Patent GB-813989-A (GB)
Title: Means for controlling static converters in a high voltage dc transmission system
IPC: H02J3/36
Filed: 1955-09-06
Abstract: 813,989. Automatic control systems for D.C. power transmission; converting. ALLMANNA SVENSKA ELEKTRISKA A.B. Sept. 6, 1955 [Sept. 7, 1954], No. 25493/55. Classes 38 (2) and 38 (4). In a high voltage D.C. power transmission system between one or more rectifier stations and one or more inverter stations in which at least one of the stations automatically regulates the transmission voltage and at least one of the other stations automatically regulates its D.C. voltage in such a manner that a " controlled quantity " is held at a predetermined value, the regulators at both stations are additionally controlled to oppose changes in the line current. This reduces current surges due to system faults until protective gear has had time to operate. The " controlled quantity " shown is the frequency of the receiving A.C. network; but it is said that it might be the D.C. power transmitted, the D.C. current transmitted or the transmission voltage, and it may, Fig. 4 (not shown), or it may not, Figs. 2, 3, 5 (not shown), have a differential component added. The current surge reduction signal may either be directly proportional to the line current, Figs. 3 and 4 (not shown), or be proportional to the rate of change of line current, Figs. 2 and 5 (not shown). Normal service. Power is transmitted from an A.C. network 7 to an A.C. network 8 by means of grid controlled rectifiers 3, a high voltage D.C. line 1 (and earth return 2, 2) and grid controlled inverters 4. The inverters are controlled to maintain constant line voltage and the output of the rectifiers 3 is controlled to supply sufficient power to maintain the frequency of the network 8 constant by means of a frequency control signal sent from network 8 by unspecified telemetering equipment (not shown) to unspecified apparatus 20 which controls the rectifier 3 through an unspecified regulator 10. The line current is measured by a transductor 14 and a current (or rate of change of current) signal is obtained from apparatus 12, various versions of which are described with reference to Figs. 2 to 5 (not shown). This signal controls the regulator 10 in such a sense as to oppose the change in current. At the inverter station, unspecified apparatus 21 is also responsive to the frequency of the network 8 and could control the inverter 4 through an unspecified regulator 11. However, under normal service conditions, the output signal from 21 is over-ridden by a signal from a bias source 23 and this controls the regulator 11 in a sense to raise the output voltage of the inverter 4. An undisclosed limiter prevents the regulator 11 from raising the output voltage above a set value. The regulator 11 is also responsive to the line current (or rate of change of current) in a manner similar to regulator 10 except that the undisclosed limiter prevents any increase in inverter output voltage, and the bias source 23 ensures that only a large decrease in line current provides sufficient signal to overcome the bias and reduce the inverter output voltage. Reserve service.-If the telemetering between network 8 and the device 20 fails, then contacts 27 and 30 are opened to remove the device 20 and the bias source 23 from service, and contacts 28, 29 are closed to add a limiter 25 and a bias source 22. The rectifier 3 is now said to control the D.C. line voltage by means of an undisclosed regulator; and the inverter 4 controls the frequency of the network 8 under control of the frequency responsive device 21 which is now free to take charge as bias source 23 has been disconnected, and the maximum voltage limiter is no longer effective as the working point of the inverter has to be dropped below the " normal service " value in order that current can flow from the rectifier 3 which now determines the line voltage. The regulator 11 is still subject to control by the line current (or rate of change of line current) and so tends to prevent current surges. An upper limit to the inverter current is set by the biased rectifier limiter 25 which shunts the frequency responsive device 21 to limit the magnitude of any " raise current " signal applied to the regulator 11. At the rectifier station, a bias source 22 raises the rectifier voltage to a level determined by an undisclosed limiter so that although signals proportional to the line current (or rate of change of line current) are still applied to the regulator 10, those tending to raise the voltage still further are ineffective, and those tending to reduce the voltage are only effective if large enough to overcome the bias source 22, i.e. due to a large increase in current.