connection of load or generation to existing bulk transmission lines. The key factors are transmission system reliability, operational constraints, equipment capability (especially with respect to the ability to withstand transient overvoltages), and impact on existing protection systems. When the elements connected to a tap branch include a significant proportion of large motors or generators, excessive overvoltages on unfaulted phases during unbalanced ground faults are possible. Overvoltages can be mitigated by effectively grounding the neutral(s) of the interconnected windings of the tap branch step-down transformer(s). However, the tap branch ground connection, as well as fault current infeed from motors or generators connected to the branch, will desensitize existing transmission line protection; so a balance between acceptable overvoltages and protection sensitivity must be found. The report identifies key criteria, includes some case studies using example criteria, and describes various connection and protection alternatives to a simple tap that may improve the acceptability of a tap option.
Tapped transmission, Generation-transmission interconnection, Load-transmission interconnection.