In the transmission and distribution network of the future, there is expected to be a mix of both Alternating Current (AC) and Direct Current (DC). In the high voltage division owing to technical and economic aspects, there is a wider use of High Voltage Direct Current (HVDC) ins
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In the transmission and distribution network of the future, there is expected to be a mix of both Alternating Current (AC) and Direct Current (DC). In the high voltage division owing to technical and economic aspects, there is a wider use of High Voltage Direct Current (HVDC) instead of High Voltage Alternating Current (HVAC). However, when observing the medium voltage network, it is noticed that it is completely AC in nature. One major reason for this preference of AC in the medium voltage network is the better know-how of an AC network and the presence of well established and reliable MVAC components and the ability to transform (step up and down) voltage in AC. A switch to Medium Voltage Direct Current (MVDC) is expected as the cost of power electronic components is decreasing with time, and there is an improvement in their performance. Therefore, switching to MVDC would provide advantages in the form of improved transfer capacity and better power control. MVDC grids are debated to be a vital member of the future distribution network. In the present scenario, there is an absence of such an MVDC grid. Thereby, there are also no MVDC accessories available which can be used in such a grid. Therefore, there is also the absence of a testing procedure for the same. CIGRE TB 496 provides the testing strategies for DC cable systems up to 500 kV, but it does not take into consideration the difference between an MVDC system and an HVDC system. The systems may have a striking difference in construction, such as concerning materials. Additionally, there is also a difference concerning max field stresses and thickness of the insulation. The possibility of using the AC accessories for DC application needs to be analysed, and it needs to be verified how such an AC accessory would behave under the influence of prolonged DC stresses. The use of existing MVAC accessories for DC would be beneficial given the high production standards and the voluminous supply chain of MVAC systems. Additionally, this also opens possibilities of reusing existing AC cable system for DC stress. It needs to be noticed that in DC, the field distribution would depend on the conductivity of the material which is different from AC where the field distribution depends on the permittivity of the material. The permittivity of insulation is virtually independent of the temperature. However, conductivity has a strong relation to temperature and electric field, which makes DC field distribution more complex when compared to AC field distribution for any geometry. The test criteria of MVAC and pre-qualification test for HVDC are well known and need to be utilised in proposing and motivating the test sequence and test voltages for the accessories to be used in the future MVDC network. These accessories to be used in this future MVDC cable system needs to be analysed using Finite Element Method (FEM). The field simulations would give identification of locations in the joint which are undergoing maximum stresses during DC application. These maximum values of stresses are used to calculate the voltage life of the system based on electro-thermal life laws. The test results based on the representative testing procedure would help in understanding the performance and lifetime of the cable systems under DC stress. Therefore, to understand all items previously mentioned, a representative testing procedure needs to be proposed and motivated to test the use of existing MVAC accessories in the future MVDC network.