Patent GB-461498-A (GB)
Title: Improvements in oil insulated electric power cables
IPC: H02G15/20
Filed: 1935-07-17
Abstract: 461,498. Automatic control systems. TELEFONAKTIEBOLAGET L.M. ERICSSON. July 17, 1935, No. 20389. Addition to 361,528. [Class 38 (iv)] [Also in Group XXXVI] Relates to improvements in or modifications of the invention described in the parent Specification, viz. a heavy current oil insulated cable or other current carrying apparatus in which expansion or contraction of the oil insulation caused by temperature variations is prevented by supplying the cable or apparatus with an amount of heat adjusted in relation to the current loading of the cable or apparatus so that the temperature of the latter is maintained substantially constant. According to one modification, the additional heat is generated in the cable by connecting to the cable a variable wattless load, such as choke coils or induction regulators, so that the wattless load current passes through the cable. A regulating ammeter may be provided which automatically controls the magnitude of the wattless current so that the copper losses in the cable due to the useful and wattless loads connected thereto are maintained constant. In another embodiment, the cable has a split conductor and the distribution of the load current between the two portions of the conductor may be controlled to vary the copper losses in the cable and hence the heat generated therein. For this purpose the two portions of the conductor are connected directly together at one end, the other ends being adjustably connected to different tappings on a supply transformer, or to the ends of a choke coil having a number of tappings to any of which the supply may be connected. Alternatively, a variable reactance may be connected in series with one portion of the split conductor. Fig. 6 shows an arrangement employing a separate heating transformer. The cable is fed from a generator 25 through a supply transformer 26 and the conductor of each phase is split, the two portions 27, 28 of each conductor being connected directly together at the ends adjacent the step-down transformer 31 and being connected together at their other ends through the secondary winding 32 of a heating transformer. The secondary windings of the supply transformer 26 are connected to tappings on the secondary windings 32 of the heating transformers. The primary windings 33 of the heating transformers are connected to a source of current 32<a> which is adjustable in order that the heating current flowing through the split conductors may be varied. In another arrangement, the primary windings of the heating transformers are excited by the supply transformer. To reduce the operating potential of the auxiliary apparatus supplying the heating current, the secondary windings 77, 771, 7711, Fig. 14, of the transformer supplying the split conductors 78, 78<1>, 78<11> may be split and the heating current supplied by an earthed device 80 connected between the two star connected ends of the secondary windings. The device 80 may be dispensed with if the halves of the secondary windings are tapped, the difference of voltage induced in the two portions of the secondary windings causing a heating current to flow in the split conductors. The automatic regulation of the temperature of the cable may be effected by bringing the load and heating currents individually to act on a measuring system in which the two currents produce torques acting in the same direction, the measuring system being arranged to vary the heating current when the torque on the measuring system deviates from a predetermined value. In another modification, the cable is provided with insulated measuring wires having different temperature coefficients, the wires being included in a bridge circuit which is balanced at the normal operating temperature of the cable, but which operates upon a deviation from the normal temperature to increase or decrease the supply of heating current to the cable.