Samenvatting

The Dutch Transmission System Operator (TSO) TenneT TSO B.V. is planning to build a 2 km to 4 km long 150 kV superconducting cable connection in the Dutch high voltage grid. Superconducting cable systems allow for connections with no external magnetic field which is beneficial due to society's concerns about magnetic fields. These cables also allow for a reduced right of way compared to conventional cable systems. In locations that already have many underground systems, for example city centres, superconducting cables have the added benefit that they don't influence nearby infrastructure, either thermally or magnetically.

The demand TenneT places on this system is that a superconducting cable should behave just like a conventional connection, not only during regular operation, but also in fault situations. This means that the connection should be able to carry a full fault current of 30 kA for 600 ms and immediately resume operation after the fault is cleared. This demand is not yet demonstrated internationally.
To evaluate the feasibility of this demand this study has been performed. A physical model of a cable system is made using Microsoft Excel. The construction of the cable is then optimized for most efficient operation. The fault current recovery is then evaluated for this cable design. In this manner numeral cable specifications are studied and compared. The cable design that is studied is a counter flown cable. This means that each phase has its own cryostat which contains both a forward and a return flow.
It is demonstrated that at a copper cross section of 300 mm2 all the designs studied offer immediate fault recovery. Some of the designs offer this recovery at a copper cross section of 250 mm2. Depending on the cable configuration and system parameters this is the minimum amount of copper required to offer fault current recovery.

Not only the fault current survivability is calculated for the different cable designs, also the no load and zero load cooling power that is required is calculated. A worst case, realistic case and best case scenario are drafted. For these scenario's the cooling power required is respectively, 329 ± 5 kW, 187 ± 5 kW and 126 ± 5 kW at nominal load and 244 ± 5 kW, 148 ± 5 kW and 112 ± 5 kW at no load.
A comparison is made between these superconducting cable systems and conventional connections. It is shown that the superconducting cable systems can only be competitive in connections that have a high load all of the time. This is due to the high no load losses of superconducting cable systems. For a 1000 A connection the continuous load should be above 550 A for the best case scenario and 650 A for the realistic scenario.

For a 2 km cable that offers immediate fault recovery the realistic scenario is that of a heat intrusion from the exterior of 4 W∙m-2 and a critical current of 3000 A. Such a cable would have a copper cross section of 300 mm2, a dielectric field strength of 10 kV∙mm-1 and a former diameter of 36 mm. This cable would operate at a pressure drop over the cable length of 5∙105 Pa and will have double sided cooling.