Patent GB-1297639-A (GB)
IPC: H02J3/36
Filed: 1969-12-09
Abstract: 1297639 HVDC converter systems BROWN BOVERI & CO Ltd 9 Dec 1969 [11 Dec 1968] 60003/69 Heading H2F A converter station for tapping an HVDC transmission link intermediate its ends, to feed power to or receive power from an A.C. network, comprises converter bridges 3-6 (Fig. 2) connected in series on their D.C. sides and through which the main direct current of the link passes, and coupled to the A.C. network via a transformer having windings 7-11. The D.C. link comprises lines 1 and 2 (Fig. 1, not shown) which are at the same voltage and respectively positive and negative with respect to earth, the switching out of one line necessitating the use of earth return for the other line. A first control circuit including a switch unit 31 applies a first control value successively to respective grid control units St 3 -St 6  of the respective bridges and a second control circuit including a regulator 36 provides a second control value which is applied simultaneously to all of the grid control units. Opposed diodes at the input to each grid control unit ensure that only the higher potential control value is effective to control the respective grid control unit and bridge. The first control circuit 26-31 functions as the main control, the frequency of the A.C. network being detected by a frequency detector 26 controlling the exchange of energy between the A.C. and D.C. systems and a current P significant of the nominal power of the A.C. network occurs at the output of 26 when the A.C. frequency has its rated value. The current P is fed to device 27 together with the actual D.C. line current value I detected by D.C. transformer 28. From the values of P and I the device 27 produces a voltage value Vd. soll  which is applied to a D.C. voltage regulator 29 together with an actual D.C. voltage value from a transformer 30. The resultant output from regulator 29 is applied to a control switch unit 31 together with the voltage value Ud. soll . which consequently determines the control angle of the current converters and hence the amount of power fed therethrough. Switch 31 determines which bridge is to be affected by the regulator 29 output, the remaining three bridges being simultaneously operated with the minimum permissible control angle resulting in minimum reactive power requirements for the bridges. The second control circuit simultaneously controls the bridges in order to keep the alternating voltage output of the station substantially constant. This circuit includes a regulator 36 which compares the actual value V ist  of the A.C. voltage with a nominal value V soll  and produces respective outputs to the grid control units St 3  to St 6 . In the event of a major fall in A.C. voltage and power factor occurring with only a small change in frequency then regulator 36 overrides the control by switch 31. Operation.-For zero power flow between the systems, bridges 3, 4 are operated fully in the inverter mode and 5, 6 in the rectifier mode. If a fall in the A.C. network frequency is detected the inputs to the switch 31 alter. The switch 31 then firstly drives the bridge 5 fully into the inverter range i.e. to γmin. This achieves an increase in the D.C. voltage between points 24 and 25. If further increase is called for the switch 31 applies an output to the next grid control unit St 6  so that bridge 6 is driven into its inverter mode its control angle being varied until the desired total output voltage is achieved. The converter bridges may be selectively connected to either line 1 or line 2 (Fig. 1, not shown) via polarity reversing switches.