Due to manufacturing tolerances in equipment

the parameters of the upper and lower arms of modular multilevel converters are not entirely identical

resulting in a DC bias current on the valve side of the transformer. In severe cases

this may lead to transformer damage. To clarify the correspondence between DC bias and arm parameter asymmetry and improve transformer operating conditions

this paper derives the expression for DC bias current under asymmetries in the inductance

resistance

capacitance

as well as number of submodules of the upper and lower arms by decoupling the DC

50Hz

and 100Hz components of the switching function. Then

to avoid controller saturation caused by the bias current measurement deviations

this paper proposes a two-dimension DC bias suppression strategy

effectively reducing the DC bias current under arm parameter asymmetry. Finally

based on the real-time hardware-in-the- loop simulation platform of a voltage source converter-high voltage direct current (VSC-HVDC) transmission project

the DC current under different influencing factors is verified. Additionally

the practical application effect of the proposed strategy in the field is demonstrated. After implementing the strategy

the DC current on the valve side of the transformer is significantly reduced

and the transformer vibration and noise are also noticeably improved

providing a typical paradigm for the optimization of control strategies in subsequent flexible DC transmission projects.