Power conversion system, and a method of power conversion using such a power conversion system

A power conversion system, comprising:

• first and second line commutated converters for converting between a.c. and d.c. power;
• transforming means for connection to an a.c. system, said transforming means having a first part which is YY connected and a second part which is Y.DELTA. connected, said first part of said transforming means being connected to said first line commutated converter, and said second part of said transforming means being connected to said second line commutating converter;
• and
• control means for controlling said first and second line commutated converters respectively, said control means having first and second phase control circuits arranged to generate first and second phase control signals respectively, said first and second phase control signals being respectively a YY minimum voltage signal and a Y.DELTA. minimum voltage signal and being derived from said first and second parts of said transforming means, respectively, said first line commutated converter being connected to said first phase control circuit and being arranged to receive said first phase control signal for controlling the power conversion thereof and said second line commutated converter being connected to said second phase control circuit and being arranged to receive said second phase control signal for controlling the power conversion thereof independently of said power conversion by said first line commutated converter.

A method of power conversion in which power is converted between a.c. and d.c. power by first and second line commutated converters and a transforming means connected to an a.c. system, said transforming means having a first part which is YY connected and a second part which is Y.DELTA. connected, said first part of said transforming means being connected to said first line commutated converter and said second part of said transforming means being connected to said second line commutated converter said first and second line commutated converters being connected in series;

• said method comprising:
• deriving a YY minimum voltage signal and a Y.DELTA. minimum voltage signal from said first and second parts of said transforming means respectively, said YY and Y.DELTA. minimum voltage signals being independent;
• detecting the d.c. current through said first and second line commutated converters and generating a d.c. current signal;
• comparing said d.c. current signal with a reference signal and generating a comparison signal;
• generating first and second control signals, said first control signal being generated using said YY minimum voltage signal and said comparison signal and said second control signal being generated using said Y.DELTA. minimum voltage signal and said comparison signal;
• and
• operating said first and second line commutated converters on the basis of said first and second control signals respectively, so as to convert between a.c. and d.c. power.