Project: T213700 #3730

This project is to justify the rational which determines the transmission structure impedance target limit as well as measurement and mitigation methods.

This project is to investigate the method to determine the Transmission Tower footing resistance target which should be used by utilities. The project will provide guidelines for calculating and establishing target limits for transmission tower impedances. The project is to provide answers to the following questions:

  • What are the present impedance limits used by utilities?
  • What considerations determine or should determine these limits?
  • What test methods and equipment are suitable for measuring the tower impedance?
  • What is the risk and/or consequence of allowing higher impedance results to be present on one or perhaps some of the structures?
  • Can target impedance levels be adjusted under certain difficult conditions?
  • What mitigation methods can be applied to achieve target impedance levels?

The project is to include consideration of the following:

  • Transmission lines are to be separated into two categories. One is with shield wire and grounded at each tower and the other is with insulated shield wire or no shield wire.
  • A discussion on different target of tower footing impedance and determine which is the governing conditions. For example, system operation might allow max footing impedance before their relay/breaker cannot pick up the fault condition. The lightning performance might have different requirements.
  • A discussion on what happens if there is no way to reduce the impedance to meet the target value at certain location. What is the recommended best practice?