Patent GB-1018823-A (GB)
Title: Arrangement for load-switching between taps on a tapped transformer
Inventor: GNEGEL FRANZ
IPC: H01F29/04
Filed: 1962-07-31
Abstract: 1,018,823. Transformers. SIEMENSSCHUCKERTWERKE A.G. July 31, 1962, No. 29454/62. Heading H1T. A transformer tap changing system incorporates a load switch 6 (Fig. 1) from or to which the load current is transferred to or from controllable rectifier devices 410, 420 during tap changing (or movement of contacts over bared turns), unblocking of the controllable rectifier devices 410, 420 being effected at the appropriate times by transformers such as 20, 30, 40, 50. preselected before tap changing by a single or multi-pole switch 8. To change from tap 2 to tap 3, switch 8 is set as shown and before the load switch contact leaves contact 1 transformer 20 is energized through contacts 22, 73 and an arm 70 mechanically linked to switch 6 to unblock 410 through rectifier 21. The load current then passes through 410 when the main circuit at contact 1 is opened. When arm 70 leaves contact 22 the load current passes through transformer 50 so that device 420 is unblocked and passes load current from tap 3 at the next half-cycle through contact 72. Device 420 is short-circuited when the load switch makes connection at contact 11 and is open-circuited when arm 70 leaves 72 but remains unblocked until switch 8 is moved to the left preparatory to a change back to tap selectors 4. The switch 7 may be omitted by modifying switch 6. For operation at a higher voltage the load switch may be a single pole double break switch, the control transformers being supplied by an auxiliary transformer (Fig. 2, not shown) which may be connected to an external voltage. The two control transformers 20 and 30 may be replaced by current transformers 65, 66 in the leads from the tap selectors (Fig. 3, not shown). The load switch may be a double snap-action switch with two contact arms moving in sequence and the auxiliary switch 7 being omitted (Fig. 4, not shown). The controllable rectifier devices may be protected against excess voltages by inductive capacitative and voltage dependent resistances and possibly by Zener diodes or controllable rectifier devices. In addition the transformers 40 and 50 may be saturable. Protection against excessive currents in the control electrode circuits may also be provided. Alternatively one of the controllable rectifying devices 410 (Fig. 5, not shown) comprises two controllable semi-conductor rectifiers whilst 420 comprises one controllable rectifier and one non-controllable rectifier the circuit being such that irrespective of the direction of tap-changing rectifier devices 410 becomes conductive first. Better protection is obtained by using both load current and stage voltage to secure rapid opening of rectifier device, e.g. 420, using a saturable and a non-saturable transformer 500, 5000 (Fig. 6, not shown) (or a single three-logged transformer) having their secondaries connected in opposition. In an alternative arrangement (Fig. 7) the load current flows simultaneously during tapchanging from both tap selector contacts 4, 5, the auxiliary switch 7 being omitted for reasons of clarity. The controllable rectifier device which takes over the load current first is blocked after the other rectifier device has been unblocked and the current distribution between the tap selector circuits is controlled by chokes 14, 13 which may have their windings on separate legs of a three-legged core. Protective impedances resistances may be used in series with the short circuiting windings 650, 660 of current transformers 65, 66 and may be ohmic, inductive, capacitive or voltage dependent or may be conductors having a negative temperature coefficient.