Method and system of controlling high voltage direct current power transmission plants

• US 4,210,956 A
• Filed: 12/30/1977
• Issued: 07/01/1980
• Est. Priority Date: 01/05/1977
• Status: Expired due to Term

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Watanabe, Atsumi
• Primary Examiner(s): Miller, J. D.
• Assistant Examiner(s): Wong, Peter S.
• Application Number:
  US05/865,874
• Time in Patent Office: 914 Days
• Field of Search: 363/35, 363/36, 363/51, 363/34, 363/37, 363/54, 363/57-58, 363/74, 363/78-79, 307/252 R
• US Class Current: 363/51
• CPC Class Codes: H02J 3/36 Arrangements for transfer of electric power between ac networks via a high-tension dc 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

In a method and system for controlling high voltage direct current power transmission plants, the delay angle for the each converter is calculated directly by a calculating arrangement, which is provided for controlling all converters totally, from data set by an operator and data transmitted by suitable sending-receiving apparatus from each converter. Each converter receives the delay angle signal by suitable sending-receiving apparatus from the calculating arrangement and operates in accordance with the received delay angle value.

The delay angle value is given the following equation. ##EQU1## where α is a delay angle value. E is either a measured or calculated A.C. voltage, and if it is a measured voltage, it is the voltage from an A.C. system to which each converter is connected, V is either a set or calculated D.C. voltage, I_d is either a set or calculated D.C. current and X is a converter commutation reactance.

Claims

1. A method of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, a first data sending means for sending the outputs of said measuring means to a central control means, a first data receiving means for receiving the outputs of the central control means, and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means, and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which method comprises the steps;

(a) setting an operational D.C. voltage Vds, current I_d and a commutation reactance X for each converter station,

(b) calculating by said calculating means a delay angle value α

for each converter in accordance with the following equation ##EQU8## where E is a measured A.C. voltage, and (c) controlling each converter in accordance with the delay angle value calculated therefor by said calculating means of the central control means.

12. A method of controlling an HVDC-power transmission plant according to claim 1, wherein, each converter station further includes a D.C. current measuring means for said HVDC transmission line;

and the method further includes the steps of;

calculating by said calculating means the current deviation Δ

I_d for each converter station, and

calculating by said calculating means a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU20## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α and

the calculated delay angle value α

'"'"'.

13. A method of controlling an HVDC-power transmission plant according to claim 12, each station further including a D.C. voltage measuring means for said HVDC transmission line, and the method further including the steps of,

calculating by said calculating means a voltage deviation Δ

V for each converter station,

calculating by said calculating means a delay angle value α

" for each converter in accordance with the following equation ##EQU21## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α

, the calculated delay angle α

'"'"', and the calculated delay angle value α

".

14. A method of controlling an HVDC-power transmission plant according to claim 12, the method further including;

the steps of,

calculating by said calculating means a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU22## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station and the step of controlling each converter includes selectively controlling the one converter in accordance with the calculated delay angle value α

".

2. A method of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, a first data sending means for sending the outputs of said measuring means to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the coverter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which method comprises the steps;

(a) setting an operational D.C. voltage Vds for one point on the HVDC transmission line, an operational D.C. current I_d and a commutation reactance X for each converter station and a resistance R for each portion of the HVDC transmission line,

(b) calculating by said calculating means a D.C. voltage V for each converter station, each D.C. voltage being calculated by correcting said set operation D.C. voltage with the voltage drop on the HVDC transmission line between said one point and the converter station for which a D.C. voltage is being calculated, calculating by said calculating means a delay angle value α

for each converter in accordance with the following equation ##EQU9## where E is a measured A.C. voltage, and (c) controlling each converter in accordance with the delay angle value calculated therefore by said calculating means of the central control means.

3. A method of controlling an HVDC-power transmission plant according to claim 2, further comprising the step of detecting that the A.C. voltage of each converter station is larger than a predetermined value.

4. A method of controlling an HVDC-power transmission plant according to claim 2, wherein each converter station further includes a D.C. current measuring means for said HVDC transmission line;

and the method further includes the steps of;

calculating by said calculating means the current deviation Δ

I_d for each converter station, and

calculating by said calculating means a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU10## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α and

the calculated delay angle value α

'"'"'.

5. A method of controlling an HVDC-power transmission plant according to claim 4, each station further including a D.C. voltage measuring means for said HVDC transmission line, and the method further including the steps of:

calculating by said calculating means a voltage deviation Δ

V for each converter station,

calculating by said calculating means a delay angle value α

" for each converter in accordance with the following equation, ##EQU11## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α

, the calculated delay angle α

'"'"', and the calculated delay angle value α

".

6. A method of controlling an HVDC-power transmission plant according to claim 4, the method further including the steps of:

calculating by said calculating means a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU12## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station, and

the step of controlling each converter includes selectively controlling the one converter in accordance with the calculated delay angle value α

".

7. A method of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, a first data sending means for sending the outputs of said measuring means to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which method comprises the steps;

(a) setting a minimum delay angle value α

_O for the converter in each rectifier station, a margin angle value γ

_O for the converter in each inverter station, an operational D.C. current I_d and a commutation reactance X for each converter station and a resistance R for each portion of the HVDC transmission line,

(b) calculating by said calculating means a maximum D.C. voltage Vdmax for each converter station in accordance with the following equations, ##EQU13## where, E is a measured A.C. voltage, (c) selecting the converter station which can be operated with the smallest maximum D.C. voltage calculated in the above steps,

(d) calculating by said calculating means a D.C. voltage V for each converter station not selected in step (c) above, each D.C. voltage being calculated by correcting said smallest maximum D.C. voltage with the voltage drop on the HVDC transmission line between said selected station and the station for which a D.C. voltage is being calculated,

calculating by said calculating means a delay angle value α

for each converter in accordance with the following equation ##EQU14## where, E is a measured A.C. voltage, V is a selected smallest D.C. voltage or calculated D.C. voltage in above steps, and

(f) controlling each converter in accordance with the delay angle value calculated therefore by said calculating means of the central control means.

8. A method of controlling an HVDC-power transmission plant according to claim 7, wherein, each converter station further includes a D.C. current measuring means for said HVDC transmission line;

and the method further includes the steps of;

calculating by said calculating means the current deviation Δ

I_d for each converter station, and

calculating by said calculating means a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU15## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α and

the calculated delay angle value α

'"'"'.

9. A method of controlling an HVDC-power transmission plant according to claim 8, each station further including a D.C. voltage measuring means for said HVDC transmission line, and the method further including the steps of:

calculating by said calculating means a voltage deviation Δ

V for each converter station,

calculating by said calculating means a delay angle value α

" for each converter in accordance with the following equation, ##EQU16## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α

, the calculated delay angle α

'"'"', and the calculated delay angle value α

".

10. A method of controlling an HVDC-power transmission plant according to claim 8, the method further including the steps of:

calculating by said calculating means a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU17## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station, and

the step of controlling each converter includes selectively controlling the one converter in accordance with the calculated delay angle value α

".

11. A method of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, an encoder for coding the the opening or closing of circuit breakers in said A.C. system, a first data sending means for sending the outputs of said measuring means and said encoder to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which method comprises the steps;

(a) setting an operational D.C. current I_d and a commutation reactance X for each converter station, a resistance R for each portion of the HVDC transmission line, some constants of said A.C. system, a minimum delay angle value α

_O for the converter in each rectifier station and a margin angle value γ

_O for the converters in each inverter station,

(b) calculating by said calculating means each A.C. voltage E of each said A.C. system in accordance with said some constants of each said A.C. system in the case where each said operational D.C. current is changed,

(c) calculating by said calculating means a maximum D.C. voltage Vdmax for each converter station in accordance with the following equations, ##EQU18## (d) selecting the converter station which can be operated with the smallest maximum D.C. voltage calculated in the above steps,

(e) calculating by said calculating means a D.C. voltage V for each converter station not selected in step (d) above, each D.C. voltage being calculated by correcting said smallest D.C. voltage with the voltage drop on the HVDC transmission line between said selected station and the station for which a D.C. voltage is being calculated,

(f) calculating by said calculating means a delay angle value Δ

for each converter in accordance with the following equation, ##EQU19## where, V is the selected smallest maximum D.C. voltage or the D.C. voltage calculated in the above steps, and

(g) controlling each converter in accordance with the delay angle value calculated therefore by said calculating means of the central control means.

15. A method of controlling an HVDC-power transmission plant according to claim 11, wherein, each converter station further includes a D.C. current measuring means for said HVDC transmission line;

and the method further includes the steps of;

calculating by said calculating means the current deviation Δ

I_d for each converter station, and

calculating by said calculating means a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU23## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α and

the calculated delay angle value α

'"'"'.

16. A method of controlling an HVDC-power transmission plant according to claim 15, each station further including a D.C. voltage measuring means for said HVDC transmission line, and the method further including the steps of:

calculating by said calculating means a voltage deviation Δ

V for each converter station,

calculating by said calculating means a delay angle value α

" for each converter in accordance with the following equation, ##EQU24## the step of controlling each converter includes selectively controlling each converter in accordance with at least one of the calculated delay angle value α

, the calculated delay angle α

'"'"', and the calculated delay angle value α

".

17. A method of controlling an HVDC-power transmission plant according to claim 15, the method further including the steps of:

calculating by said calculating means a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU25## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station, and

the step of controlling each converter includes selectively controlling the one converter in accordance with the calculated delay angle value α

".

18. A system of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected onits A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, a first data sending means for sending the outputs of said measuring means to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which system further comprises;

(a) means for setting an operational D.C. voltage Vds, current I_d and a commutation reactance X for each converter station, and

(b) said delay angle value calculating means calculating a delay angle value α

for each converter in accordance with the following equation ##EQU26## where, E is a measured A.C. voltage.

19. A system of controlling an HVDC-power transmission plant according to claim 18;

wherein, each converter station further includes a D.C. current measuring means for said HVDC transmission line;

means for calculating the current deviation Δ

I_d for each converter station, and said delay angle value calculating means including means for calculating a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU27##

20. A system of controlling an HVDC-power transmission plant according to claim 19, wherein each converter station further includes a D.C. voltage measuring means for said HVDC transmission line, means for calculating a voltage deviation Δ

V for each converter station, said delay angle value calculating means including means for calculating a delay angle value α

" for each converter in accordance with the following equation, ##EQU28##

21. A system of controlling an HVDC-power transmission plant according to claim 19, wherein said delay angle value calculating means includes means for calculating a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU29## where, X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station.

22. A system of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, a first data sending means for sending the outputs of said measuring means to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which system further comprises;

(a) means for setting an operational D.C. voltage Vds for one point on the HVDC transmission line, an operational D.C. current I_d and a commutation reactance X for each converter station and a resistance R for each portion of the HVDC transmission line,

(b) means for calculating a D.C. voltage V for each converter station, each D.C. voltage being calculated by correcting said set operation D.C. voltage with the voltage drop on the HVDC transmission line between said one point and the converter station for which a D.C. voltage is being calculated, and

(c) said delay angle value calculating means calculating a delay angle value α

for each converter in accordance with the following equation, ##EQU30## where, E is a measured A.C. voltage.

23. A system of controlling an HVDC-power transmission plant according to claim 22, further comprising means for detecting that the A.C. voltage of each converter station is larger than a predetermined value.

24. A system of controlling an HVDC-power transmission plant according to claim 22, wherein each converter station further includes a D.C. current measuring means for said HVDC transmission line;

means for calculating the current deviation Δ

I_d for each converter station, and said delay angle value calculating means including means for calculating a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU31##

25. A system of controlling an HVDC-power transmission plant according to claim 24, wherein each converter station further includes a D.C. voltage measuring means for said HVDC transmission line, means for calculating a voltage deviation Δ

V for each converter station, said delay angle value calculating means including means for calculating a delay angle value α

" for each converter in accordance with the following equation, ##EQU32##

26. A system of controlling an HVDC-power transmission plant according to claim 24, wherein said delay angle value calculating means includes means for calculating a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU33## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station.

27. A system of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. current measuring means for said D.C. transmission line, an encoder for coding the opening or closing of circuit breakers in an said A.C. system, a first data sending means for sending the outputs of said measuring means and said encoder to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which system further comprises;

(a) means for setting a minimum delay angle value α

₀ for the converter in each rectifier station, a margin angle value γ

₀ for the converter in each inverter station, an operational D.C. current I_d and a commutation reactance X for each converter station and a resistance R for each portion of the HVDC transmission line,

(b) means for calculating a maximum D.C. voltage Vdmax for each converter station in accordance with the following equations, ##EQU34## where, E is a measured A.C. voltage, (c) means for selecting the converter station which can be operated with the smallest maximum D.C. voltage calculated in the above steps,

(d) means for calculating a D.C. voltage V for each converter station not selected in step (c) above, each D.C. voltage being calculated by correcting said smallest maximum D.C. voltage with the voltage drop on the HVDC transmission line between said selected station and the station for which a D.C. voltage is being calculated, and

(e) said delay angle value calculating means for calculating a delay angle value α

for each converter in accordance with the following equation ##EQU35## where, E is a measured A.C. voltage, V is a selected smallest D.C. voltage or calculated D.C. voltage in above steps.

28. A system of controlling an HVDC-power transmission plant according to claim 27, wherein each converter station further includes a D.C. current measuring means for said HVDC transmission line;

means for calculating the current deviation Δ

I_d for each converter station, and said delay angle value calculating means including means for calculating a delay angle α

'"'"' for each converter in accordance with the following equation, ##EQU36##

29. A system of controlling an HVDC-power transmission plant according to claim 28, wherein each converter station further includes a D.C. voltage measuring means for said HVDC transmission line, means for calculating a voltage deviation Δ

V for each converter station, said delay angle value calculating means including means for calculating a delay angle value '"'"'" for each converter in accordance with the following equation, ##EQU37##

30. A system of controlling an HVDC-power transmission plant according to claim 28, wherein said delay angle value calculating means includes means for calculating a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU38## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station.

31. A system of controlling an HVDC power transmission plant, which comprises a D.C. transmission line, said transmission line comprising two D.C. conductors;

at least two converter stations, at least one of said stations being a rectifier and at least one an inverter station, each station being connected on its A.C. side to an A.C. system and on its D.C. side in parallel with the other stations through a D.C. reactor, and being provided with an A.C. voltage measuring means for said A.C. system, a D.C. voltage measuring means for said D.C. transmission line, a D.C. current measuring means for said D.C. transmission line, an encoder for coding the opening or closing of circuit breakers in an said A.C. system, a first data sending means for sending the outputs of said measuring means and said encoder to a central control means, a first data receiving means for receiving the outputs of the central control means and an automatic pulse phase shifter for sending a firing pulse to the converter in the station in accordance with the outputs of said first data receiving means;

the central control means being provided with a second data receiving means for receiving the outputs of said first data sending means, a setting means for setting certain data, a calculating means for calculating a delay angle value for each converter in accordance with the outputs of said second data receiving means and said setting means and a second data sending means for sending the outputs of said calculating means to said first receiving means;

which system further comprises;

(a) means for setting an operational D.C. current I_d and a commutation reactance X for each converter station, a resistance R for each portion of the HVDC transmission line, some constants of said A.C. system, a minimum delay angle value α

₀ for the converter in each rectifier station and a margin angle value γ

₀ for the converters in each inverter station,

(b) means for calculating each A.C. voltage E of each said A.C. system in accordance with said some constants of each said A.C. system in the case where each said operational D.C. current is changed,

(c) means for calculating a maximum D.C. voltage Vdmax for each converter station in accordance with the following equations, ##EQU39## (d) means for selecting the converter station which can be operated with the smallest maximum D.C. voltage calculated in the above steps,

(e) means for calculating a D.C. voltage V for each converter station not selected in step (d) above, each D.C. voltage being calculated by correcting said smallest D.C. voltage with the voltage drop on the HVDC transmission line between said selected station and the station for which a D.C. voltage is being calculated, and

(f) said delay angle value calculating means calculating a delay angle value α

for each converter in accordance with the following equation, ##EQU40## where, V is the selected smallest maximum D.C. voltage or the D.C. voltage calculated in the above steps.

32. A system of controlling an HVDC-power transmission plant according to claim 31, wherein each converter station further includes a D.C. current measuring means for said HVDC transmission line;

means for calculating the current deviation Δ

I_d for each converter station, and said delay angle value calculating means including means for calculating a delay angle value α

'"'"' for each converter in accordance with the following equation, ##EQU41##

33. A system of controlling an HVDC-power transmission plant according to claim 32, wherein each converter station further includes a D.C. voltage measuring means for said HVDC transmission line, means for calculating a voltage deviation Δ

V for each converter station, said delay angle value calculating means including means for calculating a delay angle value α

" for each converter in accordance with the following equation, ##EQU42##

34. A system of controlling an HVDC-power transmission plant according to claim 32, wherein said delay angle value calculating means includes means for calculating a delay angle value α

" for only one converter, which evidences a voltage change that is contrary to the changes evidenced by the all other stations, with the following equation, ##EQU43## where X'"'"' is the commutation reactance and Δ

I_d '"'"' is the D.C. current deviation of another converter station.