DC electric field control of ending box-air (EB-A) type outdoor termination under DC voltage stress

An Ending Box-Air (EB-A) type outdoor termination installed at the outdoor substation is required to connect underground cables and overhead transmission lines. The outdoor termination for AC transmission system is already developed and widely used to interconnect overhead and underground systems. Whereas, the development of the outdoor termination for DC transmission system was only introduced from China and Japan, but their development status and core technologies were not fully reported. In order to implement HVDC systems connecting overhead transmission lines and underground cables, a EB-A type outdoor termination should be developed, but the detailed specifications and information of this device were not reported. Cable accessories such as EB-A type outdoor termination are the most critical part of cable system since they are made of many subcomponents that will be assembled in installation area. In order to connect cables, the screen must be stripped off, resulting in a disruption of the uniform geometry. And then, if stress relief cone for AC type cable termination is applied to cable termination under DC voltage, there will be a substantial stress concentration in a triple junction between two insulation materials and semiconductor, unlike electric field distribution under AC voltage. Therefore, in order to reduce the electric field intensification of EB-A type outdoor termination, we applied the field grading material around existing triple junction as a method of DC electric field control. An electrical conductivity determining resistive field is field dependent in such a way that the electrical conductivity increases with increasing electric field. This will give a more uniform distribution of the field and local stress concentrations are essentially avoided. As a results, DC electric field control could be successfully controlled by using field grading material in the vicinity of triple junction. A field mitigation effect of 84% at maximum was obtained when applying field grading material on EB-A outdoor termination.