Abstract: In the multi terminal flexible direct current（DC）transmission system，the flexible direct inner and

outer loop control can not provide inertial support for the receiving end passive network. Traditional virtual

synchronous generator（TVSG）control can provide inertia support for passive network，but it can only realize

primary frequency regulator of passive network. To solve this problem，the virtual frequency regulator（VFR）control was proposed. In the VFR control ，the angle frequency and DC voltage deviation were introduced into the TVSG control，so that the converter station connected to the passive network had unbiased frequency modulation

performance，and the power change was adjusted according to the change of DC voltage to achieve power balance. In PSCAD/EMTDC，a multi terminal flexible DC system model with passive network was built for simulation，and the effectiveness of VFR control strategy was verified.

      Key words: multi terminal flexible direct current（DC）；passive network；virtual synchronous generator（VSG）；virtual frequency regulator control；unbiased frequency regulator