Electromagnetic transient will induce a relative potential difference between earthing conductors and current in secondary cable shielding in substation. The current can couple to the secondary ports through control cables

which may cause operation failure even damage of secondary equipment. Consequently

we obtained the characteristics of the voltage and current coupled to the secondary ports through the experimental simulation

and analyzed the coupling mechanism. First

we employed established test platforms for simulating switching

short circuit and lightning transients to carry out numeral experiments. The experimental results indicate that the voltage waveforms at both terminal ports have the same profile but the opposite polarity

under the condition of the same component at the terminals for all kinds of transients. Then

only common-mode voltage with no differential-mode voltage exists for the twin core cable. Based on the results

an equivalent circuit of secondary cable was proposed. It is revealed that the current in the cable shielding layer will induce the resistance voltage drop

i.e. the voltage at the secondary port. The results also show that the coupling coefficient decreases with the increase of the frequency

and the coupling impedance

which is independent of frequency

is close to the resistance of the cable shielding layer. All these results will be helpful to further research on the electromagnetic protection of the secondary ports.