1 22 b 2 is made via a power electronic convertor 24 a 1 24 a 2 24 b 1 24 b 2 and electrical isolators 26 a 1 26 a 2 26 b 1 26 b 2 which are all connected by electrical cables the bus bars 20 a 20 b and thus electrical generators 14 a 14 b are connected via an isolatable link 28 which extends between the two buses 20 a 20 b the cryogenic system includes a plurality of refrigeration units 30 a 30 b 32 a 32 b which maintain control the supply of a coolant to the various items of superconducting electrical equipment there are four refrigeration units 30 a 30 b 32 a 32 b in the embodiment shown in fig1 each generator 14 a 14 b has a single dedicated refrigeration unit 30 a 30 b and the electrical motors 22 a 1 22 a 2 22 b 1 22 b 2 each share a refrigeration unit 32 a 32 b with one other motor 22 a 1 22 a 2 22 b 1 22 b 2 of course it will be appreciated that the number and distribution of the refrigeration units will be determined by the type and distribution of the electrical equipment which is in turn determined by the application of the electrical network each refrigeration unit 30 a 30 b 32 a 32 b is connected to its respective piece or pieces of electrical equipment via a coolant pathway in the.