Patent GB-1241674-A (GB)
Title: Improvements in or relating to h.v.d.c. transmission systems
IPC: H02J3/36
Filed: 1968-08-02
Abstract: 1,241,674. High voltage direct current transmission systems. BROWN BOVERI & CO. Ltd. 2 Aug., 1968 [5 Aug., 1967], No. 36983/68. Heading H2F. Relates to the control of a H.V.D.C. transmission network (e.g. Fig. 2) having more than two converter stations some of which operate as rectifiers and others as inverters. All of the converter stations have current regulators to limit the maximum direct current therein to a set value. All the inverter stations have voltage regulators which under fault-free conditions are ineffective but in the event of certain faults prevent the station input D.C. voltage falling below a selected minimum value. All the rectifier stations have voltage regulators which are normally ineffective, but in certain fault conditions act to limit the maximum output D.C. voltage of the station. As shown, Fig. 6, the rectifying stations have a summated current/voltage characteristic #GR, and the inverting stations have a summated current/voltage characteristic #WR in which the current increases from zero with a negligible change in voltage to a point equal to or beyond the rated system current and at which the inverter station D.C. voltage rises sharply, the max. value thereof before said rise being equal to or less than the nominal D.C. voltage U A  of the H.V.D.C. system. The two characteristics intersect one another always at one stable duty point, thereby determining the system-voltage and at which duty point the current delivered by the rectifying stations is equal to the current drawn by the inverting stations. Both the current regulators and voltage regulators act on grid control systems of the individual converter stations. As shown, Fig. 7, a converter unit 5 is coupled via a tap changeable transformer 6 to the A.C. lines and via a D.C. bus-bar 7 to the H.V.D.C. network. A current transformer 8 and a voltage transformer 9 detect the D.C. system current and voltage respectively, the values of which are fed to a current regulator 11 and a voltage regulator 10 respectively. The regulators inputs also receive reference values of required system voltage U<SP>*</SP> and current I<SP>*</SP>. The regulators' outputs are fed via decoupling diodes 12, 13 to a common input of a grid control system 14 which feeds a control grid of converter 5 and in addition an angle measuring device 15. The output of device 15 (which is significant of the firing angle a or extinction angle γ) is fed to a follow-up controller 16 which adjusts the tap changer of transformer 6 in one or other direction when, for example, a certain set max. or min. value of firing angle or extinction angle is exceeded, thus restoring the control margin to a region having the most favourable reactive power requirement. One selected converter station determines the nominal voltage of the D.G; network; by means of its voltage regulator the reference value U<SP>*</SP> of which is set to the said nominal value. The reference value of its current regulator is set higher, by a safety margin, than the value corresponding to a system condition that the sum of all direct currents is zero. The current regulators of all the other stations define the pre-set reference current. Either a rectifier station or an inverter station may determine the system D.C. voltage. The set reference voltages of the regulators in the rectifier stations are 5% above the reference value in the voltage determining station while in the inverter stations they are 5% below. If for example the voltage determining station operates as an inverter, on failure thereof, current can no longer be accepted from the rectifying stations so that the rectifying stations' voltages rise. The rectifying station in which the voltage regulator first intervenes then takes over the voltage determining function. Other fault conditions are briefly described. Characteristic curves of multi-point converter operation are given.