Nowadays high voltage/ low voltage substations in significant percentage are established with cable infeed due to the urbanization. The rate of earth potential rise of these substations differs from substations fed only by aerial conductor. Its reason is the significant current conducted by the cable sheath. Because of this the higher earth potential rise of the substation grounding mat and the transfer station can occur.
The effect to the earth potential rise of a cable section between overhead lines was evaluated in case of an earth fault in general case. The earth potential rise and the transfer potential of substations feeded by a cable section connecting to aerial conductors were evaluated by an approximation method and by computer in case of an earth fault. The results of the two methods were close to each other, the difference was only three percent in the worst case. Besides the earth fault the earth potential rise in case of normal operation condition was also evaluated. The maximal value of the earth potential rise of the transfer station was almost 4 kV in the worst case. The values of the earth potential rise were compared to the following standards and directives: IEEE Std. 80, IEC 61936-1, ITU-T K 68. Recommendation and IEC TS 61000-6-5.
The most important reason of this is the inductive coupling between the conductor of the cable and the cable sheath. The value of the earth potential rise is efficiently reducible by using a bare copper underground conductor.
The earth potential rise of a cable section between overhead lines was also evaluated in case an of earth fault. In that case the value of the earth potential rise was so low that it met the requirements of the standards.
In the next chapter the effects of the earth potential rise of two adjacent grounding mats were examined.
The most important conclusion is, that the earth potential of the transfer station in case of substations feeded by a cable section connecting to aerial conductors can be high. So it may cause shocks and equipment damage. It’s important the earth potential rise to be considered.