Patent GB-1240931-A (GB)
Title: Dc power transmission system connected between an ac supply and an ac load
IPC: H02M7/757
Filed: 1969-01-15
Abstract: 1,240,931. Static converter systems. COMPAGNIE GENERALE D'ELECTRICITE. 15 Jan., 1969 [16 Jan., 1968], No. 2499/69. Heading H2F. A two-pole D.C. transmission line linking two A.C. networks, has a 3-phase, 6-rectifier cell Graetz bridge at each end (Fig. 1, not shown) and identical control arrangements (Fig. 2) for each bridge whereby one functions as an invertor and the other as a rectifier according to the desired direction of energy transmission. The control arrangement calculates the necessary firing angles taking into account essential factors such as the "rectifier limiting striking instant "-defined as the minimum delay from the theoretical zero-going positive passage of a rectifier element and the instant it can be fired with safety; also the "inverter limiting striking instant "-defined as the angle separating the firing of a given rectifier element from the instant when a conducting rectifier element passes through zero-i.e. the supplement of the min. angle of advance. Control arrangement (Fig. 2).-A detector 21 senses the zero going positive passages of voltage across a reference rectifier element in the respective bridge producing a pulse (33) (Fig. 3, not shown) in response to the first zero passage only, being provided with a locking arrangement (not shown) to exclude spurious oscillations around zero. This pulse starts a counter 23 which determines the length of a cycle T of the voltage at the reference rectifier by counting the number of cycles from an oscillator 22 during said voltage cycle, the counter being reset by each pulse. A unit 7 which includes a further counter calculates <SP>1</SP>/ 6 th of the measured cycle this value being equal to the spacing between theoretical zero going + ve voltage passages of the other five rectifier elements. A detector 6 detects the actual zero passages and a calculating member 8 determines the interval in time between actual and theoretical zeros. The five difference values are compared in a logic unit 9 which memorizes the largest interval #  max . Variations in the length of the reference rectifier voltage cycles or failure of the voltage across that rectifier are taken into account by unit 7. A unit 4 calculates T/2 from which is subtracted a constant " extinction angle " γ in a unit 5. A function generator -24 calculates the commutation angle " U " (defined as the time interval during which two consecutive rectifier elements are simultaneously conductive) from the value .of .the instantaneous current in the bridge, the value of the peak V between phases, and X 1  of the transformer. The generator 24 .emits an analogue signal which is converted to digital in a unit 25 after passing through a chopper 26. A logic unit 27 effects the choice of " U " adopting the most unfavourable value recorded over at least one cycle. A control signal " r " is established by a regulator 28, proportional to the difference between a reference current and the measured current in the line. A small signal causes a given bridge to function in rectifier mode and a large signal effects invertor mode. The signal r is. numerically coded by the unit 25. The striking angle is determined as follows: The reference zero and each. of the calculated zero passages trigger a counter 29. An adding arrangement 31 adds the number # max  equal to the rectifier limiting striking angle and the control signal r; and secondly calculates the invertor limiting striking angle namely T/2 - # - U - # max . The striking angle should be between these two calculated values. The content of counter 29 is continuously compared in a comparator 30, with firstly the rectifier limiting striking angle #max + signal r, and secondlywith the invertor limiting striking angle: T/2 - γ - U - # max . If quantity # max  + r is lower than the invertor limiting striking instant then striking occurs when the content of counter 29 equals this quantity. If r is nil or negative then striking occurs at the " rectifier limiting striking instant." If r + #max is higher than the invertor stop then striking occurs when counter 29 content equals a value corresponding to the " invertor limiting striking instant." A unit 32 selectively switches the firing pulses to the rectifier cells. In the event of certain fault conditions such as calculation failure, striking defect &c., a member 39 provides. six safety pulses at intervals of 90 degrees which are applied to the rectifier elements trigger electrodes via a gate 40.