TRANSMISSION SYSTEM AND A METHOD FOR CONTROL THEREOF

• US 20090219737A1
• Filed: 06/15/2006
• Published: 09/03/2009
• Est. Priority Date: 01/18/2006
• Status: Active Grant

• Current Assignee: ABB Power Grids Switzerland AG (ABB Limited)
• Original Assignee: ABB Technology Limited (ABB Ltd.)
• Inventors: Lungqvist, Rolf, Bjorklund, Hans
• Granted Patent: US 8,300,435 B2
• Time in Patent Office: Days
• US Class Current: 363/35
• CPC Class Codes: H02J 3/36 Arrangements for transfer of electric power between ac networks via a high-tension dc link
• 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

An HVDC transmission system including a rectifier station and an inverter station each having a series connection of at least two converters. A by-pass DC breaker is connected in parallel with each converter. A control device is adapted to deblock a blocked converter by starting to control the converter at high delay angle and gradually decreasing the delay angle until substantially all DC current flows through the converter and to then control the by-pass breaker to open at substantially zero current, and to stop the operation of a converter by controlling the converter at gradually increasing delay angle until the voltage across the converter is substantially zero and to then control the converter to be blocked by firing a by-pass pair thereof and to then control the by-pass breaker to close for taking over all the DC current when the voltage between a neutral bus and a pole of a transmission line between the stations is to be increased and reduced, respectively.

Claims

1. A high voltage direct current transmission system, comprising:

a converter station arranged at each end of an HVDC high voltage direct current transmission line, each converter station being configured to connect said transmission line to an AC system, each said station comprising a series connection of at least two converters having a DC side thereof connected to a pole of said transmission line on high potential and to a neutral bus on zero potential by being grounded, a first of said converter stations being adapted to operate as a rectifier and, a second of said converter stations being adapted to operate as an inverter, each converter comprising a by-pass DC breaker connected in parallel in a current path forming a by-pass of said converter when said DC breaker is closed, each converter station comprising an arrangement adapted to control DC current flowing through said pole and comprising for each converter of each station a device for controlling that converter and the current therethrough for either increasing the voltage between said neutral bus and said pole and thereby power transmitted between said stations by starting operation of the converter when being blocked or reducing the voltage between said neutral bus and said pole and thereby the power transmitted between said stations by stopping operation of the converter when being active, wherein each said control device is adapted to deblock a blocked said converter by starting to control the converter at a high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through that converter and to then control said by-pass breaker to open at substantially zero current, and to stop the operation of said converter by controlling the controller at a gradually increasing delay angle until the voltage across the converter is substantially zero and to then control the converter to be blocked by firing a by-pass pair thereof and to then control said by-pass breaker to close for taking over all the DC current.

2. The transmission system according to claim 1, wherein each said station comprises two said converters connected in series between said neutral bus and said pole.

3. The transmission system according to claim 1, wherein each said station comprises more than two said converters connected in series between said neutral bus and said pole, and wherein said control devices of each station are adapted to increase or reduce the power transmitted between the stations by starting or stopping the operation of one converter of that converter station at a time.

4. The transmission system according to claim 1, wherein said arrangement is adapted to control the DC current to be on a substantially constant, full current value under normal conditions of the system irrespectively of how many of the series connected converters in each station are in operation, wherein said control devices of said converters are co-ordinated to increase the power transmitted by starting operation of a converter being inactive in said series connection of converters for by that raising the voltage between the neutral bus and the pole as follows:

a control device belonging to a converter, called starting converter, in said rectifier being inactive is adapted to deblock said converter by starting to control the converter at high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through that converter and then to control the by-pass breaker in parallel therewith to open at substantially zero current, the control device belonging to an inactive converter of the inverter is adapted to then deblock said converter by starting to control it at high delay angle and gradually decreasing the delay angle until substantially all the DC-current flows through that converter and to then open the by-pass breaker in parallel therewith at substantially zero current, and wherein said arrangement is adapted to then control said starting converter to increase the voltage between the neutral bus and the pole to a voltage corresponding to the increased number of converters in operation.

5. The transmission system according to claim 1, wherein said control devices of said converters are co-ordinated for reducing the power transmitted between the stations at a substantially constant direct current flowing in said transmission line by reducing the voltage between said neutral bus and said pole as follows:

a control device belonging to a converter in said inverter station is adapted to control that converter at a gradually increasing delay angle until the voltage across this converter is close to zero, to then block the converter by firing a by-pass pair thereof and to then close the by-pass breaker in parallel with that converter, the control devices of the converters of the rectifier station are adapted to then compensate for the voltage drop between said pole and neutral bus by increasing the delay angle at which these converters are controlled, a control device belonging to a converter of the rectifier station is adapted to then control said converter at a gradually increasing delay angle until the voltage across this converter is close to zero, to then control the converter to be blocked by firing a by-pass pair thereof and to then close said by-pass breaker for taking over all the DC current.

6. The transmission system according to claim 1, wherein each said converter comprises twelve converter valves arranged in a 12-pulse configuration with three legs of four converter valves connected in series.

7. The transmission system according to claim 1, wherein said converters of the converter station operating as rectifier are adapted to generate, when all in operation, a direct voltage above 600 kV, between said neutral bus and pole.

8. The transmission system according to claim 1, wherein the transmission system is designed to be able to conduct a DC current being above 500 A, in said pole between the converter stations.

9. A method for controlling a high voltage direct current transmission system, said system comprising at each end of a high voltage direct current transmission line a converter station for connecting said transmission line to an AC system, each converter station comprising a series connection of at least two converters having a DC side thereof connected on one hand to a pole of said transmission line on high potential and on the other to a neutral bus on zero potential by being grounded, a first of said converter stations being adapted to operate as rectifier and a second of said converter stations being adapted to operate as inverter, each converter comprising a by-pass DC breaker connected in parallel therewith in a current path forming a by-pass of said converter when said breaker is closed, said method comprising:

controlling DC current through said pole to be on a substantially constant, full current value under normal conditions of said transmission system,

increasing a voltage between said neutral bus and said pole and thereby a power transmitted between said stations by starting operation of the converters being blocked, or

reducing the voltage between said neutral bus and said pole and thereby the power transmitted between said converter stations by stopping operation of converters being active,

wherein increasing the voltage between the neutral bus and the pole comprises deblocking a blocked converter by starting to control the blocked converter at a high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through that converter and to then control said by-pass breaker in parallel with that converter to open at substantially zero current, and

wherein reducing the voltage between the neutral bus and the pole the operation of said converter is stopped by controlling the converter at a gradually increasing delay angle until the voltage across the converter is substantially zero and then controlling the converter to be blocked by firing a by-pass pair thereof and then controlling said by-pass breaker in parallel with that converter to close for taking over all the DC current.

10. The method according to claim 9, wherein the method is carried out for a said transmission system in which each said station has two said converters connected in series between said neutral bus and said pole.

11. The method according to claim 9, wherein the method is carried out on a said transmission system having in each said converter station more than two said converters connected in series between said neutral bus and said pole, and wherein the power transmitted between the stations is increased or reduced by starting or stopping the operation of one converter of each converter station at a time.

12. The method according to claim 9, wherein increasing the voltage between the neutral bus and the pole comprises increasing the power transmitted between said converter stations by deblocking a converter, here called starting converter, in said rectifier being inactive by starting to control the starting converter at high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through the starting converter, wherein the by-pass breaker in parallel with the starting converter is then opened at substantially zero current, wherein an inactive converter of the inverter is then deblocked by starting to control it at high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through the starting converter, wherein the by-pass breaker in parallel with the starting converter is then opened at substantially zero current, and wherein said starting converter is then controlled to increase the voltage between the neutral bus and the pole to a voltage corresponding to the increased number of converters in operation.

13. The method according to claim 9, wherein reducing the voltage between the neutral bus and the pole the power transmitted between the stations is reduced in that a converter in said inverter station is controlled at a gradually increasing delay angle until the voltage across this converter is close to zero, wherein this converter is then blocked by firing a by-pass pair thereof, that the by-pass breaker in parallel with that converter is then closed, wherein the converters of the rectifier station are then controlled to compensate for the voltage drop between said pole and the neutral bus by increasing the delay angle at which these converters are controlled, wherein a converter of the rectifier station is then controlled at a gradually increasing delay angle until the voltage across this converter is close to zero, wherein this converter is then blocked by firing a by-pass pair thereof, and wherein the by-pass breaker in parallel with the converter last mentioned is closed for taking over all the DC current.

14. A computer program product, comprising

a computer readable medium;

and

computer program instructions recorded on the computer readable medium and executable by a processor for carrying out a method for controlling a high voltage direct current transmission system, said system comprising at each end of a high voltage direct current transmission line a converter station for connecting said transmission line to an AC system, each converter station comprising a series connection of at least two converters having a DC side thereof connected on one hand to a pole of said transmission line on high potential and on the other to a neutral bus on zero potential by being grounded, a first of said converter stations being adapted to operate as rectifier and a second of said converter stations being adapted to operate as inverter each converter comprising a by-pass DC breaker connected in parallel therewith in a current path forming a by-pass of said converter when said breaker is closed said method comprising;

controlling a DC current through said pole to be on a substantially constant, full current value under normal conditions of said transmission system,

increasing a voltage between said neutral bus and said pole and thereby a power transmitted between said stations by starting operation of the converters being blocked, or

reducing the voltage between said neutral bus and said pole and thereby the power transmitted between said converter stations by stopping operation of converters being active,

wherein increasing the voltage between the neutral bus and the pole comprises deblocking a blocked converter by starting to control the blocked converter at a high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through that converter and to then control said by-pass breaker in parallel with that converter to open at substantially zero current, and

wherein reducing the voltage between the neutral bus and the pole the operation of said converter is stopped by controlling the converter at a gradually increasing delay angle until the voltage across the converter is substantially zero and then controlling the converter to be blocked by firing a by-pass pair thereof and then controlling said by-pass breaker in parallel with that converter to close for taking over all the DC current.

15. The computer program product according to claim 14, wherein the computer program instructions further comprise providing the computer program instructions at least partially through a network.

16. The computer program product according to claim 15, wherein the network is the internet.