Three wire DC transmission circuit with modulated pole

• US 9,893,649 B2
• Filed: 10/24/2016
• Issued: 02/13/2018
• Est. Priority Date: 10/22/2015
• Status: Active Grant

• Current Assignee: Qatar Foundation For Education, Science And Community Development, Qatar University
• Original Assignee: Qatar University
• Inventors: Ahmed, Shehab, Elserougi, Ahmed A., Massoud, Ahmed M., Abdel-Khalik, Ayman S.
• Primary Examiner(s): Laxton, Gary L
• Assistant Examiner(s): NOVAK, PETER MICHAEL
• Application Number:
  US15/333,136
• Publication Number:
  US 20170117817A1
• Time in Patent Office: 477 Days
• CPC Class Codes: H02J 1/08 Three-wire systems; Systems having more than three wires
• CPC Class Codes: H02J 3/02 using a single network for simultaneous distribution of power at different frequencies; using a single network for simultaneous distribution of ac power and of dc power
• CPC Class Codes: H02J 4/00 Circuit arrangements for mains or distribution networks not specified as ac or dc
• CPC Class Codes: H02M 7/7575 for high voltage direct transmission link
• CPC Class Codes: Y02E 60/60 Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The method of converting high voltage AC lines into bipolar high voltage DC systems makes use of the three transmission lines (referred to as the positive pole, the negative pole, and the modulating pole) in an existing high voltage AC system as transmission lines in a bipolar high voltage DC system. When current from the power source is up to the thermal current limit of the transmission lines, the transmission lines operate in two-wire mode, where current is delivered in the positive pole and returned in the negative pole, the modulating pole being open. When power source current exceeds the thermal current limit, operation is in three-wire mode, alternating for predetermined periods between parallel configuration of the positive pole and the modulating pole to divide current for delivery to the load, and parallel configuration of the negative pole and the modulating pole, dividing the return current.

Claims

1. A method of converting high voltage AC lines into bipolar high voltage DC systems, the high voltage AC lines including three transmission lines designated as a positive pole, a negative pole, and a modulating pole, comprising the steps of:

providing a system for converting high voltage AC lines into bipolar high voltage DC systems, the system comprising;

three transmission lines designated as a positive pole, a negative pole, and a modulating pole;

a first switch (S_g) having an (S_g) pole and an (S_g) throw;

a second switch (S_r) having an (S_r) pole and an (S_r) throw, the (S_r) pole being connected to the (S_g) throw, the (S_r) throw being connected to an input of the negative pole transmission line, the (S_g) pole being connected to an input of the positive pole transmission line;

a current limiting resistor (R_Limit);

a plurality of step resistors (R_st1, R_st2 . . . , R_stn) connected in series, the (R_stn) resistor having a terminal lead connected to an input of the modulating pole, the (R_st1) resistor having a terminal lead connected to the limit resistor (R_Limit), and the current limiting resistor (R_Limit) having a terminal lead connected to the (S_r) pole-(S_g) throw connection;

a bypass switch (S_L) selectively shunting the limit resistor (R_Limit), bypass switches (S_st1, S_st2 . . . , Sstn) selectively shunting corresponding resistors (R_st1, R_st2 . . . , R_stn);

a plurality of diodes (D_r) connected in series, the series of diodes having an anode end connected to an output of the negative pole transmission line and a cathode end connected to an output of the modulating pole transmission line;

a plurality of diodes (D_g) connected in series, the series of diodes having an anode end connected to the output of the modulating pole transmission line and a cathode end connected to an output of the positive pole transmission line; and

a controller sequencing switching of the switches (S_g), (S_r), (S_L), and (S_st1, S_st2 . . . , Sstn), wherein the sequencing controller includes;

i) means for opening switches (S_g) and (S_r) when the source current is at or below the rated thermal current limit of the positive pole and negative pole transmission lines, the circuit then being operated in two-wire mode with no current flowing through the modulating pole transmission line;

ii) means for operating in three-wire-mode when the source current is above the rated thermal current limit, the circuit alternating between a first state and a second state;

iii) means for configuring switch (S_g) in a closed position and switch (S_r) in an open position during the first state, the flow of source current to a connected grid being divided between the positive pole transmission line and the modulating pole transmission line, and current returning from the grid to the source being carried only by the negative pole transmission line, the first state having a duration based on a length of time the transmission lines can safely carry the source current given the rated thermal current limit;

iv) means for configuring switch (S_g) in an open position and switch (S_r) in a closed position during the second state, the flow of source current to the connected grid being solely through the positive pole transmission line, but current returning from the grid to the source being split between the modulating pole transmission line and the negative pole transmission line;

v) means for successively inserting and removing the step resistors (R_st1, R_st2 . . . , R_stn) connected to the modulating pole transmission line using corresponding switches (S_st1, S_st2 . . . , S_stn) during a transient period between the first and second states in order to provide for a smooth transition in current;

vi) means for initiating the means for successively inserting the step resistors (R_st1, R_st2 . . . , R_stn) during a transition period from the first state to the second state to gradually decrease the current of the modulating pole transmission line;

vii) means for inserting the limit resistor (R_Limit) during the transition period from the first state to the second state;

viii) means for removing the limit resistor (R_Limit) during a transition period from the second state to the first state; and

ix) means for initiating the means for successively removing the step resistors (R_st1, R_st2 . . . , R_stn) during the transition period from the second state to the first state to gradually increase the current of the modulating pole transmission line;

wherein a positive terminal of the bipolar high voltage DC output is formed at the output of the positive pole transmission line and a negative terminal of the bipolar high voltage DC output is formed at the output of the negative pole transmission line;

operating the transmission lines in two-wire mode when current supplied by a power source is up to a rated thermal current limit of the transmission lines, the current being carried to a load in the positive pole and returned from the load to the power source in the negative pole, the modulating pole being unused and left open; and

operating the transmission lines in three-wire mode when current supplied by a power source exceeds the rated thermal current limit of the transmission lines, the three-wire mode alternating between a first state and a second state, the first state including the step of switching the positive pole and the modulating pole into parallel configuration to divide current delivered from the power source to the load between the positive pole and the modulating pole for a period of time 0.5 T where T is a length of time a pole may carry current higher than limit current without exceeding a maximum permissible temperature of the transmission line, current being returned to the power source only through the negative pole, the second state including the step of switching the modulating pole and the negative pole into parallel configuration to divide current returned from the load to the power source between the modulating pole and the negative pole for a period of time 0.5T, the current being delivered from the power source to the load solely in the positive pole.

2. The method of converting high voltage AC lines into bipolar high voltage DC systems according to claim 1, wherein the root mean square (rms) current of the positive and negative poles, I₁ and I₂, are equal, being characterized by the relation:

3. The method of converting high voltage AC lines into bipolar high voltage DC systems according to claim 2, wherein the rms currents of the positive and the negative pole are set equal to their thermal limit, the result being characterized by the relation:

I_1(rms)=I_2(rms)=0.79I=I_L,

where, I_L is the thermal limit of the lines.

4. The method of converting high voltage AC lines into bipolar high voltage DC systems according to claim 3, wherein the method provides power transfer increase of approximately 27% over a bipolar structure without exceeding the thermal limit (IL).

5. A system for converting high voltage AC lines into bipolar high voltage DC systems, comprising:

three transmission lines designated as a positive pole, a negative pole, and a modulating pole;

a first switch (S_g) having an (S_g)_pole and an (S_g) throw;

a second switch (S_r) having an (S_r) pole and an (S_r)throw, the (S_r) pole being connected to the (S_g) throw, the (S_r) throw being connected to an input of the negative pole transmission line, the (S_g) pole being connected to an input of the positive pole transmission line;

a current limiting resistor (R_Limit),

a plurality of step resistors (R_st1, R_st2 . . . , R_stn) connected in series, the (R_stn) resistor having a terminal lead connected to an input of the modulating pole, the (R_st1) resistor having a terminal lead connected to the limit resistor (R_Limit), and the current limiting resistor (R_Limit) having a terminal lead connected to the (S_r) pole-(S_g) throw connection;

a bypass switch (S_L) selectively shunting the limit resistor (R_Limit), switches (S_st1, S_st2 . . . , Sstn) selectively shunting corresponding resistors (R_st1, R_st2 . . . , R_stn);

a plurality of diodes (D_r) connected in series, the series of diodes having an anode end connected to an output of the negative pole transmission line and a cathode end connected to an output of the modulating pole transmission line;

a plurality of diodes (D_g) connected in series, the series of diodes having an anode end connected to the output of the modulating pole transmission line and a cathode end connected to an output of the positive pole transmission line; and

a controller sequencing switching of the switches (S_g), (S_r), (S_L), and (S_st1, S_st2 . . . , Sstn), wherein the sequencing controller includes;

i) means for opening switches (S_g) and (S_r) when the source current is at or below the rated thermal current limit of the positive pole and negative pole transmission lines, the circuit then being operated in two-wire mode with no current flowing through the modulating pole transmission line;

ii) means for operating in three-wire-mode when the source current is above the rated thermal current limit, the circuit alternating between a first state and a second state;

iii) means for configuring switch (S_g) in a closed position and switch (S_r) in an open position during the first state, the flow of source current to a connected grid being divided between the positive pole transmission line and the modulating pole transmission line, and current returning from the grid to the source being carried only by the negative pole transmission line, the first state having a duration based on a length of time the transmission lines can safely carry the source current given the rated thermal current limit;

iv) means for configuring switch (S_g) in an open position and switch (S_r) in a closed position during the second state, the flow of source current to the connected grid being solely through the positive pole transmission line, but current returning from the grid to the source being split between the modulating pole transmission line and the negative pole transmission line;

v) means for successively inserting and removing the step resistors (R_st1, R_st2 . . . , R_stn) connected to the modulating pole transmission line using corresponding switches (S_st1, S_st2 . . . , S_stn) during a transient period between the first and second states in order to provide for a smooth transition in current;

vi) means for initiating the means for successively inserting the step resistors (R_st1, R_st2 . . . , R_stn) during a transition period from the first state to the second state to gradually decrease the current of the modulating pole transmission line;

vii) means for inserting the limit resistor (R_Limit) during the transition period from the first state to the second state;

viii) means for removing the limit resistor (R_Limit) during a transition period from the second state to the first state; and

ix) means for initiating the means for successively removing the step resistors (R_st1, R_st2 . . . , R_stn) during the transition period from the second state to the first state to gradually increase the current of the modulating pole transmission line;

wherein a positive terminal of the bipolar high voltage DC output is formed at the output of the positive pole transmission line and a negative terminal of the bipolar high voltage DC output is formed at the output of the negative pole transmission line.

6. The system according to claim 5, wherein the switches are bidirectional semiconductor switches.

7. The system according to claim 5, wherein the switches are fast switches.

8. The system according to claim 5, wherein the resistors R_st1 through R_stn have values according to a formula characterized by:

9. The system according to claim 5, wherein the modulating pole transmission line comprises a first wire having a resistance R_m connected in parallel with a second wire having a resistance R_m/1.732.

10. The system according to claim 5, further comprising:

a connection of the positive pole input and the negative pole input of the three transmission lines to a two-pole undersea cable fed by an offshore wind power generating source;

a first voltage source converter (VSC) connected to the positive pole and negative pole transmission line outputs;

a connection of inputs to a bipolar transmission line to the two pole undersea cable;

a second VSC connected to outputs of the bipolar transmission line;

means for switching a droop gain constant associated with the three transmission lines from K_1(3w) to K_1(2w) when the three transmission line circuit is being operated in two-wire mode; and

means for switching the droop gain constant from K_1(2w) to K_1(3w) when the three transmission line circuit is being operated in three-wire mode.