Operating characteristics of unit and group connected generator-HVDC converter schemes

The two alternatives for the direct connection of generators to HVDC power converters, i.e. the unit and group connections, are compared under varying steady-state operating conditions at nominal operating frequency. Direct connection is defined as the connection of synchronous generators and converters without AC filters and with the transformer having simultaneously the function of converter and step-up transformer. With all the units in service both schemes display the same characteristics. By reducing the number of generators in service the group connection becomes more efficient at lower power levels. The opposite effect occurs with the unit connection where the efficiency reduces with the power levels, particularly with fewer generators in service. Moreover, in the last-mentioned case the inverter end of the link requires extra reactive power compensation. Therefore for schemes operating at varying power levels and fixed frequency the group connection should be the preferred alternative