Harmonics caused by imbalanced transformer impedances and imperfect twelve-pulse operation in HVDC conversion

In substituting a degree of imbalance between the phase impedances of HVdc converter transformers for the usual assumption of equal impedances, the generated ac harmonics are modified and triplen (non-characteristic) harmonics are produced. They are analysed and assessed; and are not expected to be serious provided due attention is paid to impedance balancing. Certain normal harmonics for 6-pulse operation are ideally eliminated by 12-pulse operation. An analysis is made of the extent of residual 6-pulse harmonics due to the imperfect balance of 12-pulse bridge operation. The paper provides a basis for specifying certain design tolerances.