True/false: cables suppress switching overvoltage

TRUE (it is used mainly to reduce the magnitude and steepness of incoming surges at substation transformers).

Justification:

• A cable has a low surge impedance (30–60 Ω), against 300–500 Ω for an overhead line. When a surge travels from the line into a short cable, the transmitted voltage $\dfrac{2Z_c}{Z_c + Z_0}V$ is much smaller than the incident wave. For example, a 400 Ω line and a 50 Ω cable pass only 22% of the wave.
• The cable's large capacitance slopes the wavefront. This reduces $dV/dt$ and the voltage stress on the first turns of the transformer winding.
• In the substation, a cable section with its high capacitance lowers the natural frequency of switching transients and damps them.

 OH line (400 ohm) --> short cable (50 ohm) --> Tr
   V                   ~0.22 V, sloped front

Limitation: the cable is not a complete cure. Repeated reflections at the transformer (open end) can build the voltage back up, and cable capacitance can cause high charging currents and restrikes when switched. So surge arresters are still placed at the transformer, and the cable is used as an additional measure.