Transient Sources - Utility

Lightning is one of the leading causes of power outages as well as transients on utility power lines. Lightning accounts for a quarter of the outages on 230-kV lines, two-thirds of the outages on 345-kV lines and half the outages on circuits up to 33 kV.

The surge currents associated with lightning strikes interact with the distribution system's impedance creating voltage transients. In this way, the effects of the lightning strikes are transmitted to remote parts of the grid.

Lightning can also induce voltages on power lines without even hitting them. The large electric fields generated during a discharge can couple into the power system, creating induced transients. A cloud-to-cloud discharge can generate a 70 Volts per meter electric field. On a 1/2-mile length of transmission line this is equal to a 56,000-volt transient.

Properly designed transmission towers and other distribution system components can minimize lightning-generated transients.

Transients may also be imposed on power lines through normal utility operations. Switching of facility loads, opening and closing of disconnects on energized lines, switching of capacitor banks, reclosure operations and tap changing on transformers can all cause transients.

Poor or loose connections in the distribution system can also generate transients. They may be caused by high winds, which can blow one power line into another or blow tree limbs into the lines causing arcing. You'll probably be able to hear a buzzing sound and see sparks when the arc occurs, or you may even be able to smell the burnt insulation around the arc.