Long distance power transmission and integration of renewable energy utilize usually HVDC land and submarine cables. To connect HVDC cable with each other and to substations like gas insulated metal enclosed switchgear (GIS), cable joints and cable connection assemblies need to be used. These components present a higher complexity than cables and GIS alone due to their geometry, and their thermal, mechanical, and electrical design. Indeed, the electric field distribution under HVDC voltage is less predictable than the one in AC voltage. It is changing over time, starting from a capacitive distribution controlled by the permittivity of the insulating materials, to a resistive distribution, driven by the conductivities. Those in their turn depend on the electric field and on the temperature. Moreover, surface and space charges accumulation can modify the electric field distribution. The time needed for the transition from capacitive to resistive electric field distribution is commonly known as DC time constant.
For the qualification of an electrical equipment, dielectric tests should be representative of the electric field stresses at which the equipment is subjected during the entire life. Some differences are evident in voltage designations and testing levels for HVDC cable and HVDC GIS test recommendations. The knowledge of the DC time constant for these systems is thus of fundamental importance for the definition of pertinent test programs.
It is well known that for GIS the DC time constant can be very long in order of days to months depending on the geometry and insulating material conductivity. On the other hand, for cables alone the DC time constant is faster and generally in the order of some hours to days. For GIS cable connection assemblies, due to the coexistence of different insulating materials in the insulating system, the DC time constant cannot be easily determined and it can be longer than the one for the cable alone. Accurate simulation models are thus needed in order to have a better understating and mapping of the AC-DC field transition in these type of accessories.
DC time constants in different parts are computed and presented in this paper for a cable GIS fluid filled connection assembly. It has been shown that, according to the different zones and the different insulating materials, the time constant can be as fast as for the cable alone or can be much longer. Indeed the stabilisation of the electric field is dependent on the conductivity of insulating materials and space and surface charge accumulation. Moreover the test voltage value and the test temperature are also an important influencing parameter. The presented results serve to underline the importance of the evaluation of the AC-DC time constant through simulations with accurate material properties in order to be able to guide for the determination of pertinent test program for HVDC GIS cable connection assembly.
Caterina TOIGO, Cong Thanh VU, Frank JACQUIER, Alain GIRODET
Presented at Jicable’23