Transient response of crossbonded cable systems

When a step voltage is applied to only one phase, the transient overvoltages seen at the first joint of a cross bonded system may be calculated by equivalent circuits which are developed in terms of the surge impedance of the cable in the coaxial mode, the sheath-earth and the sheath-sheath surge impedances. The results are tabulated for a `tight-earth¿ model in which the three phases are confined within an overall metal sheath, and the treatement is extended to the second and third joints of a major section to obtain voltages with respect to earth. The results obtained from the time-scaled models of the earth-free and tight-earth situations are shoqn to crosscheck with calculations and to give some insight into the general pattern of behaviour as the number of sections is increased. Tests carried out on the tight-earth model which was extended to six major sections indicate that the maximum sheath voltages arise from the first reflection at the first crossbonded joint. The basic parameters of 132kV cables measured by the authors are used for the calculation of voltaged for the extreme cases from open-circuit sheaths, via crossbonding, to the use of resistors for surge suppression.