Abstract In the transmission of direct current from a rectifier to an inverter one method of control has been to provide a constant current at the rectifier and a constant extinction angle at the inverter controlled by a fixed extinction angle order signal. Instability results with this form of control under certain operating conditions. In the present invention the tendency to instability is reduced by controlling the extinction angle of the inverter in accordance with the fixed order signal when the inverter output current is at least equal to a required level, but increasing the extinction angle when the inverter current is less than the required level. The extinction angle is increased under the control of an auxiliary order signal generated by a summing unit which sums a signal from a control circuit representing the actual extinction angle, the fixed order signal, and a signal from an amplifier dependent on the difference between the actual and required current levels.
A control circuit for controlling a static invertor, including control means for controlling the extinction angle of the invertor at a fixed predetermined value in accordance with a first extinction angle order signal when the level of output current from the invertor is at least as great as a predetermined level, and means for applying to said control means a second extinction angle order signal to cause said control means to increase the extinction angle when the output current level is less than said predetermined level.
Claim 2
Dependent Claim
86 words
A control circuit for controlling a static invertor, including control means for controlling the extinction angle of the invertor at a fixed predetermined value in accordance with a first extinction angle order signal when the level of output current from the invertor is at least as great as a predetermined level, and means for applying to said control means a second extinction angle order signal to cause said control means to increase the extinction angle when the output current level is less than said predetermined level.
Claim 3
Dependent Claim
55 words
A circuit as claimed in claim 1, in which the means for applying a second signal includes means for summing said first signal with a third signal representing the actual extinction angle of the inverter and with a fourth signal derived from comparison of the inverter output current level with said predetermined output current level.
Claim 4
Dependent Claim
43 words
A circuit as claimed in claim 1, including means to generate a fifth signal representing the difference between said inverter output current level and said predetermined current level, and gating means for selectively gating said second and fifth signals to the control means.
Claim 5
Dependent Claim
65 words
A circuit as claimed in claim 3, arranged for use in a DC link in which the inverter is fed by rectifying apparatus, in which circuit said means for generating said fifth signal summates a signal representing a required rectifier output current, a signal representing the inverter output current level, and a current correction signal depending upon the difference between the rectifier and inverter characteristics.
Claim 6
Dependent Claim
31 words
A circuit as claimed in claim 1, in which said second extinction angle order signal is substantially proportional to the difference between the inverter output current level and said predetermined level.
Claim 7
Dependent Claim
14 words
A high-voltage DC transmission system including a control circuit as claimed in claim 1.